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Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities
Chapter 21
Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities Competition.............................................................................................. 572 Mutualism and Facilitation..................................................................... 581 Other Species Interactions...................................................................... 583 Summary.................................................................................................. 584 Questions for Thought............................................................................ 585
Figure 21.1 An experimental stream mesocosm facility used to study interactions among organisms. Each cylinder represents a pool and each rectangular portion a riffle, with water recirculating in each channel. In the panel below, two herbivorous fish species (Campostoma anomalum and Phoxinus erythrogaster) are being used in a behavioral experiment in the same mesocosm. © 2010 Elsevier Inc. All rights reserved. Doi: 10.1016/B978-0-12-374724-2.00021-0 10.1016/B978-0-12-374724-2.00001-5
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Competition is central to understanding many aspects of aquatic communities and, thus, has been the focus of much research. Other ways that species interact (including indirect interactions, succession, mutualism, and the effects of keystone species) also have consequences in determining community structure. Some of these concepts were discussed in previous chapters including basic definitions of types of interactions (Chapter 8), classification of interaction types (e.g., trait-mediated versus density-mediated interactions; Chapter 19), and interactions involving microorganisms (Chapter 19). This chapter focuses on interactions among larger organisms.
Competition Competition (both species have a negative effect on each other) has been described as a dominant community interaction. It can occur between organisms from any taxon, but it is more likely to occur among organisms in the same functional group or guild. We discuss competition among species of plants and animals in this section. Competition occurs within a species (intraspecific) or between species (interspecific). Intraspecifc competition is density-dependent in that competition for a given resource such as food becomes more intense as the relative availability decreases (density-independent factors are not a function of population size). At some level of competition, mortality increases or natality decreases, which results in reduced or negative population growth. This type of interaction is important in natural systems, as well as aquaculture and fisheries management, where the goal is to maximize growth of desirable species. Intraspecific competition can also reduce individual growth rates. For example, tadpoles can reach very high densities in breeding ponds. In experiments where different total numbers of tadpoles are placed in the same sized rearing tanks, reduced growth occurs with increasing densities (Fig. 21.2). In natural ponds in Sweden, experimentally increased densities of Rana temporaria tadpoles also resulted in significantly higher mortality rates (Loman, 2004). Density manipulations of other amphibian species have found that tadpoles reared at high densities metamorphose earlier, and at a smaller size, than those reared at lower densities, which has implications for individual fitness (Richter et al., 2009). Intraspecific competition can be a cause of disruptive selective pressure as different morphologies or strategies of resource use are selected for and the intermediate form is at a disadvantage. For example the three-spine stickleback, Gasterosteus aculeatus, lives in lakes and individuals within the same species and the same lake specialize on pelagic or benthic food sources. The morphology of the gill rakers reflects this specialization. In experimental manipulations, when both morphologies were present the difference between the morphologies was more pronounced. The effect was accentuated at higher
Instantaneous growth rate (d�1)
Competition
0.12 0.10 0.08 0.06 0.04 0.02 0.00
5
40
60
160
Density (# per container)
Figure 21.2 Daily instantaneous growth rates of Rana tigrina tadpoles reared at different densities. Numbers below each bar represent densities of individuals that were reared in containers of the same size. (Data replotted from Dash and Hota, 1980).
densities as would be expected if it was a result of intraspecific competition for resources (Bolnick, 2004). Both intraspecific and interspecific competition can be divided into two general types. Scramble competition (exploitative competition) occurs when all individuals are “scrambling” to acquire as much as they can of the limiting resource; as availability of the resource decreases, each individual gets less, and all are equally affected. This type of competition does not involve direct interactions among members of the population and is quite common when population densities are high. In contest competition (interference competition), some individuals get more than others by directly interfering with the ability of others to acquire the resource. Examples of intraspecific contest competition include competition among male dragonflies for territories and mates and competition among male tree frogs for calling perches. Contest competition involves direct interactions among members of the population (e.g., individuals interfering with another individual’s ability to acquire the resource). Allelopathy among plant species and territoriality in animals are common forms of interspecific interference competition. Competition is often difficult to establish directly in the field, in part because evolution leads to decreased overlap in resource use. Intraspecific competition might not be apparent in cases where organisms inhabit defined territories. Interspecific competitors that are usually found in the same habitat at the same time rarely specialize on the same resource. For example, three-spined sticklebacks (Gasterosteus spp.) in three lakes of British Columbia have recently evolved from a single marine species. In all these cases, the new pairs of species found in each lake partition the habitat by feeding on benthic or limnetic invertebrates. They have evolved mechanisms of reproductive isolation to ensure that
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their offspring will inherit their parents’ behavioral traits. Limnetic adults prefer to mate with limnetic adults and benthic adults prefer to mate with benthic adults (Rundle et al., 2000). The concept of competition is often tightly interwoven with that of the niche, the ecological role of an organism, particularly as defined by its resource consumption. The idea of niche is somewhat controversial with respect to variability in types of resources used over time, and how much overlap occurs in diets of various species over time. Regardless, species often specialize on resources and partition those resources among species based on varied attributes of those resources. The idea that competition can be difficult to find in extant communities because evolution has led to species partitioning niches and not competing now is called the “ghost of competition past.” It is very difficult to prove that this effect is real (Connel, 1980). Still, the idea provides one good explanation of why it could be difficult to document competition in field experiments. An additional reason that competition may be difficult to document is that competitive exclusion may lead to conditions where potential competitors do not cooccur. We discussed competitive exclusion in Chapter 17, in the section “The Paradox of the Plankton and Nutrient Limitation.” The concept behind the competitive exclusion principle is that under equilibrium conditions, the superior competitor is expected to completely eliminate the inferior competitor. The principle also assumes that both competitors specialize on the same resources. The exclusion may take many generations, but eventually the superior competitor will come to dominate. The discussion on the paradox of the plankton explored how nonequilibrium conditions, predation, mutualism, disease, and multiple limiting factors could explain coexistence of potentially competitive species of phytoplankton. Many of these concepts can apply equally well to interspecific and intraspecific competitors in any aquatic habitat. For example, if only one genotype was the superior intraspecific competitor, then natural selection should lead to little genetic variation within a species. However, the factors that cause the competitive exclusion principle to fail also select for increased variability within species. Likewise, the types of factors that cause the competitive exclusion principle not to apply to phytoplankton also can increase diversity of potential competitors in other habitats. Nonequilibrium conditions are expected to be more prevalent in benthic habitats, streams, and wetlands than in the pelagic zones of large lakes. Predation and disease are common features of all habitats (see prior chapter). The assumptions of the competitive exclusion principle thus may not always apply and coexistence of interspecific competitors could be a potential feature of many habitats. Intraspecific competition can lead to strong selective pressures on organisms. These interactions can be avoided by organisms in some instances. For example,
Competition
intraspecific competition can lead to “Alee effects,” where organisms self-limit their population growth so as not to crash the entire population. The interplay between success of individuals and success of the group has led to substantial controversy in the field of evolution. Sexual selection can result from intraspecific competition. For example, male fishes can have broadly different morphology from females ultimately related to competition for mates (Fig. 21.3). In a more complex example, water striders (Aquarius remigis) have intense competition for mates. The water striders congregate in pools and males compete to mate with females. The intense competition to mate actually can harm the females. In conditions where there is only
Figure 21.3 Breeding-age adult central stonerollers (Campostoma anomalum). The male, in the top panel, has obvious tubercles and fin markings. These features could play a part in aggressive interactions among males competing for mates as well as female mate choice. The female is plain. (Photograph courtesy of Gerold Sneegas).
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one habitat, the most aggressive males win because they are the most successful at mating. However, when there are several habitats available (e.g., a stream with connected pools, some with many water striders and others with few), females will leave pools with many males to avoid being overwhelmed by mating attempts. If less aggressive males inhabit the pools with few water striders, then they will also mate successfully when females enter their pools to avoid the higher density pools. This is a case where population structure can alter the selective pressures of intraspecific competition (Eldakar et al., 2009). We now document several types of interspecific competition that commonly occur in freshwater environments. Primary producers compete for nutrients and light. Competition between macrophyte species has been well documented (Gopal and Goel, 1993). Macrophytes can form dense stands that shade all primary producers below them (Haslam, 1978). Floating macrophytes, such as water hyacinth (Eichhornia) or duckweed (Lemna), can blanket the surface of a lentic habitat and essentially remove all light. Macrophytes can intercept 90% of the incident light before it reaches a lake bottom, leading to a 65% decrease in benthic algal production (Lassen et al., 1997). If systems are sufficiently shallow, emergent primary producers intercept light before it reaches the water’s surface. Competition for light has been documented among macrophytes inhabiting a stream in North Carolina (Everitt and Burkholder, 1991). In this case, riparian vegetation and macrophyte assemblages were characterized in 10 stream segments. The red alga Lemanea australis or the aquatic moss Fontanalis dominated low-light assemblages in the winter. High-light sites were dominated by L. australis and the angiosperm Podostemum ceratophyllum. These sites were on the same stream and had similar water velocity and depth, indicating that light availability as influenced by riparian shading was the major abiotic difference between them. Apparently, competition for light structured the macrophyte assemblages. Macrophytes and phytoplankton compete for light and nutrients in shallow lakes. Several species of macrophytes are capable of interference competition; they release allelopathic chemicals that inhibit phytoplankton species (Gross et al., 2007). In shallow lakes dominated by macrophytes, the deeper portions are often characterized by relatively clear water (low phytoplankton concentrations). Thus, allelopathy can stabilize macrophyte dominance in shallow lakes (Hilt and Gross, 2008). Wetland plant assemblages can be shaped by competition. Competition between two species of wetland plants may occur aboveground (for light) or belowground (for nutrients), and both may be important simultaneously (Twolan-Strutt and Keddy, 1996). Competitive rankings of individual plants can remain stable against changes in nutrients and flooding in some instances (Keddy et al., 1994). However, wetland plants that occur in disturbed
Competition
habitats may be released from competition (Keddy, 1989). These studies suggest that competition can determine which plant species dominate in a particular wetland. An example of competition determining spatial patterns in macrophytes is found in two species of the cattail, Typha latifolia and T. angustifolia. Typha latifolia has broad leaves and can outcompete T. angustifolia in shallow water, but not in deeper water (Grace and Wetzel, 1998). As the season progresses, the two species are almost completely segregated (Fig. 21.4). This segregation
�20 0
Typha latifolia Typha angustifolia
Depth (cm)
20 40 60 80 100 0
(A)
200
400
600
800
1000
1200
Biomass (grams m�2)
�20 0
Depth (cm)
20 40 60 80 100 0
(B)
200
400
600
800
1000
1200
1400
1600
1800
Biomass (grams m�2)
Figure 21.4 Competitive exclusion of two species of Typha. Distributions at the same locations in the same wetland in May (A) and September (B). (Data plotted from Grace and Wetzel, 1998).
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CHAPTER 21: Nonpredatory Interspecific Interactions among Plants and Animals
occurs even though both species have optimum growth rates at 50cm depth when occurring in isolation. Zooplankton species often compete for food resources. In Chapter 19, we discussed the idea that ratios of nutrients may alter competitive ability or lead to coexistence of phytoplankton species. Similar resource partitioning could lead to coexistence of potentially competing zooplankton species (Lampert, 1997; DeMott, 1995). Such application of resource ratio theory has vastly expanded in application (Sterner and Elser, 2002). Resource ratio theory predicts how trade-offs associated with using two or more limited resources influence how species coexist (Tilman, 1982). For example, two species that are differentially limited by two resources will coexist at intermediate ratios of resource supply. At more skewed ratios of supply, the best competitor for the most limiting resource can exclude other species. In some cases, a trade-off may occur in competitive ability. For example, one species of rotifer (Keratella cochlearis) may compete better at a lower food concentration and the other (Keratella earlinae) at higher concentrations (Fig. 21.5). Both species could coexist in a spatially or temporally variable environment. Competitive interactions among zooplankton species probably vary over space and time. In Chapter 20, we discussed the idea that large zooplankton are superior competitors for phytoplankton cells. However, this competitive interaction may not be so simplistic. Large zooplankton may slow or cease
0.3 0.2 Growth (d–1)
578
0.1 0.0 –0.1 Keratella earlinae K. cochlearis
–0.2 –0.3 0.0
0.5
1.0
1.5
Cell concentration (mg L–1)
Figure 21.5 Competitive ability of two species of rotifers (Keratella cochlearis and K. earlinae) that feed on the cryptomonad Rhodomonas. Note that K. cochlearis is able to grow more rapidly at lower concentrations of food, but K. earlinae grows better at higher concentrations of food. (Redrawn from Stemberger and Gilbert, 1985).
Competition
feeding in blooms of large inedible algae while smaller zooplankton continue to feed unhindered on subdominant small phytoplankton and bacteria (Gliwicz, 1980). Similarly, cladocerans may be superior competitors for food compared to rotifers, but suspended clay can decrease the growth of cladocerans while having little effect on rotifers. Thus, the clay allows rotifers to be released from competition with cladocerans (Kirk and Gilbert, 1990). There are situations in which competitive exclusion should operate but may not be strong enough to drive out weaker competitors. An example occurred with five species of herbivorous zooplankton in a small humic lake (Hessen, 1990). These species coexisted, but bottle experiments showed no evidence of significant predation or a single dominant competitor. Weak competition for abundant nutrient-poor food in this lake was hypothesized to allow coexistence of the species. This study and those noted previously are a few of many that suggest that competition is an important but context-dependent feature of zooplankton assemblages.
Mean plant species abundance (% cover)
Invasive species are often very strong competitors. For example, purple loosestrife (Lythrum salicaria) has expanded rapidly in the United States, displacing other wetland species (Blossey et al., 2001). The reed canary grass (Phalaris arundinacea) is also a recent invasive species. In the wetlands where they have invaded, they often become dominant; purple loosestrife makes up about 30% of the total abundance, and canary grass about 8% overall (Fig. 21.6). In most cases where both were present (6 of 7), purple loosestrife was dominant. In 6 of 24 wetlands, reed canary grass was the most abundant species, and in 7 of 24 cases 100 80 60 40 20 0
Reed canary grass
Purple loostrife
Figure 21.6 Relative abundance of two invasive species (purple loosestrife Lythrum salicaria and reed canary grass Phalaris arundinacea) in Pacific Northwest US wetlands. Box plots show median, the box the 25th and 75th percentiles, and the whiskers the 90th and 10th percentiles. While overall neither species dominates in all wetlands, there is a high degree of dominance in some wetlands. (Data replotted from Schooler et al., 2006).
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purple loosestrife was dominant (Schooler et al., 2006). These data indicate that the invaders are strong competitors and that in situations where they attain high abundance they likely decrease plant diversity of the wetlands they have invaded. Competition for food has led to clear specialization of benthic freshwater macroinvertebrates over evolutionary time. Thus, invertebrates can be classified into functional feeding groups based on mode of food acquisition, such as collectors, filterers, shredders, and scrapers. Within these groups, competition can still occur. For example, net-spinning hydropsychid caddisflies collect particles from the water column with silk nets of different mesh sizes, with each mesh size characteristic of a species. Smaller mesh nets are more efficient at collecting small particles, whereas large meshes capture large particles more effectively (Loudon and Alstad, 1990). Thus, species with fine nets will likely compete more effectively for fine particles. Such competitive specialization may determine which species dominate. Although competition among coexisting species using similar resources, such as assemblages of net-spinning caddisflies, might be expected, interactions can be complex and sometimes counterintuitive. Laboratory experiments using different numbers of species of net-spinning caddisflies found that the particle capture efficiency of individual species was actually enhanced by the presence of other species because of changes in near bed flow dynamics (Cardinale et al., 2002). Competition among pond-breeding amphibian species has been studied. In some ponds, predators remove the competitive dominant frogs, and predation thus allows less competitive species to coexist with stronger competitors (Morin, 1983). The chytrid fungus Batrachochytrium dendrobatidis, which has been linked to amphibian declines around the world (Sidebar 11.2), can alter competitive interactions among amphibian species that do not die from infection. In the presence of the fungus, both fowler’s toad (Bufo fowleri) and gray tree frog (Hyla versicolor) tadpoles metamorphosed at smaller body masses when reared together in tanks compared to when they were reared separately (Parris and Cornelius, 2004). The toad tadpoles also had strong negative effects on tree frog tadpole development, but only in the presence of the fungus. Competition among fishes may be an important consideration for fisheries managers. For instance, Hodgson et al. (1991) demonstrated that introduction of rainbow trout (Oncorhynchus mykiss) into lakes with 2- and 3-year-old largemouth bass (Micropterus salmoides) can lower the condition (weight to length ratio) of bass. The diet of the bass shifted from Daphnia to odonate naiads, and bass condition was lower after the introduction of trout into one lake, particularly when compared to a nearby lake with no trout. Cage experiments suggest that sunfishes (Centrarchidae) compete when predators drive them to take refuge in macrophyte beds (Mittelbach, 1988), and competition
Mutualism and Facilitation
may occur with juveniles and translate into lower production of adult fish (Osenberg et al., 1992).
Mutualism and facilitation Mutualisms (both species have a positive effect on each other) are less conspicuous in freshwater than in marine systems, possibly because the continuous time for evolution of mutualisms has been less in freshwaters than in marine systems (i.e., many freshwater habitats have a shorter continuous history than marine or terrestrial habitats). However, it seems that some of the conditions for mutualism occur in freshwater. For example, fish that clean other fishes are common on marine reefs, and this mutualism involves many species of fishes from diverse taxonomic groups, but the same interaction has not been identified in freshwaters despite comparable benefits to freshwater fishes and the long geological age of some river systems. Many of the mutualisms that occur in freshwaters involve microorganisms and were discussed in Chapter 19. Mutualisms based on behavior require coevolved systems and organisms capable of complex behavioral patterns, such as fishes. Cichlids from Lake Tanganyika demonstrate parental care, including guarding eggs and fry from predators, and two species can brood in the same region and mutually defend their broods (Keenleyside, 1991). Fishes in the same lake have evolved cooperation in which predators hunt in mixed groups and this cooperation increases success (Nakai, 1993). Mixed-feeding schools may occur in other fish assemblages but have not been well studied. This is not a completely unique adaptation; birds have demonstrated a similar cooperative strategy of mixed-feeding flocks. Facilitation is any unidirectional positive effect of one species on another. Facilitation can include mutualism and commensalism, as well as some exploitation relationships. Facilitation may be an important and overlooked aspect of community interactions (Bertness and Callaway, 1994). Plants in stressful environments can facilitate each other (Callaway, 1995; Callaway and Walker, 1997). Few macrophyte and wetland plant assemblages have been studied with regard to facilitation, but it could be important in stressful freshwater habitats, as has been demonstrated for estuarine marshes (Bertness and Hacker, 1994). For instance, emergent freshwater marsh plants that are aerenchymous (transport oxygen to their roots) can facilitate other emergent plants living nearby by aerating the sediments (Callaway and King, 1996). As more research is done on aquatic plant assemblages, more examples of facilitation will likely be documented, given the importance of facilitation among terrestrial plants (Brooker et al., 2008). Indirect facilitation may occur in streams. Crayfish exclude the green alga Cladophora from pools. The exclusion of Cladophora facilitates the growth of
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Table 21.1 Influence of Mutualistic Midge Larvae on Nostoc Nitrogen Fixation Rates, O2 Concentrations, and Photosynthetic Rates Measurement
Units
Condition
Mean (95% confidence band)
15
Del 15N of Nostoc
With midge Without midge Over midge Away from midge
0.188 (0.005) 0.153 (0.009) 0.67 (0.09) 0.81 (0.06)
Over midge Away from midge
1.4 (0.3) 1.1 (0.2)
N2 incorporation
mmol L1 Microscale O2 concentration with microelectrodes Photosynthetic rate mol O2 L1 s1 with microelectrodes (Data from Dodds, 1989)
epilithic diatoms. The diatoms in turn support an increased biomass of grazing insect larvae (Hart, 1992; Creed, 1994). Such complex interactions may be a common feature of communities. An interesting mutualism occurs between the chironomid midge larva Cricotopus nostocicola and the cyanobacterium Nostoc parmeliodes (Fig. 19.15C). The midge receives sustenance from the Nostoc and lives inside it until pupation and emergence as an adult (Brock, 1960). The midge is a poor swimmer and highly susceptible to predation without the shelter in the tough leathery Nostoc colony. In turn, the midge increases the photosynthetic rate of the Nostoc (Ward et al., 1985) by altering its morphology and by attaching it more firmly onto rocks so it can extend into flow and have a smaller diffusive boundary layer (Dodds, 1989). Respiration by the midge lowers O2 concentration in the vicinity of the larva and enhances nitrogen fixation rates as well as photosynthetic rates (Table 21.1). This example is particularly curious because the eukaryotic organism is endosymbiotic to the bacterial organism. Snails could facilitate other grazing snails of different species by providing substrata for algae to grow on their shells (Abbot and Bergey, 2007). In this case the nutrients excreted by grazing snails can stimulate growth of diatoms that attach to their shells. Other snails then graze the diatoms off of the snail shells. Grazing tadpoles in tropical streams facilitate small grazing mayflies by removing sediments from substrata and exposing underlying periphyton; grazing mayflies respond by increasing their densities in tadpole-grazed patches (Ranvestel et al., 2004; Whiles et al., 2006). Facilitation was demonstrated in a group of three oligochaete species (Fig. 21.7), but the mechanisms were not clear (Brinkhurst et al., 1972). In this case, two of three species tested had greater weight gains when grown with the others than when grown in a single-species culture. Facilitation may also be a
Other Species Interactions
Relative growth (final g/initial g)
14 12 10 8 6 4 2 0
L +T +P +P L L +T L
Limnodrilus
T +L +P +P T T +L T
Tubifex
P +L +T +T P P +L P
Peloscolex
Figure 21.7 Growth of three tubificid oligochaetes alone and in culture with the other species. L, Limnodrilus hoffmeisteri; T, Tubifex tubifex; and P, Peloscolex multisetosus. Note that in two of three experiments, growth was greater in the presence of the other two species. (Data from Brinkhurst et al., 1972).
common feature of stream invertebrates that process litter. Shredders excrete fine particulate organic material that is ingested by collectors. Collectors may remove fine material that interferes with shredders or excrete nutrients that stimulate the microbes and make litter usable for shredders. Such facultative links merit additional study. Facilitation could be more important for some predators in streams than thought previously. For example, it has been assumed that wading birds compete with bass for prey, but a cage experiment challenged this assumption. In a prairie stream in Illinois where the dominant predatory wading birds are great blue heron (Ardea herodias), green heron (Butorides virescens), and great egret (Ardea alba), cages were used to manipulate access by birds and smallmouth bass (Micropterus dolomieu) (Steinmetz et al., 2008). The birds are effective predators only in shallow waters, whereas bass can consume prey in deeper water. In this case, more small prey fishes were consumed with both types of predators (wading birds and fish) present than either alone, and the effect was greater than additive. This experiment suggests facilitation occurs between the predators because prey fishes could not find refuge in deep or shallow waters.
Other species interactions In addition to predation (/), competition (/), and mutualism (/), other species interactions (neutralism (0/0), amensalism (/0), and commensalism (/0)) may be important but are rarely studied. Several
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examples are given here to explain potentially important interactions in which one species has an influence on a species that does not have an influence in return. Tadpoles of a common frog (Rana temporaria) have a negative effect on the snail, Lymnaea stagnalis, by competing for microalgae (Brönmark et al., 1991). The snail then consumes lower quality Cladophora and excretes nutrients that stimulate microalgal growth. The tadpole has a strong negative effect on the snail, but the snail has a weak positive effect on the tadpole. Macrophytes in lakes and ponds may provide a vital habitat for survival of small fishes. The macrophytes apparently receive little direct benefit from the fishes that live in them. However, the macrophytes could receive some mineralized nutrients from fish excretion. Also, indirect relationships could occur if the small fishes eat invertebrates that remove competitive epiphytes or prey upon herbivorous insects that eat the macrophytes. Macrophytes can also provide nutrients to epiphytes growing on them. The macrophyte Najas flexilis was planted in sediments labeled with radioactive phosphorus. The amount of the radioisotope taken into epiphytes was quantified using track autoradiography. A substantial portion of epiphyte phosphorus was derived from the macrophytes. Furthermore, different species obtained their phosphorus in varied proportions with the filamentous and more erect forms relying more heavily on the water column for nutrients, which indicated niche separation (Moeller et al., 1988). Similarly, nitrogen-fixing microbes could leak nitrogen and stimulate nearby organisms that are unable to use N2. This interaction is manipulated in rice paddies under traditional cultivation as discussed in Chapter 19. Most interactions of aquatic organisms with humans are amensal. Humans have negative effects on many aquatic species, but the effects of most of these species on humans are negligible. With some thought, the reader could identify more examples.
Summary 1. Competition has been well documented for species from a variety of taxonomic groups found in freshwaters. 2. Competition can occur between members of the same species (intraspecific) and members of different species (interspecific). 3. Competition can be divided into two general types; scramble competition, where individuals exploit the same resource, and inference competition, where individuals interfere with the ability of others to use a resource. 4. It can be difficult to establish competition in field studies.
Questions for Thought
5. Mutualism, amensalism, and commensalism have been studied much less than predation or competition, but they may be important at times. 6. Facilitation among plant species in harsh environments is an example of positive interactions that may be important in wetland communities. 7. Facilitation may be much more common than previously thought.
Questions for Thought 1. How can disturbance in a habitat act as an agent of natural selection? 2. Are indirect interactions so strong and numerous that they complicate predicting the effects of interactions within an ecological community? 3. Is there a maximum diversity based on competition? 4. How might temporal patterns of disturbance act so that competition becomes unimportant? 5. Are positive or negative interactions more important in natural communities? 6. Why are both per capita interaction strength and the number of species interacting important? 7. Can you think of additional cases of facilitative interactions that were not included earlier?
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Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities Competition.............................................................................................. 572 Mutualism and Facilitation..................................................................... 581 Other Species Interactions...................................................................... 583 Summary.................................................................................................. 584 Questions for Thought............................................................................ 585
Figure 21.1 An experimental stream mesocosm facility used to study interactions among organisms. Each cylinder represents a pool and each rectangular portion a riffle, with water recirculating in each channel. In the panel below, two herbivorous fish species (Campostoma anomalum and Phoxinus erythrogaster) are being used in a behavioral experiment in the same mesocosm. © 2010 Elsevier Inc. All rights reserved. Doi: 10.1016/B978-0-12-374724-2.00021-0 10.1016/B978-0-12-374724-2.00001-5
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Competition is central to understanding many aspects of aquatic communities and, thus, has been the focus of much research. Other ways that species interact (including indirect interactions, succession, mutualism, and the effects of keystone species) also have consequences in determining community structure. Some of these concepts were discussed in previous chapters including basic definitions of types of interactions (Chapter 8), classification of interaction types (e.g., trait-mediated versus density-mediated interactions; Chapter 19), and interactions involving microorganisms (Chapter 19). This chapter focuses on interactions among larger organisms.
Competition Competition (both species have a negative effect on each other) has been described as a dominant community interaction. It can occur between organisms from any taxon, but it is more likely to occur among organisms in the same functional group or guild. We discuss competition among species of plants and animals in this section. Competition occurs within a species (intraspecific) or between species (interspecific). Intraspecifc competition is density-dependent in that competition for a given resource such as food becomes more intense as the relative availability decreases (density-independent factors are not a function of population size). At some level of competition, mortality increases or natality decreases, which results in reduced or negative population growth. This type of interaction is important in natural systems, as well as aquaculture and fisheries management, where the goal is to maximize growth of desirable species. Intraspecific competition can also reduce individual growth rates. For example, tadpoles can reach very high densities in breeding ponds. In experiments where different total numbers of tadpoles are placed in the same sized rearing tanks, reduced growth occurs with increasing densities (Fig. 21.2). In natural ponds in Sweden, experimentally increased densities of Rana temporaria tadpoles also resulted in significantly higher mortality rates (Loman, 2004). Density manipulations of other amphibian species have found that tadpoles reared at high densities metamorphose earlier, and at a smaller size, than those reared at lower densities, which has implications for individual fitness (Richter et al., 2009). Intraspecific competition can be a cause of disruptive selective pressure as different morphologies or strategies of resource use are selected for and the intermediate form is at a disadvantage. For example the three-spine stickleback, Gasterosteus aculeatus, lives in lakes and individuals within the same species and the same lake specialize on pelagic or benthic food sources. The morphology of the gill rakers reflects this specialization. In experimental manipulations, when both morphologies were present the difference between the morphologies was more pronounced. The effect was accentuated at higher
Instantaneous growth rate (d�1)
Competition
0.12 0.10 0.08 0.06 0.04 0.02 0.00
5
40
60
160
Density (# per container)
Figure 21.2 Daily instantaneous growth rates of Rana tigrina tadpoles reared at different densities. Numbers below each bar represent densities of individuals that were reared in containers of the same size. (Data replotted from Dash and Hota, 1980).
densities as would be expected if it was a result of intraspecific competition for resources (Bolnick, 2004). Both intraspecific and interspecific competition can be divided into two general types. Scramble competition (exploitative competition) occurs when all individuals are “scrambling” to acquire as much as they can of the limiting resource; as availability of the resource decreases, each individual gets less, and all are equally affected. This type of competition does not involve direct interactions among members of the population and is quite common when population densities are high. In contest competition (interference competition), some individuals get more than others by directly interfering with the ability of others to acquire the resource. Examples of intraspecific contest competition include competition among male dragonflies for territories and mates and competition among male tree frogs for calling perches. Contest competition involves direct interactions among members of the population (e.g., individuals interfering with another individual’s ability to acquire the resource). Allelopathy among plant species and territoriality in animals are common forms of interspecific interference competition. Competition is often difficult to establish directly in the field, in part because evolution leads to decreased overlap in resource use. Intraspecific competition might not be apparent in cases where organisms inhabit defined territories. Interspecific competitors that are usually found in the same habitat at the same time rarely specialize on the same resource. For example, three-spined sticklebacks (Gasterosteus spp.) in three lakes of British Columbia have recently evolved from a single marine species. In all these cases, the new pairs of species found in each lake partition the habitat by feeding on benthic or limnetic invertebrates. They have evolved mechanisms of reproductive isolation to ensure that
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their offspring will inherit their parents’ behavioral traits. Limnetic adults prefer to mate with limnetic adults and benthic adults prefer to mate with benthic adults (Rundle et al., 2000). The concept of competition is often tightly interwoven with that of the niche, the ecological role of an organism, particularly as defined by its resource consumption. The idea of niche is somewhat controversial with respect to variability in types of resources used over time, and how much overlap occurs in diets of various species over time. Regardless, species often specialize on resources and partition those resources among species based on varied attributes of those resources. The idea that competition can be difficult to find in extant communities because evolution has led to species partitioning niches and not competing now is called the “ghost of competition past.” It is very difficult to prove that this effect is real (Connel, 1980). Still, the idea provides one good explanation of why it could be difficult to document competition in field experiments. An additional reason that competition may be difficult to document is that competitive exclusion may lead to conditions where potential competitors do not cooccur. We discussed competitive exclusion in Chapter 17, in the section “The Paradox of the Plankton and Nutrient Limitation.” The concept behind the competitive exclusion principle is that under equilibrium conditions, the superior competitor is expected to completely eliminate the inferior competitor. The principle also assumes that both competitors specialize on the same resources. The exclusion may take many generations, but eventually the superior competitor will come to dominate. The discussion on the paradox of the plankton explored how nonequilibrium conditions, predation, mutualism, disease, and multiple limiting factors could explain coexistence of potentially competitive species of phytoplankton. Many of these concepts can apply equally well to interspecific and intraspecific competitors in any aquatic habitat. For example, if only one genotype was the superior intraspecific competitor, then natural selection should lead to little genetic variation within a species. However, the factors that cause the competitive exclusion principle to fail also select for increased variability within species. Likewise, the types of factors that cause the competitive exclusion principle not to apply to phytoplankton also can increase diversity of potential competitors in other habitats. Nonequilibrium conditions are expected to be more prevalent in benthic habitats, streams, and wetlands than in the pelagic zones of large lakes. Predation and disease are common features of all habitats (see prior chapter). The assumptions of the competitive exclusion principle thus may not always apply and coexistence of interspecific competitors could be a potential feature of many habitats. Intraspecific competition can lead to strong selective pressures on organisms. These interactions can be avoided by organisms in some instances. For example,
Competition
intraspecific competition can lead to “Alee effects,” where organisms self-limit their population growth so as not to crash the entire population. The interplay between success of individuals and success of the group has led to substantial controversy in the field of evolution. Sexual selection can result from intraspecific competition. For example, male fishes can have broadly different morphology from females ultimately related to competition for mates (Fig. 21.3). In a more complex example, water striders (Aquarius remigis) have intense competition for mates. The water striders congregate in pools and males compete to mate with females. The intense competition to mate actually can harm the females. In conditions where there is only
Figure 21.3 Breeding-age adult central stonerollers (Campostoma anomalum). The male, in the top panel, has obvious tubercles and fin markings. These features could play a part in aggressive interactions among males competing for mates as well as female mate choice. The female is plain. (Photograph courtesy of Gerold Sneegas).
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one habitat, the most aggressive males win because they are the most successful at mating. However, when there are several habitats available (e.g., a stream with connected pools, some with many water striders and others with few), females will leave pools with many males to avoid being overwhelmed by mating attempts. If less aggressive males inhabit the pools with few water striders, then they will also mate successfully when females enter their pools to avoid the higher density pools. This is a case where population structure can alter the selective pressures of intraspecific competition (Eldakar et al., 2009). We now document several types of interspecific competition that commonly occur in freshwater environments. Primary producers compete for nutrients and light. Competition between macrophyte species has been well documented (Gopal and Goel, 1993). Macrophytes can form dense stands that shade all primary producers below them (Haslam, 1978). Floating macrophytes, such as water hyacinth (Eichhornia) or duckweed (Lemna), can blanket the surface of a lentic habitat and essentially remove all light. Macrophytes can intercept 90% of the incident light before it reaches a lake bottom, leading to a 65% decrease in benthic algal production (Lassen et al., 1997). If systems are sufficiently shallow, emergent primary producers intercept light before it reaches the water’s surface. Competition for light has been documented among macrophytes inhabiting a stream in North Carolina (Everitt and Burkholder, 1991). In this case, riparian vegetation and macrophyte assemblages were characterized in 10 stream segments. The red alga Lemanea australis or the aquatic moss Fontanalis dominated low-light assemblages in the winter. High-light sites were dominated by L. australis and the angiosperm Podostemum ceratophyllum. These sites were on the same stream and had similar water velocity and depth, indicating that light availability as influenced by riparian shading was the major abiotic difference between them. Apparently, competition for light structured the macrophyte assemblages. Macrophytes and phytoplankton compete for light and nutrients in shallow lakes. Several species of macrophytes are capable of interference competition; they release allelopathic chemicals that inhibit phytoplankton species (Gross et al., 2007). In shallow lakes dominated by macrophytes, the deeper portions are often characterized by relatively clear water (low phytoplankton concentrations). Thus, allelopathy can stabilize macrophyte dominance in shallow lakes (Hilt and Gross, 2008). Wetland plant assemblages can be shaped by competition. Competition between two species of wetland plants may occur aboveground (for light) or belowground (for nutrients), and both may be important simultaneously (Twolan-Strutt and Keddy, 1996). Competitive rankings of individual plants can remain stable against changes in nutrients and flooding in some instances (Keddy et al., 1994). However, wetland plants that occur in disturbed
Competition
habitats may be released from competition (Keddy, 1989). These studies suggest that competition can determine which plant species dominate in a particular wetland. An example of competition determining spatial patterns in macrophytes is found in two species of the cattail, Typha latifolia and T. angustifolia. Typha latifolia has broad leaves and can outcompete T. angustifolia in shallow water, but not in deeper water (Grace and Wetzel, 1998). As the season progresses, the two species are almost completely segregated (Fig. 21.4). This segregation
�20 0
Typha latifolia Typha angustifolia
Depth (cm)
20 40 60 80 100 0
(A)
200
400
600
800
1000
1200
Biomass (grams m�2)
�20 0
Depth (cm)
20 40 60 80 100 0
(B)
200
400
600
800
1000
1200
1400
1600
1800
Biomass (grams m�2)
Figure 21.4 Competitive exclusion of two species of Typha. Distributions at the same locations in the same wetland in May (A) and September (B). (Data plotted from Grace and Wetzel, 1998).
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CHAPTER 21: Nonpredatory Interspecific Interactions among Plants and Animals
occurs even though both species have optimum growth rates at 50cm depth when occurring in isolation. Zooplankton species often compete for food resources. In Chapter 19, we discussed the idea that ratios of nutrients may alter competitive ability or lead to coexistence of phytoplankton species. Similar resource partitioning could lead to coexistence of potentially competing zooplankton species (Lampert, 1997; DeMott, 1995). Such application of resource ratio theory has vastly expanded in application (Sterner and Elser, 2002). Resource ratio theory predicts how trade-offs associated with using two or more limited resources influence how species coexist (Tilman, 1982). For example, two species that are differentially limited by two resources will coexist at intermediate ratios of resource supply. At more skewed ratios of supply, the best competitor for the most limiting resource can exclude other species. In some cases, a trade-off may occur in competitive ability. For example, one species of rotifer (Keratella cochlearis) may compete better at a lower food concentration and the other (Keratella earlinae) at higher concentrations (Fig. 21.5). Both species could coexist in a spatially or temporally variable environment. Competitive interactions among zooplankton species probably vary over space and time. In Chapter 20, we discussed the idea that large zooplankton are superior competitors for phytoplankton cells. However, this competitive interaction may not be so simplistic. Large zooplankton may slow or cease
0.3 0.2 Growth (d–1)
578
0.1 0.0 –0.1 Keratella earlinae K. cochlearis
–0.2 –0.3 0.0
0.5
1.0
1.5
Cell concentration (mg L–1)
Figure 21.5 Competitive ability of two species of rotifers (Keratella cochlearis and K. earlinae) that feed on the cryptomonad Rhodomonas. Note that K. cochlearis is able to grow more rapidly at lower concentrations of food, but K. earlinae grows better at higher concentrations of food. (Redrawn from Stemberger and Gilbert, 1985).
Competition
feeding in blooms of large inedible algae while smaller zooplankton continue to feed unhindered on subdominant small phytoplankton and bacteria (Gliwicz, 1980). Similarly, cladocerans may be superior competitors for food compared to rotifers, but suspended clay can decrease the growth of cladocerans while having little effect on rotifers. Thus, the clay allows rotifers to be released from competition with cladocerans (Kirk and Gilbert, 1990). There are situations in which competitive exclusion should operate but may not be strong enough to drive out weaker competitors. An example occurred with five species of herbivorous zooplankton in a small humic lake (Hessen, 1990). These species coexisted, but bottle experiments showed no evidence of significant predation or a single dominant competitor. Weak competition for abundant nutrient-poor food in this lake was hypothesized to allow coexistence of the species. This study and those noted previously are a few of many that suggest that competition is an important but context-dependent feature of zooplankton assemblages.
Mean plant species abundance (% cover)
Invasive species are often very strong competitors. For example, purple loosestrife (Lythrum salicaria) has expanded rapidly in the United States, displacing other wetland species (Blossey et al., 2001). The reed canary grass (Phalaris arundinacea) is also a recent invasive species. In the wetlands where they have invaded, they often become dominant; purple loosestrife makes up about 30% of the total abundance, and canary grass about 8% overall (Fig. 21.6). In most cases where both were present (6 of 7), purple loosestrife was dominant. In 6 of 24 wetlands, reed canary grass was the most abundant species, and in 7 of 24 cases 100 80 60 40 20 0
Reed canary grass
Purple loostrife
Figure 21.6 Relative abundance of two invasive species (purple loosestrife Lythrum salicaria and reed canary grass Phalaris arundinacea) in Pacific Northwest US wetlands. Box plots show median, the box the 25th and 75th percentiles, and the whiskers the 90th and 10th percentiles. While overall neither species dominates in all wetlands, there is a high degree of dominance in some wetlands. (Data replotted from Schooler et al., 2006).
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purple loosestrife was dominant (Schooler et al., 2006). These data indicate that the invaders are strong competitors and that in situations where they attain high abundance they likely decrease plant diversity of the wetlands they have invaded. Competition for food has led to clear specialization of benthic freshwater macroinvertebrates over evolutionary time. Thus, invertebrates can be classified into functional feeding groups based on mode of food acquisition, such as collectors, filterers, shredders, and scrapers. Within these groups, competition can still occur. For example, net-spinning hydropsychid caddisflies collect particles from the water column with silk nets of different mesh sizes, with each mesh size characteristic of a species. Smaller mesh nets are more efficient at collecting small particles, whereas large meshes capture large particles more effectively (Loudon and Alstad, 1990). Thus, species with fine nets will likely compete more effectively for fine particles. Such competitive specialization may determine which species dominate. Although competition among coexisting species using similar resources, such as assemblages of net-spinning caddisflies, might be expected, interactions can be complex and sometimes counterintuitive. Laboratory experiments using different numbers of species of net-spinning caddisflies found that the particle capture efficiency of individual species was actually enhanced by the presence of other species because of changes in near bed flow dynamics (Cardinale et al., 2002). Competition among pond-breeding amphibian species has been studied. In some ponds, predators remove the competitive dominant frogs, and predation thus allows less competitive species to coexist with stronger competitors (Morin, 1983). The chytrid fungus Batrachochytrium dendrobatidis, which has been linked to amphibian declines around the world (Sidebar 11.2), can alter competitive interactions among amphibian species that do not die from infection. In the presence of the fungus, both fowler’s toad (Bufo fowleri) and gray tree frog (Hyla versicolor) tadpoles metamorphosed at smaller body masses when reared together in tanks compared to when they were reared separately (Parris and Cornelius, 2004). The toad tadpoles also had strong negative effects on tree frog tadpole development, but only in the presence of the fungus. Competition among fishes may be an important consideration for fisheries managers. For instance, Hodgson et al. (1991) demonstrated that introduction of rainbow trout (Oncorhynchus mykiss) into lakes with 2- and 3-year-old largemouth bass (Micropterus salmoides) can lower the condition (weight to length ratio) of bass. The diet of the bass shifted from Daphnia to odonate naiads, and bass condition was lower after the introduction of trout into one lake, particularly when compared to a nearby lake with no trout. Cage experiments suggest that sunfishes (Centrarchidae) compete when predators drive them to take refuge in macrophyte beds (Mittelbach, 1988), and competition
Mutualism and Facilitation
may occur with juveniles and translate into lower production of adult fish (Osenberg et al., 1992).
Mutualism and facilitation Mutualisms (both species have a positive effect on each other) are less conspicuous in freshwater than in marine systems, possibly because the continuous time for evolution of mutualisms has been less in freshwaters than in marine systems (i.e., many freshwater habitats have a shorter continuous history than marine or terrestrial habitats). However, it seems that some of the conditions for mutualism occur in freshwater. For example, fish that clean other fishes are common on marine reefs, and this mutualism involves many species of fishes from diverse taxonomic groups, but the same interaction has not been identified in freshwaters despite comparable benefits to freshwater fishes and the long geological age of some river systems. Many of the mutualisms that occur in freshwaters involve microorganisms and were discussed in Chapter 19. Mutualisms based on behavior require coevolved systems and organisms capable of complex behavioral patterns, such as fishes. Cichlids from Lake Tanganyika demonstrate parental care, including guarding eggs and fry from predators, and two species can brood in the same region and mutually defend their broods (Keenleyside, 1991). Fishes in the same lake have evolved cooperation in which predators hunt in mixed groups and this cooperation increases success (Nakai, 1993). Mixed-feeding schools may occur in other fish assemblages but have not been well studied. This is not a completely unique adaptation; birds have demonstrated a similar cooperative strategy of mixed-feeding flocks. Facilitation is any unidirectional positive effect of one species on another. Facilitation can include mutualism and commensalism, as well as some exploitation relationships. Facilitation may be an important and overlooked aspect of community interactions (Bertness and Callaway, 1994). Plants in stressful environments can facilitate each other (Callaway, 1995; Callaway and Walker, 1997). Few macrophyte and wetland plant assemblages have been studied with regard to facilitation, but it could be important in stressful freshwater habitats, as has been demonstrated for estuarine marshes (Bertness and Hacker, 1994). For instance, emergent freshwater marsh plants that are aerenchymous (transport oxygen to their roots) can facilitate other emergent plants living nearby by aerating the sediments (Callaway and King, 1996). As more research is done on aquatic plant assemblages, more examples of facilitation will likely be documented, given the importance of facilitation among terrestrial plants (Brooker et al., 2008). Indirect facilitation may occur in streams. Crayfish exclude the green alga Cladophora from pools. The exclusion of Cladophora facilitates the growth of
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Table 21.1 Influence of Mutualistic Midge Larvae on Nostoc Nitrogen Fixation Rates, O2 Concentrations, and Photosynthetic Rates Measurement
Units
Condition
Mean (95% confidence band)
15
Del 15N of Nostoc
With midge Without midge Over midge Away from midge
0.188 (0.005) 0.153 (0.009) 0.67 (0.09) 0.81 (0.06)
Over midge Away from midge
1.4 (0.3) 1.1 (0.2)
N2 incorporation
mmol L1 Microscale O2 concentration with microelectrodes Photosynthetic rate mol O2 L1 s1 with microelectrodes (Data from Dodds, 1989)
epilithic diatoms. The diatoms in turn support an increased biomass of grazing insect larvae (Hart, 1992; Creed, 1994). Such complex interactions may be a common feature of communities. An interesting mutualism occurs between the chironomid midge larva Cricotopus nostocicola and the cyanobacterium Nostoc parmeliodes (Fig. 19.15C). The midge receives sustenance from the Nostoc and lives inside it until pupation and emergence as an adult (Brock, 1960). The midge is a poor swimmer and highly susceptible to predation without the shelter in the tough leathery Nostoc colony. In turn, the midge increases the photosynthetic rate of the Nostoc (Ward et al., 1985) by altering its morphology and by attaching it more firmly onto rocks so it can extend into flow and have a smaller diffusive boundary layer (Dodds, 1989). Respiration by the midge lowers O2 concentration in the vicinity of the larva and enhances nitrogen fixation rates as well as photosynthetic rates (Table 21.1). This example is particularly curious because the eukaryotic organism is endosymbiotic to the bacterial organism. Snails could facilitate other grazing snails of different species by providing substrata for algae to grow on their shells (Abbot and Bergey, 2007). In this case the nutrients excreted by grazing snails can stimulate growth of diatoms that attach to their shells. Other snails then graze the diatoms off of the snail shells. Grazing tadpoles in tropical streams facilitate small grazing mayflies by removing sediments from substrata and exposing underlying periphyton; grazing mayflies respond by increasing their densities in tadpole-grazed patches (Ranvestel et al., 2004; Whiles et al., 2006). Facilitation was demonstrated in a group of three oligochaete species (Fig. 21.7), but the mechanisms were not clear (Brinkhurst et al., 1972). In this case, two of three species tested had greater weight gains when grown with the others than when grown in a single-species culture. Facilitation may also be a
Other Species Interactions
Relative growth (final g/initial g)
14 12 10 8 6 4 2 0
L +T +P +P L L +T L
Limnodrilus
T +L +P +P T T +L T
Tubifex
P +L +T +T P P +L P
Peloscolex
Figure 21.7 Growth of three tubificid oligochaetes alone and in culture with the other species. L, Limnodrilus hoffmeisteri; T, Tubifex tubifex; and P, Peloscolex multisetosus. Note that in two of three experiments, growth was greater in the presence of the other two species. (Data from Brinkhurst et al., 1972).
common feature of stream invertebrates that process litter. Shredders excrete fine particulate organic material that is ingested by collectors. Collectors may remove fine material that interferes with shredders or excrete nutrients that stimulate the microbes and make litter usable for shredders. Such facultative links merit additional study. Facilitation could be more important for some predators in streams than thought previously. For example, it has been assumed that wading birds compete with bass for prey, but a cage experiment challenged this assumption. In a prairie stream in Illinois where the dominant predatory wading birds are great blue heron (Ardea herodias), green heron (Butorides virescens), and great egret (Ardea alba), cages were used to manipulate access by birds and smallmouth bass (Micropterus dolomieu) (Steinmetz et al., 2008). The birds are effective predators only in shallow waters, whereas bass can consume prey in deeper water. In this case, more small prey fishes were consumed with both types of predators (wading birds and fish) present than either alone, and the effect was greater than additive. This experiment suggests facilitation occurs between the predators because prey fishes could not find refuge in deep or shallow waters.
Other species interactions In addition to predation (/), competition (/), and mutualism (/), other species interactions (neutralism (0/0), amensalism (/0), and commensalism (/0)) may be important but are rarely studied. Several
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examples are given here to explain potentially important interactions in which one species has an influence on a species that does not have an influence in return. Tadpoles of a common frog (Rana temporaria) have a negative effect on the snail, Lymnaea stagnalis, by competing for microalgae (Brönmark et al., 1991). The snail then consumes lower quality Cladophora and excretes nutrients that stimulate microalgal growth. The tadpole has a strong negative effect on the snail, but the snail has a weak positive effect on the tadpole. Macrophytes in lakes and ponds may provide a vital habitat for survival of small fishes. The macrophytes apparently receive little direct benefit from the fishes that live in them. However, the macrophytes could receive some mineralized nutrients from fish excretion. Also, indirect relationships could occur if the small fishes eat invertebrates that remove competitive epiphytes or prey upon herbivorous insects that eat the macrophytes. Macrophytes can also provide nutrients to epiphytes growing on them. The macrophyte Najas flexilis was planted in sediments labeled with radioactive phosphorus. The amount of the radioisotope taken into epiphytes was quantified using track autoradiography. A substantial portion of epiphyte phosphorus was derived from the macrophytes. Furthermore, different species obtained their phosphorus in varied proportions with the filamentous and more erect forms relying more heavily on the water column for nutrients, which indicated niche separation (Moeller et al., 1988). Similarly, nitrogen-fixing microbes could leak nitrogen and stimulate nearby organisms that are unable to use N2. This interaction is manipulated in rice paddies under traditional cultivation as discussed in Chapter 19. Most interactions of aquatic organisms with humans are amensal. Humans have negative effects on many aquatic species, but the effects of most of these species on humans are negligible. With some thought, the reader could identify more examples.
Summary 1. Competition has been well documented for species from a variety of taxonomic groups found in freshwaters. 2. Competition can occur between members of the same species (intraspecific) and members of different species (interspecific). 3. Competition can be divided into two general types; scramble competition, where individuals exploit the same resource, and inference competition, where individuals interfere with the ability of others to use a resource. 4. It can be difficult to establish competition in field studies.
Questions for Thought
5. Mutualism, amensalism, and commensalism have been studied much less than predation or competition, but they may be important at times. 6. Facilitation among plant species in harsh environments is an example of positive interactions that may be important in wetland communities. 7. Facilitation may be much more common than previously thought.
Questions for Thought 1. How can disturbance in a habitat act as an agent of natural selection? 2. Are indirect interactions so strong and numerous that they complicate predicting the effects of interactions within an ecological community? 3. Is there a maximum diversity based on competition? 4. How might temporal patterns of disturbance act so that competition becomes unimportant? 5. Are positive or negative interactions more important in natural communities? 6. Why are both per capita interaction strength and the number of species interacting important? 7. Can you think of additional cases of facilitative interactions that were not included earlier?
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