Differential predation pressure: A general mechanism for structuring plant communities along complex environmental gradients?

TREE vol. 4, no. 6, June 7989

Differential Predation Pressure: A GeneralMechanism forStructuring PlantCo unitiesalongComplex Environmetital Gradients? Svata M. Louda SPATIAL VARIATION in species abundancesalongcomplexenvironmental gradients, such as species replacement and zonation patterns, represents a nearly universal phenomenon. Such patterns are found in most ecosystems, and are quite independent of either the type of ecosystem (e.g. terrestrial versus marine) or category of organism (e.g. plant versus animal) (see, for instance, Fiefs 1 and 2). Understanding the general processes that determine such variation in abundance and distribution of organisms in nature clearly forms a fundamental goal of ecological research. There are four main hypotheses for processes determining spatial variation of species abundances

SM. Louda is at the School of Biological Sciences, University of Nebraska, Lincoln, NE 68588-0343.USA. Avicennia alba off icianalis

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Fig. 1. Mean (+s.E.) percentage of mangrove propagules consumed following four-day exposure to seed predators, in forests where each species is either dominant (shaded bars) or subdominant (open bars) in the canopy, and in high (H) and low (L) intertidal areas. Below each bar is the number of replicate plots used in each forest. ND, no data. Reproduced with permission from Ref. 7 1.

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along gradients: (I) physiological specialization to different portions of the gradient (e.g. Refs 1 and 3); (2) differential dispersal on the gradient, based on size and movement3; (3) altered relative, interspecific competitive abilities with position on the gradient (e.g. Refs 4 and 5); and (4) changing predation pressure among species along the gradients-+. While the predation hypothesis is an intriguing one, too little experimental evidence exists in terrestrial angiosperm plant communities to assess either the relative importance of differential predation in species replacements or the conditions under which predator influence should be predicted along a portion of the gradientlO. A recent paper by Smith et al.” represents a major advance in our understanding of predation as a process that can contribute to a widely recurring, natural pattern. Smith and colleagues succeeded in using a clever experimental ploy tethered propagules -to do a clean, straightforward comparison of relative predation pressure on dominant versus nondominant mangrove species along characteristic wet-todry shoreline gradients. Furthermore, by repeating the experiment on three continents (Australia, North America, Asia), these investigators were able to evaluate the generality of their experimental results (a rare accomplishment!). Mangroves are ‘self-maintaining coastal landscape units’, or common tidal shore plant communities throughout the tropics and subtropics12. The term mangrove can refer specifically to a group of halophytic tree species, in 12 genera in eight families, that varies in species richness from less than ten species in the New World tropics to 36 species in the Indo-West-Pacific3. More generally, the term is used to refer to a vegetation formation, dominated by mangrove species, that often fringes sheltered tropical shores below the high-tide mark. In Florida, these formations have been shown to be critical in the maintenance of key coastal fisheries, as a source of both energy inputs and juvenile habitat’*. Zonation among

species is typical, and each of the four hypotheses has been advanced to explain variation in relative abundance of mangrove tree species along the gradient from seaward inundated ground to inland dry areas. In a previous paper, Smiths verified reports of high seed predation by grapsid crabs, and he established major differences in loss among the mangrove species in Australia (Avicennia marina > Bruguiera exaristata > Ceriops tagal > Bruguiera gymnorrhiza > Rhizophora stylosa). This order correlated with nutritive quality (protein, sugar, fiber and tannin contents) rather than with propagule size. In the new paper, Smith et a/. test the generality of the original pattern, by comparing the vulnerabilities of mangrove tree species to grapsid and snail predation along physical and species dominance gradients in Malaysia, Panama and Florida with those from Australia. Smith eta/. found that four species studied in the most vulnerable genus, Avicennia, were always heavily utilized (46-72% cumulative predation) and, more important, were disproportionately more vulnerable where they were subdominant, rather than dominant, in the canopy (87% versus 30%, respectively). This pattern was repeated for species of both Bruguiera and Rhizophora in the Indo-Pacific, but not in the neotropics (Fig. 1). Since the composition of the predator guilds changed between regions, Smith et al. suggest that the difference among regions represents a decline in the importance of grapsid crabs in the predator guilds in Florida and Panama. This suggestion, however, remains an intriguing, still untested hypothesis. The results of Smith et al. complement and substantially extend the limited experimental data relevant to the evaluation of this major hypothesis for plant community structure. Directly comparable experimental results are very scarce. They include those of Lubchenco13 for algae in the temperate intertidal, Hay14,15 for algae in the tropical subtidal, and my own for two shrubs in temperate chaparral8pl6. No previous data exist for trees, yet most of the gradient work done in plant communities has been done in forests (see, for instance, Ref. 1). Hay’s test protocol was most similar to that of Smith et al.; he provided an array of algal species on lines placed in different subtidal reef habitats. Hay found both differential vulnerabilities to herbivores among co-occurring plant (algal) species and variation in the intensity of predation pressure

TREE vol. 4, no. 6, June

1989

by fish in adjacent habitats. He has also extended his evaluation among tropical regions and among reefs with different levels of fishing pressure. I know of no previous comparable experimental studies for trees in terrestrial ecosystems. Thus, while differential consumption pressure among plant species and among portions of an environmental gradient may be common, the Smith et al. paper is the first to document it experimentally for trees, and in particular for conspecific trees in several regions of the tropics. The experimental data now available suggest that the predation hypothesis needs to be given more consideration in investigations of the factors creating spatial patterns of abundance of terrestrial plants along gradients. In sum, the paper by Smith eta/. is particularly important for several reasons. First, it illustrates the use of a clever, yet simple, experiment to understand a process that is difficult to observe ‘in action’ in the field. Second, it shows the promise of

comparative experiments for giving increased insight into both similarities and differences in the functional relations and the dynamics of interactions underlying spatial patterns - on local, regional and geographic scales. Finally, it lends strong support to the hypothesis that the relative intensity of pressure, exerted by varying predator guilds on potentially co-occurring species of plants (or animals!), often contributes to shifting community structure along a complex environmental gradient. The results of Smith eta/., along with the others supporting this hypothesis, represent a challenge to the more traditional hypotheses for such patterns and necessitate further tests of the influence of consumers in plant community structure along gradients. References 1 Whittaker, R.H. (1975) Communities and Ecosystems (2nd edn), Macmillan 2 Begon, M., Harper, J.L. and Townsend, CR. (1986) Ecblogy: Individuals, Populations and Communities, Sinauer Associates

3 Macnae, W. (I 968) Adv. Mar. Biol. 6, 73-270 4 Cody, M.L. (1978)Am. J. 8ot. 65, 1107-1116 5 Ball, M.C. (1980) Oecologia 57,328-332 6 Harper, J.L. (1969) Brookhaven Syrnp. Biol. 22,48-62 7 Connell, J.H. (1971) in Dynamicsof Populations: Proceedings of the Advanced Study Institute in Dynamics of Numbers in Populations (den Boer, P.J. and Gradwell, G.R., eds), pp. 298-310, Centre for Agricultural Publishing and Documentation (Wageningen) 6 Louda, SM. (1982) Ecol. Monogr. 52, 25-41 9 Smith, T.J., Ill (1987) Ecology68, 266-273 10 Louda, S.M. in Ecology ofSeed Banks (Leek, M.A., Parker, V.T. and Simpson, R.L., eds), Academic Press (in press) 11 Smith, T.J., Ill, Chan, H.T., Mclvor, C.C. and Robblee, M.B. (1989) Ecology70, 146-151 12 Lugo, A.E. and Snedake;, SC. (1974) Annu. Rev. Ecol. Syst. 5,39-64 13 Lubchenco, J.L. (1978) Am. Nat. 112, 23-39 14 Hay, M.E. (1981) Aquat. Bat. 11,97-109 15 Hay, M.E. (1984) Ecology65.446-454 16 Louda, S.M. (1983) Ecology64, 51 l-521

Announcement

Ecological and Evolutionary Plant-Pathogen Systems Co-operative Interest in the study of ecological and coevolutionary interactions between plants and their pathogens is growing. However, because of the diverse backgrounds of workers entering this area, communication is often poor. To assist the development of the field, an informal and loose-knit group called the Ecological and Evolutionary Plant-Pathogen Systems Co-operative has been formed. The main aims of this co-operative are to foster interaction and co-operation between researchers with common interests in the ecological and evolutionary consequences of plant-pathogen interactions. As a first step in this process, a directory of workers actively involved four areas is being developed: 1. Empirical studies of pathogen-plant 2. Theoretical

interactions

communities.

models of I.

3. Ecological and evolutionary changes posites and their pathogens. 4. Theoretical

in non-agricultural

in the following

in varietal

mixtures,

multilines

and com-

models of 3.

The first directory is now being distributed,

with updates to be made as necessary.

All researchers with active interests in this area are invited to become involved. For further information contact: J. J. Burdon, Division of Plant Industry, CSIRO, GPO 1600, Canberra, ACT 2601, Australia.

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