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Biodiversity dynamics: from population and community ecology approaches to a landscape ecology point of view
LANDSCAPE AND URBAN PLANNING
ELSEVIER
Landscape and Urban Planning 31 (1995) 89-98
Biodiversity dynamics: from population and community ecology approaches to a landscape ecology point of view Robert Barbault Laboratoire d'Ecologie (VA 258 du CNRS), Ecole Normale Supérieure, 46 rue d'Ulm, Paris, 75230, France
Abstract Species richness and species diversity are classic concepts in ecology. What is new in the science ofbiodiversity after the Convention on Biological Diversity is: (1) that the emphasis has moved from the species to the ecosystem; (2) that the functional significance ofbiodiversity has been stressed. Thus, population and community ecology along with landscape ecology, should offer the best theoretical framework to analyse what can be called 'biodiversity dynamics'. Sorne promising pathways and areas are emphasized and the very concept of functional diversity is discussed. Species richness, genetic variability and extinction probability are closely linked with landscape traits such as habitat diversity, structural heterogeneity, patch dynamics and perturbations. Thus, it is suggested that landscape ecology hold a central role, since it will allow the response to biodiversity issues in the framework of environmental heterogeneity and patchiness. Keywords: Biodiversity; Corn munit y; Landscape; Population
1. Introduction In response to a growing concern for biodiversity, unprecedented efforts are being made by the scientific community and scientific institutions to organize research into large programs. Biodiversity is characteristically defined on three levels: genetic diversity, species diversity and ecosystem diversity. It is widely recognized as essential, whether for agricultural and forestry systems, pharmaceutical products, biotechnology development, aesthetics, evolutionary processes, or intrinsic worth of aU species (Wilson, 1992 ). Biologists are weU aware of the importance of understanding diversity, at least with respect to the increasing loss of species due to the growing
influence of human activities (Wilson, 1988, 1989). Since ecological systems are tirst and foremost networks of interacting populations, it is becoming increasingly evident that there is a need to promote a substantial research program devoted to this dimension of the biosphere (Barbault, 1990; Di Castri and Younès, 1990; Solbrig, 1991; Grassle et al., 1991; Walker, 1992; Barbault and Hochberg, 1992). The idea and appeal ofbiological diversity are certainly not new in ecology: for decades textbooks have presented hypotheses accounting for species richness and its changes in space and time (see, e.g. Begon et al., 1986). Numerous methods have been developed to measure what we caU species di versity (e.g. Pielou, 1975), and efforts towards developing fundamental theory to ex-
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plain diversity are steadily on the increase (e.g. May, 1973; Pimm, 1980, 1982, 1991; Tilman, 1988; Walker, 1992; Hochberg and Hawkins, 1992 ). Recently, following the Convention on Biological Diversity, a number of studies have sought to estimate the number of existing species (see May, 1992). It seems difficult, however, to justify the approaches aimed simply at refining the total species count. More pertinent a problem is to find out why there are so many species in the first place (Hutchinson, 1959; May, 1973) and how their diversity is maintained. In other words, focusing issues exclusively on species numbers, and even on species per se, does not seem the best way to address issues of biodiversity-unless one is directly interested for example in maintaining particular species (e.g. whales, elephants, the spotted owl) or managing the genetic diversity of a given food crop species. Even in this case, proposals to shift emphasis from species to ecosystems and even landscapes have taken on a new intensity (Franklin, 1993). In fact, interest in biological diversity is on the rise, in part, as a result of increasing concern for the preservation of the whole biosphere: we are becoming increasingly aware that understanding how the biosphere functions is important to its eventual management (Di Castri, 1989; Di Castri and Younès, 1990; Lubchenco et al., 1991; Solbrig, 1991 ). One way ofrepresenting biodiversity issues in such a perspective is presented in Fig. 1. From a biological point of view, human societies affect, directly and indirectly, biosphere-geosphere interactions and biosphere function through ecosystem and landscape dynamics. Thus, one aspect of these relationships concerns biogeochemical cycles (also emphasized by IGBP) and the other deals with population dynamics and population interactions. It is here that a biodiversity program should formally enter the agendas of the IGBP or the SBI (Sustainable Biosphere Initiative) (Lubchenco et al., 1991). However, research on biodiversity should not be viewed merely as a by-product of the logical connections schematized in Fig. 1; biodiversity is the tangible currency which is influenced by,
:::::: 1
GLOBAL CHANGES
H U M A
N
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E
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SUSTAINABLE BIOSPHERE
Fig. 1. From a biological point of view, human societies affect, directly or indirectly, biosphere-geosphere interactions and biosphere function through population and ecosystem dynamics. Biodiversity is the tangible currency which is influenced, and reflects the state of the biosphere itself (adapted from Di Castri and Younès, 1990).
and reflects, the state of the biosphere itself. Developing fundamental and general theory with the goal of linking diversity patterns with population dynamics and community structure is, in my opinion, the most sensible route towards understanding and predicting biodiversity dynamics. In any case, this is far preferable to a collection of descriptive studies, with little common ground. Let us illustrate some avenues that could help to develop a biodiversity program at the community level (for a discussion of the ecosystem function dimension see Di Castri and Younès, 1990; Schultze and Mooney, 1993).
2. Tackling the issues from an ecological point of view Population and community ecology together with landscape ecology should offer the best the-
R. Barbault / Landscape and Urban Planning 31 (1995) 89-98
oretical framework to analyze what can be called biodiversity dynamics (Barbault, 1992). Species richness and species diversity are c1assic concepts in ecology which are based in part on recording of the diversity of animaIs and plants, in the past and the present by paleontologists and taxonomists. From an ecological point of view, the study of biodiversity expands upon the concepts of species diversity and species richness in that: (i) the emphasis has moved from the level of species to that of the system (metapopulations, communities, ecosystems, landscapes); (ii) it is the functional significance of biodiversity which is now being stressed, taking into account explicitly its various components.
2.1. Trying to link the various components of biodiversity Although biological diversity can be studied at every level of organization ofbiological and ecological systems, it looks most promising to unite the various approaches in the following network (Fig. 2). This means distinguishing and linking: (1) intraspecific diversity, genetic and phenotypic, evaluated at the scale of populations or species; (2) species diversity, evaluated at the scale of functional groupings (guilds, trophic levels ); (3) ecological or functional diversity, evaluated at the scale of trophic networks and landscapes. However, representations such as in Fig. 2 have three main drawbacks: they ignore the temporal and historical dimensions of the problem; they ignore the central question of phylogenetic relationships between taxa; they do not represent the spatial framework, with its heterogeneity at various scales, from habitats to landscape. Thus, these three points should be taken as the main limitations of CUITent approaches of community studies. Moreover, the concept of a functional group, as well as the real meaning of so-called 'functional diversity', deserve further consideration.
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2.2. Movingfrom the species to the systems Despite these drawbacks the drawing used for Fig. 2 has the great advantage of allowing the substitution of a system point of view in place of an 'inventory' one in which the species is 'the tree which hides the forest'. There are many reasons to set biodiversity issues at the level of communities or trophic networks (Pimm, 1991; Barbault, 1992). A community ecological approach to biodiversity means: stressing the links between population biology and conservation biology with the ecological pressures which give them sorne generality (and their true meaning); encouraging approaches which take into account the hierarchical structure of biodiversity as well as its functions from individual organisms to whole landscapes; emphasizing the mechanisms which le ad locally to the build up, the maintenance or the erosion of biological diversity, linking in this way patterns and processes. Species diversity of communities has been related to several factors, ranging from geographic characteristics to biological factors, as well as historical events (see Ross, 1972; Begon et al., 1986; Diamond, 1988; Mitter et al., 1988; Brooks and McLennan, 1991; Barbault, 1992). Although the study of community organization was revived during the 1960s and the early 1970s under the impetus of Hutchinson and MacArthur, it focused attention on the role of biotic interactions, specially interspecific competition (Cody and Diamond, 1975; Roughgarden, 1983). This resulted in a flowering of theoretical and empirical work ranging from relating guild species richness to resource abundance and diversity, to niche breadth and to niche overlap analyses. In the 1980s a new era for community ecology began, in which birds were no longer the main reference, competition became only one factor among many, and spatial and temporal variability rose to the forefront of attention (see Strong et al., 1984; Diamond and Case, 1986). Parallei to this, literature on landscape ecology has rapidly developed (Naveh and Lieberman, 1984; Forman and Godron, 1986; Turner, 1989). AlI these points are of central interest for is-
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Fig. 2. The hierarchical structure ofbiodiversity from individual variability to ecosystem complexity using a food web representation. Each element, Ri (resource-species), Si (consuming species) and Pi (predatory species) is a population.
sues dealing with biodiversity in relation to direct and indirect human effects. In fact, the past decade has been marked by the development of more mechanistic approaches to investigate communities. These used behavioural ecology, physiological ecology and ecomorphology as the basis for a theoretical framework with which to interpret community patterns and dynamics (Schoener, 1986a). This approach can also be developed at a landscape level, as recently advocated by Wiens et al. (1993). Mechanistic approaches, in connection with the development of individual based models (e.g. Huston et al., 1988; Mills et al., 1993), should enable the realization of the three goals stressed above (Barbault, 1991 ). To take into account the drawbacks emphasized above, it is useful to set the issues as depicted by Wiens (1989: see Fig. 3) and to promote a landscape ecology approach, which would allow spatial patterns and processes to be linked.
3. Functional groupings and functional diversity
Genetic diversity, phenotypic plasticity (reaction norms), species richness-these are well defined terms. We know how to describe, and to measure them. But what really is functional diversity? Let us first begin with what 1 consider to be weIl defined concepts: genetic diversity, species richness. It is indeed possible to quantify genetic diversity in a sample of a population. But in the functional perspective the following questions emerge. What part of this diversity is important in the survival or the fitness of the systems displaying it (individuals, populations)? How is genetic diversity transferred at the scale of community and the ecosystem? Of course, the significance of a mutation will depend upon the genetic framework in which it occurs as weIl as upon the environment experienced by individuals having it (neutral here, favorable there, unfavorable somewhere else-and
R. Barbault / Landscapeand Urban Planning 31 (1995) 89-98 Speciation
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Fig. 3. Analysing biodiversity dynamics along time and space (from Wiens, 1989).
all this changing in time and space). Thus, it is not easy to answer the questions posed above; there is room for interesting research issues, and we start approaching what can be called the functional meaning of diversity. With respect to species richness we meet the same kind of problems, and possibly more. Let me put aside the difficulties in applying the species concept itself and consider only the meaning of the number S: at what scale should one measure S, which increases with the area studied? The biological diversity of a set of species Sx depends as much on the differences between the species inc1uded as on its mere number (ten species of the same genera display a lower diversity than ten other species belonging to different genera or different families); thus the relationships between taxonomic diversity and ecological diversity (diversity ofthe roles or functions played by the species in the system-community, ecosystem, landscape) must be studied. MacArthur stressed this in the 1950s! Therefore, the functional meaning of biologi-
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cal diversity is questioned at the level ofthe species as well as at the level ofthe community. The idea of functional groups appeared at various times in the 'phylogeny' of ecological concepts. It appears in the mind of ecologists who, following Hutchinson and MacArthur, have been concemed with the organization of communities. Working with assemblages of species defined on a functional basis they spoke of guilds (set of related species exploiting the same kind of resources: e.g. nectarivorous birds) or trophic levels (community of nectarivorous). It emerges also in the works of theoreticians interested in the structure and complexity of food webs. Species occupying at first sight the same place in the food web are lumped as 'trophic species' (Briand and Cohen, 1987; Lawton, 1989; Cohen et al., 1990). Independently of all this and much earlier, other scientists, such as naturalists or physiologists, developed the concept of functional type, and did define, for instance, functional types of plants. In fact, since the writings of Raunkiaer ( 1934) the concept of life-forms has proved to be very useful in attempting to analyse the formative influences of c1imate upon vegetation composition and dynamics (see Box, 1981 ). Because it relies upon morphological criteria which are readily available for all plant species this approach has been much appreciated. However, because important aspects of vegetation are not detectable by reference to plant morphology alone, it appeared necessary to recognize additional plant characteristics which are predictably related to habitat and ecology (see Grime, 1974, 1977, 1979; Noble and Slatyer, 1979). AlI these approaches are interesting and should contribute in promoting true research on functional diversity. Last but not least, 1 would like to mention an important aspect of this exploration, which relates fully concems about biodiversity with those of conservation: it is the question of so-called functional redundancy. ln fact "if scientists are to contribute usefully to the inevitable increase in management and political decisions relating to biodiversity, they need to address the issue of functional diversity
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and ecological redundancy in community composition" (Walker, 1992). Further, "a policy that places equal emphasis on every species is ecologically inachievable". What species are functionally equivalent? How are the functioning and persistence of an ecosystem affected if, for instance, half ofthe species of each functional group disappear (see also Lawton and Brown, 1993)? Of course, this relates to the concept ofkeystone species or keystone guild (Bond, 1993; Mills et al., 1993). 4. Some promising pathways and areas
With regard to global climatic change and its effects on biodiversity we should focus on how population level processes translate into community changes (Kareiva et al., 1993). For instance, relatively small shifts in local climate may exacerbate the extinctions of sorne species and promote the predominance of others, since many species will undoubtedly benefit physiologically from small changes in the climate and the decline of populations of sorne of their competitors, predators and parasites. Taking this to a spatial scale, we can ask: how would the geographical ranges of species change in response to small shifts in the environment? The problem is then both a community one as well as a largescale spatial one. If scientists restrict themselves to the study of only a few species in a given community, or only a small part of a given species' geographical range, then we are in danger of biasing our perception of diversity to extinction or proliferation of a limited number of species in a limited area. So, in our idiosyncratic view of global climate change, we think that sorne of the most important aspects to be explored are: (1) the local population dynamics of a limited number of species and its effects on community structure; (2) spatial knock-on effects on other communities; (3) the appearance of new pests and the decline of others; (4) the impoverishment of rich communities and enrichment of species-poor ones; (5) the role of landscape features on population dynamics and species interactions.
The recent emphasis put on three-Ievel interactions (Price et al., 1980, 1986; Polis et al., 1989) has improved our understanding of distributional patterns usually attributed to competitive exclusion. For instance, the colonization success of introduced birds in Hawaii is governed by interactions between introduced avian malaria, habitat preferences of vector (a mosquito), and differences in the evolved resistance of native and introduced species to the disease (Dobson and Hudson, 1986). In fact, the implications of parasites and infectious disease to community-Ievel patterns are generally neglected, although in theory they may have a considerable impact on host population dynamics and, more broadly, on the species composition and the structure of plant and animal communities (Levin, 1970; Anderson and May, 1979, 1981; Price et al., 1986; Hochberg and Holt, 1990). "Although considerable progress has been made in recent years by applying concepts and tools originally developed to examine the community structure of free-living organisms to parasite data, a full consideration of the dynamics and structure of parasite communities is likely to require a better understanding ofthe factors that determine the lifetime reproductive success of different parasite species and their interactions with their hosts" (Dobson, 1990) Indeed, the study of parasite-ho st communities is an important area to deal with in a biodiversity program. As such, the effects of small changes in the climate or the landscape structure on parasite geographic range are of central interest. Exploring food webs properties is another promising avenue, provided focus would be on dynamic properties rather than on static ones (Paine, 1980, 1988). As Paine (1980) said: "food webs along with their associated cross-links pro vide a realistic framework for understanding complex, highly interactive, multispecies re1ationships. 1 believe the next generation of models must be more sensitive to interaction strength, less so to trophic complexity, for the answers to questions on the stability properties of complex, natural communities increasingly violated by mankind are vital, and our time is short".
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In this area two more points should be emphasized. One is to focus, as Paine asks (after May, 1973), on the strength of links between species. Strong links can be demonstrated experimentally and their presence allows the explanation of the cascading changes that characterize certain altered ecosystems. Another is to identify keystone species (Paine, 1966; Terborgh, 1986; Gautier-Hion and Michaloud, 1989; Bond, 1993; Mills et al., 1993) or even keystone guilds (Brown and Heske, 1990), rather than initially dispersing research efforts over the myriad of species or links that make up a community. In other words, we need to relate the richness of functional species groups with that of larger species networks (Lawton, 1989; Schoener, 1989; Cohen et al., 1990; Pimm et al., 1991 ), as well as with subsystem dynamics and hierarchical organization (Kolasa, 1989). Studying and modelling the population dynamics properties of various kinds of ecological systems, be they single or multispecies associations, should be a central path toward the goal ofunderstanding biodiversity in the perspective of climate changes or landuse changes. In particular the population/ community level approach should help in exploring the responses of the whole ecological system to various kinds of disturbance (Sibly and Calow, 1989; Petraitis et al., 1989; Barbault, 1991; Hanski and Gilpin, 1991; Mills et al., 1993). Moreover, since we are mostly interested in human effects upon biodiversity, habitat disturbances require our particular attention. In fact, much of the picture given by present biodiversity, particularly in Europe, is due to the impacts both of the quaternary glaciation cycle and of human populations. The formation of most present continental ecosystems has occurred, to sorne extent, in the last 10 000 years. It is well known that landscape fragmentation is an increasing threat on biodiversity in many countries. In this respect, Europe is unique in the extent to which its landscape is fragmented into a patchwork pattern, with profound consequences for biodiversity. As Hanski and Gilpin (1991) have stressed, many species are being turned into metapopulations (populations of more or less
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isolated subpopulations) by habitat fragmentation. The fragmentation of habitats tends to reduce species diversity and increase risks of population extinction. Therefore, the dynamics of such fragmented populations need to be understood in relation with landscape ecology, so that relevant management solutions might be taken to prevent extinctions. Since the metapopulation concept is closely linked with the island-biogeography theory through processes of population turnover (extinction and establishment of new populations over a fragmented landscape), it is likely to become a central focus in conservation biology and biodiversity research (Case, 1990, 1991; Hanski, 1991; Hanski and Gilpin, 1991). Movement to and from habitat patches, i.e. dispersal, is one of the key processes which should enable us to predict the species that are prone to establish themselves or to become extinct. AIthough the physical mechanisms of dispersal are well established, the study of the ecology and genetics of dispersal has been largely overlooked until recently (Bunce and Howard, 1990). However, to understand and predict the dynamics of fragmented populations and communities, it is essential to study immigration and emigration processes with both an evolutionary and a landscape ecology perspective (paying attention not only to the degree of habitat fragmentation-size and isolation of patches-but also to their spatial arrangements and interdependence ). Although the extent of the applicability of general rules for community structure has been put in doubt (Strong et al., 1984: Schoener, 1986b; Diamond and Case, 1986), appreciation of the variability among communities has revived interest in comparative studies (Schluter, 1988), e.g. food limited vs. space-limited assemblages (Roughgarden, 1986), vertebrate vs. in vertebrate communities (Schoener, 1986c) or among different trophic levels (Connell, 1983; Schoener, 1986b). One goal of comparative studies is to classify communities on the basis of underlying processes into a small number of types (Schulter, 1988). A second goal, in the framework of a biodiversity program linked with global change issues, would be to understand the pop-
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ulation-Ievel mechanisms driving species diversity patterns. Many other issues could be raised in the broad field of biodiversity. For instance, the crucial need for taxonomical description and documentation as a basis for population and community level research. 1 chose to focus on population dynamics and community structure as a sensible way to unravel processes from observed biodiversity patterns and to relate geosphere-biosphere interactions with basic biological mechanisms. This may indeed be the only way to predict the effects of global changes on species diversity with the aim of curbing losses in diversity through conservation and biosphere management.
5. Conclusion To date, as stressed by Pimentel et al. (1992), the prime focus of world biological conservation has been on protecting particular areas such as national parks or biosphere reserves. Equally vital is the conservation of the biological diversity of our vast managed agricultural and forest ecosystems, even that still existing in urban areas, which, if combined, coyer approximately 95% of the terrestrial environment (Western and Pearl, 1989). If concerns about biodiversity changes or management are to prevail, a space-rooted approach would appear more convenient and efficient than accumulating species-centered studies. Species richness, as well as genetic variability are c10sely linked with landscape traits: area, habitat diversity, structural heterogeneity, patch dynamics, perturbations, etc. Thus, landscape ecology should hold here a central place: dealing with environmental heterogeneity and patchiness in spatially explicit terms (Wiens et al., 1993) it should allow the efficient response to some biodiversity issues. Key questions inc1ude: how do fragmentation and changes in agricultural practices affect population viabilÏty or dispersal, and hence biodiversity? How do they affect genetic variability of populations, and hence interactions between
species and ecosystem function? What are the effects of the size and configuration of patches on biodiversity from local to regional scales? Answering such questions should be useful in a planning context. Since it is not likely to succeed in convincing decision-makers to support more species-centered approaches, emphasizing that landscape approaches can allow for managing biodiversity-even unknown components of that biodiversity-is an efficient alternative. In fact, the landscape approach appears to be the best way to conserve the vast majority ofbiological diversity (Franklin, 1993). But it remains true, as stressed in Wiens' figure 3, that the current state and changes in biodiversity have to be understood by taking into account population dynamics processes: landscape approaches should inc1ude these ingredients.
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Pimentel, D., Stachow, U., Takacs, D.A., Brubaker, H.W., Dumas, A.R., Meaney, J.J., O'Neil, J.A.S., Onsi, D.E. and Corzilius, D.B., 1992. Conserving biological diversity in agricultural/forestry systems. Bioscience, 42:354-362. Pimm, S.L., 1980. PropertiesofFood Webs. Ecology, 61:219225. Pimm, S.L., 1982. Food Webs. Chapman and Hall, London. Pimm, S.L., 1991. The Balance of Nature? The University of Chicago Press, Chicago. Pimm, S.L., Lawton, J.M. and Cohen, J.E., 1991. Food web patterns and their consequences. Nature, 350:669-674. Polis, G.A., Myers, C.A. and Holt, R.D., 1989. The ecology and evolution of intraguild predation: potential competitors that eat each other. Annu. Rev. Ecol. Syst., 20:297330. Price, P.W., Bouton, C.E., Gross, P., McPheron, B.A., Thompson, J.N. and Weis, A.E., 1980. Interactions among three trophic levels: Influence of plants on interactions between insect, herbivorous and natural enemies. Annu. Rev. Ecol. Syst., II:47-65. Price, P.W., Westoby, M., Rice, B., Atsatt, P.R., Fritz, R.S., Thompson, J.N. and Mobley, K., 1986. Parasite mediation in ecological interactions. Annu. Rev. Ecol. Syst., 17:487-505. Raunkiaer, C., 1934. The Life Forms of Plants and Statistical Plant Geography (Translated Papers ). Clarendon, Oxford. Ross, H.H., 1972. The origin of species diversity in ecological communities. Taxonomy, 21:253-259. Roughgarden, J., 1983. Competition and theory in community ecology. Am. Nat., 122: 583-601. Roughgarden, J., 1986. A comparison of food-limited and space-limited animal competition communities. In: J. Diamond and T.J. Case (Editors), Community Ecology, 30:492-516. Schluter, D., 1988. The evolution of finch communities on islands and continents: Kenya vs. Ga1apagos. Eco!, Monogr., 58:229-249. Schoener, T.W., 1986a. Mechanistic approaches to community eco10gy: a new reductionism? Am. Zoo!., 26:81-106. Schoener, T. W., 1986b. Overview: kinds of eco10gical communities-ecology becomes pluralistic. In: J. Diamond
and T.J. Case (Editors), Community Ecology. Harper and Row, New York, pp. 467-479. Schoener, T. W., 1986c. Patterns in terrestrial vertebrate versus arthropod communities: do systematic differences in regularity exist? In: J. Diamond and T.J. Case (Editors), Community Ecology. Harper and Row, New York, pp. 556-586. Schoener, T.W., 1989. Food websfrom the small to the large. Ecology, 70:1559-1589. Schulze, E.D. and Mooney, H.A., 1993. Biodiversity and Ecosystem Function. Springer, Berlin. Sibly, R.M. and Calow, P., 1989. A life-cycle theory of responses to stress. Bio!' J. Linn. Soc., 37:101-116. Solbrig,O.T. (Editor), 1991. From Genes to Ecosystems: A Research Agenda for Biodiversity. IUBS, Paris. Strong, D.R., Jr., Sirnherloff, D., Abele, L.G. and Thistle, A.B. (Editors), 1984. Ecological Communities: Conceptual Issues and the Evidence. Princeton University Press, NJ. Terborgh, J., 1986. Keystone plant resources in the tropical forest. In: M.E. Soule (Editor), Conservation Biology: The Science of Scarcity and Diversity. Sinauer Associates, Sunderland, MA, pp. 330-334. Tilman, D., 1988. Plant strategies and the dynamics and structure of plant communities. Princeton University Press, NJ. Turner, M.G., 1989. Landscape ecology: the effect of pattern on process. Annu. Rev. Eco!, Syst., 20: 171-197. Walker, B.H., 1992. Biodiversity and ecological redundancy. Conserv. Biol., 6:18-23. Western, D. and Pearl, M.C. (Editors), 1989. Conservation for the Twenty-first Century. Oxford University Press, New York. Wiens, J.A., 1989. The Ecology of Bird Communities. Cambridge University Press, Cambridge. Wiens, J.A., Stenseth, N.C., van Home, B. and Ims, R.A., 1993. Ecological mechanisms and landscape ecology. Oikos, 66:369-380. Wilson, E.O. (Editor), 1988. Biodiversity. National Academy Press, Washington DC, 521 pp. Wilson, E.O., 1989. Threats to biodiversity. Sei. Am., 261: 108-116. Wilson, E.O., 1992. The Diversity of Life. Allen Lane, Penguin, London, 424 pp.
ELSEVIER
Landscape and Urban Planning 31 (1995) 89-98
Biodiversity dynamics: from population and community ecology approaches to a landscape ecology point of view Robert Barbault Laboratoire d'Ecologie (VA 258 du CNRS), Ecole Normale Supérieure, 46 rue d'Ulm, Paris, 75230, France
Abstract Species richness and species diversity are classic concepts in ecology. What is new in the science ofbiodiversity after the Convention on Biological Diversity is: (1) that the emphasis has moved from the species to the ecosystem; (2) that the functional significance ofbiodiversity has been stressed. Thus, population and community ecology along with landscape ecology, should offer the best theoretical framework to analyse what can be called 'biodiversity dynamics'. Sorne promising pathways and areas are emphasized and the very concept of functional diversity is discussed. Species richness, genetic variability and extinction probability are closely linked with landscape traits such as habitat diversity, structural heterogeneity, patch dynamics and perturbations. Thus, it is suggested that landscape ecology hold a central role, since it will allow the response to biodiversity issues in the framework of environmental heterogeneity and patchiness. Keywords: Biodiversity; Corn munit y; Landscape; Population
1. Introduction In response to a growing concern for biodiversity, unprecedented efforts are being made by the scientific community and scientific institutions to organize research into large programs. Biodiversity is characteristically defined on three levels: genetic diversity, species diversity and ecosystem diversity. It is widely recognized as essential, whether for agricultural and forestry systems, pharmaceutical products, biotechnology development, aesthetics, evolutionary processes, or intrinsic worth of aU species (Wilson, 1992 ). Biologists are weU aware of the importance of understanding diversity, at least with respect to the increasing loss of species due to the growing
influence of human activities (Wilson, 1988, 1989). Since ecological systems are tirst and foremost networks of interacting populations, it is becoming increasingly evident that there is a need to promote a substantial research program devoted to this dimension of the biosphere (Barbault, 1990; Di Castri and Younès, 1990; Solbrig, 1991; Grassle et al., 1991; Walker, 1992; Barbault and Hochberg, 1992). The idea and appeal ofbiological diversity are certainly not new in ecology: for decades textbooks have presented hypotheses accounting for species richness and its changes in space and time (see, e.g. Begon et al., 1986). Numerous methods have been developed to measure what we caU species di versity (e.g. Pielou, 1975), and efforts towards developing fundamental theory to ex-
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plain diversity are steadily on the increase (e.g. May, 1973; Pimm, 1980, 1982, 1991; Tilman, 1988; Walker, 1992; Hochberg and Hawkins, 1992 ). Recently, following the Convention on Biological Diversity, a number of studies have sought to estimate the number of existing species (see May, 1992). It seems difficult, however, to justify the approaches aimed simply at refining the total species count. More pertinent a problem is to find out why there are so many species in the first place (Hutchinson, 1959; May, 1973) and how their diversity is maintained. In other words, focusing issues exclusively on species numbers, and even on species per se, does not seem the best way to address issues of biodiversity-unless one is directly interested for example in maintaining particular species (e.g. whales, elephants, the spotted owl) or managing the genetic diversity of a given food crop species. Even in this case, proposals to shift emphasis from species to ecosystems and even landscapes have taken on a new intensity (Franklin, 1993). In fact, interest in biological diversity is on the rise, in part, as a result of increasing concern for the preservation of the whole biosphere: we are becoming increasingly aware that understanding how the biosphere functions is important to its eventual management (Di Castri, 1989; Di Castri and Younès, 1990; Lubchenco et al., 1991; Solbrig, 1991 ). One way ofrepresenting biodiversity issues in such a perspective is presented in Fig. 1. From a biological point of view, human societies affect, directly and indirectly, biosphere-geosphere interactions and biosphere function through ecosystem and landscape dynamics. Thus, one aspect of these relationships concerns biogeochemical cycles (also emphasized by IGBP) and the other deals with population dynamics and population interactions. It is here that a biodiversity program should formally enter the agendas of the IGBP or the SBI (Sustainable Biosphere Initiative) (Lubchenco et al., 1991). However, research on biodiversity should not be viewed merely as a by-product of the logical connections schematized in Fig. 1; biodiversity is the tangible currency which is influenced by,
:::::: 1
GLOBAL CHANGES
H U M A
N
s o c E
T
E
S
SUSTAINABLE BIOSPHERE
Fig. 1. From a biological point of view, human societies affect, directly or indirectly, biosphere-geosphere interactions and biosphere function through population and ecosystem dynamics. Biodiversity is the tangible currency which is influenced, and reflects the state of the biosphere itself (adapted from Di Castri and Younès, 1990).
and reflects, the state of the biosphere itself. Developing fundamental and general theory with the goal of linking diversity patterns with population dynamics and community structure is, in my opinion, the most sensible route towards understanding and predicting biodiversity dynamics. In any case, this is far preferable to a collection of descriptive studies, with little common ground. Let us illustrate some avenues that could help to develop a biodiversity program at the community level (for a discussion of the ecosystem function dimension see Di Castri and Younès, 1990; Schultze and Mooney, 1993).
2. Tackling the issues from an ecological point of view Population and community ecology together with landscape ecology should offer the best the-
R. Barbault / Landscape and Urban Planning 31 (1995) 89-98
oretical framework to analyze what can be called biodiversity dynamics (Barbault, 1992). Species richness and species diversity are c1assic concepts in ecology which are based in part on recording of the diversity of animaIs and plants, in the past and the present by paleontologists and taxonomists. From an ecological point of view, the study of biodiversity expands upon the concepts of species diversity and species richness in that: (i) the emphasis has moved from the level of species to that of the system (metapopulations, communities, ecosystems, landscapes); (ii) it is the functional significance of biodiversity which is now being stressed, taking into account explicitly its various components.
2.1. Trying to link the various components of biodiversity Although biological diversity can be studied at every level of organization ofbiological and ecological systems, it looks most promising to unite the various approaches in the following network (Fig. 2). This means distinguishing and linking: (1) intraspecific diversity, genetic and phenotypic, evaluated at the scale of populations or species; (2) species diversity, evaluated at the scale of functional groupings (guilds, trophic levels ); (3) ecological or functional diversity, evaluated at the scale of trophic networks and landscapes. However, representations such as in Fig. 2 have three main drawbacks: they ignore the temporal and historical dimensions of the problem; they ignore the central question of phylogenetic relationships between taxa; they do not represent the spatial framework, with its heterogeneity at various scales, from habitats to landscape. Thus, these three points should be taken as the main limitations of CUITent approaches of community studies. Moreover, the concept of a functional group, as well as the real meaning of so-called 'functional diversity', deserve further consideration.
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2.2. Movingfrom the species to the systems Despite these drawbacks the drawing used for Fig. 2 has the great advantage of allowing the substitution of a system point of view in place of an 'inventory' one in which the species is 'the tree which hides the forest'. There are many reasons to set biodiversity issues at the level of communities or trophic networks (Pimm, 1991; Barbault, 1992). A community ecological approach to biodiversity means: stressing the links between population biology and conservation biology with the ecological pressures which give them sorne generality (and their true meaning); encouraging approaches which take into account the hierarchical structure of biodiversity as well as its functions from individual organisms to whole landscapes; emphasizing the mechanisms which le ad locally to the build up, the maintenance or the erosion of biological diversity, linking in this way patterns and processes. Species diversity of communities has been related to several factors, ranging from geographic characteristics to biological factors, as well as historical events (see Ross, 1972; Begon et al., 1986; Diamond, 1988; Mitter et al., 1988; Brooks and McLennan, 1991; Barbault, 1992). Although the study of community organization was revived during the 1960s and the early 1970s under the impetus of Hutchinson and MacArthur, it focused attention on the role of biotic interactions, specially interspecific competition (Cody and Diamond, 1975; Roughgarden, 1983). This resulted in a flowering of theoretical and empirical work ranging from relating guild species richness to resource abundance and diversity, to niche breadth and to niche overlap analyses. In the 1980s a new era for community ecology began, in which birds were no longer the main reference, competition became only one factor among many, and spatial and temporal variability rose to the forefront of attention (see Strong et al., 1984; Diamond and Case, 1986). Parallei to this, literature on landscape ecology has rapidly developed (Naveh and Lieberman, 1984; Forman and Godron, 1986; Turner, 1989). AlI these points are of central interest for is-
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R. Barbau/t / Landscape and Urban Planning 31 (1995) 89-98
- - - - - - - - - - - - - -G- - - -,: 8
---------------------------------------------------------------------~
1. Genetic diversity population scale _ w~ genes as ekmentary
_ al the
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:
P z : ",.
----::~--------------7 ____ 1
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-------------------------------2. Species div.nity - atthescaleojthe ju?ctiona!groups - wilh spectes as
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----
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Fig. 2. The hierarchical structure ofbiodiversity from individual variability to ecosystem complexity using a food web representation. Each element, Ri (resource-species), Si (consuming species) and Pi (predatory species) is a population.
sues dealing with biodiversity in relation to direct and indirect human effects. In fact, the past decade has been marked by the development of more mechanistic approaches to investigate communities. These used behavioural ecology, physiological ecology and ecomorphology as the basis for a theoretical framework with which to interpret community patterns and dynamics (Schoener, 1986a). This approach can also be developed at a landscape level, as recently advocated by Wiens et al. (1993). Mechanistic approaches, in connection with the development of individual based models (e.g. Huston et al., 1988; Mills et al., 1993), should enable the realization of the three goals stressed above (Barbault, 1991 ). To take into account the drawbacks emphasized above, it is useful to set the issues as depicted by Wiens (1989: see Fig. 3) and to promote a landscape ecology approach, which would allow spatial patterns and processes to be linked.
3. Functional groupings and functional diversity
Genetic diversity, phenotypic plasticity (reaction norms), species richness-these are well defined terms. We know how to describe, and to measure them. But what really is functional diversity? Let us first begin with what 1 consider to be weIl defined concepts: genetic diversity, species richness. It is indeed possible to quantify genetic diversity in a sample of a population. But in the functional perspective the following questions emerge. What part of this diversity is important in the survival or the fitness of the systems displaying it (individuals, populations)? How is genetic diversity transferred at the scale of community and the ecosystem? Of course, the significance of a mutation will depend upon the genetic framework in which it occurs as weIl as upon the environment experienced by individuals having it (neutral here, favorable there, unfavorable somewhere else-and
R. Barbault / Landscapeand Urban Planning 31 (1995) 89-98 Speciation
0
~
Extinction
Species pool Biogeographie losses
Biogeographie additions
Area Distance Chance
-
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Population size
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[
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Population size
~
----110-
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1
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Species interactions [
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Fig. 3. Analysing biodiversity dynamics along time and space (from Wiens, 1989).
all this changing in time and space). Thus, it is not easy to answer the questions posed above; there is room for interesting research issues, and we start approaching what can be called the functional meaning of diversity. With respect to species richness we meet the same kind of problems, and possibly more. Let me put aside the difficulties in applying the species concept itself and consider only the meaning of the number S: at what scale should one measure S, which increases with the area studied? The biological diversity of a set of species Sx depends as much on the differences between the species inc1uded as on its mere number (ten species of the same genera display a lower diversity than ten other species belonging to different genera or different families); thus the relationships between taxonomic diversity and ecological diversity (diversity ofthe roles or functions played by the species in the system-community, ecosystem, landscape) must be studied. MacArthur stressed this in the 1950s! Therefore, the functional meaning of biologi-
93
cal diversity is questioned at the level ofthe species as well as at the level ofthe community. The idea of functional groups appeared at various times in the 'phylogeny' of ecological concepts. It appears in the mind of ecologists who, following Hutchinson and MacArthur, have been concemed with the organization of communities. Working with assemblages of species defined on a functional basis they spoke of guilds (set of related species exploiting the same kind of resources: e.g. nectarivorous birds) or trophic levels (community of nectarivorous). It emerges also in the works of theoreticians interested in the structure and complexity of food webs. Species occupying at first sight the same place in the food web are lumped as 'trophic species' (Briand and Cohen, 1987; Lawton, 1989; Cohen et al., 1990). Independently of all this and much earlier, other scientists, such as naturalists or physiologists, developed the concept of functional type, and did define, for instance, functional types of plants. In fact, since the writings of Raunkiaer ( 1934) the concept of life-forms has proved to be very useful in attempting to analyse the formative influences of c1imate upon vegetation composition and dynamics (see Box, 1981 ). Because it relies upon morphological criteria which are readily available for all plant species this approach has been much appreciated. However, because important aspects of vegetation are not detectable by reference to plant morphology alone, it appeared necessary to recognize additional plant characteristics which are predictably related to habitat and ecology (see Grime, 1974, 1977, 1979; Noble and Slatyer, 1979). AlI these approaches are interesting and should contribute in promoting true research on functional diversity. Last but not least, 1 would like to mention an important aspect of this exploration, which relates fully concems about biodiversity with those of conservation: it is the question of so-called functional redundancy. ln fact "if scientists are to contribute usefully to the inevitable increase in management and political decisions relating to biodiversity, they need to address the issue of functional diversity
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and ecological redundancy in community composition" (Walker, 1992). Further, "a policy that places equal emphasis on every species is ecologically inachievable". What species are functionally equivalent? How are the functioning and persistence of an ecosystem affected if, for instance, half ofthe species of each functional group disappear (see also Lawton and Brown, 1993)? Of course, this relates to the concept ofkeystone species or keystone guild (Bond, 1993; Mills et al., 1993). 4. Some promising pathways and areas
With regard to global climatic change and its effects on biodiversity we should focus on how population level processes translate into community changes (Kareiva et al., 1993). For instance, relatively small shifts in local climate may exacerbate the extinctions of sorne species and promote the predominance of others, since many species will undoubtedly benefit physiologically from small changes in the climate and the decline of populations of sorne of their competitors, predators and parasites. Taking this to a spatial scale, we can ask: how would the geographical ranges of species change in response to small shifts in the environment? The problem is then both a community one as well as a largescale spatial one. If scientists restrict themselves to the study of only a few species in a given community, or only a small part of a given species' geographical range, then we are in danger of biasing our perception of diversity to extinction or proliferation of a limited number of species in a limited area. So, in our idiosyncratic view of global climate change, we think that sorne of the most important aspects to be explored are: (1) the local population dynamics of a limited number of species and its effects on community structure; (2) spatial knock-on effects on other communities; (3) the appearance of new pests and the decline of others; (4) the impoverishment of rich communities and enrichment of species-poor ones; (5) the role of landscape features on population dynamics and species interactions.
The recent emphasis put on three-Ievel interactions (Price et al., 1980, 1986; Polis et al., 1989) has improved our understanding of distributional patterns usually attributed to competitive exclusion. For instance, the colonization success of introduced birds in Hawaii is governed by interactions between introduced avian malaria, habitat preferences of vector (a mosquito), and differences in the evolved resistance of native and introduced species to the disease (Dobson and Hudson, 1986). In fact, the implications of parasites and infectious disease to community-Ievel patterns are generally neglected, although in theory they may have a considerable impact on host population dynamics and, more broadly, on the species composition and the structure of plant and animal communities (Levin, 1970; Anderson and May, 1979, 1981; Price et al., 1986; Hochberg and Holt, 1990). "Although considerable progress has been made in recent years by applying concepts and tools originally developed to examine the community structure of free-living organisms to parasite data, a full consideration of the dynamics and structure of parasite communities is likely to require a better understanding ofthe factors that determine the lifetime reproductive success of different parasite species and their interactions with their hosts" (Dobson, 1990) Indeed, the study of parasite-ho st communities is an important area to deal with in a biodiversity program. As such, the effects of small changes in the climate or the landscape structure on parasite geographic range are of central interest. Exploring food webs properties is another promising avenue, provided focus would be on dynamic properties rather than on static ones (Paine, 1980, 1988). As Paine (1980) said: "food webs along with their associated cross-links pro vide a realistic framework for understanding complex, highly interactive, multispecies re1ationships. 1 believe the next generation of models must be more sensitive to interaction strength, less so to trophic complexity, for the answers to questions on the stability properties of complex, natural communities increasingly violated by mankind are vital, and our time is short".
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In this area two more points should be emphasized. One is to focus, as Paine asks (after May, 1973), on the strength of links between species. Strong links can be demonstrated experimentally and their presence allows the explanation of the cascading changes that characterize certain altered ecosystems. Another is to identify keystone species (Paine, 1966; Terborgh, 1986; Gautier-Hion and Michaloud, 1989; Bond, 1993; Mills et al., 1993) or even keystone guilds (Brown and Heske, 1990), rather than initially dispersing research efforts over the myriad of species or links that make up a community. In other words, we need to relate the richness of functional species groups with that of larger species networks (Lawton, 1989; Schoener, 1989; Cohen et al., 1990; Pimm et al., 1991 ), as well as with subsystem dynamics and hierarchical organization (Kolasa, 1989). Studying and modelling the population dynamics properties of various kinds of ecological systems, be they single or multispecies associations, should be a central path toward the goal ofunderstanding biodiversity in the perspective of climate changes or landuse changes. In particular the population/ community level approach should help in exploring the responses of the whole ecological system to various kinds of disturbance (Sibly and Calow, 1989; Petraitis et al., 1989; Barbault, 1991; Hanski and Gilpin, 1991; Mills et al., 1993). Moreover, since we are mostly interested in human effects upon biodiversity, habitat disturbances require our particular attention. In fact, much of the picture given by present biodiversity, particularly in Europe, is due to the impacts both of the quaternary glaciation cycle and of human populations. The formation of most present continental ecosystems has occurred, to sorne extent, in the last 10 000 years. It is well known that landscape fragmentation is an increasing threat on biodiversity in many countries. In this respect, Europe is unique in the extent to which its landscape is fragmented into a patchwork pattern, with profound consequences for biodiversity. As Hanski and Gilpin (1991) have stressed, many species are being turned into metapopulations (populations of more or less
95
isolated subpopulations) by habitat fragmentation. The fragmentation of habitats tends to reduce species diversity and increase risks of population extinction. Therefore, the dynamics of such fragmented populations need to be understood in relation with landscape ecology, so that relevant management solutions might be taken to prevent extinctions. Since the metapopulation concept is closely linked with the island-biogeography theory through processes of population turnover (extinction and establishment of new populations over a fragmented landscape), it is likely to become a central focus in conservation biology and biodiversity research (Case, 1990, 1991; Hanski, 1991; Hanski and Gilpin, 1991). Movement to and from habitat patches, i.e. dispersal, is one of the key processes which should enable us to predict the species that are prone to establish themselves or to become extinct. AIthough the physical mechanisms of dispersal are well established, the study of the ecology and genetics of dispersal has been largely overlooked until recently (Bunce and Howard, 1990). However, to understand and predict the dynamics of fragmented populations and communities, it is essential to study immigration and emigration processes with both an evolutionary and a landscape ecology perspective (paying attention not only to the degree of habitat fragmentation-size and isolation of patches-but also to their spatial arrangements and interdependence ). Although the extent of the applicability of general rules for community structure has been put in doubt (Strong et al., 1984: Schoener, 1986b; Diamond and Case, 1986), appreciation of the variability among communities has revived interest in comparative studies (Schluter, 1988), e.g. food limited vs. space-limited assemblages (Roughgarden, 1986), vertebrate vs. in vertebrate communities (Schoener, 1986c) or among different trophic levels (Connell, 1983; Schoener, 1986b). One goal of comparative studies is to classify communities on the basis of underlying processes into a small number of types (Schulter, 1988). A second goal, in the framework of a biodiversity program linked with global change issues, would be to understand the pop-
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ulation-Ievel mechanisms driving species diversity patterns. Many other issues could be raised in the broad field of biodiversity. For instance, the crucial need for taxonomical description and documentation as a basis for population and community level research. 1 chose to focus on population dynamics and community structure as a sensible way to unravel processes from observed biodiversity patterns and to relate geosphere-biosphere interactions with basic biological mechanisms. This may indeed be the only way to predict the effects of global changes on species diversity with the aim of curbing losses in diversity through conservation and biosphere management.
5. Conclusion To date, as stressed by Pimentel et al. (1992), the prime focus of world biological conservation has been on protecting particular areas such as national parks or biosphere reserves. Equally vital is the conservation of the biological diversity of our vast managed agricultural and forest ecosystems, even that still existing in urban areas, which, if combined, coyer approximately 95% of the terrestrial environment (Western and Pearl, 1989). If concerns about biodiversity changes or management are to prevail, a space-rooted approach would appear more convenient and efficient than accumulating species-centered studies. Species richness, as well as genetic variability are c10sely linked with landscape traits: area, habitat diversity, structural heterogeneity, patch dynamics, perturbations, etc. Thus, landscape ecology should hold here a central place: dealing with environmental heterogeneity and patchiness in spatially explicit terms (Wiens et al., 1993) it should allow the efficient response to some biodiversity issues. Key questions inc1ude: how do fragmentation and changes in agricultural practices affect population viabilÏty or dispersal, and hence biodiversity? How do they affect genetic variability of populations, and hence interactions between
species and ecosystem function? What are the effects of the size and configuration of patches on biodiversity from local to regional scales? Answering such questions should be useful in a planning context. Since it is not likely to succeed in convincing decision-makers to support more species-centered approaches, emphasizing that landscape approaches can allow for managing biodiversity-even unknown components of that biodiversity-is an efficient alternative. In fact, the landscape approach appears to be the best way to conserve the vast majority ofbiological diversity (Franklin, 1993). But it remains true, as stressed in Wiens' figure 3, that the current state and changes in biodiversity have to be understood by taking into account population dynamics processes: landscape approaches should inc1ude these ingredients.
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