Frontiers of Ecology

Ecological research is entering a new era of integration and collaboration as we meet the challenge of understanding the great complexity of biological systems. Ecological subdisciplines are rapidly combining and incorporating other biological, physical, mathematical, and sociological disciplines. The burgeoning base of theoretical and empirical work, made possible by new methods, technologies, and funding opportunities, is providing the opportunity to reach robust answers to major ecological questions. In December 1999 the National Science Foundation convened a white paper committee to evaluate what we know and do not know about important ecological processes, what hurdles currently hamper our progress, and what intellectual and conceptual interfaces need to be encouraged. The committee distilled the discussion into four frontiers in research on the ecological structure of the earth's biological diversity and the ways in which ecological processes continuously shape that structure (i.e., ecological dynamics). This article summarizes the discussions of those frontiers and explains why they are crucial to our understanding of how ecological processes shape patterns and dynamics of global biocomplexity. The frontiers are Dynamics of coalescence in complex communities Evolutionary and historical determinants of ecological processes: The role of ecological memory Emergent properties of complex systems: Biophysical constraints and evolutionary attractors Ecological topology: Defining the spatiotemporal domains of causality for ecological structure and processes Each of the four research frontiers takes a different approach to the overall ecological dynamics of biocomplexity, and all require integration and collaboration among those approaches. These overlapping frontiers themselves are not necessarily new. Within each frontier, however, are emerging questions and approaches that will help us understand how ecological processes are interconnected over multiple spatial and temporal scales, from local community structure to global patterns. Research frontier 1: Dynamics of coalescence in complex communities We use the term community coalescence to refer to the development of complex ecological communities from a regional species pool. This coalescence depends on interactions among species availability, physical environment, evolutionary history, and temporal sequence of assembly. Ecologists have made important strides in understanding the process of community coalescence, but it will take even greater integration of approaches for us to be able to confidently predict the pathways or endpoints of community assembly (Belyea and Lancaster 1999, Gotelli 1999, Weiher and Keddy 1999). So far, we cannot predict which species are likely to invade or to be lost from particular natural communities, although some patterns are beginning to emerge (Petchey et al. 1999). We also have much to learn about community responses to perturbations at different stages in development. Much of what we do know comes from a handful of easily studied systems, whereas the functioning of communities undoubtedly also depends heavily on little-studied hidden players, such as microbes, fungi, and soil invertebrates (de Ruiter et al. 1995). Indeed, many attempts to create or restore communities (e.g., freshwater wetlands or salt marshes) fail for reasons that remain poorly understood. Until we can fill these fundamental gaps in our knowledge, human impacts on community patterns will remain hard to predict. Research conducted over the last decade suggests what kinds of studies are needed to fill the gaps. Progress will almost certainly depend on developing new ways to simplify the study of complex communities. Focusing on functional groups or other as yet undeveloped constructs may help. We are also aware that we need to learn far more about the systematics of many smaller or cryptic organisms, which may form links that are crucial to our understanding of community coalescence. The difficulty of linking important ecosystem functions to key taxa and interspecific interactions is a particular challenge. Research in five key areas, described below, could provide important insights into the community assembly process. Functional traits and community composition As we study communities that are increasingly a mix of native and introduced taxa, we need to find out whether coevolved species or local populations differ from coevolutionarily naive populations in the strength or nature of their interactions during or after community assembly. We need to improve our ability to pinpoint the traits of species that affect the probability of invasion or extinction within developing communities (Pimm 1989, Rejmánek and Richardson 1996, Belyea and Lancaster 1999). Moreover, we need to further our understanding of how the importance of those traits depends on existing species composition, critical thresholds (e.g., species richness, functional group composition), and history of a community (Rejmánek 1989, Burke and Grime 1996, Levine and D'Antonio 1999). Pathways toward community coalescence For any pool of potential community members there are many possible assembly sequences and endpoints. The diversity of initial sequences is less problematic if many of the alternative pathways tend to converge on a limited subset of endpoints. To that end, we need to find out whether developing communities move toward single or multiple points or states (attractors). Past some stage of assembly, community composition may become canalized, but it may be highly susceptible to perturbations in the early stages of coalescence. Anthropogenic change may limit pathways of community development by changing the physicochemical environment, altering biogeochemical cycles, or changing the genetic structure of populations. Global homogenization of the species pool may itself alter the pathways or endpoints of community assembly, reshaping both short-term and long-term successional patterns across landscapes. Functional groups Ecologists recognize that the concept of functional groups is a valuable tool for simplifying community complexity to manageable levels. Currently, most groups are defined in a system-specific way (Wilson 1999) based on biochemical, morphological, or trophic criteria (Vitousek and Hooper 1993, Hooper and Vitousek 1997, Naeem and Li 1997, Tilman et al. 1997), although some attempts have been made to develop general classifications (Grime et al. 1997). Each of these attempts has helped us identify the advantages and disadvantages of different approaches. Together, they are moving us toward the development of robust frameworks that work across multiple taxa and ecological communities. Those advances should ultimately help us to understand how functional group structure shapes assembly dynamics, as well as to evaluate how different configurations of functional groups affect the development of communities or the dynamics of ecosystems (e.g., rates and trajectories of community development with and without nitrogen-fixing species). Hidden players Community assembly almost certainly depends upon cryptic invertebrate, microbiological, and mycological groups, which include a vast array of free-living species, parasites, and mutualistic symbionts. These groups and interactions have historically been ignored or unrecognized because of technological limitations and biases in the training of ecologists. Many microbial species cannot yet even be cultured. Nevertheless, they may play a keystone role in community development and function, and their absence may be responsible for some failed attempts at community restoration (Wall Freckman et al. 1997, Brussaard 1998). The importance of these hidden players may become apparent only when they become problems, as sometimes happens when we alter community structure. Lyme disease, for example, may be the result of the emergence of a hidden player in response to changes in community composition and landscape patterns (Jones et al. 1998). Part of our challenge will be to determine how the structure of microbial diversity should be incorporated into different kinds of analyses of the local, regional, and global dynamics of ecological processes. Traditional definitions of species and traditional metrics of distribution and abundance often make a poor fit with the structure of microbial populations and communities. Our research choices have been biased not only toward particular taxa but also toward particular communities. We have detailed community assembly information on only a small number of well-studied kinds of communities (e.g., successional fields, temperate lakes, rocky intertidal shores). Studies of underrepresented communities, taxa, and interactions will together provide important insights into general patterns in the assembly process (e.g., roles of particular kinds of microbial interaction or mutualistic interaction). Restoration The best measure of our understanding of any complex system is whether we can reconstruct it from its parts (Jordan et al. 1987). Successful restoration of sustainable communities, and de novo creation of persistent complex systems that provide essential ecosystem services in novel environments, are therefore the true tests of our understanding of community dynamics. Those tasks, in short, constitute the shared frontier of community ecology and ecological restoration. We need to know which aspects of community structure are restorable once disassembled and which are not. Although our understanding of what is restorable is limited, our societies are nonetheless moving apace to manipulate community assemblages worldwide (Simberloff et al. 1997). We must therefore continue to work toward a more comprehensive theory of community coalescence if we are to succeed in efforts to mitigate the effects of invasive species on local communities and to make those communities less to The of species, which takes in biological will be less once we understand how to the of species the assembly of persistent but efforts will from understanding how to develop assemblages of species that or Research frontier Evolutionary and historical determinants of ecological processes The genetic and evolutionary structure of organisms, in with the history of shape the patterns of community coalescence in frontier is a by historical and history on communities, in ecological processes, and the trajectories of community coalescence in ways that are poorly understood. 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These could us to determine whether species, and communities that a history of properties different from those from or restoration will help us determine whether long-term of taxa community properties such as and of evolutionary dynamics do landscape structure and genetic structure together in ecological Ecologists are beginning to hard at how the spatial of and shapes ecological dynamics. are groups of populations by of within local communities can to populations in other communities, altering local ecological dynamics. and of populations the of among communities both historically and We are beginning to understand how the landscape structure of with the genetic of species and to shape ecological processes at local, regional, and We need a theory of evolutionary dynamics to the overall effects of on communities. diversity and ecological dynamics genetic diversity within species shape the temporal dynamics of trophic and in incorporating the effects of genetic diversity into our of the temporal dynamics of ecosystem structure and function, we may understand and predict ecological systems. The study of has us a of the effects of the structure of genetic diversity on community dynamics et al. 1999). or of some species are much more to or are more other or of the Moreover, some and greater diversity and of and other species, local of diversity and of ecological dynamics do and patterns of shape species species and responses to changing We are to evaluate the ways in which number shapes patterns in community structure and dynamics. a small number of studies we know that number is to important of taxa, such as species (e.g., the of species with and of to (e.g., on et al. 1997). the evolutionary of species may have impacts on the ecological functioning of communities. The role of history and temporal Ecological research to on efforts to processes across different at from to of temporal we often cannot understand communities without understanding or processes. 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