Synecology

E R Pianka, University of Texas, Austin, TX, USA © 2008 Elsevier B.V. All rights reserved.

Further Reading

The Encyclopedia Britannica defines synecology as the study of a group or community of organisms and their relationships to each other and to their common environment. Whereas autecology is the study of interrelationships between organisms and their environments at the level of an individual, a population, or an entire species (see Autecology), synecology is concerned with the highest level of biological organization: entire systems of interacting

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Kondrashov AS and Kondrashov FA (1999) Interactions among quantitative traits in the course of sympatric speciation. Nature 400: 351-354.

Maynard Smith J (1966) Sympatric speciation. American Naturalist 104: 487-490.

MUntzing A (1930) Über chromosomenvermehrung in Galeopsis-Kreuzungen and ihre phylogenetische bedeutung. Hereditas 14: 153-172.

Orr AH (1990) 'Why polyploidy is rarer in animals than in plants' revisited. American Naturalist 136: 759-770.

Otto SP and Whitton J (2000) Polyploid incidence and evolution. Annual Reviews in Genetics 34: 401-437.

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populations in a complex and dynamic physical environmental setting. Synecology is the study of ecosystems, which include both the abiotic nonliving physical environment as well as a biotic component, the community (or biome) of living microbes, fungi, plants, and animals that occur together at any given spot. The concept of a community is itself an abstraction; communities are seldom clear cut and distinct but almost always grade into one another. By considering ecological systems as 'open' rather than 'closed', and by allowing for continual inflow and outflow of materials, energy, and organisms, this difficulty can be partially overcome and the community concept can be quite useful. Although communities change both in space and in time, they can be examined from an instantaneous view of a fairly localized portion of a larger community. Community ecology is the study of the distribution, abundance, demography, and interactions between populations of coexisting species. Synecology is the most abstract and most difficult kind of ecology, but it is also exceedingly tantalizing and vitally important, as well as extremely urgent as humans complete their domination and usurpation of planet Earth.

As in almost any academic endeavor, two, diametrically opposed, approaches to ecosystem ecology exist: one approach views ecological systems in terms of their component parts, nutrient pools coupled to complex networks of interacting populations. The other approach is more holistic, coming at ecosystems from the top down, rather than from the bottom up. These two perspectives each have their own advantages and limitations, but both are useful.

Community structure concerns all the various ways in which members of communities relate to and interact with one another, as well as any community-level properties that emerge from these interactions. Just as populations have properties that transcend those of the individuals comprising them, communities have both structure and properties that are not possessed by their component populations.

Community ecologists are still in the process of developing a vocabulary. Identification of appropriate aggregate variables or macrodescriptors is essential, but constitutes a double-edged sword; macrodescriptors allow progress but simultaneously constrain direction(s) that can be pursued. To be most useful, such macrodescriptors must simplify population-level processes while retaining their essence without fatal oversimplification. Examples include trophic structure (food webs), connectance, rates of energy fixation and flow, efficiency, diversity, stability, distributions of relative importance among species, niche relationships, resource partitioning, guild structure, assembly 'rules', suc-cessional stages, and so on. At this early stage in community ecology, it seems prudent not to become overly 'locked in' by words and concepts. Even the trophic level concept (producers, consumers (herbivores, omnivores, and carnivores), and decomposers) should not be inviolate (Figure 1).

Clements saw communities as organized superorgan-isms, whereas Gleason envisioned them as merely statistical ensembles of individualistic species. This debate lives on today, albeit in a somewhat different form. Some putative system-level properties are undoubtedly simply epiphenomena that arise from pooling components; examples would presumably include

Respiration

Figure 1 A simple trophic level 'compartment' model of a community, with arrows indicating flow of energy through the system. Widths of arrows reflect rate of flow of energy between particular parts of the system.

Respiration

Figure 1 A simple trophic level 'compartment' model of a community, with arrows indicating flow of energy through the system. Widths of arrows reflect rate of flow of energy between particular parts of the system.

Rate of resource consumption and renewal

Rate of resource consumption and renewal

Resource gradient

Figure 2 Niche relationships among members of competitive communities can be represented with bell-shaped utilization curves along a resource spectrum such as height above ground or prey size. Among the seven hypothetical species shown, those toward the tails have broader utilization curves because their resources renew more slowly. In such a competitive community, consumers are at equilibrium with their resources and the rate of resource consumption is equal to the rate of renewal along the entire resource gradient (uppermost curve).

Resource gradient

Figure 2 Niche relationships among members of competitive communities can be represented with bell-shaped utilization curves along a resource spectrum such as height above ground or prey size. Among the seven hypothetical species shown, those toward the tails have broader utilization curves because their resources renew more slowly. In such a competitive community, consumers are at equilibrium with their resources and the rate of resource consumption is equal to the rate of renewal along the entire resource gradient (uppermost curve).

trophic levels, subwebs, nutrient cycles, and ecological pyramids. But, do communities also possess truly emergent properties that transcend those of mere statistical collections of populations? For example, do patterns of resource utilization among coexisting species become coadjusted so that they mesh together in meaningful ways (see Figure 2)?

If such resource partitioning occurs, truly emergent community-level properties arise as a result of orderly interactions among component populations. The null model approach has been exploited to search for such patterns by Winemiller and Pianka, who constructed randomized replicates of real communities termed

'pseudo-communities' and compared these with their prototypes to detect guild structure and resource partitioning.

Such transcendent phenomena or epiphenomena simply cannot be studied at individual or population levels, but must be approached at the level of an entire assemblage or community.

A major problem for community and ecosystem ecol-ogists is that communities are not acted upon directly by natural selection (as individual organisms are). We must keep clearly in mind that natural selection operates by differential reproductive success of individual organisms. It is tempting but dangerously misleading to view organisms or ecosystems as having been 'designed' for orderly and efficient function. Antagonistic interactions at the level of individuals and populations (competition, predation, parasitism) must frequently impair certain aspects of ecosystem performance. Effective studies of community organization thus require a pluralism of approaches, including all of the following levels: individuals, family groups, populations, trophic levels, and community networks, as well as historical and biogeographic studies. All these approaches have something useful to offer. The approach taken must be fitted to the questions asked as well as to the peculiarities of the system under study. Much more effort needs to be devoted to connecting community-level attributes and phenomena to how natural selection operates on the behavior and ecology of individual organisms.

For example, efficiencies of transfer of energy from one trophic level to the next have been estimated to average ~10—15%. Natural selection operating on individual prey organisms favors escape ability, which in turn reduces the rate of flow of matter and energy through that trophic level, decreasing ecological efficiency but increasing community stability. In contrast, predators evolve so as to be better able to capture their prey, which increases the efficiency of flow of energy through trophic levels but reduces a system's stability. In the coevolution of a predator and its prey, to avoid extinction, the prey must remain a step ahead of its predator. As a corollary, community-level properties of ecological efficiency and community stability may in fact be inversely related because natural selection operates at the level of individual predators and prey. Moreover, the apparent constancy and low level (10-15%) of ecological efficiency could be a result ofthe 'compromise' that must be reached between prey and their predators.

Ecosystem-level studies are also plagued by difficult problems of scale in both space and time. Patch size and dynamics, climatic events and climate change, nutrient cycles, disturbance frequency, and dispersal ability are just a few of many factors that vary widely within and among systems, as well as over space from local to geographic areas and through time from the short term to the long term.

A plethora of interesting questions can be asked about communities: What structure do they have that transcends population-level processes? What are the effects of community-level attributes on the component organisms living in a given community? What are the roles of parasitism, predation, mutualism, and interspecific competition in shaping community structure? How important are indirect interactions among species (see Autecology) and to what extent do such interactions balance out direct effects? How many niche dimensions separate species, and which ones? To what extent are species spread out evenly in niche/resource space? (Such an overdispersion in niche space might be predicted under a competitive null hypothesis, with each species minimizing its interactions with all others.) Do clusters of functionally similar species ('guilds') exist? If so, how can such guild structure be detected and measured? What are its components? Are such guilds merely a result of built-in design constraints on consumer species, and/or do guilds simply reflect natural gaps in resource space? Can guild structure evolve even when resources are continuously distributed as a means of reducing diffuse competition? (A community without guild structure would presumably have greater diffuse competition than one with guild structure.) Do more diverse communities have more guild structure than simpler communities? What factors determine the diversity and stability of communities and what is their relationship to one another?

Ultimately, we must be able to answer such fundamental questions about 'how' natural systems are put together before we will even begin to be able to ask more interesting questions about 'why' ecosystems have any particular observed properties, such as ''What are the effects of indirect interactions among populations and/or guild structure on the assembly, structure, stability, and diversity of communities?''

The extreme complexity of most ecosystems makes their study quite difficult but at the same time extremely challenging. Humans now dominate all of Earth's biomes -pristine natural ecosystems no longer exist. Unfortunately, we still know very little about how natural communities function, much to our peril as we continue to usurp whatever limited resources this planet has to offer. Community ecology has thus become exceedingly urgent: humans sorely need to understand how natural ecosystems function and evolve, if only so that we can manage our artificial human-engineered ecosystems more sensibly than we have so far.

See also: Ecological Niche; Ecosystems; Ecotoxicology: The Focal Topics.

Further Reading

Clements FE (1916) Publication No. 242: Plant Succession: Analysis of the Development of Vegetation. Washington, DC: Carnegie Institute.

Cody M and Diamond JM (eds.) (1975) Ecology and Evolution of

Communities. Boston: Harvard University Press. Diamond J and Case T (eds.) (1986) Community Ecology. New York: Harper & Row.

Gleason HA (1926) The individualistic concept of the plant association.

Bulletin of the Torrey Botanical Club 53: 7-26. Gotelli NJ and Graves GR (1996) Null Models in Ecology. Washington,

DC: Smithsonian Institution Press. Kikkawa J and Anderson DJ (eds.) (1986) Community Ecology: Pattern and Process. London: Blackwell Scientific Publications. Morin PJ (1999) Community Ecology. Oxford: Blackwell. Odum EP (1959) Fundamentals of Ecology. Philadelphia: W. B. Saunders.

Pianka ER (1980) Guild structure in desert lizards. Oikos 35: 194-201.

Planka ER (1981) Competition and niche theory. In: May RM (ed.) Theoretical Ecology, 2nd edn., ch. 8, pp. 167-196. Oxford: Blackwell.

Planka ER (1987) The subtlety, complexity, and importance of population interactions when more than two species are involved. Revista Chilena de Historia Natural 60: 351-362.

Pianka ER (1992) The state of the art in community ecology. In: Adler K (ed.) Proceedings of the First World Congress of Herpetology at Canterbury. Contributions to Herpetology, No. 9: Herpetology. Current Research on the Biology of Amphibians and Reptiles, pp. 141-162. Society for the Study of Amphibians and Reptiles.

Pianka ER (2000) Evolutionary Ecology, 6th edn. San Francisco: Benjamin-Cummings, Addison-Wesley-Longman.

Putman RJ (1994) Community Ecology. London: Chapman and Hall.

Weiher E and Keddy P (1999) Ecological Assembly Rules. Cambridge: Cambridge University Press.

Winemiller KO and Pianka ER (1990) Organization in natural assemblages of desert lizards and tropical fishes. Ecological Monographs 60: 27-55.

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