Overview of Community Ecology - Basic Models General objectives and questions addressed Historical trends Some people Some terms Basics of species interactions: Predation, Competition, Cooperation, Host-parasite, Food webs Survey of recent papers - topic coverage Papers to read: Hutchinson, G. E. 1959. Homage to Santa Rosalia or why are there so many kinds of animals? Am. Nat. 93:145-159. Kohler, Steven L. 1992. Competition and the structure of a benthic stream community. Ecol. Monog. 62:165-188. Schoener, T. W. 1974. Resource partitioning in ecological communities. Science 185:27-39. General objectives of community ecology: Determine the relative importance of competition, predation and other interactions between populations in determining the structure of a community - e.g. which species eat which, the number of connections between species, etc. Determine if there are general food web patterns that are similar in differing habitats. Determine what factors are responsible for the relative abundances of species within a community. What geographic, environmental and historical factors determine the diversity of species in a community and how do these factors interact? How do populations partition available resources in a community? Main historical trends: There are a variety of general opposing views of the nature of communities. One view (superorganism) is that community assemblages are organized systems having parallel structures of species presence and abundance and associated functional similarities. Evolution has driven these assemblages to take on particular patterns of interactions so the community consists of tightly coupled populations - this is typically called the Clementsian view after F. Clements. Alternatively, one can view communities as random assemblages with species present based upon their physiological tolerances, but no close linking between the species. This is the Gleasonian view (H. A. Gleason). Simberloff (1980) argues that there have been 3 successive paradigms: (1) Essentialism with communities in an ideal, deterministic form (Clementsian); (2) Materialism, in which communities are not in one of a few climax types but with a focus on the variation in composition and organization, and the processes which lead to this; (3) Probabalism, in which community structure arises as outcome of a small set of probabalistic pathways. Several major contyroversies over the last 30 years have arisen regarding the importance of competition in structuring communities. Following the concept of niche developed by Hutchinson, Robert MacArthur combined this with the theory of competitive exclusion to produce a view of communities consisting of species tightly packed within niche space, leading to a theory of limiting similarity of species within a community. This view was challenged by T. Schoener (1982) among others due to (1) Null model results which indicated that some patterns ascribed to competitive interactions could be explained by modelw ith no competition (Strong) (2) models showed that simple limits to similarity were very sensitive to the model formulation (Abrams) (3) the density-independent view of population regulation viewed animal populations at typical levels far away from carrying capacity with competitive interactions only significant during short periods (Wiens) (4) empirical studies which indicated predation was often a stronger biotic interaction than competition (Connell). Roughgarden replied to this with a defense of competition models and a crtique of null models, that led to a wide variety of responses involving many theoretical ecologists (Strong, Simberloff, Connor, Diamond, Gilpin). See Gotelli and Graves text "Null Models in Ecology" for more on this controversy. Some people: F. Clements - superorganism view of communities H. A. Gleason - individualistic view of communities G. E. Hutchinson - niche concept D. Lack - avian communities R. H. MacArthur - mathematical foundations of geographical ecology R. H. Whittaker - gradient analysis of plant communities E. O. Wilson - island biogeography Some Terms: Competition coefficients - the parameters in Lotka-Volterra competiotion equations which determine the realtive reduction in one competitors grwoth rate due to the presence of the other competitor. Exploitative competition - competition for a shared resource Food chain - a linearly ordered food web, e.g. Species A eats species B which eats species C, etc. Functional and numerical response - in the system for predator and prey with V the prey and P the predator: dV/dt = r V - f(V) P dP/dt = g(V) P - q P the functional response of the predator is f(V) giving the rate of capture of the prey as a function of prey density, and the numerical response of the predator is g(V) giving the growth rate of the predators as a function of prey density. Interference competition - occurs when an individual or populatiuon behaves in a way that reduces the ability of the competitor to exploit a resource, so affect the competitor directly rather than indirectly through a resource - territoriality, allelopathy Lotka-Volterra equations - equations for two species interactions in which the per capita growth rates for each species are linear functions of the densities of other species present in the community. Niche - the functional role of a species within a community Null model - a statistical test that is designed to distinguish pattern from randomness in a partricular biological context. Paradox of enrichment - for appropriate functional responses it is possible that increasing the prey population's carrying capacity leads to overexploitation of the prey by the predator, destabilizing the sytem and leading to extinction of both predator and prey. Predator mediated coexistence - a situation in which the presence of a predator allows for coexistence of two or more prey species which otherwise would not coexist. Pre-emptive competition - competition for space as a limiting resource, or another resource that is available immediately upon becoming unoccupied. Principle of competitive exclusion - must be some differences in two species resource use in order for them to coexist. Competition: Basic model results for spatially homogeneous system of two or more competing species are: one species or the other dominates the system due to being the superior competitor, both species coexist, or one of the two species dominates but which one does depends upon initial conditions. The competitive exclusion principle can be proved in generality for multiple species systems by showing tha if there are n available resources, then at most n species can coexist. Adding spatial effects modifies these results as superior dispersers can remain in a system although being inferior competitors. Predation: Simple two species models lead to both species going to extinction (overexploitation of the prey), the two species going to an equilibrium, or cyclical patterns of species density which can be stable or unstable. Taking space into account leads to possibilties of refuges in which the prey avoids the predator, with long-term coexistence occuring in situations for which without space the system would crash to extinction. Cooperation: Simplest models for facultative cooperation (e.g. non-obligatory) lead to either coexistence or an orgy of mutual benefaction in which each speciers grows without bound. For obligatory cooperation (in which each species would go extinct without the other present) the simplest models lead to carzy cases (populations go to infinity in finite time) or coexistence or extinction. There is a well developed theory for multiple species which gives easily checked conditions for existence of an equilibrium.