Community Ecology

Community Ecology What is community ecology?

Community Ecology Individuals are usually limited by abiotic constraints (potential niche), but their actual distribution is usually a subset of the potential space it can occupy (realized niche) What are some constraints?

Community Ecology

Community Ecology Organisms do not live in isolation They are part of a larger system in which individuals are interacting with other species The major interspecific interactions (between 2 sp) include competition, predation, and mutualism

Community Ecology More complex interactions can arise when 3 or more species start interacting There are also many abiotic forces that can have dramatic affects on an individuals reproductive ecology

Community Ecology The precise pattern of resource use varies across time and space and subsequently manifests itself as differences in demographic parameters (e.g. population size, schedules of mortality or maturity, growth rates)

Community Ecology Within the context of biological theatre (ecosystem) the players (species) interact Ecosystem: the smallest self-contained ecological unit of function Most studies do not take place at the ecosystem level… why?

Community Ecology Consequently, the restricted subgroup of interest is the community(the assemblage of interacting species and the relationships in which they take part)

Community Ecology Communities are somewhat difficult to define Many have described ‘communities’ as predictable associations of species (or repeatedly encountered species-sets which are found commonly to co-occur) Thus communities are frequently described to certain ‘groups’ e.g. bird communities, fish, dragonfly, microbes

Community Ecology Can you see any potential problems with restricting analysis to a single group? What about an analysis of desert rodent community structure? What about examining nectivorous birds on a body size spectrum?

Community Ecology However, given enough species in a series of communities, isn’t it likely that some species will commonly co-occur even if they have no relationship? Consequently, one important aspect of community ecology is to determine if assemblages are random or not (structured)

Community Ecology To do this, we will typically restrict our analyses to those groups that have a geographic commonality, interact (directly or indirectly), and are in some way, interdependent

Biotic Relationships Many of the biotic relationships that link species are subtle or ephemeral (e.g. food) The most common of these is competition for food can you think of what else plants may compete for? however, there are also cases of facultative or obligatory mutualism

the Search for Pattern Communities are not static in time or space but rather dynamic Some of the parameters we will use to describe the structure of communities include: composition (the species present and their relative abundances), the nature and form of the relationships (e.g. the relative strength and/or direction), and their dynamics (fluxes in time and space)

the Search for Pattern Perhaps there should be no a priori presumption of structure within a community However, many studies have documented the non-random, predictable nature of some assemblages (e.g. limited membership, restricted patterns of relative abundance, overdispersion of co-occurrence, correlations between body-size and abundance, patterns in food webs, chain length)

the Search for Pattern If such patterns exist, they suggest some mechanism(s) responsible for said pattern Thus community ecology is the search for patterns and corresponding mechanisms within groups of interacting species

the Search for Pattern One of the goals of community ecology is to understand the origin, maintenance, and consequences of biodiversity within local communities

the Search for Pattern In an attempt to explore community patterns, most modern ecologists are no longer just describing communities but examining mechanisms generating To do so, they utilize mathematical models, statistical tests and experiments

Historical Approaches Ecologists generally approached community structure from one of two ways: One option was a reductionist approach (individual species or populations) where attributes were believed to be governed by intrinsic forces problem? Conversely, a holistic approach attempts to study the community in its entirety (but simplified)

Historical Findings Early communities were thought to be perhaps deterministic (Clements) vs. more probabilistic (Gleason) Clements view largely won

Historical Findings Early theory suggested that communities should largely be governed by competition Competition would result in species occupying distinct niches (based upon Gause’s early work) and limited niche overlap (resource use) Communities should be ‘packed’ tightly like a crystal

Historical Findings MacArthur extended competition to not just local communities but islands and perhaps continents with EO Wilson, formulated the ‘theory of island biogeography’ By the late 1970’s, others were suggesting these ‘patterns’ could be derived from processes other than competition (or any other)

Historical Findings MAcArthur’s view was largely a compromise between both Gleason and Clements Taken a step further, Simberloff suggested communities are probabilistic

Historical Findings Other findings include: 1) Null models … competition free 2) Mathematical analysis…limits to similarity and very sensitive 3) ‘variable’ environments 4) predation can be a very strong structuring force

Historical Findings Communities

Community Ecology