Nature of Communities

Learning goals:
* be able to describe what an ecological community is and what attributes define it
* understand the factors that go in to determining ecological diversity and how ecologists calculate it
* know the factors that go into figuring out the net interaction among species determining their presence and abundance in communities

An ecological community is a group of interacting species that occur together at the same place and time
The interactions result in synergies that make communities more than just a sum of their component species- i.e. the composition and functioning are closely tied to biological interactions such as competition, predation, and facilitation

What differentiates a community from an ecosystem?
Communities are defined by a subset of the species within a uniform area, while ecosystems include the entire biological and physical features that constitute a unit of the landscape (e.g. watershed)
Community ecology focuses on the factors that determine what species occur in a given area and how they change through time

What determines which organisms are included into a community?
1) Taxonomic affinity - how closely related the species are; closely related species have similar requirements for resources and physiological tolerances, and live in similar habitats (constituting the niche of the organism)
2) Guilds – species (usually animals) that use the same resources; enhanced potential for interaction (usually competition)
3) Functional groups – similar function, but different resources

Community Structure
The diversity of species and their identity (composition) describe the structure of a community
Species diversity has 2 components:
1) the number of species = species richness; (often expressed per unit area, species density)
2) relative abundance of all species = evenness (equitability)
Richness and evenness can be combined using diversity indices; most common one is Shannon-Wiener Index:

            H’ = - Σ pi ln (pi)

where H’ is the diversity index, and pi is the proportion of the community made up by species i; summed (Σ) for all species.


“Biodiversity" = biological diversity at all spatial scales, including genetic diversity, species diversity, and landscape diversity

Descriptors of species abundance: dominance and rank abundance
Dominance describes the importance of a species to a community, based on several possible attributes:

1) abundance- total numbers
2) area covered by the species (usually plants)
3) function- amount of resources captured / competitive ability, growth

Some species have disproportionately large effects on communities relative to their abundance; they are known as keystone species

Rank abundance curves (graphs of proportion of species cover vs. their rank order in the community) and Shannon-Wiener index are both useful for evaluating variation in diversity between communities and in response to environmental manipulation

The spatial scale and amount of sampling affect estimates of diversity
Sampling greater area should yield grater numbers of species
The rate at which new species appear should decline as more area is sampled, eventually leveling off
Greater sampling effort is required to estimate diversity in some groups, particularly microbes

Multiple species interactions in communities
Organisms are subjected to multiple, simultaneous interactions that can influence their presence and abundance in a community
Both direct and indirect interactions are important
Direct interactions include all of the pairwise interactions we’ve discussed over the past few weeks (competition, predation, facilitation)
Indirect interactions include effects mediated by a third, interacting species, and can be either inhibitory or facilitative
Examples of an indirect interaction include trophic facilitation and  apparent competition- increase in abundance of one species results in a decrease in another as a result of increase in a shared enemy (e.g. herbivore)