Conceptual Definition: A community is an assemblage of co-existing populations of different species living and (potentially) interacting in the same area.

Species may be permanent members of the community, or temporary members (e.g. migrants, accidentals, invaders . . .).

Gleason's Individualistic Concept: Communities are chance assemblages of species that live in the same place only because they share similar abiotic tolerances and need similar resources (i.e. their niches overlap):

• Most species live wherever conditions are suitable for them
• Most species live up to the limits of their fundamental niches (i.e. realized niche @ fundamental)
• Species co-exist wherever their fundamental niches (sensu physiological "place" and "resource use") overlap
• Presence of each species is independent of other species (although animals do require presence of food species)
• Total number of species present approaches the maximum number of species with overlapping physiological/resource niches available in region
• Communities do not have discrete boundaries - they simply intergrade gradually along environmental gradients into other communities
• Each species evolves more or less independently of the others

Clement's Interactive Concept: Communities consist of predictable groups of species that are linked by more or less obligatory interactions with one another (i.e. a community can be viewed as a "super-organism"):

• Species co-exist because they depend on one another
• Presence of a species is a consequence of positive interactions with certain other species that are present
• Absence of a species may be due to negative interactions with species already present in the community (e.g. potential predators, competitors, pathogens . . .)
• Species coexist only when their fundamental niches (sensu ecological "role") are compatible
• Total number of species is likely to be much fewer than predicted from knowledge of physiological/resource fundamental niches (i.e. realized niche < fundamental)
• Repeatable sets of interacting species tend to have discrete geographic boundaries where adjacent communities come together
• Evolutionary histories are closely linked via co-evolution, in which the main selective pressures on each species are exerted by other species

Empirical Definition: As with populations, it is usually unnecessary, impractical, or impossible to define and work with the entire community. In practice, one usually defines and works with a subset of the community in a convenient area. Empirical communities may be defined by one or more of the following criteria:

• Taxon (bird, fish, bacterial . . . communities)
• Morphology (tree, shrub, grass . . . communities)
• Feeding (trophic) mode (predator, herbivore, decomposer . . . communities)
• Habitat (alpine, marsh, reef . . . communities)
• Dominant organism(s) (redwood, live oak, mussel . . . communities)
• Locality (UCSC, Santa Cruz County, Salinas Valley . . . communities)

Each species in a community may be involved in many interactions simultaneously. A species' interactions can be classified by how its density (N) is affected ( + , 0 , - ) by the presence of each of the other species.:

[ - - ] COMPETITION: Both species decline because they are using the same limiting resource. There are two main kinds of competition:

• Exploitation: Resource (often food) is used by whoever gets it first (and hence denies it to others)
• Interference: One species prevents the other from gaining access to the resource (often space or food), even if the first species is not using it all. (e.g. by arriving first, territoriality, aggression, toxins, accumulated wastes . . . )

Competitive Exclusion Principle: If two similar species are limited by the same resource, one will be excluded from the community by competition. As the species become more similar (i.e. their resource niches overlap more), the intensity of competition increases, and the probability of competitive exclusion increases. This is predicted by Lotka-Volterra Inter-specific Competition models, and is supported by field observations.

o-evolutionary natural selection between competitors favors adaptations that:

• increase ability to detect, sequester and use resources
• deter competing species
• overcome deterrents of other species
• minimize intensity of competition

Competition commonly leads to co-evolutionary divergence in which resource niches have less overlap and the intensity of competition is reduced. This is commonly expressed as:

• Resource partitioning (niche partitioning): This reduces niche overlap, and allows co-existence by sharing the resource spectrum. It may produced by non-genetic (ecological) phenotypic plasticity as well as genetic adaptation.
• Character displacement: Species characteristics closely associated with resource use diverge genetically so that each species "specializes" on different parts of the resource spectrum. Character displacement is always associated with resource partitioning.

[ + - ] PREDATION (includes Herbivory, Parasitism (è Parasitoidism, Hyperparasitoidism), Disease): One species increases; the other species declines. In all these interactions, one species (predator, herbivore, parasite, pathogen etc.) consumes the tissue of the other species (prey, host), either with or without causing its death.

Co-evolutionary adaptations common among consumers include better ways to:

• detect the prey/host (visual, chemical, tactile, auditory, thermal . . . )
• pursue the prey/host (speed, camouflage, locomotion . . . )
• handle, subdue the prey/host (claws, stings, toxins, mucus . . . )
• overcome prey/host defences (armor, behavior, guile, enzymes, thick skin . . . )
• digest/use prey/host efficiently (saliva, gastric enzymes, detoxifying molecules, gut symbionts . . . )

Co-evolutionary adaptations common among prey or host species include better:

• ways to avoid detection (behavior, diurnal cycles, avoid consumer habitats . . . )
• disguises (camouflage (= cryptic coloration), mimicry (Batesian = copy a toxic model; Mullerian = both species toxic)
• defences (armor, scales, spines, toughness, mucus, chemical deterrents, toxins, immunity, aposomatic (warning) coloration . . . )
• escape mechanisms (flee, jump, play "possum", aggression, refuges, lookouts. . .
• interference with consumers use/digestion (plant "secondary chemistry" interferes with herbivore digestion, tannins bind proteins and enzymes, acute toxins (opiates, nicotine, strychnine . . . )

Co-evolution between a predator and a prey species (or plant and herbivore, etc) has been likened to an "Arms Race" in which the prey is perpetually evolving new adaptations to avoid or minimize predation, while the predator is constantly evolving ways to overcome the prey's latest defences.

[ 0 - ] ASSYMETRICAL Competition, Predation, etc: One species declines; other not affected. Frequently involves species of very different body sizes or population densities (e.g. occasional predation by a common predator on a rare prey may reduce the prey's N, without increasing the predator's N). The adversely affected species may evolve adaptations to minimize effects (see above), if encounters are frequent enough.

[ + + ] MUTUALISM ( = "Symbiosis"): Both species benefit. Co-evolution favors adaptations that enhance detection of the other species and establishment of a secure interaction with it. Over time, complex, specialized interdependencies may evolve.

[ + 0 ] COMMENSALISM: One species benefits; other is not affected ("doesn't notice"). The commensal evolves ways to detect its host, and then live with it in ways that don't adversely affect the host.

[ 0 0 ] NO INTERACTION (even though species co-exist): Neither species is affected by the other, therefore they have no evolutionary impacts on each other.