Lecture 11: COMMUNITIES AND NICHES Ecology of a community: - Biological community: assemblage of populations of various species living close enough for potential interaction - assemblage of populations of various species in a given place, at a given time, which are close enough for potential interaction. - Communities differ in species richness (no. of species) abundance, and diversity. - Diversity can be considered a measure of richness and abundance - Integrated hypothesis: community is an assemblage of closely- linked species, locked together by mandatory biotic interactions - Clemence: communities are closely linked and locked together by mandatory interactions. - Individualistic hypothesis: communities are loosely organized associations of independently-distributed species with the same abiotic requirements - Gleason: communities we recognise are loosely organised, they share the same abiotic requirements, however there is independent distribution of species due to an environmental gradient. Presence or absence of particular species depends on presence or absence of other species Integrated hypothesis Population densities of individual species Environmental gradient (eg. temperature or moisture) . In most actual cases, composition of communities seems to change continuously Ā· Each species more or less independently distributed Individualistic hypothesis Population densities of - The individualistic hypothesis is the one which is more supported. Some communities have a large turnover. - We base conservation programmes today on a blinkered view of individual species Environmental gradient (eg. temperature or moisture) communities, e.g. wildcat which was pushed into Scotland, but 1000 years ago lived in Southern Britain in deciduous woodland. Rivet and Redundancy Models: investigating the effects of stress on communities - Rivet model: all species in a community are linked in a tight web of interactions. Therefore, the loss of even a single species has strong repercussions for the community. This model assumes that species are specialised in their roles, so the loss of species is critical for the entire ecosystem and may result in ecosystem collapse. - Redundancy model: if a species is lost, other species will fill the gap, therefore assumes that more than one specie within the ecosystem carries out the same specialised role, therefore if this one is lost, another will be able to step in, meaning the ecosystem is not subject to collapse. However, increased dependency may be placed on compensating species which may cause further stress to that species. - Most communities probably lie somewhere in the middle. - A few impact species exert strong control on community structure. - Some species are more generalised and others are highly specialised. Impact species: they have elevated importance within the community - Dominant Species: o Those which are the most abundant or have the highest biomass. o Exert powerful control over occurrence and distribution of other species o Can be due to successful avoidance of predators or most competitive in exploiting resources o Invasive species problems - e.g. invasive species becoming the dominant species can result in ecosystem instability. - Keystone Species: - related to keystone in an arch O Not necessarily very common but disproportionate effect on