a community is a group of species that occupy a given area and interact either directly or indirectly the biological structure of a community is defined by its species composition that is the set of species present in their relative abundances historically there have been two contrasting concepts of the community the organismal concept views the community as a unit an association of species in which each species is a component of the integrated whole the individualistic concept views the co-occurrence of species as a result of similarities in requirements and tolerances the number of species in the community defines species richness species diversity involves two components species richness and species evenness which reflect how individuals are apportioned among the species when a single or a few species predominate within a community they are referred to as dominant the dominance are often defined as the most American abundant however in populations or among species in which individuals can vary widely in size abundance alone is not always a sufficient indicator of dominance keystone species are species that function in a unique and significant man and their effect on the community is disproportionate to their numerical abundance their removal initiates changes in the community structure and often results in a significant loss of diversity their role in the community may be to create modify habitats or to influence the interactions among other species feeding relationships can be graphically represented as a food chain a series of arrows each pointing from one species to another that is a source of food within a community many food chains mesh into a complex footprint with names leading from primary producers to an array of consumers species that are fed on but that do not feed on others are turned basal species species that feed on others but are not prey for other species are termed top predators species that are both predators and prey are term intermediate species groups of species that exploit a common resource in a similar fashion are term guilds functional group or functional type is a more general term used to define a group of species based on their common response to the environment life history characteristics or role within the community communities are characterized by a physical structure in terrestrial communities structure is largely defined by the vegetation vertical structure on land reflects the life forms of plants in aquatic environments communities are largely defined by physical features such as light temperature and oxygen profiles all communities have an auto traffic and heterotrophic layers the ODA traffic layer carries out photosynthesis the heterotrophic players uses carbon storage where the autotrophs as a food source vertical layering provides the physical structure in which many forms of life live changes in the physical structure and biological communities across the landscape result in zonation zonation is common to all environments both aquatic and terrestrial zonation is most pronounced where sharp changes occur in the physical environment as in aquatic communities in most cases transitions between communities are gradual and defining a boundary between communities is difficult the way we classify a community depends on the scale we use the goal of restoration ecology is to return a community or ecosystem to a close approximation of its condition before disturbance by applying ecological principles restoration ecology requires an understanding of the basic processes influencing the structure and dynamics of ecological communities [Music] you