BIOLOGY 157: LIFE SCIENCE: AN ENVIRONMENTAL APPROACH (Communities and Succession)

COMMUNITIES I DEFINITION: a community is a unit composed of two or more interacting species in a given area How many communities are there on the earth? One ??? Hundreds ??? Thousands ??? Millions ??? It all depends on how you ‘define’ it.

COMMUNITIES II Community Diversity: the number of species and strains in a community Community Complexity: (= Ecological Diversity ) the number of trophic levels and how the species are distributed among the trophic levels Communities have boundaries in space and time.

ECOTONES Ecotone zone between communities where composition very rapidly changes from that typical of one community to that typical of the other Ecotones are ‘pressure’ or ‘tension’ zones Ecotones often have a greater biodiversity than either of the adjacent communities Ecotones can be narrow or wide Ecotones and adjacent communities can shift over time

ECOTONES (narrow)

EDGE EFFECT (I) Related to ecotones Size of the area and shape of the area interact Two areas of the same size may have very different shapes and thus very different amounts of ‘edge’. How does this affect species in those areas?

EDGE EFFECT (II)

COMMUNITY PRODUCTIVITY

EMERGING CHARACTERISTICS IN COMMUNITIES Dominant (= Keystone) Species and Subordinate Species (may not always be related to abundance) Stratification (= Layering) Succession: a non-cyclic process during which species / communities in a particular area are replaced by other species / communities over time = the ‘life history’ of a community

ECOLOGICAL SUCCESSION (I)

ECOLOGICAL SUCCESSION (II)

ECOLOGICAL SUCCESSION (III) What causes it? Organisms and the changes they create are the driving force. Types of Succession Primary occurs on a primary bare area Secondary occurs on a secondary bare area

ECOLOGICAL SUCCESSION (IV) Pioneer (species or community) the first species to invade an area Seral (species or community) those which occupy intermediate positions in a succession (early, mid, late, etc.) Climax (species or community) those occupying the final position in a succession

CLIMAX COMMUNITY (I) DEFINITION: a steady state community whose characteristics are determined by the characteristics of its habitat & environment Primary Climax does not depend on recurrent disturbances by fire or animals to maintain floristic / faunal composition Climatic Climax --- forms on normal soils, with average topography, and thus only the macroclimatic conditions are controlling it. Edaphic Climax --- develops differently from what one expects for the macroclimatic conditions due to an unusual soil Topographic Climax --- develops differently from others in the same region due to a distinctive microclimate

CLIMAX COMMUNITY (II) Disclimax all climaxes that maintain their floral / faunal composition only as a result of persistent disturbances of the same kind, frequency and intensity - Fire NJ Pine Barrens Some Grasslands - Zootic Some Grasslands

TRENDS DURING SUCCESSION (I) Stage in Ecosystem Development Attribute Young Mature Biomass low high Trophic Relationships simple complex Food Chains short, grazing long, detritus Food Webs simple complex Stratification less more Species Diversity low high Niche Specialization broad narrow Feeding Relations general specialized Life Cycles short, simple long, complex Population Control Mechanisms physical biological

TRENDS DURING SUCCESSION (II) Stage in Ecosystem Development Attribute Young Mature Fluctuations greater lesser Stability low high Potential Yield to Humans high low Net Community Production higher lower (0 ??) Community Respiration lower higher Community P / R Ratio P > R P = R