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COMMUNITY ECOLOGY I: BIODIVERSITY Community: Any assemblage of populations [of plants and/or animals] in a given area or habitat. Community Biodiversity: Number of species, relative abundance of each species, kinds of species present How can we describe biodiversity quantitatively? Species Richness: number of different species present in the community Species Evenness: relative abundance of the different species present Species Diversity: number and relative abundance of each species Which Forest is More Diverse? Each forest has the same four tree species (same species richness), but they differ in species evenness (relative abundance of each species). How can we describe biodiversity quantitatively? Shannon-Weiner Diversity Index Ranges from 0 (only one species present) to infinity (though usually less than 3 in temperate habitats). GREEN OAKS LAB Green Oaks Field Station Two forest habitats were sampled: Old Growth: relatively undisturbed, but selectively logged in the 50’s and 60’s (avoided cutting White Oaks) H’ = 2.499 Spoil Banks: completely clear-cut for strip-mining purposes in 1940-1941 - however, some non-local trees planted during 60-year recovery period H’ = 2.191 How can we describe biodiversity qualitatively? Marine Food Web Bottom-Up and Top-Down Controls • V H: increasing V increases H, but not vice versa (Bottom-Up) • V H: increasing H decreases V, but not vice versa (Top-Down) • V H P: “Trophic Cascade” What Factors Affect Biodiversity? Some species have major influences on community composition... • Foundation species have major positive or negative influences because of their physical effects on the environment. Beaver Beaver dam • Dominant species have major (usually negative) influences because of their high abundance. e.g. Ponderosa Pine • Keystone species influence ecological communities more than would be expected from their abundances. • Effect is positive (enhances biodiversity) Keystone predator: - a species whose predatory behavior has regulating effects on other species in the community -maintains higher species richness by altering competitive relationships Classic Example: Rocky Intertidal Zone Sessile invertebrates Acorn barnacle Gooseneck barnacle Mussel Mobile invertebrates Whelk Starfish Chiton • Intense competition for space among sessile invertebrates; one is dominant competitor • One mobile invertebrate species keeps dominant competitor in check and maintains biodiversity: Keystone predator Expt: Removed and excluded different mobile invertebrate species to see the effect on biodiversity Keystone - # species dropped from 17 to 2.5 in 3 years Keystone Keystone (Expt’l) Recent Example: Sea Otters Like the rocky intertidal zone, kelp forests are communities of extremely high biodiversity. Sea otters feed on sea urchins, which, in turn, feed on kelp. However, orcas have recently turned to feeding on sea otters along west coast. As a result, sea urchins have increased and kelp forests have declined. Disturbance Events that damage communities, remove organisms from them, and alter resource availability (storms, floods, fire, etc.) Intermediate Disturbance Hypothesis: Moderate levels of disturbance can create conditions that foster greater species diversity than low or high levels of disturbance. Example of Intermediate Disturbance Succession The change in species richness and species composition of a community over time, usually after a disturbance of some kind. Primary Succession Secondary Succession Primary Succession - succession on newly exposed, soil-free areas e.g. glacial retreat ( community gradients) Primary Succession at Glacier Bay e.g. volcanic eruption 1981 1999 Primary succession around Mount St. Helens Secondary Succession -succession in disturbed areas, where at least soil remains (usually due to clear-cutting or fire) e.g. old field succession Old growth Spoil banks Biodiversity can increase with forest age. Another factor that influences biodiversity: LATITUDE Latitudinal Gradients Species richness increases along polar-equatorial gradients. Bird Species WHY? Evolutionary History -tropics are older than temperate zones Climate -solar energy, water, temperature all higher in tropics Tree Species Another factor that influences biodiversity: AREA “Species-Area Relationship” Species richness increases with the size of the habitat.
