COMMUNITY ECOLOGY I: BIODIVERSITY 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 in that habitat.

Why is community biodiversity so important? Communities with high biodiversity are more productive. Communities with high biodiversity are more stable. Communities with high biodiversity are better able to withstand and recover from environmental stresses. Higher-diversity communities are more resistant to invasive species.

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).

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”

Consider a grassland with five trophic levels: plants, grasshoppers, snakes, raccoons, and bobcats. If you released additional bobcats into the grassland, how would biomass change if the Top-Down model applied? Plants would increase Plants would decrease Raccoons would increase Snakes would decrease Grasshoppers would increase

What Factors Affect Biodiversity? 1) Certain types of species have major influences on community composition... a) Foundation species have major positive or negative influences because of their physical effects on the environment. Beaver Beaver dam

b) Dominant species have major (usually negative) influences because of their high abundance. e.g. Ponderosa Pine

Effect is positive (enhances biodiversity) c) 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 (Expt’l) Keystone

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.

What Factors Affect Biodiversity? 2) Disturbance Events that damage communities, remove organisms from them, and alter resource availability (storms, floods, fire, glaciers, volcanoes)

Succession i) Primary Succession ii) Secondary Succession The change in species richness and species composition of a community over time, usually after a disturbance of some kind. i) Primary Succession ii) Secondary Succession

i) 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

ii) 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.

3) “Species-Area Relationship” What Factors Affect Biodiversity? Species richness increases with the size of the habitat.