Ecology – Community Interactions As populations interact with one another and influence each other’s survival and reproduction, they serve as agents of natural selection – leads to Coevolution Biotic Interactions: Predator-prey – very close relationship Results in coevolution because each exerts intensive environmental pressure on the other Generally fewer predators than prey Most predators are larger than prey or hunt collectively (wolf pack)

Includes herbivores eating plants (coevolve also) Produce a “saw-tooth” curve

Symbiosis – means “living together”, close interaction between organisms of different species for extended period of time Mutualism – both benefit – ex. Plants and pollinators, cows and intestinal bacteria Commensalism – one benefits, the other is unaffected – ex. Barnacles riding on a whale Parasitism – parasite benefits, host is harmed Parasite harms and weakens host but does not kill it immediately In parasitism and mutualism, both species coevolve since they exert very intense natural selection pressures on each other

Keystone species – some communities have certain key species that play a major role in determining community structure Role is out of proportion to its abundance in community Removal drastically alters community Ex. Starfish (Pisaster) in rocky intertidal (overgrowth of mussels eliminates other species) otters along Calif. coast (prevent overgrowth of urchins and destruction of kelp forests) elephants in the savanna prevent encroachment of forest in grasslands

Succession Structural change in a community and it’s nonliving environment over time Occurs because successional communities tend to alter the area in which they occur in such a way as to make it less favorable for themselves and more favorable for other communities Two types: Primary and Secondary Succession

Primary Succession Community gradually colonizes bare rock, sand, or clear glacial pool where there is no trace of a previous community (“from scratch”) Typically requires thousands or tens of thousands of years First species to arrive are the pioneers – very hardy, lichens, mosses – change the environment (breakdown bare rock) so new life can enter Pioneer community replaced by seral communities – intermediate communities such as woody shrubs, trees (that need sunlight) Shade-tolerant, faster growing tree seedlings replace seral communities leading to climax community Climax community – most diverse and stable community, permanent

Primary Succession

Secondary Succession New community develops after an existing ecosystem is disturbed (forest fire, volcano eruption, abandoned farm field) Occurs much more rapidly (only a few hundred years) Pioneer species are typically grasses, weeds, wildflowers

Succession occurs in lakes and ponds Start as clear, clean pools of water – oligotrophic Sediments and nutrients begin to accumulate and fill water – eutrophication Eventually water fills and becomes a marsh, then a meadow, then a forest

Common trends in succession: Species composition changes continuously but more rapid in earlier stages Total number of species represented increases initially, declines slightly, and finally stabilizes in older stages Net primary productivity increases and is highest in climax community Store of inorganic nutrients in organisms and plants increase with older stages Total biomass and amount of nonliving organic matter increases with older stages Height and massiveness of plants increases Food webs become more complex

Climax communities found in similar climates are somewhat alike Factors such as annual temperature and precipitation along with wind, humidity, latitude, altitude, and topography result in biotic regions called biomes Biomes are characterized by different climates, plants and animals Read about different biomes in textbook