BIOL202 - Lectures 8-11

BIOL202 - Lectures 8-11 - LECTURE 8 THE NATURE OF...

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LECTURE 8 THE NATURE OF BIOLOGICAL COMMUNITIES Biological community: an assemblage of populations of different species living in a prescribed area DEFINING CHARACTERISTICS OF COMMUNITIES 1. Species Composition A. Membership list - who's present and who's not? B. Species richness and diversity species richness : refers simply to the number of different species in community, ignoring relative abundances species diversity : refers not only the number of different species but also takes into account the relative number of individuals belonging to each species; a community with more even numbers of individuals in different species is more diverse Imagine have two communities each w/ four spp.: A, B, C, D Number of individuals of each species: Community 1: 25 A's 22 B's 30 C's 23 D's Community 2: 6 A's 84 B's 1 C 9 D's Two communities are equal in species richness (4) but which community is most diverse ?
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2. Physical structure or "physiognomy" of community: = overall "physical structure" of community Determined by: - "growth form" of predominant plant species - number of vertical layers of vegetation - amount of surface covered by vegetation 3. Trophic structure - basically, who eats whom? Descriptions of food chains and food webs 4. Productivity and pattern of energy flow through community How much new "biomass" (living tissue) is produced per year by plants and by animals? 5. Input, output and recycling of essential nutrients All organisms need certain key nutrients, e.g. N, P, Ca, Mg, S, Fe Animals get essential nutrients from food (plants or other animals) Where do the plants get the nutrients they need?
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A key point about the nature of biological communities: Populations in communities are tied together in a very complex web of both direct and indirect interactions Each species in a community interacts directly with a variety of other species: Direct interaction: where the activity of one species directly affects the survival, growth and/or reproduction of individuals in another species e.g.,: bison population of Wind Cave National Park, Black Hills, SD Microparasites Predators Mutualists (viruses, bacteria (coyotes, cougars) (cellulose-digesting & protozoans) micro-organisms in gut) Macroparasites Prey Competitors (ticks, lice, fleas, (grasses, herbs, (mice, grasshoppers, flies, flukes, worms) shrubs, tree seeds) deer, beetles, antelope)
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Indirect Interactions Between Species Indirect interaction: where the activity of one species affects the survival, growth, and/or reproduction of another species even though two species don't engage in any type of direct interaction. One example of an indirect interaction between species: a trophic cascade Definition: where activity of a consumer species not only has a direct effect its prey but effects also “cascade down” to have indirect effects on the trophic levels below for example: predator herbivore plant Note: Depending on the system, the consumer species could be a predator, herbivore, parasite, pathogen, etc.
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Well-studied trophic cascade in the Aleutian Islands, Alaska Sea Otters Sea Urchins + Brown Kelp
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BIOL202 - Lectures 8-11 - LECTURE 8 THE NATURE OF...

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