1. competitive exclusion => local elimination of one => competitive exclusion principle (Gause and Paramecia)

Transcription

1 Chapter 54: Community Ecology A community is defined as an assemblage of species living close enough together for potential interaction. Each member of same community has a particular habitat and niche. Habitat => home Ecological niche => sum total of what an organism does (role) 54.1 Interspecific Interactions and Structure Interactions between populations are symbolized by positive (+), negative ( ) or 0 effects Competition => demand for a resource in short supply (-/-) Strong competition can lead to: 1. competitive exclusion => local elimination of one => competitive exclusion principle (Gause and Paramecia) 2. resource partitioning => modification of niche => this can happen because an organism s fundamental niche (potential) may differ from the realized niche (actual) 3. Character displacement => tendency for characteristics to be more divergent in sympatric ( same country ) populations of two closely related species than in allopatric ( different country ) populations of the same two species. Community Ecology 54-1

2 Predation => predator kills and eats prey (+/-) Predator adaptations: acute senses, weaponry (claws, fangs, stingers, poison), speed, agility Prey adaptations: fleeing, hiding, self-defense, alarm calls, cryptic coloration (camouflage), aposematic colorationwarning Batesian mimicry => harmless mimics harmful Müllerian mimicry => harmful resemble each other (avoidance is quickly learned) Predators may also use mimicry (tongue of snapping turtle looks like worm) Herbivory => organism eats plant material or algae (+/-) Herbivore adaptations: chemical sensors, dentition and digestive systems to process vegetation Plant adaptations: chemical toxins (strychnine, nicotine), spines/thorns Parasitism => parasite derives nourishment from host (+/-) Types: endoparasites => live within body of host ectoparasites => live on external surface of host parasitoidism => insect lays eggs on or in living host (larvae then eat through flesh) Some parasites change behavior of host to finish their life cycle. (suicidal snail; wasp turning spider into zombie) Pathogens => disease causing agents (+/-) Typically bacteria, viruses, or protists (also fungi, prions) Can be particularly virulent in new habitats. Emerging diseases are a cause for concern. (community structure could be severely altered.) Zoonotic pathogens => transferred to humans from other animals either directly or through a vector ex. lyme from ticks; malaria from mosquitoes Mutualism => both species benefit (+/+) ex. nitrogen fixing bacteria in root nodules of legumes digestion of cellulose by microorganisms in termites Commensalism => one benefits, other is unaffected (+/0) Difficult to document because there could be a minimal effect ex. shark and remora, barnacles on whale, orchid on tree Community Ecology 54-2

3 Community Structure Species diversity is important to community structure. Two components: 1. Species richness => number of different species 2. Relative abundance => the proportion of each species Which community is more diverse? Why? (No need to know how to calculate indexes of diversity) Potential benefits of diversity: More productive Higher stability Faster recovery More resistant to change and invasion Trophic structure => food chain => autotrophs herbivores (primary consumers) carnivores (secondary and tertiary consumers) decomposers Food chains are linked together into food webs (arrows show direction of energy flow). Community Ecology 54-3

4 Each food chain within a web is usually only a few links long (4-5). Length is limited by inefficiency of energy transfer (10% to next level) Represented by energy pyramid: Certain species have a large impact on community structure. Dominant species => most abundant or have the highest biomass; competitively successful, avoiding predation or disease Keystone species => key ecological niches (ex. Pisaster, sea otters) Foundation species => cause physical changes (ex. beaver) Video: Wolves in Yellowstone: The structure of a community may be controlled from the bottom up by nutrients (N V H P ) or from the top down by predators (trophic cascade model as discussed in wolves video). Other models are reciprocal. Understanding these models are useful in using biomanipulation to solve ecological problems such as improving water quality. Community Ecology 54-4

5 Disturbance and Community Structure Many communities characterized by change rather than stability. A disturbance can have a beneficial effect on a community. create opportunities for species not present enhance environmental patchiness maintaining diversity small-scale disturbances may prevent a large-scale disturbance Research supports that the best disturbances seem to be intermediate ones. Ecological succession => sequence of community changes (transition in species) after a disturbance Primary succession => begins in a lifeless, soil-less area (volcanic) Mosses/lichens colonize and cause the development of soil Grasses shrubs trees (and others that live in them) Secondary succession => disturbance removes existing community (soil remains intact) Ex. forest fire, abandoned field Video succession: Although disturbances occur naturally, it seems that human activity is often the most severe. Community Ecology 54-5

6 Biogeographic Factors That Affect Community Diversity Two key factors of biodiversity: geographic location and size. Biogeographic patterns in biodiversity conform to a set of basic principles: 1. Biodiversity generally declines along an equatorial-polar gradient. Tropical habitats support more species than do temperate and polar regions. Tropics are generally older and growing season is longer. (More time for speciation to occur). Species richness correlates with measures of evapotranspiration. 2. The larger the geographic area, the greater the number of species. (species-area curve) 3. Small islands or far from mainland (more isolation): lower immigration, less potential for new colonizers, less diversity, higher extinction (because of fewer resources). Community Ecology 54-6