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Community Ecology Chapter 53 Community An assemblage of populations in an area or habitat Communities differ in Species richness # they contain Relative abundance of diff. species Different Views of Communities An individualistic hypothesis depicts a community as a chance assemblage of species found in the same area because they happen to have similar abiotic requirements.(Gleason.1900s) An interactive hypothesis depicts a community as an assemblage of closely linked species locked in by mandatory biotic interactions. (Clements 1900s) In most actual cases, the composition of communities does seem to change continuously Rivet Model & Redundancy Model The rivet model of communities is a reincarnation of the interactive model. The redundancy model states that most species in a community are not closely associated with one another. No matter which model is correct, it is important to study species relationships in communities. Interspecific Interactions Populations may be linked by Competition, Predation, Mutualism and Commensalism Interspecific Interactions A. Interspecific Competition Resources are in short supply Potential for competition between any two species that need the same limited resource. Competitive Exclusion Principle 1934, G.F. Gause P. aurelia & P. caudatum, grown separately-did well but tog. P. aurelia drove P. caudatum to extinction Two species with similar needs for same limiting resources cannot coexist in the same place The better adapted of the two will replace the other Ecological Niche All the biotic and abiotic factors needed to maintain the species. An organism’s niche is its role in the environment. Hawks and Owls have the same niche but can coexist b/c hawks are diurnal, owls are nocturnal The competitive exclusion principle can be restated to say that two species cannot coexist in a community if their niches are identical. Resource Partitioning Differentiation of niches that enables two similar species to coexist in a community Character Displacement The tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species. Hereditary changes evolve that bring about resource partitioning B. Predation (-/+) A predator eats prey. Herbivory, in which animals eat plants. In parasitism, predators live on/in a host and depend on the host for nutrition. Predator adaptations: many important feeding adaptations of predators are both obvious and familiar. Claws, teeth, fangs, poison, heat-sensing organs, speed, and agility Plant Defenses Chemical compounds that are toxic Thorns Secondary compounds not part of normal met, ex poisonous berries Animal Defenses Behavioral defenses include fleeing, hiding, self-defense, noises, and mobbing. a. Camouflage includes cryptic coloration, deceptive markings. b. Mechanical defenses include spines. c. Chemical defenses include odors and toxins Aposematic coloration is indicated by warning colors, and is sometimes associated with other defenses (toxins). Fig. 53.6 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Mimicry- organisms resemble other species 1. Batesian mimicry is where a harmless species mimics a harmful one Hawkmoth larva Snake 2. Müllerian Mimicry Two or more unpalatable species resemble each other Both produce a toxin& predators learn quickly to avoid them Cuckoo Bee Yellow jacket Batesian mimicry Convergent evolution Monarch male poisonous Viceroy male edible Parasites and pathogens as predators A parasite derives nourishment from a host, which is harmed in the process. Endoparasites live inside the host and ectoparasites live on the surface of the host. Parasitoidism is a special type of parasitism where the parasite eventually kills the host. Pathogens are disease-causing organisms that can be considered predators. C. Mutualism (+) Two species benefit from their interaction Acacia & ants Acaciafood & cover Ants sting any intruder D.Commensalism (+/o) One species benefits from the interaction, but other is not affected. An example would be barnacles that attach to a whale Bird’s nests in trees E. Coevolution Two species with intertwined life history- the evolution of one affects the other When one species evolves, it exerts selective pressure on the other to evolve to continue the interaction Ex humming bird & flower F.Trophic structure Feeding relationships Food chains food chain usually 4 or 5 links = trophic levels length of food chain limited by inefficiency of energy transfer Food webs Food chains are hooked together into food webs Who eats whom? a species may weave into web at more than 1 trophic level bears “there’s always a bigger fish” What limits the length of a food chain? G. Dominant & Keystone Species Dominant species most abundant species or highest biomass (total weight) community If we remove a dominant species from a community, it can change the entire community structure. Keystone species exert important regulating effect on other species in community Keystone species Influential ecological role not necessarily dominant or most abundant keystone species increases diversity of habitat Keystone species Sea otter is keystone predator in North Pacific What is the impact of the whale? II. Disturbances Most communities are in a state of non-equilibrium due to disturbances fire, weather, human activities, etc. not all are negative Disturbances Disturbances are often necessary for community development & survival Ecological cycle fire as part of a natural community cycle IIB. Ecological succession The sequence of community changes after a disturbance transition in species composition over ecological time years or decades Mt. St. Helens Primary succession Begins with virtually lifeless area without soil, then… bacteria { make lichens soil & mosses grasses shrubs trees Succession from mosses & lichens = pioneer species to shrubs & trees Secondary succession Existing community cleared, but soil is intact Climax forest Plant community dominated by trees representing final stage of natural succession for specific location stable plant community developed through stages remains essentially unchanged in species composition for as long as site remains undisturbed birch, beech, maple, hemlock oak, hickory, pine Climax Forest The species mix of climax forest is dependent on the abiotic factors of the region solar energy levels temperature rainfall fertility & depth of soil birch, beech, maple, hemlock III. Biodiversity of Communities Two components 1. Species Richness 2. Relative abundance Greater diversity = greater stability Which is more diverse? Species richness is related to a community’s geographic size The species-area curve : the larger the geographic area, the greater the number of species Patterns of Biodiversity 1. Equatorial/Polar gradient: greatest at equator, less &less as move to poles, because Climate: poles frozen at times,long growing season @eq, more solar energy & precip.@ eq 2. Age: eq comm. Are older, more established 1. Patterns of Biodiversity 2. Island Biogeography: diff islands have diff # of species b/c of immigration & extinction. Immigration & extinction determined by the size of the island & distance from the mainland Factors that determine the number of species that eventually inhabit the island The rate at which new species immigrate to the island. The rate at which species become extinct. New Island Size of Island Distance from Mainland