Ch18 Community Structure.docx

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Spencer Barrett

BIO120H © Li| Page 12011 chapter 18: COMMUNITY STRUCTURE  Frederic E. Clements  communities are superorganisms  the functions of various species are connected and have evolved to enhance their interdependence  communities are discrete entities that can be distinguished from one another  it is aholistic conceptt (the idea that the organisms in a community form a discrete, complex unit analogous to a superorganism)  Henry A. Gleason  advocated an inindividualistic conceptf community organization (the idea that a community is not a discrete unit, but merely a fortuitous association of species whose adaptations and requirements enable them to live together under the physical and biological conditions of a particular place)  natural selection acts on the fitness of the indiv. to maximize its own reproductive success, not the benefit the community  community: an association of interacting pop. usu. defined by the nature of their interaction or the place in which they live A BIOLOGICAL COMMUNITY IS AN ASSOCIATION OF INTERACTING POPULATIONS  a community can be spatially defined, including all the pop. w/in its boundaries  Braun-Blanquet sys.: a taxonomy of communities based on a method of sampling plant species composition  places each community in a hierarchy of types when organized by their similarity  closed community: a community in which the distributions of several species coincide closely, but are largely separated from those of other sets of species  consistent w/ holistic view  species closely related, and share ecological tolerance limites  ecotone: a region of rapid turnover of species along a spatial transect or ecological gradient; a zone of transition b/w communities (boundaries of closed comm.)  open community: a local association of species having independent and only partially overlapping ecological distributions  no natural boundaries Ecotones  where species reach the edges of their distributions  prominent at sharp community boundaries ex. terrestrial & aquatic, diff.-facing slopes  some plants only grow in ecotones, or on either sides of ecotones, and others are not affected by the differing conditions around the ecotones  dominant plants can create ecotones by competition AND through altering the envmt  ex. sharp boundary b/w broad- and coniferous- leaved forests  decomposition of conifer needles makes soil acidic, resulting in diff. soil conditions  diff. distributions of herbs and shrubs  ex. fire maintains ecotones b/w prairies and forests  perennial prairie grasses resist fire that kills tree seedlings, but fire can’t penetrate deeply into moist forest BIO120H © Lis| Page 2011 The Continuum Concept and Gradient Analysis  continuum: a gradient of envmtal characteristics or of change in the composition of communities  gradient analysis: the plotting and interpretation of the abundance of species along an gradient analysis envmtal gradient  gradient: ex. moisture, temp., salinity, exposure, light level  Robert Whittaker  puts to rest Clements’ view of extreme closed communities  Whittaker found that dominant trees in the Great Smoky Mountains of Tennessee have distinct but partially overlapping ecological distributions on a gradient MEASURES OF COMMUNITY STRUCTURE INCLUDE NUMBERS OF SPECIES AND TROPHIC LEVELS  species richness: a simple count of the number of species in an area  more species live in tropical regions than temperate and boreal  one way to partition species is according to their feeding relationships  trophic levels: a position in a food web, determined by the number of energy transfer steps from primary producers to that level  autotrophic: an organism that assimilates energy either from sunlight or from inorganic autotrophic compounds  primary producer: a plant or other autotroph that assimilates the energy of sunlight (a photoautotroph) or reduced inorganic compounds (a chemoautotroph) and uses it to synthesize organic compounds  primary consumer: an herbivore; an organism at the lowest consumer level in a food web  secondary consumer: a carnivore; a consumer of primary consumers  guild: a group of species that occupy similar ecological positions within the same community  grouped together within trophic levels by types of resources consumed, and methods or locations of foraging FEEDING RELATIONSHIPS ORGANIZE COMMUNITIES IN FOOD WEBS  food web: a representation of the various interconnected paths of energy flow through populations in a community, taking into account the fact that each population shares resources and consumers with other populations Effects of Species Richness on Food Web Structure  omnivory: feeding on more than one trophic level  complexity of a food web is based on the # of feeding links and trophic levels  # of trophic levels (and guilds) increases w/ species richness  increasing species richness is associated w/ increasing food web complexity  # of feeding links per species is independent of the species richness of the community BIO120H © Lis| Page 3011 Effect of Food Web Structure on Species Diversity  species diversity increases food web diversity  feeding relationships affect species diversity w/in a community  John Terborgh et al. (consumer effect on species diversity) John Terborgh  unintended predator removal in rain forests of Venezuela by dam  dam created patches – too small to support predators of the herbivores  howler monkeys, green iguanas –pop. skyrocketed  ecosystem “meltdown” occurred and affected regeneration of trees and decreased productivity and plant diversity  keystone consumer: species, often a predator, that has a dominant influence on the structure of a community, which may be revealed when that species is removed A Variety of Food Web Types  Robert Paine  described diff. types of food webs that describe diff. ways in which species influence one another w/in communities  connectedness web: a food web that emphasizes the feeding relationships among species
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