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Lecture 32: Community Structure
Reading: None.
Gaia Hypothesis: Biosphere as self-regulating superorganism This hypothesis is not supported by the evidence on the composition of communities, the distributions of species in communities, or the interactions that occur between organisms in communities. Interspecific interactions can range from obligate exclusion to obligate association, but most species may have no interactions or non-obligate interactions. Obligate associations are not common but they do occur as symbiotic interactions.
If most species do not interact, or have non-obligate interactions, then well defined functional communities, communities based on obligate interspecific interactions, may not be possible. Biological communities are a human construct. Community classification is informal and flexible, and typically based on dominance (abundance of a conspicuous species, form of vegetation, or physical conditions. This is the basis for the major plant formations (terrestrial community types) which can be organized on the basis of temperature and precipitation characteristics (Ricklefs, 1996, p 114, Fig. 5.11). Keystone Species Some communities are characterized by one species whose abundance (or presence) influences many other species, typically this is a predator-prey interaction. Marine intertidal environments: Pisaster (starfish, predator) - Mytilus (bivalve mollusk, prey) Marine subtidal environments: lobster - urchin - algae Marine nearshore environments: sea otters - urchins - Laminaria algae Terrestrial boreal forest: wolf - moose - fir trees A keystone species need not be abundant to have a significant influence on a community.
Community Variables Stratification or Vertical Structure Terrestrial communities are often light limited, and stratification is a function of competition for sunlight. Light intensity decreases from the top of forest trees to the ground level (after Krebs, 1994, p 467, Fig. 21.6).
Light limitation puts a premium on plant height, but there are tradeoffs between plant height and leaf biomass and coverage (after Krebs, 1994, p 469, Fig. 21.8).
Aquatic Stratification Temperate lakes are thermally stratified in the summer but mix each spring and autumn (Ricklefs, 1996, p 87, Fig. 4.11).
Thermal stratification prevents mixing of surface water (epilimnion) with the deeper waters (hypolimnion) and can result in a dramatic decrease in dissolved oxygen concentration with depth (Ricklefs, 1996, p 182, Fig. 8.12), and the accumulation of nutrients in the hypolimnion that are typically limiting when oxygen concentration is adequate (Ricklefs, 1996, p 183, Fig. 8.13). Tropical lakes are thermally stratified all year. In all lakes, phytoplankton are limited to surface waters for photosynthesis. Zooplankton often perform vertical migrations to feed and avoid predators. Marine and Intertidal Zonation Stratification among marine autotrophs is a function of light, and among heterotrophs it is a function of food, predation, temperature, and pressure. |
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