BIO 115 1st Edition Lecture 34 Topic Discussed: Abiotic/Biotic Factors in a CommunityInteraction network- dots are species, lines are interactions, length indicates strength of interactionsDominant species -most abundant or have the highest biomasso exert powerful control over the occurrence and distribution of other specieso ie. sugar maples limit the understoryHypotheses on why dominant species are dominant- dominant species are most competitive in exploiting resources- they are most successful at avoiding predators- Invasive species, lack predators or diseaseKeystone Species and Ecosystem Engineers- exert strong control on a community by their ecological roles, or niches- In contrast to dominant species, they are not necessarily abundant in a community- Ie. sea starsEcosystem engineers (“foundation species”)-cause physical changes in the environment that affect community structure- Ie. beavers and leaf rollersBottom-Up and Top-Down ControlsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- The bottom-up model -unidirectional influence from lower to higher trophic levels- presence or absence of mineral nutrients determines community structure, including the abundance of primary producers- top-down model - control comes from the trophic level above- predators control herbivores, which in turn control primary producersDisturbance influences species diversity and composition- Decades ago, most ecologists favored the view that communities are in a state of equilibriumo F. E. Clements- suggested that species in a climax community function as a superorganism- Other ecologists, including A. G. Tansley and H. A. Gleason, challenged whether communities were at equilibriumo Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbancesCharacterizing Disturbance- Fire is a significant disturbance - A high level of disturbance = high intensity and high frequency of disturbanceintermediate disturbance hypothesis-moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance- High levels of disturbance exclude many slow-growing species- low levels of disturbance allow dominant species to exclude less competitive speciesEcological Succession- Ecological succession - sequence of community and ecosystem changes after a disturbance- Primary succession -where no soil exists when succession begins- Secondary succession -begins in an area where soil remains after a disturbanceEarly-arriving species and later-arriving species may be linked:- Early arrivals may facilitate the appearance of later species by making the environment favorable- They may inhibit the establishment of later species- They may tolerate later species but have no impact on their establishmentBiogeographic factors affect community diversity- Latitude and area are two key factors that affect a community’s species diversityLatitudinal Gradients- Species richness is especially great in the tropics and generally declines along an equatorial-polar gradient- Two key factors -evolutionary history and climate- Tropical habitats are “older” because glaciers restart communities- Temperate and polar communities have started over repeatedly following glaciations- The greater age of tropical environments may account for their greater species richness- In the tropics, the growing season is longer, so biological time runs fasterThe species-area curve- all other factors being equal, a larger geographic area has more speciesIsland Equilibrium Model- Species richness on islands depends on island size, distance from the mainland, immigration, and extinction- The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium
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