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Chapter 6: Population and Community Ecology Key Ideas There are clear patterns in the distribution and abundance of species across the globe. Understanding the factors that generate these patterns can help us preserve global biodiversity. These factors include the ways in which populations increase and decrease in size and the ways in which species interact with one another in their communities Population Characteristics • • • • • population size (N) population density population distribution population sex ratio population age structure Population Dispersal Patterns • Random • Clumped • Uniform Factors that Influence Population Size • Density-dependent factors- the size of • • the population will influence an individual’s probability of survival. Density-independent factors- the size of the population has no effect on the individual’s probability of survival. Carrying capacity (K) population growth • birth • immigration population decline • death • emigration population growth environmental resistance carrying capacity population logistic growth biotic potential exponential growth time Logistic Growth of Sheep Population Number of sheep (millions) 2.0 Overshoot 1.5 Carrying Capacity dieback 1.0 .5 1800 1825 1850 1875 1900 1925 Year Fig. 6-12, p. 121 Variations of the Logistic Model • If food becomes scarce, the population will experience an overshoot by becoming larger than the spring carrying capacity and will result in a die-off, or population crash. Estimating Population in the Wild • sampling- • mark and recapture- Assumptions for validity of mark and recapture techniques • tagging does not change the behavior of the animal • tagged animals must have time to freely mix with the rest of the population • animals must be captured randomly Species Interactions Keystone Species • play critical ecological roles • top predators • pollinators • engineers Competition • interspecific • intraspecific • resource partitioning reduces competition Mutualism • Mutualism- A type of interspecific interaction where both species benefit. Ecological Succession Change Over Time primary succession secondary succession Aquatic Succession