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What factors affect population growth? What limits the distribution and abundance of organisms? Abiotic environment Thermal limitations Ability to respond to change Interactions (intraspecific and interspecific) Characteristics inherent within the individual Not how an individual responds, but how an individual lives Life-history Importance of density Some factors that regulate population sizes are directly related to density In other words, as density changes, the factor will change with density Density dependent factors Some factors that regulate populations are not related to density at all In other words, as density changes, the factor does not change Density independent factors Density-dependent factors Higher proportion of population is affected as population density increases Tend to reduce population size by decreasing natality or increasing mortality Interspecific interactions Predator-Prey oscillations Intraspecific interactions Territoriality Stress and crowding Stress-related diseases Crowding Annual plant Erophila verna Varied planting density Found shift in survivorship curves Intraspecific competition Density can affect all BIDE factors Allee effect Density-independent factors Factors unaffected by population density Examples include drought, hurricanes and floods Density-independent factor Decline in the population of one of Darwin's finches (Geospiza fortis) on Galapagos Islands due to drought Community interactions Hare fluctuations due to both Food plants abundance Predator abundance Why don’t all populations maximize their growth? i.e., why don’t all organisms reproduce like rabbits? Cost of reproduction Due to limited resources, increased reproduction may decrease survival and chances of future production Therefore, there is a cost of reproduction Remember that natural selection will favor the life history that maximizes lifetime reproductive success Life history A life history is the complete life cycle of an organism “Choices” in life history How much to invest in each child How often to reproduce When to reproduce All “choices” involve significant trade-offs! So, organisms must make choices that maximize their reproductive success overall given ecological conditions Cost of reproduction Investment per offspring Key reproductive tradeoff concerns the amount of resources to invest in producing any single offspring. number of offspring versus size of each offspring – In many species, offspring size critically affects chances of survival Parental investment Tradeoff between number of offspring and size of offspring Number of reproductive events Trade off between current and future reproductive success Cost of reproduction Reproductive events per lifetime Semelparity - organisms focus all reproductive efforts on a single, large event Iteroparity - organisms produce offspring several times over many seasons Cost of reproduction Age at first reproduction Longer-lived animals tend to reproduce later, and provide more parental care than shorter-lived animals Life-history strategies All of these trade-offs – and more -occur simultaneously Can not maximize one trait without losing in another Results in some common strategies for dealing with life r vs K Life history strategies K-selected species tend to produce relatively few, large offspring large investment in parental care r-selected species tend to produce many, small offspring small investment in parental care r/K species Species at near exponential phase of life = r Rapid growth “r-selecting” habitats Species at more constant density = K Growth more limited by resources “K-selecting” habitats r-selection vs. K-selection