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Biol 121- 2010.04.09- Ecology- Population Dynamics (Ch. 52).docx

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BIOL 121
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Biol 121 225 Freeman 1181-1193 Apr 9, 10 Ecology: Population Dynamics Population dynamics -population dynamics = change in populations through time -find patterns in natural populations in addition to exponential and logistic growth -why do all populations stop growing? What limits growth rate and 1) Density-independent factors population size? -when b or d is not influenced by population density -e.g. abiotic environmental changes such as weather patterns and disturbances 2) Density-dependent factors -increase in d; decrease in b as population density increases – negative feedback -there are extrinsic and intrinsic factors that are density-dependent Extrinsic factors -sensitivity to some limiting resource which limits reproduction e.g. decrease in the # eggs laid -territory boundaries – need certain space between nests, etc -crowding – increases mortality (e.g. smaller, frailer individuals) -predation – more predators leads to less prey Intrinsic factors -hormonal changes – can lead to aggressive behaviour, delay in sexual maturation time, as population density increases Density dependent factors: -density-dependent factors define K for a particular habitat Relationship to K -K depends on the quality of habitat (food, space, accumulation of waste) -K varies with time – can change year to year -characteristics of species are also looked at: e.g. larger sized species need more food, space, etc Density dependent factors: -rmaxis the intrinsic rate of increase Relationship to r max -it is genetically determined, but can vary with the environment -e.g. in L. vivipara, at low elevations they begin breeding younger but in populations at high elevation they breed at an older age -in general, species with a relatively long lifespan and larger body size will have a lower rmax -to summarize, r max is genetically determined but may be modified by the environment Patterns of population change – -0.1 to 0.2 ha plots in a meadow with uniform soil and vegetation Rothamstead, UK Park Grass -some plots treated with fertilizer, some left as controls study, 1856 -analyzed data from 1920 to 1979: which species were present in which plot -then used as an index for population size and growth -results were that different spp had different growth patterns (some spiked then declined, some increased then reached plateau, some did not change) -in several cases, patterns of change over time appeared to correlate with life-history traits: -spp that maintained constant population size have longer life spans and lower r max -possibly b/c they allocate more energy and resources to competitive ability than to reproduction -conversely, species that spiked and declined or that increased and maintained high abundance had a higher r max, an earlier age at first reproduction, and shorter life spans than did species that maintained a constant population size Population Cycles -regular fluctuations in population size, normally in animal populations -certain population cycles can be explained with a density-dependent factor: -idea is that predation, disease or food shortage intensify dramatically at high population density and cause population numbers to crash but not much evidence 1 Biol 121 225 Freeman 1181-1193 Apr 9, 10 -competition between male crabs (intraspecific competition) could lead to fluctuations Boom-bust cycle: Lynx and hare -snowshoe hare and lynx populations in northern Canada population -the hare-lynx populations cycle every 11 years; the population size for lynx lags behind that of hares (the graphs of # vs. time is 90 degrees out of phase if considered sinusoidal) Hypotheses for these population dynamics: 1. H
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