Abstract
Intraspecific density effects are generally associated with other factors, like disturbance. Therefore, the waysinwhichdensityeffectsmightinteractwithdisturbancetomodifytherelationshipsbetweenvitalratesandpopu- lationgrowthmustbeunderstood.Iquantifiedtheeffectsofdensityonthelife-historystagesoftheperennialcompos- itePityopsisasperaover3years,thespanofwhichincludedyearsinwhichfiredidanddidnotoccur.Inanexperimental study, I estimated the survival, growth and reproduction for shoots in plots established across a natural range of dens- itiesinFlorida,USA.Inanovelanalysis,aregression-designlife-tableresponseexperimentwasusedtodeterminewhich transitionswereassociatedwithdensity,howtheycontributedtodifferencesinestimatedpopulationgrowthratesand how this relationship differed as a result of fire. The shape of the relationship between population growth rate (l) and density was modified by fire, primarily as a result of contributions from adult flowering stasis and survival, and first- year survival probabilities. Fire modified and even reversed the effect of extreme densities on adult flowering stasis and survival and of first-year survival, resulting in more positive contributions from these transitions to l at the lowest and highest density values. These results demonstrate the first application of a regression-design life-table response experiment toelucidatingthe interactiveeffects of densityandfire. They highlight the utilityof this approach forbothcapturingthecomplexdynamicsofpopulationsandestablishingameansofdetermininghowvitalratesmight contribute to differences in demography across densities.
Highlights
The regulatory causes and effect of within-population density are among the most important and most documented topics in ecology
Fecundities of plants that withstood the burning had a positive relationship with density in the fire year and were, in general, lower in the fire than in the non-fire year
Neither first-year survival nor flowering adult survival appeared to be related to density in the fire year
Summary
The regulatory causes and effect of within-population density are among the most important and most documented topics in ecology. Differences in population density across sites are expected to be caused by a variety of factors, including differences in habitat quality (Holt 1987), settlement costs (Greene and Stamps 2001) and dispersal (Amarasekare 2004). Density often plays a role in the modification of resources and both intra- and interspecific interactions Competition for limiting resources becomes more intense (Antonovics and Levin 1980; Miller 1996), leading to decreases in the size and often the number of individuals (e.g. the self-thinning rule; Hutchings 1983).
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