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Abstract
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<p class="first" id="P2">In species with complex life cycles, population dynamics
result from a combination
of intrinsic cycles arising from delays in the operation of negative density-dependent
processes (e.g., intraspecific competition) and extrinsic fluctuations arising from
seasonal variation in the abiotic environment. Abiotic variation can affect species
directly through their life history traits and indirectly by modulating the species’
interactions with resources or natural enemies.
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<p class="first" id="P3">We investigate how the interplay between density-dependent
dynamics and abiotic variability
affects population dynamics of the bordered plant bug (
<i>Largus californicus</i>), a Hemipteran herbivore inhabiting the California coastal
sage scrub community.
Field data show a striking pattern in abundance: adults are extremely abundant or
nearly absent during certain periods of the year, leading us to predict that seasonal
forcing plays a role in driving observed dynamics.
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<p class="first" id="P4">We develop a stage-structured population model with variable
developmental delays,
in which fecundity is affected by both intra-specific competition and temporal variation
in resource availability and all life history traits (reproduction, development, mortality)
are temperature-dependent. We parameterize the model with experimental data on temperature-responses
of life history and competitive traits and validate the model with independent field
census data.
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<p class="first" id="P5">We find that intra-specific competition is strongest at temperatures
optimal for reproduction,
which theory predicts leads to more complex population dynamics. Our model predicts
that while temperature or resource variability interact with development-induced delays
in self-limitation to generate population fluctuations, it is the interplay between
all three factors that drive the observed dynamics. Considering how multiple abiotic
factors interact with density-dependent processes is important both for understanding
how species persist in variable environments and predicting species’ responses to
perturbations in their typical environment.
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Ecological changes in the phenology and distribution of plants and animals are occurring in all well-studied marine, freshwater, and terrestrial groups. These observed changes are heavily biased in the directions predicted from global warming and have been linked to local or regional climate change through correlations between climate and biological variation, field and laboratory experiments, and physiological research. Range-restricted species, particularly polar and mountaintop species, show severe range contractions and have been the first groups in which entire species have gone extinct due to recent climate change. Tropical coral reefs and amphibians have been most negatively affected. Predator-prey and plant-insect interactions have been disrupted when interacting species have responded differently to warming. Evolutionary adaptations to warmer conditions have occurred in the interiors of species' ranges, and resource use and dispersal have evolved rapidly at expanding range margins. Observed genetic shifts modulate local effects of climate change, but there is little evidence that they will mitigate negative effects at the species level.
For at least 200 years, since the time of Malthus, population growth has been recognized as providing a critical link between the performance of individual organisms and the ecology and evolution of species. We present a theory that shows how the intrinsic rate of exponential population growth, rmax, and the carrying capacity, K, depend on individual metabolic rate and resource supply rate. To do this, we construct equations for the metabolic rates of entire populations by summing over individuals, and then we combine these population-level equations with Malthusian growth. Thus, the theory makes explicit the relationship between rates of resource supply in the environment and rates of production of new biomass and individuals. These individual-level and population-level processes are inextricably linked because metabolism sets both the demand for environmental resources and the resource allocation to survival, growth, and reproduction. We use the theory to make explicit how and why rmax exhibits its characteristic dependence on body size and temperature. Data for aerobic eukaryotes, including algae, protists, insects, zooplankton, fishes, and mammals, support these predicted scalings for rmax. The metabolic flux of energy and materials also dictates that the carrying capacity or equilibrium density of populations should decrease with increasing body size and increasing temperature. Finally, we argue that body mass and body temperature, through their effects on metabolic rate, can explain most of the variation in fecundity and mortality rates. Data for marine fishes in the field support these predictions for instantaneous rates of mortality. This theory links the rates of metabolism and resource use of individuals to life-history attributes and population dynamics for a broad assortment of organisms, from unicellular organisms to mammals.
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