Abstract
BackgroundLong-lived marine megavertebrates (e.g. sharks, turtles, mammals, and seabirds) are inherently vulnerable to anthropogenic mortality. Although some mathematical models have been applied successfully to manage these animals, more detailed treatments are often needed to assess potential drivers of population dynamics. In particular, factors such as age-structure, density-dependent feedbacks on reproduction, and demographic stochasticity are important for understanding population trends, but are often difficult to assess. Lemon sharks (Negaprion brevirostris) have a pelagic adult phase that makes them logistically difficult to study. However, juveniles use coastal nursery areas where their densities can be high.ResultsWe use a stage-structured, Markov-chain stochastic model to describe lemon shark population dynamics from a 17-year longitudinal dataset at a coastal nursery area at Bimini, Bahamas. We found that the interaction between delayed breeding, density-dependence, and demographic stochasticity accounts for 33 to 49% of the variance in population size.ConclusionsDemographic stochasticity contributed all random effects in this model, suggesting that the existence of unmodeled environmental factors may be driving the majority of interannual population fluctuations. In addition, we are able to use our model to estimate the natural mortality rate of older age classes of lemon sharks that are difficult to study. Further, we use our model to examine what effect the length of a time series plays on deciphering ecological patterns. We find that—even with a relatively long time series—our sampling still misses important rare events. Our approach can be used more broadly to infer population dynamics of other large vertebrates in which age structure and demographic stochasticity are important.ReviewersThis article was reviewed by Yang Kuang, Christine Jacob, and Ollivier Hyrien.
Highlights
Long-lived marine megavertebrates are inherently vulnerable to anthropogenic mortality
For combinations that fit the data based on our criteria, we found that, while demographic stochasticity alone can correctly predict the mean population size in a sizable region of parameter space, at best it can account for only 33% to 49% of the observed variance—nowhere in parameter space was the observed variance in the inner quartile range of the sampling distribution of the variance
Variance in Juvenile Population Size insight about expected behavior of the population, under a suitable definition of “expected”, that insight is limited because such models provide no measure of the fluctuations about this ensemble average one can expect to see in any real instance [19]. We addressed this shortcoming by developing a model of the lemon shark population at Bimini incorporating both demographic stochasticity and age structure
Summary
Long-lived marine megavertebrates (e.g. sharks, turtles, mammals, and seabirds) are inherently vulnerable to anthropogenic mortality. Many large marine megavertebrates (e.g. sharks, turtles, mammals, seabirds) are vulnerable to anthropogenic mortality due to their complex life history characteristics, including long lifespans, delayed maturity, low fecundity, and extended migrations [1,2,3,4] These animals often act as ecological keystones, and their removal can lead to considerable ecosystem changes such as Physiologically structured population models [13,14] that incorporate delayed breeding [15,16], densitydependent mechanisms [13,17], demographic stochasticity [18,19,20,21,22], or some combination of these processes, have been applied to many ecological systems to answer questions related to population dynamics, conservation, and management. Fecundity and early juvenile mortality rates have been estimated precisely using mark-recapture and genetic methods [33,34,35]
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