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, 6 (11), 3496-3512
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The Relative Importance of Reproduction and Survival for the Conservation of Two Dolphin Populations

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The Relative Importance of Reproduction and Survival for the Conservation of Two Dolphin Populations

Oliver Manlik et al. Ecol Evol.

Abstract

It has been proposed that in slow-growing vertebrate populations survival generally has a greater influence on population growth than reproduction. Despite many studies cautioning against such generalizations for conservation, wildlife management for slow-growing populations still often focuses on perturbing survival without careful evaluation as to whether those changes are likely or feasible. Here, we evaluate the relative importance of reproduction and survival for the conservation of two bottlenose dolphin (Tursiops cf aduncus) populations: a large, apparently stable population and a smaller one that is forecast to decline. We also assessed the feasibility and effectiveness of wildlife management objectives aimed at boosting either reproduction or survival. Consistent with other analytically based elasticity studies, survival had the greatest effect on population trajectories when altering vital rates by equal proportions. However, the findings of our alternative analytical approaches are in stark contrast to commonly used proportional sensitivity analyses and suggest that reproduction is considerably more important. We show that in the stable population reproductive output is higher, and adult survival is lower;the difference in viability between the two populations is due to the difference in reproduction;reproductive rates are variable, whereas survival rates are relatively constant over time;perturbations on the basis of observed, temporal variation indicate that population dynamics are much more influenced by reproduction than by adult survival;for the apparently declining population, raising reproductive rates would be an effective and feasible tool to reverse the forecast population decline; increasing survival would be ineffective. Our findings highlight the importance of reproduction - even in slow-growing populations - and the need to assess the effect of natural variation in vital rates on population viability. We echo others in cautioning against generalizations based on life-history traits and recommend that population modeling for conservation should also take into account the magnitude of vital rate changes that could be attained under alternative management scenarios.

Keywords: Bottlenose dolphin; PVA; Tursiops; cetacean; population dynamics; population viability; sensitivity analysis; wildlife management.

Figures

Figure 1
Figure 1
Population trajectories of standard models; effect of substituting survival rates, reproductive rates and initial population sizes (N 0) on population trajectories, stochastic growth rates (r), and forecasts for 300‐year probabilities of extinction (PE 300). (A) Effect of application of Bunbury vital rates and N 0 to Shark Bay standard model. (B) Effect of application of Shark Bay vital rates and N 0 to Bunbury standard model. Values for extant population size forecasts were log‐transformed for better visual comparison of models with large differences in N 0. Note that standard errors are too small to be shown on the graph – see Table S8 (Appendix S8) for standard errors for each population size forecast plotted here.
Figure 2
Figure 2
Sensitivity analyses. Panels A, B: relative effect of fixed‐proportion perturbations (standard value ± 1%; 3.14 lethal equivalents ± 1%) of parameters on stochastic growth rate (r). Panels C, D: relative effect of observed, temporal variation in reproductive rates and survival rates (standard value, ±1 SDEV), as well as perturbations of inbreeding levels (0, 3.14, 6.28 lethal equivalents) on stochastic growth rate (r) forecasts. Each box plot shows median, upper, and lower quartile growth rate forecasts of 81 simulations across all (34) combinations. The white, gray, and dark‐shaded boxes show the output from scenarios run with high, medium, and low input values, respectively. Whiskers display minimum and maximum output value.
Figure 3
Figure 3
Approach to assessing the importance of vital rates for the conservation of the two bottlenose dolphin populations. We combined (1) fixed‐proportion and (2) observed‐variation sensitivity analyses with (3) methods of vital rate substitutions between contrasting populations and threat assessment. This approach provided insight into the relative importance of reproduction and survival for the two dolphin populations, and guidance to wildlife management.

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