The utility of bioenergetics modelling in quantifying predation rates of marine apex predators: Ecological and fisheries implications
Adam BarnettMatías BracciniChristine L. DudgeonNicholas L. PayneKátya G. AbrantesMarcus SheavesEdward P. Snelling
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Abstract Predators play a crucial role in the structure and function of ecosystems. However, the magnitude of this role is often unclear, particularly for large marine predators, as predation rates are difficult to measure directly. If relevant biotic and abiotic parameters can be obtained, then bioenergetics modelling offers an alternative approach to estimating predation rates, and can provide new insights into ecological processes. We integrate demographic and ecological data for a marine apex predator, the broadnose sevengill shark Notorynchus cepedianus , with energetics data from the literature, to construct a bioenergetics model to quantify predation rates on key fisheries species in Norfolk Bay, Australia. We account for the uncertainty in model parameters by incorporating parameter confidence through Monte Carlo simulations and running alternative variants of the model. Model and parameter variants provide alternative estimates of predation rates. Our simplest model estimates that ca. 1130 ± 137 N . cepedianus individuals consume 11,379 (95% CI: 11,111–11,648) gummy sharks Mustelus antarcticus (~21 tonnes) over a 36-week period in Norfolk Bay, which represents a considerable contribution to total predation mortality on this key fishery species. This study demonstrates how the integration of ecology and fisheries science can provide information for ecosystem and fisheries management.Keywords:
Marine ecosystem
Bioenergetics
Author(s): Selden, Rebecca | Advisor(s): Gaines, Steven D; Warner, Robert R | Abstract: Body size has long been recognized as a key driver of species interactions and of an individual’s role in the ecosystem. Body size determines the amount, species, and sizes of prey resources an individual can consume, as well as its own susceptibility to predators. Human harvest of predators can result in severe truncations in predator body size that can have cascading consequences on food webs. Where small and large individuals of the same species differ greatly in their diets, as is common in aquatic systems, the absence of large predators may functionally eliminate a key predator-prey linkage. Recently management agencies have begun to include size-based metrics as targets. As various harvest strategies differentially affect predator size and biomass, the research presented in this dissertation aims to understand the conditions under which truncations in predator size structure will result in additional loss of predator function than would be predicted from predator biomass alone, and where it will therefore be important to maintain predator size distributions. I specifically examine how the type of ontogenetic shift in diet (e.g. prey species or size class), and the shape of the diet switching function (e.g. gradual or abrupt) will affect the consequences of the loss of the largest predators, and the relative utility of various management strategies in maintaining predator function. In Chapter 1, I examined the tradeoffs between fishery yield and predator function in the ecosystem when preferentially fishing the largest predators. I found that fisheries that delay harvest until large predator sizes maximize fishery yield but that this virtually eliminates predation on focal prey eaten late in life history when diet shifts are abrupt and occur at or after the size at maturity. In this case, there is a clear tradeoff between fisheries and ecosystem objectives. Instead, where shifts in diet toward late prey are more gradual, targeting the largest predators can achieve a win-win by maximizing yield and achieving predation rates similar to that with other strategies that harvest predators earlier. As such, the optimal fishing strategy to achieve both single-species and ecosystem benefits depends strongly on the interaction between the fishery selectivity pattern and the changes in predator diet with size. In Chapter 2, I quantified the size-dependence of the predator-prey interaction between herbivorous sea urchins and one of their important predators in southern California kelp forests, California Sheephead. I further examined the consequences of changes in sheephead size and abundance in marine reserves at Catalina Island on size-specific urchin mortality in field predation trials. In my observations of predation of sheephead on urchins, sheephead smaller than 20cm TL do not eat urchins of any size. Thereafter, small sheephead only consumed small urchins, with larger sheephead sizes needed to successfully consume larger urchins, and the largest sheephead preferentially targeted the largest urchins. Inside marine reserves at Catalina, the greater abundance of large sheephead in combination with the observed size-specific capacities for urchin predation led to higher urchin mortality with marine reserve protection, particularly for the largest urchins. Ultimately, by restoring predator size structure, reserves may serve to enhance the resilience of southern California kelp forests. In Chapter 3, I examined how variation in predator body size distributions and biomass affects the likelihood of size escapes in situations where predators begin eating prey at some threshold size and thereafter consume increasingly larger prey. We focus on California sheephead because of the size dependence of its interaction with herbivorous sea urchins (Chapter 2), and the natural variation in demography where sheephead achieve smaller maximum sizes but higher biomass in the south of its range. We evaluate the consequences of smaller predator body size on top-down control of urchin populations in two scenarios: 1) when overall predator abundance is the same as the population with larger body size, and 2) when predator biomass is the same. With the same numbers of predators, top-down control was significantly weakened by the lack of large sheephead. However, when sheephead biomass was maintained, the absence of large sheephead did not lead to greater urchin abundance, despite lower predation rates overall and much lower predation on large urchins. Higher predation rates on the smallest urchin size classes served as a bottleneck that kept total urchin population at similar levels and prevented a size escape for the largest urchins. This suggests that where predators switch prey size classes in the same species, the loss of the largest individuals does not inherently result in weaker top-down control, if biomass is maintained, but effective control is sensitive to prey growth rates. The results of this research suggest that the ignoring shifts in predator size structure can under-estimate the effects of fishing on predator function, especially when large predators eat different species than their smaller counterparts. High predator biomass can compensate when diet shifts are to different prey size classes of the same species. Concordance between diet shifts and fishery selectivity can help identify where it will be important to consider changes in predator size in addition to biomass.
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The loss of a predator from an ecological community can cause large changes in community structure and ecosystem processes, or have very little consequence for the remaining species and ecosystem. Understanding when and why the loss of a predator causes large changes in community structure and ecosystem processes is critical for understanding the functional consequences of biodiversity loss. We used experimental microbial communities to investigate how the removal of a large generalist predator affected the extinction frequency, population abundance and total biomass of its prey. We removed this predator in the presence or absence of an alternative, more specialist, predator in order to determine whether the specialist predator affected the outcome of the initial species removal. Removal of the large generalist predator altered some species’ populations but many were unaffected and no secondary extinctions were observed. The specialist predator, though rare, altered the response of the prey community to the removal of the large generalist predator. In the absence of the specialist predator, the effects of the removal were only measurable at the level of individual species. However, when the specialist predator was present, the removal of the large generalist predator affected the total biomass of prey species. The results demonstrate that the effect of species loss from high trophic levels may be very context‐dependent, as rare species can have disproportionately large effects in food webs.
Microcosm
Trophic cascade
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Maximum food–chain length has been correlated with resource availability, ecosystem size, environmental stability and colonization history. Some of these correlations may result from environmental effects on predator–prey body size ratios. We investigate relationships between maximum food–chain length, predator–prey mass ratios, primary production and environmental stability in marine food webs with a natural history of community assembly. Our analyses provide empirical evidence that smaller mean predator–prey body size ratios are characteristic of more stable environments and that food chains are longer when mean predator–prey body size ratios are small. We conclude that environmental effects on predator–prey body size ratios contribute to observed differences in maximum food–chain length.
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This paper studies a nonautonomous predator-prey system with stagestructure for predator.it shows this system is persistence.By means of Liapunov function,a sufficient conditions of global stability is obtained.
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Large carnivores' fear of the human ‘super predator’ has the potential to alter their feeding behaviour and result in human-induced trophic cascades. However, it has yet to be experimentally tested if large carnivores perceive humans as predators and react strongly enough to have cascading effects on their prey. We conducted a predator playback experiment exposing pumas to predator (human) and non-predator control (frog) sounds at puma feeding sites to measure immediate fear responses to humans and the subsequent impacts on feeding. We found that pumas fled more frequently, took longer to return, and reduced their overall feeding time by more than half in response to hearing the human ‘super predator’. Combined with our previous work showing higher kill rates of deer in more urbanized landscapes, this study reveals that fear is the mechanism driving an ecological cascade from humans to increased puma predation on deer. By demonstrating that the fear of humans can cause a strong reduction in feeding by pumas, our results support that non-consumptive forms of human disturbance may alter the ecological role of large carnivores.
Trophic cascade
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Рассматривается модель, описывающая пространственно-временную динамику сообщества, состоящего из трех популяций, представляющих звенья трофической цепи. Локальные взаимодействия популяций строятся по типу «хищник – жертва», причем хищник потребляет не только жертву, но и ресурс, составляющий рацион жертвы. В предыдущей работе автором был проведен анализ модели без учета пространственной неоднородности. Данное исследование продолжает модельное изучение сообщества, учитывая диффузию особей, а также направленные перемещения хищника. Предполагается, что хищник реагирует на пространственное изменение ресурса и жертвы, занимая области с более высокой плотностью или избегая их. В модели такое поведение описывается адвективным членом со скоростью, пропорциональной градиенту плотности ресурса и жертвы. Система рассматривается в одномерной области в предположении нулевых потоков через границу. Динамика модели определяется устойчивостью системы в окрестности пространственно-однородного равновесия к малым пространственно-неоднородным возмущениям. В работе проведен анализ возможности возникновения в системе волновой неустойчивости, приводящей к возникновению автоволн и неустойчивости Тьюринга, в результате которой образуются стационарные структуры. Получены достаточные условия существования обоих видов неустойчивости, определяющие границы области значений коэффициентов таксиса, при которых система может потерять устойчивость. Анализ влияния параметров локальной кинетики модели на возможность образования пространственных структур показал, что при положительном таксисе на ресурс возможна лишь неустойчивость Тьюринга, а при отрицательном — оба вида неустойчивости. Для поиска численного решения системы использован метод линий с расщеплением разностного оператора по физическим процессам. Пространственно-временная динамика системы представлена в нескольких вариантах, реализующих один из типов неустойчивости. В случае положительного таксиса на жертву в областях меньшего размера возможно как реализация автоволнового режима, так и образование стационарных структур; с увеличением области тьюринговы структуры не образуются. Если же таксис на жертву отрицательный, то стационарные структуры возникают в областях любого размера, периодические структуры появляются только в более крупных областях.
Omnivore
Taxis
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Mixotroph
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Abstract Historical resurveys of ecological communities are important for placing the structure of modern ecosystems in context. Rarely, however, are snapshot surveys alone sufficient for providing direct insight into the rates of the ecological processes underlying community functioning, either now or in the past. In this study, I used a statistically reasoned observational approach to estimate the feeding rates of a New Zealand intertidal predator, Haustrum haustorium , using diet surveys performed at several sites by Robert Paine in 1968–1969 and by me in 2004. Comparisons between time periods reveal a remarkable consistency in the predator's prey‐specific feeding rates, which contrasts with the changes I observed in prey abundances, the predator's body‐size distribution, and the prey's proportional contributions to the predator's apparent diet. Although these and additional changes in the predator's per‐capita attack rates seem to show adaptive changes in its prey preferences, they do not. Rather, feeding‐rate stability is an inherently statistical consequence of the predator's high among‐prey variation in handling times which determine the length of time that feeding events will remain detectable to observers performing diet surveys. Though understudied, similarly high among‐prey variation in handling (or digestion) times is evident in many predator species throughout the animal kingdom. The resultant disconnect between a predator's apparent diet and its actual feeding rates suggests that much of the temporal, biogeographic, and seemingly context‐dependent variation that is often perceived in community structure, predator diets, and food‐web topology may be of less functional consequence than assumed. Qualitative changes in ecological pattern need not represent qualitative changes in ecological process.
Community
Predator avoidance
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