Dangerous prey and daring predators: a review Mukherjee, Shomen; Heithaus, Michael R.
Biological reviews of the Cambridge Philosophical Society,
August 2013, Letnik:
88, Številka:
3
Journal Article
Recenzirano
ABSTRACT
How foragers balance risks during foraging is a central focus of optimal foraging studies. While diverse theoretical and empirical work has revealed how foragers should and do manage food ...and safety from predators, little attention has been given to the risks posed by dangerous prey. This is a potentially important oversight because risk of injury can give rise to foraging costs similar to those arising from the risk of predation, and with similar consequences. Here, we synthesize the literature on how foragers manage risks associated with dangerous prey and adapt previous theory to make the first steps towards a framework for future studies. Though rarely documented, it appears that in some systems predators are frequently injured while hunting and risk of injury can be an important foraging cost. Fitness costs of foraging injuries, which can be fatal, likely vary widely but have rarely been studied and should be the subject of future research. Like other types of risk‐taking behaviour, it appears that there is individual variation in the willingness to take risks, which can be driven by social factors, experience and foraging abilities, or differences in body condition. Because of ongoing modifications to natural communities, including changes in prey availability and relative abundance as well as the introduction of potentially dangerous prey to numerous ecosystems, understanding the prevalence and consequences of hunting dangerous prey should be a priority for behavioural ecologists.
Ontogenetic niche shifts are common among animals, yet most studies only investigate niche shifts at the population level, which may overlook considerable differences among individuals in the timing ...and dynamics of these shifts. Such divergent behaviors within size-/age-classes have important implications for the roles a population—and specific age-classes—play in their respective ecosystem(s). Using acoustic telemetry, we tracked the movements of juvenile bull sharks in the Shark River Estuary of Everglades National Park, Florida, and found that sharks increased their use of marine microhabitats with age to take advantage of more abundant resources, but continued to use freshwater and estuarine microhabitats as refuges from marine predators. Within this population-level ontogenetic niche shift, however, movement patterns varied among individual sharks, with 47 % of sharks exhibiting condition-dependent habitat use and 53 % appearing riskaverse regardless of body condition. Among sharks older than age 0, fifty percent made regular movements between adjacent regions of the estuary, while the other half made less predictable movements that often featured long-term residence in specific regions. Individual differences were apparently shaped by both intrinsic and extrinsic factors, including individual responses to food-risk trade-offs and body condition. These differences appear to develop early in the lives of bull sharks, and persist throughout their residencies in nursery habitats. The widespread occurrence of intraspecific variation in behavior among mobile taxa suggests it is important in shaping population dynamics of at least some species, and elucidating the contexts and timing in which it develops and persists is important for understanding its role within communities.
Ecology Letters (2010) 13: 1055–1071
Whereas many land predators disappeared before their ecological roles were studied, the decline of marine apex predators is still unfolding. Large sharks in ...particular have experienced rapid declines over the last decades. In this study, we review the documented changes in exploited elasmobranch communities in coastal, demersal, and pelagic habitats, and synthesize the effects of sharks on their prey and wider communities. We show that the high natural diversity and abundance of sharks is vulnerable to even light fishing pressure. The decline of large predatory sharks reduces natural mortality in a range of prey, contributing to changes in abundance, distribution, and behaviour of small elasmobranchs, marine mammals, and sea turtles that have few other predators. Through direct predation and behavioural modifications, top‐down effects of sharks have led to cascading changes in some coastal ecosystems. In demersal and pelagic communities, there is increasing evidence of mesopredator release, but cascading effects are more hypothetical. Here, fishing pressure on mesopredators may mask or even reverse some ecosystem effects. In conclusion, large sharks can exert strong top‐down forces with the potential to shape marine communities over large spatial and temporal scales. Yet more empirical evidence is needed to test the generality of these effects throughout the ocean.
Rays (superorder Batoidea) are the most diverse group of elasmobranchs, and many are threatened with extinction. However, there remain areas where research on the ecology of this group is lacking, ...from trophic interactions to their importance to ecosystem structure and function. Such ecological insights are critical for predicting the potential consequences of changes in their population sizes. Our aim was to synthesize the existing ray ecology literature and identify key knowledge gaps in order to provide a framework for future research. Numerous studies describe ray diets, and the number of studies using biochemical methods to address ray trophic interactions is increasing. The implications of ray predator–prey interactions on population dynamics of prey and how ray foraging might influence ecosystem dynamics through bioturbation remain relatively unexplored, despite claims that rays are ecologically important because they are bioturbators and because of their potential to deplete stocks of commercially important bivalves. Therefore, to better integrate rays in our understanding of marine community dynamics, there is a need to: (i) combine behavioural data with dietary information to describe predator–prey interactions; (ii) understand how ray bioturbation affects biogeochemical cycles and infaunal communities; (iii) elucidate conditions under which rays might initiate or transmit trophic cascades through consumptive and non‐consumptive pathways; and (iv) consider anthropogenic influences on the ecological roles and importance of rays.
1. Apex predators are often assumed to be dietary generalists and, by feeding on prey from multiple basal nutrient sources, serve to couple discrete food webs. But there is increasing evidence that ...individual level dietary specialization may be common in many species, and this has not been investigated for many marine apex predators. 2. Because of their position at or near the top of many marine food webs, and the possibility that they can affect populations of their prey and induce trophic cascades, it is important to understand patterns of dietary specialization in shark populations. 3. Stable isotope values from body tissues with different turnover rates were used to quantify patterns of individual specialization in two species of ‘generalist' sharks (bull sharks, Carcharhinus leucas, and tiger sharks, Galeocerdo cuvier). 4. Despite wide population-level isotopic niche breadths in both species, isotopic values of individual tiger sharks varied across tissues with different turnover rates. The population niche breadth was explained mostly by variation within individuals suggesting tiger sharks are true generalists. In contrast, isotope values of individual bull sharks were stable through time and their wide population level niche breadth was explained by variation among specialist individuals. 5. Relative resource abundance and spatial variation in food-predation risk tradeoffs may explain the differences in patterns of specialization between shark species. 6. The differences in individual dietary specialization between tiger sharks and bull sharks results in different functional roles in coupling or compartmentalizing distinct food webs. 7. Individual specialization may be an important feature of trophic dynamics of highly mobile marine top predators and should be explicitly considered in studies of marine food webs and the ecological role of top predators.
Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how ...predators influence communities by inflicting mortality on prey and inducing behavioral modifications (risk effects). Both mechanisms are important in marine communities, and a sole focus on the effects of predator-inflicted mortality might severely underestimate the importance of predators. We outline direct and indirect consequences of marine predator declines and propose an integrated predictive framework that includes risk effects, which appear to be strongest for long-lived prey species and when resources are abundant. We conclude that marine predators should be managed for the maintenance of both density- and risk-driven ecological processes, and not demographic persistence alone.
Extreme climatic events can trigger abrupt and often lasting change in ecosystems via the reduction or elimination of foundation (i.e., habitat‐forming) species. However, while the ...frequency/intensity of extreme events is predicted to increase under climate change, the impact of these events on many foundation species and the ecosystems they support remains poorly understood. Here, we use the iconic seagrass meadows of Shark Bay, Western Australia – a relatively pristine subtropical embayment whose dominant, canopy‐forming seagrass, Amphibolis antarctica, is a temperate species growing near its low‐latitude range limit – as a model system to investigate the impacts of extreme temperatures on ecosystems supported by thermally sensitive foundation species in a changing climate. Following an unprecedented marine heat wave in late summer 2010/11, A. antarctica experienced catastrophic (>90%) dieback in several regions of Shark Bay. Animal‐borne video footage taken from the perspective of resident, seagrass‐associated megafauna (sea turtles) revealed severe habitat degradation after the event compared with a decade earlier. This reduction in habitat quality corresponded with a decline in the health status of largely herbivorous green turtles (Chelonia mydas) in the 2 years following the heat wave, providing evidence of long‐term, community‐level impacts of the event. Based on these findings, and similar examples from diverse ecosystems, we argue that a generalized framework for assessing the vulnerability of ecosystems to abrupt change associated with the loss of foundation species is needed to accurately predict ecosystem trajectories in a changing climate. This includes seagrass meadows, which have received relatively little attention in this context. Novel research and monitoring methods, such as the analysis of habitat and environmental data from animal‐borne video and data‐logging systems, can make an important contribution to this framework.
Unmanned aerial vehicles (UAVs) are being increasingly used in studies of marine fauna. Here, we tested the use of a UAV (DJI Phantom II®) to assess fine-scale variation in densities of 2 ...elasmobranchs (blacktip reef sharks Carcharhinus melanopterus and pink whiprays Himantura fai) on reef systems off Moorea (French Polynesia). We flew parallel transects designed to sample reef habitats (fringing, channel and sandflat habitats) across 2 survey blocks. Block 1 included a shark and ray provisioning site with potentially higher elasmobranch densities, whereas Block 2 most likely had lower densities with no provisioning activities. Across 10 survey days in July 2014, we flew 3 transects (400 m) within each survey block (n = 60 total transect passes). As expected, densities (animals ha−1) were significantly higher in Block 1 than in Block 2, particularly where provisioning activities occur. Differences between habitats surveyed were also found. Our study provides the first direct estimates of shark and ray densities in coral-reef ecosystems and demonstrates that UAVs can produce important fishery-independent data for elasmobranchs, particularly in shallow-water habitats.
Habitat selection decisions by consumers has the potential to shape ecosystems. Understanding the factors that influence habitat selection is therefore critical to understanding ecosystem function. ...This is especially true of mesoconsumers because they provide the link between upper and lower tropic levels. We examined the factors influencing microhabitat selection of marine mesoconsumers - juvenile giant shovelnose rays (Glaucostegus typus), reticulate whiprays (Himantura uarnak), and pink whiprays (H. fai) - in a coastal ecosystem with intact predator and prey populations and marked spatial and temporal thermal heterogeneity. Using a combination of belt transects and data on water temperature, tidal height, prey abundance, predator abundance and ray behavior, we found that giant shovelnose rays and reticulate whiprays were most often found resting in nearshore microhabitats, especially at low tidal heights during the warm season. Microhabitat selection did not match predictions derived from distributions of prey. Although at a course scale, ray distributions appeared to match predictions of behavioral thermoregulation theory, fine-scale examination revealed a mismatch. The selection of the shallow nearshore microhabitat at low tidal heights during periods of high predator abundance (warm season) suggests that this microhabitat may serve as a refuge, although it may come with metabolic costs due to higher temperatures. The results of this study highlight the importance of predators in the habitat selection decisions of mesoconsumers and that within thermal gradients, factors, such as predation risk, must be considered in addition to behavioral thermoregulation to explain habitat selection decisions. Furthermore, increasing water temperatures predicted by climate change may result in complex trade-offs that might have important implications for ecosystem dynamics.
1. Understanding how natural and anthropogenic drivers affect extant food webs is critical to predicting the impacts of climate change and habitat alterations on ecosystem dynamics. 2. In the Florida ...Everglades, seasonal reductions in freshwater flow and precipitation lead to annual migrations of aquatic taxa from marsh habitats to deep-water refugia in estuaries. The timing and intensity of freshwater reductions, however, will be modified by ongoing ecosystem restoration and predicted climate change. 3. Understanding the importance of seasonally pulsed resources to predators is critical to predicting the impacts of management and climate change on their populations. As with many large predators, however, it is difficult to determine to what extent predators like bull sharks (Carcharhinus leucas) in the coastal Everglades make use of prey pulses currently. 4. We used passive acoustic telemetry to determine whether shark movements responded to the pulse of marsh prey. To investigate the possibility that sharks fed on marsh prey, we modelled the predicted dynamics of stable isotope values in bull shark blood and plasma under different assumptions of temporal variability in shark diets and physiological dynamics of tissue turnover and isotopic discrimination. 5. Bull sharks increased their use of upstream channels during the late dry season, and although our previous work shows long-term specialization in the diets of sharks, stable isotope values suggested that some individuals adjusted their diets to take advantage of prey entering the system from the marsh, and as such this may be an important resource for the nursery. 6. Restoration efforts are predicted to increase hydroperiods and marsh water levels, likely shifting the timing, duration and intensity of prey pulses, which could have negative consequences for the bull shark population and/or induce shifts in behaviour. 7. Understanding the factors influencing the propensity to specialize or adopt more flexible trophic interactions will be an important step in fully understanding the ecological role of predators and how ecological roles may vary with environmental and anthropogenic changes.
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