A comparison of the seasonal movements of tiger sharks and green turtles provides insight into their predator-prey relationship

During the reproductive season, sea turtles use a restricted area in the vicinity of their nesting beaches, making them vulnerable to predation. At Raine Island (Australia), the highest density green turtle Chelonia mydas rookery in the world, tiger sharks Galeocerdo cuvier have been observed to feed on green turtles, and it has been suggested that they may specialise on such air-breathing prey. However there is little information with which to examine this hypothesis. We compared the spatial and temporal components of movement behaviour of these two potentially interacting species in order to provide insight into the predator-prey relationship. Specifically, we tested the hypothesis that tiger shark movements are more concentrated at Raine Island during the green turtle nesting season than outside the turtle nesting season when turtles are not concentrated at Raine Island. Turtles showed area-restricted search behaviour around Raine Island for ~3–4 months during the nesting period (November–February). This was followed by direct movement (transit) to putative foraging grounds mostly in the Torres Straight where they switched to area-restricted search mode again, and remained resident for the remainder of the deployment (53–304 days). In contrast, tiger sharks displayed high spatial and temporal variation in movement behaviour which was not closely linked to the movement behaviour of green turtles or recognised turtle foraging grounds. On average, tiger sharks were concentrated around Raine Island throughout the year. While information on diet is required to determine whether tiger sharks are turtle specialists our results support the hypothesis that they target this predictable and plentiful prey during turtle nesting season, but they might not focus on this less predictable food source outside the nesting season.

Incorporating anthropogenic effects into trophic ecology: predator-prey interactions in a human-dominated landscape

Apex predators perform important functions that regulate ecosystems worldwide. However, little is known about how ecosystem regulation by predators is influenced by human activities. In particular, how important are top-down effects of predators relative to direct and indirect human-mediated bottom-up and top-down processes? Combining data on species' occurrence from camera traps and hunting records, we aimed to quantify the relative effects of top-down and bottom-up processes in shaping predator and prey distributions in a human-dominated landscape in Transylvania, Romania. By global standards this system is diverse, including apex predators (brown bear and wolf), mesopredators (red fox) and large herbivores (roe and red deer). Humans and free-ranging dogs represent additional predators in the system. Using structural equation modelling, we found that apex predators suppress lower trophic levels, especially herbivores. However, direct and indirect top-down effects of humans affected the ecosystem more strongly, influencing species at all trophic levels. Our study highlights the need to explicitly embed humans and their influences within trophic cascade theory. This will greatly expand our understanding of species interactions in human-modified landscapes, which compose the majority of the Earth's terrestrial surface.

Not so fast: swimming behavior of sailfish during predator-prey interactions using high-speed video and accelerometry

Billfishes are considered among the fastest swimmers in the oceans. Despite early estimates of extremely high speeds, more recent work showed that these predators (e.g., blue marlin) spend most of their time swimming slowly, rarely exceeding 2 m s(-1). Predator-prey interactions provide a context within which one may expect maximal speeds both by predators and prey. Beyond speed, however, an important component determining the outcome of predator-prey encounters is unsteady swimming (i.e., turning and accelerating). Although large predators are faster than their small prey, the latter show higher performance in unsteady swimming. To contrast the evading behaviors of their highly maneuverable prey, sailfish and other large aquatic predators possess morphological adaptations, such as elongated bills, which can be moved more rapidly than the whole body itself, facilitating capture of the prey. Therefore, it is an open question whether such supposedly very fast swimmers do use high-speed bursts when feeding on evasive prey, in addition to using their bill for slashing prey. Here, we measured the swimming behavior of sailfish by using high-frequency accelerometry and high-speed video observations during predator-prey interactions. These measurements allowed analyses of tail beat frequencies to estimate swimming speeds. Our results suggest that sailfish burst at speeds of about 7 m s(-1) and do not exceed swimming speeds of 10 m s(-1) during predator-prey interactions. These speeds are much lower than previous estimates. In addition, the oscillations of the bill during swimming with, and without, extension of the dorsal fin (i.e., the sail) were measured. We suggest that extension of the dorsal fin may allow sailfish to improve the control of the bill and minimize its yaw, hence preventing disturbance of the prey. Therefore, sailfish, like other large predators, may rely mainly on accuracy of movement and the use of the extensions of their bodies, rather than resorting to top speeds when hunting evasive prey.

Floods and famine: climate-induced collapse of a tropical predator-prey community

Will climate change threaten wildlife populations by gradual shifts in mean conditions, or by increased frequency of extreme weather events? Based on long-term data (from 1991 to 2014), the aim of the present study was to analyse and compare the sensitivity of predator-prey demography to extreme climatic events versus normal, albeit highly variable, annual deviations in climatic conditions in the Australian wet-dry tropics. From 1991 to 2005, predators (water pythons, Liasis fuscus) and their main prey (dusky rats, Rattus colletti) showed significant climate-driven fluctuations in numbers. These fluctuations were, however, trivial compared to the impact of two massive but brief deluges in 2007 and 2011, which virtually eliminated the dusky rats. The two floods resulted in the pythons experiencing an unprecedented famine in seven out of the last 8 years causing a massive shift in python demography, that is a significant reduction in feeding rates, reproductive output, growth rates, relative body mass, survival, mean body length and numbers (from 3173 in 1992 to 96 in 2013). Our results demonstrate that attempts to predict faunal responses to climate change, even if based on long-term studies, may be doomed to failure. Consequently, biologists may need to confront the uncomfortable truth that increased frequency of brief unpredictable bouts of extreme weather can influence populations far more than gradual deviations in mean climatic conditions.

Crying wolf: limitations of predator-prey studies need not preclude their salient messages.

A rapidly growing body of the literature reveals the important roles apex predators play in shaping the composition and functioning of ecological communities worldwide. The principal effects of apex predators—namely herbivore and mesopredator population suppression—are often evident following their removal from environments, or their reintroduction, including rewilding initiatives. What remains less clear, however, is to what extent humans versus other apex predators affect ecosystems, how both interact across gradients of anthropogenic pressure and how such interactions can be affected by underlying bottom-up processes. Such questions are critical to answer in the Anthropocene, where effective management of ecosystems and conservation of biodiversity requires a better understanding of how top-down and bottom-up processes vary according to anthropogenic influences.

Ratio-dependent response of a temperate Australian estuarine system to sustained nitrogen loading

Classical resource- and the less studied ratio-dependent models of predator–prey relationships provide divergent predictions as to the sustained ecological effects of bottom-up forcing. While resource-dependent models, which consider only instantaneous prey density in modelling predator responses, predict community responses that are dependent on the number of trophic levels in a system, ratio-dependent models, which consider the number of prey per consumer, predict proportional increase in each level irrespective of chain length. The two models are only subtly different for systems with two or three trophic levels but in the case of four trophic levels, predict opposite effects of enrichment on primary producers. Despite the poor discriminatory power of tests of the models in systems with two or three trophic levels, field tests in estuarine and marine systems with four trophic levels have been notably absent. Sampling of phytoplankton, macroinvertebrates, invertebrate-feeding fishes, piscivorous fishes in Kooloonbung Creek, Hastings River estuary, eastern Australia, subject to over 20 years of sewage discharge, revealed increased abundances in all four trophic levels at the disturbed location relative to control sites. Increased abundance of phytoplankton at the disturbed site was counter to the predictions of resource-dependent models, which posit a reduction in the first trophic level in response to enrichment. By contrast, the increase in abundance of this first trophic level and the proportionality of increases in abundances of each of the four trophic groups to nitrogen loading provided strong support for ratio dependency. This first evidence of ratio dependence in an estuarine system with four trophic levels not only demonstrates the applicability of ecological theory which seeks to simplify the complexity of systems, but has implications for management. Although large nutrient inputs frequently induce mortality of invertebrates and fish, we have shown that smaller inputs may in fact enhance biomass of all trophic levels.

Population consequences of a predator-induced habitat shift by trout in whole-lake experiments

In a replicated whole-lake experiment, we (a) tested for the existence of a flexible habitat shift in response to predator presence in age-0 rainbow trout (Oncorhynchus mykiss) at risk of cannibalism and (b) evaluated the population-level consequences of habitat shifts in terms of growth and survival over their first growing season. Daphnid food and adult trout predators were substantially more abundant in pelagic than in littoral habitats. Age-0 trout used all habitats in populations without adult trout predators, whereas age-0 trout were observed only in the less profitable littoral habitat in populations with adult trout. Consequently, mean fall mass of age-0 trout in the presence of predators was almost half that observed in populations without adult trout. Despite the shift in habitat use, age-0 trout experienced 90% mortality when adult trout predators were present, in comparison to only 36% mortality when absent. We conclude that the commonly observed habitat shifts by fish at risk of predation, observed at smaller scales, do in fact occur at the whole-system scale over long time intervals. These results suggest that fish are able to perceive risk at large spatial scales and thus take advantage of profitable (but normally risky) habitats when predators are absent, or move to less profitable refuge habitats when predators are present.

How well do predators adjust to climate-mediated shifts in prey distribution? A study on Australian water pythons

Climate change can move the spatial location of resources critical for population viability, and a species' resilience to such changes will depend upon its ability to flexibly shift its activities away from no-longer-suitable sites to exploit new opportunities. Intuition suggests that vagile predators should be able to track spatial shifts in prey availability, but our data on water pythons (Liasis fuscus) in tropical Australia suggest a less encouraging scenario. These pythons undergo regular long-range (to >10 km) seasonal migrations to follow flooding-induced migrations by their prey (native dusky rats, Rattus colletti). However, when an extreme flooding event virtually eliminated rats for a three-year period, the local pythons did not disperse despite the presence of abundant rats only 8 km away; instead, many pythons starved to death. This inflexibility suggests that some vagile species that track seasonally migrating prey may do so by responding to habitat attributes that have consistently predicted prey availability over evolutionary time, rather than reacting to proximate cues that signal the presence of prey per se. A species' vulnerability to climate change will be increased by an inability to shift its activities away from historical sites toward newly favorable areas.

Preference for feeding at habitat edges declines among juvenile blue crabs as oyster reef patchiness increases and predation risk grows

Both habitat patchiness and behaviorally-mediated indirect effects (BMIEs; predator- induced changes in prey behavior that affect the prey's resources) are important in many food webs, but the relationships between these 2 factors have yet to be investigated. To explore effects of habitat patchiness and variation in perceived risk of predation on food-web dynamics, we conducted a factorial experiment in a model aquatic food chain of predator-prey-resource using 2 contrasting predators (adult blue crab Callinectes sapidus and toad fish Opsanus tau), juvenile blue crab as prey, and mussel Geukensia demissa as resource. Both predator presence and habitat patchiness influenced the prey's preference for consuming resources at patch edges instead of interiors. The preference of prey for consuming resources at habitat edges was 4 times stronger in continuous oyster reef habitat than in smaller habitat patches. This suggests that interior resources in continuous habitat experience a refuge from consumption, but this refuge is largely lost in patchy habitat. The mere presence of predators reduced the prey's preference for consuming resources at habitat edges. This BMIE was significant for the ambush predator (toadfish) and the treatment containing both predators, but not for the actively hunting predator (adult blue crab). We conclude that habitat patchiness and predator presence can jointly affect resource distribution by inducing shifts in prey foraging behavior, revealing a need to incorporate BMIEs into habitat fragmentation studies. This conclusion has broad and growing relevance as anthropogenic factors increasingly modify predator abundances and fragment coastal habitats. © Inter-Research 2012.

Introgression of domesticated alleles into a wild trout genotype and the impact on seasonal survival in natural lakes

We tested the fitness consequences of introgression of fast-growing domesticated fish into a wild population. Fry from wild and domesticated rainbow trout (Oncorhynchus mykiss) crosses, F1 hybrids, and first- and second-generation backcrosses were released into two natural lakes. Parentage analysis using microsatellite loci facilitated the identification of survivors, so fitness was estimated in nature from the first-feeding stage. Results indicated that under certain conditions, domesticated fish survived at least as well as wild fish within the same environment. Relative growth and survival of the crosses, however, were highly dependent on environment. During the first summer, fastest-growing crosses had the highest survival, but this trend was reversed after one winter and another summer. Although the F1 hybrids showed evidence of outbreeding depression because of the disruption of local adaptation, there was little evidence of outbreeding depression in the backcrosses, and the second-generation backcrosses exhibited a wild-type phenotype. This information is relevant for assessing the multigenerational risk of escaped or released domesticated fish should they successfully interbreed with wild populations and provides information on how to minimize detrimental impacts of a conservation breeding and/or management programme. These data also further understanding of the selection pressures in nature that maintain submaximal rates of growth.

Asymmetric impact of piscivorous birds on size-structured fish populations

Fish are frequently considered the top predator in freshwater food web models despite evidence that predatory birds can impact fish populations. In this study, we quantified bird predation rates on experimental populations of rainbow trout (Oncorhynchus mykiss (Walbaum, 1792)) created by stocking nine small lakes in British Columbia, Canada. Combining estimates of fish mortality with estimated bird predation rates allowed us to partition fish mortality into that due to birds versus cannibalism. Our results indicated that bird predators had significant impacts on age-1 trout populations, but little impact on age-0 trout. Common loons (Gavia immer Brunnich, 1764) were the principle predator among eight predatory bird species present, apparently consuming nearly 50% of all stocked age-1 trout and explaining almost 50% of variation in mortality rates. Age-1 trout mortality did not differ significantly from zero in lakes without loons. Birds consumed a small proportion of age-0 trout, and estimated consumption explained none of the variation in age-0 trout mortality among lakes. We conclude that birds affect fish populations by asymmetric predation on different age (size) classes and can be important top predators that should not be ignored when characterizing freshwater food webs in lakes.

Trophic cul-de-sac, Pyrazus ebeninus, limits trophic transfer through an estuarine detritus-based food web

The importance to food-webs of trophic cul-de-sacs, species that channel energy flow away from higher trophic levels, is seldom considered outside of the pelagic systems in which they were first identified. On intertidal mudflats, inputs of detritus from saltmarshes, macroalgae or microphytobenthos are generally regarded as a major structuring force underpinning food-webs and there has been no consideration of trophic cul-de-sacs to date. A fully orthogonal three-factor experiment manipulating the density of the abundant gastropod, Pyrazus ebeninus, detritus and macrobenthic predators on a Sydney mudflat revealed large deleterious effects of the gastropod, irrespective of detrital loading or the presence of predators. Two months after experimental manipulation, the standing-stock of microphytobenthos in plots with high (44 per m²) densities of P. ebeninus was 20% less than in plots with low (4 per m²) densities. Increasing densities of P. ebeninus from low to high halved the abundance of macroinvertebrates and the average number of species. In contrast, the addition of detritus had differing effects on microphytobenthos (positively affected) and macroinvertebrates (negatively affected). Over the two-months of our experiment, no predatory mortality of P. ebeninus was observed and high densities of P. ebeninus decreased impacts of predators on macroinvertebrate abundances. Given that the dynamics of southeast Australian mudflats are driven more by disturbance than seasonality in predators and their interactions with prey, it is likely that Pyrazus would be similarly resistant to predation and have negative effects on benthic assemblages at other times of the year, outside of our study period. Thus, in reducing microphytobenthos and the abundance and species richness of macrofauna, high abundances of the detritivore P. ebeninus may severely limit the flow of energy up the food chain to commercially-important species. This study therefore suggests that trophic cul-de-sacs are not limited to the eutrophied pelagic systems in which they were first identified, but may exist in other systems as well.