Top Predator Populations Research Articles

Sympatric prey responses to lethal top-predator control: predator manipulation experiments

Introduction: Many prey species around the world are suffering declines due to a variety of interacting causes such as land use change, climate change, invasive species and novel disease. Recent studies on the ecological roles of top-predators have suggested that lethal top-predator control by humans (typically undertaken to protect livestock or managed game from predation) is an indirect additional cause of prey declines through trophic cascade effects. Such studies have prompted calls to prohibit lethal top-predator control with the expectation that doing so will result in widespread benefits for biodiversity at all trophic levels. However, applied experiments investigating in situ responses of prey populations to contemporary top-predator management practices are few and none have previously been conducted on the eclectic suite of native and exotic mammalian, reptilian, avian and amphibian predator and prey taxa we simultaneously assess. We conducted a series of landscape-scale, multi-year, manipulative experiments at nine sites spanning five ecosystem types across the Australian continental rangelands to investigate the responses of sympatric prey populations to contemporary poison-baiting programs intended to control top-predators (dingoes) for livestock protection. Results: Prey populations were almost always in similar or greater abundances in baited areas. Short-term prey responses to baiting were seldom apparent. Longer-term prey population trends fluctuated independently of baiting for every prey species at all sites, and divergence or convergence of prey population trends occurred rarely. Top-predator population trends fluctuated indep endently of baiting in all cases, and never did diverge or converge. Mesopredator population trends likewise fluctuated independently of baiting in almost all cases, but did diverge or converge in a few instances. Conclusions: These results demonstrate that Australian populations of prey fauna at lower trophic levels are typically unaffected by top-predator control because top-predator populations are not substantially affected by contemporary control practices, thus averting a trophic cascade. We conclude that alteration of current top-predator management practices is probably unnecessary for enhancing fauna recovery in the Australian rangelands. More generally, our results suggest that theoretical and observational studies advancing the idea that lethal control of top-predators induces trophic cascades may not be as universal as previously supposed.

A Model for the Effect of Toxicant on a Three Species Food-Chain System with "Food-Limited" Growth of Prey Population

In this paper, a mathematical model is proposed and analyzed to study the effect of toxicant in a three species food chain system with food-limited growth of prey population. The mathematical model is formulated using the system of non-linear ordinary differential equations. In the model, there are seven state variables, viz, prey density, intermediate predator density, density of top predator, concentration of toxicant in the environment, concentration of toxicant in the prey, concentration of toxicant in the intermediate predator and concentration of toxicant in the top predator. In the model, it is assumed that the carrying capacity and growth rate of prey is affected by environmental toxicant. Toxicant is transferred to intermediate predator and top predator populations through food chain pathways.All the feasible equilibria of the system are obtained and the conditions are determined for the survival or extinction of species under the effect of toxicant. The local and global stability analysis of all the feasibleequilibria are carried out.Further, the results are compared with the case when toxicant is absent in the system. Finally, we support our analytical findings with numerical simulations. Keywords: Food chain; Toxicant; Food-Limited Growth; Stability; Lyapunov function.

Effects of supplying alternative food in a predator–prey model with harvesting

In this paper, we propose a tritrophic predator–prey model with harvesting where the top-predator population is partially supported with alternative food. We report the consequences of providing alternative food to the top-predator in a top-predator harvested model. The extinction criterion for top-predator population, local stability of equilibrium points and persistence conditions are discussed theoretically. Pontryagins maximum principle is used to characterize the optimal control of harvesting. We have derived the condition of Hopf bifurcation by varying harvesting effort. The bifurcation diagrams of the system with respect to harvesting effort in presence of alternative food are given. Our analysis show that alternative food can prevent top-predator extinction risk at higher harvesting effort and plays a vital role for biological conservation of species.

Atlantic Rainforest's Jaguars in Decline

In her News Focus story “Predators in the 'hood” (20 September, p. [1332][1]), V. Morell reported that top predator populations are coming back across much of North America. Meanwhile, predators in Brazil continue to decline. A recent meeting of wildlife experts indicated that the Atlantic

Intra-population variation in activity ranges, diel patterns, movement rates, and habitat use of American alligators in a subtropical estuary

Movement and habitat use patterns are fundamental components of the behaviors of mobile animals and help determine the scale and types of interactions they have with their environments. These behaviors are especially important to quantify for top predators because they can have strong effects on lower trophic levels as well as the wider ecosystem. Many studies of top predator movement and habitat use focus on general population level trends, which may overlook important intra-population variation in behaviors that now appear to be common. In an effort to better understand the prevalence of intra-population variation in top predator movement behaviors and the potential effects of such variation on ecosystem dynamics, we examined the movement and habitat use patterns of a population of adult American alligators (Alligator mississippiensis) in a subtropical estuary for nearly four years. We found that alligators exhibited divergent behaviors with respect to activity ranges, movement rates, and habitat use, and that individualized behaviors were stable over multiple years. We also found that the variations across the three behavioral metrics were correlated such that consistent behavioral types emerged, specifically more exploratory individuals and more sedentary individuals. Our study demonstrates that top predator populations can be characterized by high degrees of intra-population variation in terms of movement and habitat use behaviors that could lead to individuals filling different ecological roles in the same ecosystem. By extension, one-size-fits-all ecosystem and species-specific conservation and management strategies that do not account for potential intra-population variation in top predator behaviors may not produce the desired outcomes in all cases.

Genetic network and breeding patterns of a sicklefin lemon shark (Negaprion acutidens) population in the Society Islands, French Polynesia.

Human pressures have put many top predator populations at risk of extinction. Recent years have seen alarming declines in sharks worldwide, while their resilience remains poorly understood. Studying the ecology of small populations of marine predators is a priority to better understand their ability to withstand anthropogenic and environmental stressors. In the present study, we monitored a naturally small island population of 40 adult sicklefin lemon sharks in Moorea, French Polynesia over 5 years. We reconstructed the genetic relationships among individuals and determined the population’s mating system. The genetic network illustrates that all individuals, except one, are interconnected at least through one first order genetic relationship. While this species developed a clear inbreeding avoidance strategy involving dispersal and migration, the small population size, low number of breeders, and the fragmented environment characterizing these tropical islands, limits its complete effectiveness.

Intraguild relationships between sympatric predators exposed to lethal control: predator manipulation experiments

IntroductionTerrestrial top-predators are expected to regulate and stabilise food webs through their consumptive and non-consumptive effects on sympatric mesopredators and prey. The lethal control of top-predators has therefore been predicted to inhibit top-predator function, generate the release of mesopredators and indirectly harm native fauna through trophic cascade effects. Understanding the outcomes of lethal control on interactions within terrestrial predator guilds is important for zoologists, conservation biologists and wildlife managers. However, few studies have the capacity to test these predictions experimentally, and no such studies have previously been conducted on the eclectic suite of native and exotic, mammalian and reptilian taxa we simultaneously assess. We conducted a series of landscape-scale, multi-year, manipulative experiments at nine sites spanning five ecosystem types across the Australian continental rangelands to investigate the responses of mesopredators (red foxes, feral cats and goannas) to contemporary poison-baiting programs intended to control top-predators (dingoes) for livestock protection.ResultShort-term behavioural releases of mesopredators were not apparent, and in almost all cases, the three mesopredators we assessed were in similar or greater abundance in unbaited areas relative to baited areas, with mesopredator abundance trends typically either uncorrelated or positively correlated with top-predator abundance trends over time. The exotic mammals and native reptile we assessed responded similarly (poorly) to top-predator population manipulation. This is because poison baits were taken by multiple target and non-target predators and top-predator populations quickly recovered to pre-control levels, thus reducing the overall impact of baiting on top-predators and averting a trophic cascade.ConclusionsThese results are in accord with other predator manipulation experiments conducted worldwide, and suggest that Australian populations of native prey fauna at lower trophic levels are unlikely to be negatively affected by contemporary dingo control practices through the release of mesopredators. We conclude that contemporary lethal control practices used on some top-predator populations do not produce the conditions required to generate positive responses from mesopredators. Functional relationships between sympatric terrestrial predators may not be altered by exposure to spatially and temporally sporadic application of non-selective lethal control.

Seabird and marine-mammal attendance and by-catch in semi-industrial trawl fisheries in near-shore waters of northern Argentina

Seabird and marine-mammal attendance and by-catch in mid-water and bottom otter semi-industrial coastal pair-trawl fisheries were assessed for the first time in northern Argentina. Observers were placed onboard trawlers between autumn 2007 and autumn 2008. Fifteen marine top-predator species were associated with the vessels. The most abundant and frequent seabirds (trawl fisheries combined) were the kelp gull, Larus dominicanus (~70% of total birds and >96% occurrence), and the Olrog’s gull, L. atlanticus (~12% and >50%, respectively). Other seabird taxa such as Procellariiforms and Sphenisciforms, among others, were represented in very low numbers. The only mammal species recorded was the South American sea lion, Otaria flavescens (1% and 2%, respectively). Analysis of environmental and operational variability affecting the abundance of gulls indicated a significant increase in abundance with fishing depth, time of day, seasonality, wind intensity and wind direction. Incidental mortality of top predators was low and comprised only Magellanic penguins, Spheniscus magellanicus, in the mid-water gear. The results of the present study showed that the semi-industrial trawl fisheries operating in coastal waters in northern Argentina may a have a relatively minor impact on marine top-predator populations, at least in terms of incidental capture.

Dynamical effects of weak trophic interactions in a stochastic food web simulation

Network models are traditional in community ecology. For example, they provide a rich analytical toolkit to put higher predators into a multispecies context. Better understanding their top-down effects and the potential bottom-up control on them would be of key importance for predictive ecosystem management. Food web architecture may be used to predict community dynamics, but it is an old question how reliable are the studies considering only static information. A general and intuitive assumption is that stronger links (with larger weights) mediate stronger effects. We study this statement and use an illustrative case study. We investigate the trophic structure of the Prince William Sound food web in terms of biomass flows, and study its simulated dynamics in a stochastic modelling framework. We aim to understand bottom-up effects of preys on consumers: we focus on the fluctuations of top predator populations, following disturbance on their prey. Several disturbance regimes are studied and compared. Food web structure and link weight generally predict well the average impacts of preys on top-predators, with larger flows mediating stronger effects. Most exceptions appear for weak links: these are less predictable, some of them can be surprisingly important.

The role of top predator interference on the dynamics of a food chain model

In this paper, the effects of top predator interference on the dynamics of a food chain model involving an intermediate and a top predator are considered. It is assumed that the interaction between the prey and intermediate predator follows the Volterra scheme, while that between the top predator and its favorite food depends on Beddington–DeAngelis type of functional response. The boundedness of the system, existence of an attracting set, local and global stability of non-negative equilibrium points are established. Number of the bifurcation and Lyapunov exponent bifurcation diagrams is established. It is observed that, the model has different types of attracting sets including chaos. Moreover, increasing the top predator interference stabilizes the system, while increasing the normalization of the residual reduction in the top predator population destabilizes the system.

Indirect facilitation of a native mesopredator by an invasive species: are cane toads re-shaping tropical riparian communities?

Top predators can suppress mesopredators both by killing them and by motivating changes in their behavior, and there are numerous examples of mesopredator release caused by declines in top predator populations. Demonstrated cases of invasive species triggering such releases among vertebrate trophic linkages (indirect facilitation), however, are rare. The invasive cane toad, Bufo marinus, has caused severe population-level declines in some Australian predators via lethal toxic ingestion. During a long-term study of the direct impacts of cane toads on predatory monitor lizards in tropical Australia, we documented significant, marked increases in annual counts of a mesopredator, the common tree snake (Dendrelaphis punctulatus). Mean snake counts during surveys of 70-km river transects at two sites increased from <1 individual per survey during 2001–2006, to 8–18 per survey in 2007. These increases occurred approximately 3 years following the arrival of cane toads, and 1–3 years after 71–96 % population declines in three species of predatory monitor lizards (Varanus panoptes, V. mertensi, and V. mitchelli). These data suggest a mesopredator release: the dramatic reduction of predatory monitor lizards caused increases in the tree snake by decreasing predation risk. The increases in tree snake counts were not attributable to either abiotic factors, or a trophic subsidy. The severe declines of predatory monitor lizards, coupled with recent evidence of cascading effects on their prey, suggest that cane toads are re-shaping riparian communities in tropical Australia through both direct negative effects and indirect facilitation.

Seabird and marine mammal attendance in the Chub mackerel Scomber japonicus semi-industrial Argentinian purse seine fishery

We assessed for the first time marine bird and mammal attendance in the Chub mackerel Scomber japonicus semi-industrial coastal purse seine fishery in northern Argentina. Information was gathered during a fishery run from late spring 2007 to early summer 2008. Sixteen marine top predator species attended vessels, and the most abundant seabirds (percent of overall individuals and percent of occurrence) were the Kelp Gull Larus dominicanus (25% and 40% respectively), and White-chinned Petrel Procellaria aequinoctialis (24% and 47% respectively). Other attending bird species were the Black-browed Albatross Thalassarche melanophrys (18.5% and 12% respectively) and South American Tern Sterna hirundinacea (30% and 22% respectively). The most abundant marine mammal's species was the South American Sea Lion Otaria flavescens (2% and 8% respectively). Analysis of environmental and operational variability affecting the abundance of the four main seabird species indicated a clear spatial variability in the numbers of seabirds attending coastal purse seiners. Other factors included fishing operation, time of day, seasonality, and wind intensity and wind direction. Neither seabird nor marine mammal species were observed to be incidentally taken during 82 fishing operations. The results of this study showed that the semi-industrial purse seine fishery operating in coastal waters in northern Argentina may have a relatively minor impact on some marine top predator populations mainly because (1) the fishing grounds are close to shore, and at relatively shallow depths, and (2) for economic reasons, the fishing fleet operates at considerable distance from dense schooling areas. Still, it has to be stressed that data were obtained during operations in which the levels of discards (<2%) were by far lower than the average of 30% reported in the literature. Given this background, further studies including long-term monitoring are needed for an adequate modeling of the interactions and the identification of core areas for seabirds interacting with purse seiners in the waters of northern Argentina.

Temporal variation in distribution and density of ice-obligated seals in western Hudson Bay, Canada

Recent unidirectional climatic trends and changes in top predator population ecology suggest that long-term modifications may be happening in Hudson Bay, Canada. Effects of such changes on ice-obligated seal populations are expected but long-term studies are required to differentiate climate-induced changes from natural variation. We conducted strip-transect surveys in late spring in 1995–1997, 1999–2000 and 2007–2008 to estimate distribution, density and abundance of ice-obligated ringed (Phoca hispida) and bearded (Erignathus barbatus) seals in western Hudson Bay. When hauled out, ringed seals preferred land-fast and consolidated pack ice, whereas bearded seals preferred unconsolidated pack ice. Bearded and ringed seal density estimates varied from 0.0036 to 0.0229 seals/km2 of ice and from 0.46 to 1.60 seals/km2 of ice, respectively. Strong inter-annual variations were recorded in the abundance estimates of both species, with the largest abundance estimates in 1995 (104,162 and 1,494 ringed and bearded seals, respectively) and the lowest in 2008 for ringed seals (33,701) and 1997 for bearded seals (278). A sine function best described seal density estimates in western Hudson Bay and suggested a decadal cycle. Previous studies that reported low ringed seal demographic parameters in the 1990s and a recovery in the 2000s supported our interpretation of the survey results. We discuss our results in the context of climate warming and suggest that a long-term decline in ice-obligated seal density estimates may overlay a possible natural decadal cycle.

The ecological importance of intact top-predator populations: a synthesis of 15 years of research in a seagrass ecosystem

The worldwide decline of large-bodied marine taxa has made it difficult to draw conclusions about the relative importance of top-down control, and the mechanisms through which it might operate, in coastal marine ecosystems. Since 1997, the Shark Bay Ecosystem Research Project has used the relatively pristine seagrass community of Shark Bay, Australia, to investigate the potential for tiger sharks, the apex predator in the ecosystem, to have an impact on their large-bodied prey through non-consumptive (‘risk’) effects. Here, we synthesise nearly 15 years of data to demonstrate that tiger sharks have widespread risk effects on both large-bodied herbivores and mesopredators in Shark Bay and explore the possibility that these impacts may cascade to lower trophic levels. Although much work remains to be done, our studies suggest that losses of top predators in subtropical estuaries may have greater consequences than generally appreciated and that efforts to conserve and restore their populations should be a priority. Furthermore, future management strategies and studies must explicitly consider the potential for predators to influence behaviour of even large-bodied marine taxa.

Simple Life-History Omnivory: Responses to Enrichment and Harvesting in Systems with Intraguild Predation

This article analyzes the nature of top-down and bottom-up effects and alternative states in systems characterized by life-history omnivory. The analysis is based on a three-species food web with intraguild predation (IGP). The top predator population has juvenile and adult stages, which consume the basal resource and the intermediate prey, respectively; the prey consumes only the resource. The per capita reproduction of the adult predators depends on their consumption rate of prey, while the maturation rate of the juvenile predators depends on their resource consumption rate. Enriching the resource can increase or decrease the abundances of one or both of the two consumer species; an increased density is more likely in the intermediate species than in the systems where IGP is not based on stage differences. Alternative states that have or lack the predator occur frequently, particularly when the prey population is capable of reducing the resource to very low densities. These results differ from those of several other recent models of life-history omnivory. They suggest that life-history omnivory may be one of the primary reasons why exploited populations undergo sudden collapses and why collapsed populations fail to recover in spite of large reductions in the exploitation rate.

Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica

The West Antarctic Peninsula (WAP) and adjacent Scotia Sea support abundant wildlife populations, many of which were nearly extirpated by humans. This region is also among the fastest-warming areas on the planet, with 5-6 °C increases in mean winter air temperatures and associated decreases in winter sea-ice cover. These biological and physical perturbations have affected the ecosystem profoundly. One hypothesis guiding ecological interpretations of changes in top predator populations in this region, the "sea-ice hypothesis," proposes that reductions in winter sea ice have led directly to declines in "ice-loving" species by decreasing their winter habitat, while populations of "ice-avoiding" species have increased. However, 30 y of field studies and recent surveys of penguins throughout the WAP and Scotia Sea demonstrate this mechanism is not controlling penguin populations; populations of both ice-loving Adélie and ice-avoiding chinstrap penguins have declined significantly. We argue in favor of an alternative, more robust hypothesis that attributes both increases and decreases in penguin populations to changes in the abundance of their main prey, Antarctic krill. Unlike many other predators in this region, Adélie and chinstrap penguins were never directly harvested by man; thus, their population trajectories track the impacts of biological and environmental changes in this ecosystem. Linking trends in penguin abundance with trends in krill biomass explains why populations of Adélie and chinstrap penguins increased after competitors (fur seals, baleen whales, and some fishes) were nearly extirpated in the 19th to mid-20th centuries and currently are decreasing in response to climate change.

Does a top predator suppress the abundance of an invasive mesopredator at a continental scale?

Aim We examined evidence for the mesopredator release hypothesis at a subcontinental scale by investigating the relationship between indices of abundance of the dingo Canis lupus dingo (top-order predator) and the invasive red fox Vulpes vulpes (mesopredator) in three large regions across mainland Australia. The red fox is known to be one of the major threats to the persistence of small and medium-sized native vertebrates across the continent. Location Australia. Methods Indices of abundance were calculated from three independently collected datasets derived from bounty returns and field surveys. Data were analysed using univariate parametric, semi-parametric and nonparametric techniques. Results Predator abundance indices did not conform to a normal distribution and the relationships between dingo and fox abundance indices were not well described by linear functions. Semi-parametric and nonparametric techniques revealed consistently negative associations between indices of dingo and fox abundance. Main conclusions The results provide evidence that mesopredator suppression by a top predator can be exerted at very large geographical scales and suggest that relationships between the abundances of top predators and mesopredators are not linear. Our results have broad implications for the management of canid predators. First, they suggest that dingoes function ecologically to reduce the activity or abundance of red foxes and thus are likely to dampen the predatory impacts of foxes. More generally, they provide support for the notion that the mesopredator-suppressive effects of top predators could be incorporated into broad-scale biodiversity conservation programmes in many parts of the world by actively maintaining populations of top predators or restoring them in areas where they are now rare. Determining the population densities at which the interactions of top predators become ecologically effective will be a critical goal for conservation managers who aim to maintain or restore ecosystems using the ecological interactions of top predators.

Proximate composition and energy content of forage species from the Bay of Biscay: high- or low-quality food?

Abstract Spitz, J., Mourocq, E., Schoen, V., and Ridoux, V. 2010. Proximate composition and energy content of forage species from the Bay of Biscay: high- or low-quality food? – ICES Journal of Marine Science, 67: 909–915. Collapses of high-energy dense concentrations of prey species induce negative effects on populations of top predators. Knowledge of prey quality appears to be crucial in ecosystem modelling and management. The aim here was to provide baseline data of forage species quality in the Bay of Biscay. Proximate composition (water, ash, protein, and lipid) and energy content have been determined to assess the quality of 78 species, including jellyfish, crustaceans, cephalopods, cartilaginous fish, and bony fish. Results show broad variations between species, with energy densities ranging from 2 to 10 kJ g−1. Lipids are the most structuring component and largely determined prey quality, and prey species are not necessarily interchangeable for the fulfilment of a predator's energy and food requirements. In ecosystem models, therefore, multispecies compartments of forage organisms would ideally be constituted using prey species of equivalent quality and hence of equivalent benefit to top predators.

Do inter-colony differences in Cape fur seal foraging behaviour reflect large-scale changes in the northern Benguela ecosystem?

The northern Benguela ecosystem adjoining Namibia has undergone considerable changes in recent decades, with reductions and northwards shifts of key prey species that have had severe implications for marine top predator populations. We investigated how such environmental variability may impact foraging behaviour of the Cape fur seal Arctocephalus pusillus pusillus, using satellite telemetry on animals in northern, central and southern Namibia. We expected that seal foraging behaviour would reflect a gradient of deteriorating feeding conditions from north to south. Results showed that foraging trips were shorter in the central region, where seals fed over the continental shelf, than in the north or south, where seals fed at the shelf edge. However, whereas seals in the north showed strong fidelity to the colony at which they were tagged and to persistent, clustered foraging areas, seals in the south showed weak fidelity both to the colony at which they were tagged and to foraging areas, which were scattered and variable. Seals in the south also tended to migrate northwards to other colonies while concurrently adapting their foraging behaviour to local conditions. Flexible use of foraging space and colonies of Cape fur seals during the three-year period (2002-2004) demonstrates that the species is adapted for variable environments over time and space.

The Rise of the Mesopredator

Apex predators have experienced catastrophic declines throughout the world as a result of human persecution and habitat loss. These collapses in top predator populations are commonly associated with dramatic increases in the abundance of smaller predators. Known as “mesopredator release,” this trophic interaction has been recorded across a range of communities and ecosystems. Mesopredator outbreaks often lead to declining prey populations, sometimes destabilizing communities and driving local extinctions. We present an overview of mesopredator release and illustrate how its underlying concepts can be used to improve predator management in an increasingly fragmented world. We also examine shifts in North American carnivore ranges during the past 200 years and show that 60% of mesopredator ranges have expanded, whereas all apex predator ranges have contracted. The need to understand how best to predict and manage mesopredator release is urgent—mesopredator outbreaks are causing high ecological, economic, an...