Predator-free Areas Research Articles

Top-predator removal does not cause trophic cascades in Australian rangeland ecosystems

Increased demand for livestock products is exacerbating conflict with predators in many parts of the world, fuelling an increase in predation management practices in many grazing systems. In Australia, exclusion fences are being erected across broad areas to facilitate the sustained eradication of dingoes, an apex predator, prompting concern for possible cascading effects on extant wildlife populations. We experimentally assessed population trends of mammals, reptiles and ground-dwelling birds inside and outside of two large, fenced areas over a five-year period subsequent to the erection of the fences and the removal of dingoes. Wildlife population abundances inside the fences were always higher or the same as outside the fences, and all wildlife species exhibited similar population trends within each treatment over time. The only exception to this was kangaroo populations, which fluctuated widely inside fenced areas but remained relatively low and stable outside fenced areas. Kangaroos exhibited negative relationships with dingoes, although unmeasured variables limited our ability to quantify specific causes for kangaroo population trends. Vegetation trends were virtually identical between treatments, indicating that wildlife population trends are closely linked to vegetation conditions and not predator abundances. These applied, experimental results demonstrate that top-predator removal does not routinely cause trophic cascades (including mesopredator releases) in Australian rangeland ecosystems. We therefore encourage exploration of opportunities for targeted threatened species recovery in predator-free areas on land used for livestock production.

The ‘Goldilocks Zone’ of predation: the level of fox control needed to select predator resistance in a reintroduced mammal in Australia

A large component of the anthropogenic biodiversity crisis is the loss of animal species. In response, there has been significant investment in reintroductions of species to their historical ranges. Predation by native and exotic predators, however, remains a barrier to success. Over the past 200 years, Australia has seen the highest rate of mammal extinction on earth, with mammals within a critical weight range (CWR: 35 g–5.5 kg) most affected due to predation by exotic predators. Populations of some threatened species now exist only in Tasmania, offshore islands, or predator-proof sanctuaries. The next critical step is to return native populations outside of predator-free areas, ‘beyond-the-fence’, on the continental mainland. Given our current inability to completely remove exotic predators from mainland ecosystems, how can we achieve successful mammal reintroductions? A potential solution is to drive adaptation of reintroduced animals towards predator-resistance by exposing them to low levels of predation. We propose the concept of a ‘Goldilocks Zone’—the ‘just right’ levels of predation needed to drive selection for predator-resistant native species, while ensuring population viability. We experimentally reintroduced a mammal, the eastern bettong (Bettongia gaimardi), to mainland Australia, 100 years after its local extinction. Using an intense baiting regime, we reduced the population density of the red fox (Vulpes vulpes), the main factor behind the eastern bettong’s extirpation from the continent. Reducing bait take to 15% of previous levels allowed differential survival among bettongs; some surviving under 100 days and others over 450 (~ 4 times longer than some similar trials with related species). Surviving individuals were generally larger at release than those that died earlier, implying selection for larger bettongs. Our results suggest that reducing predation could establish a Goldilocks Zone that could drive selection for bettongs with predator-resistant traits. Our work contributes to a growing body of literature that explores a shift towards harnessing evolutionary principles to combat the challenges posed by animal management and conservation.

Antipredator behaviour of a native marsupial is relaxed when mammalian predators are excluded

Context Predator-controlled environments can lead to prey species losing costly antipredator behaviours as they exploit their low-risk environment, creating a ‘predator-naïve’ population. If individuals lacking suitable antipredator behaviours are used as source populations for reintroductions to environments where predators are present, their behaviour could result in high post-release predation. In contrast, animals sourced from environments with predators (‘predator-exposed’) may show effective antipredator behaviours and thus higher survival post-release. Aims The aim was to compare the antipredator behaviour of brushtail possums (Trichosurus vulpecula) at predator-exposed and predator-naïve source populations, and then compare post-release survival after their reintroduction to a low predator environment. Methods Data were collected from possums at two sites, one with and one without mammalian predators. The behavioural responses of possums to a spotlighter, their willingness to use supplementary feeders at ‘safe’ and ‘risky’ heights, whether they avoided predator odour at traps and their general willingness to enter traps were recorded. Key results Predator-naïve possums showed weaker antipredator responses, were often found at ground level, engaged with novel objects, did not avoid predator scents and utilised different habitats regardless of associated predation risk. In contrast, predator-exposed possums had higher antipredator responses, chose connected trees, were rarely found at ground level and were generally difficult to capture. Post-translocation survival was high for both source populations. Predator-naïve-sourced female possums began to avoid predator urine (feral cat; Felis catus) 12 months after translocation. Conclusions Our research demonstrates that environmental predation risk can predict prey naïvety in brushtail possums. Some aspects of prey naïvety behaviour appear to be able to change in response to altered predation risk. Implications With many threatened species now existing only in feral predator-free areas, these results have implications for future reintroductions into unbounded areas where feral predators are present, and for the management of fenced reserves. The addition of a small number of predators to fenced reserves may aid in retaining antipredator behaviours in fenced prey populations.

Herd Size-Dependent Effects of Restricted Foraging Time Allowance on Cattle Behavior, Nutrition, and Performance

We tested the influence of herd size on the effects of restricted foraging time on cattle (Bos indicus) foraging behavior, nutrition, and performance in a Kenyan savanna rangeland. Using a randomized block design, we compared weight gain, forage intake, diet selection, dietary crude protein (CP) and digestible organic matter (DOM), bite and step rates, distance travelled, and activity time budgets between steers allowed unlimited foraging time (DNG) in predator-free areas with those herded diurnally in predator-accessible areas in large (200 steers; LDG), medium (150 steers; MDG), or small (100 steers; SDG) herds and corralled at night. Daily weight gain was greater (P < 0.01) in DNG (0.61 kg) or SDG (0.56 kg) than in LDG (0.19 kg) or MDG (0.29 kg) but did not differ (P = 0.591) between DNG and SDG. Likewise, daily organic matter intake was greater (P < 0.05) in DNG (6.2 kg) or SDG (5.4 kg) than in LDG (3.7 kg) or MDG (3.7 kg) but did not differ (P = 0.288) between DNG and SDG. Grazing time was lower (P < 0.01) in DNG (42.2%) than in LDG (71.3%), MDG (72.2%), or SDG (69.5%), while the reverse was the case for ruminating and/or resting time (47.1%, 12.1%, 11.9%, and 10.3% in DNG, LDG, MDG, and SDG, respectively). Bite rate was lower in DNG (13.1 bites · min-1) than LDG (21.0 bites · min-1; P = 0.068), MDG (27.7 bites · min-1; P = 0.13) or SDG (26.2 bites · min-1; P = 0.007). However, diet selection, CP, DOM, step rate, and distance travelled did not differ among treatments. Our findings demonstrate subdued negative effects of restricted foraging time when cattle are herded diurnally in small-sized herds. Application of this strategy could reduce the need for eliminating wild carnivores to facilitate unrestricted foraging time for cattle.

Restricting Prey Dispersal Can Overestimate the Importance of Predation in Trophic Cascades

Predators can affect prey populations and, via trophic cascades, predators can indirectly impact resource populations (2 trophic levels below the predator) through consumption of prey (density-mediated indirect effects; DMIEs) and by inducing predator-avoidance behavior in prey (trait-mediated indirect effects; TMIEs). Prey often employ multiple predator-avoidance behaviors, such as dispersal or reduced foraging activity, but estimates of TMIEs are usually on individual behaviors. We assessed direct and indirect predator effects in a mesocosm experiment using a marine food chain consisting of a predator (toadfish – Opsanus tau), prey (mud crab - Panopeus herbstii) and resource (ribbed mussel – Geukensia demissa). We measured dispersal and foraging activity of prey separately by manipulating both the presence and absence of the predator, and whether prey could or could not disperse into a predator-free area. Consumption of prey was 9 times greater when prey could not disperse, probably because mesocosm boundaries increased predator capture success. Although predator presence did not significantly affect the number of crabs that emigrated, the presence of a predator decreased resource consumption by prey, which resulted in fewer resources consumed for each prey that emigrated in the presence of a predator, and reduced the overall TMIE. When prey were unable to disperse, TMIEs on mussel survival were 3 times higher than the DMIEs. When prey were allowed to disperse, the TMIEs on resource survival increased to 11-times the DMIEs. We found that restricting the ability of prey to disperse, or focusing on only one predator-avoidance behavior, may be underestimating TMIEs. Our results indicate that the relative contribution of behavior and consumption in food chain dynamics will depend on which predator-avoidance behaviors are allowed to occur and measured.

Summer bed-site selection by roe deer in a predator free area

For many animals, bed-site selection is influenced by anti-predator strate- gies, as they are forced to cope with high risk of predation. I examined summer bed-site selection by roe deer (Capreolus capreolus bedfordi) in an area - Baxianshan Nature Re- serve, north China -, where predators were extirpated 22 years before, predicting that roe deer would select the bed-sites which allowed them to maximize fawn survival, indepen- dently from predation risk. Among three available forest types, roe deer showed a strong preference for deciduous fo- rests. Roe deer bedded at sites with higher elevation, denser shrub, higher herb biomass and cover and higher percentage of new shoots to the total twigs compared to negative sites. They selected bed-sites relatively far from trees, shrubs, and human features. Furthermore, roe deer avoided areas with tall shrubs and steep slopes. Principal component analysis (PCA) revealed that food availability, visibility, space, and human disturbance were the most important factors affecting bed-site selection by roe deer. Bed-site selection by roe deer was still the result of anti-predator strategies. This implies that two decades were a too short period to let deer lose the memory of predation pressure. This study suggests that the nonrandom bed-site selection of roe deer aimed to increase reproductive success by enhanc- ing fawn survival.

Shifter and resident red deer: intrapopulation and intersexual behavioural diversities in a predator-free area

Context Population management of meso-large game species; knowledge of seasonal movements and sexual segregation. Aims We predicted (1) a similar mixed spatial tactic in the two sexes (shifters and residents); (2) that reproductive success could be a key factor eliciting a mixed spatial tactic in hinds; and (3) that hunting activities may affect the rutting season, influencing rutting site location. Methods We studied the spatial behaviour of 41 red deer on the Italian north-eastern Alps, from December 2003 to October 2007, by VHF radio-tracking. Key results For both sexes, two main spatial tactics were described, including residents, remaining in the same area all year long, and shifters, using non-overlapping seasonal home ranges. Most deer showed consistent spatial behaviour. In all, 87% of stags and 49% of hinds were shifters. Sexual segregation was greater in winter than in summer. All hinds wintered in the same valley, with residents in the qualitatively poorer, but protected, area. Resident hinds showed a variable yearly reproductive success, whereas usually all shifters had a calf at heel. Rutting areas of hinds overlapped with their winter or summer ranges, whereas stags occupied up to three different areas yearly. Deer distribution determined a different harvesting pressure on the two sexes. Conclusions Although shifters and residents were found in both sexes, differences in spatial behaviour of stags and hinds were consistent for seasonal occupancy, habitat selection and number of seasonal home ranges. For hinds, a mixed spatial tactic, seasonality and hunting activities could be key factors of reproductive success. Implications Hunting could affect the proportions of shifter and resident hinds at different levels, with the latter benefiting from the protection accorded. This unequally distributed selective harvesting may alter the local social structure and population dynamics in the long term. Shifter behaviour, seasonal movements and sexual segregation may bias population estimates, if counts are conducted when individuals make seasonal movements. Furthermore, the size of a hunting unit should be function of the extent of local movements of deer.

Vigilance and foraging patterns of American elk during the rut in habitats with and without predators

Animal behavior is often optimized as a trade-off between survival and reproduction. During the breeding season, mammals tend to maximize their reproductive effort within the constraints of predation pressure. When predation pressure is reduced, greater effort can be allocated to reproductive behavior and less to vigilance and predator avoidance. The objective of this study was to test the hypothesis that elk, Cervus elaphus, in Yellowstone National Park (YNP), with predators, would spend more time in vigilance and risk-avoidance behavior than would elk in Rocky Mountain National Park (RMNP), a predator-free environment. We further predicted that elk at Mammoth Hot Springs (MAM) in YNP would behave similarly to those at RMNP because predators were absent in that area of the park. Cow elk in YNP spent more time in vigilance and less in foraging during activity periods than did cows in RMNP or MAM. Also, elk in YNP retreated to forest cover during the midday inactive period, whereas elk in RMNP and MAM remained in open habitat. Vigilance was not correlated with group size at either site. Cows with calves spent more time in vigilance and less in foraging than did cows without calves in RMNP and YNP. Bull elk spent most of their time in courtship at all sites, but foraged more at RMNP than in YNP or MAM. Mean harem sizes were similar among the three sites: 17.0 in RMNP, 15.7 in YNP, and 19.0 in MAM. The proportion of cows with calves was significantly lower in the area with predators, YNP (0.10), than in the predator-free areas (0.24 in RMNP and 0.37 in MAM), probably because of greater calf mortality in YNP. Elk in YNP behaved in accordance with a predation risk, whereas those in RMNP and MAM showed less vigilance behavior.

Olfactory predator recognition: wallabies may have to learn to be wary

AbstractMany species modify their behaviour in response to the scents of their predators, but species or populations living without predators may lose such abilities. This loss has been suggested to be irreversible, and to constitute a significant hurdle in restoring historical ecosystems. Olfactory predator recognition was studied in two macropodid marsupials ‐ the tammar wallaby (Macropus eugenii) and the red‐necked pademelon (Thylogale thetis). Both species are in the ‘critical weight range’ of Australian native mammals that have been negatively affected by the introduction of novel predators since European settlement. Predator‐naïve animals were tested by exposing subjects simultaneously to two feeders with either a predator or a herbivore faecal or urine sample beneath the food tray. The presence of predator olfactory cues beneath the feeder did not affect foraging behaviour or feeder use when compared to control stimuli (herbivore faeces or urine). Previous studies have found that predator‐experienced herbivorous marsupials modify their behaviour in the presence of predator scents. In contrast, our studies of predator‐naïve individuals found no evidence of such selectivity, suggesting that marsupial herbivores may have to learn to modify their behaviour in response to olfactory cues from predators. This implies that the loss of olfactory predator recognition may not be irreversible. Animals translocated from predator‐free areas could potentially be trained to recognise the smells of their predators.

Predator detection and avoidance by lotic mayfly nymphs of different size.

We studied antipredatory responses of lotic mayfly (Baetis) nymphs in a factorial experiment with four levels of fish presence: (1) a freely foraging fish (the European minnow,Phoxinus phoxinus), (2) a constrained fish, (3) water from a fish stream, (4) water from a fishless stream. LargeBaetis nymphs drifted mainly during night-time in treatments involving either the chemical or actual presence of fish, whereas no diel periodicity was observed when the water was not conditioned with fish odour. The response was strongest when the fish was uncaged, which suggests that visual or hydrodynamic cues are needed in addition to chemical ones for an accurate assessment of predation risk. Fish presence had no effect on the drift rates of small nymphs. Instead, they increased their refuge use in the presence of a live fish. Chemical cues alone did not have any effect on the refuge use of any of theBaetis size classes. Our results indicate active drift entry by mayfly nymphs. Because predation pressure is spatially and temporally variable, nymphs must sample the environment in order to locate predator-free areas or areas with low predation risk. Drifting should be the most energy-saving way to do this. To avoid the risk from visually feeding fish, large individuals can sample safely (i.e. enter drift) only at night-time, while the small ones can also do this safely during the day. We suggest that, contrary to some earlier assumptions, mayfly drift is not a fixed prey response. Instead,Baetis nymphs are able to assess the prevailing predation pressure, and they adjust their foraging behaviour accordingly.

Risk of predation and feeding rate in tropical freshwater fishes: field evidence.

In a previous study (A. Prejs and K. Prejs in press) we found that at the end of the dry season small fish species trapped together with piscivorous fish in small, shallow pool varied in their level of food intake. The two smallest of these species fed little, whereas larger species fed at high rate. By examining the species and size composition of the diet and gape limitations of predatory species, I found that the decreased feeding rate of small fish was in response to a high risk of predation by gape-limited predators. The reduction of feeding rate was very substantial when compared to that of the same species in predator free area.

Competition conditional on recruitment and temporary escape from predators on a tropical rocky shore

On a rocky headland on the Pacific Coast of Costa Rica we document a period of heavy recruitment of the barnacle, Chthamalus fissus Darwin, in an area infrequently visited by its major predator, the muricid gastropod, Acanthina brevidentata (Wood). For much of 1984, Chthamalus fissus occupied most available free space in the area, appearing to surround and imprison pulmonate limpets, Siphonaria gigas (Sowerby), on their home scars. Limpets lost weight when imprisoned, a situation which persisted until barnacles were removed by Acanthina brevidentata predation. Limpets had little effect on barnacles. Heavy barnacle recruitment in predator-free areas occupied by the limpet is unpredictable in space and time. As a result the negative effect of barnacles on limpets probably has little evolutionary consequence for the limpet.