Predator Exclosures Research Articles

CHANGING RECRUITMENT IN CONSTANT SPECIES ASSEMBLAGES: IMPLICATIONS FOR PREDATION THEORY IN INTERTIDAL COMMUNITIES

Ecological theory for benthic communities emphasizes intense species interactions that depend on the high productivity of sedentary invertebrates. The keystone predator hypothesis maintains that intense predation by one consumer species is necessary to prevent a prolific, competitively dominant prey species from eliminating other species using the same resource. This study considers the consequences of extreme spatial and temporal variation in the recruitment of a prey species supporting keystone and diffuse predation. Prior experiments on rocky shores of Santa Catalina Island, California, USA, demonstrated that predation by spiny lobsters (Panulirus interruptus) maintained a distinctive red algal turf by killing juvenile mussels (Mytilus californianus and M. galloprovincialis) that otherwise overgrow and replace the algae. In the present study, long-term surveys revealed that high recruitment of the predominant mussel, M. californianus, occurred only on the most wave-exposed sites in certain years; mussel recruitment was slight to nil on relatively protected sites in most years. A predator exclosure experiment consisting of seven replicates placed along the gradient of wave exposure demonstrated that the effects of predation depended upon the spatial differences in recruitment rates. Lobsters on wave-exposed sites functioned as keystone predators; on more sheltered sites, little or no predation, whether by lobsters or the fishes and whelks also foraging on the sheltered sites, was necessary to maintain the algal assemblage. Similar species assemblages can be maintained by markedly different relative levels of crucial ecological rates. In the mid-intertidal zone of Santa Catalina Island, the intense species interactions depicted in the keystone predator hypothesis occurred only at productive, high wave exposure locations; low recruitment of mussels elsewhere preempts both predation and the competition between the mussel and algal assemblages. Thus, red algae dominates rocky shores through different mechanisms over a range of physical conditions. The occurrences of low mussel recruitment do not appear to be anomalies, but rather a consequence of the life history of Mytilus californianus.

Changing Recruitment in Constant Species Assemblages: Implications for Predation Theory in Intertidal Communities

Ecological theory for benthic communities emphasizes intense species interactions that depend on the high productivity of sedentary invertebrates. The keystone predator hypothesis maintains that intense predation by one consumer species is necessary to prevent a prolific, competitively dominant prey species from eliminating other species using the same resource. This study considers the consequences of extreme spatial and temporal variation in the recruitment of a prey species supporting keystone and diffuse predation. Prior experiments on rocky shores of Santa Catalina Island, California, USA, demonstrated that predation by spiny lobsters (Panulirus interruptus) maintained a distinctive red algal turf by killing juvenile mussels (Mytilus californianus and M. galloprovincialis) that otherwise overgrow and replace the algae. In the present study, long-term surveys revealed that high recruitment of the predominant mussel, M. californianus, occurred only on the most wave-exposed sites in certain years; mussel recruitment was slight to nil on relatively protected sites in most years. A predator exclosure experiment consisting of seven replicates placed along the gradient of wave exposure demonstrated that the effects of predation depended upon the spatial differences in recruitment rates. Lobsters on wave-exposed sites functioned as keystone predators; on more sheltered sites, little or no predation, whether by lobsters or the fishes and whelks also foraging on the sheltered sites, was necessary to maintain the algal assemblage. Similar species assemblages can be maintained by markedly different relative levels of crucial ecological rates. In the mid-intertidal zone of Santa Catalina Island, the intense species interactions depicted in the keystone predator hypothesis occurred only at productive, high wave exposure locations; low recruitment of mussels elsewhere preempts both predation and the competition between the mussel and algal assemblages. Thus, red algae dominates rocky shores through different mechanisms over a range of physical conditions. The occurrences of low mussel recruitment do not appear to be anomalies, but rather a consequence of the life history of Mytilus californianus.

Effects of predation on Zostera marina L. seed abundance

Predator effects on Zostera marina L. seed abundance were studied in the York River, VA, USA, using enclosure and exclosure caging experiments. Seeds were placed in cages in two concurrent experiments. The first experiment was a predator exclosure experiment to test the effects of excluding predators, using a full predator exclosure cage, a partial exclosure top-only cage, a partial exclosure side-only cage and uncaged plots. The second experiment was a predator enclosure experiment, using two highly abundant macro-benthic predators in the Chesapeake Bay: the decapod crustacean Callinectes sapidus Rathbun and the sciaenid fish Micropogonias undulatus L. Additionally, two-week long trials of sequentially protected and exposed seeds were also performed. Replicate treatment plots were sampled by removing the top 5–10 cm of the sediment surface with a suction sampler and still viable seeds in each plot were counted. Full exclosure cages contained significantly higher numbers of seeds than the uncaged or partial caged treatments. Seed abundances in the C. sapidus enclosure cages were significantly less than the full exclusion cage, but not significantly different than the uncaged treatments. Seed abundances in the M. undulatus cages were not significantly different than the full exclusion cage. The least number of seeds were found in the uncaged and partial cage treatments. Results of the sequentially protected and exposed trials were similar to results from the one-week uncaged treatments. These experiments suggest that seed predation can affect the abundance of Z. marina seeds, possibly causing up to 65% of the seed losses observed in these experiments. Results suggest that seed predation has the potential to be an important force governing the sexual reproductive success and propagation of eelgrass beds and that the degree of seed loss via predation may be related to predator and primary food abundances.

Effects of predators on reef fishes: separating cage artifacts from effects of predation

Predators are thought to play prominent roles in determining population abundance and dynamics of many species, but it is often difficult to document effects of predators in the field. For lack of any other effective means of manipulating predator densities, many studies use predator exclosures, however, the results of these studies are difficult to interpret because the exclosures may introduce artifacts that cannot be separated from predatory effects. Using a design common in tests of predatory effects, exclosure cages (− predators), “cage controls” = partial cages ( + predators), and no cages ( + predators), patterns of survival were found for two reef fishes that were consistent with both predator and cage effects. To separate cage artifacts from effects of predators, direct tests of cage effects in an environment free of predators were conducted, within a large predator exclosure. Cage artifacts had dramatic effects on one fish species but not the other. Moreover, the way the affected species responded to cages was exactly what would be expected if predators had strong negative effects on these fish: growth and survivorship were much lower on uncaged reefs than on caged reefs. To circumvent the cage effects, rather than compare uncaged reefs ( + predators) to reefs in exclosure cages (− predators), exclosures were compared to partial cages ( + predators). In the absence of predators, the partial cages did not differ from exclosure cages in their effects on the prey. Using the partial cages and exclosure cages in an area where predators were present, predator effects on the two fishes were tested. Survivorship of one species was greatly reduced by predators, but the other species was not affected. The lack of a predator effect on the one species was apparently an artifact of comparing completely caged reefs to partially caged reefs; another experiment documented strong predator effects on this species. Partial cages probably greatly reduced the ability of predators to consume the “unaffected” species. When cage artifacts affect prey, partial cages may be usefully employed to avoid confounding predator effects with cage effects at the cost of underestimating the magnitude of predator effects. Direct tests of cage effects are necessary to interpret the results of any study that uses cages to test for predatory effects.

Impact of Food and Predation on the Snowshoe Hare Cycle

Snowshoe hare populations in the boreal forests of North America go through 10-year cycles. Supplemental food and mammalian predator abundance were manipulated in a factorial design on 1-square-kilometer areas for 8 years in the Yukon. Two blocks of forest were fertilized to test for nutrient effects. Predator exclosure doubled and food addition tripled hare density during the cyclic peak and decline. Predator exclosure combined with food addition increased density 11-fold. Added nutrients increased plant growth but not hare density. Food and predation together had a more than additive effect, which suggests that a three-trophic-level interaction generates hare cycles.

Prédation on red squirrels during a snowshoe hare decline

We examined the extent and impact of predation on red squirrels (Tamiasciurus hudsonicus) during a cyclic decline of snowshoe hares in the southwestern Yukon, Canada. We monitored survival of squirrels on three control grids and a predator exclosure from March 1991 through August 1993. On controls, adult survival during the breeding season decreased from 1991, when snowshoe hare populations were high, to 1992, when hare populations declined rapidly. Survival increased slightly in 1993, when hare and predator populations were very low. Similarly, adult survival during winter was lower in 1992 – 1993 than in 1991 – 1992. Adult survival on the exclosure remained similar in each breeding season but declined during winter 1992 – 1993. Adult survival was similar on the controls and the exclosure in each year except during winter 1991–1992 and the 1992 breeding season, when it was lower on the controls. There was no difference in juvenile survival between the controls and the exclosure. Despite the decrease in adult survival due to predation, there was no population decline on any of the control grids. We conclude that predation did not have a measurable impact on red squirrel densities at Kluane and that it is unlikely red squirrels show 10-year population cycles in conjunction with snowshoe hares.

Does the activity of cockles, Cerastoderma edule (L.) and lugworms, Arenicola marina L., make Corophium volutator Pallas more vulnerable to epibenthic predators: a case of interaction modification?

In the field, both lugworms, Arenicola marina L., and cockles, Cerastoderma edule (L.), suppress the numbers of Corophium volutator Pallas. By aquarium-experiments it has been shown that this is not caused by direct lethal effects, but by stimulation of migration. In the field, the more serious reductions of Corophium numbers suggest an additional, lethal, factor. Both aquarium and field-experiments were performed to test the hypothesis that the induced migration enhanced vulnerability to epibenthic predators. Aquarium-experiments showed that survival of Corophium in the presence of shrimps as predators declined by an extra ~50% when also cockles and lugworms were present. Observations of the reaction of Corophium after disturbance showed a real flight behaviour, with swimming away and crawling on the sediment. The time spent out of the sediment was longer for adults ( ~ 5 min) than for juveniles ( ~ 2 min). Cage-experiments in shallow-water bays on the Swedish west coast showed positive effects of predator exclosure on Corophium survival and this effect was stronger in the presence than in the absence of cockles and lugworms. These positive effects were stronger for adult than for juvenile Corophium. In areas with low natural densities of Corophium volutator (but high numbers of lugworms and epibenthic predators), the introduction of lugworm-and predator-free exclosures resulted in Corophium densities comparable to those in (nearby shallower) areas with low natural lugworm and predator densities. The results of these experiments provide an example of interaction modification: the disturbing activities of cockles and lugworms alter the risk of Corophium to be taken by predators and restrict its abundance and distribution more than either factor (disturbance, predation) would do.

Effects of vertebrate predation on freshwater gastropods: an exclosure experiment

A field experiment with predator exclosures was performed in a pond in Southern Sweden to evaluate the effects of vertebrate predators on the diversity, density and distribution of freshwater gastropods. Ten exclosures (1 × 1 × 1.5 m) were erected in the nearshore vegetation at a waterdepth of 0.5–0.7 m in early June 1983 so that they included different amounts and species of macrophytes. The exclosures and cageless controls were sampled for macrophytes and gastropods at the end of the summer. The density of gastropods in the controls, exposed to predators, was positively correlated to the abundance of macrophytes. No such correlation existed in the exclosures. Further, the density of gastropods was significantly higher in the exclosures than predicted by the regression between gastropods and macrophytes in the controls. There was no difference between species composition of gastropods between controls and exclosures, but the number of species and the density of gastropods were lower in the macrophyte stands dominated byScirpus lacustris than in the more complex stands dominated bySparganium erectum. These results indicate that vertebrate predation is a major structuring force of benthic freshwater gastropod communities.

Population Dynamics of White-Tailed Deer (Odocoileus virginianus) on the Welder Wildlife Refuge, Texas

Reproduction among Welder Refuge deer was density independent at densities from 26 to 60 deer per km2. At densities up to 84 deer per km2 resulting from intensive predator control, nonlinear decreases occurred in reproductive rates. Timing of conception was inversely correlated with rainfall prior to and during the breeding season, presumably through a link with forage quantity and quality. Preimplantation and prenatal mortality was low. Fawn survival to 3 months of age was determined primarily by coyote predation. Fawn survival was correlated with rainfall during the gestation period. Adult mortality up to 4.5 years of age was higher among males than among females, even in the absence of selective removals by hunters. The result was a preponder- ance of adult females in the Welder Refuge herd. Basic population information for white-tailed deer, Odocoileus virginia- nus texanus, has been collected since 1962 on the 3,157-ha Rob and Bessie Welder Wildlife Refuge in south Texas. Early studies conducted on the Welder Refuge indicated that rainfall, and the resulting forage conditions, affected deer reproduction and mortality (Knowlton, 1964; White, 1966). Also documented was a high mortality rate among newborn fawns, primarily from predation by coyotes, Canis latrans (Cook et al., 1971). Recent research showed that intensive coyote control, begun in 1973 within a fenced, 391 ha predator exclosure, led to increased fawn survival and a substantial short- term increase in deer population density (Kie, 1977; Kie et al., 1979). Adverse changes in food habits and nutritional levels at the higher population densi- ties (Kie et al., 1980) resulted in poor body condition (Kie et al., 1983), reduced reproductive rates, increased mortality, and a population crash. The purposes of this paper are: 1) to summarize reproductive and mortal- ity patterns of deer on the Welder Refuge over the last 20 years and 2) to examine how these patterns were influenced by rainfall, population density, and other factors.

Condition Parameters of White-Tailed Deer in Texas

Physical condition in 2 herds of white-tailed deer (Odocoileus virginianus texanus) was assessed through postmortem analysis of weight, fat, antler, whole blood, and blood serum. One herd was existing at a normal population density and the other was artificially maintained at a high density within a 391-ha predator exclosure. Adult deer within the predator exclosure had lower values for bled carcass weight, eviscerated carcass weight, kidney fat index, femur marrow fat, paired adrenal gland weight, number of antler points, hemoglobin, serum creatinine, iron, albumin, and alkaline phosphatase than those outside the exclosure. Fawns within the exclosure had lower eviscerated carcass weights and higher relative adrenal weights and blood urea nitrogen levels. Body weights, kidney fat, and femur marrow fat were the best indicators of physical condition when postmortem sampling was done. J. WILDL. MANAGE. 47(3):583-594 Previous studies of white-tailed deer on the 3,157-ha Rob and Bessie Welder Wildlife Foundation Refuge in south Texas documented high mortality rates among newborn fawns, primarily because of coyote (Canis latrans) predation (Knowlton 1964, White 1966, Cook et al. 1971). An intensive predator control program, initiated in 1973 within a fenced 391-ha study area, was successful in increasing fawn survival. As a result, deer population density increased. Counts by helicopter in January 1975 yielded density estimates of 81 deer/km2 inside the predator exclosure, but only 40 deer/km2 on surrounding areas outside the exclosure. In 1976, estimated densities were 64 deer/km2 inside the predator exclosure and 49 deer/ km2 outside (Kie 1977, Kie et al. 1979). Deer inside the predator exclosure exhibited lower reproductive rates, delays in conception and parturition, longer antler velvet retention, earlier shedding of hardened antlers (Kie et al. 1979), greater grass and less forb consumption, lower levels of rumen-content crude protein, and increases in rumen calcium-to-phosphorus ratios (Kie et al. 1980). These differences suggested that deer within the exclosure were on suboptimal nutrition and were in poorer condition. Measures of body weight were among the 1st-used indicators of physical condition in deer. Body weight was related to range conditions by Johnson (1937), Park and Day (1942), Severinghaus (1955), and others. These authors also suggested that antler measurements were useful indicators of the relationship between range quality and deer numbers. Qualitative and quantitative measurements of fat stores have also been used to assess physical condition in deer and other mammals. However, fat may be deposited in several forms and locations throughout the body (Pond 1978). In the past, analyses have included examination of rump and other subcutaneous fat, total abdominal fat, fat surrounding the kidneys (Riney 1955), combined indices of subcutaneous and visceral fat (Kistner et al. 1980), lipid content of bone marrow (usually from the femur in ungulates) (Cheatum 1949), and blood serum lipid concentraIA contribution of the Pacific Southwest Forest and Range Experiment Station and the Welder Wildlife Foundation (WWF #271). J. Wildl. Manage. 47(3):1983 583 This content downloaded from 157.55.39.102 on Mon, 03 Oct 2016 06:25:35 UTC All use subject to http://about.jstor.org/terms 584 WHITE-TAILED DEER CONDITION PARAMETERS * Kie et al. Table 1. Monthly and weighted yearly mean age, weight, fat, and blood values of adult white-tailed deer outside and inside a predator exclosure, Welder Wildlife Refuge, July 1975-May 1976.

Disturbance and Community Structure in a Shallow Water Sand Flat

Diopatra cuprea, an onuphid polychaete, is a large and conspicuous component of semiprotected sand flats along the east coast of North America. It builds a large tube which projects above the surface of the sediment. Infaunal abundances are greater around such tubes than elsewhere. To test the hypothesis that tubes provide a local refuge from predation, I manipulated the abundances of the conspicuous predators and sediment disruptors, Callinectes sapidus the blue crab and Limulus polyphemus the horseshoe crab. In the absence of all predators >0.625 cm, infaunal abundances no longer increased as a function of Diopatra abundance. Thus, the tube of Diopatra appears to act as a refuge from predation for infauna. Further manipulations of the predators indicated that Callinectes was far more important than Limulus. Species which were numerically dominant in the presence of predators predominated in the predator removals as well. These experiments were repeated three times over a 3—yr period (1974, 1975, 1976) and in general the results were consistent over that period. Densities of juvenile Diopatra increased dramatically inside predator exclosures. Densities of adult Diopatra did not. The abundance of this refuge—forming organism, therefore, is at least partially dependent upon the frequency with which individuals pass through the size—dependent period of susceptibility.

Changes in Diet and Nutrition with Increased Herd Size in Texas White-Tailed Deer

A high-density deer herd within a 3Yl-ha predator exclosure on the Welder Wildlife Refuge was sampled to investigate diet and nutritional levels. Annual diet of the exclosure herd averaged 76% forbs, 21% grasses, and 3% browse. Deer from the surrounding area consumed 87% forbs, 10% grasses, and 3% browse. Fawns from both herds consumed less grass during the first 9 months of life. The exclosure herd also exhibited lower ruminal levels of crude protein, higher levels of calcium, and higher calcium to phosphorus ratios than the surrounding herd. It was hypothesized that with increased herd size, deer within the predator exclosure overutilized the most desirable forb species and were forced to consume more grasses. The resulting decrease in nutritional level was responsible for changes in health, condition, and population parameters.