Compensatory Predation Research Articles

Cumulative Effects and Boreal Woodland Caribou: How Bow-Tie Risk Analysis Addresses a Critical Issue in Canada's Forested Landscapes

Boreal caribou (Woodland Caribou, boreal population; Rangifer tarandus caribou) is a prominent mammal at the heart of a decades-long conflict between a growing resource sector and the associated risks to biodiversity. We employed the ISO 31010 Bow-tie Risk Assessment Tool (BRAT) to evaluate the cumulative effects of anthropogenic and natural factors that may affect the risk of self-sustainability to the boreal caribou herds of Northeastern British Columbia. We used the BRAT to produce a visual synthesis of the cumulative effects causing the growth rate of boreal caribou herds to persistently fall below a level corresponding to a 60% chance of being self-sustaining (λ < 1.025). The BRAT diagram provided the basis for a quantitative Layers of Protection Analysis (LOPA) of risk probabilities for three caribou herds. We combined threat assessments from the Species at Risk Act recovery strategy (Environment Canada 2012) with data from published landscape experiments (e.g. restoration of seismic traces, maternal penning, and wolf culls) to parameterize the LOPA in three study areas. We report the implications of a combination of mitigation options versus current risk conditions, as well as the implications of uncertainty in threat prevention. Our analysis indicates that a combination of mitigation scenarios will best facilitate caribou herd recovery, that barriers preventing predation threats could also aid in recovery success, and that compensatory predation may account for a significant proportion of both adult and juvenile female mortality across different herds. We estimated the minimum annual cost for effective mitigation and recovery to be $CDN 224K within any of the study areas. Bow-tie diagrams are a flexible and quantifiable tool that can translate resource management solutions to the diverse audience involved in conservation decision-making: scientists, land managers, policy makers, and concerned stakeholders.

Reducing nest predation of ground-nesting birds through conditioned food aversion

Populations of many ground-nesting bird species have declined substantially due to several factors, and predation can be a leading contributor to these declines. As a method for reducing the nest predation on ground-nesting birds, we tested whether conditioned food aversion (CFA) can reduce red-legged partridge (Alectoris rufa) nest predation by red foxes (Vulpes vulpes). With a before-after control-impact design we deployed 1329 artificial nests in two different habitats in Central Spain using thiram as an aversive. Artificial nests were monitored by camera traps to identify the red fox individuals during the predation events. 26 foxes were GPS-tagged to monitor their spatial movements and feeding behavior. Partridge density and productivity were monitored to assess the thiram treatment effect on partridge population. Thiram treatment decreased artificial nest predation by foxes in both study sites by 26.8–50.1%, but this was compensated by an increased predation by other predators, possibly enhanced by the availability of our artificial nests. 78% of identified foxes that ingested thiram-treated eggs, stopped nest predation after treatment. Foxes maintained stable territories during the whole study period. Partridge productivity was 132–677% higher in thiram-treatment areas than in control areas, and partridge density after treatment increased more in thiram-treated areas (193–292%) than in control areas (1.8–99%). Our study shows that CFA reduced ground nest predation by foxes, and had a positive effect on the partridge population despite the compensatory predation. This method could be used as a non-lethal tool for conservation of endangered ground-nesting bird species.

Effect of predation on adult pronghornAntilocapra americana(Antilocapridae) in New Mexico, Southwestern USA

AbstractThe effect of predation on ungulate populations remains contentious, despite a lack of evidence showing impacts in arid Southwestern USA populations where low precipitation and frequent drought limit ungulate nutritional condition. These conditions can increase predisposition of prey to mortality, which is prerequisite for predation to be compensatory. Consequently, we tested the effect of predation on adult pronghornAntilocapra americana(Ord 1815) in two populations in arid New Mexico by modeling transformed annual survival rates as a function of predation rates. For this conservative test, a slope=0 indicates complete compensation, whereas a slope=−1 indicates complete additivity. The corrected slope of mortality potentially attributable to predation was &gt;−0.14, and this result was consistent among individual populations. Thus, predation was primarily compensatory. Primarily compensatory predation was related to the relatively low condition of pronghorn individuals, as predated individuals were all below the mean condition of the population, similar to results seen in previous tests of the compensatory versus additive predation hypothesis in the arid Southwest USA. Conditions that predispose individual ungulates to mortality are present more often than not in arid environments, and thus managers should not assume that predation is limiting, regardless of predation rates.

Compensatory puma predation on adult female mule deer in New Mexico

Pumas ( Puma concolor ) are the primary predator of adult mule deer ( Odocoileus hemionus ) throughout most of arid New Mexico, and predation by pumas is popularly believed to limit mule deer populations. Predation can limit population size of prey if it is primarily additive or acts in addition to other mortality factors. Thus, clarifying the impact of predation is important because of misconceptions regarding the act of predation (i.e., killing an individual) rather than the effect (i.e., effects on population demography) cloud discussions of the issue. Herein, we tested the population-level question of whether puma predation on adult female mule deer is compensatory or additive mortality in multiple mule deer populations in New Mexico. We modeled annual survival rate as a function of cause-specific mortality rate. For this conservative test, a slope = 0 indicated complete compensation, whereas a slope = −1 indicated complete additivity. For all populations, the corrected slope of predation ultimately attributable to pumas was 0.00, ultimate and proximate puma predation combined was 0.10, ultimate puma predation and unknown causes of mortality combined was −0.28, and ultimate and proximate puma predation and unknown deaths combined was −0.23. Thus, puma predation appeared primarily compensatory. Despite small sample sizes, individual populations showed similar patterns indicating the compensatory nature of puma predation. Primarily compensatory predation for mule deer reflected the relatively low condition of deer. Survival was most strongly tied to individual condition in these populations, and a priori condition of puma-predated mule deer was lower than the population means. Compensatory mortality requires predisposition whether caused by decreased condition, high population density, or other factors. If conditions that predispose individual deer to mortality are present, managers should not assume that predation is limiting, regardless of predation rates.

A review of the impacts of corvids on bird productivity and abundance

Corvids are often viewed as efficient predators capable of limiting prey species populations. Despite this widely held belief, a comprehensive review quantifying the effect of corvids on the demography of prey species is lacking. We examine the impacts of crows, ravens Corvus spp. and Eurasian Magpies Pica pica on the population parameters of other bird species. We summarize results from 42 studies, which included 326 explicit evaluations of relationships between a corvid and a potential prey species. Population parameters of studied prey species were categorized as abundance‐related (numbers, nest density) or productivity‐related (nest success, brood size). Information from both experimental removal studies and correlative studies was examined. Combining all studies, no negative influence of corvids on either abundance or productivity of prey species was found in 81% of cases. Negative impacts were significantly more likely in cases examining productivity rather than abundance (46 vs. 10%). Experimental studies that removed only corvid species were significantly less likely to show a positive impact on productivity than those removing corvids alongside other predators (16 vs. 60%). This suggests that the impact of corvids is smaller than that of other predators, or that compensatory predation occurs. The impact of corvids was similar between diverse avian groups (such as gamebirds, passerines and waders; or ground‐nesting and other species). Crows were found to be significantly more likely to have a negative impact on prey species productivity than were Magpies (62 vs. 12%), but no differences were found in relation to prey abundance. We conclude that while corvids can have a negative impact on bird species, their impact is small overall, and nearly five times more frequent for productivity than for abundance. These results suggest that in most cases bird populations are unlikely to be limited by corvid predation and that conservation measures may generally be better targeted at other limiting factors. However, negative impacts were found in a minority of cases, and those may require further investigation to develop management tools to mitigate such impacts where they are of economic or conservation concern.

Salt marshes

Salt marshes are unique ecosystems that are brimming with diversity, and provide crucial shelter to coastal communities.

Absence of effects of predator control on nesting success of Northern Lapwings Vanellus vanellus: implications for conservation

The control of generalist predator populations is increasingly adopted as a management tool to combat declines in ground-nesting bird populations. However, compensatory predation by uncontrolled species frequently occurs, so determining the relative impacts of different predatory species, and hence the relative benefits of their control, can be difficult. Islands, with their reduced faunas, provide natural experimental units for investigating specific predator–prey interactions in detail. We studied Northern Lapwing Vanellus vanellus breeding success on an island where feral Ferrets Mustela furo and Hooded Crows Corvus cornix were subjected to trial control regimes over 2 years. In both years, Lapwing hatching success was >80%, with neither Ferret nor Crow control selected as important predictors. Fledging rates in both years were 0.7 young per pair, despite highly effective predator removal, although Crow control potentially resulted in compensatory predation by Common Ravens C. corax. Neither mustelid nor corvid control produced significant immediate benefits for Lapwings. This suggests that mesopredator release of mustelids in mainland situations is unlikely to be a consistent threat to Lapwing, and provides further evidence that declines in this species are unlikely to be tackled successfully through predator management alone.

Effects of mesopredators and prescribed fire on hispid cotton rat survival and cause‐specific mortality

AbstractControl of mid‐sized mammalian predators (hereafter, mesopredators) is sometimes advocated in an attempt to reduce their impact on wildlife populations, particularly economically important (i.e., game) or endangered species. However, mesopredators may play a role in regulating small mammal populations; thus, lethal control of mesopredators may have unintended consequences. The hispid cotton rat (Sigmodon hispidus; hereafter, cotton rat) is one of the most common small mammals in the southeastern United States and is an important prey species for several of the region's predators. Within fire‐maintained communities, such as the longleaf pine (Pinus palustris) forests of the Coastal Plain, cotton rat populations dramatically, yet temporarily, decline following prescribed fire. To evaluate the effects of mesopredator removal on cotton rat survival and cause‐specific mortality, we conducted a large‐scale mesopredator exclusion experiment that incorporated a prescribed fire during the winter of study. Between 18 May 2006 and 20 June 2007, we used radio‐telemetry to monitor 252 cotton rats (131 in exclosures and 121 in controls) and documented 184 mortalities. During the 37‐week period of monitoring prior to the prescribed fire event, weekly survival of cotton rats was greater in mesopredator exclusion plots. During the 19 weeks following the prescribed fire, there were no differences in weekly survival relative to mesopredator treatment, but fire caused a short‐term reduction in weekly survival within both exclosures and controls. Of 36 cotton rats monitored on the date of prescribed fire, 18 were depredated within 1 month, 4 emigrated, and 5 were killed by the fire event. Overall, raptors preyed on cotton rats more in exclosures than in controls, but evidence for compensatory predation (raptor‐caused morality greater in exclosures than in controls although survival rates were similar between treatments) only occurred following the prescribed fire event. Our results suggest that managing mesopredators may result in a temporary increase in cotton rat survival, but dormant season prescribed fire removes this effect until well into the following growing season. © 2011 The Wildlife Society.

The effect of disturbance on an ant–plant mutualism

Protective ant–plant mutualisms—where plants provide food or shelter to ants and ants protect the plants from herbivores—are a common feature in many ecological communities, but few studies have examined the effect of disturbance on these interactions. Disturbance may affect the relationship between plants and their associated ant mutualists by increasing the plants’ susceptibility to herbivores, changing the amount of reward provided for the ants, and altering the abundance of ants and other predators. Pruning was used to simulate the damage to buttonwood mangrove (Conocarpus erectus) caused by hurricanes. Pruned plants grew faster than unpruned plants, produced lower levels of physical anti-herbivore defenses (trichomes, toughness), and higher levels of chemical defenses (tannins) and extrafloral nectaries. Thus, simulated hurricane damage increased plant growth and the amount of reward provided to ant mutualists, but did not have consistent effects on other anti-herbivore defenses. Both herbivores and ants increased in abundance on pruned plants, indicating that the effects of simulated hurricane damage on plant traits were propagated to higher trophic levels. Ant-exclusion led to higher leaf damage on both pruned and upruned plants. The effect of ant-exclusion did not differ between pruned and unpruned plants, despite the fact that pruned plants had higher ant and herbivore densities, produced more extrafloral nectaries, and had fewer physical defenses. Another common predator, clubionid spiders, increased in abundance on pruned plants from which ants had been excluded. I suggest that compensatory predation by these spiders diminished the effect of ant-exclusion on pruned plants.

Effects of Mesopredators on Nest Survival of Shrub‐Nesting Songbirds

ABSTRACT Although nest predation is often the single largest source of mortality in avian populations, manipulative studies to determine predator impacts on nest survival are rare, particularly studies that examine impacts of mid‐size mammalian predators (hereafter, mesopredators) on nest survival of shrub‐nesting birds. We quantified nest survival and identified nest predators of shrub‐nesting songbirds within 4 large (approx. 40‐ha) exclosures and 4 control sites within a longleaf pine (Pinus palustris) ecosystem. During 2003–2006, we located and monitored 535 shrub nests (222 with videography) for 4,804 nest‐days to quantify daily nest survival and document predation events. We found no support for a treatment effect, suggesting mesopredators had little impact on daily nest survival (0.9303 in controls and 0.9260 in exclosures) of shrub‐nesting songbirds. For the 5 most commonly monitored species, daily nest survival within species was constant. Our analysis suggested that shrub nests were most vulnerable during the nestling stage and presence of cameras on nests increased survival with the increase in survival being more pronounced during the incubation stage. We filmed 107 nest predation events, identifying predators at 88 nests. Of these 88 nests, snakes caused 33%, red imported fire ants (hereafter fire ants, Solenopsis invicta) 28%, raptors 17%, corvids 8%, mesopredators 6%, and small mammals 8% of nest predations. Cause‐specific nest predation in controls and exclosures did not differ from expectation, providing evidence that compensatory predation did not occur. Nest predators differed from expectation with regard to nest stage; fire ants and raptors only depredated nests during the nestling stage. Presence of cameras had no effect on nest abandonment. Fire ants were the most prevalent nest predator, and nest predation by fire ants was only observed on nestlings, potentially reducing likelihood of renesting. Magnitude and timing of fire ant predation suggests that fire ants may be the most influential nest predator of shrub‐nesting birds within the longleaf pine ecosystem. Our data suggest that controlling mesopredators will have no effect on nest success of shrub‐nesting birds within longleaf pine forests.

ALTERATION OF ISLAND FOOD-WEB DYNAMICS FOLLOWING MAJOR DISTURBANCE BY HURRICANES

Major abiotic disturbance can be an important factor influencing food-web dynamics, particularly in areas impacted by the recent increase in hurricane activity. We present a unique set of data on key food-web processes occurring on 10 small islands for three relatively calm years and then four subsequent years during which two hurricanes passed directly over the study site. Herbivory, as measured by leaf damage, was 3.2 times higher in the year after the first hurricane (2000) than in the previous year and was 1.7 times higher in the year after the second hurricane (2002) than in 2001. The effect of a top predator (the lizard, Anolis sagrei) on herbivory strengthened continuously after the first hurricane and overall was 2.4 times stronger during the disturbance period than before. Overall abundance of lizards was 30% lower during the disturbance period than before, and abundances of web spiders and hymenopteran parasitoids were 66% and 59% lower, respectively. We suggest that increased herbivory observed on all islands was caused, at least in part, by the overall reduction in predation by both lizards and arthropods, whereas magnification of the lizard effect on herbivory was caused by reduced compensatory predation by arthropods.

The Effect of Supplemental Prey and Prescribed Fire on Success of Artificial Nests

Nest predation hinders recruitment of avian species and may be affected by availability of alternative prey and amount of nesting cover. Therefore, we evaluated effects of food abundance (i.e., supplemental prey) and time since prescribed fire on nest success of artificial ground nests. We monitored the fate of 759 artificial ground nests from June to July 2000. No interaction (P= 0.74) occurred between fire and supplemental prey treatments. Nest success in prey-supplemented plots (37.6%) did not differ (P= 0.70) from control plots (44.9%), and nest success in burned plots (41.8%) did not differ (P = 0.86) from nonburned plots (40.7%). Motion-sensitive cameras placed on feeders revealed that mesomammals accounted for >80% of visits to feeders, indicating that supplemental prey was detected and consumed by mesomammals. Nest predators differed as a function of food abundance, with combined avian and small-mammal predation being greater in prey-supplemented (46.5%) than in control (25.9%) plots. Nest predators also differed as a function of prescribed fire Avian predation of nests was greater in burned (13.7%) than nonburned (9.9%) plots. whereas small-mammal predation was greater in nonburned (30.9%) than in burned (15.1%) plots. Altering food and cover to manage nest success may result in compensatory predation. Further work to quantify the extent of compensatory predation is needed to fully understand trade-offs of various practices for managing nest predation.

PREDATION ON A COMMON ANOLIS LIZARD: CAN THE FOOD-WEB EFFECTS OF A DEVASTATING PREDATOR BE REVERSED?

We artificially invaded a set of small islands with the large lizard Leioce- phalus carinatus to determine effects on food-web elements including an intermediate predator, the lizard Anolis sagrei; the latter was previously found to have major, mostly direct effects on web spiders, as well as detectable indirect effects on aerial arthropods (including parasitoids) and leaf damage. In addition to these food-web elements, we mon- itored ground-surface arthropods; they are expected to be affected directly, as well as indirectly via A. sagrei ,b yL. carinatus. Five islands were randomly selected for invasion while six others served as controls. In addition, four islands without A. sagrei were mon- itored to determine the natural state of the sagrei-free food web. We also monitored a variety of A. sagrei's traits, from behavioral through physiological and demographic to morphological, to elucidate the mechanisms whereby L. carinatus might affect the smaller lizard and to evaluate trait-mediated as well as density-mediated effects. Comparison of unmanipulated islands with and without A. sagrei showed that A. sagrei appeared to be affecting web-spider density and diversity, numbers of small aerial arthro- pods, number of large ground-surface arthropods, and two types of leaf damage, scars and mines. All effects were negative save one: Small aerial arthropods were more abundant with A. sagrei; this variable and parasitoid abundance were negatively related to web-spider density, implicating an indirect effect pathway from A. sagrei via web spiders. Introduction of L. carinatus had major and immediate effects on A. sagrei density and perch height; effects on perch diameter, percentage hatchlings, and the width of the body- size distribution followed over time. Behavioral shifts in habitat use continued to the end of the experiment, after changes in population density had mostly leveled off. Adult-male body condition became poorer after introduction of L. carinatus. Among islands, percentage use of the ground was correlated negatively with percentage vegetated area of the island and positively with premanipulation relative hindlimb length. We hypothesize that longer- legged individuals do better on the ground because they are faster there. Conditional ev- idence for an increase in adult-male relative hindlimb length suggested that, when the ground-inhabiting L. carinatus was introduced, such individuals were differentially able to escape predation. Introduction of L. carinatus significantly reversedA. sagrei's effect on number of species and density of web spiders but had no other major effects on lower level food-web elements. Thus, a relatively short-chained (two links) effect, with a relatively strong second link ( A. sagrei on spiders), showed the greatest indirect effect of the manipulation. Another poten- tially strong indirect effect, that on large ground-surface arthropods, did not occur, possibly because of compensatory (direct) predation by L. carinatus. Thus presence of omnivory may also be relevant to whether reversal of effects occurs. Although most A. sagrei effects were not reversed in the time available for the experiment, this may be typical, as natural L. carinatus colonizations can dwindle away to extinction over about the same time span, producing at best only short-term indirect effects. Population-density estimates from other sites and comparisons to other experimental studies suggest that, in general, predators on Anolis may have erratic effects in space and time.

Effects of Red Squirrel (Tamiasciurus hudsonicus) Removal on Survival of Artificial Songbird Nests in Boreal Forest Fragments

The importance of red squirrels (Tamiasciurus hudsonicus) as predators of songbird nests in the boreal forest was determined by monitoring survival of artificial nests before and after squirrel removal. In eight conifer-dominated forest fragments surrounded by agricultural fields we placed artificial ground and shrub nests along the edge and interior of each fragment. Nests contained one quail and one clay egg, which were monitored for 12 d. We then removed squirrels from four forest fragments and repeated the experiment. Nests located in reference plots, where squirrel numbers were not manipulated, had similar survival rates among trials (2 ± 7% change between trials). In contrast, the percentage of quail eggs surviving 12 d increased 32 ± 9% after squirrels were removed. The survival of clay eggs increased 22 ± 10% after squirrel removal, although this was not significantly different from changes in clay egg survival between trials in reference plots (−2 ± 6%). The increase in nest survival when squirrels were removed was consistent for nests on edge and interior transects and ground and shrub nests. Unlike previous studies, we did not observe compensatory predation by other predators, although the relative frequency of mice destroying nests increased after squirrel removal.

Kleptoparasitism of Herring Gulls Taking Eider Eggs by Canada Geese

Nesting in association with species that aggressively attack predators may reduce nest predation in some birds. For example, nesting ducks sometimes benefit from aggressive defence by nesting Canada Geese (Branta canadensis). Although Common Eiders (Somateria mollissima) commonly nest with gulls (Larus spp.), the costs and benefits for eiders of this association remain uncertain. Over two years, 32 instances of kleptoparasitism of Herring Gulls (Larus argentatus) by Canada Geese were recorded in arctic Canada. The Canada Geese nested amongst Common Eiders and interrupted Herring Gulls while taking eider eggs. The geese displaced Herring Gulls from approximately ten percent of all eider eggs taken from nests, and ate egg contents themselves. Kleptoparasitism may provide Canada Geese with an important exogenous food resource, but the small overall number of eggs stolen from gulls (less than five percent of all eider eggs laid) limits the potential consequences of additive compensatory predation pressure by gulls within the eider colony.

DIRECT AND INDIRECT EFFECTS OF PREDATION BY COMMON EIDERS AND ABIOTIC DISTURBANCE IN AN INTERTIDAL COMMUNITY

Predation and disturbance have been well studied in intertidal communities. However, the impact of vertebrate predators, particularly waterfowl, has been only infre- quently determined in long-term intertidal studies. Using predator exclusion cages and simulated abiotic disturbance, I studied the direct and indirect effects of predation by Common Eiders (Somateria mollissima) on an intertidal mussel bed community in Passa- maquoddy Bay, New Brunswick, Canada. Eiders fed heavily on blue mussels (Mytilus edulis), the dominant invertebrate present, reducing their abundance in ?+duck (exposed to predation) relative to -duck (exclosure) plots by nearly 50% within eight months of initiation of the experiment. Based on counts and estimated consumption rates, eiders appeared to be responsible for most predation observed in this system. Although ducks fed heavily on the dominant intertidal invertebrate, they had little effect on species diversity or richness in the community. However, relative abundance of the most common invertebrates did vary. In particular, exclusion of eiders led to an increase in abundance of dogwhelks (Nucella lapillus) one year into the experiment. Whelks, in turn, fed heavily on mussels under cages and obscured the longer-term effect of ducks in the system. This indirect effect appears to be an example of asymmetric exploitation compe- tition, with ducks influencing the food supply of whelks, but whelks having little effect on ducks. When ducks were excluded, whelks were released from this competition and acted as compensating predators. No other indirect effects developed following duck exclusion, presumably due to the increased whelk effect. Disturbance, in contrast, did lead to an increase in species diversity, which later returned to predisturbance levels as the community recovered. Predation delayed the recovery of disturbed sites, because ducks began feeding in these plots before mussel abundance had completely rebounded. Disturbance, while initially deterring predation somewhat, ulti- mately allowed the effects of predators to persist longer. This interaction of predation and disturbance resulted from compensatory growth of mussels under exclosures in disturbed sites. Mussels protected from eider predation grew quickly after disturbance and rapidly became larger than the preferred prey for whelks. As a consequence, whelks did not feed as heavily on mussels in disturbed sites as in undisturbed sites, where more mussels were of the preferred size, and the observed effect of ducks on mussel biomass persisted. This experiment demonstrates that eiders were significant, and, probably, keystone pred- ators in this system. Eiders directly reduced abundance of mussels, thereby indirectly increasing whelk density in plots where ducks were excluded. Compensatory predation by whelks following duck exclusion blocked other potential indirect effects, thus preventing eiders from having more widespread effects in the system. Such compensatory predation may act to stabilize communities and evidently can occur in relatively simple systems as well as the more species-rich communities with which it is usually associated.

Effects of removing duck-nest predators on nesting success of grassland songbirds

During 1995 and 1996 we monitored natural and simulated nests of grassland songbirds following removal of duck-nest predators, primarily raccoons (Procyon lotor), striped skunks (Mephitis mephitis), and red foxes (Vulpes vulpes), in North Dakota, U.S.A. We were unable to detect an effect of predator removal on survival of natural (n = 153 nests) or simulated (n = 2264 nests) nests of grassland songbirds in either year. However, the assemblage of predators destroying simulated nests differed between removal and non-removal sites in 1995, when medium-sized carnivores and small mammals were the most important predators on non-removal sites, whereas ground squirrels were most important on removal sites. Our study indicates that grassland songbirds were not negatively affected, at least in the short term, by removal of duck nest predators. However, changes in the relative importance of predators on removal sites in 1995 suggest that smaller predators that were not targeted by removal efforts (i.e., ground squirrels) may have caused compensatory predation. Long-term studies are necessary to determine if the removal of duck-nest predators affects the numerical or functional response of predators at a lower trophic level, such as ground squirrels and other small mammals.

Effects of Top and Intermediate Predators in a Terrestrial Food Web

To determine the effects of lizards and web spiders on species in lower trophic levels, we manipulated their abundances within large field enclosures on Staniel Cay, Bahamas, from 1989 to March 1992. The experimental design (2 x 2 factorial) measured the separate effects of lizards and spiders and compensatory predation (lizard x spider interaction). In treatments where web spiders were unaltered, mean number of web spider individuals was 1.4 times higher in enclosures with lizards removed than in those with lizards present at natural densities. Total biomass of aerial arthropods caught in sticky traps was 1.4 times higher in treatments with web spiders removed than in treatments with web spiders unaltered. Lizards had no significant effect on aerial arthropods. Total amount of herbivore damage on sea grape leaves was 3.3 times higher in treatments with lizards removed than in treatments with lizards unaltered. Web spiders had no significant effect on leaf damage. The lizard x spider interaction was not significant in each analysis, indicating that compensatory predation was weak. Our results support a model in which the interaction between top predators (lizards) and herbivores is strong, whereas the interaction between intermediate predators (web spiders) and herbivores is weak. Consequently, the net effect of top predators on producers is positive.