High Predator Densities Research Articles

Can extreme morphology in Bosmina reduce predation risk from Leptodora ? An experimental test

Bosmina (Cladocera) populations, especially within the subgenus Eubosmina, show a variety of phenotypes that exhibit large differences in body size and shape and antennule length. In some populations, the morphological traits also vary during the season, with the most extreme forms occurring in periods with high densities of certain invertebrate predators. However, while temporal phenotypic variation in other cladocerans, as in the family Daphnidae, has been shown to be an adaptation to reduce the risk of predation by invertebrate predators, the reason for such changes in Bosmina is much less clear. We examined whether certain morphological traits in Bosmina species could act as a defence against invertebrate predators. We tested three Bosmina forms (subgenus Eubosmina), differing in morphology from each other, which are found in lakes together with the predator Leptodora kindtii (Cladocera). Bosmina (E.) longispina has a relatively low and elongated carapace with a caudal mucro, and short antennule, B. (E.) coregoni gibbera has a higher and more protruding carapace without caudal mucro, and a much longer antennule. Finally, B. (E.) coregoni retro extensa has a carapace like that of B. longispina but with no caudal mucro and a much longer antennule. In one experiment, B. longispina and B. gibbera were exposed for 12 h to Leptodora in Petri dishes. In a second experiment, we observed directly the escape efficiency of B. longispina, B. gibbera and B. retro extensa, and the handling time of Leptodora. The two Bosmina forms with more extreme morphological features had a lower death rate and higher escape efficiency than B. longispina. Prey that escaped did so, in most cases, within 5 min. Predator handling time was correlated to predator body length and antennule length of the prey. The results suggests that Bosmina species with extreme morphological traits may be less vulnerable to invertebrate predators.

Spatial distribution and coincidence of Neoseiulus californicus and Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae) on strawberry

In strawberry greenhouses in La Plata (Buenos Aires, Argentina), Tetranychus urticae is a major pest and Neoseiulus californicus is its most important established phytoseiid predator. The purpose of this study was to determine the spatial distribution and coincidence of N. californicus and T. urticae on strawberry. T. urticae populations exhibited density variations that were followed, with some temporal delay, by those of the predator. In general terms, N. californicus exhibited lower aggregation than T. urticae. The index of dispersion (I) of T. urticae had values significantly greater than 1 in 100% of the sampling dates. The percentage of infested leaflets increased with prey density in a curvilinear way, ranging from 80 to 100% when density was higher than 100 individuals/leaflet. N. californicus had values of I higher than unity in 86.7% (G1) and 53.8% (G2) of the cases. TaylorOs b and IwaoOs β values were greater than 1 for both populations, this indicating aggregation. IwaoOs results showed larger aggregation units of T. urticae (α=53.24) than of N. californicus (α=3.61), probably due to its higher fecundity and oviposition behaviour of laying eggs in clumps. Both populations were overdispersed, although in general terms, N. californicus exhibited lower aggregation than T. urticae. This would create refuges for the prey, thus increasing the persistence of the system. Index of coincidence of predator (Ic) was high most of the time even at low densities of both populations, decreased abruptly at the end of an interaction cycle and was similar at greenhouse and plant spatial scales. The high spatial coincidence of N. californicus with T. urticae suggests an important dispersal capacity of the predator and a high ability to detect leaflets with prey. The refuge index for the prey decreased with predator density and was higher at greenhouse scale than at plant scale, both at similar predator densities. At greenhouse scale and despite high predator density, the prey had a 10%-refuge, which would lead to the system persistence. Results of the present study suggest that N. californicus is a promising established natural enemy for controlling T. urticae on strawberry.

Predictable changes in predation mortality as a consequence of changes in food availability and predation risk

Theory predicts that animals will have lower activity levels when either the risk of predation is high or the availability of resources in the environment is high. If encounter rates with predators are proportional to activity level, then we might expect predation mortality to be affected by resource availability and predator density independent of the number of effective predators. In a factorial experiment, we tested whether predation mortality of larval wood frogs, Rana sylvatica, caused by a single larval dragonfly, Anax junius, was affected by the presence of additional caged predators and elevated resource levels. Observations were consistent with predictions. The survival rate of the tadpoles increased when additional caged predators were present and when additional resources were provided. There was no significant interaction term between predator density and food concentration. Lower predation rates at higher predator density is a form of interference competition. Reduced activity of prey at higher predator density is a potential general mechanism for this widespread phenomenon. Higher predation rates at low food levels provides an indirect mechanism for density-dependent predation. When resources are depressed by elevated consumer densities, then the higher activity levels associated with low resource levels can lead to a positive association between consumer density and consumer mortality due to predation. These linkages between variation in behaviour and density-dependent processes argue that variation in behaviour may contribute to the dynamics of the populations. Because the capture rate of predators depends on the resources available to prey, the results also argue that models of food-web dynamics will have to incorporate adaptive variation in behaviour to make accurate predictions.

Nutritional condition and starvation in Sardina pilchardus (L.) larvae off southern Portugal compared with some environmental factors

The aim of this study was to determine the condition and the incidence of starvation among field-caught Sardina pilchardus larvae, using RNA/DNA ratios and to compare them with various abiotic and biotic factors. S. pilchardus larvae were collected off southern Portugal, during April and May 1992. A highly sensitive fluorometric method for quantifying larvae nucleic acid was applied. Sardine larvae were generally in good condition. The means of the RNA/DNA ratios were relatively high and low percentages of starving larvae (RNA/DNA ratio <1.3) were recorded, ranging from 0 to 12.5%. These two results were associated with high prey availability, which contributes to enhanced condition, and with a high density of potential predators, which may result in fewer larvae in poor condition.

Cannibal–prey dynamics in young juveniles and postlarvae of the blue crab

Although cannibalism can act as a density-dependent regulator of population size in terrestrial systems, little is known of its effects in the marine environment. Herein we investigate the influence of cannibalism upon the early life history stages of the blue crab, Callinectes sapidus Rathbun, emphasizing cannibalism between juveniles and postlarvae (i.e. megalopae) of the same year class. In laboratory mesocosms we examined various factors modulating cannibal–prey dynamics, specifically: (1) the effects of habitat and presence of conspecifics on postlarval metamorphosis rate; (2) the effect of metamorphosis rate on the mortality of postlarvae from both intra- and inter-cohort cannibalism; (3) the effects of habitat and predator density on the functional response of young juvenile blue crab predators to varying densities of postlarval prey, and (4) the effects of prey size and habitat on predation mortality.Inter-cohort cannibalism caused significant mortality in every crab size and habitat type combination, and was lower in grass than sand for all prey smaller than fifth instar. Cannibalism between postlarvae was associated with metamorphosis and was density-dependent in sand, but not present in grass. Metamorphosis rates of postlarvae were inversely density-dependent in sand, but density-independent and higher in grass, indicating that habitat and intra-cohort agonism likely affects postlarval metamorphosis rates. Inter-cohort cannibalism was negatively correlated with metamorphosis rates of postlarvae. The functional response of young juvenile cannibalistic blue crabs differed significantly between sand and grass habitats, and between medium and high predator densities. Juvenile crabs displayed a type II, inversely density-dependent functional response in sand, resulting in very high mortality at low densities of postlarval prey. In grass, the crabs displayed a weak type III, density dependent response, yielding significantly lower mortality at low prey densities. Thus, habitat complexity changes the form of the functional response in cannibal–prey interactions and grass provides a relative habitat refuge from cannibalism. Doubling the number of predators in grass decreased the consumption rates per predator significantly and eliminated the density-dependence, indicating that intraspecific density can qualitatively change the form of the functional response. In the crab size experiment, only prey smaller than fifth instars received a habitat refuge from cannibalism in grass, whereas fifth instars received a relative size refuge in sand.Our results demonstrate that intra-year class cannibalism can cause mortality upon settling megalopae and first juvenile instars that is dependent on prey density. We expect inter-cohort cannibalism to cause local extinction of cohorts settling in sand, especially at low settlement densities, and high mortality at moderate settlement densities in grass. Satiation of predators at high settlement densities in grass suggests that episodic settlement can overwhelm predators locally. Furthermore, density-dependent mutual interference within large cohorts in the grass beds likely reduces their predation efficiency, indicating that aggregation of conspecific predators in grass habitats does not necessary lead to an increase in predation pressure. Finally, a relative size-refuge from inter-cohort cannibalism for fifth instar crabs supports an ontogenetic habitat shift around this crab size, which may be influenced by density-dependent agonistic behavior within cohorts. We suggest that intra-year class cannibalism is a major process regulating both survival and dispersal in megalopae and juvenile blue crabs.

Population Limitation in Arctic Ground Squirrels: Effects of Food and Predation

1. We examined the relative importance of food and predators in limiting Arctic ground squirrel (Spermophilus parryii plesius Richardson) populations in the boreal forest of the southwestern Yukon during the peak and early decline of a snowshoe hare cycle (Lepus americanus Erxleben). 2. Squirrels were live-trapped from 1990 to 1992 on two control grids and three experimental treatments (food addition, mammalian and avian predator exclosure, and food addition plus mammalian predator exclosure). Adult squirrels were radiocollared on all areas in 1992. 3. Food addition increased densities 3-8 times, generally increased reproductive traits (increased proportion of females lactating, doubled recruited litter sizes, resulted in earlier emergence of juveniles), increased immigration rates (but only in 1992), resulted in heavier females though not males at emergence in spring, and resulted in more rapid growth rates of juvenile males, but not of juvenile females. It had no effect on active season or overwinter survival rates. 4. Exclusion of predators had virtually no effect on any demographic variable measured, except for population densities in 1991 when they were approximately double those of the control populations. 5. Food addition plus exclusion of mammalian predators resulted in demographic changes that were comparable to those of food addition alone. 6. Thus, it appeared that food, not predators, limited ground squirrel populations at this stage of the hare cycle. However, independent of experimental treatment, active season survival of adult squirrels declined markedly from 1990 (high hare numbers) to 1992 (low hare numbers). Most of the radiocollared squirrels disappearing in 1992 were killed by predators and this was coincident with high densities of predators. In the predator exclosures, all predation mortalities resulted from avian predators which we could not exclude. 7. We conclude that both food and predators interact to limit Arctic ground squirrel populations during the peak and early decline of the hare cycle.

Predators Use Volatiles to Avoid Prey Patches with Conspecifics

1. Simple models of optimal foraging, such as ideal free distribution models, are based on the assumption that foragers are omniscient with respect to the quality of all patches in the environment; they know how much food and how many competitors are present in each patch. 2. In contrast, simple population dynamic models treat predator-prey distributions in a phenomenological way, and do not take fitness consequences for individual foragers into account. Yet, the precise way in which these distributions come into being is what really matters to population dynamics. It is therefore necessary to study the behavioural mechanisms underlying the distributions of foragers over patches. 3. We studied the behaviour of a predatory mite, Phytoseiulus persimilis, in response to prey patches occupied by conspecifics. It is well known that high predator densities in prey patches promote dispersal of these predatory mites. Our question was to what extent predators can assess the presence of conspecifics from a distance. 4. Experiments with a Y-tube olfactometer showed that predatory mites avoid patches occupied by conspecifics. 5. This avoidance cannot be attributed to odours of conspecific predators, or of prey damaged by predation, as these odour sources both appear to be attractive. 6. Separating the prey patch from the conspecific predators in the odour source led to the avoidance response only when the predators in the odour source were positioned upwind from the prey patch, and not when they were positioned downwind. This suggests that predators release an odour that elicits the production of yet another odour by the prey. This was supported by the observation that removal of adult prey led to a quick disappearance of the avoidance response. 7. We argue that distant discrimination between patches with and without competing conspecifics may be quite common among predators and parasitoids, and that the use of odours instead of physical inspection of patches allows predators to instantaneously integrate information on the distribution of food and competitors. 8. This behavioural mechanism may bring predators and parasitoids closer to behaving as ideal free foragers than was previously thought possible.

Keystone Predation and Interaction Strength: Interactive Effects of Predators on Their Main Prey

The application of basic ecological concepts to fields of conservation biology and applied environmental sciences is a healthy sign, but before these concepts are widely used, ecology must provide operational definitions and quantifiable methods. Keystone species and interaction strength are concepts with deep practical and theoretical implications. We studied the strength of predation on mussels (Mytilus trossulus) by the keystone seastar Pisaster ochraceus and the whelks Nucella emarginata and N. canaliculata under different environmental conditions in the Oregon intertidal zone. We attempted to determine: (1) the sensitivity of keystone predation to the presence of other predators in the system; (2) the role of other predators in the presence and absence of a keystone species; and (3) the per capita and population—level variability in interaction strengths of strong (keystone) vs. weak interactors. Predation intensity on mussels was measured by recording the survival of mussels transplanted to areas from which seastars, whelks, or both, had been either manually removed or left undisturbed at natural densities. Whelk experimental units were nested within those for the seastar treatment to account for the much larger body size and greater mobility of seastars. Each combination of seastar and whelk treatment was replicated four times in both wave—exposed and wave—protected habitats of two sites that differed in predator densities, primary productivity, and recruitment and growth rates of prey species. Predation intensity by the keystone predator was strong under all site $\times$ wave exposure combinations, and was unaffected by the presence of whelks. Whelks, in contrast, had ecologically important effects on mussel survival in the absence, but not in the presence, of the keystone predator. Population (total) interaction strength between seastars and mussels was 2—10 times stronger than that between whelks and mussels across sites and wave exposures. Per capita interaction strength of seastars was two to three orders of magnitude larger than that of whelks. However, per capita effects of seastars were more variable between sites and wave exposures, probably because simple density values grossly underestimate the ability of mobile predators to localize prey. Such interactive effects and variability in interaction strengths between keystone and weak predators may characterize all keystone predator—dominated systems, but data currently are insufficient to test this proposition. Negative effects of seastars on whelk density were observed &lt;4 mo following initiation of Pisaster removals. Seastars also had a negative effect on whelk sizes, which took longer to appear, after 6 mo of continuous Pisaster removal. Negative effects of seastars on whelks appeared to be stronger in places with higher densities of predators, partially explaining the reduced predation intensity of whelks observed in the presence of seastars. Our results support the idea that in keystone—dominated systems, species other than the keystone species have only minor, if any, effects on the rest of the community, and thus might be cited by some as redundant species. However, our results also indicate that, after the loss of a keystone species, previously redundant species can partially compensate for the reduced predation and adopt a major role in the altered system. Such responses are potentially an important force in stabilizing communities. Further, such possible compensatory capabilities of alternative consumers suggests that, at least for predators, the term redundant species conveys an inaccurate image of the potential importance of weak interactors and should be abandoned.

Managing for Caribou Survival in a Partitioned Habitat

Forest management guidelines for woodland caribou (Rangifer tarandus caribou) in Ontario need to be re-examined in light of the finding that caribou partition habitat with moose (Alces alces), partly to find virtual refuges from predation by gray wolves (Canis lupus). Forest-wide guidelines seem inappropriate for a species that is widely scattered and little known. Management should concentrate on and around currently used virtual refuges to ensure their continued habitability. Cutting these areas may force the caribou into places with higher densities of predators; winter use of roads might bring poachers, increased wolf entry, and accidents. A proposal for 100 km2 clear-cuts scheduled over 60+ years across the forest landscape would probably minimize moose/wolf densities in the long run as intended, but because of habitat partitioning might forfeit any benefits to caribou in the short-term. Sharply reducing moose densities near areas where caribou have sought refuge might incline wolves to switch to caribou. Cutting beyond caribou winter refuge areas should aim at maintaining current moose densities to prevent wolves from switching prey species. Operations level manipulation of the forest around each wintering area should provide winter habitat for the future, while treatment replications with controls across the whole forest would provide reliable knowledge about which approaches work best. The remainder of the forest should be managed to maintain suitable densities of all other species.

Adaptation of inducible defense in Euplotes daidaleos (Ciliophora) to predation risks by various predators

The extent of induced morphological defense in Euplotes daidaleos correlates to this ciliate's predation risk from the defense-inducing predator species. Euplotes daidaleos responded by morphological transformation only to organisms that are able to feed on typically formed Euplotes cells (63 ± 5 μm cell width in E. daidaleos). Three of those potential predator species caused defensive changes to various degrees (Student's t-test, P < 0.1 to P < 0.0001): Lembadion bullinum (Ciliata) induced 82 ± 6 μm cell width in E. daidaleos; Chaetogaster diastrophus (Oligochaeta) induced 85 = 6 μm width; and Stenostomum sphagnetorum (Turbellaria) induced 89 ± 8 μm width (at a density of 10 predators per milliliter, respectively). At higher predator densities (50 or 100 organisms per milliliter), Euplotes developed a correspondingly larger width (to a maximum of 103 ± 10 μm in the presence of S. sphagnetorum). Euplotes did not respond to organisms (e.g., Blepharisma japonicum, Colpidium campylum, Didinium nasutum, Paramecium caudatum, Spirostomum ambiguum, Stentor coeruleus) that cannot feed on this ciliate species. Daphnia longispina and Bursaria truncatella predators, which can feed on large prey of ≥125, or ≥200 μm in diameter, respectively, also had no effect on the morphology of Euplotes. The extent of defense in Euplotes that was induced by 10 predators per milliliter during 24 h decreased the predation risk from those predators to 67% in the presence of S. sphagnetorum, to 50% with L. bullinum, and to 15% with C. diastrophus, compared to the typical form of Euplotes. In a natural population, the defensive form of E. daidaleos was found with average cell widths of 88 ± 8 μm. The results indicate that predator-induced defense in natural Euplotes populations is beneficial to this prey and that it is adapted to the predation abilities of Euplotes predators, whereby energetical costs related to defensive changes may be saved.

BIOLOGICAL CONTROL OF THE COLORADO POTATO BEETLE LEPTINOTARSA DECEMLINEATA (COLEOPTERA: CHRYSOMELIDAE) IN QUEBEC BY AUGMENTATIVE RELEASES OF THE TWO-SPOTTED STINKBUG PERILLUS BIOCULATUS (HEMIPTERA: PENTATOMIDAE)

AbstractField plot tests during two seasons show that augmentative releases of Perillus bioculatus can control first-generation eggs and larvae of the Colorado potato beetle (CPB) under Quebec short-season conditions. Stinkbugs mass-reared on CPB were introduced manually as 2nd- and 3rd-stage nymphs during spring oviposition of the beetle at densities of one nymph per 50–225 CPB eggs recruited per plant, in 0.025-ha plots. Sampling at intervals of 1–3 days measured predator establishment, CPB egg and larval mortality, and foliage protection level. Direct observations were used to document predator-prey relationships and to determine weather conditions favourable to P. bioculatus.In the 1 st-year test, with predators released over all plants within plots, CPB control was compared among: (1) stinkbugs alone, (2) the bioinsecticide Bacillus thuringiensis (BT) as Trident™, (3) a combination of both control alternatives, and (4) no protection against the CPB. BT and stinkbugs, both alone and in combination, produced significant control as measured by percentage reductions in prepupal drop to the ground, new adult emergence, and foliage protection. Only treatments involving P. bioculatus reduced CPB egg eclosion which was the key control factor; it is strongly correlated with overall control level.In the 2nd-year test, predators were released in a central area comprising only 25% of all plants within plots. Predator and CPB egg mass densities were manipulated to produce predator: prey ratios of 0:1, 0:2, 1.5:2, 1.5:1, 3:2, and 3:1, representing average ratios of 0, 1.5,or 3 P. bioculatus per plant, and 1 or 2 CPB egg masses per plant at release time. As expected, central release produced complex interactions between control, and predator and prey densities. High beetle density slowed predator emigration from the centre, more so at low predator density than at high predator density. Predators moved more quickly within the same row than across rows. CPB eggs were destroyed in large numbers in both years but in 1993, final control correlated with larval rather than egg predation. A maximum 80% reduction in prepupal drop to the ground was obtained at the 3:1 predator:prey ratio (1 P. bioculatus:95 CPB eggs recruited per plant).The results prove the efficacy of P. bioculatus for biological control of first-generation L. decemlineata populations, at densities resulting in approximately 300 eggs laid per plant during spring oviposition in the study area. Despite low natural populations of P. bioculatus and unclear prospects for commercial availability at reasonable cost, it is evident that stinkbug releases have potential as a non-chemical control measure within an IPM program in Quebec potato production.

Intraspecific competition in immature Amblyseius fallacis, Amblyseius andersoni, Typhlodromus occidentalis and Typhlodromus pyri (Acari: Phytoseiidae)

Intraspecific competition in immature Amblyseius fallacis, Amblyseius andersoni, Typhlodromus occidentalis and Typhlodromus pyri was examined in the laboratory using small cages at five different predator densities (two, four, eight, 16 and 32) in the absence and presence of prey 100 eggs of two-spotted spider mite, Tetranychus urticae (Koch), at 25 ± 1°C, ∼80% RH and 16L:8D photoperiod. In the absence of spider mite prey, some individuals of immature phytoseiids showed increased development and surival with increasing predator densities up to certain limits, but none survived to the adult stage, except for a single male each of A. andersoni and A. fallacis who completed development by cannibalizing on conspecifics at a density of 32 predators per cage. In the absence of spider mite prey, the mean immature survival time was independent of the initial predator density, but the variance of survival time increased with predator density. In the presence of prey, the proportion of immatures surviving to adulthood generally decreased with initial predator density and dropped sharply to almost none at the predator density of 32 for A. fallacis, eight for A. andersoni, 16 for T. occidentalis and four for T. pyri. The number of prey consumed per predator during the first day generally decreased with predator density in all four species, as prey available per predator decreased and the competition for food increased with predator density. Our data indicate that scramble competition is operating in these four species. Although cannibalism was occasionally observed, especially after the exhaustion of prey and in the generalist predators such as A. andersoni, the immatures of these phytoseiids were less influenced by the interference of conspecifics than by the increasing difficulty of finding food at high predator densities. The implications of this study for understanding phytoseiid population dynamics and their use in biological control are discussed.

The Selfish Crouton

Abstract We studied predator prey interactions on a model system with gulls as predators and bread croutons as prey. The advantage to prey grouping appeared even at low numbers of prey and high predator density. Moreover, individual predators gained by preying upon a group rather than on a solitary prey. Rate of predation was the same for homogeneous (uni-coloured) and heterogeneous (multicoloured) groups of croutons.

Biology and predation rate of the sarcophagid fly,Agria mamillataa predator of European small ermine moths

Abstract The biology of the univoltine, predatory sarcophagid fly Agria mamillata (Pandellé) was studied in a population of Yponomeuta evonymellus(L.) (Yponomeutidae) attacking Prunuspadus L. in the Valais, Switzerland. Clarification of the literature records suggests that A. mamillata is oligophagus, attacking five Yponomeuta species in the Palaearctic. The adult flies emerged in May. The presence of host odour was advantageous to elicit mating. After a preoviposition period of 19 days, the ovolarviparous females deposited eggs/first instar larvae on the cuticle of late fifth instar prey larvae, or on cocoons of prey pupae. The potential fecundity averaged 40 eggs per female. Reid data indicated that females of the predator laid more eggs in tents with higher prey density. Larval development lasted 11 days. Rain stimulates mature predator larvae to leave the tents of Y. evonymellus to pupate in the soil. Pupation was observed between early July and mid August. Predation rates ( = number of prey eaten per predator) were estimated from laboratory food consumption tests and from the number of predator larvae and predated cocoons in prey tents in the field. It was shown that a single predator consumed an average of five Y. evonymellus individuals. Field data indicated that predation rates cannot increase with higher numbers of prey available. It was shown that the number of prey eaten per predator decrease with higher predator densities per tent in the field.

A Behavioral Trade‐Off and Its Consequences for the Distribution of Pseudacris Treefrog Larvae

Larval chorus frogs (Pseudacris triseriata) and spring peepers (P. crucifer) show marked differences in their distribution patterns, with chorus frogs tending to occur in less permanent ponds containing lower predator densities, and spring peepers tending to occur in more permanent ponds with higher predator densities. In order to understand this pattern I performed experiments at two levels. First, at the community level, I evaluated the importance of competition, predation, and pond permanence for patterns of larval performance. Second, at the behavioral level, I asked whether a hypothesized trade—off between gain and risk underlies ecological interactions, and ultimately, patterns of performance and distribution of these species. I employed a field manipulation to asses the contribution of competition and predation within different ponds that varied in permanence. In order to examine the effect of interspecific competition, enclosures contained each tadpole species either alone or in the presence of an equal density of the other species; these enclosures were placed in two temporary ponds, two intermediate ponds, and two permanent ponds. Tadpole stocking densities reflected local natural densities. This competition treatment was crossed with a predator treatment manipulating the absence or presence of predators (with "presence" representing the naturally occurring density within each pond). Patterns of relative performance were consistent with relative distribution patterns across the pond permanence gradient: chorus frogs performed best relative to spring peepers in temporary ponds while spring peepers performed best relative to chorus frogs in permanent ponds (performance was measured as survivorship and growth). The dominant sources of mortality were pond drying in temporary ponds and predation in more permanent ponds. In contrast to most previous experimental studies of larval anurans, competition had small effects on tadpole performance. Results from the field experiment suggest that pond drying and predation may be important determinants of performance, and that the two species are differentially susceptible to these mortality agents. In a set of laboratory experiments I evaluated the hypothesis that a trade—off between foraging gain and predation risk mediated through activity (proportion of time spent moving) might explain patterns of Pseudacris distribution. In both species, individuals facultatively reduced activity in the present of a caged predator with a corresponding drop in growth rate. In general, larval chorus frogs also tend to be more active and faster growing and developing than spring peepers. These results provide support for the existence of the trade—off and suggest that interspecific differences in activity may contribute to patterns of larval performance and distribution among ponds. The general nature of this trade—off suggests that it may also be important in other systems.

Factors influencing the population density of the hanuman langur (Presbytis entellus) in Sariska Tiger Reserve

We carried out a study of Hanuman langurs in Sariska Tiger Reserve, Rajasthan, India during three months in 1990. The area surveyed included habitat disturbed by human activity, with both the grazing of livestock and firewood collecting resulting in degraded forest. Langur population density was estimated to be between 19 and 36 animals/km2. The density of langurs in disturbed areas was significantly lower than in undisturbed areas. Both tree cover, total tree, and shrub cover were positively correlated with langur density, suggesting that a lack of trees in disturbed areas may have caused the lower langur densities. Although bisexual groups were significantly more common in undisturbed areas the distribution of all male groups did not appear to be affected by human disturbance, suggesting that male bands are likely to inhabit more disturbed habitats than bisexual groups. We suggest that either a lack of food trees or the high density of predators in Sariska may prevent bisexual groups from inhabiting areas where tree cover is low, although the more mobile male groups can survive in these areas. Although Hanuman langurs are a relatively common species in India, these results suggest that they are not, as is commonly assumed, immune to the effects of human disturbance. In areas where langurs appear to survive in disturbed habitats the different response of bisexual and all male groups to disturbance may still result in serious disruption to the population structure.

Estimates of hominid and carnivore involvement in the FLK Zinjanthropus fossil assemblage: some socioecological implications

Abstract Studies of Plio-Pleistocene zooarchaeological assemblages indicate that early Homo transported and processed carcass-parts with tools for meat and marrow. Because hominid and carnivore damage frequencies have not been estimated, however, debate continues on the extent and nature of hominid and carnivore involvement in many assemblages, thereby hindering reconstructions of early Homo socioecology. Analysis of hammerstone- and carnivore-induced damage frequencies on all large mammal limb pieces in the FLK Zinjanthropus , Olduvai Gorge, Tanzania, fossil assemblage provides direct estimates of hominid and carnivore involvement. Three patterns are clear: (1) hominid-induced damage on 75% of the limb MNE demonstrates that early Homo habitually processed carcass-parts; (2) carnivore damages and fractures occur on 54% and 8·5% of the limb MNE, respectively, indicating a significant carnivore role in assemblage modification, but not breakage; (3) the lack of carnivore damage on 60% of hominid-damaged specimens implies that early Homo had regular access to meat-rich carcasses. Because social carnivores also habitually transport food, the biological and ecological forcing mechanisms behind these behaviors are examined for implications for early Homo socioecology. Anti-predator defense is a primary factor in the evolution of sociality in the Carnivora (Gittleman, 1989). Thus, food transport from a carcass by a foraging group is an anti-predator strategy that minimizes interspecific competition at carcasses. Similarly, maintenance of a refuge area to which food is transported to mother and altricial young reflects a risk-reduction strategy to protect the young. Early Homo shared these ecological and biological conditions, occupying with its altricial young (Stanley, 1992) semi-open habitats with high predator densities. Thus, the habitual food transport and processing documented at FLK Zinjanthropus suggest that early Homo was subject to these same socioecological mechanisms that require similar group division into two functional units (foraging and care-giving) when infants are present. Processed animal remains at places like FLK Zinjanthropus may not represent provisioning or food sharing, per se, but merely the intersection of two anti-predator strategies. Nevertheless, the ecological and biological conditions, and prerequisite subsistence behaviors were apparently in place for the development of social behaviors originally envisioned by Isaac (1978 a ).

Oviposition habitat selection by the mosquito, Culiseta longiareolata: effects of conspecifics, food and green toad tadpoles

Abstract. 1. Ovipositing females should choose egg‐laying sites which maximize the growth and survivorship of their offspring. Accordingly, they should avoid habitats with high densities of competitors and predators and should choose sites with adequate food. We tested experimentally the oviposition response of the mosquito, Culiseta longiareolata Macquart, to extra food and two species which are both potential competitors and predators of its offspring: conspecific larvae and green toad (Bufo viridis) tadpoles.2. We conducted the experiment in outdoor artificial pools where wild C. longiareolata females were presented with choices of all combinations of the presence or absence of: (1) C.longiareolata larvae, (2) B.viridis tadpoles, and (3) extra food (a mixture of ground fish food and mouse chow).3. Oviposition rates averaged approximately 4 times greater in response to extra food.4. The presence of the tadpoles, particularly later in the experiment when they were larger, caused a significant reduction (approximately 50%) in oviposition rate.5. We did not detect a statistically significant effect of conspecific larvae on oviposition habitat selection.6. Our results indicate that Culiseta females choose habitats for oviposition based on food availability. They also tend to avoid habitats containing Bufo tadpoles, presumably to avoid predation and/or competition.

Hunting Associations between Badgers (Taxidea taxus) and Coyotes (Canis latrans)

Coyotes (Canis latrans) associating with badgers (Taxidea taxus) appeared to hunt Uinta ground squirrels (Spermophilus armatus) more effectively than lone coyotes. Coyotes with badgers consumed prey at higher rates (P = 0.09) and had an expanded habitat base and lower locomotion costs. Badgers with coyotes spent more time below ground and active (P = 0.02), and probably had decreased locomotion and excavation costs. Overall, prey vulnerability appeared to increase when both carnivores hunted in partnership. Complementary morphological adaptations and predatory strategies, interspecific tolerance, and behavioral flexibility allowed them to form temporary hunting associations. The following ecological circumstances may have increased the likelihood of this interaction in our study area: relatively high densities of predators and prey; relatively long-lived predator populations; a vegetative structure that impeded solitary hunting by coyotes; a high connectivity of prey burrows that decreased hunting success of badgers; an absence of interaction with humans; a stressful physical environment.

Field margins: can they enhance natural enemy population densities and general arthropod diversity on farmland?

The interactions of predatory arthropods between natural habitats and arable land were explored. Three questions were asked: (1) Does habitat quality in field margins determine the density of natural enemies overwintering in field margins? (2) Are good sites for natural enemies also habitats which support greater general arthropod diversity? (3) Does the density of predators overwintering in field margins determine their distribution and abundance in adjacent cereal fields in spring? The value of field margins to agriculture was assessed as the density of arthropods overwintering in these habitats. The arthropod groups used for the assessment were known aphid predators which moved out into cereal crops in the spring. This assessment was compared with general arthropod diversity at each site to test whether field margins supporting high densities of predators were also of value to nature conservation in supporting greater arthropod biodiversity. In spring, the spatial dynamics of predatory arthropod species were investigated in cereal crops adjacent to field-margin overwintering sites. The influence of field-margin habitats on natural enemy populations within adjacent crops was evaluated. The enhancement of natural enemies and general arthropod conservation are discussed as components of overall biological diversity at the farm scale.