Effects of chronic exposure to a toxic prey in a generalist predator

Abstract.  Generalist predators experience large differences in prey quality, and eating toxic prey may affect their fitness even if the toxic prey constitutes only a small fraction of a mixed diet. Feeding and life history parameters were examined in the wolf spider Pardosa prativaga during continuous exposure to the toxic collembolan Folsomia candida . Spiderlings were divided into a control group fed adult Drosophila melanogaster , a group fed only F. candida , and a mixed diet group fed both types of prey. Folsomia candida reduced survival, developmental stage attained and growth rate of all exposed spiders. Spiders chronically exposed to F . candida increased their consumption of D. melanogaster compared to control spiders. Folsomia candida thus inhibited the utilization of the high-quality food and/or increased the respiration rate. The intake of F . candida remained at a constant low level throughout the experiment, indicating that P. prativaga was unable to develop an absolute aversion against this prey. Half of the control group was also given the mixed diet after a large juvenile instar was attained. These spiders were not affected to the same extent as the newly hatched spiderlings, indicating an ontogenetically increased tolerance. Comparison of laboratory-raised and field-caught spiders confirmed that tolerance to F. candida was size-dependent and thus not an induced response. A differential survival in the mixed diet group, based partly on maternal effects, indicated possible genetic variability in the physiological tolerance to F. candida .     

Behavioral and ecophysiological responses of a generalist predator to single- and mixed-species diets of different quality

Prey of different quality can be distinguished by their effects on fitness parameters (e.g., survival, growth, development) of the predator. This paper describes a series of experiments with wolf spiders, Schizocosa sp., designed to analyze the behavioral and physiological mechanisms by which these effects are brought about. Schizocosa responded to prey of inferior quality by developing an aversion. The number of prey encounters needed for the aversion to develop depended on the prey type and the quality of alternative prey available. As expected, fewer low-quality prey were accepted if prey of higher quality were available; however, acceptance was also reduced if alternative prey were of even lower quality. An aversion disappeared within a few hours to about 1 day. Consumption by Schizocosa of even small amounts of some “toxic” prey species reduced growth by inhibiting feeding rate and impairing utilization of food derived from prey of higher quality. These results have implications for understanding the interactions of generalist predators in community food webs. Received: 29 July 1998 / Accepted: 1 February 1999     

Short‐term foraging responses of a generalist predator to management‐driven resource pulses

Generalist predators characteristically switch prey in response to resource pulses. Ungulate population reinforcement through translocation provides stressed and vulnerable prey, and apex predator reintroduction provides carrion, both resources that can be exploited by generalist predators. We investigated the dietary response of black‐backed jackal to two management interventions in the Karoo National Park, South Africa, namely the reinforcement of the springbok population, and then the reintroduction of lions. By analysing jackal diets from scats collected before and after each management intervention, we show that the relative per cent occurrence and biomass consumed of springbok increased in the diet following the population reinforcement of springbok. In contrast, large ungulates were more prevalent in the diet following apex predator reintroduction. These results suggest that jackals took advantage of a potentially vulnerable abundant springbok population following their population reinforcement, and then switched to foraging on carrion provided by reintroduced lions. These results provide insights into the dietary response of a generalist mesopredator to management interventions. A key lesson from the study is that the diet of generalist predators is context‐dependent and should be interpreted in that light.     

When host choice is a problem for a generalist herbivore: experiments with the whitefly, Bemisia tabaci

1. Experiments with adult females of the whitefly, Bemisia tabaci, demonstrated that when mixtures of host plants were present, performance was reduced relative to that on the best of the host plants when presented alone. 2. Behavioural observations indicated that in the presence of a variety of plants, individuals moved more, switched between plants more frequently, and stayed feeding in one place for shorter periods of time, compared with the situation when single plant species were available. 3. The data are interpreted as a constraint on attentiveness in the presence of complex, mixed-sensory stimuli.     

Coral snake mimicry: live snakes not avoided by a mammalian predator

The occurrence of coral snake coloration among unrelated venomous and non-venomous New World snake species has often been explained in terms of warning coloration and mimicry. The idea that snake predators would avoid coral snakes in nature seems widely established and is postulated in many discussions on coral snake mimicry. However, the few workers that have tested a potential aposematic function of the conspicuous colour pattern focused exclusively on behaviour of snake predators towards coloured abstract models. Here we report on behaviour of temporarily caged, wild coatis (Nasua narica) when confronted with co-occurring live snakes, among which were two species of venomous coral snakes. Five different types of responses have been observed, ranging from avoidance to predation, yet none of the coatis avoided either of the two coral snake species or other species resembling these. As in earlier studies coatis appeared to avoid coral snake models, our findings show that results from studies with abstract snake models cannot unconditionally serve as evidence for an aposematic function of coral snake coloration.     

Capture efficiency and trophic adaptations of a specialist and generalist predator: A comparison

Specialist true predators are expected to exhibit higher capture efficiencies for the capture of larger and dangerous prey than generalist predators due to their possession of specialized morphological and behavioral adaptations. We used an araneophagous spider (Lampona murina) and a generalist spider (Drassodes lapidosus) as phylogenetically related model species and investigated their realized and fundamental trophic niches and their efficacy with respect to prey capture and prey handling. The trophic niche of both species confirmed that Lampona had a narrow trophic niche with a predominance of spider prey (including conspecifics), while the niche of Drassodes was wide, without any preference. DNA analysis of the gut contents of Lampona spiders collected in the field revealed that spiders form a significant part of its natural diet. Lampona captured significantly larger prey than itself and the prey captured by Drassodes. As concerns hunting strategy, Lampona grasped the prey with two pairs of legs possessing scopulae, whereas Drassodes immobilized prey with silk. Lampona possess forelegs equipped with scopulae and a thicker cuticle similar to other nonrelated araneophagous spiders. Lampona fed for a longer time and extracted more nutrients than Drassodes. We show that specialized behavioral and morphological adaptations altogether increase the hunting efficiency of specialists when compared to generalists.     

Age and sex differences in numerical responses,dietary shifts,and total responses of a generalist predator to population dynamics of main prey

Oecologia - Fluctuations in the abundance of main prey species might shape animal communities, by inducing numerical responses and dietary shifts in predators. Whether numerical responses and...     

Prey discrimination by a generalist coccinellid predator: effect of prey age or parasitism?

1. Patterns of prey discrimination by the generalist predatory coccinellid Coleomegilla maculata lengi Timb. (Coleoptera: Coccinellidae) were studied in relation to prey quality, under laboratory conditions. 2. Choice experiments were performed in which second‐ and fourth‐instar coccinellids had the choice between Trichoplusia ni (Lepidoptera: Noctuidae) eggs that were young or old, unparasitised or parasitised by Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae). 3. Coleomegilla maculata larvae did not exhibit any preference for parasitised or unparasitised eggs at a similar stage of development but always preferred younger eggs, regardless of whether or not they were parasitised. Furthermore, the percentage of rejection was higher and handling time longer on the less preferred egg type. Pre‐imaginal development time, food intake to reach adulthood, and survival of coccinellid immatures were altered when coccinellid larvae were fed with parasitised and old unparasitised eggs. 4. These results indicate that C. maculata larvae select eggs based on their stage of development regardless of parasitism, and that prey quality of aged prey is lower. 5. Fourth‐instar coccinellid larvae spent less time in patches containing solely parasitised old eggs, and their level of exploitation was greatly reduced, compared with homogeneous patches containing unparasitised young eggs. This suggests that C. maculata larvae respond to variable patch quality by using flexible decision rules that reflect the payoff of the patch.     

Foraging responses of predators to novel toxic prey: effects of predator learning and relative prey abundance

Learning to avoid toxic prey items may aid native predators to survive the invasion of highly toxic species, such as cane toads Bufo marinus in tropical Australia. If the predators’ initial aversion is generalized, native prey that resemble the toxic invader may receive a benefit through accidental mimicry. What ecological factors influence the acquisition of learned avoidance (and hence, the impact of invasion on both predators and native prey)? We conducted laboratory experiments to evaluate how the relative abundance of toad tadpoles compared to palatable native tadpoles (Litoria caerulea and L. rubella) affected the ability of native aquatic predators to discriminate between these two prey types. Both fish (northern trout gudgeon, Mogurnda mogurnda) and frogs (Dahl's aquatic frog, Litoria dahlii) learned to discriminate between toads and frogs within an eight‐day period. Higher abundance of toad tadpoles relative to frog tadpoles enhanced rates of predator learning, and thus reduced predation on toads and increased predation on native tadpoles. In the field, spatial and temporal variation in the relative abundance of cane toads compared to native frogs may influence the rates at which these novel toxic items are deleted from predator diets, and the duration of predator protection afforded to natives that resemble the invader.     

Experimental assessment of trophic ecology in a generalist spider predator: Implications for biocontrol in Uruguayan crops

Conservative biological control promotes the use of native natural enemies to limit the size and growth of pest populations. Although spiders constitute one of the most important groups of native predators in several crops, their trophic ecology remains largely unknown, especially for several generalist taxa. In laboratory, we assessed the predatory behaviour of a wandering spider (the wolf spider Lycosa thorelli (Keyserling, 1877) against several arthropods varying in size and trophic positions, all found in South American soybean and rice crops. As prey we used the bug Piezodorus guildinii (Westwood, 1837) as well as larvae and adults of the moth Spodoptera frugiperda (Smith, 1797), both being considered important pests in Uruguayan crops. We also used several non-pest arthropods as prey, sarcophagid flies, carabid beetles and wolf spiders. All prey were attacked in more or less high, although not statistically differing, proportions. However, carabids were not consumed, and bugs were consumed in significantly lower proportions than flies. A negative correlation was found between prey size and acceptance rate. Immobilization times were longer against larvae when compared to moths and flies, while predatory sequences were longer for bugs when compared to flies, moths and spiders. In addition, we found a positive effect of prey size on predatory sequence length and complexity. Our results confirm the ability of spiders to attack and feed upon prey with different morphologies, included well-defended arthropods, and their potential use as natural enemies of several pests in South American crops.     

Individual dietary specialization in a generalist predator: A stable isotope analysis of urban and rural red foxes

Some carnivores are known to survive well in urban habitats, yet the underlying behavioral tactics are poorly understood. One likely explanation for the success in urban habitats might be that carnivores are generalist consumers. However, urban populations of carnivores could as well consist of specialist feeders. Here, we compared the isotopic specialization of red foxes in urban and rural environments, using both a population and an individual level perspective. We measured stable isotope ratios in increments of red fox whiskers and potential food sources. Our results reveal that red foxes have a broad isotopic dietary niche and a large variation in resource use. Despite this large variation, we found significant differences between the variance of the urban and rural population for δ¹³C as well as δ¹⁵N values, suggesting a habitat‐specific foraging behavior. Although urban regions are more heterogeneous regarding land cover (based on the Shannon index) than rural regions, the dietary range of urban foxes was smaller compared with that of rural conspecifics. Moreover, the higher δ¹³C values and lower δ¹⁵N values of urban foxes suggest a relatively high input of anthropogenic food sources. The diet of most individuals remained largely constant over a longer period. The low intraindividual variability of urban and rural red foxes suggests a relatively constant proportion of food items consumed by individuals. Urban and rural foxes utilized a small proportion of the potentially available isotopic dietary niche as indicated by the low within‐individual variation compared to the between‐individual variation. We conclude that generalist fox populations consist of individual food specialists in urban and rural populations at least over those periods covered by our study.     

Effect of plants on the searching efficiency of a generalist predator: the importance of predator-prey spatial association

In most studies of tritrophic interactions, the effect of plants on predators is confounded with changes in prey and predator behaviors after an encounter event. Here, we estimate how the effect of plants on prey distribution (in the absence of the predator) and on predator foraging behavior (in the absence of prey) may influence predation rate of Orius insidiosus (Say) (Heteroptera: Anthocoridae) in 11 plant by prey species combinations. The within-leaf distributions of O. insidiosus and its prey overlapped most on bean plants. The predator's foraging behavior (e.g., walking speed, turning rate) also differed among plant species. Simulations, using the prey distribution data and predator's foraging patterns on leaf surfaces of each plant species, show that, overall, the searching efficiency of O. insidiosus was higher on leaves of bean and corn than of tomato. However, the predator's searching efficiency was not consistent within plant species. Thus, the combined effect of plants directly on the predator and indirectly through the prey influenced the predator's searching efficiency.     

Variation of prey responses to cues from a mesopredator and an apex predator

Detection and avoidance of predator cues can be costly, so it is important for prey to balance the benefits of gaining food against the costs of avoiding predators. Balancing these factors becomes more complicated when prey are threatened by more than one type of predator. Hence, the ability to recognize species‐specific predator odours and prioritize behaviours according to the level of risk is essential for survival. We investigated how rock rats, Zyzomys spp. modify their foraging behaviour and giving‐up density (GUD) in the presence of an apex predator, the dingo Canis dingo, a mesopredator, the northern quoll Dasyurus hallucatus, a herbivore, the rock wallaby Petrogale brachyotis as a pungency control and water as a procedural control. Both dingoes and quolls consume rock rats, but because quolls can enter small crevices inhabited by rock rats, they pose a greater threat to rock rats than dingoes. Rock rats demonstrated a stronger avoidance to quoll odour than dingo odour, and no avoidance of the pungency control (rock wallaby) and the procedural control (water). GUD values declined significantly over the duration of the study, but did not differ between odour treatments. Our results support the hypothesis that prey vary behaviour according to perceived predator threat, and show stronger responses to potentially more dangerous predators.     

Interaction strength between a predator and dangerous prey: Sinistrofulgur predation on Mercenaria

The lack of direct empirical evidence of predator evolution in response to prey adaptation is a fundamental weakness of the arms race analogy of predator-prey coevolution. I examined the interaction between the predatory busyconine whelk Sinistrofulgur sinistrum and its bivalve prey Mercenaria mercenaria to evaluate whether reciprocal adaptation was likely in this predator-prey system. Thick-lipped whelks use their shell lip to chip open the shell of their prey, often resulting in breakage to their own shell. Thus, hard-shelled prey, such as Mercenaria, may be considered dangerous because they are able to inflict damage to the predator as a consequence of the interaction. The strength of interaction between whelks and their bivalve prey was viewed by regressing predator performance (the incidence of shell breakage in encounters with prey) on prey phenotype (a function of size). Interaction with Mercenaria of varying sizes has strong and predictable consequences (r²=0.946; p=0.028) for Sinistrofulgur. Predators that select large, thick bivalve prey increase the likelihood that their shell lip will be broken in the process of attempting to open their prey. Ecological consequences of feeding-induced breakage may include reduced growth rate, reproductive success, and survivorship. These results suggest that natural selection should favor predator phenotypes that reduce feeding-induced breakage when interactions with damage-inducing prey occur.     

Pharmacological activities of a toxic phospholipase A isolated from the venom of the snake Bothrops asper

A toxic phospholipase A was isolated from the venom of Bothrops asper. It induced skeletal muscle damage, anticoagulant effects and edema in the foot pad. The toxin had an intravenous LD50 of 95 micrograms/16-18 g mouse body wt and an intraventricular LD50 of 0.42 micrograms/16-18 g mouse body wt. Upon intramuscular and intravenous injections, the toxin induced a prominent increase in serum creatine kinase (CK) levels; only the CK-MM isozyme increased markedly. The toxin induced CK and creatine release from skeletal muscle incubated in vitro. The rate of efflux of creatine was higher than that of CK, although both markers were partially released as early as 15 min after incubation. The toxin also induced elevation of serum levels of lactic dehydrogenase isozymes. However, histological examination of skeletal muscle, kidneys, heart and lungs revealed cell damage only in skeletal muscle. The toxin was not cytotoxic to erythrocytes, lymphocytes or macrophages. In addition, it did not induce a mitogenic response on lymphocytes. In the absence of albumin in the medium, there was no significant difference between myotoxic activities in Ca2+-free and Ca2+-containing bathing solutions. However, when albumin was added, there was a significantly higher myotoxic effect in the presence of Ca2+. Thus, although phospholipolytic activity of the toxin plays a role in muscle damage when albumin is present, the toxin induces muscle damage even when phospholipase A activity is inhibited.