Michael I. Sitvarin

The importance of intraguild predation in predicting emergent multiple predator effects

Prey typically coexist with multiple predator species, each of which presents a predation risk related to its habitat domain and foraging mode. These predator characteristics can be used to predict how the risk from multiple predators will combine to create emergent multiple predator effects for shared prey. Interactions between predators, particularly intraguild predation, can strongly alter prey suppression, though the importance of intraguild predation in multiple predator effects has not been explicitly explored. Furthermore, the vast majority of studies on multiple predator effects has focused on shared prey that are herbivorous, thus experiments focused on mesopredators are needed to evaluate the generality of conclusions made about multiple predator effects. We used a suite of carnivorous arthropods to test a predictive framework of multiple predator effects and to evaluate the role of intraguild predation in shaping these effects. We allowed the wolf spiders Pardosa milvina, Tigrosa helluo, and Rabidosa rabida and the ground beetle Scarites quadriceps to interact and recorded the outcome of all predatory events. We used two tests of multiple predator effects to determine whether predators created risk enhancement, risk reduction, or were substitutable. We found that the occurrence of intraguild predation decreased the overall risk to prey, causing observed multiple predator effects to deviate from predictions. Additionally, we highlight the importance of considering predator identity, as predators were capable of increasing, decreasing, or not altering the success of their competitors. Our study demonstrates that intraguild predation is a critical factor in determining how the risk from multiple predators will combine to affect their prey.

Fear of predation alters soil carbon dioxide flux and nitrogen content.

Predators are known to have both consumptive and non-consumptive effects (NCEs) on their prey that can cascade to affect lower trophic levels. Non-consumptive interactions often drive these effects, though the majority of studies have been conducted in aquatic- or herbivory-based systems. Here, we use a laboratory study to examine how linkages between an above-ground predator and a detritivore influence below-ground properties. We demonstrate that predators can depress soil metabolism (i.e. CO2 flux) and soil nutrient content via both consumptive and non-consumptive interactions with detritivores, and that the strength of isolated NCEs is comparable to changes resulting from predation. Changes in detritivore abundance and activity in response to predators and the fear of predation likely mediate interactions with the soil microbe community. Our results underscore the need to explore these mechanisms at large scales, considering the disproportionate extinction risk faced by predators and the importance of soils in the global carbon cycle.

Nonconsumptive Predator—Prey Interactions: Sensitivity of the Detritivore Sinella curviseta (Collembola: Entomobryidae) to Cues of Predation Risk from the Spider Pardosa milvina (Araneae: Lycosidae)

ABSTRACT Predators can affect prey indirectly when prey respond to cues indicating a risk of predation by altering activity levels. Changes in prey behavior may cascade through the food web to influence ecosystem function. The response of the collembolan Sinella curviseta Brook (Collembola: Entomobryidae) to cues indicating predation risk (necromones and cues from the wolf spider Pardosa milvina (Hentz) (Araneae: Lycosidae)) was tested. Additionally, necromones and predator cues were paired in a conditioning experiment to determine whether the collembolan could form learned associations. Although collembolans did not alter activity levels in response to predator cues, numerous aspects of behavior differed in the presence of necromones. There was no detectable conditioned response to predator cues after pairing with necromones. These results provide insight into how collembolans perceive and respond to predation threats that vary in information content. Previously detected indirect impacts of predator cues on ecosystem function are likely due to changes in prey other than activity level.

Information from familiar and related conspecifics affects foraging in a solitary wolf spider

As neighbours become familiar with one another, they can divert attention away from one another and focus on other activities. Since familiarity is a likely mechanism by which animals recognise relatives, both kinship and prior association with conspecifics should allow individuals to increase foraging. We attempted to determine if the interference observed among conspecific foragers could be mitigated by familiarity and/or kinship. Because Pardosa milvina wolf spiders are sensitive to chemotactile cues deposited on substrates by other spiders, we used cues to manipulate the information available to focal spiders. We first verified that animals could use these cues to differentiate relatives and familiar conspecifics. We then documented foraging in the presence of all combinations of related and familiar animal cues. Test spiders were slower foragers, less likely to capture prey, and consumed less of each prey item when on cues from unfamiliar kin, but were faster and more effective foragers on cues from familiar non-kin. Their reactions to familiar kin and unfamiliar non-kin were intermediate. High foraging intensity on familiar cues is consistent with the idea that animals pay less attention to neighbours after some prior association. Lower foraging effort in the presence of cues from relatives may be an attempt to reduce kin competition by shifting attention toward dispersal or to provide increased access to prey for hungry relatives nearby. These findings reveal that information from conspecifics mediates social interactions among individuals and affects foraging in ways that can influence their role in the food web.

The wolf spider Pardosa milvina detects predator threat level using only vibratory cues

Predators may inadvertently signal their presence and threat level by way of signals in multiple modalities. We used a spider, Pardosa milvina, known to respond adaptively to chemotactile predator cues (i.e., silk, faeces and other excreta) to evaluate whether it could also discriminate predation risk from isolated vibratory cues. Vibrations from its prey, conspecifics, and predators (Tigrosa helluo and Scarites quadriceps) were recorded and played back to Pardosa. In addition, we recorded predator vibrations with and without access to chemotactile cues from Pardosa, indicating the presence of prey. Pardosa did not appear to discriminate between vibrations from prey or conspecifics, but the response to predators depended on the presence of cues from Pardosa. Vibrations from predators with access to chemotactile cues from prey induced reductions in Pardosa activity. Predator cues typically occur in multiple modalities, but prey are capable of imperfectly evaluating predation risk using a limited subset of information.

Predator cues have contrasting effects on lifespan of Pardosa milvina (Araneae: Lycosidae)

Abstract Predators can affect prey indirectly by eliciting changes in behavior, morphology, and life history. These nonconsumptive effects are often mediated by predator cues used by prey to avoid capture. However, predator cues can cause stress responses in prey that negatively impact survival and reproduction. We explored responses of the wolf spider Pardosa milvina (Hentz 1844) to cues from the larger wolf spider Tigrosa helluo (Walckenaer 1837) and the ground beetle Scarites quadriceps Chaudoir 1843. We exposed Pardosa to cues from both predators and measured changes in body size, weight, consumption, and lifespan. We found significant effects of predator cues only on female longevity: females exposed to Tigrosa cues had shorter lifespans than those exposed to cues from Scarites. The lack of treatment effects on energy intake suggests that predator cues act through physiological pathways. Future experiments may uncover opposing hormonal mechanisms underlying the observed differences in lifespan.

Selectivity underlies the dissociation between seasonal prey availability and prey consumption in a generalist predator

Generalist predators are capable of selective foraging, but are predicted to feed in close proportion to prey availability to maximize energetic intake especially when overall prey availability is low. By extension, they are also expected to feed in a more frequency‐dependent manner during winter compared to the more favourable foraging conditions during spring, summer and fall seasons. For 18 months, we observed the foraging patterns of forest‐dwelling wolf spiders from the genus Schizocosa (Araneae: Lycosidae) using PCR‐based gut‐content analysis and simultaneously monitored the activity densities of two common prey: springtails (Collembola) and flies (Diptera). Rates of prey detection within spider guts relative to rates of prey collected in traps were estimated using Roualdes’ cst model and compared using various linear contrasts to make inferences pertaining to seasonal prey selectivity. Results indicated spiders foraged selectively over the course of the study, contrary to predictions derived from optimal foraging theory. Even during winter, with overall low prey densities, the relative rates of predation compared to available prey differed significantly over time and by prey group. Moreover, these spiders appeared to diversify their diets; the least abundant prey group was consistently overrepresented in the diet within a given season. We suggest that foraging in generalist predators is not necessarily restricted to frequency dependency during winter. In fact, foraging motives other than energy maximization, such as a more nutrient‐focused strategy, may also be optimal for generalist predators during prey‐scarce winters.

Laboratory and Field Investigation of Biological Control for Brown Marmorated Stink Bug (Halyomorpha halys (Stål) (Hemiptera: Pentatomidae))

Abstract Conservation biological control manipulates habitat characteristics to enhance natural enemy populations and ultimately reduce pest density. These practices can be most effective early in the growing season when pest populations are low. Early season predator impacts on these pests can include both direct consumption of herbivores and non-consumptive effects such as superfluous killing, both of which provide pest suppression. We combined laboratory feeding trials, and a field cage experiment with molecular gut-content analysis to explore the effects of striped lynx spiders (Oxyopes salticus Hentz) on brown marmorated stink bugs (Halyomorpha halys (Stål)). The laboratory feeding trials revealed that lynx spiders did attack stink bugs but that stink bug DNA had a short DNA detectability half-life within lynx spider guts. To simulate field conditions where these two species could interact in both early and late season, we manipulated the density and relative abundance of stink bug nymphs and adults in the presence and absence of lynx spiders. There was no effect of treatment on recovery of either adult stink bugs or nymphs. Although dead stink bugs were recovered, we found no evidence of consumption through molecular gut-content analysis. Contrary to expectations that generalist predators would have the greatest impact on pests early in the growing season, our results suggest that lynx spiders are unlikely to exert substantial early season control. Biological control might be most effective when utilizing multiple predator species as part of a complex of natural enemies, so spiders acting in concert with other generalist predators could be capable of suppressing pest populations.