Regulation of wolf spider populations

Aim of this study was to identify major regulatory mechanisms for wolf spider populations. Field and laboratory experiments focussed on the importance of prey availability (autochthonous & allochthonous), food quality and habitat heterogeneity on performance of individual wolf spiders or whole populations and on intra- (cannibalism) and interspecific (intraguild predation) relationships in wolf spiders. Wolf spider populations on xeric grasslands near Darmstadt (Germany) are increased on grazed sites as compared to sites without sheep grazing. Since potential prey was also more abundant on grazed sites, I hypothesised that spiders were bottom-up controlled. Therefore, alternative prey (Drosophila melanogaster; Diptera) was added to fenced plots on grazed and non-grazed sites in the field and spider as well as prey densities (detritivores & herbivores) were compared to plots without prey addition. In order to document integration of D. melanogaster into the arthropod food web, stable isotopes in the most abundant animals were analysed. Even though D. melanogaster was integrated into the diet of most predators as documented by stable isotope analysis, neither spiders nor ground beetles increased in densities when provided with additional alternative prey. Similarly, densities of potential prey, such as Collembola and Auchenorrhyncha, were not affected by prey addition, indicating that prey populations were not effectively controlled by predator populations. Stable isotope analysis documented the dependence of Lycosidae and Carabidae on prey from the detritivore food. Collembola were strongly increased on grazed sites, presumably being responsible for increased wolf spider densities indicating that wolf spider populations were bottom-up controlled. Probably, the amount of alternative prey added was not sufficient to significantly affect predator and prey densities in addition to the beneficial effect of grazing. The importance of allochthonous resources in food webs has been documented and spider populations are often sustained by prey from the detritivore food web (see above). In coastal ecosystems, predators often benefit from allochthonous input from the marine ecosystem. Kelp and sea grass deposition is common on sandy beaches on the east coast of Tasmania (Australia) providing marine input for cursorial predators on the beach and foredunes. I hypothesised that the terrestrial predators benefit from these allochthonous resources. Animals from the beach and the foredune were collected and most abundant taxa were selected for stable isotope analysis in order to trace the importance of marine subsidies in animal diet. This is possible, because stable isotope ratios in algae differ strongly from those in terrestrial plants that determine the baseline of the terrestrial food web. According to activity abundance of animals, arthropods could be separated into the beach and the foredune community that hardly overlapped. Stable isotope analysis documented that kelp deposits were used by amphipods and isopods that were the most abundant animals on the beach. Despite of their high abundance, only few predators, such as Tetralycosa oraria (Lycosidae), Cafius 2 (Staphylinidae) and Tuoba laticeps (Geophilomorpha) preyed on these isopods and amphipods. Stable isotope analyses revealed feeding niches of predators and documented that the marine subsidies were not incorporated into the foredune food web. Thus, the availability of allochthonous resources is not necessarily important for arthropod food webs in the vicinity of ecosystem borders. However, some predators specialize on prey depending on marine allochthonous resources. Dietary mixing and food quality have become major issues in foraging theory. Both prey availability and prey quality play an important role in growth and reproduction of generalist predators. In general, food mixing is beneficial for predators, since it may optimise nutrient uptake and dilute toxins. The springtail Folsomia candida has been used in many laboratory experiments and was proven toxic for various predators. In contrast, Drosophila melanogaster and Heteromurus nitidus (Collembola) are considered to be intermediate to high quality prey. Performance and reproduction of female Pardosa lugubris (Lycosidae) fed with prey of different quality were investigated. Spiders were fed either a single diet of D. melanogaster, H. nitidus or F. candida, or a mixed diet of D. melanogaster and H. nitidus, or of D. melanogaster and F. candida. Feeding on high quality prey (D. melanogaster only, H. nitidus only and both combined) resulted in similar growth and reproduction of female P. lugubris as well as in similar offspring number, size and survival. There was no positive effect of mixing high quality prey. In contrast, feeding on toxic prey (F. candida) in single and mixed diet was detrimental and none of the females survived. P. lugubris did not develop acquired aversion against F. candida and offering high quality prey (D. melanogaster) mixed with F. candida did not improve survival. Nutrient flow from females into offspring during egg production was investigated using stable isotope analysis. D. melanogaster was enriched in 13C and this allowed tracing the carbon flow from prey into females and into their offspring. Furthermore, both D. melanogaster and H. nitidus were enriched in 15N compared to female spiders so that nitrogen flow from prey into offspring could be followed. The enrichment in 13C and 15N differed strongly between juveniles and their mothers. The analyses suggest that adult females invested dietary nutrients almost exclusively into egg production. Mortality of juvenile Pardosa palustris (Lycosidae) is high on xeric grasslands near Darmstadt likely resulting from predation by conspecifics (cannibalism) or other predators (intraguild predation). Cannibalism is common among spiders and likely is a major mortality factor for juvenile wolf spiders in the field. Potential driving factors for intraspecific predation in P. palustris were investigated conducting two laboratory experiments to evaluate the role of availability of alternative prey, habitat structure and spider density for cannibalism between and within developmental stages. Availability of alternative insect prey strongly reduced cannibalism between adult spiders and juveniles as well as among juveniles. For juvenile spiders habitat complexity reduced predation by adult females, whereas cannibalism among second instar spiderlings was not affected by habitat structure suggesting that complex habitat structure only provides shelter from cannibalism by large conspecifics. High density of juvenile spiders increased cannibalism only when alternative prey was available, suggesting that alternative prey increased aggression and interference at higher density. High mortality and low growth of spiders in treatments without alternative prey likely resulted from starvation indicating that intraspecific prey is of low food quality and does not allow spider development. Intraguild predation is also common among generalist predators and an important issue for food web theory. Juveniles of Alopecosa cuneata (Lycosidae) are likely to be successful intraguild predators of the smaller P. palustris, adding to juvenile mortality on xeric grasslands near Darmstadt. Intraguild predation of third instar A. cuneata on second instar P. palustris was investigated in a laboratory experiment manipulating availability of alternative prey and microhabitat complexity. Both, the presence of alternative prey and complex microhabitat structure reduced mortality of juvenile P. palustris during the first week. During the second week, mortality increased in complex structure without alternative prey presumably due to enhanced activity of starving P. palustris and cannibalism among second instar juveniles. Stable isotope analysis documented predation of A. cuneata on P. palustris and predation on alternative prey by both juveniles. Thus, using stable isotope analysis intraguild predation among arthropod predators was documented for the first time under semi-natural conditions in the laboratory. In conclusion, intraguild predation can be an important mortality factor for juvenile wolf spiders in the field, if resources are limited and shelter by a complex structured microhabitat is missing. Since shelter is abundant on xeric grasslands near Darmstadt, intraguild predation by A. cuneata can be excluded as an important mortality factor for juvenile P. palustris in the field suggesting that rather cannibalism and/or intraguild predation by other spider species of similar size cause substantial mortality in P. palustris. Results of this study document the importance of prey availability, food quality and habitat heterogeneity for wolf spider populations. Cannibalism and intraguild predation among wolf spiders depended on the availability of alternative prey and microhabitat structure and may be major factors regulating spider populations. When juveniles hatch in close proximity, cannibalism and intraguild predation can contribute to population regulation by imposing greater per capita mortality at high densities. Furthermore, cannibalism and intraguild predation increase population synchrony by exerting size-specific mortality on smaller individuals throughout development