David H. Wise

Terrestrial trophic cascades: how much do they trickle?

Although more consensus is now emerging on the magnitude and frequency of cascading trophic effects in aquatic communities, the debate over their terrestrial counterparts continues. We used meta‐analysis to analyze field experiments on trophic cascades in terrestrial arthropod‐dominated food webs to evaluate the overall magnitude of trophic cascades and conditions affecting their occurrence and strength. We found extensive support for the presence of trophic cascades in terrestrial communities. In the majority of experiments, predator removal led to increased densities of herbivorous insects and higher levels of plant damage. Cascades in which removing predators led to decreased herbivory also were detected but were less frequent and weaker, suggesting a predominantly three‐trophic‐level behavior of arthropod‐dominated terrestrial food webs. Despite the clear evidence that cascades often decreased plant damage, residual effects of predation produced either no or only minimal changes in overall plant biomass. Agricultural systems and natural communities exhibited similarly strong effects of predation on herbivore abundance. However, resulting effects on plant damage and community‐wide effects of trophic cascades on plant biomass usually were highly variable, and only in the managed agricultural systems did predators occasionally have strong indirect effects on plant biomass. Our meta‐analysis suggests that the effects of trophic cascades on the biomass of primary producers are weaker in terrestrial than aquatic food webs.

BOTTOM-UP LIMITATION OF PREDACEOUS ARTHROPODS IN A DETRITUS-BASED TERRESTRIAL FOOD WEB

Knowing how an increase in the resource base of a food web produces effects that propagate through the web is central to developing a clearer understanding of food- web structure and dynamics. In a detritus-based terrestrial food web, we measured the responses of predaceous arthropods to increases in prey arthropods that occurred in response to experimentally enhancing the webs resource base. Open 2 X 5 m plots on the floor of a deciduous forest were randomly assigned to either a Food Enhancement or Control treat- ment. We supplemented the resource base of the arthropod community of the leaf litter layer for 3.5 mo by periodically adding chopped mushrooms, potatoes, and instant fruit fly medium to the Food Enhancement plots. Major taxa of detritivores and fungivores increased in response to added food. Densities of springtails (Collembola) were on average 3 X higher in the Food Enhancement than Control plots. Numbers of adult fungus gnats (Diptera: Sciaridae and Mycetophilidae) did not differ significantly between treatments after 6 wk but were >2X higher in Food Enhancement plots at the end of the experiment. Total Diptera were twice as abundant in Food Enhancement plots on both census dates. Arthropod groups that include a range of feeding strategies also increased. Mites (Ac- arina), which include detritivores, fungivores, and predators, were twice as abundant in the experimental treatment. Staphylinid and carabid beetles (Coleoptera), which are primarily predaceous but include omnivorous species, were several times more numerous in the Food Enhancement plots. Effects of increasing the resource base propagated through the food web, leading to higher densities of the major strictly predaceous arthropod taxa. Centipedes (Chilopoda), pseudoscorpions (Pseudoscorpionida), and spiders (Araneae) were -2X as abundant in the Food Enhancement treatment. Thus, our experiment uncovered substantial bottom-up lim- itation in this detritus-based food web, expressed as responses by predaceous arthropods at least two trophic links removed from the experimentally elevated resource.

Growth, development, and survival of a generalist predator fed single- and mixed-species diets of different quality

Abstract It is often assumed that prey species consumed by generalist predators are largely, though not entirely, equivalent in terms of their value to the predators. In contrast to this expectation, laboratory feeding experiments uncovered distinctly varied developmental responses of a generalist predator, the wolf spider Schizocosa, to different experimental diets. Naive Schizocosa attacked and fed upon all the prey species offered; however, highly divergent patterns of survival, development, and growth of Schizocosa spiderlings reared on different single-prey diets revealed a wide spectrum of prey qualities. Spiderlings fed the collembolan Tomocerus bidentatus sustained the highest overall rates of survival, growth, and development. Fruit flies (Drosophila melanogaster) were intermediate-quality prey: spiders fed Drosophila initially exhibited rates of survival, growth, and development equal to those of spiders on a diet of T. bidentatus, but after about 3 months, rates declined markedly. Fungus gnats (Sciaridae; Bradysia sp.) and conspecfic spiderlings are low-quality prey for Schizocosa: a sole diet of either of these prey types resulted in positive but markedly submaximal rates of growth, retarded rates of development, and survival rates much lower than that supported by a diet of Drosophila. Worst were the collembolans Folsomia candida and Isotoma trispinata, and the aphid Aphis nerii: spiderlings fed solely one of these species did not grow and died without molting. A. nerii is classified as poor quality because survival was no better than that of starved controls. F. candida and I. trispinata were toxic: survival of Schizocosa hatchlings fed these collembolans was lower than that of starved controls. A mixed diet of T. bidentatus and fruit flies yielded positive synergistic effects with respect to growth, but development and rate of survival were similar to those of spiders on a sole diet of T. bidentatus. Including toxic prey did not produce a better diet, while inclusion of toxic prey with prey of higher quality created diets that were no better than the toxic prey alone. The results of these experiments suggest that prey species that are similar in morphology and behavior, and that are initially killed and consumed, may differ dramatically in their suitability as food for generalist arthropod predators.

Food Limitation of the Spider Linyphia marginata: Experimental Field Studies

Field experiments were performed to determine whether the spider Linyphia marginate C. L. Koch is food limited, and, if so, whether a shortage of prey acts as a density- independent or density-dependent limiting factor. The experiments uncovered the effects of food supply, spider density, and the interaction of food and density upon survival (combined effects of migration and mortality), rate of increase in weight, and fecundity. Replicate, un- enclosed spider populations were established at two densities, and natural food levels were supplemented by fruit flies added to the spider webs in one-half of the populations at each density. Adding food to the webs of immature spiders did not improve survival at either low or high spider density. Survival was lower in the high-density populations. Supplementing the food supply of immatures did increase the rate at which they gained weight. Density, how- ever, had no negative effect on growth rate, indicating that though a shortage of food limits growth, immature spiders are not competing for food. Hence, for immature L. marginata, food supply acts as a density-independent limiting factor. For mature 9 9, supplementing the food supply improved survival and increased fecundity. Spider density had a negative effect upon both survival and fecundity. There were no statis- tically significant interactions between food supply and density, probably because not enough food was added. It appears that mature L. marginate compete for food; hence, for them, food supply acts as a density-dependent limiting factor.

IMPACT OF A DETRITAL SUBSIDY ON TROPHIC CASCADES IN A TERRESTRIAL GRAZING FOOD WEB

Theory predicts that allochthonous energy subsidies can modify the strength of trophic cascades in food webs. We tested the hypothesis that a detrital subsidy would enhance trophic cascades in the grazing food web of an agroecosystem in which generalist predators, wolf spiders and carabid beetles, feed both on detritivores and herbivores. We crossed detritus-addition with predator-removal treatments in replicated 8 × 8 m open and fenced plots of cucumbers and squash. Collembola, a major detritivore, became 2–4 times more abundant in detritus-addition than control plots; densities of other detritivores also increased. Although the detrital subsidy initially increased ground beetle numbers and produced a 2–3-fold increase in wolf spider densities, we found no consistent evidence of strong trophic cascades measured as changes in fruit yield in either detritus-addition or predator-removal treatments. This result contrasts with predator-removal experiments in previous years in the same study area, which revealed strong cascades affecting cucumber yield. We hypothesize that (1) competition between cucurbits and fungi for nitrogen in the detritus-addition plots, (2) unusually high herbivore densities in the spring cucumber crop, and low densities on the summer squash crop, (3) changes in patterns of intraguild predation in the detritus-addition plots, and (4) perhaps a switch by generalist predators to prey of the detrital food web all may have contributed to preventing the detrital subsidy from benefiting primary producers via an enhanced trophic cascade.

Spider predation on forest-floor Collembola and evidence for indirect effects on decomposition

Summary Spiders are major predators in forest-floor leaf litter, yet little is known about their impact on prey populations and on forest-floor processes such as litter decomposition. This experiment investigated the effect of removing spiders on Collembola densities. We also examined the potential indirect effect of spider removal on rate of litter disappearance. Twenty-eight 1-m 2 plots were randomly assigned to one of four treatments: O – open, no manipulations; F – fenced, no litter sifted, no spiders removed; FS – fenced, litter sifted, no spiders removed; and FSR – fenced, litter sifted, spiders removed. In early August, we sifted the litter in the FS and FSR plots, removing all encountered spiders from the FSR treatment. A month later, we placed into each plot one 15 × 15-cm litterbag filled with a known amount (ca. 3 g) of dried straw. After six weeks, litterbags were collected and fauna were extracted in a Kempson-McFadyen apparatus. Decreasing spider predation increased densities of Collembola, and increased the rate at which straw disappeared from litterbags. These results indicate that spider predation may reduce Collembola densities enough to lower rates of litter disappearance on the forest floor.

CLIMBING TO REACH FEMALES: ROMEO SHOULD BE SMALL

Abstract The race for reaching mates by the time they are receptive, or sexual selection by scramble competition, has received little attention. We argue that smaller males are favored in species in which the male must climb to reach females located in high habitat patches. This new explanation we term the “gravity hypothesis” of sexual size dimorphism (SSD). We show that a simple biomechanical model of animal movement predicts that: (1) selection should favor a comparatively smaller size in the searching sex when searching involves climbing; and (2) this effect should be stronger in larger species than in smaller species. In reaching high habitats, smaller, faster searchers will be favored either through sexual selection by scramble competition and/or by escaping predation easier by running faster on vertical surfaces. Different spider species are found at a wide range of heights. We compiled a dataset of spider taxa and arranged their habitats according to four height categories, ranked from soil surface to trees. We show that, after controlling for phylogeny, both predictions of the gravity hypothesis of SSD are met. Thus, it appears that the constraint imposed by gravity on climbing males is a selective factor in determining male dwarfism.

Inferring trophic positions of generalist predators and their linkage to the detrital food web in agroecosystems: A stable isotope analysis

Summary We investigated the feasibility of using stable isotope analysis of carbon and nitrogen to infer the trophic level of generalist arthropod predators and the relative strengths of their linkages to detrital and grazing food webs in agroecosystems. Generalist predators are potential biocontrol agents because they prey on herbivores in the grazing food web. Many of these predators also feed on detritivores and fungivores in the soil food web; thus, knowledge of this detrital trophic link may be instrumental to the effective manipulation of generalist predators to enhance their effectiveness in biological control. We analyzed patterns of isotopic concentrations of δ 13 C and δ 15 N in several groups of arthropod predators and their potential prey in replicated cucurbit gardens to which a detrital supplement had been added. Similarity in δ 13 C values between spiders and Collembola suggests that detritivores in this crop system may represent a key prey resource for small spiders, including sheet-web weavers (Linyphiidae) and juvenile wolf spiders (Lycosidae). Isotopic values of δ 15 N place spiders more than one trophic level above Collembola, perhaps due to substantial intraguild predation and cannibalism. Patterns of δ 13 C and δ 15 N for carabid beetles reveal large interspecific variation in the extent of omnivory, in agreement with documented broad ranges of carabid feeding habits. We conclude that stable isotope analysis is a promising tool for investigating trophic connections in arthropod-dominated food webs in agroecosystems.

Predicted climate change alters the indirect effect of predators on an ecosystem process

Changes in rainfall predicted to occur with global climate change will likely alter rates of leaf-litter decomposition through direct effects on primary decomposers. In a field experiment replicated at two sites, we show that altered rainfall may also change how cascading trophic interactions initiated by arthropod predators in the leaf litter indirectly influence litter decomposition. On the drier site there was no interaction between rainfall and the indirect effect of predators on decomposition. In contrast, on the moister site spiders accelerated the disappearance rate of deciduous leaf litter under low rainfall, but had no, or possibly a negative, indirect effect under high rainfall. Thus, changes resulting from the more intense hydrological cycle expected to occur with climate change will likely influence how predators indirectly affect an essential ecosystem process.

Prey availability limits fecundity and influences the movement pattern of female fishing spiders

We conducted a food supplementation field experiment to test two hypotheses: (1) fecundity of the fishing spider Dolomedes triton is limited by a shortage of prey, and (2) the increased movement of adult female D. triton exhibited upon maturation is a foraging adaptation to lessen the impact of food limitation on egg production. Free ranging, uniquely marked adult female fishing spiders were assigned either to a food-supplemented group that received crickets in addition to their natural diet, or to a control group. Juvenile female spiders were also marked and their movement patterns were recorded, but juveniles were not offered supplemental food. Food-supplemented adult females gained weight at a faster rate and hatched more than twice as many offspring as control females. Adult females in the control group moved greater distances per day than did juvenile females. Supplemented adult females moved shorter distances per day than control females, and the movement pattern of fed adults did not differ from that of juveniles. These results support the hypotheses that adult female D. triton are food limited, and that the increased movement of adult females is a switch in foraging behavior that occurs during the reproductive period. Our finding that natural prey shortages limit egg production contrasts with laboratory-based studies of food limitation in the genus Dolomedes, and contradicts a basic assumption of a recent hypothesis that sexual cannibalism in Dolomedes is non-adaptive. These discrepancies highlight the importance of insights gained from field experiments with natural populations.