We present the results of an experiment designed to examine the role predator—avoidance behaviors play in structuring a community. Previous studies have shown that small bluegills (Lepomis macrochirus) shift their habitat use in the presence of their predator, the largemouth bass (Micropterus salmoides). The addition of bass causes small bluegills to make greater use of structured (vegetated) habitats and reduces their use of open (limnetic) habitats. We hypothesized that these effects of bass on bluegill behaviors should lead to an increase in the abundance and mean size of zooplankton when bass are present. We tested this prediction in two ponds by stocking small bluegills with and without bass. Pronounced differences in zooplankton abundance and size structure developed between treatments, with three cladocerans (Daphnia, Diaphanosoma, and Ceriodaphnia), and the phantom midge Chaoborus, becoming more abundant with bass. These effects of bass were not due to changes in bluegill abundance, as very few bluegills were eaten by the bass, and bluegill densities did not differ between treatments. Calculation of bluegill feeding rates shows that the fish exerted substantial mortality on zooplankton and that cladocerans experienced lower per capita death rates (due to bluegill) in the presence of bass. We discuss these results in light of current theory predicting how foragers should deplete prey among habitats that differ in mortality risk.

This paper presents the results of a long—term study of changing predator densities and cascading effects in a Michigan lake in which the top carnivore, the largemouth bass (Micropterus salmoides), was eliminated in 1978 and then reintroduced in 1986. The elimination of the bass was followed by a dramatic increase in the density of planktivorous fish, the disappearance of large zooplankton (e.g., two species of Daphnia that had historically dominated the zooplankton community), and the appearance of a suite of small—bodied cladoceran (zooplankton) species. The system remained in this state until bass were reintroduced. As the bass population increased, the system showed a steady and predictable return to its previous state; planktivore numbers declined by two orders of magnitude, large—bodied Daphnia reappeared and again dominated the zooplankton, and the suite of small—bodied cladocerans disappeared. Within each cladoceran species there was a steady increase in mean adult body size as planktivore numbers declined. Total zooplankton biomass increased ≈ 10—fold following the return of large—bodied Daphnia, and water clarity increased significantly with increases in Daphnia biomass and total cladoceran biomass. These changes in community structure and trophic—level biomasses demonstrate the strong impact of removing a single, keystone species, and the capacity of the community to return to its previous state after the species is reintroduced.

Predators can alter the outcome of ecological interactions among other members of the food web through their effects on prey behavior. While it is well known that animals often alter their behavior with the imposition of predation risk, we know less about how other features of predators may affect prey behavior. For example, relatively few studies have addressed the effects of predator identity on prey behavior, but such knowledge is crucial to understanding food web interactions. This study contrasts the behavioral responses of the freshwater snail Physellagyrina to fish and crayfish predators. Snails were placed in experimental mesocosms containing caged fish and crayfish, so the only communication between experimental snails and their predators was via non-visual cues. The caged fish and crayfish were fed an equal number of snails, thereby simulating equal prey mortality rates. In the presence of fish, the experimental snails moved under cover, which confers safety from fish predators. However, in the presence of crayfish, snails avoided benthic cover and moved to the water surface. Thus, two species of predators, exerting the same level of mortality on prey, induced very different behavioral responses. We predict that these contrasting behavioral responses to predation risk have important consequences for the interactions between snails and their periphyton resources.

Chemical signals released by predators or injured prey often induce shifts in the traits of prey species, which may in turn affect species interactions. Here we investigate the role that chemical cues play in mediating species interactions in the littoral food web of lakes. Previous studies have shown that predators induce shifts in the morphology, life history, and behavior of the freshwater snail Physella, but the ecological consequences of developing these inducible defenses are not well documented. We observed habitat use of the freshwater snail Physella gyrina along a depth gradient in a natural lake, and found they increased their use of covered habitats with increasing depth. We hypothesized that this habitat shift was due to changes in the level and type of predation risk, and that the habitat shift would affect periphyton standing crops. These hypotheses were tested in a mesocosm experiment in which we manipulated the presence of molluscivorous fish and crayfish. Predators were confined to cages and snail density was identical in all treatments, so any effects of predators were mediated through trait shifts induced by chemical cues. In the presence of fish, Physella moved under cover, but in the presence of crayfish, Physella avoided cover and moved to the water surface. These non‐lethal effects of predators on snail habitat use influenced the interaction between snails and their periphyton resources. In the presence of fish, periphyton standing crop in covered habitats was reduced to just 8% of periphyton in the absence of fish. Crayfish had no significant effect on periphyton in covered habitats, but they reduced periphyton in near‐surface habitats to 39% of the standing crop in the absence of crayfish. The combined effects of fish and crayfish were generally intermediate to their individual effects. We conclude that because chemical cues often have strong effects on individual traits and trophic interactions are sensitive to trait values, chemical cues may play an important role in shaping the structure and dynamics of food webs.

Predators often induce shifts in the traits of nearby prey, and these trait shifts are important in mediating a variety of evolutionary and ecological processes. However, little is known about the spatial and temporal scales over which predators induce trait shifts. We empirically determined the spatial scale of predator avoidance by measuring the habitat use and growth rates of snails (Physa acuta) held at varying distances from a caged pumpkinseed sunfish (Lepomis gibbosus). Refuge use was highest near the fish and gradually decayed to background level, with a characteristic response range of 1.0 m. Snail growth rates were negligible near the predator but increased with greater separation from fish. The dependence of behavior on the age of chemical cues was measured in a mesocosm experiment in which water was withdrawn from a tank holding pumpkinseeds and held for varying lengths of time before being added to experimental mesocosms with snails. Fresh cues elicited the strongest habitat shifts relative to well‐water controls, and avoidance behavior decayed in an exponential manner with increasing cue age. The characteristic lifetime of avoidance behavior was 41 h. Taken together, these results allow us to begin to describe the behavioral landscape created by mobile predators. Corresponding Editor: S. P. Lawler

Perturbations to the density of a species can be propagated to distant members of a food web via shifts in the density or the traits (i.e. behavior) of intermediary species. Predators with differing foraging modes may have different effects on prey behavior, and these effects may be transmitted differently through food webs. Here we test the hypothesis that shifts in the type of predator present in a food web indirectly affect the prey's resource independent of changes in the density of prey. We assessed the importance of predator identity in mediating the grazing effects of the freshwater snail Physa integra on its periphyton resources using field and mesocosm studies. Field observations showed that Physa used covered habitats more in ponds containing fish than in ponds containing crayfish or no predators at all. A field experiment confirmed that snail behavior depended on predator identity. Physa placed near caged pumpkinseed sunfish (Lepomis gibbosus) selected covered habitats, but Physa placed near caged crayfish (Orconectes rusticus) moved to the surface of the water. The effects of predator identity on periphtyon were then examined in a mesocosm experiment, using caged predators. Habitat use of Physa was similar to their habitat use in the field experiment. In the presence of caged sunfish, periphyton standing crop in covered habitats was reduced to 34% of the standing crop in the presence of crayfish. In contrast, periphyton in near-surface habitats was 110% higher in the presence of fish than in the presence of crayfish. Thus, the effects of predator identity on Physa behavior cascaded through the food web to affect the abundance and spatial distribution of periphyton.

Isovalency rationalizes fundamental chemical properties of elements in the same group, but often fails to account for differences in the molecular structure due to the distinct atomic sizes and electron-pair repulsion of the isovalent atoms. With respect to main group V, saturated hydrides of nitrogen are limited to ammonia (NH3) and hydrazine (N2H4) along with ionic and/or metal-bound triazene (N3H5) and potentially tetrazene (N4H6). Here, we present a novel approach for synthesizing and detecting phosphanes formed via non-classical synthesis exploiting irradiation of phosphine ices with energetic electrons, subliming the newly formed phosphanes via fractionated sublimation, and detecting these species via reflectron time-of-flight mass spectrometry (ReTOF) coupled with vacuum ultraviolet (VUV) single photon ionization. This approach is able to synthesize, to separate, and to detect phosphanes as large as octaphosphane (P8H10), which far out-performs the traditional analytical tools of infrared spectroscopy and residual gas analysis via mass spectrometry coupled with electron impact ionization that could barely detect triphosphane (P3H5) thus providing an unconventional tool to prepare complex inorganic compounds such as a homologues series of phosphanes, which are difficult to synthesize via classical synthetic methods.

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