Ecology

Walters, Annika W.; Post, David M.

doi: 10.1890/08-0273.1pmid: 19137932

Streams experience frequent natural disturbance and are undergoing considerable anthropogenic disturbance due to dam construction and water diversion. Disturbance is known to impact community structure, but its effect on food chain length is still a matter of considerable debate. Theoretical models show that longer food chains are less resilient to disturbance, so food chain length is predicted to be shorter following a disturbance event. Here we experimentally test the effect of disturbance on food chain length in streams by diverting stream flow. We found that our experimental low-flow disturbance did not alter food chain length. We did see an effect on body-size structure in our food webs suggesting that food chain length may be an insensitive indicator of disturbance. We suggest that habitat heterogeneity and food web complexity buffer the effect of disturbance on food chain length. The theoretical predictions of disturbance on food chain length are only likely to be seen in homogeneous systems that closely approximate the linear food chains the models are based upon.

Walters, Annika W.; Post, David M.

doi: 10.1890/08-0273.1pmid: 19137932

Streams experience frequent natural disturbance and are undergoing considerable anthropogenic disturbance due to dam construction and water diversion. Disturbance is known to impact community structure, but its effect on food chain length is still a matter of considerable debate. Theoretical models show that longer food chains are less resilient to disturbance, so food chain length is predicted to be shorter following a disturbance event. Here we experimentally test the effect of disturbance on food chain length in streams by diverting stream flow. We found that our experimental low‐flow disturbance did not alter food chain length. We did see an effect on body‐size structure in our food webs suggesting that food chain length may be an insensitive indicator of disturbance. We suggest that habitat heterogeneity and food web complexity buffer the effect of disturbance on food chain length. The theoretical predictions of disturbance on food chain length are only likely to be seen in homogeneous systems that closely approximate the linear food chains the models are based upon.

Fodrie, F. Joel; Kenworthy, Matthew D.; Powers, Sean P.

doi: 10.1890/07-1679.1pmid: 19137933

We manipulated predator densities and prey vulnerability to explore how interactions between two predators affect overall mortality of their shared prey. Our three-member study system included eastern oysters ( Crassostrea virginica ) and two of its major consumers: southern oyster drills ( Stramonita haemastoma ) and stone crabs ( Menippe adina ). Field experiments demonstrated that drills and crabs foraging together generated higher than expected oyster mortality based on each species operating independently, even though crabs also killed some drills. In subsequent laboratory trials, we experimentally mimicked the handling of oysters by foraging crabs and confirmed that crabs facilitated drills by breeching oyster valves, thereby granting easy access for drills to their prey. Facilitation between co-occurring predators is uncommon and typically occurs because the behavior or habitat selection of a prey species is altered by the presence of one predator, consequently making the prey more susceptible to another predator. Whereas oysters are sedentary regardless of the predator field, we observed an entirely different mechanism that resulted in predator facilitation. This involved direct attacks on the physical defenses of oysters by one predator that ultimately increased the overall consumption rate of foraging species. These dynamics significantly enhanced mortality risk for a foundation species within an estuarine ecosystem.

Fodrie, F. Joel; Kenworthy, Matthew D.; Powers, Sean P.

doi: 10.1890/07-1679.1pmid: 19137933

We manipulated predator densities and prey vulnerability to explore how interactions between two predators affect overall mortality of their shared prey. Our three‐member study system included eastern oysters (Crassostrea virginica) and two of its major consumers: southern oyster drills (Stramonita haemastoma) and stone crabs (Menippe adina). Field experiments demonstrated that drills and crabs foraging together generated higher than expected oyster mortality based on each species operating independently, even though crabs also killed some drills. In subsequent laboratory trials, we experimentally mimicked the handling of oysters by foraging crabs and confirmed that crabs facilitated drills by breeching oyster valves, thereby granting easy access for drills to their prey. Facilitation between co‐occurring predators is uncommon and typically occurs because the behavior or habitat selection of a prey species is altered by the presence of one predator, consequently making the prey more susceptible to another predator. Whereas oysters are sedentary regardless of the predator field, we observed an entirely different mechanism that resulted in predator facilitation. This involved direct attacks on the physical defenses of oysters by one predator that ultimately increased the overall consumption rate of foraging species. These dynamics significantly enhanced mortality risk for a foundation species within an estuarine ecosystem.

Wise, Michael J.; Abrahamson, Warren G.

doi: 10.1890/08-0277.1pmid: 19137934

Many populations of goldenrod show a peculiar, genetically controlled stem dimorphism. In Solidago altissima , for instance, while most stems are erect, a sizable minority (the ““candy-cane”” stems) nod at the apex during growth. We used data from three studies to test the hypothesis that this candy-cane growth form confers resistance to herbivory. In a controlled growth trial, we showed that nodding is a temporary phenomenon that coincides with the oviposition period of at least two common apex-attacking herbivores: the tephritid galler Eurosta solidaginis and the gall midge Rhopalomyia solidaginis . In a large field survey, stems of candy-cane genets were only half as likely to be ovipunctured by E. solidaginis . In a common-garden study, candy-cane stems were less likely to be ovipunctured by E. solidaginis , and they were galled only half as often by R. solidaginis as erect stems. These results suggest that the candy-cane stems of goldenrod possess a resistance strategy that allows them to essentially duck and hide from certain herbivores.

Wise, Michael J.; Abrahamson, Warren G.

doi: 10.1890/08-0277.1pmid: 19137934

Many populations of goldenrod show a peculiar, genetically controlled stem dimorphism. In Solidago altissima, for instance, while most stems are erect, a sizable minority (the “candy‐cane” stems) nod at the apex during growth. We used data from three studies to test the hypothesis that this candy‐cane growth form confers resistance to herbivory. In a controlled growth trial, we showed that nodding is a temporary phenomenon that coincides with the oviposition period of at least two common apex‐attacking herbivores: the tephritid galler Eurosta solidaginis and the gall midge Rhopalomyia solidaginis. In a large field survey, stems of candy‐cane genets were only half as likely to be ovipunctured by E. solidaginis. In a common‐garden study, candy‐cane stems were less likely to be ovipunctured by E. solidaginis, and they were galled only half as often by R. solidaginis as erect stems. These results suggest that the candy‐cane stems of goldenrod possess a resistance strategy that allows them to essentially duck and hide from certain herbivores.

Gonzales, Emily K.; Arcese, Peter

doi: 10.1890/08-0435.1pmid: 19137935

The dominance of nonnative plants coupled with declines of native plants suggests that competitive displacement drives extinctions, yet empirical examples are rare. Herbivores, however, can alter vegetation structure and reduce diversity when abundant. Herbivores may act on mature, reproductive life stages whereas some of the strongest competitive effects might occur at early life stages that are difficult to observe. For example, competition by perennial nonnative grasses can interfere with the establishment of native seeds. We contrasted the effects of ungulate herbivory and competition by neighboring plants on the performance of native plant species at early and established life stages in invaded oak meadows. We recorded growth, survival, and flowering in two native species transplanted as established plants, six native species grown from seed, and five extant lily species as part of two 2 × 2 factorial experiments that manipulated herbivory and competition. Herbivory reduced the performance of nearly all focal native species at early and established life stages, whereas competition had few measurable effects. Our results suggest that herbivory has a greater local influence on native plant species than competition and that reducing herbivore impacts will be required to successfully restore endangered oak meadows where ungulates are now abundant.

Gonzales, Emily K.; Arcese, Peter

doi: 10.1890/08-0435.1pmid: 19137935

The dominance of nonnative plants coupled with declines of native plants suggests that competitive displacement drives extinctions, yet empirical examples are rare. Herbivores, however, can alter vegetation structure and reduce diversity when abundant. Herbivores may act on mature, reproductive life stages whereas some of the strongest competitive effects might occur at early life stages that are difficult to observe. For example, competition by perennial nonnative grasses can interfere with the establishment of native seeds. We contrasted the effects of ungulate herbivory and competition by neighboring plants on the performance of native plant species at early and established life stages in invaded oak meadows. We recorded growth, survival, and flowering in two native species transplanted as established plants, six native species grown from seed, and five extant lily species as part of two 2 ×× 2 factorial experiments that manipulated herbivory and competition. Herbivory reduced the performance of nearly all focal native species at early and established life stages, whereas competition had few measurable effects. Our results suggest that herbivory has a greater local influence on native plant species than competition and that reducing herbivore impacts will be required to successfully restore endangered oak meadows where ungulates are now abundant.

Ram, Karthik; Preisser, Evan L.; Gruner, Daniel S.; Strong, Donald R.

doi: 10.1890/08-0228.1pmid: 19137936

A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local conditions set a stage upon which larger-scale dynamics play out. Soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root-feeding herbivores. In this study, we examine the mechanisms responsible for patchy occurrence of an entomopathogenic nematode, Heterorhabditis marelatus , in a California coastal prairie. One hypothesis proposes that biotic factors such as competition and natural enemies could regulate occurrence of EPN populations. We found that fungi and other enemies of EPN, although locally potent, did not explain the patterns of incidence across sites. Abiotic factors also have strong effects on EPN persistence, especially for vulnerable free-living stages. Thus, we tested the hypothesis that patchy occurrence of EPN on a large landscape was driven by differences in soil moisture. Our research uses long-term data on nematode incidence in combination with a landscape-level experiment to demonstrate the lack of a correlation between soil moisture and long-term persistence. A year-long experiment showed EPN mortality was weakly correlated with soil moisture among our study sites. Thirteen years of data, however, showed that colonization rates were highly correlated with long-term persistence. Sites with highest long-term persistence experienced the highest rates of rhizosphere colonization, extinction, and turnover. As a result, we concluded that metapopulation dynamics override limitations set by local and short-term abiotic conditions to determine long-term persistence in this parasite-driven trophic cascade.

Ram, Karthik; Preisser, Evan L.; Gruner, Daniel S.; Strong, Donald R.

doi: 10.1890/08-0228.1pmid: 19137936

A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local conditions set a stage upon which larger‐scale dynamics play out. Soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root‐feeding herbivores. In this study, we examine the mechanisms responsible for patchy occurrence of an entomopathogenic nematode, Heterorhabditis marelatus, in a California coastal prairie. One hypothesis proposes that biotic factors such as competition and natural enemies could regulate occurrence of EPN populations. We found that fungi and other enemies of EPN, although locally potent, did not explain the patterns of incidence across sites. Abiotic factors also have strong effects on EPN persistence, especially for vulnerable free‐living stages. Thus, we tested the hypothesis that patchy occurrence of EPN on a large landscape was driven by differences in soil moisture.