摘要:

Dispersal is considered a vital life history characteristic for insects exploiting temporary habitats, and life history theorists have often hypothesized an inverse relationship between dispersal capability and habitat persistence. Most often, this hypothesis has been tested using interspecific comparisons of dispersal capability and qualitative estimates of habitat persistence. Consequently, most assessments have failed to control for possible phylogenetic nonindependence and they also lack quantitative rigor. We capitalized on existing intraspecific variation in the dispersal capability of Prokelisia planthoppers to examine the relationship between habitat persistence and dispersal, thereby minimizing possible phylogenetic effects. Two congeneric species (Prokelisia marginata and P. dolus) occur in the intertidal marshes of North America, where they feed exclusively on cordgrasses (Spartina). Because these planthoppers exhibit wing dimorphism, flight—capable adults (macropters with fully developed wings) are easily differentiated from flightless adults (brachypters with reduced wings). Thus, dispersal capability can be readily estimated by the percentage of macropters in a population. At a regional spatial scale, we found a highly significant negative relationship between dispersal capability (percent macroptery) and habitat persistence. In this system, habitat persistence is influenced by a combination of marsh elevation, winter severity, and tidal range, which interact to determine the ability of planthoppers to endure through winter in their primary habitat for development. P. marginata develops primarily in low—marsh habitats during summer, habitats that can be subjected to pronounced winter disturbance due to ice scouring and/or extensive tidal inundation. Levels of winter disturbance of the low marsh are extreme along the Atlantic coast, intermediate along the Pacific, and low along the Gulf. Both the failure of P. marginata populations to remain through winter in this habitat, and the dispersal ability of these populations (92%, 29%, and 17% macroptery, respectively), are correlated with levels of disturbance. Thus, in regions where winter disturbance is high, levels of dispersal are correspondingly high to allow for recolonization of extirpated habitats from overwintering sites on the high marsh. Unlike P. marginata, P. dolus develops primarily in high—marsh habitats, which are much less disturbed on all coasts during winter. Consequently, this species remains year—round in its primary habitat for development, and most populations exhibit relatively low levels of macroptery (<10%). When raised under common garden conditions, many more macropters of both species were produced from Atlantic compared to Gulf populations. Thus the proportion of macropters produced from the populations used in this experiment paralleled the incidence of macroptery measured in the field, providing evidence that the geographic variation in dispersal capability in both species has in part a genetic basis. The results of this study provide strong intraspecific evidence for an inverse relationship between the dispersal capability of insects and the persistence of their habitats.

ISSN:0012-9615    

DOI:10.2307/2963487

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The application of basic ecological concepts to fields of conservation biology and applied environmental sciences is a healthy sign, but before these concepts are widely used, ecology must provide operational definitions and quantifiable methods. Keystone species and interaction strength are concepts with deep practical and theoretical implications. We studied the strength of predation on mussels (Mytilus trossulus) by the keystone seastar Pisaster ochraceus and the whelks Nucella emarginata and N. canaliculata under different environmental conditions in the Oregon intertidal zone. We attempted to determine: (1) the sensitivity of keystone predation to the presence of other predators in the system; (2) the role of other predators in the presence and absence of a keystone species; and (3) the per capita and population—level variability in interaction strengths of strong (keystone) vs. weak interactors. Predation intensity on mussels was measured by recording the survival of mussels transplanted to areas from which seastars, whelks, or both, had been either manually removed or left undisturbed at natural densities. Whelk experimental units were nested within those for the seastar treatment to account for the much larger body size and greater mobility of seastars. Each combination of seastar and whelk treatment was replicated four times in both wave—exposed and wave—protected habitats of two sites that differed in predator densities, primary productivity, and recruitment and growth rates of prey species. Predation intensity by the keystone predator was strong under all site $\times$ wave exposure combinations, and was unaffected by the presence of whelks. Whelks, in contrast, had ecologically important effects on mussel survival in the absence, but not in the presence, of the keystone predator. Population (total) interaction strength between seastars and mussels was 2—10 times stronger than that between whelks and mussels across sites and wave exposures. Per capita interaction strength of seastars was two to three orders of magnitude larger than that of whelks. However, per capita effects of seastars were more variable between sites and wave exposures, probably because simple density values grossly underestimate the ability of mobile predators to localize prey. Such interactive effects and variability in interaction strengths between keystone and weak predators may characterize all keystone predator—dominated systems, but data currently are insufficient to test this proposition. Negative effects of seastars on whelk density were observed<4 mo following initiation of Pisaster removals. Seastars also had a negative effect on whelk sizes, which took longer to appear, after 6 mo of continuous Pisaster removal. Negative effects of seastars on whelks appeared to be stronger in places with higher densities of predators, partially explaining the reduced predation intensity of whelks observed in the presence of seastars. Our results support the idea that in keystone—dominated systems, species other than the keystone species have only minor, if any, effects on the rest of the community, and thus might be cited by some asredundant species. However, our results also indicate that, after the loss of a keystone species, previouslyredundant species can partially compensate for the reduced predation and adopt a major role in the altered system. Such responses are potentially an important force in stabilizing communities. Further, such possible compensatory capabilities of alternative consumers suggests that, at least for predators, the termredundant species conveys an inaccurate image of the potential importance of weak interactors and should be abandoned.

ISSN:0012-9615    

DOI:10.2307/2963488

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Risk of larval mortality is a critical component of models and debates concerning the ecology and evolution of the differing reproductive characteristics exhibited by marine invertebrates. In these discussions, predation often is assumed to be a major source of larval mortality. Despite limited empirical support, most marine larvae are thought to be palatable and broadly susceptible to generalist predators. Previous studies of larval—planktivore interactions have focused primarily on larvae that typically feed, grow, and develop for weeks to months in the plankton. Such planktotrophic species commonly produce large numbers of small larvae that disperse over vast distances. In contrast, the nonfeeding lecithotrophic larvae from sessile invertebrates that brood are often large and conspicuous, lack morphological defenses, and have limited dispersal distances because they typically are competent to settle minutes to hours after spawning. Interactions between lecithotrophic larvae and consumers are not well studied. This has limited the ability of previous authors to test broad generalities about marine larvae. We show that brooded larvae of Caribbean sponges (11 species) and gorgonians (three species) as well as brooded larvae of temperate hydroids (two species) and a bryozoan are unpalatable to co—occurring fishes. In contrast, brooded larvae of temperate ascidians (three species), a temperate sponge, and Caribbean hard corals (three species) are readily consumed by fishes, as are larvae from four of six species of synchronous broadcast—spawning gorgonians from the Florida Keys. Frequencies of survivorship for larvae attacked and rejected by fishes were high and statistically indistinguishable from frequencies for unattacked control larvae. Frequency of metamorphosis (when it occurred) of rejected larvae never differed significantly from that of unattacked control larvae. Assays testing for larval vs. adult chemical defenses for five species with distasteful larvae showed that larvae of all five species were chemically distasteful to fishes, whereas only three of five adult extracts deterred fish feeding. A comparison of larval palatability among chemically rich taxa showed that brooded larvae were significantly more likely to be unpalatable (86% of the species tested) than larvae of broadcasters (33%), and that palatable larvae were rarely released during the day (23%) while unpalatable larvae usually were (89%). Additionally, the frequency of bright coloration was high (60%) for unpalatable larvae and low (0%) for palatable larvae, suggesting that unpalatable larvae often may be aposematically colored. Results of this broad survey cast doubt on the widely accepted notion that virtually all marine larvae are suitable prey for most generalized planktivores. Among species that do not chemically or physically protect larvae against fishes, selection appears to favor the release of larvae at night, or the production of smaller more numerous offspring that grow and develop at sea as a way of escaping consumer—rich benthic habitats. Because distasteful larvae are not similarly constrained, distasteful species should exhibit reproductive and larval characteristics selected more by the fitness—related consequences of larval development mode and dispersal distance than by the necessity of avoiding benthic predators. Production of large larvae and retention of offspring in parental habitats that have proved to be suitable for growth and reproduction have both been proposed as advantageous, but these advantages often were assumed to be offset by losses due to increased larval apparency to fishes. This assumed trade—off is not mandatory because larvae can be defended chemically. Distasteful larvae tend to be conspicuous, localized dispersers that can co—occur with benthic fishes, and yet not be consumed.

ISSN:0012-9615    

DOI:10.2307/2963489

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

To test how well use of discrete trophic levels represents pelagic trophic structure, we compiled dietary data from>200 lake trout and pelagic forage fish populations and calculated a continuous (fractional) measure of trophic position for each population. Although discrete trophic levels qualitatively represent broad—scale patterns in trophic structure, pelagic food webs are characterized by complexity and omnivory, thereby limiting the ability of discrete trophic levels to quantitatively represent trophic structure in terms of mass transfer and energy flow. Lake trout trophic position, which ranged from 3.0 to 4.6, explained 85% of the between—lake variability in mean PCB levels in lake trout muscle tissue, providing a significant improvement over the use of discrete trophic levels as a predictor of contaminant levels. Having demonstrated the utility of trophic position, we developed a generalizedtrophic position model of lake trout food webs. This approach eliminates minor trophic linkages, calculates a fractional measure of each species' trophic position, and aggregates species of similar trophic position into trophic guilds. Thisrealized model represents trophic structure in terms of mass transfer and accounts for the complexity and omnivory that characterize aquatic food webs. In our trophic position model, smelt (a species of pelagic forage fish) were designated a trophic guild separate from other pelagic forage fish, due to their elevated trophic position. Separate consideration of smelt was supported by elevated lake trout trophic position, PCB, and Hg levels in lakes containing smelt. Consideration of omnivory caused biomagnification factors (BMFs) to be many times higher than BMFs that ignored omnivory. These omnivory—corrected BMF estimates appeared to be more consistent with values calculated using stable nitrogen isotopes (δ15N), an alternative continuous measure of trophic position. (δ15N) provided trophic position estimates that generally corresponded with our diet—derived estimates.

ISSN:0012-9615    

DOI:10.2307/2963490

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

In this paper, we investigate whether a stage—structured population model can explain major features of dynamics of the herbivores Daphnia galeata and Bosmina longirostris reared under controlled laboratory conditions. Model parameters are determined from independent individual—based information gleaned from the literature on feeding, growth, reproduction, and survivorship of these herbivores. We tested predictions of our model against published observations on the dynamics of laboratory populations. The feeding protocols used in these experiments present a highly dynamic food environment that rigorously challenges the ability of stage—structured models to predict the dynamics of populations as they approach equilibrium. For both herbivore species, the models correctly predict feasible equilibria and some features of their dynamics (e.g., periodicity, cycle amplitude, demography, and fecundity) for experiments in which the species were raised in isolation and food transfers were relatively frequent (at least one transfer per instar). With frequent food transfers, the model also correctly predicts coexistence of the herbivores during competition experiments and suggests a novel mechanism for coexistence. The model fails to predict correctly single—species dynamics and the outcome of competition in experiments where food transfers were infrequent and utilization of internal reserves by individuals in the populations must have been high.

ISSN:0012-9615    

DOI:10.2307/2963491

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

I compared effects of increased temperature and litter from different Alaskan tundra plant species on cycling of carbon and nitrogen through litter and soil in microcosms. Warming between 4°and 10°C significantly increased rates of soil and litter respiration, litter decomposition, litter nitrogen release, and soil net nitrogen mineralization. Thus, future warming will directly increase rates of carbon and nitrogen cycling through litter and soil in tundra. In addition, differences among species' litter in rates of decomposition, N release, and effects on soil net nitrogen mineralization were sometimes larger than differences between the two temperature treatments within a species. Thus, changes in plant community structure and composition associated with future warming will have important consequences for how elements cycle through litter and soil in tundra. In general, species within a growth form (graminoids, evergreen shrubs, deciduous shrubs, and mosses) were more similar in their effects on decomposition than were species belonging to different growth forms, with graminoid litter having the fastest rate and litter of deciduous shrubs and mosses having the slowest rates. Differences in rates of litter decomposition were more related to carbon quality than to nitrogen concentration. Increased abundance of deciduous shrubs with future climate warming will promote carbon storage, because of their relatively large allocation to woody stems that decompose slowly. Changes in moss abundance will also have important consequences for future carbon and nitrogen cycling, since moss litter is extremely recalcitrant and has a low potential to immobilize nitrogen.

ISSN:0012-9615    

DOI:10.2307/2963492

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY