Experiments were performed to determine if earlier colonists inhibited, enhanced, or were necessary for establishment of later colonists during development of an estuarine fouling community at Lewes, Delaware. We determined the significance of earlier stages on the successional process by functionally removing early colonizing species. Since settlement of sessile invertebrates onto our experimental test plates was seasonal, we were able to accomplish functional removal of early colonists by putting out clean test panels after these species had ceased settling. Comparisons between panels initially submerged at three different times in 1974 and 1975, and between panels put out at one-month intervals throughout the study (to describe seasonal settlement patterns) allowed us to determine interactions between adult populations of earlier colonists and colonizing individuals of later arriving species.The dominant sessile species in our system and their times of settlement were: a barnacle (Balanus improvisus) - April through June, a polychaete (Hydroides dianthus) - July and August, a tunicate (Molgula manhatensis) - June through October, a hydroid (Tubularia crocea) - July through October, and a mussel (Mytilus edulis) - November through April. All successional series eventually came to be dominated by M. edulis, and it persisted as the dominant for over a year.A variety of species interactions were observed. M. edulis inhibited colonization by all other dominants and B. improvisus partially inhibited settlement of M. manhattensis. The presence of adult M. manhattensis had no influence on summer settlement of T. crocea, but the hydroids enhanced settlement of tunicates in the fall. During both years of our study, larger settlements of mussels were noted on panels harboring tunicates and hydroids than on bare surfaces. H. dianthus, on the other hand, became established only on bare substrates, and colonization was almost totally inhibited by other dominants.Development in our fouling community did not conform to any single model of community development presented to date. Instead, components of several models were observed within our relatively simple (in terms of number of species) system. For example, facilitation (enhancement of later colonists by earlier ones) and inhibition (resistance of earlier colonists to invasion by later colonists) were both observed. However, we found no evidence earlier colonists were essential for establishment of the next developmental stage. In fact, inhibitory interactions appeared to be much more prevalent than facilitative interactions. The former may also have more profound effects on community development since they more often determine eventual species compositions.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Department of Biology, Washington and Lee University, Lexington, Virginia 24450 USA Abstract. We tested the hypothesis that a bitrophic (third and fourth level) arthropod predator can exert a cascading, top-down influence on other arthropods and plants in an early successional old field. First-stadium mantids, Tenodera sinensis, were added to replicated open-field plots in numbers corresponding to naturally occurring egg hatch density and allowed to remain for ;2 mo. Sticky-trap dispersal barriers around both control and mantid-addition plots allowed us to monitor emigration of arthropods continuously during the experiment. Biomass of herbivores, carnivores, and plants, and abundances of arthropod taxa within plots were determined at the beginning, middle, and end of the experiment. Ecological Society of AmericaThe impact of mantids on the community was a top-down trophic cascade, beginning at the fourth trophic level and evident at each of the lower three levels. Mantids induced marked behavioral responses in other predators, but interference among predators did not prevent the trophic cascade. The most common predators, cursorial spiders, emigrated from mantid addition plots in significantly greater numbers than from controls. This behavioral response may have resulted from avoidance of predation or competition.Mantids decreased biomass of herbivorous arthropods through predation, and this decrease in turn increased biomass of plants. Therefore, these generalist predators were able to decrease herbivory enough to affect plant growth. This and other recent studies indicate that top-down effects can be important in structuring terrestrial communities. Ours is the first example of a top-down cascade by a generalist arthropod predator in a nonagricultural ecosystem and illustrates the importance of detecting behavioral responses in studies of trophic interactions.

We examined density dependence in population attributes and community impact of a generalist predator by experimentally mimicking natural variation in initial cohort densities produced by synchronous egg hatch in Mantis religiosa (Mantodea: Mantidae). Mantid cohorts within the normal range of emergence from a single egg mass were established in a replicated, well-controlled open field experiment. On the scale of the progeny from a single female, density-dependent food limitation caused mortality and ontogenetic asynchrony to increase with increasing density. All cohorts converged to a common level of abundance and biomass because both development rate and population size declined with increasing initial density.Numbers and biomass of other arthropods generally declined with increasing initial density of mantids, although there were both positive and negative effects on different taxa. The abundance of hemipterans (almost exclusively herbivorous mirids) increased in the presence of mantids; this was an indirect effect as large in magnitude as any of the direct reductions in abundance of other taxa. Per capita interaction strengths of mantids on most taxa generally were weak except for the strong positive interaction with hemipterans. In spite of different man tid development rates among treatments, predator load (proportion of arthropod biomass present as predators) for all three treatments, attributable mainly to man tid biomass, converged to approximately five times control level by the end of the experiment. The differences in predator loads between control and treatment plots thus may represent different levels of predator saturation: one for control plots, where predator load was constant over time and in which generalists contributed relatively little to predator biomass, and a higher one for treatment plots, in which generalists comprised the bulk of predator biomass. Predator load may therefore be an indicator of the relative importance of generalist vs. specialist predators in terrestrial arthropod assemblages.

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An intensive sampling study, undertaken in a natural sandflat population of an estuarine neogastropod, Ilyanassa obsoleta, revealed a marked degree of spatial heterogeneity for a number of important characteristics. Growth rate could be inferred by observing changes in size—classes over time, but the inequitable spatial distribution of size—classes renders size/frequency analysis impractical and of dubious predictive value for this population. Overall parasitism rate in this population is high, generally increases exponentially with increasing age, and creates a relatively narrow reproductive fenestra temporalis by apparently permanent sexual impairment of infected snails. In fact, the strong influence of parasites on behavior and physiology of snails may necessitate distinguishing between apparent and functional individuals, with regard to their role in estuarine benthic communities. However, incidence of both parasitism and parasite species is highly variable in space. Nearly half of all samples taken departed significantly from the overall sex ratio, indicating that distribution of sexes is also heterogeneous. Since there is little predictive relationship between specific samples and overall population trends, it is likely that differentiating between trends that are truly temporal and those that are in reality spatial is difficult at best. Moreover, high spatial heterogeneity in the field has implications for the interpretation of laboratory studies as well as those done in the field and indicates that generalizations about the biology and ecology of this species gleaned from small, albeit random, samples should be viewed with caution.

Two adjacent, abandoned hayfields in central New York State were subjected to nutrient enrichment perturbation by means of a single application of 10-10-10 N, P, K fertilizer early in the growing season of 1970. Aboveground arthropod herbivores and carnivores were monitored with respect to net productivity (dB/dt) and diversity for one growing season (1970) in the younger field (6 yr old), and for two growing seasons (1970 and 1971) in the older field (17 and 18 yr old). Sampling of arthropods was done with a gasolinepowered suction sampler between 1200 and 1600 h, twice weekly.Stability of the old-field arthropods was defined as resistance to change imposed by external (fertilizer) perturbation and was measured in terms of (1) magnitude (amplitude of deflection from ground state, (2) rate of initial response to perturbation, and (3) rate of damping or return to ground state. The degree of deflection from ground state was determined by comparing fertilized (treated) plots with unfertilized (control) plots in each field.During the first growing season the older, more species-rich field was less stable with respect to magnitude and rate of productivity deflection than the younger, less species-rich field at both arthropod consumer levels. In addition, the magnitude of productivity deflection from ground state was lower in the carnivore level than in the herbivore trophic level.By the second growing season, the productivity of the arthropod consumers in the treated plots of the older field had not yet returned to control levels. The productivity of arthropod herbivores in the older field exhibited a relatively lower magnitude of response in 1971 than in 1970, while the arthropod carnivores in the older field exhibited a significantly greater difference from ground state in 1971 than in 1970.It was concluded that ( 1) the magnitude of deflection caused by the enrichment perturbation was decreased up the trophic levels from the herbivore to the carnivore level, (2) the rate of response to perturbation decreased up the trophic levels, and (3) although in control plots diversity increased and productivity decreased with increasing successional age for both trophic levels, stability was not positively related to either higher diversity or to increasing age in either consumer trophic level in this old-field successional ecosystem.

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