As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging ; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (' disc equation ') formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m x2 ). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.A multivariate analysis of 468 ' spot ' estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6 %, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shore...

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.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. Progressive stabilization ("fixation") of a sand dune system by vegetational succession has allowed one anuran species (Bufo bufo) to encroach and breed in areas formerly the sole preserve of another (B. calamita). We investigated interactions and the potential for competition between larvae of these two species under conditions of community transition. Survival, growth rates, and spatial and feeding niche overlaps of larvae as well as abundance of the unicellular microorganism Prototheca richardsi (a mediator of interference competition between tadpoles) were measured in three duplicated sets of ponds in the same habitat. One set was used by B. bufo only, one by B. calamita only, and one by both species. B. bufo larvae survived and grew as well, or better, in mixed-species ponds as they did when alone. By contrast, B. calamita larvae grew more slowly, suffered higher premetamorphic mortality, and emerged as smaller toadlets in mixed-species ponds, compared with single-species ponds. B. calamita changed its spatial niche in mixed-species ponds much more markedly than did B. bufo, but spatial and feeding niche overlaps remained high. P. richardsi was rare in guts and feces of larvae in single-species ponds, but it occurred in large numbers in guts and feces of larvae from mixed-species ponds. The data indicate that B. bufo is a superior competitor to B. calamita and that both resource and interference competition may occur between the two species in this transitional situation.

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging ; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (' disc equation ') formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m x2 ). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.A multivariate analysis of 468 ' spot ' estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6 %, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shore...

We investigated the roles of competition and predation in the structuring of mixed anuran communities (Rana temporaria and Bufo bufo) in an English sand‐dune system. Rana temporaria breeds earlier in spring than B. bufo. but the latter species was numerically superior in all ponds at the study site. We measured survival and growth rates of larvae in three duplicate sets of natural ponds. In one set we removed B. bufo spawn, leaving only R. temporaria: in a second sel we removed R. temporaria spawn, leaving only B. bufo: and in a third set we left boih together. The third set also contained substantial numbers of predatory fish (Perea fluviatilis and Gasterosteus aeuleatus). Rana temporaria survival was lower than that of B, bufo in all ponds, on average by sixfold. Rana temporaria fared better on its own than in the mixed‐species ponds, but lower survival in the latter ponds was probably due to fish predation rather than to Inierspecific competition. By eonlrast, B. bufo survival was sevenfold higher in mixed‐species compared with single‐species ponds, probably because they are distasteful to fish and thus benefited indirectly from fish‐predation of invertebrates, Protothieca rieharsi. a mediator of interference competition between anuran larvae under laboratory conditions, was present in some of the dune ponds but was not correlated with larval growth inhibition. Taken together the evidence implicated predation of larvae and intraspecific competition, rather than interspecific competition, as the dominant structuring forces in this community.

Two major types of indirect competition have been recognised in natural communities, notably scramble for resources and interference to gain advantage by various alternative mechanisms. Interference effects are however often difficult to demonstrate in the field and their significance in nature requires more extensive study. Larvae of the anurans Bufo bufo and Bufo calamita compete strongly both in the field and in the laboratory, with the former species generally superior to the latter. Under laboratory conditions an interference component to this competition is readily demonstrable so we carried out an experiment to determine whether interference effects between the larvae of these species are also detectable in natural ponds. Larvae were reared at natural densities in cages immersed in a sand dune pool. Survival and growth rates were measured under conditions of no interaction, partial interaction (permitting interference but not resource competition) and full interaction between the species. For partial interaction, larvae were separated by mesh such that water and faeces could pass between compartments but the animals could not. Numbers of Anurofeca [=Prototheca] richardsi, a mediator of interference competition between these anurans under laboratory conditions, food acquisition and food availability were also determined during the course of the experiment. Asymmetric interspecific competition, manifested as reduced survival and growth rates of B. calamita, occurred under conditions of both full and partial interactions between the species though overall competition strength was much greater under fully interacting compared with partially interacting conditions. Interference competition, probably mediated by A. richardsi, was implicated under the partially interacting conditions in which resource competition was prevented.