Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates.

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-2). 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 shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.

Non-behavioural interference competition between anuran larvae under semi-natural conditions.

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.

Competition between Bufo larvae in a eutrophic pond.

Competition between larvae of two anuran species (Bufo bufo and B. calamita) was investigated under field conditions likely to disfavour cell-mediated interference mechanisms. The experiment used triplicated cage treatments in an unshaded farm pond, a poor habitat for the unicellular pathogen Anurofeca richardsi implicated in interference competition between these anurans in sand dune ponds. The farm pond experienced lower maximum temperatures than a nearby dune pond but sustained larger numbers of eukaryotic algae and therefore had higher primary productivity. Survival and growth of B. calamita larvae were inversely related to density in all treatments but interspecific effects were much more severe than intraspecific ones. There was no evidence of A. richardsi in any treatment and competition between the Bufo larvae was therefore intense in the absence of Anurofeca-mediated interference effects. Anuran larvae reduced the standing crop and altered the community composition of algae in the treatment cages but larval growth rates were not simply related to food availability. Algal cell numbers in larval guts, a measure of food acquisition, were however inversely related to tadpole density in both species. Feeding niche overlap was high but decreased as larval density increased. Resource competition was implicated as the most probable major mechanism.