James N. McNair

The effects of refuges on predator-prey interactions: A reconsideration

Prey refuges are widely believed to prevent prey extinction and damp predator-prey oscillations. A review of the empirical evidence suggests that refuges are indeed capable of playing the former role. But the conditions under which they do so are not understood, nor is there any solid evidence for an effect on population fluctuations. The intuitive view that refuges act to stabilize equilibria and damp predator-prey oscillations is based in several theoretical studies of extremely simple models. Using a more realistic model, I show that several kinds of refuges can exert a locally destabilizing effect and create stable, large-amplitude oscillations which would damp out if no refuge was present. This finding contrasts sharply with the usual view. I argue that current evidence is tol weak, and the range of theoretically possible effects is too broad, to justify any simple characterization of refuge effects in nature. Manipulative empirical studies are an important first step toward correcting this situation, and I discuss some important factors to consider in their design.

A stochastic foraging model with predator training effects. II. Optimal diets

Abstract Standard optimal diet models require that a predators behavior while searching for food does not change in response to experiences with individual prey. There is evidence for rapid and reversible changes in feeding behavior caused by as few as one or two prey encounters. When these “training effects” occur, a given prey type is more likely to be captured next if it was the last type with which the predator had experience. This is not compatible with the standard foraging model. I present a stochastic model which incorporates predator training effects, and three types of training are explored: training in the ability to detect prey (search image formation), training in the probability of succeeding in an attempted capture, and training in the time to pursue, capture, and eat prey. The main result is that all three types of training can result in optimal diets which do not obey the standard optimal diet rules. Conditions under which these rules will suffice are discussed.

A generalized model of optimal diets

Abstract The scope of current optimal diet theory is greatly restricted by certain rather stringent assumptions upon which it rests. One of these is that the type of prey a predator encounters next is not influenced by the last type encountered. The purpose of this paper is to relax this and certain other assumptions and, in so doing, arrive at a set of rules for determining the structure of the optimal diet which are analogous to, but more general than, those of current theory. Once obtained, these rules are contrasted with their earlier analogues. The major findings are that (1) prey types are not necessarily added to the optimal diet in order of decreasing energy to handling time ratio, and (2) the abundance of a type initially excluded from the diet is not necessarily irrelevant in determining whether or not that type will be included in the future. These findings show that, in the more general case considered, the structure of the optimal diet may be quite different than predicted by current theory.

Mammalian Metabolic Allometry: Do Intraspecific Variation, Phylogeny, and Regression Models Matter?

Power scaling relationships between body mass and organismal traits are fundamental to biology. Compilations of mammalian masses and basal metabolic rates date back over a century and are used both to support and to assail the universal quarter‐power scaling invoked by the metabolic theory of ecology. However, the slope of this interspecific allometry is typically estimated without accounting for intraspecific variation in body mass or phylogenetic constraints on metabolism. We returned to the original literature and culled nearly all unique measurements of body mass and basal metabolism for 695 mammal species and (1) phylogenetically corrected the data using the fullest available phylogeny, (2) applied several different regression analyses, (3) resampled regressions by drawing randomly selected species from each of the polytomies in the phylogenetic hypothesis at each iteration, and (4) ran these same analyses independently on separate clades. Overall, 95% confidence intervals of slope estimates frequently did not include 0.75, and clade‐specific slopes varied from 0.5 to 0.85, depending on the clade and regression model. Our approach reveals that the choice of analytical model has a systematic influence on the estimated allometry, but irrespective of the model applied, we find little support for a universal metabolic rate–body mass scaling relationship.

HOST AND PARASITE COUNTERADAPTATIONS: AN EXAMPLE FROM A FRESHWATER SNAIL

Host-parasite interactions may lead to a variety of outcomes. Trematode infection of pulmonate snails is often associated with increased growth and/or survivorship of snail hosts. We use the freshwater pulmonate Lymnaea elodes and its trematode parasites to test whether this increase is a parasite adaptation, a host adaptation, or a side effect that serves no adaptive function for either participant. Field experiments indicate that trematode parasitism significantly reduces host fecundity and causes a temporary elevation and subsequent reduction in host growth. A 2-yr field survey of the prevalence of trematode infection in three snail populations revealed a significant positive relationship between shell size and prevalence. Apparently, L. elodes does not outlive its trematode infections. When survey data are compared with theoretical curves generated from a simple model of the system, it appears that trematode parasitism increases the survivorship of infected snails. Overall, the results suggest that increased survivorship in trematode-infected L. elodes is a parasite strategy for providing a stable, longterm resource for the parasite.

A stochastic foraging model with predator training effects: I. Functional response, switching, and run lengths.

Abstract Training effects are changes in a predators behavior while it is searching for the next prey to eat which are caused by the predators experience with the last prey encountered. A stochastic foraging model is formulated incorporating several specific types of training effects, and their impact on the functional response shape, switching, and mean prey run lengths is evaluated. The main result is that training effects such as search image formation can cause sigmoid functional responses and switching, and can result in runs of prey captures longer than expected in the absence of training.

Phylogeny, Regression, and the Allometry of Physiological Traits

Physiological and ecological allometries often pose linear regression problems characterized by (1) noncausal, phylogenetically autocorrelated independent (x) and dependent (y) variables (characters); (2) random variation in both variables; and (3) a focus on regression slopes (allometric exponents). Remedies for the phylogenetic autocorrelation of species values (phylogenetically independent contrasts) and variance structure of the data (reduced major axis [RMA] regression) have been developed, but most functional allometries are reported as ordinary least squares (OLS) regression without use of phylogenetically independent contrasts. We simulated Brownian diffusive evolution of functionally related characters and examined the importance of regression methodologies and phylogenetic contrasts in estimating regression slopes for phylogenetically constrained data. Simulations showed that both OLS and RMA regressions exhibit serious bias in estimated regression slopes under different circumstances but that a modified orthogonal (least squares variance‐oriented residual [LSVOR]) regression was less biased than either OLS or RMA regressions. For strongly phylogenetically structured data, failure to use phylogenetic contrasts as regression data resulted in overestimation of the strength of the regression relationship and a significant increase in the variance of the slope estimate. Censoring of data sets by simulated extinction of taxa did not affect the importance of appropriate regression models or the use of phylogenetic contrasts.

A reconciliation of simple and complex models of age-dependent predation

Abstract Predation intensity often varies with prey age or development stage. Several theoretical studies have considered what effect such age-dependent predation has on the stability of predator-prey interactions. The simple models suggest its effect is typically stabilizing; more complex models suggest otherwise. Current theory is therefore divided on this issue. Here I show that the conclusion of previous studies of simple models is due to biologically extreme restrictions on certain parameter values. When these are relaxed, age-dependent predation becomes equally capable of a stabilizing or destabilizing effect, just as in more complex models. Specific conditions favoring each effect are identified and discussed.

Lipid Dietary Dependencies in Zooplankton

Zooplankton accumulate large amounts of lipids, as much as 40-70% of the dry mass of their body (Goulden and Henry, 1985; Lee et al., 1972) ome polyunsaturated fatty acids and sterols are essential but required in trace amounts. By contrast, storage and membrane lipids are generally composed of nonessential lipids and make up an important energy reserve.

Stability effects of prey refuges with entry-exit dynamics

Prey refuges are widely believed to be important components of natural communities. Conventional wisdom holds that damping predator-prey oscillations is one of their most significant roles. Refuges are thus thought to reduce fluctuation and foster stable coexistence of prey and predator populations. Here I examine this view using a model which explicitly incorporates the process by which prey enter and leave their refuge, a process which nearly all previous models have ignored. Contrary to conventional wisdom, the results indicate that a prey refuge with legitimate entry-exit dynamics is quite capable of amplifying rather than damping predator-prey oscillations. Some simple conditions are identified which ensure that a refuge will act as (a) a damper and (b) an antidamper, and these are exploited in proposing a strategic way to test the model experimentally.