Richard M. Sibly

Producers and scroungers : a general model and its application to captive flocks of house sparrows

Abstract Many forms of interaction within and between species appear to be based on ‘scrounger’ individuals or species exploiting a limited resource provided ‘producers’. A mathematical model is presented which shows whether or not scroungers are maintained in a group, depending on their frequency and the group size. Some of the predictions of the model were tested in captive flocks of house sparrows Passer domesticus L. Here the scroungers obtained most of their food (mealworms) by interaction and the producers found most of their food by actively foraging: the pay-off to each type was measured as mealworm capture rate. Neither type changed strategy opportunistically in response to instantaneous flock composition but, not surprisingly, scroungers fared better when one of more producers were present. However, scrougers did much worse than expected when greatly outnumbered by producers, perhaps because producers then found the available food very quickly.

Why are organisms usually bigger in colder environments? Making sense of a life history puzzle

Environmental effects on body size are of widespread ecological and economic importance but our understanding of these effects has been obscured by an apparent paradox. Life history analysis suggests that it is adaptive for adults to emerge smaller if reared in conditions that slow down juvenile growth. However, whereas smaller adults emerge if growth is limited by food availability, the reverse is usually observed if growth is limited by temperature. The resolution of this apparent paradox may be that the response of adult size to temperature is adaptive, but is constrained by a trade-off that can be understood in terms of von Bertalanffys classic theory of growth. Alternatively, the response may be the unavoidable consequence of a fundamental relationship between cell size and temperature.

ON THE FITNESS OF BEHAVIOR SEQUENCES

Our aim is to demonstrate a method of determining the extent to which behavior maximizes fitness. We believe the temporal organization of behavior to be in part dependent on the animals genetic makeup and subject to natural selection and that behavioral strategies may be as adaptive as structural characters. The question of the adaptiveness of behavior has been found hard to study in the past (Tinbergen 1963) because of the difficulty of quantitative verification of hypotheses. The method presented here tests specifically whether deployment of behavioral options is optimally related to environmental conditions.

A Physiological Basis of Population Processes: Ecotoxicological Implications

Environmental toxicologists often want to use bioassays that can be carried out quickly, easily, and hence inexpensively on individual organisms, to make predictions about long-term impacts of toxicants at an ecological level (Maltby & Calow, 1990). More fundamentally, it is of interest for population dynamicists to understand to what extent processes within individuals, as compared with interactions between them, contribute to population changes (Metz & Diekmann, 1986). Here we explore models that provide the basis of links between physiological and population processes and point out some implications for the application of physiological bioassays in ecotoxicology.

How Rearing Temperature Affects Optimal Adult Size in Ectotherms

1. Rearing temperature may affect juvenile mortality, growth and development rates, adult mortality rate, and/or population growth rate. Increased juvenile growth rate may affect the trade-off curve relating fecundity to development period, lowering it and making it less steep. Here we identify the overall effects of temperature on optimal adult size. Larger bodies are favoured by decreased juvenile mortality rate and decreased population growth rate (the sum of these is termed discounted juvenile mortality rate). 2. The effects of the other variables depend on whether environmental heterogeneity is spatial or temporal. Under spatial heterogeneity larger adult bodies are favoured by increased juvenile growth rate but varying adult mortality rate has no effect

Shifts in metabolic scaling, production, and efficiency across major evolutionary transitions of life

The diversification of life involved enormous increases in size and complexity. The evolutionary transitions from prokaryotes to unicellular eukaryotes to metazoans were accompanied by major innovations in metabolic design. Here we show that the scalings of metabolic rate, population growth rate, and production efficiency with body size have changed across the evolutionary transitions. Metabolic rate scales with body mass superlinearly in prokaryotes, linearly in protists, and sublinearly in metazoans, so Kleiber’s 3/4 power scaling law does not apply universally across organisms. The scaling of maximum population growth rate shifts from positive in prokaryotes to negative in protists and metazoans, and the efficiency of production declines across these groups. Major changes in metabolic processes during the early evolution of life overcame existing constraints, exploited new opportunities, and imposed new constraints.

Splitting behaviour into bouts

Abstract When considering splitting behaviour into bouts, it is best to display the data in log frequency rather than in log survivorship plots. While the latter has the advantage of simplicity, the former has the advantage that the data points are independent of each other, so that a non-linear curve-fitting procedure, such as NLIN in SAS, can be applied. Traditional analysis of variance can then be used to decide whether the behavioural events are split into bouts, and least-squares estimates and standard errors can be calculated for each parameter of the fitted model. These estimates can be used in Slater & Lesters (1982 Behaviour , 79 , 153–161) formula to calculate the best criterion interval to use in splitting behaviour into bouts. Average bout length and number of bouts are, however, best estimated directly from the models parameters.

A model of mate desertion

Abstract Using a very simple model, in which the reproductive strategy of an individual is considered to be completely specified by the time he cares for each brood, we find the evolutionarily stable strategies, and characterize them in terms of the parental abilities of males and females. We predict inter alia that: (1) If at any stage the sexes are complementary, either by specialization or by the need for constant attendance with the young, so that two parents are much better than either parent alone, then only at extreme sex ratios will either parent desert. (2) If the sex ratio is unity, then the first parent deserts when both parents are twice as good as the deserted parent alone. (3) Except at extreme sex ratios, if one sex is very much worse than the other at caring for the young, then that sex will desert first. (4) Where the male deserts first, the more heavily the sex ratio is biassed towards males, the less the relative advantage of both parents over the female alone is, at the time of desertion. We conclude with some general points about modelling mate desertion and parental investment.

Metabolic Ecology:A Scaling Approach

Most of ecology is about metabolism: the ways that organisms use energy and materials. The energy requirements of individuals - their metabolic rates - vary predictably with their body size and temperature. Ecological interactions are exchanges of energy and materials between organisms and their environments. So metabolic rate affects ecological processes at all levels: individuals, populations, communities and ecosystems. Each chapter focuses on a different process, level of organization, or kind of organism. It lays a conceptual foundation and presents empirical examples. Together, the chapters provide an integrated framework that holds the promise for a unified theory of ecology.

The Evolution of Maximum Body Size of Terrestrial Mammals

How Mammals Grew in Size Mammals diversified greatly after the end-Cretaceous extinction, which eliminated the dominant land animals (dinosaurs). Smith et al. (p. 1216) examined how the maximum size of mammals increased during their radiation in each continent. Overall, mammal size increased rapidly, then leveled off after about 25 million years. This pattern holds true on most of the continents—even though data are sparse for South America—and implies that mammals grew to fill available niches before other environmental and biological limits took hold. Maximum mammal size increased at the beginning of the Cenozoic, then leveled off after about 25 million years. The extinction of dinosaurs at the Cretaceous/Paleogene (K/Pg) boundary was the seminal event that opened the door for the subsequent diversification of terrestrial mammals. Our compilation of maximum body size at the ordinal level by sub-epoch shows a near-exponential increase after the K/Pg. On each continent, the maximum size of mammals leveled off after 40 million years ago and thereafter remained approximately constant. There was remarkable congruence in the rate, trajectory, and upper limit across continents, orders, and trophic guilds, despite differences in geological and climatic history, turnover of lineages, and ecological variation. Our analysis suggests that although the primary driver for the evolution of giant mammals was diversification to fill ecological niches, environmental temperature and land area may have ultimately constrained the maximum size achieved.