Body size and food web structure: testing the equiprobability assumption of the cascade model

The cascade model successfuly predicts many patterns in reported food webs. A key assumption of this model is the existence of a predetermined trophic hierarchy; prey are always lower in the hierarchy than their predators. At least three studies have suggested that, in animal food webs, this hierarchy can be explained to a large extent by body size relationships. A second assumption of the standard cascade model is that trophic links not prohibited by the hierarchy occur with equal probability. Using nonparametric contingency table analyses, we tested this ”equiprobability hypothesis” in 16 published animal food webs for which the adult body masses of the species had been estimated. We found that when the hierarchy was based on body size, the equiprobability hypothesis was rejected in favor of an alternative, ”predator-dominance” hypothesis wherein the probability of a trophic link varies with the identity of the predator. Another alternative to equiprobabilty is that the probability of a trophic link depends upon the ratio of the body sizes of the two species. Using nonparametric regression and liklihood ratio tests, we show that a size-ratio based model represents a significant improvement over the cascade model. These results suggest that models with heterogeneous predation probabilities will fit food web data better than the homogeneous cascade model. They also suggest a new way to bridge the gap between static and dynamic food web models. Received: 3 February 1999 / Accepted: 26 October 1999     

On the role of body size in a tri-trophic metapopulation model

 A particular tri-trophic (resource, prey, predator) metapopulation model with dispersal of preys and predators is considered in this paper. The analysis is carried out numerically, by finding the bifurcations of the equilibria and of the limit cycles with respect to prey and predator body sizes. Two routes to chaos are identified. One is characterized by an intriguing cascade of flip and tangent bifurcations of limit cycles, while the other corresponds to the crisis of a strange attractor. The results are summarized by partitioning the space of body sizes in eight subregions, each one of which is associated to a different asymptotic behavior of the system. Emphasis is put on the possibility of having different modes of coexistence (stationary, cyclic, and chaotic) and/or extinction of the predator population. Received 1 August 1995; received in revised form 8 January     

The relationship between ecological segregation and sexual body size dimorphism in large herbivores

Ecological segregation (sexual differences in diet or habitat use) in large herbivores has been intimately linked to sexual body size dimorphism, and may affect both performance and survival of the sexes. However, no one has tested comparatively whether segregation occurs at a higher frequency among more dimorphic species. To test this comparatively, data on sex-specific diet, habitat use and body size of 40 species of large herbivores were extracted from the literature. The frequency of ecological segregation was higher among more dimorphic herbivores; however, this was only significant for browsers. This provides the first evidence that segregation is more common among more dimorphic species. The comparative evidence supported the nutritional-needs hypothesis over the incisor breadth hypothesis, as there was no difference in frequency of segregation between seasons with high and low resource levels, and since segregation was also evident among browsers. Whether the absence of a correlation between ecological segregation and level of sexual body size dimorphism for intermediate feeders and grazers is due to biological differences relative to browsers or to the fact that the monomorphic species included in the analysis were all browsers is discussed. Received: 18 August 1999 / Accepted: 31 January 2000     

Human morphology and temperature regulation

For nearly a century individuals have believed that there is a link between human morphology and one’s thermoregulatory response in adverse environments. Most early research was focussed on the rate of core cooling in a male adult population and the role of subcutaneous adipose tissue, surface area and the surface-area-to-mass ratio in one’s ability to withstand varying degrees of cold stress. More recently research has addressed heat tolerance in various populations, exploring the role of subcutaneous adipose tissue, surface area and the surface-area-to-mass ratio in one’s ability to maintain thermal equilibrium in warm and hot, dry and humid environments. Since the late 1970s an emphasis has been placed on the role of muscle and muscle perfusion in total-body thermal insulation. Yet, despite the history of research pertaining to human morphology and temperature regulation there is little consensus as to the impact of variations in human morphology on thermoregulatory responses. Individuals differing in body size, shape and composition appear to respond quantitatively differently to variations in both ambient and core temperatures but the interrelations between morphological components and temperature regulation are complex. It is the purpose of this paper to examine the literature pertaining to the impact of variations in muscularity, adipose tissue thickness and patterning, surface area and the surface-area-to-mass ratio on thermoregulation and thermal stability in response to both heat and cold stress. Received: 6 May 1999 / Accepted: 14 July 1999     

Reversed sexual size dimorphism in raptors: evaluation of the hypotheses in kestrels breeding in a temporally changing environment

Reversed sexual size dimorphism (RSD, females larger than males) is commonly found in birds of prey. We used kestrels (Falco tinnunculus), breeding in western Finland in a temporally varying environment of 3-year vole cycles, to assess current hypotheses for the evolution and maintenance of RSD. Our 12-year data showed only weak correlations between parental size and breeding parameters (laying date, clutch size and the number of fledglings produced). The degree of RSD per se was unrelated to breeding success, contrary to the prediction of the female dominance hypothesis. Females with small males produced larger clutches in low-vole years, independently of laying date, which supports the small male (or its equivalent inter-sexual selection) hypothesis. Small females tended to have more fledglings, particularly in low-vole years, which is inconsistent with the hypotheses for an advantage of large female size (the starvation, intra-sexual selection, reproductive effort, and supplementary feeding hypotheses). As for males, smaller females may be more efficient hunters, the importance of which should be most pronounced under harsh breeding conditions. Our results suggest that the directional selection on a particular size in kestrels may be under contrasting selection pressures by the environment, and, at least in breeding females, the advantages of large size can actually be counterbalanced during harsh environmental conditions. Received: 7 May 1999 / Accepted: 20 January 2000     

Distalless expression in crustaceans and the patterning of branched limbs

 In Drosophila, Distalless (Dll) is critical in establishing the proximal/distal axis of the leg. Lack of proper Dll expression causes distal limb structures to be truncated or lost. Dll expression was examined through the course of development in the limbs of two crustaceans, Triops and Nebalia. Because the limbs of these two species are branched, they provide a comparison to the uniramous (unbranched) leg of Drosophila. In Triops and Nebalia, development of limb branches is not tightly coupled with Dll expression: in some cases, branches can arise prior to Dll expression and in others, certain branches never express Dll. These data suggest that, while Dll may indeed initiate overall limb outgrowth, limb branches are unlikely to be patterned by a simple iteration of the mechanism patterning the unbranched leg of Drosophila. Received: 14 May 1997 / Accepted: 25 September 1997     

Effects of <Emphasis Type="Italic">Acanthocephalus</Emphasis> sp. (Acanthocephala: Echinorhynchidae) on the body size and reproduction of isopods (<Emphasis Type="Italic">Asellus hilgendorfi</Emphasis>)

 Isopods Asellus hilgendorfi were collected from a small lake in northern Japan and examined to determine whether their body size and reproduction were affected by infection with larval acanthocephalans (Acanthocephalus sp.). Seasonal changes in the breeding ratio of isopods and the prevalence of larval acanthocephalan infection showed a reverse trend. Acanthocephalan larvae occurred mainly in males and immature females and were rarely found in mature females. In late immature females, the body size, as indicated by the width of the pleotelson, of infected isopods was significantly larger than that of uninfected ones. These results suggest that acanthocephalans can prevent female isopods from attaining sexual maturity and increasing their body size. Received: January 9, 2002 / Accepted: December 16, 2002 Acknowledgments We thank Professor Shōichi Saito, Faculty of Education, Hirosaki University, for his encouragement of the present study. Thanks are also due to the Iwasaki Village Office and the Fukaura Forestry Office for giving us permission for the survey. Correspondence to:A. Ohtaka     

Relationships between nuclear DNA content and seed and leaf size in soybean

 A correlation between genome size and agronomically important traits has been observed in many plant species. The goal of the present research was to determine the relationship between genome size, seed size, and leaf width and length in soybean [Glycine max (L.) Merr.] Twelve soybean strains, representing three distinct seed size groups, were analyzed. Flow cytometry was used to estimate their 2C nuclear DNA contents. Data on seed size and leaf size of the 12 strains were obtained from 1994 and 1995 field experiments. Variation of 2C nuclear DNA among the 12 soybean strains was 4.6%, ranging from 2.37 pg for a small-seed strain to 2.48 pg for a large-seed strain. Strain seed size was positively associated with leaf width (r=0.92) and leaf length (r=0.93). Genome size was highly correlated with seed size (r=0.97), leaf width (r=0.90) , and leaf length (r=0.93). The results of our study indicate that there is a significant correlation between genome size and leaf and seed size in soybean. It is possible that selection for greater seed size either leads to, or results from, greater genome size. If so, this relationship might be worth exploring at a more fundamental level. Received: 5 April 1997 / Accepted: 9 January 1998     

Body size and invasion success in marine bivalves

The role of body size in marine bivalve invasions has been the subject of debate. Roy et al . found that large-bodied species of marine bivalves were more likely to be successful invaders, consistent with patterns seen during Pleistocene climatic change, but Miller et al . argued that such selectivity was largely driven by the inclusion of mariculture species in the analysis and that size-selectivity was absent outside of mariculture introductions. Here we use data on non-mariculture species from the north-eastern Pacific coast and from a global species pool to test the original hypothesis of Roy et al . that range limits of larger bivalves are more fluid than those of smaller species. First, we test the hypothesis that larger bivalve species are more successful than small species in expanding their geographical ranges following introduction into new regions. Second, we compare body sizes of indigenous and non-indigenous species for 299 of the 303 known intertidal and shelf species within the marine bivalve clade that contains the greater number of non-mariculture invaders, the Mytilidae. The results from both tests provide additional support for the view that body size plays an important role in mediating invasion success in marine bivalves, in contrast to Miller et al . Thus range expansions in Recent bivalves are consistent with patterns seen in Pleistocene faunas despite the many differences in the mechanisms.     

Body size and microclimate use in Neotropical granivorous ants

The stability of tropical microclimates has left microclimate use by tropical species little unexplored. At La Selva Costa Rica, I related foraging activity at seed baits to humidity in two forests types. I recorded 38 and 35 ant species at seed baits in closed and open canopy forest. The microclimate 5 cm above the forest floor in the younger, Open Forest was warmer, drier, more variable, and more sensitive to current weather than in the older Closed Forest. Ant species within both forests foraged at different Vapor Pressure Deficits (kPa), a measure of the drying power of the air. VPD use was not confounded with diel activity patterns. Body size explained 46% of the variance in mean VPD use among ant species. Small ant species tended to forage in moist microclimates; large species tended to be microclimate generalists. Larger species were also more active in the drier Open Forest. Foraging activity by these assemblages varies 4-fold, and peaks close to the mean VPD for each habitat. The behavior of these assemblages suggest that 1) small ant species at La Selva potentially compete with the entire range of ant body sizes, whereas large ants forage when and where small ants are inactive; and 2) seeds dispersed to the forest floor at dawn will be consumed or further dispersed by a larger suite of ants species than those falling in the heat of the tropical afternoon.     

Niche division and abundance: an evolutionary perspective

 In recent years, biodiversity has become an issue of broad academic interest, and its assessment and maintenance are now recognized as an important area of ecological research. While the concept of biodiversity encompasses, first and foremost, the total number of species co-occurring in a locality, it has increasingly been realized that information on the relative abundances of co-occurring species is also required for a better understanding of the patterns and dynamics of biodiversity. In many areas of ecological research, “abundance” constitutes a key variable that characterizes populations and communities. The relative abundances of species in natural communities reflect evolutionary and contemporary processes occurring on different spatiotemporal scales. The idea of niche apportionment has been developed to provide an integrated conceptual framework for the study of species abundance patterns in communities. This article reviews a number of important issues surrounding the concept of niche apportionment, including some aspects that have received very little or no consideration in previous ecological literature. The main emphasis here is on possible evolutionary implications and backgrounds. Further, as a universal factor which affects species abundance in one way or another, body size is highlighted and its relationship with abundance (“density–body-size relation”) is considered, referring in particular to a recent comprehensive analysis of freshwater benthic data. Consideration of this and other studies has led to the formulation of the biomass equivalence rule, that suggests the independence of the biomass measure of abundance from body size, which strengthens the logical basis of niche apportionment models. It is suggested that, compared with Hubbell's neutral theory of biodiversity, niche apportionment with the biomass equivalence rule represents a conceptually more sound and widely applicable approach to elucidating species abundance patterns. Received: February 4, 2002 / Accepted: October 25, 2002 Correspondence to:M. Tokeshi     

Genetic control of macro-and micro-environmental sensitivities in Populus

Understanding the genetic mechanisms for the phenotypic plasticity and developmental instability of a quantitative trait has important implications for breeding and evolution. Two clonally replicated plantations of two 3-generation inbred pedigrees derived from the highly divergent species Populus trichocarpa and P. deltoides were used to examine the genetic control of macro- and micro-environmental sensitivities and their genetic relationships with the trait mean across two contrasting environments. For all stem-growth traits studied, the trait mean had a higher broad-sense heritability (H²) level than macroenvironmental sensitivity, both with much higher values than microenvironmental sensitivity. Genetic correlation analyses indicated that the trait mean was more or less independent of macro- or micro-environmental sensitivity in stem height. Thus, for this trait, the genetic difference in response to the two environments might be mainly due to epistasis between some regulatory loci for plasticity and loci for trait mean. However, for basal area and volume index, pleiotropic loci might be more important for their genetic differences between the two environments. No evidence was found to support Lerner’s (1954) homeostasis theory in which macro- or micro-environmental sensitivity is the inverse function of heterozygosity. Received: 8 March 1996 / Accepted: 31 May 1996     

The cost of mutualism in a fly-fungus interaction

The movement ability of individuals has become increasingly important to a variety of ecological questions. In this study, I investigate how plant structure and changes in body size through development affect the movement ability of a predaceous stinkbug, Podisus maculiventris, on three species of goldenrod (Solidago spp.) representing a wide range of surface complexities. I adapt existing techniques for quantifying movement in two dimensions to the study of movement on natural plant structures in three dimensions. These experiments indicate that plant structure and insect size are significant factors affecting the movement ability of P. maculiventris. Changes in movement ability due to factors of ontogeny and different habitat structures suggest that the scale of an individual’s ambit or ecological sphere of influence may vary within its lifespan. Considering the influence of ontogeny and habitat structure on movement ability may be useful to investigations of population dynamics, foraging behavior, and pest management. Received: 14 July 1999 / Accepted: 23 March 2000     

Population size and structure of giant clams at Rose Atoll, an important refuge in the Samoan Archipelago

Rose Atoll is an important refuge for giant clams (Tridacna maxima) that have been heavily exploited elsewhere in Samoa. During an extensive survey of six islands in the archipelago (50.5?ha surveyed in 420 transects), 97% of a total of 2853 clams were recorded at the atoll (42% of area surveyed). Clam densities were highest in the atoll lagoon, especially around the bases of the pinnacles (mean density=8870?ha^-1). Estimated population size for the small atoll (615?ha) was approximately 27800 clams. Twenty four percent of the population consisted of mature clams (?12?cm), 70% of which occupied the pinnacles and shallow lagoon habitat. Estimated mortality was low (Z=0.3) and primarily due to natural mortality (M=0.3). Maximum recorded size (L _max ) and asymptotic mean size (L _∞) were 25.0?cm and 27.8?cm respectively.     

Body size and fecundity in the waterstrider Aquarius remigis: a test of Darwin's fecundity advantage hypothesis

The general female bias in body size of animals is usually attributed to fecundity selection. While many studies have demonstrated a positive relationship between body size and fecundity, the most common interpretation of fecundity selection is that larger females have larger abdomens and can hold more eggs, yet the relationship between abdomen size and fecundity has rarely been examined. For the waterstrider, Aquarius remigis, we find a significant relationship between body size and fecundity and demonstrate that the target of fecundity selection is abdomen size. Thus, larger females have higher fecundities because they have larger abdomens and not because of their total size per se. The rate at which fecundity increases with increasing abdomen size exceeds that which would be expected due to a simple volume constraint and suggests that other factors, such as increased ability to obtain resources, may contribute to the increase in fecundity with body size. Selection intensities estimated from our data indicate that fecundity selection could be a significant selective force on both total and abdomen lengths. Previous studies have found that abdomen size increased faster than body size and thus, larger females had relatively larger abdomens. The relationship of abdomen length and thorax length in A. remigis is hypoallometric and indicates that larger females have relatively smaller abdomens. We hypothesize that this may reflect conservation of abdomen size in females developing under poor conditions. Finally, while egg size is not directly related to body size, we find a trade-off between egg size and number when female abdomen length is held constant, suggesting that selection on egg size may influence abdomen length only indirectly through its effects on fecundity.     

Sexual size dimorphism in the ontogeny of the solitary predatory wasp Symmorphus allobrogus (Hymenoptera: Vespidae)

Sex-specific patterns of individual growth, resulting in sexual size dimorphism (SSD), are a little studied aspect of the ontogeny related to the evolutionary history and affected by the ecology of a species. We used empirical data on the development of the predatory wasp Symmorphus allobrogus (Hymenoptera, Vespidae) to test the hypotheses that sexual differences of growth resulting in the female-biased SSD embrace the difference in (1) the egg size and the starting size of larva, (2) the larval development duration, and (3) the larval growth rate. We found that eggs developing into males and females have significant differences in size. There was no significant difference between the sexes in the duration of larval development. The relative growth rate and the food assimilation efficiency of female larvae were significantly higher than compared to those of male larvae. Thus, the SSD of S. allobrogus is mediated mainly by sexual differences in egg size and larval growth rate.     

pH regulation in apoplastic and cytoplasmic cell compartments of leaves

 The regulation of pH in the apoplast, cytosol and chloroplasts of intact leaves was studied by means of fluorescent pH indicators and as a response of photosynthesis to acid stress. The apoplastic pH increased under anaerobiosis. Aeration reversed this effect. Apoplastic responses to CO₂, HCl or NH₃ differed considerably. Whereas HCl and ammonia caused rapid acidification or alkalinization, the return to initial pH values was slow after cessation of fumigation. Addition of CO₂ either did not produce the acidification expected on the basis of known apoplastic buffering or even caused some alkalinization. Removal of CO₂ shifted the apoplastic pH into the alkaline range before the pH returned to initial steady-state levels. In the presence of vanadate, the alkaline shift was absent and the apoplastic pH returned slowly to the initial level when CO₂ was removed from the atmosphere. In contrast to the response of the apoplast, anaerobiosis acidified the cytosol or, in some species, had little effect on its pH. Acidification was rapidly reversed upon re-admission of oxygen. The CO₂-dependent pH changes were very fast in the cytosol. Considerable alkalinization was observed after removal of CO₂ under aerobic, but not under anaerobic conditions. Rates of the re-entry of protons into the cytosol during recovery from CO₂ stress increased in the presence of oxygen with the length of previous exposure to high CO₂. Effective pH regulation in the chloroplasts was indicated by the recovery of photosynthesis after the transient inhibition of photosynthetic electron flow when CO₂ was increased from 0.038% to 16% in air. As photosynthesis became inhibited under high CO₂, reduction of the electron transport chain increased transiently. The time required for recovery of photosynthesis from inhibition during persistent CO₂ stress was similar to the time required for establishing steady-state pH values in the cytosol under acid stress. The high capacity of leaf cells for the rapid re-attainment of pH homeostasis in the apoplast and the cytoplasm under acid or alkaline stress suggested the rapid activation or deactivation of membrane-localised proton-transporting enzymes and corresponding ion channel regulation for co-transport of anions or counter-transport of cations together with proton fluxes. Acidification of the cytoplasm appeared to activate energy-dependent proton export primarily into the vacuoles whereas apoplastic alkalinization resulted in the pumping of protons into the apoplast. Proton export rates from the cytosol into the apoplast after anaerobiosis were about 100 nmol (m² leaf area)⁻¹ s⁻¹ or less. Proton export under acid stress into the vacuole was about 1200 nmol m⁻² s⁻¹. The kinetics of pH responses to the addition or withdrawal of CO₂ indicated the presence of carbonic anhydrase in the cytosol, but not in the apoplast. Received: 19 July 1999 / Accepted: 29 December 1999     

Ontoaenetic correlates of diet in anthropoid primates

This study assesses ontogenetic correlates of diet in anthropoid primates. Associations between body weight growth, adult size, and diet are evaluated for a sample of 42 primate species, of which 8 are classifiable as “folivores.” The hypothesis that folivores show a pattern of growth that differs from “nonfolivores” is tested. Ontogenetic variation is summarized through use of parametric and nonparametric regression analysis. Several analytical techniques, including broad interspecific and detailed comparisons among species of similar adult size, are applied. This investigation indicates a clear association between body weight ontogeny and diet: folivorous species grow more rapidly over a shorter duration than comprably sized nonfolivorus species. A positive correlation between adult size and diet is not unambiguously established in this sample. A threshold (at around 1 kg) below which insectivory is very common may adequately characterize the association between adult size and diet in anthropoid primates. Above this threshold, adult size does not appear to covary predictably with diet. Evolutionary correlates of the ontogenetic pattern seen in folivores may include a variety of factors. The distinctive pattern of development in folivores may relate to the profile of ecological and social risks that these species face. Morphophysiological advantages to rapid growth may relate to a need for accelerated alimentary (dental and gut) development. The implications of ontogenetic variation in folivores are discussed. © 1994 Wiley-Liss, Inc.