GEOGRAPHIC VARIATION IN LIFE-HISTORY TRAITS OF THE ANT LION,MYRMELEON IMMACULATUS: EVOLUTIONARY IMPLICATIONS OF BERGMANN'S RULE

In eastern North America, body size of the larval ant lion Myrmeleon immaculatus increases from south to north, following Bergmann's rule. We used a common-garden experiment and a reciprocal-transplant experiment to evaluate the effects of food and temperature on ant lion growth, body size, and survivorship. In the laboratory common-garden experiment, first-instar larvae from two southern (Georgia, South Carolina) and two northern (Connecticut, Rhode Island) populations were reared in incubators under high- and low-food and high- and low-temperature regimes. For all populations, high food increased final body mass and growth rate and decreased development time. Growth rates were higher at low temperatures, but temperature did not affect larval or adult body mass. Survivorship was highest in high-food and low-temperature treatments. Across all food and temperature treatments, northern populations exhibited a larger final body mass, shorter development time, faster growth rate, and greater survivorship than did southern populations. Results were similar for a field reciprocal-transplant experiment of third-instar larvae between populations in Connecticut and Oklahoma: Connecticut larvae grew faster than Oklahoma larvae, regardless of transplant site. Conversely, larvae transplanted to Oklahoma grew faster than larvae transplanted to Connecticut, regardless of population source. These results suggest that variation in food availability, not temperature, may account for differences in growth and body size of northern and southern ant lions. Although northern larvae grew faster and reached a larger body size in both experiments, northern environments should suppress growth because of reduced food availability and a limited growing season. This study provides the first example of countergradient selection causing Bergmann's rule in an ectotherm.     

Latitudinal clines in the grasshopper Dichroplus elongatus: Coevolution of the A genome and B chromosomes?

Argentine populations of Dichroplus elongatus (Orthoptera: Acrididae) are polymorphic for B chromosomes. Previous studies showed that B chromosomes affect body size and some fitness components in Northwestern populations. We studied phenotype and B′s variation patterns along a latitudinal cline as well as the relationship between karyotype and body size related traits in 17 populations from East. Body size related traits showed a ‘saw tooth’ pattern of variation being small at low and high latitudes and large at intermediate latitudes in most of the analysed populations. Analyses of variance and principal components demonstrated that in most analysed populations B carrier males are associated with a decrease in body size related traits with respect to individuals with standard karyotype. Accordingly with the relationship between karyotype and body size, an opposite pattern of latitudinal variation in the frequencies of B′s with respect to body size variation was observed in this area. i.e. smaller individuals tend to have a higher frequency of B chromosomes. The comparison of the differentiation of both karyotype and body size traits with molecular neutral markers demonstrated the relative importance of selection moulding chromosome and phenotype variation. The observed pattern of phenotypic variation is likely to be the result of local adaptation to season length along the latitudinal gradient. The observed contrary pattern of B′s clinal variation may reflect the population ability to maintain this chromosome in relation to the local adaptation. The available evidence indicates that the distribution of B chromosome frequency was shaped by selective factors.     

Wood ants are wood ants: deforestation causes population declines in the polydomous wood ant Formica aquilonia

Abstract.  1. One of the main themes in ecology is adaptation for survival in different habitats and the potential of the environment to regulate populations.
2. The effects of clear-cutting on nest-abandonment rate and local population sizes in the polydomous wood ant Formica aquilonia was studied, using uncut forest stands as controls.
3. The nest-abandonment rate was clearly higher in clear-cuts than in forest interiors. In clear-cuts, 39% of pre-deforestation nests and 73% of new bud-nests were abandoned 4–5 years after deforestation, whereas in forest interiors fewer than 2% of nests were abandoned at the same time period. Local population size decreased 30% in clear-cuts, but fewer than 2% in forest interiors.
4. The results demonstrate that despite modern logging practices in which mechanical harming of nest mounds is reduced, nest mounds are abandoned at high rate, and despite frequent establishment of new bud-nests, populations start to decline.
5. The likely reason for the high nest-abandonment rate in clear-cuts is a combination of changed abiotic conditions, resource limitation, and disturbed reproduction.
6. Species that are sensitive to changes in the size of habitat patch, such as F. aquilonia , likely are harmed by logging, even employing biodiversity oriented management practices. Hence there is a need for conservation actions that are based on the size of protection areas.     

What determines conditionality in ant–Hemiptera interactions? Hemiptera habitat preference and the role of local ant activity

Abstract.  1. This paper describes spatial variation in density of a mutualist Hemiptera, and attempts to elucidate an understanding of the spatial variation in conditionality of its mutualism with ants.
2. Aggregations of the membracid treehopper, Campylenchia sp., occurred more frequently, and with higher number of individuals, in isolated trees as compared with their occurrence in the same tree species in wooded patches.
3. Using treehopper aggregations as the level of replication, there was a habitat × ant presence interaction associated with colony survival time. However, when the median survival time of all aggregations within each tree was used, only an additive effect of both habitat type and ant presence were apparent. This suggests that treehopper aggregations in some isolated trees experienced more benefit from ant presence than other trees.
4. The ant:treehopper ratio averaged over each tree was correlated with effect of ant presence on survivorship (survival time without ants/survival time with ants). Therefore, conditionality in the ant–membracid mutualism appears related to ant attendance level at the scale of individual trees.
5. Ant attendance at treehopper aggregations in isolated and matorral trees did not differ, and therefore there is no clear mechanism by which ants create the contrast in treehopper densities between the two habitat types.
6. The main force shaping initial treehopper densities in the two habitats is likely plant quality. Results suggest that growing situation (or more specifically plant health) is likely a good predictor of membracid density, while general ant activity within a tree is the best indicator of interaction strength.     

Variation in juvenile growth rates among and within latitudinal populations of the medaka

In ectotherms, lower temperatures at high latitudes would theoretically reduce annual growth rates of individuals. If slower growth and resulting smaller body size reduce fitness, individuals at high latitudes may evolve compensatory growth. This study compares individual growth rates among and within 12 latitudinal populations of the medaka (Oryzias latipes). Growth rates during juvenile stage were measured in a common, temperature-controlled (28°C) environment. The results revealed that juvenile growth rates differed significantly among the populations. Growth rates were, moreover, significantly correlated with latitudes of source populations, such that higher-latitude individuals grew faster. Significant variation in growth rates among full-sib families within populations was also demonstrated. The results strongly suggest that higher-latitude O. latipes have acquired a greater capacity for growth as an adaptation to shorter growing seasons (which would reduce annual growth rates), thus refuting probability processes, i.e., genetic drift, founder, or bottleneck effects, as a cause of the among-population variation.     

Invasion success depends on invader body size in a size-structured mixed predation–competition community

1.  The size of an individual is an important determinant of its trophic position and the type of interactions it engages in with other heterospecific and conspecific individuals. Consequently an individual's ecological role in a community changes with its body size over ontogeny, leading to that trophic interactions between individuals are a size-dependent and ontogenetically variable mixture of competition and predation.
2.  Because differently sized individuals thus experience different biotic environments, invasion success may be determined by the body size of the invaders. Invasion outcome may also depend on the productivity of the system as productivity influences the biotic environment.
3.  In a laboratory experiment with two poeciliid fishes the body size of the invading individuals and the daily amount of food supplied were manipulated.
4.  Large invaders established persistent populations and drove the resident population to extinction in 10 out of 12 cases, while small invaders failed in 10 out of 12 trials. Stable coexistence was virtually absent. Invasion outcome was independent of productivity.
5.  Further analyses suggest that small invaders experienced a competitive recruitment bottleneck imposed on them by the resident population. In contrast, large invaders preyed on the juveniles of the resident population. This predation allowed the large invaders to establish successfully by decreasing the resident population densities and thus breaking the bottleneck.
6.  The results strongly suggest that the size distribution of invaders affects their ability to invade, an implication so far neglected in life-history omnivory systems. The findings are further in agreement with predictions of life-history omnivory theory, that size-structured interactions demote coexistence along a productivity gradient.     

Environmental influences on spatial and temporal patterns of body-size variation in California ground squirrels (Spermophilus beecheyi)

Aim  In order to understand how ground squirrels ( Spermophilus beecheyi ) may respond to future environmental change, we investigated five biotic and environmental factors potentially responsible for explaining body-size variation in this species across California. We examined the concordance of spatial patterns with temporal body-size change since the last glacial maximum (LGM).
Location  California, western North America.
Methods  We quantified body size of modern populations of ground squirrels ( n  = 81) and used a model-selection approach to determine the best variables (sex, vegetation, number of congeners, temperature and/or precipitation) explaining geographical variation in body size among modern populations. We also quantified body size of one fossil population in northern California ( n  = 39) and compared temporal body-size change in S. beecheyi at this location since the LGM with model predictions.
Results  Body size of modern populations conformed to Bergmann's rule, with larger individuals in northern (wetter and cooler) portions of California. However, the models suggest that precipitation, rather than temperature or other variables, may best explain variation in body size across modern spatial gradients. Our conclusion is supported by the temporal data, demonstrating that the body size of S. beecheyi has increased in northern California since the LGM, concordant with precipitation but not temperature change in the region.
Main conclusions  Precipitation, rather than temperature, vegetation or number of congeneric species, was the main factor explaining both spatial and temporal patterns of body-size variation in S. beecheyi . The integration of space and time provides a powerful mechanism for predicting how local populations may respond to current and future climatic changes.     

Relative Importance of Sex,Pre-Starvation Body Mass and Structural Body Size in the Determination of Exceptional Starvation Resistance of Anchomenus dorsalis (Coleoptera: Carabidae)

In nature, almost all animals have to cope with periods of food shortage during their lifetimes. Starvation risks are especially high for carnivorous predatory species, which often experience long intervals between stochastic prey capturing events. A laboratory experiment using the common predatory carabid beetle Anchomenus dorsalis revealed an exceptional level of starvation resistance in this species: males survived up to 137 days and females up to 218 days without food at 20°C. Individual starvation resistance was strongly positively affected by pre-starvation body mass but only slightly by beetle structural body size per se. Females outperformed males even when the effect of gender was corrected for the effects of structural body size and pre-starvation body mass. The better performance of females compared to males and of beetles with higher relative pre-starvation body mass could be linked to higher fat content and lean dry mass before starvation, followed by a greater decrease in both during starvation. There was also a difference between the sexes in the extent of body mass changes both during ad libitum feeding and following starvation; the body masses of females fluctuated more compared to males. This study stresses the need to distinguish between body mass and structural body size when investigating the ecological and evolutionary consequences of body size. Investigation of the net effects of body size and sex is necessary to disentangle the causes of differences in individual performances in studies of species with significant sexual size dimorphism.     

The influences of host plant and genetic resistance to Bacillus thuringiensis on trade-offs between offspring number and growth rate in cabbage loopers, Trichoplusia ni

Abstract.  1. Variation in progeny size and quality is common among insects and this variation can strongly influence individual fitness. Larger progeny typically survive better and develop faster under adverse conditions and may have higher fecundity. Due to resource limitations, however, trade-offs may arise between having fewer large offspring or more smaller ones.
2. For cabbage loopers, Trichoplusia ni , pepper leaves are a poorer larval host than cucumber or tomato leaves as indicated by survival, development rate, and body size. Moths reared on cucumber produced more slower growing offspring than those that had been reared on pepper, which produced fewer, faster growing progeny.
3. Traits conferring resistance to Bacillus thuringiensis ( Bt ) generally are associated with strong deleterious effects that may influence resource allocation and reproductive trade-offs between progeny size and number.
4. Unlike the host-plant related trade-off between progeny size and fecundity observed among susceptible control moths, Bt -resistant parents had both the lowest fecundity and smallest progeny size on all host plants. This finding suggests that the progeny size–number relationship is constrained in resistant individuals.     

Shaking a leg and hot to trot: the effects of body size and temperature on running speed in ants

Abstract.  1. Data were compiled from the literature and our own studies on 24 ant species to characterise the effects of body size and temperature on forager running speed.
2. Running speed increases with temperature in a manner consistent with the effects of temperature on metabolic rate and the kinetic properties of muscles.
3. The exponent of the body mass-running speed allometry ranged from 0.14 to 0.34 with a central tendency of approximately 0.25. This body mass scaling is consistent with both the model of elastic similarity, and a model combining dynamic similarity with available metabolic power.
4. Even after controlling for body size or temperature, a substantial amount of inter-specific variation in running speed remains. Species with certain lifestyles [e.g. nomadic group predators, species which forage at extreme (>60 °C) temperatures] may have been selected for faster running speeds.
5. Although ants have a similar scaling exponent to mammals for the running speed allometry, they run slower than predicted compared with a hypothetical mammal of similar size. This may in part reflect physiological differences between invertebrates and vertebrates.     

Development time and body size in Eupolyphaga sinensis along a latitudinal gradient from China

The responses of organisms to temperature variations may be via short term responses of the phenotype (phenotypic plasticity), or they could involve long-term evolutionary change and adaptation (via selection) to the genotype. These could involve changes to the mean size of the animal or to the thermal reaction norm. We examined the effects of various temperatures (of 22, 25, 28, and 31 °C) on development time, adult body size and preadult survivorship in three populations of the cockroach, Eupolyphaga sinensis (Walker), collected at different latitudes. We found substantial temperature-induced plasticity in development time, body size, and preadult survivorship, indicating that developmental temperatures have strong impacts on growth and life history traits of E. sinensis. Genetic differences for development time, body size, and preadult survivorship were detected among populations, and the three traits exhibited highly significant variations in the responses of different populations to various temperature conditions, indicating genetic differences among populations in terms of thermal reaction norms. We also found that two populations seem to support the beneficial acclimation hypothesis whereas the third mid-latitude population does not. The results are likely because of differences in season length and voltinism, indicating that not only temperature regime but also its interactions with generation time (and development time), voltinism, and season length are likely to have considerable effects on insect development time and body size. Overall, changes in development time, body size, and preadult survivorship in E. sinensis can all be regarded as adaptations to changing thermal regimes.     

Emergent body size of mayfly survivors

1. Employing field-deployed mesocosms, we examined the effects of 12-h pulse and 20-day press (continuous) exposures of the common agricultural insecticide, imidacloprid, on nymph abundance, emergence patterns and adult body size of Epeorus spp. (Heptageniidae) and Baetis spp. (Baetidae).
2. In press exposures, reduced nymph density was driven by reduced survivorship; in pulse exposures, reduced nymph density may reflect increased emergence because of stress.
3. Once exposed to imidacloprid, Epeorus and Baetis mayflies developed less and emerged smaller than their control counterparts. Concentrations as low as 0.1  μ g L⁻¹ (12-h pulse) reduced head length in Baetis and thorax length in Epeorus .
4. In all of the Baetis and Epeorus examined, effects were only found in males. Male survivorship and body size can affect population dynamics. Sublethal doses of this widely applied agricultural insecticide have the potential to reduce reproductive success of mayfly populations.     

ENVIRONMENTAL STRESS AND THE MAINTENANCE OF SEX IN A FRESHWATER SNAIL

Synergism among mutations can lead to an advantage to sexual reproduction, provided mutation rates are high enough (the mutational deterministic hypothesis). Here we tested the idea that competition for food can increase the advantage to sexual reproduction, perhaps by increasing the synergism among mutations in asexual individuals. We compared the survivorship of sexual and asexual snails (Potamopyrgus antipodarum) under two treatments: starved and fed. We predicted higher mortality for asexual snails when starved, but found that sexual and asexual individuals survived at the same rate, independent of treatment. These results suggest that the distribution of sex in this snail may not be explained by variation in competition among populations.     

Development time and size-related traits in the oriental blowfly, Chrysomya megacephala along a latitudinal gradient from China

Organisms can respond to variation in temperature through the direct effect of temperature on phenotypes (phenotypic plasticity), or through long-term adaptation to temperature (and thus evolution of either mean size or thermal reaction norm). We examined the effects of various temperatures (of 20 and 30 °C) on development time, adult body size (body length and body width) and pre-adult survivorship in six populations of Chrysomya megacephala, collected at different latitudes. We found that temperature changes induced substantial plasticity in terms of development time, body size and pre-adult survivorship, indicating that developmental temperature significantly affects growth and life history traits of C. megacephala. We also detected genetic differences among populations for body size and development time, and these two traits exhibited highly significant variations in the responses of different populations to various temperature conditions, indicating genetic differences among populations in terms of thermal reaction norms. The latitude of origin of the different populations (and hence mean temperature regimes in the environments from where the populations originated) did not appear to fully explain these genetic differences. In short, changes in development time and body size in C. megacephala can be regarded as adaptations to changing thermal regimes.     

Starvation mortality and body condition of Goshawks Accipiter gentilis along a latitudinal gradient in Norway

Relative starvation risk and body condition were investigated in 599 Goshawks that had died in collision accidents or of starvation. Specimens were collected by the public along a 1300-km north–south (58°N–71°N) gradient in Norway, representing the northernmost geographical range of the species. The probability of a Goshawk's death being caused by starvation as opposed to by a collision accident increased with latitude with juvenile males at a disproportionately higher risk than others. Of birds killed in accidents, females generally were in better condition than males, and adults in better condition than juveniles. A season-by-latitude interaction indicated that males from northern latitudes were in poorer condition during winter and spring than males from southern parts of the country. This could also be modelled as a curvilinear relationship with daylength. There were no significant relationships between weather factors in the weeks prior to the deaths of the birds and the relative starvation probability or the condition of trauma victims. The results suggest that food limitation plays a relatively higher role in northern populations, affecting young males especially. This was also supported by the fact that the sex ratio of accidentally killed birds was increasingly female biased with increasing latitudes. It is suggested that the relatively higher mortality risk of males is due to their smaller average body size, and that selection for starvation resistance during winter is the reason behind the clinal increase of body size in Goshawks towards the northern and eastern parts of Europe.     

Migration and Allee effects in the six-spot burnet moth Zygaena filipendulae

Abstract 1. Migration into local populations may increase the likelihood of persistence but emigration may decrease the persistence of small and isolated populations. The dispersal behaviour of a day-flying moth Zygaena filipendulae was examined to determine whether emigration is correlated positively or negatively with population size and host plant density.
2. A mark–release–recapture study showed that most moths moved small distances (< 40 m on average) and only 6% of movements were > 100 m.
3. Twenty-five individuals moved between populations, a measured exchange rate of 8%. Moths were more likely to move between patches that were close together and they moved to relatively large patches.
4. The fraction of residents increased with increasing population size in the patch and increasing host plant cover. Relatively high proportions of individuals left small patches with small moth populations.
5. Moths released in grassland lacking Lotus corniculatus (the host plant) tended to leave the area and biased their movement towards host plant areas, whereas those released within an area containing L. corniculatus tended to stay in that area.
6. Biased movement away from small populations and areas of low host plant density (normally with low population density) was found. This migration-mediated Allee effect is likely to decrease patch occupancy in metapopulations, the opposite of the rescue effect. The effects on metapopulation persistence are not known.     

Fattening strategies of wintering great tits support the optimal body mass hypothesis under conditions of extremely low ambient temperature

1.  In a seasonal environment, subcutaneous energy reserves of resident animals often increase in winter and decline again in summer reflecting gradual seasonal changes in their fattening strategies. We studied changes in body reserves of wintering great tits in relation to their dominance status under two contrasting temperature regimes to see whether individuals are capable of optimizing their body mass even under extreme environmental conditions.
2.  We predicted that dominant individuals will carry a lesser amount of body reserves than subordinate great tits under mild conditions and that the body reserves of the same dominant individuals will increase and exceed the amount of reserves of subordinates under conditions of extremely low ambient temperatures, when ambient temperature dropped down to −37 °C.
3.  The results confirmed the predictions showing that dominant great tits responded to the rising risk of starvation under low temperatures by increasing their body reserves and this was done at the expense of their safety.
4.  Removal experiments revealed that lower body reserves of subordinate flock members are due to the increased intraspecific competition for food under low ambient temperatures.
5.  Our results also showed that fattening strategies of great tits may change much quicker than previously considered, reflecting an adaptive role of winter fattening which is sensitive to changes in ambient temperatures.     

Immune Activity,Body Condition and Human-Associated Environmental Impacts in a Wild Marine Mammal

Within individuals, immunity may compete with other life history traits for resources, such as energy and protein, and the damage caused by immunopathology can sometimes outweigh the protective benefits that immune responses confer. However, our understanding of the costs of immunity in the wild and how they relate to the myriad energetic demands on free-ranging organisms is limited. The endangered Galapagos sea lion (Zalophus wollebaeki) is threatened simultaneously by disease from domestic animals and rapid changes in food availability driven by unpredictable environmental variation. We made use of this unique ecology to investigate the relationship between changes in immune activity and changes in body condition. We found that during the first three months of life, changes in antibody concentration were negatively correlated with changes in mass per unit length, skinfold thickness and serum albumin concentration, but only in a sea lion colony exposed to anthropogenic environmental impacts. It has previously been shown that changes in antibody concentration during early Galapagos sea lion development were higher in a colony exposed to anthropogenic environmental impacts than in a control colony. This study allows for the possibility that these relatively large changes in antibody concentration are associated with negative impacts on fitness through an effect on body condition. Our findings suggest that energy availability and the degree of plasticity in immune investment may influence disease risk in natural populations synergistically, through a trade-off between investment in immunity and resistance to starvation. The relative benefits of such investments may change quickly and unpredictably, which allows for the possibility that individuals fine-tune their investment strategies in response to changes in environmental conditions. In addition, our results suggest that anthropogenic environmental impacts may impose subtle energetic costs on individuals, which could contribute to population declines, especially in times of energy shortage.     

Preference and Performance in Plant–Herbivore Interactions across Latitude–A Study in U.S. Atlantic Salt Marshes

High-latitude plants are often more palatable to herbivores than low-latitude conspecifics. Does increased plant palatability lead to better herbivore performance? Our field and laboratory work investigated (A) whether high-latitude plants have traits indicating that they should be higher-quality foods for herbivores; (B) whether geographic differences in plant quality are more important than local adaptation of herbivores. We studied 3 plant species and 6 invertebrate herbivores in U.S. Atlantic Coast. Past studies had shown high-latitude individuals of these plants are more palatable than low-latitude conspecifics. We documented plant traits and herbivore performance (body size) in the field across latitude. We collected individuals from different latitudes for factorial (plant region x herbivore region) laboratory experiments, examining how herbivore performance was affected by plant region, herbivore region, and their interaction (i.e., local adaptation). Field surveys suggested high-latitude plants were likely of higher quality to herbivores. Leaf nitrogen content in all plant species increased toward high latitudes, consistent with lower leaf C/N and higher leaf chlorophyll content at high latitudes. Furthermore, leaf toughness decreased toward higher latitudes in 1 species. The body size of 4 herbivore species increased with latitude, consistent with high-latitude leaves being of higher quality, while 2 grasshopper species showed the opposite pattern, likely due to life-history constraints. In the laboratory, high-latitude plants supported better performance in 4 herbivore species (marginal in the 5th). The geographic region where herbivores were collected affected herbivore performance in all 6 species; however, the pattern was mixed, indicating a lack of local adaptation by herbivores to plants from their own geographic region. Our results suggest that more-palatable plants at high latitudes support better herbivore growth. Given that geographic origin of either plants or herbivores can affect herbivore performance, the nature of plant-herbivore interactions is likely to change if climate change “reshuffles” plant and herbivore populations across latitude.     

The effects of tail autotomy on survivorship and body growth of Uta stansburiana under conditions of high mortality

We examined the effects of tail autotomy on survivorship and body growth of both adult and juvenile Uta stansburiana by directly manipulating tail condition. Tail loss decreased neither survivorship nor rate of body growth for individuals in two natural populations. Lack of an influence of tail loss on survivorship in these two populations may be the result of high mortality. Under high mortality any differential effects of tail loss will be lower than in populations facing lower mortality. Growth experiments in the laboratory demonstrated that, under conditions of minimal environmental variation and social interactions, there is no tradeoff between body growth and tail regeneration as has been suggested for other species of lizards. One possible reason for this difference is that U. stansburiana does not use the tail as a storage organ for lipids. The original and regenerated tails are composed mainly of protein. In general, any differential body growth between tailed and tailless individuals may be due to social interactions and not a diversion of limited energy into tail regeneration.