Multidimensional resource partitioning by Serengeti herbivores

An understanding of the habitat selection patterns by wild herbivores is critical for adaptive management, particularly towards ecosystem management and wildlife conservation in semi arid savanna ecosystems. We tested the following predictions: (i) surface water availability, habitat quality and human presence have a strong influence on the spatial distribution of wild herbivores in the dry season, (ii) habitat suitability for large herbivores would be higher compared to medium-sized herbivores in the dry season, and (iii) spatial extent of suitable habitats for wild herbivores will be different between years, i.e., 2006 and 2010, in Matetsi Safari Area, Zimbabwe. MaxEnt modeling was done to determine the habitat suitability of large herbivores and medium-sized herbivores. MaxEnt modeling of habitat suitability for large herbivores using the environmental variables was successful for the selected species in 2006 and 2010, except for elephant (Loxodonta africana) for the year 2010. Overall, large herbivores probability of occurrence was mostly influenced by distance from rivers. Distance from roads influenced much of the variability in the probability of occurrence of medium-sized herbivores. The overall predicted area for large and medium-sized herbivores was not different. Large herbivores may not necessarily utilize larger habitat patches over medium-sized herbivores due to the habitat homogenizing effect of water provisioning. Effect of surface water availability, proximity to riverine ecosystems and roads on habitat suitability of large and medium-sized herbivores in the dry season was highly variable thus could change from one year to another. We recommend adaptive management initiatives aimed at ensuring dynamic water supply in protected areas through temporal closure and or opening of water points to promote heterogeneity of wildlife habitats.

The relationship between environmental gradients and patterns of geographic variation in body size has been a controversial topic for ectothermic organisms globally. To examine whether the patterns that generally hold in more temperate species also hold for tropical ones, we examined the intraspecific body size variation in three species of Neotropical frogs, Dendropsophus minutus, Hypsiboas faber and Physalaemus cuvieri, along different environmental gradients (e.g. temperature, precipitation and topography). We analysed four competing hypotheses: (i) the water availability hypothesis that predicts a negative relationship between body size and precipitation; (ii) the heat balance hypothesis that predicts a negative relationship between body size and temperature; (iii) the topography hypothesis that predicts a negative relationship between body size and altitude; and (iv) the mixed-effect hypothesis that predicts that individuals occurring in wet and cold sites would be larger than individuals occurring in dry and warm sites. The spatial pattern of geographic variation in body size among populations of H. faber was associated with the mixed-effect hypothesis. In localities with low precipitation seasonality and cold conditions, H. faber individuals were larger than in localities with high precipitation seasonality and warm conditions. Variation in the body size of D. minutus was the opposite of that predicted by the heat balance hypothesis. Individuals in localities with high temperatures were larger than in localities with low temperatures. On the other hand, variation in the body size of P. cuvieri was not associated with the variables used in this study. Our results suggest that intraspecific variation in anuran body size is more dependent on species-specific response than on the region (i.e. temperate or tropical) where they occur.

SummaryHerbivores may negatively impact plant populations by reducing the survival, growth and reproduction of individual plants.In African savannas, browsing by large mammalian herbivores has been shown repeatedly to reduce adult survival and growth of members of the genusAcacia, the dominant trees in these systems. However, the potential costs of mammalian herbivory toAcaciareproduction are largely unexplored, and have never been investigated experimentally in African savannas.Using a long‐term, large‐scale field experiment, we demonstrate that, in the absence of native herbivores, individual trees were twice as likely to reproduce, and those that reproduced produced a greater biomass of seeds. In addition, spine length (an indicator of past herbivory) was correlated with reduced reproduction ofAcacia drepanolobium, the dominant tree across large areas of East Africa. Browsing by native herbivores triggers the production of longer spines, an induced resistance, and spine length was significantly and negatively related both to the occurrence and magnitude of reproduction.Induced resistance appeared to mitigate the negative effects of herbivory where large herbivores were present: trees with long spines reproduced at levels comparable to trees with similarly long spine lengths in the absence of browsers.Large mammalian herbivores kill and suppress the growth of adultAcacia, and therefore often are regarded as critical in maintaining the co‐dominance of trees and grasses in African savannas. Our results provide the first experimental evidence that large mammalian herbivores can suppress reproduction inAcacia, thus highlighting an additional pathway through which these herbivores might impact population dynamics of this important and widespread genus.In savanna systems, declining populations of large herbivores may trigger a series of complex demographic responses for trees, with potentially strong consequences for the structure and function of savannas.

The viability of large herbivore populations in the face of climate change, environmental variability, disease and predation will be determined by their freedom to assess and respond to these factors through access to a range of functional seasonal resources and habitats. Their responses will be contingent upon various organismal traits, such as body size, mouth and digestive anatomy, which also facilitate coexistence among sympatric species. In this paper we develop a functional adaptive forage resource framework based on plant biomass, quality and phenology responses on ecological productivity gradients. We show how large herbivores coexist and respond to environmental variability, disease and predation by their foraging responses in relation to functional adaptive resources, as mediated by their anatomical traits. Below a critical body size, where predation limits population size, large herbivores adopt a variety of predation avoidance strategies, which are linked to their anatomical traits and foraging strategies. Mouth anatomy, and its interaction with body size, appears to be the major anatomical trait determining large herbivore selection for grass height. Body size is the major trait determining vulnerability to predators. Ecological productivity gradients underlain by variation in soil moisture availability over the annual cycle support high quality forage in the least productive (driest) regions, which promote growth and reproduction (a fecundity resource). Reserves of adequate quality forage in moderate productivity regions and buffers of low-quality forage in wetter and more productive regions of the gradient prevent loss of body stores over the dry season and starvation during droughts. Fire and grazing contribute towards providing high quality forage by removing old, low-quality material and preventing forage maturation. Consuming a high diversity of plant species distributed across ecological gradients promotes phytochemical diversity in the diet, which functions as medicinal resources to promote health while combating disease and parasites. Large herbivores are becoming increasingly restricted by ecosystem fragmentation in their access to the full range of these functional resource classes. The negative consequences for large herbivore populations of reduced access to these resource classes is compounded by climate change, where conditions are hotter and drought frequency and intensity is expected to be higher.

Semiarid rangelands often respond slowly to rest/relaxation of grazing pressure by large herbivores, and the effects of grazing are most often inferred from this direction of study because the imposition of grazing onto previous ungrazed/lightly grazed areas occurred prior to the age of scientific studies. These rangelands host a diversity of small and large herbivores, but grazing studies most often concern effects of the large generalists. Here, the effects of herbivore body size on plant species richness and dominant species, and imposition and relaxation of grazing by large herbivores were studied by opening half of exclosures established in 1939 and building new exclosures to large herbivores, and to small-plus-large herbivores. Plant richness using sensitive species-area sampling was studied in a dry and a wet year, about 62 years after initiating the long-term experiment and about 6-10 years after initiating the altered designs. Convergence of the newly opened to large herbivore grazing treatment to the long-term grazed treatment occurred within 10 years, but convergence of the newly excluded to large herbivore treatment to the long-term excluded treatment had only partly occurred. This indicated that recovery from grazing is slow relative to imposition of grazing by large herbivores, but effects of the additional exclusion of small-plus-large herbivores occurred relatively rapidly. These results were mirrored by trajectories of convergence of the dominant species, and this is discussed with respect to implications for state-and-transition models. Short-term exclusion of small-plus-large herbivores resulted in greater richness than even long-term exclusion of only large herbivores, even though quantities consumed by small herbivores are much less than by large. Grazing effects on plant richness were large in the wet year, but the very dry year suppressed richness on all treatments. When sampling effort and area are the same, the numbers and attributes of species unique to a treatment are indicators of rareness of the richness and traits selected for by the treatment. More unique species were sampled in the small-plus-large herbivore exclosures when comparing body size, and the long-term large herbivore exclosures when comparing time of exclosure. Unique species encountered during sampling the ungrazed treatments were generally forbs, exotic and/or weedy invasive species, and often tall, annual species.

Although variation in body size has been recently reported in stingless bees (Meliponini), empirical evidence evaluating possible factors related to such variation is lacking, and thus it is not clear if it may have an adaptive significance. We evaluated if variation in the body size and weight of workers of stingless bees fluctuates across a seasonal pattern and if this could be related to characteristics of the food consumed during the larval stage. The weight of larval provisions, their protein, and sugar content were evaluated in four colonies of Nannotrigona perilampoides every 2 months across 1 year. Worker-destined larvae from the same combs were allowed to develop and were sampled as callow workers to determine their weight and size using morphometric data. The weight and size of workers were highly correlated and varied across the seasons in established colonies, suggesting that size variation cycles across the year in stingless bees. An increase in the protein content and, to a lesser degree, the quantity of larval food were positively linked to variation in body weight and size; food with richer protein content resulted in larger and heavier workers. This study provides the first evidence of an effect of the quantity and composition of larval food on the size of workers in stingless bees. Although body weight and size of workers differed across seasons, they were not readily noticeable as changes seem to occur as a continuum across the year. Since size polymorphism was of a larger magnitude across time but not within age cohorts and as it was highly determined by food resources, it may not be an adaptive feature in stingless bees. However, more studies are needed to determine the role of the cyclical change in worker body size on colony performance and thus its adaptive significance in stingless bees.

Density-related pattern of variation in body growth, body size and annual productivity in the common hamster

Abstract:Litter-feeding termites influence key aspects of the structure and functioning of semi-arid ecosystems around the world by altering nutrient and material fluxes, affecting primary production, foodweb dynamics and modifying vegetation composition. Understanding these complex effects depends on quantifying spatial heterogeneity in termite foraging activities, yet such information is scarce for semi-arid savannas. Here, the amount of litter that was removed from 800 litterbags in eight plots (100 litterbags per plot) was measured in Hluhluwe–iMfolozi Park (HiP) South Africa. These data were used to quantify variation in litter removal at two spatial scales: the local scale (within 450-m2 plots) and the landscape scale (among sites separated by 8–25 km). Subsequently, we attempted to understand the possible determinants of termites’ foraging patterns by testing various ecological correlates, such as plant biomass and bare ground at small scales and rainfall and fences that excluded large mammalian herbivores at larger scales. No strong predictors for heterogeneity in termite foraging intensity were found at the local scale. At the landscape scale termite consumption depended on an interaction between rainfall and the presence of large mammalian herbivores: litter removal by termites was greater in the presence of large herbivores at the drier sites but lower in the presence of large herbivores at the wetter sites. The effect of herbivores on termite foraging intensity may indicate a switch between termites and large herbivore facilitation and competition across a productivity gradient. In general, litter removal decreased with increasing mean annual rainfall, which is in contrast to current understanding of termite consumption across rainfall and productivity gradients. These results generate novel insights into termite ecology and interactions among consumers of vastly different body sizes across spatial scales.

ABSTRACTAim To determine how well variation in median body size of avian assemblages is predicted by (1) the environmental models usually employed in analyses of Bergmann's rule and (2) random sampling from the regional body size frequency distribution. If body size frequency distributions of local assemblages represent a random sample of a regional frequency distribution, then geographical variation in body sizes of assemblages might be a consequence of the determinants of spatial variation in species richness rather than direct influences on body size per se.Location Southern Africa.Methods Median body masses (as a measure of body size) of avian assemblages were calculated for quarter‐degree grid cells across South Africa and Lesotho. The relationship between median body mass and four environmental variables (minimum and maximum monthly temperatures, precipitation and seasonality in the normalized difference vegetation index, as a measure of seasonality in productivity) was examined using general linear models first without taking spatial autocorrelation into account, and then accounting for it by fitting an exponential spatial covariance structure. Model fit was assessed using the Akaike information criterion and Akaike weights. At each species richness value, random assemblages were sampled by either drawing species randomly from the regional body mass frequency distribution, or drawing species from the regional body mass frequency distribution with a probability proportional to their geographical distribution in the area. The ability of randomizations to predict actual body masses was examined using two‐tailed Fisher exact tests.Results Seasonality in productivity was the only environmental variable that remained a significant predictor of body mass variation in spatially explicit models, though the positive relationship was weak. When species richness was included in the models it remained the only significant predictor of size variation. Randomizations predicted median body mass poorly at low species richness, but well at high richness.Main conclusions Environmental models that have previously been proposed explain little of the variation in body mass across avian assemblages in South Africa. However, much of the variation in the median mass of assemblages could be predicted by randomly drawing species from the regional body mass frequency distribution, particularly using randomizations in which all species were drawn from the regional body mass frequency distribution with equal probability and at high species richness values. This outcome emphasizes the need to consider null expectations in investigations of the geographical variation in body size together with the probable environmental mechanisms underlying spatial variation in average size. Moreover, it suggests that in the South African avifauna, spatial variation in the body sizes of assemblages may be determined indirectly by the factors that influence geographical variation in species richness.

1. The egg size of insects can vary depending on maternal body size or resource status, and it may influence offspring body size by determining initial resource level.2. The giant rhinoceros beetle Trypoxylus dichotomus exhibits considerable variation in body size, some of which is attributed to the variation in larval food (humus) quality, although a substantial amount of variation in body size remains unexplained. In the present study, changes in the egg size and offspring body size in response to several maternal variables were examined (i.e. body size, age, and, nutritional status).3. Nutritional intake of the females during the adult stage did not affect the egg size. Larvae hatched from small eggs partially recovered from the initial disadvantage during their ontogenetic processes by increasing growth rate (i.e. compensatory growth); however, there was still a positive relationship between egg size and pupal body size.4. Older females produced small eggs, but because of compensatory growth, the pupae were no longer small. By contrast, due to a lack of compensatory growth, small females produced small eggs as well as small pupae.5. These results suggest that maternal body size affects offspring body size through effects on egg size. This transgenerational effect may account for some of the variation in adult body size of T. dichotomus.

This thesis examines the mutual interactions between the population dynamics of large herbivores and wood-pasture cycles in eutrophic wetlands. Therefore, habitat use and population dynamics of large herbivores, the effects of large herbivores on vegetation development, and the mutual interactions between vegetation development and herbivore population dynamics were studied in the eutrophic wetland the Oostvaardersplassen. At the Oostvaardersplassen cattle, horses and red deer were introduced in a fenced area with no predators, and population numbers are bottom-up controlled by food supply. The study showed that high densities of cattle, horses and red deer were able to break down woody vegetation and create grasslands. As the populations of large herbivores increased, the amount of the preferred grass available per animal decreased. This forced the large herbivores to use other food plants in other vegetation types, such as scrub, and transforming these into grasslands. In this way, the large herbivores facilitated high numbers of geese. As geese can clip the grass very short (<2 cm), they forced the large herbivores even more to forage in alternative vegetation types. Cattle, the largest herbivore in the system, were the first to experience the negative consequences of this strong competition, and their numbers declined. This raises the question whether an assemblage of bottom-up regulated populations of cattle, horses and red deer, or other large herbivores, can sustainably coexist under these circumstances. The results of our modelling study and experiences in the field suggest that resource partitioning may be a more reliable mechanism for long term coexistence than temporal variability due to climatic extremes or disease outbreaks. The best way to provide opportunities for resource partitioning in the Oostvaardersplassen is to enlarge the area and connect it to other reserves in order to increase the heterogeneity of the grazed system. Although the results of our model suggest that weather variability and presence of geese gave minor opportunities for the coexistence of large herbivores, both factors were necessary for creating windows of opportunity for the establishment of thorny shrubs. Weather variability creates strong reductions of the large herbivore populations while geese influence the maximum and minimum numbers, which are lower when geese are present. The effects of geese on the minimum numbers are small, but apparently sufficient to make the wood-pasture cycle operate. This raises another question whether a large predator, such as the wolf, could have similar effects on these ecosystems as the geese in the model. The impact of geese combined with a possible positive effect of wolves on wood-pasture cycles could perhaps increase the frequency of the windows of opportunity and increase the survival of established thorny shrubs. Until now, we have seen that a few conditions for the wood-pasture cycle are met by the herbivores. However, a few important requirements are not satisfied: (a) a temporary reduction of large herbivore numbers allowing the establishment of light demanding thorny shrubs and the development of thorny scrubland within the created grasslands; (b) the establishment of palatable trees within these thorny scrubs; (c) the formation of closed canopies which shade out the shrubs and lead to unprotected groups of trees and groves. This means that we still cannot conclude if the large herbivores are a driving force for the whole cycle in a highly productive environment. As long as we have not experienced a complete wood-pasture cycle in the Oostvaardersplassen or any other area, it remains to be seen what will happen in the future. Whatever the outcome will be, the results of our study suggest that some adjustments would benefit the Oostvaardersplassen-system such as increasing heterogeneity through connecting the area with other large nature reserves. This will not only increase opportunities for resource and space partitioning and thus increase opportunities for the coexistence of the large herbivores, but also for wood-pasture cycles and increased biodiversity.

Hypotheses that explain geographic variation in body size were examined using cranial measurements of 950 bobcats (Lynx rufus) from western North America. Bobcats were divided into 25 geographic localities of similar habitats and landform (based on ecoregions). Principal component analyses were used to derive a single estimate of size from scores on the first principal component. Males and females were examined separately, because they were significantly dimorphic in body size and because sex and locality exhibited a significant interaction. We expected that female body size would best reflect environmental influences, because male size may be influenced by sexual selection. We found significant geographic variation in bobcat body size, with about 44% of the variation in males and 47% of the variation in females accounted for by comparison among the localities. We also found that variation in body size was associated with Bergmann's rule, as indicated by significant multiple regression of body size of males (R2 = 0.426) and females (R2 = 0.480) on latitude and elevation. Using correlation and regression analyses, we examined the association of body size with selected environmental variables that represent the classical physiological explanation of Bergmann's rule, James' moisture-humidity modification of Bergmann's rule, Rosenzweig's productivity hypothesis, and Boyce's seasonality hypothesis. Only the productivity hypothesis was not supported. The relative strengths of associations suggested, however, that James' modification was better supported than the classical explanation for Bergmann's rule. Path analyses permitted further discrimination of hypotheses, and only the seasonality hypothesis received significant support. As expected, this support was only evident for females. Path analysis may provide a tool for evaluating relative strengths of competing but correlated explanations of geographic variation.

An increase in genetic variation in body size has often been observed under stress; an increase in dominance variance and interaction variance as well as in additive genetic variance has been reported. The increase in genetic variation must be caused by physiological mechanisms that are specific to adverse environments. A model is proposed to explain the occurrence of an increase in genetic variation in body size in Drosophila at extreme temperatures. The model has parameters specific to the low- and high-temperature regions of the viable range. Additive genetic variation in the boundary temperatures leads to a marked increase in additive genetic variation in development rate and body size at extreme temperatures. Additive genetic variation in the temperature sensitivity in the low- and high-temperature regions adds non-additive genetic variation. Development rate shows patterns in additive genetic variation that differ from the patterns of genetic variation in body size; therefore, the genetic correlation between development rate and body size changes sign repeatedly as a function of temperature. The existence of dominance in the genetic variation in the boundary temperatures or in the low- and high-temperature sensitivities leads to a higher total genetic variance due to higher dominance and interaction variance, for both development rate and body size.

Intraspecific variability and species turnover drive variations in Collembola body size along a temperate-boreal elevation gradient

Body size of large herbivores is a crucial life history variable influencing individual fitness‐related traits. While the importance of this parameter in determining temporal trends in population dynamics is well established, much less information is available on spatial variation in body size at a local infra‐population scale. The relatively recent increase in landscape fragmentation over the last century has lead to substantial spatial heterogeneity in habitat quality across much of the modern agricultural landscape. In this paper, we analyse variation in body mass and size of roe deer inhabiting a heterogeneous agricultural landscape characterised by a variable degree of woodland fragmentation. We predicted that body mass should vary in relation to the degree of access to cultivated meadows and crops providing high quality diet supplements. In support of our prediction, roe deer body mass increased along a gradient of habitat fragmentation, with the heaviest deer occurring in the most open sectors and the lightest in the strict forest environment. These spatial differences were particularly pronounced for juveniles, reaching &gt;3 kg (ca 20% of total body mass) between the two extremes of this gradient, and likely have a marked impact on individual fates. We also found that levels of both nitrogen and phosphorous were higher in deer faecal samples in the more open sectors compared to the forest environment, suggesting that the spatial patterns in body mass could be linked to the availability of high quality feeding habitat provided by the cultivated agricultural plain. Finally, we found that adults in the forest sector were ca 1 kg lighter for a given body size than their counterparts in the more open sectors, suggesting that access to nutrient rich foods allowed deer to accumulate substantial fat reserves, which is unusual for roe deer, with likely knock‐on effects for demographic traits and, hence, population dynamics.