Why do population density and inverse home range scale differently with body size?

No evidence for the consistent effect of supplementary feeding on home range size in terrestrial mammals

Food availability and distribution are key drivers of animal space use. Supplemental food provided by humans can be more abundant and predictable than natural resources. It is thus believed that supplementary feeding modifies the spatial behaviour of wildlife. Yet, such effects have not been tested quantitatively across species. Here, we analysed changes in home range size owing to supplementary feeding in 23 species of terrestrial mammals using a meta-analysis of 28 studies. Additionally, we investigated the moderating effect of factors related to (i) species biology (sex, body mass and taxonomic group), (ii) feeding regimen (duration, amount and purpose), and (iii) methods of data collection and analysis (source of data, estimator and spatial confinement). We found no consistent effect of supplementary feeding on changes in home range size. While an overall tendency of reduced home range was observed, moderators varied in the direction and strength of the trends. Our results suggest that multiple drivers and complex mechanisms of home range behaviour can make it insensitive to manipulation with supplementary feeding. The small number of available studies stands in contrast with the ubiquity and magnitude of supplementary feeding worldwide, highlighting a knowledge gap in our understanding of the effects of supplementary feeding on ranging behaviour.

Island Hopping through Urban Filters: Anthropogenic Habitats and Colonized Landscapes Alter Morphological and Performance Traits of an Invasive Amphibian

Simple Summary

Invasive species are common on islands and, increasingly so, in urban ecosystems. They can pose serious ecological and socioeconomic impacts, making research on how invasions are promoted critically important. We examined different traits of guttural toads (Sclerophrys gutturalis) in their natural and invasive ranges (both natural and urban populations in native and invasive sites) to understand if divergences in habitats in their native range could increase their invasive potential. We found that invasive island populations on Mauritius and Réunion (Indian Ocean) have reduced body sizes, proportionally shorter limbs, slower escape speeds, and reduced endurance capacities compared to the native South African populations. In short, these changes occurred post-invasion. However, increase climbing ability was seen within the urban-native toads, a trait maintained within the two invasions, suggesting that it may have been an advantageous prior adaptation. Becoming climbers may have benefited the toad during colonization, increasing navigation and hunting ability within the urbanized areas where they were introduced, prior to their spread into natural areas. This change in climbing performance is an example of how the urbanization of native taxa may be increasing the ability of certain species to become better invaders should they be introduced outside their native range.

Abstract

A prominent feature of the modern era is the increasing spread of invasive species, particularly within island and urban ecosystems, and these occurrences provide valuable natural experiments by which evolutionary and invasion hypotheses can be tested. In this study, we used the invasion route of guttural toads (Sclerophrys gutturalis) from natural-native and urban-native populations (Durban, South Africa) to their urban-invasive and natural-invasive populations (Mauritius and Réunion) to determine whether phenotypic changes that arose once the toads became urbanized in their native range have increased their invasive potential before they were transported (i.e., prior adaptation) or whether the observed changes are unique to the invasive populations. This urban/natural by native/invasive gradient allowed us to examine differences in guttural toad morphology (i.e., body size, hindlimb, and hindfoot length) and performance capacity (i.e., escape speed, endurance, and climbing ability) along their invasion route. Our findings indicate that invasive island populations have reduced body sizes, shorter limbs in relation to snout-vent length, decreased escape speeds, and decreased endurance capacities that are distinct from the native mainland populations (i.e., invasion-derived change). Thus, these characteristics did not likely arise directly from a pre-transport anthropogenic “filter” (i.e., urban-derived change). Climbing ability, however, did appear to originate within the urban-native range and was maintained within the invasive populations, thereby suggesting it may have been a prior adaptation that provided this species with an advantage during its establishment in urban areas and spread into natural forests. We discuss how this shift in climbing performance may be ecologically related to the success of urban and invasive guttural toad populations, as well as how it may have impacted other island-derived morphological and performance phenotypes.

Spatial variance-mass allometry of population density in felids from camera-trapping studies worldwide

Power laws are cornerstone relationships in ecology and evolutionary biology. The density-mass allometry (DMA), which predicts an allometric scaling of population abundance, and Taylor’s law (TL), which predicts a decrease in the population abundance variation along with a decrease in population density, have enhanced our knowledge of inter- and intra-specific variation in population abundance. When combined, these two power laws led to the variance-mass allometry (VMA), which states that larger species have lower spatial variation in population density than smaller species. The VMA has been predicted through theoretical models, however few studies have investigated if this law is also supported by empirical data. Here, to formally test the VMA, we have used the population density estimates obtained through worldwide camera trapping studies for an emblematic and ecologically important carnivorous taxa, the Felidae family. Our results showed that the VMA law hold in felids, as well as the TL and the DMA laws; bigger cat species showed less variation for the population density than smaller species. These results have important implications for the conservation of wildlife population and confirm the validity of important ecological concepts, like the allometric scaling of population growth rate and the slow-fast continuum of life history strategies.

Articulated remains of the extinct shark Ptychodus (Elasmobranchii, Ptychodontidae) from the Upper Cretaceous of Spain provide insights into gigantism, growth rate and life history of ptychodontid sharks

Due to their cartilaginous endoskeleton and the continuous tooth replacement, the chondrichthyan fossil record predominantly consists of isolated teeth, which offer diagnostic features for taxonomic identifications, but only provide very limited information of an organism's life history. In contrast, the calcified vertebral centra of elasmobranchs (sharks, skates and rays) yield important information about ecological and biological traits that can be utilized for constructing age-structured population dynamic models of extant species and palaeoecological reconstructions of such aspects in extinct groups. Here, we describe two large shark vertebrae from the Santonian (Upper Cretaceous) of Spain, which show a unique combination of characters (asterospondylic calcification pattern, with concentric lamellae and numerous parallel bands that are oriented perpendicular) that is only known from ptychodontid sharks, a distinct, extinct group of giant durophagous sharks of the Cretaceous era. Based on linear regression models for large extant sharks a total length between 430 and 707cm was estimated for the examined specimen. Our results indicate that ptychodontid sharks were large viviparous animals, with slow growth rates, matured very late and, therefore, show typical traits for K-selected species. These traits combined with a highly specialized feeding ecology might have played a crucial role for the success but also, eventually, extinction of this group.

Home ranges, habitat and body mass: simple correlates of home range size in ungulates

The spatial scale of animal space use, e.g. measured as individual home range size, is a key trait with important implications for ecological and evolutionary processes as well as management and conservation of populations and ecosystems. Explaining variation in home range size has therefore received great attention in ecological research. However, few studies have examined multiple hypotheses simultaneously, which is important provided the complex interactions between life history, social system and behaviour. Here, we review previous studies on home range size in ungulates, supplementing with a meta-analysis, to assess how differences in habitat use and species characteristics affect the relationship between body mass and home range size. Habitat type was the main factor explaining interspecific differences in home range size after accounting for species body mass and group size. Species using open habitats had larger home ranges for a given body mass than species using closed habitats, whereas species in open habitats showed a much weaker allometric relationship compared with species living in closed habitats. We found no support for relationships between home range size and species diet or mating system, or any sexual differences. These patterns suggest that the spatial scale of animal movement mainly is a combined effect of body mass, group size and the landscape structure. Accordingly, landscape management must acknowledge the influence of spatial distribution of habitat types on animal behaviour to ensure natural processes affecting demography and viability of ungulate populations.

Home-range interactions of three sympatric mesopredators in east Texas

Spatial interactions among predators are central to their behavioral ecology. Although coyotes (Canis latrans Say, 1823), bobcats (Lynx rufus (Schreber, 1777)), and raccoons (Procyon lotor (L., 1758)) occur sympatrically throughout much of the USA, little is known about the nature of inter- and intra-specific spatial interactions. We used kernel analysis to estimate home ranges and core areas for these species. We estimated home-range overlap within and among species using the utilization distribution overlap index (UDOI) and percent overlap. Bobcat and coyote home ranges were similar in extent, whereas raccoons had substantially smaller home ranges. Only bobcats displayed seasonal differences in home-range size. Male bobcats had larger home ranges than female bobcats; there was no sex-based differentiation in home range or core area for coyotes or raccoons. Home-range overlaps calculated using percent overlap were consistently larger than UDOI estimates. Percent overlap seemed to exaggerate the extent to which individuals of the same species share space. Bobcats and coyotes shared space more than raccoons did with either bobcats or coyotes, which indicates that raccoons may have avoided areas used by trophically higher ranking mesopredators or that species with similar feeding strategies had lower levels of overlap to avoid competition.

Energy and the Scaling of Animal Space Use

Daily animal movements are usually limited to a discrete home range area that scales allometrically with body size, suggesting that home-range size is shaped by metabolic rates and energy availability across species. However, there is little understanding of the relative importance of the various mechanisms proposed to influence home-range scaling (e.g., differences in realm productivity, thermoregulation, locomotion strategy, dimensionality, trophic guild, and prey size) and whether these extend beyond the commonly studied birds and mammals. We derive new home-range scaling relationships for fishes and reptiles and use a model-selection approach to evaluate the generality of home-range scaling mechanisms across 569 vertebrate species. We find no evidence that home-range allometry varies consistently between aquatic and terrestrial realms or thermoregulation strategies, but we find that locomotion strategy, foraging dimension, trophic guild, and prey size together explain 80% of the variation in home-range size across vertebrates when controlling for phylogeny and tracking method. Within carnivores, smaller relative prey size among gape-limited fishes contributes to shallower scaling relative to other predators. Our study reveals how simple morphological traits and prey-handling ability can profoundly influence individual space use, which underpins broader-scale patterns in the spatial ecology of vertebrates.

Taylor's law and body size in exploited marine ecosystems

Taylor's law (TL), which states that variance in population density is related to mean density via a power law, and density-mass allometry, which states that mean density is related to body mass via a power law, are two of the most widely observed patterns in ecology. Combining these two laws predicts that the variance in density is related to body mass via a power law (variance-mass allometry). Marine size spectra are known to exhibit density-mass allometry, but variance-mass allometry has not been investigated. We show that variance and body mass in unexploited size spectrum models are related by a power law, and that this leads to TL with an exponent slightly <2. These simulated relationships are disrupted less by balanced harvesting, in which fishing effort is spread across a wide range of body sizes, than by size-at-entry fishing, in which only fish above a certain size may legally be caught.

Influence of habitat on home-range size in the short-beaked echidna

The size of an animal’s home range is strongly influenced by the resources available within it. In productive, resource-rich habitats sufficient resources are obtainable within a smaller area, and for many species, home ranges are smaller in resource-rich habitats than in habitats with lower resource abundance. Location data on 14 male and 27 female echidnas (Tachyglossus aculeatus) fitted with tracking transmitters, in the southern midlands of Tasmania, were used to test the influence of habitat type on home-range size. We hypothesised that as woodland should offer more shelter, food resources and refuges than pasture, echidnas living in woodland would have smaller home ranges than those living in pasture areas. We found significant differences between the sexes. Male echidnas had a significantly larger mean home range than females and a quite different relationship between home-range size and habitat type from females. There was no relationship between the proportion of woodland within male home ranges and home-range size whereas female echidnas had a highly significant negative relationship. This suggests that home-range size of female echidnas is highly influenced by the amount of woodland within it, but the home-range size of male echidnas is controlled by factors other than habitat. This pattern is consistent with the spatial ecology of many other solitary species with a promiscuous mating system. The home ranges of females are scaled to encompass all necessary resources for successfully raising their young within a minimal area, whilst the large home ranges of males are scaled to maximise access to females.

Animal interactions and the emergence of territoriality

Inferring the role of interactions in territorial animals relies upon accurate recordings of the behaviour of neighbouring individuals. Such accurate recordings are rarely available from field studies. As a result, quantification of the interaction mechanisms has often relied upon theoretical approaches, which hitherto have been limited to comparisons of macroscopic population-level predictions from un-tested interaction models. Here we present a quantitative framework that possesses a microscopic testable hypothesis on the mechanism of conspecific avoidance mediated by olfactory signals in the form of scent marks. We find that the key parameters controlling territoriality are two: the average territory size, i.e. the inverse of the population density, and the time span during which animal scent marks remain active. Since permanent monitoring of a territorial border is not possible, scent marks need to function in the temporary absence of the resident. As chemical signals carried by the scent only last a finite amount of time, each animal needs to revisit territorial boundaries frequently and refresh its own scent marks in order to deter possible intruders. The size of the territory an animal can maintain is thus proportional to the time necessary for an animal to move between its own territorial boundaries. By using an agent-based model to take into account the possible spatio-temporal movement trajectories of individual animals, we show that the emerging territories are the result of a form of collective animal movement where, different to shoaling, flocking or herding, interactions are highly heterogeneous in space and time. The applicability of our hypothesis has been tested with a prototypical territorial animal, the red fox (Vulpes vulpes).

How animals succeed in sharing and occupying space in an efficient way has always fascinated biologists. When occupying space involves marking and defending a given area, the animal is said to be territorial. By scent marking the locations that an animal visits, it conveys to a potential intruder that the area is claimed by another animal. Once an intruder encounters a foreign scent, it typically retreats from it to avoid an aggressive response by the resident animal. This is the so-called mechanism of conspecific avoidance. By considering this mechanism and the movement of the individual animals, we predict how territorial patterns are formed and maintained. Data and information on the red fox has served as a benchmark to test our predictions and has provided the experimental support to our theory. The implications of our results reach far beyond behavioural ecology, encompassing fields from epidemiology and conservation biology to social and state boundary dynamics in human society and ‘divide and conquer’ approaches to collective robotics.

Spatial organization of Molina’s hog-nosed skunk (Conepatus chinga) in two landscapes of the Pampas grassland of Argentina

We radio-tracked 16 individuals (6 males, 10 females) of the little known Molina’s hog-nosed skunk ( Conepatus chinga (Molina, 1782)) and compared home-range dynamics, movement rates, and densities between a protected area and a landscape fragmented by agriculture. The mean home-range size (95% fixed kernel) was 166.7 ha (SD = 107.5 ha), without significant differences between areas. Home-range size varied significantly between males (mean = 243.7 ha, SD = 76.5 ha) and females (mean = 120.4 ha, SD =77.6 ha). Overlap between home range and core area was extensive between and within sexes in the protected area and more limited in the cropland area. Mean distance traveled between two consecutive resting sites was 269.5 m (SD = 365 m) and did not differ between areas, although movements were greater for males than females. Distance moved was influenced by seasons, being greater during the cold period. Finally, density estimates were consistently greater at the protected area. We argue that home-range size in Molina’s hog-nosed skunks is an inherent species property, whereas population density and territoriality are more flexible parameters that could reflect how the ecosystem state was affected. In our study, the greater dispersion of food patches in the cropland area than in the protected area may be the major factor influencing these parameters.

Is spatial autocorrelation an intrinsic property of territory size?

In animals, competition for space and resources often results in territorial behaviour. The size of a territory is an important correlate of fitness and is primarily determined by the spatial distribution of resources and by interactions between competing individuals. Both of these determinants, alone or in interaction, could lead to spatial non-independence of territory size (i.e. spatial autocorrelation). We investigated the presence and magnitude of spatial autocorrelation (SAC) in territory size using Monte Carlo simulations of the most widely used territory measures. We found significant positive SAC in a wide array of competition-simulated conditions. A meta-analysis of territory size data showed that SAC is also a feature of territories mapped based on behavioural observations. Our results strongly suggest that SAC is an intrinsic trait of any territory measure. Hence, we recommend that appropriate statistical methods should be employed for the analysis of data sets where territory size is either a dependent or an explanatory variable.

Incorporating dispersal distance into the disperser effectiveness framework: frugivorous birds provide complementary dispersal to plants in a patchy environment

Fleshy-fruited plants are usually dispersed by an array of frugivores, differing in the effectiveness of the dispersal service they provide to the plant. Body size differences among frugivores are hypothesized to affect seed dispersal distances and consequently their effectiveness as dispersers. We tested this hypothesis by comparing the effectiveness of two passerine birds, grackles (Onychognathus tristramii) and bulbuls (Pycnonotus xanthopygos), dispersing the desert shrub Ochradenus baccatus. Laboratory experiments, quantifying gut retention time and the effect on germination, were combined with field observations quantifying bird movements and fruit consumption rates. An empirically parameterized mechanistic model showed that the two dispersers switch roles as a function of spatial-scale: while most seeds within the local habitat were dispersed by bulbuls, the larger grackles were exclusively responsible for between-patches, long-distance dispersal. We suggest that distance-related differences are common and important to plant fitness, and thus should explicitly be considered in studies of disperser effectiveness.

Bee foraging ranges and their relationship to body size

Bees are the most important pollinator taxon; therefore, understanding the scale at which they forage has important ecological implications and conservation applications. The foraging ranges for most bee species are unknown. Foraging distance information is critical for understanding the scale at which bee populations respond to the landscape, assessing the role of bee pollinators in affecting plant population structure, planning conservation strategies for plants, and designing bee habitat refugia that maintain pollination function for wild and crop plants. We used data from 96 records of 62 bee species to determine whether body size predicts foraging distance. We regressed maximum and typical foraging distances on body size and found highly significant and explanatory nonlinear relationships. We used a second data set to: (1) compare observed reports of foraging distance to the distances predicted by our regression equations and (2) assess the biases inherent to the different techniques that have been used to assess foraging distance. The equations we present can be used to predict foraging distances for many bee species, based on a simple measurement of body size.

SPECIES ABUNDANCE DISTRIBUTIONS RESULT FROM BODY SIZE–ENERGETICS RELATIONSHIPS

Abundance distributions are a central characteristic of ecosystems. Certain distributions have been derived from theoretical models of community organization, and therefore the fit of data to these distributions has been proposed as a test of these theories. However, it is shown here that the geometric sequence distribution can be derived directly from the empirical relationship between population density and body size, with the assumption of random or uniform body size distributions on a log scale (as holds at local scales). The geometric sequence model provides a good to excellent fit to empirical data. The presence of noise in the relationship between population density and body size creates a curve that begins to approximate a lognormal species abundance distribution as the noise term increases. For continental-scale data in which the body size distribution is not flat, the result of sampling tends again toward the lognormal. Repeat sampling over time smooths out species population fluctuations and damps out the noise, giving a more precise geometric sequence abundance distribution. It is argued that the direct derivation of this distribution from empirical relationships gives it priority over distributions derived from complex theoretical community models.