Active density-dependent habitat selection in a controlled population of small mammals

Habitat selection in a fluctuating ground squirrel population: Density‐dependence and fitness consequences

Investigating individual‐based habitat settlement decisions is a central theme in ecology, yet studies that quantify density‐dependent habitat selection or tie fitness to resource selection decisions remain rare. We quantified habitat selection in golden‐mantled ground squirrels (Callospermophilus lateralis) across two spatial scales (home‐range placement, and occurrence within the home range) by using 11 consecutive years of data on individual space use, and we used resource selection functions and multilevel modeling to address how habitat preferences may be influenced by density or linked to fitness outcomes. Squirrels preferred dry meadow over other habitat types (wet meadow, aspen, spruce, and willow) at both spatial scales. Squirrels were more likely to use dry meadow that contained shorter vegetation and vision‐enhancing prominences such as rocks (“perches”). The use of dry meadow at each scale was not influenced by changes in density. The use of dry meadow did not lead to increased litter size, pre‐hibernation mass, or survival. However, squirrels that experienced a greater number of perches or lower local densities had higher survival rates. Our results suggest that a lack of visual obstruction, probably facilitating detection of predators, drives habitat selection in this system. Surprisingly, squirrels maintained their preference for dry meadow as density increased, and they experienced reduced survival as a result. This work furthers our understanding about the causes and consequences of changes in habitat use, informing wildlife management and conservation.

Site-dependent regulation of breeding success: evidence for the buffer effect in the common guillemot, a colonially-breeding seabird

Density-dependent regulation can offer resilience to wild populations experiencing fluctuations in environmental conditions because, at lower population sizes, the average quality of habitats or resources is predicted to increase. Site-dependent regulation is a mechanism whereby individuals breed at the highest quality, most successful, sites, leaving poorer quality, less successful sites vacant. As population size increases, higher quality sites become limiting but when populations decline, lower quality sites are vacated first, offering resilience. This process is known as the 'buffer effect'. However, few studies have tested whether such regulation operates in populations experiencing changes in size and trend. We used data from a population of common guillemots Uria aalge, a colonially breeding seabird, to investigate the relationship between site occupancy probability, site quality and population size and trend. These data were collected at five sub-colonies spanning a 38-year period (1981-2018) comprising phases of population increase, decrease and recovery. We first tested whether site quality and population size in sub-colonies explained which sites were occupied for breeding, and if this was robust to changes in sub-colony trend. We then investigated whether disproportionate use of higher quality sites drove average site quality and breeding success across sub-colony sizes and trends. Finally, we tested whether individuals consistently occupied higher quality sites during periods of decline and recovery. Higher quality sites were disproportionality used when sub-colony size was smaller, resulting in higher average site quality and breeding success at lower population sizes. This relationship was unaffected by changes in sub-colony trend. However, contrary to the predictions of the buffer effect, new sites were established at a similar rate to historically occupied sites during sub-colony decline and recovery despite being of lower quality. Our results provide support for the buffer effect conferring resilience to populations, such that average breeding success was consistently higher at lower population size during all phases of population change. However, this process was tempered by the continued establishment of new, lower quality, sites which could act to slow population recovery after periods when colony size was low.

Habitat associations of small mammal communities in a restored prairie system in southern Illinois

Extensive habitat loss and degradation have transformed the grasslands of North America into one of the continent’s most vulnerable ecosystems and have greatly imperiled the plants and animals that depend on them. Small mammal communities play a pivotal role in maintaining the health and function of grasslands, and managing small mammal populations is an important, though overlooked, part of grassland restoration and management. We identified habitat variables in restored tallgrass prairie patches that were most strongly associated with variation in small mammal abundance and community composition, with the goal of aiding management efforts to manipulate small mammal populations. We found that small mammal abundance at the local vegetation scale was negatively related both to litter depth and plant diversity. At the landscape scale, small mammal abundance was positively associated with the amount of water surrounding a patch, and negatively associated with the amount of grassland surrounding a patch. Variation in small mammal community composition largely was governed by differences in habitat structure at the landscape scale, rather than differences in vegetation structure at the local scale. We suggested that managers interested in influencing small mammal abundance in grasslands employ tools such as prescribed fire to decrease litter depth if increased small mammal abundance is desired, or increase plant diversity by sowing a high diversity of seeds to depress small mammal abundance.

Colony dynamics and Plant Community Associations of the Harvester Ant, Pogonomyrmex salinus (Hymenoptera: Formicidae) in Sagebrush-Steppe Habitat

We present the results of a 5-yr field study on colony dynamics and plant community associations of Owyhee harvester ants, Pogonomyrmex salinus (Olsen), in sagebrush-steppe habitat in southwestern Idaho. Over a 5-yr period, the total number of ant colonies across 16 sites increased from 843 to 878 (4.15%) as a result of 315 colony deaths and 350 colony initiations. Up to 7.1% of colony initiations may have been instances of nest relocation. Colonies had a higher rate of failure in their first year than in subsequent years (12.4 vs 4.6-8.4% over the next 3 yr). Of the 11 sites analyzed for colony dispersion, one was significantly clumped and the others did not differ from random. Population density in the final year of study ranged from 0.1 to 62.9 colonies/ha and was best described by an inverse relationship with sagebrush cover and positive relationship with coverage of non-Bromus (non-cheatgrass) understory vegetation. We interpret these results both in terms of 1) food resources-harvester ants avoid cheatgrass seeds in their diet, preferring instead small-seeded grasses and forbs, and 2) habitat structure-harvester ants prefer nesting in open areas where the ground is exposed to sun and they can clear vegetation from the vicinity of their nests. Given the habitat associations we report, the transition from sagebrush-dominated habitat to open grasslands that is occurring rapidly throughout much of the western United States may prove costly to native plant species whose seeds are readily consumed by harvester ants.

Time-averaging voles match density with long-term habitat quality

An optimal habitat-selecting organism should use a dispersal strategy that enables occupation of the habitat yielding greatest fitness. The strategy is complicated when habitat quality varies through time. Theory predicts that the long-term distribution of individuals will match mean habitat quality while undermatching current habitat quality. I tested the prediction with experiments on controlled populations of meadow voles occupying two pairs of field enclosures. I released equal numbers, and equal sexes, of voles in each enclosure, and varied resource abundance between enclosures by supplemental feeding. I measured the voles' response with giving-up densities (GUDs) in artificial foraging patches, and with live-trapping at the end of the experiment. The data were consistent with only one of four a priori dispersal models. Giving-up densities declined with resource supply because short-term supply had no effect on population density. GUDs were invariant to the time course of the experiment because densities were proportional to each enclosure's long-term mean quality. Similar patterns in sex ratios and patterns of habitat occupation by juvenile voles born during the experiment reinforce the interpretation of time-averaged habitat matching. This study adds to the cumulating evidence that strategies of space use converge toward behavioral and evolutionary optima.

Spatial ecology of the quokka (Setonix brachyurus) in the southern forests of Western Australia: implications for the maintenance, or restoration, of functional metapopulations

We used radio-telemetry to investigate the home-range size and movement patterns of the quokka (Setonix brachyurus) in the southern forests of Western Australia to assess the ability of animals to move between increasingly segregated habitat patches and to identify implications for metapopulation function. We found that quokkas in this region have a much larger home range (71 ± 5.8 ha) and move larger distances (up to 10 km per night) than previously reported for this species in other regions. Temporal and sex variations in home-range size, overlap and movement patterns provided insights into the social structure, reproductive strategies and resource availability for the species in this part of its range. Quokkas moved up to 14 km between habitat patches, where these patches were connected by dense riparian vegetation. While riparian vegetation was used exclusively for movement between habitat patches, quokkas spent only 40% of their time in this ecotype. The current management paradigm of protecting linear riparian vegetation as habitat for quokkas is important for maintaining habitat connectivity, but is unlikely to meet broader habitat and spatial requirements. Management of preferred habitat as well as riparian corridors is necessary for the maintenance of a functional metapopulation.

Temporal changes in species composition affect a ubiquitous species' use of habitat patches

Across landscapes, shifts in species composition often co-occur with shifts in structural or abiotic habitat features, making it difficult to disentangle the role of competitors and environment on assessments of patch quality. Using over two decades of rodent community data from a long-term experiment, we show that a small, ubiquitous granivore (Chaetodipus penicillatus) shifted its use of different experimental treatments with the establishment of a novel competitor, C. baileyi. Shifts in residency, probability of movement between patches, and the arrival of new individuals in patches altered which treatment supported the highest abundances of C. penicillatus. Our results suggest that the establishment of a new species worsened the quality of the originally preferred treatment, likely by impacting resource availability. Paradoxically, the presence of the new species also increased C. penicillatus' use of the less preferred treatment, potentially through shifts in the competitive network on those plots.

Advances in population ecology and species interactions in mammals

The study of mammals has promoted the development and testing of many ideas in contemporary ecology. Here we address recent developments in foraging and habitat selection, source–sink dynamics, competition (both within and between species), population cycles, predation (including apparent competition), mutualism, and biological invasions. Because mammals are appealing to the public, ecological insight gleaned from the study of mammals has disproportionate potential in educating the public about ecological principles and their application to wise management. Mammals have been central to many computational and statistical developments in recent years, including refinements to traditional approaches and metrics (e.g., capture-recapture) as well as advancements of novel and developing fields (e.g., spatial capture-recapture, occupancy modeling, integrated population models). The study of mammals also poses challenges in terms of fully characterizing dynamics in natural conditions. Ongoing climate change threatens to affect global ecosystems, and mammals provide visible and charismatic subjects for research on local and regional effects of such change as well as predictive modeling of the long-term effects on ecosystem function and stability. Although much remains to be done, the population ecology of mammals continues to be a vibrant and rapidly developing field. We anticipate that the next quarter century will prove as exciting and productive for the study of mammals as has the recent one.

Processes driving short-term temporal dynamics of small mammal distribution in human-disturbed environments

As the impact of anthropogenic activities intensifies worldwide, an increasing proportion of landscape is converted to early successional stages every year. To understand and anticipate the global effects of the human footprint on wildlife, assessing short-term changes in animal populations in response to disturbance events is becoming increasingly important. We used isodar habitat selection theory to reveal the consequences of timber harvesting on the ecological processes that control the distribution dynamics of a small mammal, the red-backed vole (Myodes gapperi). The abundance of voles was estimated in pairs of cut and uncut forest stands, prior to logging and up to 2 years afterwards. A week after logging, voles did not display any preference between cut and uncut stands, and a non-significant isodar indicated that their distribution was not driven by density-dependent habitat selection. One month after harvesting, however, juvenile abundance increased in cut stands, whereas the highest proportions of reproductive females were observed in uncut stands. This distribution pattern appears to result from interference competition, with juveniles moving into cuts where there was weaker competition with adults. In fact, the emergence of source-sink dynamics between uncut and cut stands, driven by interference competition, could explain why the abundance of red-backed voles became lower in cut (the sink) than uncut (the source) stands 1-2 years after logging. Our study demonstrates that the influences of density-dependent habitat selection and interference competition in shaping animal distribution can vary frequently, and for several months, following anthropogenic disturbance.

Local density and group size interacts with age and sex to determine direction and rate of social dispersal in a polygynous mammal

Movement away from an area or social group in response to increasing density (density-dependent dispersal) is known for most species; why it evolves is fundamental to our understanding of ecology and evolution. However, we have yet to fully appreciate how individuals of varying conditions (e.g., age and sex) might differently consider effects of density (quorum) when deciding to disperse or not, and scale dependence in their sense of quorum. We tracked movements of all individuals of a naturalized population of feral horses (Equus ferus caballus; Sable Island National Park Reserve, Nova Scotia, Canada) during a period of rapid population growth (N increased from 375 to 484 horses from 2008 to 2010). Permanent dispersal from breeding groups (bands) was positively density dependent for all age and sex categories with respect to local density (horses/km², bounded by the 99th percentile of individual movements [8000 m]), but was negatively and positively density dependent for males and females, respectively, in relation to group (band) size. Dispersal was generally female biased, with the exception of foals which moved with their mothers (no sex effect), and for yearlings and subadults when band sizes were smaller than average, in which case males dispersed at higher rates than females. Dispersal distance was positively related to local density. We conclude that dispersal rate can be both positively and negatively density dependent for feral horses, contingent on the state of individuals and the scale at which quorum with respect to choosing to disperse or not is assessed. Scale effects and interactions of density-dependent and sex- and age-biased dispersal may have both ecological and evolutionary consequences through effects on resource and mate competition.

Riparian habitat disturbed by reservoir management does not function as an ecological trap for the Yellow Warbler (Setophaga petechia)

Ecological traps arise when anthropogenic change creates habitat that appears suitable but when selected reduces the fitness of an individual. We evaluated whether riparian habitat within the drawdown zone of the Arrow Lakes Reservoir, British Columbia, creates an ecological trap for Yellow Warblers ( Setophaga petechia (L., 1766)) by investigating habitat preferences and the fitness consequences of habitat selection decisions. Preferences were inferred by examining how habitat variables influenced settlement order, and comparing habitat at nest sites and random locations. Males preferred to settle in territories with more riparian shrub and tree cover, higher shrub diversity, and less high canopy cover. Females built nests in taller shrubs surrounded by a greater density of shrub stems. Habitat preferences were positively associated with fitness: nest sites in taller shrubs surrounded by higher shrub-stem densities were more likely to avoid predation and fledge young, whereas territories with more riparian cover, higher shrub diversity, and less high canopy cover had higher annual productivity. We therefore found no evidence that riparian habitat affected by reservoir operations functions as an ecological trap. Current habitat selection decisions may be associated with fitness because Yellow Warblers are adapted to breeding in a heterogeneous environment subject to periodic flooding.

Patterns and processes of dispersal behaviour in arvicoline rodents

A good understanding of mammalian societies requires measuring patterns and comprehending processes of dispersal in each sex. We investigated dispersal behaviour in arvicoline rodents, a subfamily of mammals widespread in northern temperate environments and characterized by a multivoltine life cycle. In arvicoline rodents, variation in life history strategies occurs along a continuum from precocial to delayed maturation that reflects seasonal and ecological fluctuations. We compared dispersal across and within species focusing on the effects of external (condition-dependent) and internal (phenotype-dependent) factors. Our data revealed substantial, unexplained variation between species for dispersal distances and a strong variation within species for both dispersal distance and fraction. Some methodological aspects explained variation across studies, which cautions against comparisons that do not control for them. Overall, the species under consideration display frequent short-distance dispersal events and extremely flexible dispersal strategies, but they also have hitherto unexpected capacity to disperse long distances. Female arvicolines are predominantly philopatric relative to males, but we found no clear association between the mating system and the degree of sex bias in dispersal across species. Dispersal is a response to both various proximate and ultimate factors, including competition, inbreeding avoidance, mate searching and habitat quality. In particular, our review suggests that costs and benefits experienced during transience and settlement are prime determinants of condition dependence. Patterns of phenotype-dependent dispersal are idiosyncratic, except for a widespread association between an exploration/activity syndrome and natal dispersal. Consequences for population dynamics and genetic structures are discussed.

The ability of individuals to assess population density influences the evolution of emigration propensity and dispersal distance

We analyze the simultaneous evolution of emigration and settlement decisions for actively dispersing species differing in their ability to assess population density. Using an individual-based model we simulate dispersal as a multi-step (patch to patch) movement in a world consisting of habitat patches surrounded by a hostile matrix. Each such step is associated with the same mortality risk. Our simulations show that individuals following an informed strategy, where emigration (and settlement) probability depends on local population density, evolve a lower (natal) emigration propensity but disperse over significantly larger distances - i.e. postpone settlement longer - than individuals performing density-independent emigration. This holds especially when variation in environmental conditions is spatially correlated. Both effects can be traced to the informed individuals' ability to better exploit existing heterogeneity in reproductive chances. Yet, already moderate distance-dependent dispersal costs prevent the evolution of multi-step (long-distance) dispersal, irrespective of the dispersal strategy.