242 research outputs found
Non-linear density dependence in time series is not evidence of non-logistic growth
No full textTime series of population density are often used to seek deviations from logistic regulation by testing for a non-linear decline in per capita growth rate with density. Here I show that this method fails when the interval between observations is not matched by the timing of density impacts on growth. Time series overestimate instantaneous density impacts at low density and underestimate them at high density. More generally, logistic growth produces a deterministically decelerating decline in per capita growth with density if the interval between measures of population size exceeds any lag in density response. Deceleration arises independently out of stochastic density fluctuations, and under-compensating regulation. These multiple influences lead to the conclusion that sequential density estimates provide insufficient information on their own to reveal the identity of non-logistic growth processes. They can yield estimates of density compensation, however, which may suggest time lags in density dependence. Analysis of an empirical time series illustrates the issues
Roads as barriers to movement for hedgehogs
No full text1. Movements of hedgehogs, Erinaceus europaeus L., were analysed in relation to roads by recording their nocturnal foraging trajectories in urban areas adjacent to major road systems. Four male and four female hedgehogs at each of two sites were each tracked over five replicate trajectories of more than 3 h, using radio-telemetry and
direct observation.2. Frequency of road crossings and use of habitat were compared to simulated random trajectories. For each observed trajectory, 100 trajectories were simulated with the same distribution of speeds as observed, but with random direction.3. Observed trajectories had lower rates of crossing large but not small roads than simulated, indicating that hedgehogs treat large roads as barriers during nightly foraging activities. Most hedgehogs were never observed to cross roads, whereas almost all simulated trajectories did cross both large and small roads.4. Crossing behaviour differed by site, but not by sex.5. Roads and road verges had the lowest rank in a habitat preference analysis at both sites, while playing fields, gardens and urban areas were preferred habitats. These results are discussed in relation to the regional scale preference for urban areas, and the attraction to road verges while dispersing, shown by hedgehogs
Technical comment. "On the Regulation of Populations of Mammals, Birds, Fish, and Insects" III
No full textStochasticity in time series explains concave responses of per capita growth rate to population size. The gradients with the natural log of population size have more biological importance because they measure strength of density compensation. Its weakening with increasing body size across taxa (Sibly et al., Reports, 22 July 2005, p. 607) is consistent with slower responses in ascent than descent toward carrying capacity. Time series therefore suggest that populations of large-bodied animals underfill their environments
Density dependence triggers runaway selection of reduced senescence
No full textIn the presence of exogenous mortality risks, future reproduction by an individual is worth less than present reproduction to its fitness. Senescent aging thus results inevitably from transferring net fertility into younger ages. Some long-lived organisms appear to defy theory, however, presenting negligible senescence (e.g., hydra) and extended lifespans (e.g., Bristlecone Pine). Here, we investigate the possibility that the onset of vitality loss can be delayed indefinitely, even accepting the abundant evidence that reproduction is intrinsically costly to survival. For an environment with constant hazard, we establish that natural selection itself contributes to increasing density-dependent recruitment losses. We then develop a generalized model of accelerating vitality loss for analyzing fitness optima as a tradeoff between compression and spread in the age profile of net fertility. Across a realistic spectrum of senescent age profiles, density regulation of recruitment can trigger runaway selection for ever-reducing senescence. This novel prediction applies without requirement for special life-history characteristics such as indeterminate somatic growth or increasing fecundity with age. The evolution of nonsenescence from senescence is robust to the presence of exogenous adult mortality, which tends instead to increase the age-independent component of vitality loss. We simulate examples of runaway selection leading to negligible senescence and even intrinsic immortality
An agent-based model of jaguar movement through conservation corridors
No full textWildlife corridors mitigate against habitat fragmentation by connecting otherwise isolated regions, bringing well established benefits to conservation both in principle and practice. Populations of large mammals in particular may depend on habitat connectivity, yet conservation managers struggle to optimise corridor designs with the rudimentary information generally available on movement behaviours. We present an agent-based model of jaguars (Panthera onca), scaled for fragmented habitat in Belize where proposals already exist for creating a jaguar corridor. We use a leastcost approach to simulate movement paths through alternative possible landscapes. Six different types of corridor and three control conditions differ substantially in their effectiveness at mixing agents across the environment despite relatively little difference in individual welfare. Our best estimates of jaguar movement behaviours suggest that a set of five narrow corridors may out-perform one wide corridor of the same overall area. We discuss the utility of ALife modelling for conservation management
Do wildlife corridors link or extend habitat? Insights from elephant use of a Kenyan wildlife corridor
No full textHuman encroachment on wildlife habitats is leading to increased fragmentation; hence, there is an increasing focus on improving connectivity between remaining habitat. Large, wide-ranging species such as the African elephant, (Loxodonta africana), are particularly vulnerable due to their extensive habitat requirements. Wildlife corridors have been created to facilitate movement, with little knowledge to date on whether they serve their intended function as transit routes, or whether they simply extend the available habitat for occupancy. We collected data on elephant behaviour in the Mount Kenya Elephant Corridor, with the aim of quantifying the utility of the corridor. A grid of 25 camera traps was used to survey the 478 ha corridor over 11 weeks. Cameras recorded over 43,000 photos with 694 separate events triggered by elephants. Patterns of use varied spatially and temporally, indicating that certain areas were treated as habitat extension, while others were predominantly for transit. These differences were likely due to variation in vegetation cover and levels of human disturbance. Corridor use differed amongst individuals, suggesting that use may depend on both the characteristics of the corridor itself and the social or resource needs of individual elephants.</p
The influence of simulated exploitation on Patella vulgata populations: protandric sex change is size-dependent
No full textGrazing mollusks are used as a food resource worldwide, and limpets are harvested commercially for both local consumption and export in several countries. This study describes a field experiment to assess the effects of simulated human exploitation of limpets Patella vulgata on their population ecology in terms of protandry (age-related sex change from male to female), growth, recruitment, migration, and density regulation. Limpet populations at two locations in southwest England were artificially exploited by systematic removal of the largest individuals for 18 months in plots assigned to three treatments at each site: no (control), low, and high exploitation. The shell size at sex change (L50: the size at which there is a 50:50 sex ratio) decreased in response to the exploitation treatments, as did the mean shell size of sexual stages. Size-dependent sex change was indicated by L50 occurring at smaller sizes in treatments than controls, suggesting an earlier switch to females. Mean shell size of P. vulgata neuters changed little under different levels of exploitation, while males and females both decreased markedly in size with exploitation. No differences were detected in the relative abundances of sexual stages, indicating some compensation for the removal of the bigger individuals via recruitment and sex change as no migratory patterns were detected between treatments. At the end of the experiment, 0–15 mm recruits were more abundant at one of the locations but no differences were detected between treatments. We conclude that sex change in P. vulgata can be induced at smaller sizes by reductions in density of the largest individuals reducing interage class competition. Knowledge of sex-change adaptation in exploited limpet populations should underpin strategies to counteract population decline and improve rocky shore conservation and resource management
Field test for environmental correlates of dispersal in hedgehogs Erinaceus europaeus
No full text1. We report a field experiment designed to explore the responses by hedgehogs to novel and unfavourable terrain, which they encounter when dispersing between fragmented local populations in farmland. We identified four replicate sites as unused by hedgehogs, but lying between existing populations and containing locally abundant food. At each site, we released six hedgehogs from distant sources at 2-day intervals and monitored their subsequent dispersals over 20 days by radio-tracking. We compared movement trajectories under this treatment to those of 29 hedgehogs released at two sites known to support abundant hedgehogs. A third treatment comprised 20 unmanipulated hedgehogs at three sites. We estimated habitat use and proximity to habitats by comparing each observed trajectory to simulated random walks of equal length. We sought differences between treatments in movement and habitat use from nested analysis of variance.2. No two trajectories of any translocated hedgehogs followed the same route; most involved little change in body weight, and took the animal into or through existing populations. Hedgehogs moved substantially further and faster on average from the unfavourable than from the favourable sites. They showed a significantly stronger attraction to habitat edges, which therefore acted as corridors, and a significant proportion of them stayed closer to roads and to urban habitat than random expectation. Habitat preferences also shifted significantly towards urban and away from arable areas. Six were predated, and two killed by road traffic. At least three returned to their release point, with P < 0·05 of this occurring by chance.3. In this region, natural dispersals between populations up to 4 km apart are rare events. We have shown, however, that hedgehogs are capable of travelling distances of up to 3·8 km from a release point, and up to 9·9 km in total, compared to an average home range span of 0·8 km, and it appears that none of the local populations in the study area are out of reach of neighbours
Food habits of sympatric jaguars and pumas across a gradient of human disturbance
No full textJaguars Panthera onca coexist with pumas Puma concolor across their entire range. In areas where they occur together their coexistence may be facilitated by differences in diet. This study compared food habits of jaguars and pumas in Belize, Central America, across a protected lowland rainforest and the neighbouring human-influenced landscape. Diets were determined from 362 jaguar scats and 135 puma scats, identified by genetic analysis. In the protected forest, dietary breadths were low for jaguars and pumas and showed little overlap. In this habitat each relied heavily on a single medium-sized (5–10 kg) prey species: armadillos Dasypus novemcinctus for jaguars, and pacas Agouti paca for pumas. Both cats also took larger prey (>10 kg), mainly white-lipped peccaries Tayassu pecari by jaguars and red brocket deer Mazama americana by pumas. In unprotected fragmented lands, jaguar scats rarely contained large wild prey species; rather, a diet of relatively small wild prey was supplemented with larger domestic species. Pumas did not take domestic species and were scarce outside the protected forest, possibly indicating competition with humans for pacas and deer, which are also prized game species in the region. This study is the largest analysis to date of sympatric jaguar and puma diets in both forest and farmland. We suggest that jaguar predation on cattle may be reduced by ensuring that game hunting is sustainable and potentially by augmenting forests within the human matrix with large wild ungulates. The supplementation could benefit both of the cat species, and the local game hunting economy.<br/
Data from: AudioMoth: evaluation of a smart open acoustic device for monitoring biodiversity and the environment
No full textGunshot data from BelizeCSV file giving the logarithmic ratio between a gunshots max peak amplitude and a reference amplitude at varying distances and orientation from source. We used the maximum possible magnitude for a .WAV file recorded on the device as our reference.gunshot_data.csv,1. The cost, usability and power efficiency of available wildlife monitoring equipment currently inhibits full ground-level coverage of many natural systems. Developments over the last decade in technology, open science, and the sharing economy promise to bring global access to more versatile and more affordable monitoring tools, to improve coverage for conservation researchers and managers. 2. Here we describe the development and proof-of-concept of a low-cost, small-sized and low-energy acoustic detector: 'AudioMoth'. The device is open-source and programmable, with diverse applications for recording animal calls or human activity at sample rates of up to 384kHz. We briefly outline two ongoing real-world case studies of large-scale, long-term monitoring for biodiversity and exploitation of natural resources. These studies demonstrate the potential for AudioMoth to enable a substantial shift away from passive continuous recording by individual devices, towards smart detection by networks of devices flooding large and inaccessible ecosystems. 3. The case studies demonstrate one of the smart capabilities of AudioMoth, to trigger event logging on the basis of classification algorithms that identify specific acoustic events. An algorithm to trigger recordings of the New Forest cicada (Cicadetta montana) demonstrates the potential for AudioMoth to vastly improve the spatial and temporal coverage of surveys for the presence of cryptic animals. An algorithm for logging gunshot events has potential to identify a shotgun blast in tropical rainforest at distances of up to 500 m, extending to 1km with continuous recording. 4. AudioMoth is more energy efficient than currently available passive acoustic monitoring (PAM) devices, giving it considerably greater portability and longevity in the field with smaller batteries. At a build cost of ~US$43 per unit, AudioMoth has potential for varied applications in large-scale, long-term acoustic surveys. With continuing developments in smart, energy-efficient algorithms and diminishing component costs, we are approaching the milestone of local communities being able to afford to remotely monitor their own natural resources.</span
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