Occupancy

About: Occupancy is a research topic. Over the lifetime, 2757 publications have been published within this topic receiving 68288 citations.

Improving the accuracy of population size estimates for burrow-nesting seabirds

Abstract: Seabird numbers can change rapidly as a result of environmental processes, both naturaland anthropogenic. Informed management and conservation of seabirds requires accurateand precise monitoring of population size. However, for burrow-nesting species this israrely achieved due to spatial and temporal heterogeneity in burrow occupancy. Here,we describe a novel method for deriving more accurate population size estimates thatemploys mark-recapture methods to correct for unknown variation in nest occupancythroughout a breeding season. We apply it to estimate breeding numbers of a colonial,burrowing seabird, the Little Penguin Eudyptula minor, on the Summerland Peninsula,Phillip Island, Australia. Estimates of active burrow numbers during the September 2008to February 2009 breeding season were adjusted to numbers of breeding birds based onburrow occupation and modelled population demographics at six, fortnightly monitoredreference sites. The population was estimated to be 26 100 (95% CI: 21 100–31 100)and 28 400 (23 800–33 000) breeding Penguins in two temporally separated surveyswithin one breeding season. We demonstrate using simulation that the method is robustto variation in burrow occupancy throughout the breeding season, providing consistentand more accurate estimates of population size. The advantage of using the correctedmethod is that confidence intervals will include the true population size. Confidence lim-its widened as burrow occupancy declined, reflecting the increased uncertainty as largeradjustments for low burrow occupancy were required. In contrast, the uncorrectedmethod that uses burrow occupancy alone as a measure of breeding numbers wasinconsistent and significantly underestimated population size across much of the breedingseason. Although requiring considerably more survey effort, the corrected approachprovides a more accurate means for monitoring population changes in colonially breedinganimals while collecting demographic data that can help diagnose the drivers ofpopulation change.Keywords: Australia, Eudyptula minor, Little Penguin, mark-recapture, nest-site occupancy.Population size is a key statistic to monitor howecological processes influence wild populations butaccurate estimates of population size are difficult toachieve for many animals. Many animals, includingseabirds, build nests, which provide a temporallyand spatially stable index of their abundance(Rayner et al. 2007). The most common techniqueto estimate the number of breeding seabirds in largepopulations has been to predict an appropriate timeto survey active nests, assume that each active nestrepresents a breeding pair, estimate density from arepresentative sample of the population, and applythat density to measured areas of occupancy(Lawton et al. 2006, Priddel et al. 2006, Rayneret al. 2007, Reyes-Arriagada et al. 2007, Barbraudet al. 2009, Bragg et al. 2009, Charleton et al.2009, McKechnie et al. 2009, Newman et al. 2009,Scott et al. 2009). While allowing a general popula-tion size to be interpreted, this technique makesnumerous assumptions that are in many casesuntested and may have a large and variable influ-ence on estimates between years. Hence, only largechanges in population size can be detected reliably.

Can camera traps monitor Komodo dragons a large ectothermic predator

Abstract: Camera trapping has greatly enhanced population monitoring of often cryptic and low abundance apex carnivores. Effectiveness of passive infrared camera trapping, and ultimately population monitoring, relies on temperature mediated differences between the animal and its ambient environment to ensure good camera detection. In ectothermic predators such as large varanid lizards, this criterion is presumed less certain. Here we evaluated the effectiveness of camera trapping to potentially monitor the population status of the Komodo dragon (Varanus komodoensis), an apex predator, using site occupancy approaches. We compared site-specific estimates of site occupancy and detection derived using camera traps and cage traps at 181 trapping locations established across six sites on four islands within Komodo National Park, Eastern Indonesia. Detection and site occupancy at each site were estimated using eight competing models that considered site-specific variation in occupancy (ψ)and varied detection probabilities (p) according to detection method, site and survey number using a single season site occupancy modelling approach. The most parsimonious model [ψ (site), p (site survey); ω = 0.74] suggested that site occupancy estimates differed among sites. Detection probability varied as an interaction between site and survey number. Our results indicate that overall camera traps produced similar estimates of detection and site occupancy to cage traps, irrespective of being paired, or unpaired, with cage traps. Whilst one site showed some evidence detection was affected by trapping method detection was too low to produce an accurate occupancy estimate. Overall, as camera trapping is logistically more feasible it may provide, with further validation, an alternative method for evaluating long-term site occupancy patterns in Komodo dragons, and potentially other large reptiles, aiding conservation of this species.

Exploring stream communities in a tropical biodiversity hotspot: biodiversity, regional occupancy, niche characteristics and environmental correlates

Abstract: Exploring and describing biodiversity and the mechanisms structuring it is fundamental to advancing ecology. This is particularly pertinent in understudied biogeographical regions, such as the Afrotropics, that are characterised by strong seasonal climatic shifts. We investigated the characteristics of stream biodiversity in the Niger Delta region of Nigeria, a tropical biodiversity hotspot, by examining patterns in 20 stream invertebrate communities across both the wet and dry seasons. For this, we took a multi-faceted approach accounting for the three levels of biodiversity (α, β and γ), including partitioning the nestedness and turnover components of β diversity, regional occupancy-abundance patterns, niche characteristics, and the environmental drivers of community structure. α diversity was low in these streams, with strong turnover between sites leading to high β diversity contributing to regional biodiversity, but there was little variation in communities between seasons. The proportion of sites occupied by taxa declined with increasing niche position, and decreasing niche breadth. Occupancy was predicted well by a combination of these two factors (niche position and breadth), but not mean local abundance, as the abundance-occupancy link was an upper-limit unimodal relationship. On average, community structure was linked more strongly to environmental variables in the wet season. Our findings demonstrate the clear role of spatial, but not temporal, turnover in assemblages, which likely reflects the environmental heterogeneity of this region. This is further supported by the fact that regional occupancy was mostly related to niche characteristics, particularly niche position. We emphasise the importance of continued basic and applied ecological work in this important biogeographic region to enable better protection of its biodiversity.

Functional responses of insectivorous bats to increasing housing density support ‘land‐sparing’ rather than ‘land‐sharing’ urban growth strategies

Abstract: 1. Debates about 'land-sparing' and 'land-sharing' strategies for conserving biodiversity in cities provide an overly simplistic characterization of alternate planning options. Increased urbanization manifests in a number of ways and sophisticated analyses of how species respond to urban environments are required before generalizations about the relative merits of either planning strategy should be made. 2. We investigated how insectivorous bats respond to housing density (a measure of urbanization intensity) and a range of habitat variables by modelling the occupancy and activity of 12 species in Melbourne, Australia. In addition to species-level analyses, species were grouped into guilds ('matrix', 'patch' or 'edge' species) depending on their ecomorphology and level of patch-dependency. 3. Housing density (dwellings per hectare: dph) was the dominant explanatory variable of occupancy and activity of most species studied. Site occupancy probabilities of patch and edge species were reduced by half at housing densities of just 1·1-4·5 dph depending on species, while their activity was halved at housing densities of 2·6-10·3 dph. The activity of two matrix species also declined, but at higher housing densities (reducing by half at 4·5-7·9 dph). Patch and edge species responded nonlinearly to tree cover at low housing densities, with peaks in occupancy and activity at 20-60%. This response broke down at medium housing densities, however, as most species were no longer present. 4. Synthesis and applications. Occupancy probability and activity of almost all bat species studied were substantially reduced even at very low housing densities. Increasing tree cover had no positive effect on the occupancy and activity of patch and edge species once medium housing densities were reached. In order for land-sharing strategies to provide good habitat for patch and edge bat species housing densities would need to be so low as to be impractical in most urban planning contexts. By contrast, land-sparing with forested habitat remaining in 20% or more of the landsc ape could provide positive conservation outcomes for the species modelled here. We discuss the relevance of our findings in planning for conservation of bats and their habitats in other recently established and expanding cities.