James J. Gilroy

How Should Beta-Diversity Inform Biodiversity Conservation?

To design robust protected area networks, accurately measure species losses, or understand the processes that maintain species diversity, conservation science must consider the organization of biodiversity in space. Central is beta-diversity--the component of regional diversity that accumulates from compositional differences between local species assemblages. We review how beta-diversity is impacted by human activities, including farming, selective logging, urbanization, species invasions, overhunting, and climate change. Beta-diversity increases, decreases, or remains unchanged by these impacts, depending on the balance of processes that cause species composition to become more different (biotic heterogenization) or more similar (biotic homogenization) between sites. While maintaining high beta-diversity is not always a desirable conservation outcome, understanding beta-diversity is essential for protecting regional diversity and can directly assist conservation planning.

Land-sharing versus land-sparing logging: reconciling timber extraction with biodiversity conservation

Selective logging is a major driver of rainforest degradation across the tropics. Two competing logging strategies are proposed to meet timber demands with the least impact on biodiversity: land sharing, which combines timber extraction with biodiversity protection across the concession; and land sparing, in which higher intensity logging is combined with the protection of intact primary forest reserves. We evaluate these strategies by comparing the abundances and species richness of birds, dung beetles and ants in Borneo, using a protocol that allows us to control for both timber yield and net profit across strategies. Within each taxonomic group, more species had higher abundances with land-sparing than land-sharing logging, and this translated into significantly higher species richness within land-sparing concessions. Our results are similar when focusing only on species found in primary forest and restricted in range to Sundaland, and they are independent of the scale of sampling. For each taxonomic group, land-sparing logging was the most promising strategy for maximizing the biological value of logging operations.

EDITOR'S CHOICE: Surrounding habitats mediate the trade-off between land-sharing and land-sparing agriculture in the tropics

1. Two strategies are often promoted to mitigate the effects of agricultural expansion on biodiversity: one integrates wildlife-friendly habitats within farmland (land sharing), and the other intensifies farming to allow the offset of natural reserves (land sparing). Their relative merits for biodiversity protection have been subject to much debate, but no previous study has examined whether trade-offs between the two strategies depend on the proximity of farmed areas to large tracts of natural habitat. 2. We sampled birds and dung beetles across contiguous forests and agricultural landscapes (low-intensity cattle farming) in a threatened hotspot of endemism: the Colombian Choco-Andes. We test the hypothesis that the relative biodiversity benefits of either strategy depend partially on the degree to which farmlands are isolated from large contiguous blocks of forest. 3. We show that distance from forest mediates the occurrence of many species within farmland. For the majority of species, occurrence on farmland depends on both isolation from forest and the proportionate cover of small-scale wildlife-friendly habitats within the farm landscape, with both variables having a similar overall magnitude of effect on occurrence probabilities. 4. Simulations suggest that the biodiversity benefits of land sharing decline significantly with increasing distance from forest, but land sparing benefits remain consistent. In farm management units situated close to large contiguous forest (<500 m), land sharing is predicted to provide equal benefits to land sparing, but land sparing becomes increasingly superior in management units situated further from forest (1500 m). The predicted biodiversity benefits of land sparing are similar across all distances, provided that sparing mechanisms genuinely deliver protection for contiguous forest tracts. 5. Synthesis and applications: the persistence of bird and dung beetle communities in low-intensity pastoral agriculture is strongly linked to the proximity of surrounding contiguous forests. Land-sharing policies that promote the integration of small-scale wildlife-friendly habitats might be of limited benefit without simultaneous measures to protect larger blocks of natural habitat, which could be achieved via land-sparing practices. Policymakers should carefully consider the extent and distribution of remaining contiguous natural habitats when designing agri-environment schemes in the tropics.

Likely effects of construction of Scroby Sands offshore wind farm on a mixed population of harbour Phoca vitulina and grey Halichoerus grypus seals

Scroby Sands offshore wind farm was built close to a haul-out and breeding site for harbour seal, a species of conservation concern. An aerial survey programme conducted during a five-year period spanning wind farm construction, revealed a significant post-construction decline in haul-out counts. Multivariate model selection suggested that the decline was not related to the environmental factors considered, nor did it mirror wider population trends. Although cause and effect could not be unequivocally established, the theoretical basis of hearing in pinnipeds and previous studies suggested that extreme noise (to 257 dB re 1 μ Pa(pp) @ 1m) generated by pile-driving of turbine bases led to displacement of seals. A lack of full recovery of harbour seal during the study was also linked to their sensitivity to vessel activity and/or rapid colonisation of competing grey seal. Any impact of offshore wind farm development upon pinnipeds would be much reduced without pile-driving.

Effects of the construction of Scroby Sands offshore wind farm on the prey base of Little tern Sternula albifrons at its most important UK colony

Despite widespread interest in the impacts of wind farms upon birds, few researchers have examined the potential for indirect or trophic (predator-prey) effects. Using surface trawls, we monitored prey abundance before and after construction of a 30 turbine offshore wind farm sited close to an internationally important colony of Little terns. Observations confirmed that young-of-the-year clupeids dominated chick diet, which trawl samples suggested were mainly herring. Multivariate modelling indicated a significant reduction in herring abundance from 2004 onwards that could not be explained by environmental factors. Intensely noisy monopile installation during the winter spawning period was suggested to be responsible. Reduced prey abundance corresponded with a significant decline in Little tern foraging success. Unprecedented egg abandonment and lack of chick hatching tentatively suggested a colony-scale response in some years. We urge a precautionary approach to the timing and duration of pile-driving activity supported with long-term targeted monitoring of sensitive receptors.

Foraging habitat selection, diet and nestling condition in Yellow Wagtails Motacilla flava breeding on arable farmland

Capsule Yellow Wagtails showed seasonally variable foraging preferences, favouring field margin and crop habitats, although habitat availability did not influence brood productivity. Aims To identify key habitats and prey types for Yellow Wagtails in arable farmland, assessing the importance of foraging habitat availability in determining breeding productivity. Methods Intensive nest searches were conducted across six areas in eastern England over two breeding seasons. Focal nest watches were used to determine foraging habitat selection. Nestling condition was analyzed in relation to surrounding habitat characteristics and weather conditions. Diet was assessed using chick faecal samples. Results Wet ditches and tracks were favoured foraging habitats, together with crop fields used for nesting. Crop preference varied seasonally, with autumn‐sown wheat preferred in May–June and potatoes preferred in July–August. Chick diet was dominated by flies (Diptera), although beetles (Coleoptera) were prevalent in early broods and damselflies (Zygoptera) in late broods. Weather and foraging habitat availability were poor predictors of nestling condition. Conclusion Our results highlight key microhabitats for Yellow Wagtails in arable farmland. The late‐season avoidance of autumn‐sown cereals implies that breeding season length could be limited in cereal‐dominated areas. The provision of alternative late‐season breeding habitats might represent an effective conservation strategy for this species.

Effects of spatial heterogeneity on feeding behaviour of Porcellio scaber (Isopoda : Oniscidea)

Foraging behaviour of Porcellio scaber was observed in laboratory arenas in which the spatial distribution of patches of high quality food (powdered dicotyledonous leaf litter) was varied within a matrix of lower quality food (powdered grass leaf litter). The hypotheses that feeding behaviour of isopods would vary with the degree of clumping of high quality food patches and with the density of conspecifics, were tested. In more clumped treatments, animals spend less time on high quality food and more on a low quality one. At higher densities more time was spent searching. This effect was more pronounced in clumped treatments, but negligible in homogeneous ones. The effects of variation in the spatial heterogeneity of high quality foods on the trade-off between searching costs and intake-rate benefits for saprophages are discussed in the context of predictions from optimal foraging theory for scenarios in which intake-rate maximisation is constrained by nutrient limitation.

Land-Sparing Agriculture Best Protects Avian Phylogenetic Diversity

The conversion of natural habitats to farmland is a major driver of the global extinction crisis. Two strategies are promoted to mitigate the impacts of agricultural expansion on biodiversity: land sharing integrates wildlife-friendly habitats within farmland landscapes, and land sparing intensifies farming to allow the offset of natural reserves. A key question is which strategy would protect the most phylogenetic diversity--the total evolutionary history shared across all species within a community. Conserving phylogenetic diversity decreases the chance of losing unique phenotypic and ecological traits and provides benefits for ecosystem function and stability. Focusing on birds in the threatened Chocó-Andes hotspot of endemism, we tested the relative benefits of each strategy for retaining phylogenetic diversity in tropical cloud forest landscapes threatened by cattle pastures. Using landscape simulations, we find that land sharing would protect lower community-level phylogenetic diversity than land sparing and that with increasing distance from forest (from 500 to >1,500 m), land sharing is increasingly inferior to land sparing. Isolation from forest also leads to the loss of more evolutionarily distinct species from communities within land-sharing landscapes, which can be avoided with effective land sparing. Land-sharing policies that promote the integration of small-scale wildlife-friendly habitats might be of limited benefit without the simultaneous protection of larger blocks of natural habitat, which is most likely to be achieved via land-sparing measures.

Mate-Finding as an Overlooked Critical Determinant of Dispersal Variation in Sexually-Reproducing Animals

Dispersal is a critically important process in ecology, but robust predictive models of animal dispersal remain elusive. We identify a potentially ubiquitous component of variation in animal dispersal that has been largely overlooked until now: the influence of mate encounters on settlement probability. We use an individual-based model to simulate dispersal in sexually-reproducing organisms that follow a simple set of movement rules based on conspecific encounters, within an environment lacking spatial habitat heterogeneity. We show that dispersal distances vary dramatically with fluctuations in population density in such a model, even in the absence of variation in dispersive traits between individuals. In a simple random-walk model with promiscuous mating, dispersal distributions become increasingly ‘fat-tailed’ at low population densities due to the increasing scarcity of mates. Similar variation arises in models incorporating territoriality. In a model with polygynous mating, we show that patterns of sex-biased dispersal can even be reversed across a gradient of population density, despite underlying dispersal mechanisms remaining unchanged. We show that some widespread dispersal patterns found in nature (e.g. fat tailed distributions) can arise as a result of demographic variability in the absence of heterogeneity in dispersive traits across the population. This implies that models in which individual dispersal distances are considered to be fixed traits might be unrealistic, as dispersal distances vary widely under a single dispersal mechanism when settlement is influenced by mate encounters. Mechanistic models offer a promising means of advancing our understanding of dispersal in sexually-reproducing organisms.

Effect of scale on trait predictors of species responses to agriculture

Species persistence in human-altered landscapes can depend on factors operating at multiple spatial scales. To understand anthropogenic impacts on biodiversity, it is useful to examine relationships between species traits and their responses to land-use change. A key knowledge gap concerns whether these relationships vary depending on the scale of response under consideration. We examined how local- and large-scale habitat variables influence the occupancy dynamics of a bird community in cloud forest zones in the Colombian Chocó-Andes. Using data collected across a continuum of forest and agriculture, we examined which traits best predict species responses to local variation in farmland and which traits best predict species responses to isolation from contiguous forest. Global range size was a strong predictor of species responses to agriculture at both scales; widespread species were less likely to decline as local habitat cover decreased and as distance from forest increased. Habitat specialization was a strong predictor of species responses only at the local scale. Open-habitat species were particularly likely to increase as pasture increased, but they were relatively insensitive to variation in distance to forest. Foraging plasticity and flocking behavior were strong predictors of species responses to distance from forest, but not their responses to local habitat. Species with lower plasticity in foraging behaviors and obligate flock-following species were more likely to decline as distance from contiguous forest increased. For species exhibiting these latter traits, persistence in tropical landscapes may depend on the protection of larger contiguous blocks of forest, rather than the integration of smaller-scale woodland areas within farmland. Species listed as threatened or near threatened on the International Union for Conservation of Nature Red List were also more likely to decline in response to both local habitat quality and isolation from forest relative to least-concern species, underlining the importance of contiguous forests for threatened taxa.