Johnny Wilson - Academia.edu (original) (raw)

Papers by Johnny Wilson

Carolina Digital Repository (University of North Carolina at Chapel Hill), 2012

Persistence of species in fragmented landscapes depends on dispersal among suitable breeding site... more Persistence of species in fragmented landscapes depends on dispersal among suitable breeding sites, and dispersal is often influenced by the ''matrix'' habitats that lie between breeding sites. However, measuring effects of different matrix habitats on movement and incorporating those differences into spatially explicit models to predict dispersal is costly in terms of time and financial resources. Hence a key question for conservation managers is: Do more costly, complex movement models yield more accurate dispersal predictions? We compared the abilities of a range of movement models, from simple to complex, to predict the dispersal of an endangered butterfly, the Saint Francis' satyr (Neonympha mitchellii francisci). The value of more complex models differed depending on how value was assessed. Although the most complex model, based on detailed movement behaviors, best predicted observed dispersal rates, it was only slightly better than the simplest model, which was based solely on distance between sites. Consequently, a parsimony approach using information criteria favors the simplest model we examined. However, when we applied the models to a larger landscape that included proposed habitat restoration sites, in which the composition of the matrix was different than the matrix surrounding extant breeding sites, the simplest model failed to identify a potentially important dispersal barrier, open habitat that butterflies rarely enter, which may completely isolate some of the proposed restoration sites from other breeding sites. Finally, we found that, although the gain in predicting dispersal with increasing model complexity was small, so was the increase in financial cost. Furthermore, a greater fit continued to accrue with greater financial cost, and more complex models made substantially different predictions than simple models when applied to a novel landscape in which butterflies are to be reintroduced to bolster their populations. This suggests that more complex models might be justifiable on financial grounds. Our results caution against a pure parsimony approach to deciding how complex movement models need to be to accurately predict dispersal through the matrix, especially if the models are to be applied to novel or modified landscapes.

Conservation Biology in Sub-Saharan Africa

Conservation Biology in Sub-Saharan Africa, 2019

Even without human influences, the size of any wildlife population may be stable, increasing, dec... more Even without human influences, the size of any wildlife population may be stable, increasing, decreasing, or even fluctuating. These population changes, combined with occasional natural perturbations, can and have driven some species and populations to extinction. Such natural extinction events generally occur at local scales, and are interspersed by long periods of little change, so that overall ecosystem stability is not compromised. Moreover, as explained by the intermediate disturbance hypothesis (e.g. Bongers et al., 2009), localised disturbances and subsequent local extinctions play an important role in maintaining regional biodiversity, as they increase opportunities for a greater variety of species to live in an area (Figure 9.1), at least until succession drives them out again. Some species that colonise the empty niches left by extinctions or extirpations may even evolve to become new species over time.

Conservation Biology in Sub-Saharan Africa, 2019

List of Acronyms xxxiii 1. What is Conservation Biology? Bibliography 6. Our Warming World 6.1 Dr... more List of Acronyms xxxiii 1. What is Conservation Biology? Bibliography 6. Our Warming World 6.1 Drivers of Climate Change 6.2 Predicting Earth's Future Climate 6.3 The Impact of Climate Change 6.3.1 Climate change's impact on people 6.3.2 Climate change's impact on terrestrial ecosystems Climate change on mountains Climate change in the lowlands Climate change and dispersal limitations Climate change and biological interactions Climate change and reptiles 6.3.3 Climate change's impact on freshwater ecosystems Warmer rivers and streams Changing flow regimes 6.3.4 Climate change's impact on marine ecosystems Ocean acidification Sea level rise Coral bleaching Ocean deoxygenation 6.3.5 Climate change interacts with habitat loss 6.4 Beneficiaries of Climate Change xi Contents 6.5 The Overall Impact of Climate Change

Conservation Biology in Sub-Saharan Africa, 2019

Conservation Biology in Sub-Saharan Africa, Sep 1, 2019

Journal of Insect Conservation, 2015

Established butterfly monitoring methods are designed for open habitats such as grasslands. Not a... more Established butterfly monitoring methods are designed for open habitats such as grasslands. Not all rare species occupy habitats that are easy to see across and navigate, in which cases a new approach to monitoring is necessary. We present a novel use of point transect distance sampling to monitor the Miami blue, a highly endangered butterfly that occupies dense shrub habitat. To monitor Miami blue density, we developed surveys consisting of butterfly counts in semi-circular plots. We examined the rate at which an observer detects new butterflies to determine the survey duration that meets the key assumption that butterflies are detected at their initial location. As a related secondary goal, we identified the determinants of adult flight phenology to target monitoring efforts during periods of high adult abundance. We observed peak Miami blue densities in April and July/August 2012, and July/August 2013. We estimated density using detections from a 10-sec survey, our most defensible and conservative estimate. Peak daily density estimates ranged from 592 to 680 butterflies per hectare. Adult density was related to precipitation patterns, with high densities occurring 4-6 weeks after particularly wet 4-week intervals. For butterfly species that exist in high enough densities, we recommend using point transect distance sampling in habitats where traditional methods are impossible to implement.

Ardea, 2010

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access t... more BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

Ostrich, 2007

We investigated the influence of forest fragment size and isolation on the bird assemblages in th... more We investigated the influence of forest fragment size and isolation on the bird assemblages in the species-and endemic-rich sand forests of the Maputaland Centre of Endemism, southern Mozambique. Point-centre surveys were conducted across 12 sand forest patches that varied in size and isolation. Patch size and isolation had little influence on bird species richness, but the number of individuals decreased significantly with increasing isolation. Furthermore, bird abundances were correlated to a combination of the size and isolation of patches. Many forest specialists, in particular large-bodied frugivores, were highly sensitive to reduced patch size and increased distances between patches. Further fragmentation of the landscape may therefore impair the ability of these forests to support viable populations of forest specialists.

Ostrich, 2008

... Primary moult patterns and morphometrics in the Green-winged Pytilia Pytilia melba Craig T Sy... more ... Primary moult patterns and morphometrics in the Green-winged Pytilia Pytilia melba Craig T Symes* and John W Wilson Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa * Corresponding author, e-mail: craig.symes@zoology.up.ac.za ...

Ostrich, 2009

There is confusion in the literature concerning the taxonomic status of the Turdus smithi—T. oliv... more There is confusion in the literature concerning the taxonomic status of the Turdus smithi—T. olivaceus species complex. Here we attempt to clarify morphological differences within this complex. In addition, we attempt to clarify identification of the respective taxa. Although mean measurements of morphometric features differed significantly between species and subspecies, these features are not useful in separating species or subspecies due to considerable overlap in measurements. Furthermore, there were often larger differences ...

Journal of Biogeography, 2007

Aim To assess the relative importance of environmental (climate, habitat heterogeneity and topog... more Aim To assess the relative importance of environmental (climate, habitat heterogeneity and topography), human (population density, economic prosperity and land transformation) and spatial (autocorrelation) influences, and the interactions between these predictor groups, on species richness patterns of various avifaunal orders.Location South Africa.Methods Generalized linear models were used to determine the amount of variation in species richness, for each order, attributable to each of the different predictor groups. To assess the relationships between species richness and the various predictor groups, a deviance statistic (a measure of goodness of fit for each model) and the percentage deviation explained for the best fitting model were calculated.Results Of the 12 avifaunal orders examined, spatially structured environmental deviance accounted for most of the variation in species richness in 11 orders (averaging 28%), and 50% or more in seven orders. However, orders comprisin...

Forest Ecology and Management, 2010

Emu - Austral Ornithology, 2010

Northern Rockhopper penguin populations, Eudyptes moseleyi, are declining globally, and at Trista... more Northern Rockhopper penguin populations, Eudyptes moseleyi, are declining globally, and at Tristan da Cunha have undergone severe declines (> 90% in the last 130 years), the cause(s) of which are unknown. There is a paucity of data on this species in the South Atlantic Ocean, therefore their trophic ecology at Tristan da Cunha was studied, specifically focusing on diet, using stomach content analysis and stable isotope analysis (SIA), in conjunction with an analysis of diving behaviour, assessed using temperature-depth recorders. In order to evaluate the influence of gender on foraging, a morphometric investigation of sexual dimorphism was confirmed using molecular analysis. Additionally, plasma corticosterone levels were measured to examine breeding stage and presence of blood parasites as potential sources of stress during the breeding season. Northern Rockhopper penguins at Tristan da Cunha displayed a high degree of foraging plasticity, and fed opportunistically on a wide variety of prey, probably reflecting local small-scale changes in prey distribution. Zooplankton dominated (by mass) the diet of guard stage females, whereas small meso-pelagic fish (predominantly Photichthyidae) dominated diet of adults of both sexes in the crèche stage, with cephalopods contributing equally in both stages. Adults consistently fed chicks on lower-trophic level prey (assessed using SIA), probably zooplankton, than they consumed themselves indicating that the increasing demands of growing chicks were not met by adults through provisioning of higher-quality prey. SIA also indicated that adults foraged in different oceanic water masses when feeding for self-maintenance and for chick provisioning, thus temporally segregating the prey consumed for different purposes. It is possible that adults 'selected' these higherquality prey for themselves, or this may be a reflection of opportunistic behaviour. ii At Tristan da Cunha sexual dimorphism was observed in culmen dimensions (length, depth, width), with males having larger beaks and feeding on larger individuals of squid and fish than females. No sexual segregation in terms of foraging habitat (i.e. different water masses, based on δ 13 C) or trophic level (δ 15 N) during the breeding season or pre-moult period was revealed through SIA, and stomach content analysis revealed no sexual differences in prey species targeted. The results of SIA of feathers indicate that during the pre-moult period birds foraged in different water masses than during the breeding period. The fact that throughout the breeding season birds foraged in similar habitats suggests no intra-specific competition, despite both sexes feeding on the same prey. Birds were generally diurnal, daily foragers (12-16 hr trips), with extended trips (maximum duration 35.5 hours) and nocturnal diving recorded in a few individuals. Birds dived well within their physiological limits, predominantly utilising the upper 20m of the water column, employing two different strategies to target different prey items. Long, deep (30-40 m), energetically costly dives were performed when targeting energy-rich prey (fish), and a greater number of shorter, shallower (5-20 m), energy-efficient dives were performed when targeting prey with a lower energy content (zooplankton). More than half of the sampled study population were infected with the intra-cellular blood parasite Babesia, but infection showed no relationship to body mass, corticosterone levels or breeding success. Fasting birds showed no signs of elevated corticosterone levels, suggesting they had acquired sufficient fat reserves prior to breeding. Failed breeders did not exhibit elevated corticosterone levels. Tristan skuas, Catharacta antarctica hamiltoni, were observed to be a significant cause of egg and chick mortality. The absence of sex-based differences in foraging, and the absence of any signs of stress in relation to body mass, presence of Babesia or breeding stage, suggest that there are iii no obvious signs of high levels of stress or food limitations during breeding at Tristan da

Ecological Applications, 2012

Persistence of species in fragmented landscapes depends on dispersal among suitable breeding site... more Persistence of species in fragmented landscapes depends on dispersal among suitable breeding sites, and dispersal is often influenced by the ''matrix'' habitats that lie between breeding sites. However, measuring effects of different matrix habitats on movement and incorporating those differences into spatially explicit models to predict dispersal is costly in terms of time and financial resources. Hence a key question for conservation managers is: Do more costly, complex movement models yield more accurate dispersal predictions? We compared the abilities of a range of movement models, from simple to complex, to predict the dispersal of an endangered butterfly, the Saint Francis' satyr (Neonympha mitchellii francisci). The value of more complex models differed depending on how value was assessed. Although the most complex model, based on detailed movement behaviors, best predicted observed dispersal rates, it was only slightly better than the simplest model, which was based solely on distance between sites. Consequently, a parsimony approach using information criteria favors the simplest model we examined. However, when we applied the models to a larger landscape that included proposed habitat restoration sites, in which the composition of the matrix was different than the matrix surrounding extant breeding sites, the simplest model failed to identify a potentially important dispersal barrier, open habitat that butterflies rarely enter, which may completely isolate some of the proposed restoration sites from other breeding sites. Finally, we found that, although the gain in predicting dispersal with increasing model complexity was small, so was the increase in financial cost. Furthermore, a greater fit continued to accrue with greater financial cost, and more complex models made substantially different predictions than simple models when applied to a novel landscape in which butterflies are to be reintroduced to bolster their populations. This suggests that more complex models might be justifiable on financial grounds. Our results caution against a pure parsimony approach to deciding how complex movement models need to be to accurately predict dispersal through the matrix, especially if the models are to be applied to novel or modified landscapes.

Austral Ecology, 2013

It is often hypothesized that two species competing for the same resource cannot stably coexist u... more It is often hypothesized that two species competing for the same resource cannot stably coexist unless they partition their resources in space and time. More recently stable isotope analyses have complemented traditional, observation-based niche research by conceptualizing many of the characteristics of communities, for example, trophic niche width and the partitioning of resources. Here we quantify resource partitioning of sympatric small mammal species in an African ecosystem by analysing stable isotope ratios of hair collected from a South African forest-grassland vegetation mosaic, and combine this with known spatial and temporal behavioural data to interpret community competition and resource partitioning.We observe niche separation to different degrees across the entire community, with different species displaying either unique isotopic dietary preferences, or partitioning resources in space and/or time. d 13 C values were more enriched in species that inhabited afromontane grassland compared with those that inhabited afromontane forest, a reflection of the dominant vegetation in each habitat. Contrary to expectations, arboreal rodents occupied higher trophic positions than terrestrial rodents and approaching d 15 N values similar to insectivorous shrews, suggesting that arboreal rodents feed on items such as arthropods enriched in 15 N. While grassland species display phenotypic plasticity in terms of dietary preferences, small mammals that occurred in forests display narrow niche preferences, suggesting these species may be particularly sensitive to habitat modifications. Our results illustrate that the use of stable isotopes can be used in conjunction with spatial and temporal behavioural knowledge to elucidate resource partitioning in small African mammal communities.

Behavioral Ecology, 2014

Factors affecting social group size in mammals are relatively well studied for females, but less ... more Factors affecting social group size in mammals are relatively well studied for females, but less is known about determinants of group size for males, particularly in species that live in sexually segregated groups. Male grouping patterns are thought to be driven more by spatial and temporal dispersion of mating opportunities than by food resources or predation risk. We evaluated the influence of 3 factors on male group sizes and number of males in mixed-sex groups in African elephants; forage availability (using Normalized Difference Vegetation Index, a satellite-based indicator of primary productivity), anthropogenic mortality risk (using distance of elephants from a protected area center), and mating opportunities (using the number of males in mixed-sex groups with and without estrous females). Using zero-truncated negative binomial regressions and a model-selection approach, we found that male elephants occurred in larger groups where primary productivity was higher and where they were further from a protected area center. However, we found an interaction between primary productivity and anthropogenic mortality risk: at low primary productivity, elephants formed larger groups further away from a protected area center, but did less so at higher primary productivity. This pattern suggests that male elephants are sensitive to seasonal variation in potential anthropogenic mortality risk, by remaining in smaller groups when risk is low, but forming larger groups when risk is high. Mating opportunities also led to an increase in male numbers in mixed-sex groups, but its relative influence on male grouping was less important because mating opportunities were rare.

Carolina Digital Repository (University of North Carolina at Chapel Hill), 2012

Persistence of species in fragmented landscapes depends on dispersal among suitable breeding site... more Persistence of species in fragmented landscapes depends on dispersal among suitable breeding sites, and dispersal is often influenced by the ''matrix'' habitats that lie between breeding sites. However, measuring effects of different matrix habitats on movement and incorporating those differences into spatially explicit models to predict dispersal is costly in terms of time and financial resources. Hence a key question for conservation managers is: Do more costly, complex movement models yield more accurate dispersal predictions? We compared the abilities of a range of movement models, from simple to complex, to predict the dispersal of an endangered butterfly, the Saint Francis' satyr (Neonympha mitchellii francisci). The value of more complex models differed depending on how value was assessed. Although the most complex model, based on detailed movement behaviors, best predicted observed dispersal rates, it was only slightly better than the simplest model, which was based solely on distance between sites. Consequently, a parsimony approach using information criteria favors the simplest model we examined. However, when we applied the models to a larger landscape that included proposed habitat restoration sites, in which the composition of the matrix was different than the matrix surrounding extant breeding sites, the simplest model failed to identify a potentially important dispersal barrier, open habitat that butterflies rarely enter, which may completely isolate some of the proposed restoration sites from other breeding sites. Finally, we found that, although the gain in predicting dispersal with increasing model complexity was small, so was the increase in financial cost. Furthermore, a greater fit continued to accrue with greater financial cost, and more complex models made substantially different predictions than simple models when applied to a novel landscape in which butterflies are to be reintroduced to bolster their populations. This suggests that more complex models might be justifiable on financial grounds. Our results caution against a pure parsimony approach to deciding how complex movement models need to be to accurately predict dispersal through the matrix, especially if the models are to be applied to novel or modified landscapes.

Conservation Biology in Sub-Saharan Africa

Conservation Biology in Sub-Saharan Africa, 2019

Even without human influences, the size of any wildlife population may be stable, increasing, dec... more Even without human influences, the size of any wildlife population may be stable, increasing, decreasing, or even fluctuating. These population changes, combined with occasional natural perturbations, can and have driven some species and populations to extinction. Such natural extinction events generally occur at local scales, and are interspersed by long periods of little change, so that overall ecosystem stability is not compromised. Moreover, as explained by the intermediate disturbance hypothesis (e.g. Bongers et al., 2009), localised disturbances and subsequent local extinctions play an important role in maintaining regional biodiversity, as they increase opportunities for a greater variety of species to live in an area (Figure 9.1), at least until succession drives them out again. Some species that colonise the empty niches left by extinctions or extirpations may even evolve to become new species over time.

Conservation Biology in Sub-Saharan Africa, 2019

List of Acronyms xxxiii 1. What is Conservation Biology? Bibliography 6. Our Warming World 6.1 Dr... more List of Acronyms xxxiii 1. What is Conservation Biology? Bibliography 6. Our Warming World 6.1 Drivers of Climate Change 6.2 Predicting Earth's Future Climate 6.3 The Impact of Climate Change 6.3.1 Climate change's impact on people 6.3.2 Climate change's impact on terrestrial ecosystems Climate change on mountains Climate change in the lowlands Climate change and dispersal limitations Climate change and biological interactions Climate change and reptiles 6.3.3 Climate change's impact on freshwater ecosystems Warmer rivers and streams Changing flow regimes 6.3.4 Climate change's impact on marine ecosystems Ocean acidification Sea level rise Coral bleaching Ocean deoxygenation 6.3.5 Climate change interacts with habitat loss 6.4 Beneficiaries of Climate Change xi Contents 6.5 The Overall Impact of Climate Change

Conservation Biology in Sub-Saharan Africa, 2019

Conservation Biology in Sub-Saharan Africa, Sep 1, 2019

Journal of Insect Conservation, 2015

Established butterfly monitoring methods are designed for open habitats such as grasslands. Not a... more Established butterfly monitoring methods are designed for open habitats such as grasslands. Not all rare species occupy habitats that are easy to see across and navigate, in which cases a new approach to monitoring is necessary. We present a novel use of point transect distance sampling to monitor the Miami blue, a highly endangered butterfly that occupies dense shrub habitat. To monitor Miami blue density, we developed surveys consisting of butterfly counts in semi-circular plots. We examined the rate at which an observer detects new butterflies to determine the survey duration that meets the key assumption that butterflies are detected at their initial location. As a related secondary goal, we identified the determinants of adult flight phenology to target monitoring efforts during periods of high adult abundance. We observed peak Miami blue densities in April and July/August 2012, and July/August 2013. We estimated density using detections from a 10-sec survey, our most defensible and conservative estimate. Peak daily density estimates ranged from 592 to 680 butterflies per hectare. Adult density was related to precipitation patterns, with high densities occurring 4-6 weeks after particularly wet 4-week intervals. For butterfly species that exist in high enough densities, we recommend using point transect distance sampling in habitats where traditional methods are impossible to implement.

Ardea, 2010

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access t... more BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

Ostrich, 2007

We investigated the influence of forest fragment size and isolation on the bird assemblages in th... more We investigated the influence of forest fragment size and isolation on the bird assemblages in the species-and endemic-rich sand forests of the Maputaland Centre of Endemism, southern Mozambique. Point-centre surveys were conducted across 12 sand forest patches that varied in size and isolation. Patch size and isolation had little influence on bird species richness, but the number of individuals decreased significantly with increasing isolation. Furthermore, bird abundances were correlated to a combination of the size and isolation of patches. Many forest specialists, in particular large-bodied frugivores, were highly sensitive to reduced patch size and increased distances between patches. Further fragmentation of the landscape may therefore impair the ability of these forests to support viable populations of forest specialists.

Ostrich, 2008

... Primary moult patterns and morphometrics in the Green-winged Pytilia Pytilia melba Craig T Sy... more ... Primary moult patterns and morphometrics in the Green-winged Pytilia Pytilia melba Craig T Symes* and John W Wilson Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa * Corresponding author, e-mail: craig.symes@zoology.up.ac.za ...

Ostrich, 2009

There is confusion in the literature concerning the taxonomic status of the Turdus smithi—T. oliv... more There is confusion in the literature concerning the taxonomic status of the Turdus smithi—T. olivaceus species complex. Here we attempt to clarify morphological differences within this complex. In addition, we attempt to clarify identification of the respective taxa. Although mean measurements of morphometric features differed significantly between species and subspecies, these features are not useful in separating species or subspecies due to considerable overlap in measurements. Furthermore, there were often larger differences ...

Journal of Biogeography, 2007

Aim To assess the relative importance of environmental (climate, habitat heterogeneity and topog... more Aim To assess the relative importance of environmental (climate, habitat heterogeneity and topography), human (population density, economic prosperity and land transformation) and spatial (autocorrelation) influences, and the interactions between these predictor groups, on species richness patterns of various avifaunal orders.Location South Africa.Methods Generalized linear models were used to determine the amount of variation in species richness, for each order, attributable to each of the different predictor groups. To assess the relationships between species richness and the various predictor groups, a deviance statistic (a measure of goodness of fit for each model) and the percentage deviation explained for the best fitting model were calculated.Results Of the 12 avifaunal orders examined, spatially structured environmental deviance accounted for most of the variation in species richness in 11 orders (averaging 28%), and 50% or more in seven orders. However, orders comprisin...

Forest Ecology and Management, 2010

Emu - Austral Ornithology, 2010

Northern Rockhopper penguin populations, Eudyptes moseleyi, are declining globally, and at Trista... more Northern Rockhopper penguin populations, Eudyptes moseleyi, are declining globally, and at Tristan da Cunha have undergone severe declines (> 90% in the last 130 years), the cause(s) of which are unknown. There is a paucity of data on this species in the South Atlantic Ocean, therefore their trophic ecology at Tristan da Cunha was studied, specifically focusing on diet, using stomach content analysis and stable isotope analysis (SIA), in conjunction with an analysis of diving behaviour, assessed using temperature-depth recorders. In order to evaluate the influence of gender on foraging, a morphometric investigation of sexual dimorphism was confirmed using molecular analysis. Additionally, plasma corticosterone levels were measured to examine breeding stage and presence of blood parasites as potential sources of stress during the breeding season. Northern Rockhopper penguins at Tristan da Cunha displayed a high degree of foraging plasticity, and fed opportunistically on a wide variety of prey, probably reflecting local small-scale changes in prey distribution. Zooplankton dominated (by mass) the diet of guard stage females, whereas small meso-pelagic fish (predominantly Photichthyidae) dominated diet of adults of both sexes in the crèche stage, with cephalopods contributing equally in both stages. Adults consistently fed chicks on lower-trophic level prey (assessed using SIA), probably zooplankton, than they consumed themselves indicating that the increasing demands of growing chicks were not met by adults through provisioning of higher-quality prey. SIA also indicated that adults foraged in different oceanic water masses when feeding for self-maintenance and for chick provisioning, thus temporally segregating the prey consumed for different purposes. It is possible that adults 'selected' these higherquality prey for themselves, or this may be a reflection of opportunistic behaviour. ii At Tristan da Cunha sexual dimorphism was observed in culmen dimensions (length, depth, width), with males having larger beaks and feeding on larger individuals of squid and fish than females. No sexual segregation in terms of foraging habitat (i.e. different water masses, based on δ 13 C) or trophic level (δ 15 N) during the breeding season or pre-moult period was revealed through SIA, and stomach content analysis revealed no sexual differences in prey species targeted. The results of SIA of feathers indicate that during the pre-moult period birds foraged in different water masses than during the breeding period. The fact that throughout the breeding season birds foraged in similar habitats suggests no intra-specific competition, despite both sexes feeding on the same prey. Birds were generally diurnal, daily foragers (12-16 hr trips), with extended trips (maximum duration 35.5 hours) and nocturnal diving recorded in a few individuals. Birds dived well within their physiological limits, predominantly utilising the upper 20m of the water column, employing two different strategies to target different prey items. Long, deep (30-40 m), energetically costly dives were performed when targeting energy-rich prey (fish), and a greater number of shorter, shallower (5-20 m), energy-efficient dives were performed when targeting prey with a lower energy content (zooplankton). More than half of the sampled study population were infected with the intra-cellular blood parasite Babesia, but infection showed no relationship to body mass, corticosterone levels or breeding success. Fasting birds showed no signs of elevated corticosterone levels, suggesting they had acquired sufficient fat reserves prior to breeding. Failed breeders did not exhibit elevated corticosterone levels. Tristan skuas, Catharacta antarctica hamiltoni, were observed to be a significant cause of egg and chick mortality. The absence of sex-based differences in foraging, and the absence of any signs of stress in relation to body mass, presence of Babesia or breeding stage, suggest that there are iii no obvious signs of high levels of stress or food limitations during breeding at Tristan da

Ecological Applications, 2012

Persistence of species in fragmented landscapes depends on dispersal among suitable breeding site... more Persistence of species in fragmented landscapes depends on dispersal among suitable breeding sites, and dispersal is often influenced by the ''matrix'' habitats that lie between breeding sites. However, measuring effects of different matrix habitats on movement and incorporating those differences into spatially explicit models to predict dispersal is costly in terms of time and financial resources. Hence a key question for conservation managers is: Do more costly, complex movement models yield more accurate dispersal predictions? We compared the abilities of a range of movement models, from simple to complex, to predict the dispersal of an endangered butterfly, the Saint Francis' satyr (Neonympha mitchellii francisci). The value of more complex models differed depending on how value was assessed. Although the most complex model, based on detailed movement behaviors, best predicted observed dispersal rates, it was only slightly better than the simplest model, which was based solely on distance between sites. Consequently, a parsimony approach using information criteria favors the simplest model we examined. However, when we applied the models to a larger landscape that included proposed habitat restoration sites, in which the composition of the matrix was different than the matrix surrounding extant breeding sites, the simplest model failed to identify a potentially important dispersal barrier, open habitat that butterflies rarely enter, which may completely isolate some of the proposed restoration sites from other breeding sites. Finally, we found that, although the gain in predicting dispersal with increasing model complexity was small, so was the increase in financial cost. Furthermore, a greater fit continued to accrue with greater financial cost, and more complex models made substantially different predictions than simple models when applied to a novel landscape in which butterflies are to be reintroduced to bolster their populations. This suggests that more complex models might be justifiable on financial grounds. Our results caution against a pure parsimony approach to deciding how complex movement models need to be to accurately predict dispersal through the matrix, especially if the models are to be applied to novel or modified landscapes.

Austral Ecology, 2013

It is often hypothesized that two species competing for the same resource cannot stably coexist u... more It is often hypothesized that two species competing for the same resource cannot stably coexist unless they partition their resources in space and time. More recently stable isotope analyses have complemented traditional, observation-based niche research by conceptualizing many of the characteristics of communities, for example, trophic niche width and the partitioning of resources. Here we quantify resource partitioning of sympatric small mammal species in an African ecosystem by analysing stable isotope ratios of hair collected from a South African forest-grassland vegetation mosaic, and combine this with known spatial and temporal behavioural data to interpret community competition and resource partitioning.We observe niche separation to different degrees across the entire community, with different species displaying either unique isotopic dietary preferences, or partitioning resources in space and/or time. d 13 C values were more enriched in species that inhabited afromontane grassland compared with those that inhabited afromontane forest, a reflection of the dominant vegetation in each habitat. Contrary to expectations, arboreal rodents occupied higher trophic positions than terrestrial rodents and approaching d 15 N values similar to insectivorous shrews, suggesting that arboreal rodents feed on items such as arthropods enriched in 15 N. While grassland species display phenotypic plasticity in terms of dietary preferences, small mammals that occurred in forests display narrow niche preferences, suggesting these species may be particularly sensitive to habitat modifications. Our results illustrate that the use of stable isotopes can be used in conjunction with spatial and temporal behavioural knowledge to elucidate resource partitioning in small African mammal communities.

Behavioral Ecology, 2014

Factors affecting social group size in mammals are relatively well studied for females, but less ... more Factors affecting social group size in mammals are relatively well studied for females, but less is known about determinants of group size for males, particularly in species that live in sexually segregated groups. Male grouping patterns are thought to be driven more by spatial and temporal dispersion of mating opportunities than by food resources or predation risk. We evaluated the influence of 3 factors on male group sizes and number of males in mixed-sex groups in African elephants; forage availability (using Normalized Difference Vegetation Index, a satellite-based indicator of primary productivity), anthropogenic mortality risk (using distance of elephants from a protected area center), and mating opportunities (using the number of males in mixed-sex groups with and without estrous females). Using zero-truncated negative binomial regressions and a model-selection approach, we found that male elephants occurred in larger groups where primary productivity was higher and where they were further from a protected area center. However, we found an interaction between primary productivity and anthropogenic mortality risk: at low primary productivity, elephants formed larger groups further away from a protected area center, but did less so at higher primary productivity. This pattern suggests that male elephants are sensitive to seasonal variation in potential anthropogenic mortality risk, by remaining in smaller groups when risk is low, but forming larger groups when risk is high. Mating opportunities also led to an increase in male numbers in mixed-sex groups, but its relative influence on male grouping was less important because mating opportunities were rare.