Spatial analysis of habitat quality in a fragmented population of little bustard (Tetrax tetrax): Implications for conservation

Little bustard populations have suffered reduction and isolation as a consequence of landscape transformations resulting from changes in traditional agricultural systems. Consequently, the species survives within reduced and fragmentary habitats, like islands isolated in a modified matrix. In this paper, we analyze the spatial variations in male density and habitat quality in a fragmented population located at the limit of the species’ Iberian range, which is affected by agricultural intensification, using a regional modelling approach. Habitat quality (quantified according to the species perception) and bird density decreased along the intensification gradient. However, in the most intensive agricultural zone, the quality of habitats selected by little bustard males increased, while density decreased, against the expected. In possible explanation, we suggest: (1) density is not necessarily a good indicator of habitat quality, (2) population could be under-saturated in this zone, (3) interannual variations in species distribution, or (4) other relevant variables related to the agricultural intensification process not included in this analysis, such as small-scale disturbances. Analysis of population distribution pattern showed a spatial configuration in which the most densely populated squares were located at the core of the biggest population patches, in contact with mid-density squares, and all surrounded by low-density squares. Fragmentation negatively affected habitat quality and male density. Largest population patches, containing higher density values, were located at the beginning of the intensification gradient. Preservation of little bustard densities is related to an adequate management of the farming system. Habitat fragmentation requires an urgent conservation strategy to prevent local and regional scale habitat deterioration, by reducing patch isolation to maintain genetic diversification and functional connectivity.     

Spatial population structure of the Cabrera vole in Mediterranean farmland: The relative role of patch and matrix effects

There is mounting evidence that both patch networks and the intervening matrix influence species persistence in fragmented landscapes, though the relative importance of each of these factors in determining spatial population structure remains poorly understood. This study examined this issue using a three-year data set on the distribution of Cabrera voles (Microtus cabrerae) in Mediterranean farmland. The spatial pattern appeared consistent with a metapopulation structure, as voles occupied discrete tall herb patches scattered across the agricultural landscape, where local extinctions and colonizations induced temporal changes in occupancy patterns. Patch dynamics determined deviations from classical metapopulation assumptions, with over half the extinctions resulting from agricultural disturbance or vegetation succession, and recolonizations often occurring after the recovery of suitable habitat conditions sometime after disturbance. Occupancy in undisturbed patches was more stable, with vole occurrence in one year strongly reflecting that in the previous year. Overall, occupancy increased with both patch size and connectivity, but the unique contribution of patch variables to explain variation in vole occurrence was far smaller than that of matrix attributes. Voles occurred more often in patches surrounded by natural pastures, while prevalence declined with increasing cover by shrubland, pine plantations, improved pastures and grazed cropland. It is hypothesised that unfavourable land uses may increase the effective isolation of habitat patches through increased predation risk of dispersing voles. Conservation of the Cabrera vole in Mediterranean farmland should thus strive to maintain lightly grazed fields surrounding well-connected networks of suitable habitat patches.     

Resource availability, matrix quality, microclimate, and spatial pattern as predictors of patch use by the Karner blue butterfly

Determination of which aspects of habitat quality and habitat spatial arrangement best account for variation in a species’ distribution can guide management for organisms such as the Karner blue butterfly (Lycaeides melissa samuelis), a federally endangered subspecies inhabiting savannas of Midwest and Eastern United States. We examined the extent to which three sets of predictors, (1) larval host plant (Lupinus perennis, wild lupine) availability, (2) characteristics of the matrix surrounding host plant patches, and (3) factors affecting a patch’s thermal environment, accounted for variation in lupine patch use by Karner blues at Indiana Dunes National Lakeshore, Indiana and Fort McCoy, Wisconsin, USA. Each predictor set accounted for 7-13% of variation in patch occupancy by Karner blues at both sites and in larval feeding activity among patches at Indiana Dunes. Patch area, an indicator of host plant availability, was an exception, accounting for 30% of variation in patch occupancy at Indiana Dunes. Spatially structured patterns of patch use across the landscape accounted for 9-16% of variation in patch use and explained more variation in larval feeding activity than did spatial autocorrelation between neighboring patches. Because of this broader spatial trend across sites, a given management action may be more effective in promoting patch use in some portions of the landscape than in others. Spatial trend, resource availability, matrix quality, and microclimate, in general, accounted for similar amounts of variation in patch use and each should be incorporated into habitat management planning for the Karner blue butterfly.     

Anthropogenic determinants of primate and carnivore local extinctions in a fragmented forest landscape of southern Amazonia

Habitat fragmentation has been shown to influence the abundance, movements and persistence of many species. Here, we examine the effects of forest patch and landscape metrics, and levels of forest disturbance on the patterns of local extinction of five primate and 14 carnivore species within 129 forest patches in a highly fragmented forest landscape of southern Brazilian Amazonia. Classic habitat area effects were the strongest predictors of species persistence, explaining between 42% and 55% of the overall variation in primate and carnivore species richness. Logistic regression models showed that anthropogenic disturbance, including surface wildfires, timber extraction and hunting pressure, had detrimental effects on the persistence of some species over and above those of fragment size. Different species ranged in their responses from highly sensitive to highly tolerant to forest fragmentation. Patterns of local extinction documented here were by no means chance events, and the nestedness of the overall species-by-site matrix was highly nonrandom in terms of the sets of species extirpated from the most to the least species-rich forest patches.     

Habitat configuration affects colonisation of epifauna in a marine algal bed

Habitat fragmentation is a threat to the preservation of both terrestrial and marine biodiversity. While terrestrial systems have been well studied, relatively few studies have considered how changes to the spatial arrangement of habitats affect fauna in marine systems. In this study, sampling and manipulative experiments examined the effects of varying the size and isolation of habitat patches on the abundance of mobile invertebrates inhabiting an algal dominated rocky reef. Variation in the size of naturally occurring patches of the brown alga Sargassum linearifolium did not impact upon the abundance of any taxonomic groups, with the exception of polychaete worms, which were most abundant in small patches. When habitat patch size and isolation were manipulated, the abundance of colonising isopods increased with increasing isolation from contiguous habitat. Amphipods and ostracods colonised small patches to greater numbers than large patches. Sampling of the matrix was undertaken to examine the rarely tested assumption that the area between habitat patches is not a potential source of colonists to the habitat in question. The matrix was found to support a strikingly different community of invertebrates than did the algal habitats and thus was unlikely to be a source of colonists to isolated algal patches. The increased abundance on small and isolated patches for some taxa are inconsistent with traditional predictions of the effects of reduced habitat patch sizes and indicate that patchy landscapes should not necessarily be considered poor habitats. The variation in responses among taxonomic groups suggests that a range of patch sizes may be necessary to maintain species diversity.     

Effects of structural and functional connectivity and patch size on the abundance of seven Atlantic Forest bird species

We studied the importance of fragment size and structural and functional connectivity on the occurrence and abundance of seven Atlantic Forest bird species in 13 patches (13-275 ha) and three sites within a continuous forest (10,000 ha). We sampled birds with point counts and evaluated structural connectivity considering the presence of corridors and the degree of isolation. We defined functional connectivity by analyzing species movements using playbacks in forest corridors between fragments and in the surrounding matrix. Species differed in their responses to fragmentation. For the frugivorous species, Trogon surrucura, Carpornis cucullatus and Triclaria malachitacea, patch size was the main factor determining abundance. Two understory insectivorous species, Basileuterus leucoblepharus and Pyriglena leucoptera, were more affected by the degree of patch connectivity, the former by the presence of corridors and the latter by the distance between patches. The capacity of P. leucoptera to use corridors and open areas (i.e. functional connectivity) shaped its abundance pattern. Fragmentation had no effect on the abundance of Chiroxiphia caudata and had a positive effect on Batara cinerea. This study emphasizes the importance of considering species’ perceptions of landscape, especially functional connectivity, in understanding the effects of habitat fragmentation.     

Thresholds in landscape parameters: occurrence of the white-browed treecreeper Climacteris affinis in Victoria, Australia

The concept of critical thresholds of habitat loss has recently received considerable attention in conservation biology and landscape ecology, yet empirical examples of thresholds are scarce. Threatened species management could benefit from recognition of thresholds because conditions under which populations are at risk can be specified. In this study, 56 woodland patches in north-west Victoria were surveyed for the white-browed treecreeper Climacteris affinis, a threatened insectivorous bird of the semi-arid zone of southern Australia. Comparisons with historic records indicate the species’ range is contracting in Victoria. Using logistic regression and hierarchical partitioning, two models of patch occupancy were developed. Tree species composition was an important factor in both models, confirming the treecreepers’ affinity for belah Casuarina pauper and slender cypress-pine Callitris gracilis-buloke Allocasuarina luehmannii woodlands in north-west Victoria. The first model emphasized the importance of demographic isolation: probability of patch occupancy decreased with distance to the nearest occupied patch. A threshold response in demographic isolation was apparent. In agricultural landscapes, most suitable woodland patches within 3 km of an occupied patch were occupied, whereas patches beyond the threshold were vacant. The threshold distance increased to a minimum of 8 km in a matrix of native vegetation, suggesting landscape context affects the response of white-browed treecreepers to habitat fragmentation. Demographic isolation is a quasi-dependent variable and therefore a second model was developed using surrogate variables for demographic isolation. A positive relationship with the proportion of woodland cover in the landscape (100 km²) emerged as the pre-eminent explanatory factor. Depending on woodland quality, a threshold of patch occupancy was apparent at levels of woodland cover between 15 and 25%. However, belah and slender cypress-pine-buloke woodlands now cover only 10% of their original extent in the region. These results highlight the inter-dependence of patch isolation with the amount and quality of habitat in the landscape and the implications this has for maintaining functional connectivity. The retention (or restoration) of suitable habitat is the critical issue for conservation of the white-browed treecreeper, but in landscapes below the threshold of habitat cover, viability of local populations may be influenced by the configuration and quality of remaining habitat.     

Development and evaluation of predictive habitat models to assist the conservation planning of a threatened lucanid beetle, Hoplogonus simsoni, in north-east Tasmania

The use of predictive habitat distribution models by land managers in the conservation management of threatened species is increasing. Few models, however, are subsequently field-checked and evaluated. This study evaluates the statistical strength and usefulness for conservation purposes of three predictive habitat models developed for a threatened stag beetle, Hoplogonus simsoni, found in the wet eucalypt forests and mixed/rainforests of north-east Tasmania. The relationship between various environmental variables for which spatial (GIS) information was available and the density, frequency of occurrence and presence/absence of the species was investigated using generalised linear modelling. Models developed were coupled with the GIS data to develop maps of predicted occurrence within the species’ range, grouped into categories of habitat quality. The models found that altitude, aspect, slope, distance to nearest stream and overstorey tree height were significantly associated with the occurrence of the species. Evaluation of the statistical strength of the models with independent data of species’ occurrence collected at 95 sites found that the density model performed poorly with little correlation between predicted and observed densities of the species. The frequency of occurrence model, however, showed a moderate ability to predict both species’ abundance and presence/absence. The presence/absence model had a similar discriminatory ability in predicting presence or absence of H. simsoni, but also showed some potential as an indirect predictor of species’ abundance. Assuming a correlation between relative abundance and habitat quality, the frequency of occurrence predictive model appeared to be the better and more direct discriminator of high quality habitat relative to the other models. The value of species’ habitat models and the need to evaluate their utility in the development of conservation strategies are discussed.     

Metapopulation dynamics and conservation of the marsh fritillary butterfly: Population viability analysis and management options for a critically endangered species in Western Europe

This study investigates the dynamics and viability of a marsh fritillary butterfly Euphydryas aurinia metapopulation in a Belgian successional landscape. Based on capture-mark-recapture and winter nest census data, we first estimated demography (survival and recruitment rates, population size, density dependence) and dispersal parameters (emigration rate, effect of patch connectivity on dispersal, mortality during dispersal). Then using RAMAS/GIS platform, we parameterised a population viability analysis (PVA) model with these parameters to simulate the future of this metapopulation under different scenarios.The metapopulation does not seem viable even if natural reforestation is controlled by adequate management. In its present state, the patch system is not able to sustain enough individuals: due to the large temporal fluctuations in demographic parameters, a carrying capacity far higher than currently would be necessary to limit extinction risk to 1%, suggesting the existence of an extinction debt for the species in Belgium. The situation of E. aurinia appears much worse compared to two other fritillary species threatened in Belgium, for which similar PVA are available. It is therefore urgent to increase the carrying capacity of the patch system. How and where it is achieved are of secondary importance for the gain in viability: improvement of habitat quality through restoration, or increase of habitat quantity via enlargement of existing patches and/or creation of new habitat in the matrix. A regime of management based on regular re-opening and maintenance of habitat patches may be the only guarantee of long-term persistence for this critically endangered species in Belgium.