Effects of habitat fragmentation on Eurasian red squirrel (Sciurus vulgaris) in a forest landscape

We studied the effects of habitat fragmentation, measured as forest stand size and isolation, on the distribution of Eurasian red squirrels (Sciurus vulgaris). Squirrel density was surveyed during four years in 46 forest stands (0.1–500 ha) in a forest landscape in south-central Sweden. The only factor that significantly influenced a density index was the proportion of spruce within a habitat fragment. Neither fragment size nor degree of isolation were significant. Furthermore, none of the interactions with year were significant, suggesting the same pattern in all four years. Thus, the effect of habitat fragmentation in this study seems to be only pure habitat loss, i.e. halving the proportion of preferred habitat in the landscape should result in a halving of the red squirrel population. Therefore, the landscape can be viewed as functionally continuous for the squirrels, although the preferred habitat was divided into fragments. The most likely explanation for the difference between this study and other studies on squirrels that found effects due to habitat fragmentation is a combination of shorter distances and less hostile surroundings in our study area. To identify landscape effects requires multiple studies because single studies usually consider only one landscape.     

Distribution and dynamics of the red squirrel (Sciurus vulgaris L.) in a landscape with fragmented habitat

In a four year study data on the presence of red squirrel were collected in an agricultural landscape by counting dreys in 49 woods ranging from 0.5 to 14 ha, and differing in quality of habitat and isolation.Logit regression analysis showed that the area per woodlot covered with conifers is a good predictor of squirrel presence for each year and during the whole period, but the significance of the regression decreases with time. During the study the number of woods occupied by red squirrel increased, and smaller woods and those without conifers also became inhabited. This trend is in accordance with the positive effect of time in regression analyses on the presence of the species and on the colonization of woods, and it suggests an increase of squirrel numbers in the area. Addition of several isolation variables in the regression analyses showed significant effects in different years, and the effect of isolation was independent of time. In the first two years the area of habitat around a woodlot, the distance to the nearest woodlot larger than 30 ha, and the density of possible movement corridors have significant effects on the presence of red squirrel.In the last two years, with presumably a high number of squirrels, the (short) distance to the nearest woodlot and also the area of habitat around woods have significant effects. It is concluded that the spatial dynamics of the population can be understood as the outcome of individual spatial behaviour, rather than as the result of metapopulation processes.     

Additive partitioning of plant species diversity in an agricultural mosaic landscape

In this paper, we quantify the effects of habitat variability and habitat heterogeneity based on the partitioning of landscape species diversity into additive components and link them to patch-specific diversity. The approach is illustrated with a case study from central Switzerland, where we recorded the presence of vascular plant species in a stratified random sample of 1'280 quadrats of 1 m² within a total area of 0.23 km². We derived components of within- and between-community diversity at four scale levels (quadrat, patch, habitat type, and landscape) for three diversity measures (species richness, Shannon index, and Simpson diversity). The model implies that what we measure as within-community diversity at a higher scale level is the combined effect of heterogeneity at various lower levels. The results suggest that the proportions of the individual diversity components depend on the habitat type and on the chosen diversity aspect. One habitat type may be more diverse than another at patch level, but less diverse at the level of habitat type. Landscape composition apparently is a key factor for explaining landscape species richness, but affects evenness only little. Before we can test the effect of landscape structure on landscape species richness, several problems will have to be solved. These include the incorporation of neighbourhood effects, the unbiased estimation of species richness components, and the quantification of the contribution of a landscape element to landscape species richness.     

Spatial pattern of coniferous and deciduous forest patches in an Eastern North America agricultural landscape: the influence of land use and physical attributes

In agricultural landscapes, most studies have investigated the influence of the spatial pattern of forest patches on other ecological phenomena and processes, such as animal movement and biodiversity. However, few have focused on explaining the spatial pattern of the forest patches themselves. Understanding how these patterns relate to the processes that generate them is fundamental in developing a sound theory of landscape ecology, and in devising rational management strategies. In this paper, the pattern of the overall forest patches, as well as the pattern of deciduous and coniferous patches in an agricultural landscape of Southern Quebec, Canada, were analyzed and related to landscape physical attributes and land use, using remote sensing, geographic information systems and statistical methods. Results show that the role of landscape physical attributes on forest patch pattern has been modified by land use. In the study area, coniferous or deciduous patches are not associated with a specific surface deposit. In addition, physical attributes explain only a small proportion of the abundance of conifers on past abandoned land compared with land-use factors. Physical attributes only indirectly influence the forest pattern because they strongly influence the land-use practices. Our results reveal a conifer recovery process with the abandonment of agricultural land. On past abandoned land, conifers expand with increasing stand age, mostly by invasion from neighboring coniferous patches. Spatially, coniferous patches are usually located on the margins of the overall forest patches, and they are connected to non-forest land-use types such as crop and pasture, the latter being the most important. By showing the importance of some coniferous forest types that did not exist in the precolonial forest, a new perspective emerges when landscape, especially, land-use dynamics are taken into account.     

Does matrix resistance influence Red squirrel (Sciurus vulgaris L. 1758) distribution in an urban landscape?

In determining isolation effects in fragmented populations, the landscape matrix is not often considered. Usually simple distance measures are used to quantify degree of isolation. We tested the effect of the matrix on the presence of red squirrels in 354 wooded patches in the Brussels Region, by comparing several isolation measures. These were 1) distance to the nearest source patch, 2) the Hanski-measure (a combination of distance to and size of all possible sources), 3) effective distances calculated from different least cost models using the ArcView grid extension ‘Cost Distance’ (a combination of distance and resistance of the landscape, with different resistances for different landscape types) and 4) some combinations of the Hanski-measure and the effective distances. Size and quality of the target patches were always included in the tests of the predictive power of different isolation measures on squirrel presence/absence. All variables examined (patch size, quality and isolation) significantly influenced squirrel presence. Models using the effective distances gave the best results. Models including the Hanski-measure improved significantly when Euclidean distance was replaced by effective distance, showing that parameterisation of matrix resistance added significant additional explanatory power when modelling squirrel presence. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction of food resources and landscape structure in determining the probability of patch use by carnivores in fragmented landscapes

Studies on the distribution of mammalian carnivores in fragmented landscapes have focused mainly on structural aspects such as patch and landscape features; similarly, habitat connectivity is usually associated with landscape structure. The influence of food resources on carnivore patch use and the important effect on habitat connectivity have been overlooked. The aim of this study is to evaluate the relative importance of food resources on patch use patterns and to test if food availability can overcome structural constraints on patch use. We carried out a patch-use survey of two carnivores: the beech marten (Martes foina) and the badger (Meles meles) in a sample of 39 woodland patches in a fragmented landscape in central Italy. We used the logistic model to investigate the relative effects on carnivore distribution of patch, patch neighbourhood and landscape scale variables as well as the relative abundance of food resources. Our results show how carnivore movements in fragmented landscapes are determined not only by patch/landscape structure but also by the relative abundance of food resources. The important take-home message of our research is that, within certain structural limits (e.g. within certain limits of patch isolation), by modifying the relative amount of resources and their distribution, it is possible to increase suitability in smaller/relatively isolated patches. Conversely, however, there are certain thresholds above which an increase in resources will not achieve high probability of presence. Our findings have important and generalizable consequences for highly fragmented landscapes in areas where it may not be possible to increase patch sizes and/or reduce isolation so, for instance, forest regimes that will increase resource availability could be implemented. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Respective roles of recent hedges and forest patch remnants in the maintenance of ground-beetle (Coleoptera: Carabidae) diversity in an agricultural landscape

We compared three kinds of habitats: small remnants of native forests, recent hedges and barley crops, in order to investigate their respective roles in the maintenance of carabid-beetle diversity in a 950-ha area of an intensive agricultural landscape. Carabid faunas in remnants differed weakly from these found in hedges and crops. In particular, small remnants had few typical forest carabid species and a large number of open-area or ubiquitous species. Different approaches in the measurement of and -diversity (classical indices, and additive partitioning of Simpson's index) showed similar results: hedges supported a high -diversity but habitat types were quite similar overall, with weak differences between open and closed or disturbed and undisturbed habitats.A comparison of species dispersal powers in the various habitat types showed that species with a low dispersal power were rare in all habitats. However, wing development measured on two dimorphic species revealed, surprisingly, that brachypterous individuals were mainly present in hedges, which were expected a priori to be more disturbed, than remnants hence less suitable for the establishment of populations with a low dispersal power.These results suggest that small remnants do not behave as 'climax' habitats in this intensive agricultural landscape, probably because of their small size and strong isolation. We discuss the interest of new undisturbed habitats, such as recent hedges, for the maintenance of carabid diversity at both the local and landscape scale.