Body size, niche breadth, and ecologically scaled responses to habitat fragmentation: mammalian predators in an agricultural landscape

The ability to make a priori assessments of a species' response to fragmentation, based on its distribution in the landscape, would serve as a valuable conservation and management tool. During 1997-1999, we monitored 717 scent stations to examine seasonal use of forest patches, corridors, and crop fields by coyotes (Canis latrans), domestic cats (Felis catus), foxes (Vulpes vulpes and Urocyon cinereoargenteus), raccoons (Procyon lotor), striped skunks (Mephitis mephitis), opossums (Didelphis virginiana), and long-tailed weasels (Mustela frenata). For each species we developed landscape-based ecologically scaled landscape indices (ELSI), and we modeled species spatial distribution across three spatial scales (landscape-level, element-level, and local habitat-level). Our results suggest that these predators view landscape fragmentation at different spatial scales and demonstrate strong interspecific differences in their response to elements of the landscape. All species except coyotes and domestic cats avoided agricultural fields. In general, predator species that were more mobile (i.e. high ESLI for landscape connectivity; coyotes) were characterized by landscape- and element-based logistic models. In contrast, models including local habitat features generally were most appropriate for less mobile or more stenophagous predators (e.g. long-tailed weasels). Our analysis extends the application of the ESLI concept to species assemblages that do not appear to function as metapopulations, and it highlights the importance of examining spatial scale and species-specific responses to habitat fragmentation. We discuss the relevance of these findings for defining ecological landscapes, understanding predator-prey interactions at multiple spatial scales, and conserving predator and prey populations in fragmented landscapes.     

Non-intertidal habitat use by shorebirds: a reflection of inadequate intertidal resources?

Many species of shorebird typically forage almost exclusively on intertidal habitats. When such strongly maritime species choose to forage on non-intertidal habitats, it may either be a response to deteriorating intertidal conditions or to the discovery of more profitable resources in non-intertidal areas. Methods which allow distinction between these two will clearly be important for identifying problems in intertidal habitats. Since January 1998, turnstone (Arenaria interpres) on the Wash estuary, eastern England have been foraging on the docksides of Port Sutton Bridge (mainly on spilt wheat and fishmeal), fields and river edges, resulting in concern that intertidal food supplies were no longer sufficient to support the population. We quantified the distribution and behaviour of turnstone within the Wash in relation to season, tidal state and weather and used a depletion model to predict the number of turnstone that could be supported by the port, under a range of resource densities and environmental conditions. Numbers of turnstone on non-intertidal habitats increased over the winter and use of the port was significantly greater around high tide and on colder days. The depletion model showed that under virtually all conditions, the port could support a much greater proportion of the turnstone population than current peak numbers. The use of non-intertidal habitats therefore suggests that the preferred intertidal food supplies are not currently capable of supporting the turnstone population throughout the winter. Habitat switches such as this can potentially be important advance warnings of ecological changes for species, which have not yet led to reductions in population size.     

Effects of ground fires on understorey bird assemblages in Amazonian forests

Avifaunal responses to understorey fire disturbance and subsequent changes in habitat structure were examined within 20 burnt and unburnt forest plots of 0.25 ha (10×250 m), 10-15 months after an unprecedented understorey fire swept through the Tapajós-Arapiuns Extractive Reserve of central Brazilian Amazonia following the severe 1997-1998 El-Niño dry season. Although these surface fires in the previously undisturbed primary forest were relatively mild, they resulted in dramatic changes in forest structure consistent with those found elsewhere in Amazonia. Bird species negatively affected by these changes tended to be the least common, the most disturbance-sensitive, and habitat specialists. Considering different guilds, ant followers, dead-leaf gleaners, terrestrial gleaners, and arboreal sallying insectivores were the most negatively affected, whereas nectivores and arboreal granivores became more abundant in the burnt forest. The results highlight the severe consequences of even relatively mild surface fires in neotropical forests, and the importance of controlling haphazard frontier expansion for the conservation of susceptible species that are endemic to fire-prone regions.     

Distribution and nest site selection of red-shouldered hawks (Buteo lineatus) in forests of northeastern Wisconsin (USA)

Red-shouldered hawks (Buteo lineatus) are threatened in Wisconsin and when nest sites are found during the cruising or marking stage of timber harvesting, the harvest is altered to accommodate the hawks. If nest site locations are known before initiation of timber harvest, foresters can employ a proactive approach to manage red-shouldered hawks while maintaining timber production. We searched for red-shouldered hawks nest sites on Marinette County Forest (MCF) which encompasses 94,000 ha in northeastern Wisconsin and is the second largest county forest in the state. We used a comparative modeling approach to evaluate distribution and habitat relationships of red-shouldered hawk nest sites in relation to a suite of environmental variables in MCF. Models were used to develop forest management recommendations for red-shouldered hawks in Wisconsin. During the spring of 2006 and 2007, we broadcasted conspecific calls to survey 1121 calling stations along forest roads and trails. We located 20 and 25 active nesting territories in 2006 and 2007, respectively (11 of which were active in both years). To understand nest site selection, we measured 22 habitat variables within 0.04-ha plots at active nest sites (n = 34) and at stratified random sites (n = 61). Logistic regression with information-theoretic model selection identified a model including greater tree species richness and closeness to forested wetland as the best-approximating model. Variable selection with Discriminant Function Analysis (DFA) indicated that nest selection was best explained by greater number of tree species, closer distance to forested wetlands, greater volume of downed woody debris, fewer small sawlogs, and increased proximity to streams. Univariate comparisons identified four of the five aforementioned variables in the DFA model as significant. Red-shouldered hawks are likely more common in Wisconsin than their state status suggests. Forest management for red-shouldered hawk nest sites should focus on increasing tree species richness, increasing down woody debris volume, and protecting forested wetlands. These recommendations may assist property managers to locate and plan for continued persistence of this species on MCF.     

Effects of grassland abandonment, restoration and management on butterflies and vascular plants

When semi-natural pastures are abandoned, specialized grassland species are lost as a consequence of succession. As a counter-measure, previously abandoned grasslands may be restored by clearing shrubs and trees and re-introducing grazing livestock. In order to examine the effects of this type of habitat restoration, we compared species richness of plants and of specialized plants thought to be dependent on continuous management and species richness and abundance of butterflies and red-listed butterflies in 12 sets of matched continuously managed, abandoned and restored grassland in southern Sweden. We found no differences in species richness or abundance between the three grassland types. There were, however, some negative effects of abandonment. The number of management-dependent plants decreased with increasing cover of trees and shrubs, and in restored sites species richness of all groups decreased with increasing cover of trees and shrubs before restoration. Also the present management significantly affected both butterflies and plants. Species richness of both groups increased with increasing vegetation height and differed between sites depending on the species of grazers, with negative effects of sheep compared to cattle or horses. Our study indicates that for grassland management to be efficient, the restoration actions should mainly be directed towards sites where the post-abandonment succession has not proceeded too far.     

Small isolated aspen stands enrich bird communities in southwestern ponderosa pine forests

Small aspen stands are disappearing from the landscape in the Southwest, so it is important to understand their contribution to the avian community. We sampled birds in 53 small, isolated aspen stands and 53 paired plots within the ponderosa pine forest in northern Arizona, during the 1996 and 1997 breeding seasons. Bird species richness and abundance were higher in aspen than in pine. However, bird species richness and abundance did not vary with size of the aspen patch or isolation index. In addition, direct ordination of species distributions with habitat factors suggested no distinct avian communities. This suggests that aspen stands do not harbor separate populations, but rather are locations where the regional avifauna reaches high local density and richness and may be crucial to birds in years of resource scarcity. Thus it is important for avian conservation to maintain many aspen stands across the landscape, encompassing a diversity of vegetation structure and composition.     

The effects of extreme forest fragmentation on the bird community of Singapore Island

Singapore Island suffered one of the highest known deforestation rates in the tropics from the mid-to-late 19th century when over 95% of its native lowland forest was cleared. We compared the current bird community structure and composition among three habitat types, i.e., old (>50 years, 7-935 ha) and young (?50 years, 29-49 ha) naturally regenerating secondary forests and abandoned wooded plantations (27-102 ha) dominated by exotic species. Forest patch area had the strongest influence on the current species richness. The overall bird richness was not greater in most mature forest patches, but 20 species were only found in the old secondary forests and five of these were found in <50% of these patches. The rapid decrease in the number of forest species in plantations was offset by an increase in the number of open habitat species. Comparisons with current bird communities in nearby mainland forest sites (Peninsular Malaysia) suggest that the forest avifauna of Singapore is depauperate. The preservation of larger mature and maturing forests is therefore required for conserving the extant forest avifauna in Singapore. Connecting isolated patches can also be envisioned to facilitate movements of forest birds that have low densities and restricted distribution.     

Habitat use and spatial dynamics of takhi introduced to Hustai National Park, Mongolia

The successful introduction of captive bred takhi or Przewalski’s horse, Equus ferus przewalskii, into Mongolia in the 1990s is a good example of the benefits of ex situ conservation and one of the few examples of the recovery of an animal after it became extinct in the wild. This is also particularly interesting because virtually nothing was known about how takhi lived before they died out, and the introductions have enabled us to study how they have settled, and their ecology and behaviour within their former natural range. In this paper, we describe the movement, home range size and shape, and habitat use of takhi at one of the release areas, the 570 km² Hustai National Park in Mongolia. Harem home ranges varied between 129 and 2399 ha, with 80% core areas of between 61 and 1196 ha. There was no relationship between range size and harem size, or length of time since release. Initially, harems stayed near their release enclosures, but over time they established home ranges further away. There was little overlap between home ranges of different harems, but neither was there evidence of exclusive range use. The more nutritious vegetation at lower elevations was preferentially selected. Thus the present situation looks good, but, as the population continues to grow, we anticipate that there will be potential problems related to intraspecific competition for water and vegetation resources, and the potential for hybridisation with domestic horses belonging to the local people. We consider the time it may take for takhi to reach carrying capacity within Hustai National Park and emphasise that continual monitoring of the population is essential because interventional management is likely to be required in the future.     

Rescue of the Seychelles warbler on Cousin Island, Seychelles: the role of habitat restoration

Management policies to save threatened species are not always successful, often due to the lack of a scientific basis and evaluation of the species response. We describe the ecological studies and the conservation actions taken between 1985 and 1992 on Cousin Island (29 ha, Seychelles) to safeguard the future of the highly threatened Seychelles warbler (Acrocephalus sechellensis), which until 1988 only occurred on this island. A detailed field study was designed to (1) identify the key processes influencing warbler demography, (2) identify appropriate management techniques to increase the warbler population, and (3), assess the influence of the resulting habitat management. Since 1980 the island has been saturated with c. 115 territories and c. 320 birds. The warbler is purely insectivorous. Morinda (Morinda citrifolia), the most insect rich tree, is preferred for foraging. The higher the insect abundance (and Morinda cover) in territories the higher the reproductive success and survival of warblers. Insect numbers were highest in the central part of Cousin and decreased towards the coast. Coastal territories protected by a salt tolerant hedge of Scaevola (Scaevola taccada) had more insects and higher reproductive success than unprotected territories. Between 1990 and 1992 Morinda trees were planted on the island and Scaevola along the coast. Although these habitat restoration measures have not resulted in higher numbers of adult warblers and territories due to habitat saturation, they have been successful in terms of improving the quality of existing breeding territories and with that the reproductive success of breeding birds (including the number of territories producing recruits), and the exchange of individuals (genetic material) between territories. We provide evidence that the high reproductive potential of this species is likely to improve the resilience of the species to catastrophic events.     

Locating and protecting critical reserve sites to minimize expected and worst-case losses

There has been much recent interest in the development of systematic reserve selection methods that are capable of incorporating uncertainty associated with site destruction. This paper makes a contribution to this line of research by presenting two different optimization models for minimizing species losses within a planning region. Given limited acquisition budgets, the first minimizes expected species losses over all possible site loss patterns outside the reserve network while the second minimizes maximum species losses following the worst-case loss of a restricted subset of nonreserve sites. By incorporating the uncertainty of site destruction directly into the decision planning process, these models allow a conservation planner to take a less defensive and more strategic view of reserve selection that seeks to minimize species losses through the targeted acquisition of high-value/high-risk sites. We compare both of these methods to a more standard approach, which simply maximizes within reserve representation without regard for the varied level of threat faced by different sites and species. Results on a realistic dataset show that significant reductions in species losses can be achieved using either of these more intelligent modeling frameworks.