Effects of landscape and patch-level attributes on regional population persistence

Species responses are influenced by processes operating at multiple scales, yet many conservation studies and management actions are focused on a single scale. Although landscape-level habitat conditions (i.e., habitat amount, fragmentation and landscape quality) are likely to drive the regional persistence of spatially structured populations, patch-level factors (i.e., patch size, isolation, and quality) may also be important. To determine the spatial scales at which habitat factors influence the regional persistence of endangered Ord's kangaroo rats (Dipodomys ordii) in Alberta, Canada, we simulated population dynamics under a range of habitat conditions. Using a spatially-explicit population model, we removed groups of habitat patches based on their characteristics and measured the resulting time to extinction. We used proportional hazards models to rank the influence of landscape and interacting patch-level variables. Landscape quality was the most influential variable followed by patch quality, with both outweighing landscape- and patch-level measures of habitat quantity and fragmentation/proximity. Although habitat conservation and restoration priorities for this population should be in maximizing the overall quality of the landscape, population persistence depends on how this goal is achieved. Patch quality exerted a significant influence on regional persistence, with the removal of low quality road margin patches (sinks) reducing the risk of regional extinction. Strategies for maximizing overall landscape quality that omit patch-level considerations may produce suboptimal or detrimental results for regional population persistence, particularly where complex local population dynamics (e.g., source-sink dynamics) exist. This study contributes to a growing body literature that suggests that the prediction of species responses and future conservation actions may best be assessed with a multi-scale approach that considers habitat quality and that the success of conservation actions may depend on assessing the influences of habitat factors at multiple scales.     

At what spatial scales does resource selection vary? A case study of koalas in a semi‐arid region

The conservation of any species requires understanding and predicting the distribution of its habitat and resource use, including the effects of scale‐dependent variation in habitat and resource quality. Consequently, testing for resource selection at the appropriate scales is critical. We investigated how the resource selection process varies across scales, using koalas in a semi‐arid landscape of eastern Australia as a case study. We asked: at what scales does tree selection by koalas vary across regions? We tested the importance of the variation of our ecological predictors at the following scales: (i) the site‐scale (a stand of trees representing an individual koala's perception of local habitat); (ii) the landscape‐scale (10 × 10 km area representing a space within which a population of koalas exists); and (iii) a combination of these scales. We used a mixed‐modelling approach to quantify variation in selection of individual trees by koalas among sites and landscapes within a 1600 km² study area. We found that tree species, and tree height, were the most important factors influencing tree selection, and that their effect did not vary across scales. In contrast, preferences for trees of different condition, which is the state of tree canopy health, did vary across landscapes, indicating spatial variation in the selection of trees with respect to tree condition at the landscape‐scale, but not at the site‐scale. We conclude that resource selection processes can depend on the quality of those resources at different scales and their heterogeneous nature across landscapes, highlighting the consequence of scale‐dependent ecological processes. Designing studies that capture the heterogeneity in habitat and resources used by species that have an extensive distribution is an important prerequisite for effective conservation planning and management.     

Landscape structure and management regime as indicators of calcareous grassland habitat condition and species diversity

This study investigates the importance of spatial landscape characteristics and habitat management on the condition of calcareous grassland in the North Down Natural Area, Kent UK. We used a digitised map of the study area containing shapefiles of all the habitats including 82 patches of calcareous grassland together with management information for each patch and data on the presence and abundance of a range of calcareous grassland indicator plant species. We defined habitat condition by presence of indicator species and used classification trees to generate models with rules for predicting habitat condition from the landscape spatial characteristics and management information. We also applied the same method to investigate the factors affecting presence or diversity of three ecological groups of positive indicator species and dominance of a negative indicator species. All the models except one showed good classification accuracy and high kappa statistic. Favourable habitat condition was predicted by presence of different types of grazing management, presence of woodland around patches of calcareous grassland and shape complexity. These results indicate that calcareous grassland in favourable condition is management-dependent but also located in less intensively managed landscapes. Unfavourable habitat condition was predicted by threat factors such as lack of management and high incidence of arable or improved grassland around patches of calcareous grassland, indicating nutrient enrichment and habitat degradation. Some of these factors also predicted high diversity of the different ecological species groups. The value of this method for predicting habitat condition and species diversity from baseline ecological data for conservation monitoring at the landscape level is emphasised.     

Habitats and Resources: The Need for a Resource-based Definition to Conserve Butterflies

Current definitions of habitat are closely allied to the concept of patch and matrix. This concept is, for instance, central to the prevailing metapopulation models of population dynamics. But, butterfly population dynamics, mobility and spatial structure can only properly be understood in the context of a resource-based definition of habitats. In criticising current definitions of habitat, we illustrate how habitat is best understood in terms of resource distributions. These transcend vegetation-based definitions of habitat and lie at the root of life history strategies, the vulnerability of butterflies to environmental changes and extinction, and govern conservation status. We emphasise the need for a resource-use database and demonstrate the shortcomings of current data for conserving butterflies; patch based definitions of habitats are inappropriate for some species and for others do not provide a universal panacea, inadequately explaining spatial occurrence when scaled over space and time. A resource-based habitat definition challenges the bipolar, patch vs. matrix view of landscape; the alternative is to view landscape as a continuum of overlapping resource distributions. We urge greater attention to the details of butterfly behaviour and resource use as the keys to understanding how landscape is exploited and therefore to successful conservation at the landscape scale.     

Landscape genetic connectivity in European wildcat (Felis silvestris silvestris): a matter of food,shelters and demographic status of populations

Understanding landscape impacts on gene flow is necessary to plan comprehensive management and conservation strategies of both the species of interest and its habitat. Nevertheless, only a few studies have focused on the landscape genetic connectivity of the European wildcat, an umbrella species whose conservation allows the preservation of numerous other species and habitat types. We applied population and landscape genetics approaches, using genotypes at 30 microsatellites from 232 genetically-identified wildcats to determine if, and how, landscape impacted gene flow throughout France. Analyses were performed independently within two population patches: the historical north-eastern patch and the central patch considered as the colonization front. Our results showed that gene flow occurred at large spatial scales but also revealed significant spatial genetic structures within population patches. In both population patches, arable areas, pastures and permanent grasslands and lowly fragmented forested areas were permeable to gene flow, suggesting that shelters and dietary resources are among the most important parameters for French wildcat landscape connectivity, while distance to forest had no detectable effect. Anthropized areas appeared highly resistant in the north-eastern patch but highly permeable in the central patch, suggesting that different behaviours can be observed according to the demographic context in which populations are found. In line with this hypothesis, spatial distribution of genetic variability seemed uneven in the north-eastern patch and more clinal in the central patch. Overall, our results highlighted that European wildcat might be a habitat generalist species and also the importance of performing spatial replication in landscape genetics studies.

     

An Examination of Scale-Dependent Resource Use by Eastern Hognose Snakes in Southcentral New Hampshire

ABSTRACT The decline of many snake populations is attributable to habitat loss, and knowledge of habitat use is critical to their conservation. Resource characteristics (e.g., relative availability of different habitat types, soils, and slopes) within a landscape are scale-dependent and may not be equal across multiple spatial scales. Thus, it is important to identify the relevant spatial scales at which resource selection occurs. We conducted a radiotelemetry study of eastern hognose snake (Heterodon platirhinos) home range size and resource use at different hierarchical spatial scales. We present the results for 8 snakes radiotracked during a 2-year study at New Boston Air Force Station (NBAFS) in southern New Hampshire, USA, where the species is listed by the state as endangered. Mean home range size (minimum convex polygon) at NBAFS (51.7 ± 14.7 ha) was similar to that reported in other parts of the species’ range. Radiotracked snakes exhibited different patterns of resource use at different spatial scales. At the landscape scale (selection of locations within the landscape), snakes overutilized old-field and forest edge habitats and underutilized forested habitats and wetlands relative to availability. At this scale, snakes also overutilized areas containing sandy loam soils and areas with lower slope (mean slope = 5.2% at snake locations vs. 6.7% at random locations). We failed to detect some of these patterns of resource use at the home range scale (i.e., within the home range). Our ability to detect resource selection by the snakes only at the landscape scale is likely the result of greater heterogeneity in macrohabitat features at the broader landscape scale. From a management perspective, future studies of habitat selection for rare species should include measurement of available habitat at spatial scales larger than the home range. We suggest that the maintenance of open early successional habitats as a component of forested landscapes will be critical for the persistence of eastern hognose snake populations in the northeastern United States.     

Broad‐scale patterns in plant diversity vary between land uses in a variegated temperate Australian agricultural landscape

Plant diversity is threatened in many agricultural landscapes. Our understanding of patterns of plant diversity in these landscapes is mainly based on small‐scale (<1000 m²) observations of species richness. However, such observations are insufficient for detecting the spatial heterogeneity of vegetation composition. In a case‐study farm on the North‐West Slopes of New South Wales, Australia, we observed species richness at four scales (quadrat, patch, land use and landscape) across five land uses (grazed and ungrazed woodlands, native pastures, roadsides and crops). We applied two landscape ecological models to assess the contribution of these land uses to landscape species richness: (i) additive partitioning of diversity at multiple spatial scales, and (ii) a measure of habitat specificity – the effective number of species that a patch contributes to landscape species richness. Native pastures had less variation between patches than grazed and ungrazed woodlands, and hence were less species‐rich at the landscape scale, despite having similar richness to woodlands at the quadrat and patch scale. Habitat specificity was significantly higher for ungrazed woodland patches than all other land uses. Our results showed that in this landscape, ungrazed woodland patches had a higher contribution than the grazed land uses to landscape species richness. These results have implications for the conservation management of this landscape, and highlighted the need for greater consensus on the influence of different land uses on landscape patterns of plant diversity.     

How Do Landscape Structure,Management and Habitat Quality Drive the Colonization of Habitat Patches by the Dryad Butterfly (Lepidoptera: Satyrinae) in Fragmented Grassland?

Most studies dealing with species distribution patterns on fragmented landscapes focus on the characteristics of habitat patches that influence local occurrence and abundance, but they tend to neglect the question of what drives colonization of previously unoccupied patches. In a study of the dryad butterfly, we combined classical approaches derived from metapopulation theory and landscape ecology to investigate the factors driving colonization from a recent refugium. In three consecutive transect surveys, we recorded the presence and numbers of imagos in 27 patches of xerothermic grassland and 26 patches of wet meadow. Among the predictors affecting the occurrence and abundance of the dryad, we considered environmental variables reflecting (i) habitat patch quality (e.g., goldenrod cover, shrub density, vegetation height); (ii) factors associated with habitat spatial structure (patch size, patch isolation and fragmentation); and (iii) features of patch surroundings (100-m buffers around patches) that potentially pose barriers or provide corridors. Patch colonization by the dryad was strongly limited by the distance from the species refugium in the region; there was a slight positive effect of shrub density in this respect. Butterfly abundance increased in smaller and more fragmented habitat patches; it was negatively impacted by invasive goldenrod cover, and positively influenced by the density of watercourses in patch surroundings. Nectar plant availability was positively related to species abundance in xerothermic grassland, while in wet meadow the effect was the reverse. We conclude that dryad colonization of our study area is very recent, since the most important factor limiting colonization was distance from the refugium, while the habitat quality of target patches had less relevance. In order to preserve the species, conservation managers should focus on enhancing the quality of large patches and should also direct their efforts on smaller and more fragmented ones, including those with relatively low resource availability, because such habitat fragments have an important role to play for specialist species.     

Thyme and isolation for the Sinai baton blue butterfly (<Emphasis Type="Italic">Pseudophilotes sinaicus</Emphasis>)

The distribution of the narrowly endemic butterfly Pseudophilotes sinaicus (Lycaenidae) was studied. Potential habitat within its range was first located and then the quality of that habitat assessed. Degree of shelter, diversity of plant species, and resource area of an individual food plant (Thymus decussatus) all affected habitat quality and together were used to develop an index of habitat suitability applicable to each site. The butterfly's distribution was then studied within the identified network of suitable habitat patches: isolated patches with a small resource area were least likely to contain butterflies. Population size in a patch (as opposed merely to patch occupancy) was affected by resource area and the quality of habitat within that patch. Metapopulation processes and variation in habitat quality therefore appear to combine to describe the distribution of patches occupied by P. sinaicus and their population sizes. This finding provides insights into some of the processes operating on an endemic species throughout its geographical range and has important implications for the conservation of this rare butterfly.     

Determinants of distribution and abundance in the clouded apollo butterfly: a landscape ecological approach

Recent studies on the determinants of distribution and abundance of animals at landscape level have emphasized the usefulness of the metapopulation approach, in which patch area and habitat connectivity have often proved to explain satisfactorily existing patch occupancy patterns. A different approach is needed to study the common situation in which suitable habitat is difficult to determine or does not occur in well‐defined habitat patches. We applied a landscape ecological approach to study the determinants of distribution and abundance of the threatened clouded apollo Parnassius mnemosyne butterfly within an area of 6 km² of agricultural landscape in south‐western Finland. The relative role of 24 environmental variables potentially affecting the distribution and abundance of the butterfly was studied using a spatial grid system with 2408 grid squares of 0.25 ha, of which 349 were occupied by the clouded apollo. Both the probability of butterfly presence and abundance in a 0.25 ha square increased with the presence of the larval host plant Corydalis solida the cover of semi‐natural grassland, the amount of solar radiation and spalial autocorrelation in butterfly occurrence. Additionally, butterfly abundance increased with overall mean patch size and decreased with maximum slope angle and wind speed. Two advantages of the employment of a spatial grid system included the avoidance of a subjective definition of suitable habitat patches and an evaluation of the relative significance of different components of habitat quality at the same time with habitat availability and connectivity. The large variation in habitat quality was influenced by the abundance of the larval host plant and adult nectar sources but also by climatological. topographical and structural factors. The application of a spatial grid system as used here has potential for a wide use in studies on landscape‐level distribution and abundance patterns in species with complex habitat requirements and habitat availability patterns.     

Extinction-colonization dynamics and host-plant choice in butterfly metapopulations

Species living in highly fragmented landscapes often occur as metapopulations with frequent population turnover. Turnover rate is known to depend on ecological factors, such as population size and connectivity, but it may also be influenced by the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host-plant species that vary in their relative abundances among distinct habitat patches (dry meadows) in a large network of approximately 1,700 patches. We found no effect of host species use on local extinction. In contrast, population establishment was strongly influenced by the match between the host species composition of an empty habitat patch and the relative host use by larvae in previous years in the habitat patches that were well connected to the target patch. This "colonization effect" could be due to spatially variable plant acceptability or resistance or to spatially variable insect oviposition preference or larval performance. We show that spatial variation in adult oviposition preference occurs at the relevant spatial scale and that the other possible causes of the colonization effect can be discounted. We conclude that the colonization effect is generated by host preference influencing the movement patterns of ovipositing females. Migrant females with dissimilar host preferences have different perceptions of relative patch quality, which influences their likelihood of colonizing patches with particular host composition.     

Biodiversity and Conservation of Diptera in Heterogeneous Land Mosaics: A Fly's Eye View

Classical landscape ecology views spatial heterogeneity of habitats at relatively large 'human scales', and it is at such scales that most decisions of land management and nature conservation are made. The present paper makes use of a wider range of spatial scales to examine land mosaics from the 'fly point of view'. Taking examples from the Diptera faunas of mountainous land mosaics, it is demonstrated that: (i) large scale, 'patch content' landscape management has a direct bearing on Diptera community structure, (ii) borders between large scale patches are not necessarily perceived by flies (or other insects) in the same way as we perceive them, (iii) border complexity between patches at any scale may be as important as patch content as an axis of habitat definition. In this sense, 'border' is not to be confused with 'edge effects'. It is concluded that attention to both patch content and patch border complexity of land mosaics, viewed at the relevant spatial scales, is necessary for future successful conservation of Diptera biodiversity and for the efficient use of these insects in environmental assessment studies.     

Remote sensing‐based landscape indicators for the evaluation of threatened‐bird habitats in a tropical forest

Avian species persistence in a forest patch is strongly related to the degree of isolation and size of a forest patch and the vegetation structure within a patch and its matrix are important predictors of bird habitat suitability. A combination of space‐borne optical (Landsat), ALOS‐PALSAR (radar), and airborne Light Detection and Ranging (LiDAR) data was used for assessing variation in forest structure across forest patches that had undergone different levels of forest degradation in a logged forest—agricultural landscape in Southern Laos. The efficacy of different remote sensing (RS) data sources in distinguishing forest patches that had different seizes, configurations, and vegetation structure was examined. These data were found to be sensitive to the varying levels of degradation of the different patch categories. Additionally, the role of local scale forest structure variables (characterized using the different RS data and patch area) and landscape variables (characterized by distance from different forest patches) in influencing habitat preferences of International Union for Conservation of Nature (IUCN) Red listed birds found in the study area was examined. A machine learning algorithm, MaxEnt, was used in conjunction with these data and field collected geographical locations of the avian species to identify the factors influencing habitat preference of the different bird species and their suitable habitats. Results show that distance from different forest patches played a more important role in influencing habitat suitability for the different avian species than local scale factors related to vegetation structure and health. In addition to distance from forest patches, LiDAR‐derived forest structure and Landsat‐derived spectral variables were important determinants of avian habitat preference. The models derived using MaxEnt were used to create an overall habitat suitability map (HSM) which mapped the most suitable habitat patches for sustaining all the avian species. This work also provides insight that retention of forest patches, including degraded and isolated forest patches in addition to large contiguous forest patches, can facilitate bird species retention within tropical agricultural landscapes. It also demonstrates the effective use of RS data in distinguishing between forests that have undergone varying levels of degradation and identifying the habitat preferences of different bird species. Practical conservation management planning endeavors can use such data for both landscape scale monitoring and habitat mapping.     

Factors at multiple scales affecting distribution patterns and their implications for animal conservation – Leadbeater's Possum as a case study

Detailed field and modelling studies have been completed at different spatial scales for the endangered arboreal marsupial, Leadbeater's Possum (Gymnobelideus leadbeateri); a species virtually confined to the ash-type eucalypt forests in the Central Highlands of Victoria, south-eastern Australia. These forests are also subject to considerable pressure to produce timber and paper products. The studies of Leadbeater's Possum highlighted the factors influencing the distribution and abundance of the species ranging from broad distribution patterns, the occupancy of habitat patches at the landscape scale, and the use of individual den sites and the quality of trees that provide food within particular stands. These scales correspond to the entire known range of the species, sub-populations within a metapopulation occupying an ensemble of patches at the landscape scale, and colonies occupying den trees in individual stands. Information on the factors influencing the distribution of Leadbeater's Possum at one spatial scale were found to be important for informing processes at another. For example, an understanding of the species habitat requirements informed the spatial distribution of habitat patches at the landscape level which, in turn, gave insights into predicted metapopulation dynamics. Each spatial scale of ecological investigation of Leadbeater's Possum had a corresponding scale of forest management, and they included: virtually the entire ash-type eucalypt resource in the region, the location of large reserves, the design of corridors and retained patches within production forest landscapes, silvicultural practices employed in individual stands, and the retention of individual trees and clusters of trees on logged sites. This range of forest management scales, and the corresponding-scaled array of ecological studies, have particular implications for the conservation of Leadbeater's Possum. The results of investigations to date have revealed that conservation efforts must focus on ash-type eucalypt forests in the Central Highlands of Victoria because populations of Leadbeater's Possum are virtually confined to these areas. Within the broadscale limits of the distribution of the species, the long-term conservation of Leadbeater's Possum will not only be dependent on large reserves, but also intermediate-scale reserves and corridors within wood production areas, as well changed silvicultural practices to better provide suitable habitat on logged sites. While each strategy will make a contribution to the persistence of Leadbeater's Possum, it also has important limitations; an outcome which highlights the need for a multi-scale approach to attempts to conserve the species. Notably, the overaching implications for the design of an array of different conservation strategies for Leadbeater's Possum could not have been derived from any single study at a particular spatial scale. Thus, the case study on Leadbeater's Possum suggests that consideration of factors at a range of scales should be an integral part of management and conservation planning, not only for this species, but for wildlife per se.     

Ecological metrics predict connectivity better than geographic distance

We use microsatellite loci to examine genetic structure of the Florida scrub lizard (Sceloporus woodi) and test for the effects of landscape variables at the scale of neighboring patches. We evaluate ecological metrics of connectivity with genetics data, which to our knowledge is the first application of these particular metrics to landscape-level genetics studies in Florida scrub. Florida scrub is a highly threatened ecosystem in which habitat patches are remnants of a previously widespread xeric landscape. Analysis of mitochondrial DNA (mtDNA) has shown that landscape structure influenced the evolutionary history of the Florida scrub lizard (S. woodi) across its range. Our results concur with these mtDNA studies in documenting divergence between xeric ridge systems and also demonstrate divergence at very local scales. Both least-cost distance and pairwise isolation (a metric used in ecological studies that includes patch size, quality and a modified isolation index) were better predictors of genetic distance than Euclidean distance, indicating that mesic and hydric habitat influence spatial patterns in genetic variation. Our results support the need for focusing on spatial distribution of scrub habitat at the scale of neighboring patches, as well as regionally, in conservation management and restoration. Also, our study points to the value of integrating landscape ecology metrics into landscape genetics.     

Effects of spatial heterogeneity on butterfly species richness in Rocky Mountain National Park, CO, USA

We investigated butterfly responses to plot-level characteristics (plant species richness, vegetation height, and range in NDVI [normalized difference vegetation index]) and spatial heterogeneity in topography and landscape patterns (composition and configuration) at multiple spatial scales. Stratified random sampling was used to collect data on butterfly species richness from seventy-six 20 × 50 m plots. The plant species richness and average vegetation height data were collected from 76 modified-Whittaker plots overlaid on 76 butterfly plots. Spatial heterogeneity around sample plots was quantified by measuring topographic variables and landscape metrics at eight spatial extents (radii of 300, 600 to 2,400 m). The number of butterfly species recorded was strongly positively correlated with plant species richness, proportion of shrubland and mean patch size of shrubland. Patterns in butterfly species richness were negatively correlated with other variables including mean patch size, average vegetation height, elevation, and range in NDVI. The best predictive model selected using Akaike’s Information Criterion corrected for small sample size (AIC_c), explained 62% of the variation in butterfly species richness at the 2,100 m spatial extent. Average vegetation height and mean patch size were among the best predictors of butterfly species richness. The models that included plot-level information and topographic variables explained relatively less variation in butterfly species richness, and were improved significantly after including landscape metrics. Our results suggest that spatial heterogeneity greatly influences patterns in butterfly species richness, and that it should be explicitly considered in conservation and management actions.     

Host–parasitoid dynamics in a fragmented landscape: Holly trees,holly leaf miners and their parasitoids

Many species inhabit fragmented landscapes, where units of resource have a patchy spatial distribution. While numerous studies have investigated how the incidence and dynamics of individual species are affected by the spatial configuration and landscape context of habitat patches, fewer studies have investigated the dynamics of multiple interacting resource and consumer species in patchy landscapes. We describe a model system for investigating host–parasitoid dynamics in a patchy landscape: a network of 166 holly trees, a specialised herbivore of holly (the leaf miner, Phytomyza ilicis (Curtis, 1948)), and its suite of parasitoids. We documented patch occupancy by P. ilicis, its density within patches, and levels of parasitism over a 6-year period, and manipulated patch occupancy by creating artificially vacant habitat patches. Essentially all patches were occupied by the herbivore in each year, suggesting that metapopulation dynamics are unlikely to occur in this system. The main determinants of densities for P. ilicis and its parasitoids were resource availability (patch size and host density, respectively). While P. ilicis is apparently not restricted by the spatial distribution of resources, densities of its parasitoids showed a weaker positive relationship with host density in more isolated patches. In patches where local extinctions were generated experimentally, P. ilicis densities and levels of parasitism recovered to pre-manipulation levels within a single generation. Furthermore, patch isolation did not significantly affect re-colonisation by hosts or parasitoids. Analysing the data at a variety of spatial scales indicates that the balance between local demography and dispersal may vary depending on the scale at which patches are defined. Taken together, our results suggest that the host and its parasitoids have dispersal abilities that exceed typical inter-patch distances. Patch dynamics are thus largely governed by dispersal rather than within-patch demography, although the role of demography is higher in larger patches.     

The influences of landscape structure on butterfly distribution and movement: a review

We review the literature on the influence of landscape structure on butterfly distribution and movement. We start by examining the definition of landscape commonly used in spatial ecology. Landscape-level processes are reviewed before focusing on the impact of the geometry and spatial arrangement of habitat patches on butterflies e.g. the nature of the matrix, patch size and shape, minimum area requirements, immigration and emigration, and temporal habitat dynamics. The role of landscape elements is reviewed in terms of corridors (and stepping-stones), barriers, nodes, environmental buffers, and prominent landmark features.     

Species‐habitat relationships and ecological correlates of butterfly abundance in a transformed tropical landscape

Tropical butterfly conservation strategies often focus on total and/or common species richness to assess the conservation value of a patch or habitat. However, such a strategy overlooks the unique dynamics of rare species. We evaluated the species‐habitat relationships of 209 common, intermediate, and rare butterfly species (including morphospecies) across four habitat types (mature, degraded, or fragmented forest, and urban parks) and two patch sizes (<400 ha, ≥400 ha) in Singapore. Common species richness was consistent across habitat types. Intermediate species richness declined by more than 50 percent in urban parks (relative to all forest habitats), and rare species richness was reduced by 50 percent in degraded and fragmented forest and by 90 percent in urban parks (relative to mature forest). Large patches had comparable overall richness to small patches, but they supported more rare species and three times as many habitat‐restricted species over a similar area. Importantly, a number of rare species were confined to single small patches. Mixed‐effects regression models were constructed to identify habitat and ecological/life history variables associated with butterfly abundance. These models revealed that species with greater habitat specialization, rare larval host plants, few larval host plant genera, and narrow global geographic ranges were more likely to be rare species. Overall, these results demonstrate that the richness of habitat‐restricted and rare species do not follow the same spatial distribution patterns as common species. Therefore, while conserving mature forests is key, effective butterfly conservation in a transformed landscape should take into account rare and habitat‐restricted species.     

Does variation in patch use among butterfly species contribute to nestedness at fine spatial scales?

A nested pattern occurs whenever the species observed in depauperate habitat patches are a subset of those found in more species‐rich patches. Ecologists have documented many instances of nestedness caused by population‐level processes such as colonization and extinction at biogeographic scales. However, few researchers have examined whether nestedness may exist at fine scales due to the ways in which individual organisms discriminate among potential habitat patches. In 1999, we experimentally fragmented an old‐field habitat into patches of varying size to test whether nestedness could exist on a fine spatial scale. Five treatments of differing patch size were replicated five times in a Latin square design by selectively mowing 15×15 m² plots within an old‐field (patch areas: 225, 180, 135, 90, and 45 m²). Specifically, we tested whether butterflies foraging within a network of patches differing in area conformed to a nested subset structure. We also classified species according to (1) their flight height while foraging (high or low), and (2) their adult habitat breadth (ubiquitous, general, or restricted) to determine whether nestedness could be explained by difference in species’ tendency to discriminate among patches differing in area.
We found significant evidence that a community of foraging Lepidoptera conformed to a nested subset structure based on the difference between the observed nestedness within the butterfly community and the nestedness obtained from randomly generated species presence/absence matrices. Poisson regression analyses demonstrated that high‐flying, habitat‐restricted species avoided the smallest patches (90 and 45 m²) in favor of larger remnants, whereas low‐flying, habitat generalists used all patch sizes. Thus, our study is one of the first to demonstrate that nestedness among species subsets can be observed at fine spatial scales (within a single 1.5 hectare field) and may be maintained by species behavioral differences: discriminating species (i.e. high‐flying, habitat restricted) avoided the smallest patches, and less discriminating species (i.e. low‐flying, ubiquitous) were distributed throughout the field without regard to patch size. Our results also suggest that nestedness should be viewed as yet another scalar pattern in ecology, generated by variation in patch use by individuals at fine‐scales as well as the more traditionally invoked processes of extinction and colonization of species at broad‐scales.