Keith S. Summerville

Partitioning Species Diversity across Landscapes and Regions: A Hierarchical Analysis of α, β, and γ Diversity

Species diversity may be additively partitioned within and among samples (α and β diversity) from hierarchically scaled studies to assess the proportion of the total diversity (γ) found in different habitats, landscapes, or regions. We developed a statistical approach for testing null hypotheses that observed partitions of species richness or diversity indices differed from those expected by chance, and we illustrate these tests using data from a hierarchical study of forest‐canopy beetles. Two null hypotheses were implemented using individual‐ and sample‐based randomization tests to generate null distributions for α and β components of diversity at multiple sampling scales. The two tests differed in their null distributions and power to detect statistically significant diversity components. Individual‐based randomization was more powerful at all hierarchical levels and was sensitive to departures between observed and null partitions due to intraspecific aggregation of individuals. Sample‐based randomization had less power but still may be useful for determining whether different habitats show a higher degree of differentiation in species diversity compared with random samples from the landscape. Null hypothesis tests provide a basis for inferences on partitions of species richness or diversity indices at multiple sampling levels, thereby increasing our understanding of how α and β diversity change across spatial scales.

Life-history traits predict species responses to habitat area and isolation: a cross-continental synthesis

There is a lack of quantitative syntheses of fragmentation effects across species and biogeographic regions, especially with respect to species life-history traits. We used data from 24 independent studies of butterflies and moths from a wide range of habitats and landscapes in Europe and North America to test whether traits associated with dispersal capacity, niche breadth and reproductive rate modify the effect of habitat fragmentation on species richness. Overall, species richness increased with habitat patch area and connectivity. Life-history traits improved the explanatory power of the statistical models considerably and modified the butterfly species-area relationship. Species with low mobility, a narrow feeding niche and low reproduction were most strongly affected by habitat loss. This demonstrates the importance of considering life-history traits in fragmentation studies and implies that both species richness and composition change in a predictable manner with habitat loss and fragmentation.

EFFECTS OF TIMBER HARVEST ON FOREST LEPIDOPTERA: COMMUNITY, GUILD, AND SPECIES RESPONSES

Two pressing questions for forestry and conservation biology are whether periodic logging in forest ecosystems significantly changes biodiversity and whether the changes can be mitigated through appropriate harvest methods. Such questions of timber resource management, however, are rarely applied to nonpest insect species, particularly in temperate forest systems. We studied the effects of timber harvest on species richness, abundance, and community composition of forest Lepidoptera (moths). Moths were sampled in 16 forest stands occurring in two watersheds (managed and wilderness) in southeastern Ohio during summer 2000. Stands were chosen from one of four management categories: clear-cut, selectively logged, unlogged, and wilderness. Specifically, we tested the following predictions: (1) shifts in moth community composition would be affected by postharvest changes in stand structure and floristic composition, (2) variation in species richness would be determined by the magnitude of the logging disturbance, and (3) unlogged stands within managed landscapes would contain different species assemblages compared to wilderness stands. Canonical correspondence analysis (CCA) revealed significant compositional differ- ences among moth communities sampled from forest stands differing in harvest regime. Variation among moth communities was mainly attributable to postharvest changes in stand floristic composition rather than stand structure. Postdisturbance shifts in moth community composition were related to the magnitude of displacement of a given forest stand to earlier successional stages. We also found that both species richness of the overall moth community and several feeding guilds were significantly lower in clear-cut stands, but species richness did not differ between selectively logged and unlogged stands. Thus, selective logging appears to be a better strategy for timber harvest when concern is for maintaining species richness of Lepidoptera within stands. Finally, although no differences were detected in overall species richness or abundance of moths sampled from stands in managed or wil- derness watersheds, the CCA suggested that the surrounding landscape influenced the var- iation in community composition within and among forest stands. Therefore, we suggest that the long-term maintenance of lepidopteran species diversity and community compo- sition within temperate deciduous forests managed for timber may ultimately depend on successful regeneration of harvested stands to precut floristic composition as well as the preservation of larger areas of unlogged forests.

EFFECTS OF EXPERIMENTAL HABITAT FRAGMENTATION ON PATCH USE BY BUTTERFLIES AND SKIPPERS (LEPIDOPTERA)

We conducted an experimental study to assess the effects of habitat frag- mentation on patch use by butterflies and skippers. Five fragmentation treatments (20%, 40%, 60%, 80%, and 100% habitat remaining) were replicated five times by selectively mowing 15 X 15 m patches of an old field. Community and species responses to frag- mentation were tested against several alternative predictions: (1) the proportional area hypothesis, in which species richness or abundance declines linearly with habitat area, (2) the patch heterogeneity hypothesis, in which richness or abundance peaks at intermediate levels of fragmentation, (3) the critical threshold hypothesis, in which a nonlinear decrease in richness or abundance occurs below some critical level of habitat loss, and (4) the patch quality hypothesis, in which declines in richness or abundance with increasing habitat loss are partially offset by the resource value of the remnant habitat. We found that fragmentation treatment significantly affected species richness and total number of patch visits for the overall community, as well as the numbers of patch visits by individual species. Community and species responses were linearly related to the amount of habitat remaining, as predicted by the proportional area model of fragmentation. We also found that plots with a higher cover of flowering forbs received greater patch use. Thus, the effects of habitat quantity (patch area) and habitat quality (flower cover) were inde- pendent and additive, suggesting that moderately sized patches of high resource quality may be equivalent to large patches of lower quality. Therefore, to maintain local population densities of Lepidoptera, it may be possible to compensate for losses in habitat area with improvements in habitat quality. Species did not uniformly respond to fragmentation. Rare species were disproportion- ately affected by fragmentation and did not occur in patches with <40% habitat remaining. Several common species appeared unaffected by fragmentation; however other common species were also significantly influenced by fragmentation treatment. These findings sup- port the notion that rare species may be especially vulnerable to fragmentation, but it could be quite difficult to predict a priori how more common species will respond to habitat loss.

Forest moth taxa as indicators of lepidopteran richness and habitat disturbance: a preliminary assessment

The conservation of hyperdiverse taxa such as arthropods may prove frustrating for land managers who lack the taxonomic expertise to perform species-level identifications. Therefore, interest has turned towards selecting indicator taxa that can be monitored as surrogates for detecting changes in entire communities. In this study, we examined whether several moth families or subfamilies were significant indicators of lepidopteran species richness and habitat disturbance in Eastern deciduous forests of North America. We screened moth families to identify assemblages that are easily sampled and primarily comprised of species that can be identified by non-experts. Using existing field data collected over 3 years, we tested four taxa that met our initial requirements with regard to their performance as an indicator of total observed lepidopteran species richness and levels of forest disturbance. We determined that the Arctiidae and Notodontidae hold promise as potential indicator taxa in Eastern deciduous forests. The Arctiidae functioned best as an indicator of moth species richness, while the Notodontidae performed best as a coarse-scale disturbance indicator. Additional research will be required, however, before species within either family have practical significance as tools for assessing forest condition.

Habitat specialization, body size, and family identity explain lepidopteran density–area relationships in a cross-continental comparison

Habitat fragmentation may strongly affect species density, species interactions, and the rate of ecosystem processes. It is therefore important to understand the observed variability among species responses to fragmentation and the underlying mechanisms. In this study, we compare density–area relationships (DARs) for 344 lepidopteran species belonging to 22 families (butterflies and moths). This analysis suggested that the DARslope is generally positive for moths and negative for butterflies. The differences are suggested to occur because moths are largely olfactory searchers, whereas most butterflies are visual searchers. The analysis also suggests that DARs vary as a function of habitat specialization and body size. In butterflies, generalist species had a more negative DARslope than specialist species because of a lower patch size threshold. In moths, the differences in DARslope between forest and open habitat species were large for small species but absent for large species. This difference is argued to occur because the DARslope in large species mainly reflects their search mode, which does not necessarily vary between moth groups, whereas the slope in small species reflects population growth rates.

Structure and conservation of lepidopteran communities in managed forests of northeastern North America: a review

We review the existing literature on patterns of moth (Lepidoptera) species richness and community composition in northeastern Nearctic forest ecosystems across hierarchical scales ranging from individual trees to entire managed ecoregions. Moths are species-rich in northeastern forests of North America, with the most diverse families being Noctuidae and Geometridae. Individual trees and forest stands, however, are often dominated by few species. Climate, stand age, disturbance regime, and landscape heterogeneity are significant predictors of abundance of dominant species. Most other moth species in the regional pool are patchily distributed and appear to occur regularly at very low abundance. Moth communities respond predictably to forest-management practices, and the outcomes of postmanagement response are largely driven by changes in the plant community. Significant reductions in moth species richness and changes in community composition are correlated with clear-cut harvesting, whereas selective logging appears to cause more moderate changes in moth community structure. Broad-scale effects of forest fragmentation on moth communities in unglaciated regions are best described by species replacement rather than species loss; moth species richness decreases slightly across a gradient of fragment sizes, but shifts in moth community composition are more important, especially in the relative importance of herbaceous-plant-feeding species in large and small fragments. Species that appear to be most sensitive to timber management or habitat loss are dietary specialists as larvae, dispersal-limited as adults, or dependent on commercially valuable tree species such as oaks, Quercus L. (Fagaceae). Restored forest stands tend to converge in terms of lepidopteran species dominance and diversity among stands, suggesting that the long-term consequences of timber management or habitat loss include a significant reduction of regional β-diversity. Finally, future research on forest Lepidoptera should include an emphasis on understanding the role of urban woodland habitat in retaining viable and diverse moth communities and how the spatial pattern of timber harvest affects the relative magnitude of α- and β-diversity components within a given ecoregion.

Community structure of arboreal caterpillars within and among four tree species of the eastern deciduous forest

Abstract.  1. A seasonally replicated experimental design was used to address the question of how differences within and among host tree species affect arboreal caterpillar communities.

Diversity of Lepidoptera in Ohio forests at local and regional scales : How heterogeneous is the fauna?

Abstract We examined the diversity of Lepidoptera in several Ohio natural areas at local and regional scales. First, we conducted a field study to assess effects of spatial and temporal extent on species accumulation in inventories of forest moth communities in the North Central Tillplain Ecoregion. We then compared the complementarity among moth communities found in nature reserves in two other Ohio ecoregions to the moth community described from the North Central Tillplain. As expected, more moth species were inventoried when multiple sites and multiple years were examined. Species accumulation curves did not saturate for any inventory, regardless of spatial or temporal extent. Within the North Central Tillplain, nature reserves of similar size and floristic composition supported equally rich moth communities but species turnover among reserves suggested that floristically similar habitats supported different species assemblages. Species turnover estimates were not significantly different between macro- and microlepidoptera; rarity was more important in determining community heterogeneity than coarse taxonomic affiliation. At a regional scale, nature reserves in different Ohio ecoregions had a species complementarity of 50–70%, indicating significant turnover of species across broad scales. This suggests that considerable heterogeneity exists among moth communities at both local and regional scales. Successful conservation of Lepidoptera in fragmented landscapes should strive to represent the natural heterogeneity of species richness at both local and regional scales.

Temporal Patterns of Species Accumulation in a Survey of Lepidoptera in a Beech-Maple Forest

One of the most significant challenges to insect conservation is lack of information concerning species diversity and distribution. Because a complete inventory of all species in an area is virtually impossible, interest has turned to developing statistical techniques to guide sampling design and to estimate total species richness within a site. We used two such techniques, diversity partitioning and non-parametric richness estimation, to determine how variation in sampling effort over time affected species accumulation for a survey of Lepidoptera in an old-growth beech-maple forest. Temporal scaling of sampling effort had significant effects on two measures of species diversity. Increases in species richness were primarily driven by changes in species occurrences with season, while Shannon diversity was largely determined at the scale of individual sampling units (i.e. by spatial effects). Variation in sampling effort affected the values of the two most widely regarded richness estimators (ICE and Chao 2); neither diversity estimator achieved stable values across a range of sampling efforts. Even after 52 trap-nights and accounting for seasonality, rare species (singletons and uniques) remained a significant component of the moth community. To the extent that moth communities in other forest systems are similarly comprised of many rare species, non-parametric richness estimators should be expected to yield variable estimates with increased effort and should only be used to provide a minimum benchmark for predicting the number of species remaining to be sampled. Our results suggest the best strategy for a short-term survey of forest Lepidoptera should emphasize spreading sampling intervals throughout a given year rather than focusing on intensive sampling during a short time period or prolonged sampling over many years.