Thomas O. Crist

Navigating the multiple meanings of β diversity: a roadmap for the practicing ecologist.

A recent increase in studies of β diversity has yielded a confusing array of concepts, measures and methods. Here, we provide a roadmap of the most widely used and ecologically relevant approaches for analysis through a series of mission statements. We distinguish two types of β diversity: directional turnover along a gradient vs. non-directional variation. Different measures emphasize different properties of ecological data. Such properties include the degree of emphasis on presence/absence vs. relative abundance information and the inclusion vs. exclusion of joint absences. Judicious use of multiple measures in concert can uncover the underlying nature of patterns in β diversity for a given dataset. A case study of Indonesian coral assemblages shows the utility of a multi-faceted approach. We advocate careful consideration of relevant questions, matched by appropriate analyses. The rigorous application of null models will also help to reveal potential processes driving observed patterns in β diversity.

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.

Animal movements and population dynamics in heterogeneous landscapes

Organisms respond to environmental heterogeneity at different scales and in different ways. These differences are consequences of how the movement characteristics of animals—their movement rates, directionality, turning frequencies, and turning angles—interact with patch and boundary features in landscape mosaics. The interactions of movement patterns with landscape features in turn produce spatial patterns in individual space-use, population dynamics and dispersion, gene flow, and the redistribution of nutrients and other materials. We describe several theoretical approaches for modeling the diffusion, foraging behavior, and population dynamics of animals in heterogeneous landscapes, including: (1) scaling relationships derived from percolation theory and fractal geometry, (2) extensions of traditional patch-based metapopulation models, and (3) individual-based, spatially explicit models governed by local rules. We conclude by emphasizing the need to couple theoretical models with empirical studies and the usefulness of ‘microlandscape’ investigations.

Disentangling the Drivers of β Diversity Along Latitudinal and Elevational Gradients

Increases in species turnover of woody plants at low latitudes and elevations are explained by the size of species pools alone. Understanding spatial variation in biodiversity along environmental gradients is a central theme in ecology. Differences in species compositional turnover among sites (β diversity) occurring along gradients are often used to infer variation in the processes structuring communities. Here, we show that sampling alone predicts changes in β diversity caused simply by changes in the sizes of species pools. For example, forest inventories sampled along latitudinal and elevational gradients show the well-documented pattern that β diversity is higher in the tropics and at low elevations. However, after correcting for variation in pooled species richness (γ diversity), these differences in β diversity disappear. Therefore, there is no need to invoke differences in the mechanisms of community assembly in temperate versus tropical systems to explain these global-scale patterns of β diversity.

The spread of invasive species and infectious disease as drivers of ecosystem change

Invasive species, disease vectors, and pathogens affect biodiversity, ecosystem function and services, and human health. Climate change, land use, and transport vectors interact in complex ways to determine the spread of native and non-native invasive species, pathogens, and their effects on ecosystem dynamics. Early detection and in-depth understanding of invasive species and infectious diseases will require an integrated network of research platforms and information exchange to identify hotspots of invasion or disease emergence. Partnerships with state and federal agencies that monitor the spread and impacts of invasive species and pathogens will be critical in developing a national data and research network that can facilitate a full understanding of the resulting effects on ecosystems and society. Citizen science can also play a role; individuals can report new invasions, record phenological changes associated with invasions or disease outbreaks, and can participate in efforts such as the Breeding Bir...

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.

Fractal Patterns of Insect Movement in Microlandscape Mosaics

How individuals move, whether in short-term searching behavior or long-term dispersal influences the probability that individuals will experience physiological stress or encounter appropriate habitat, potential mates, prey, or predators. Because of variety and complexity, it is often difficult to make sense of movements. Because the fractal dimension of a movement pathway is scale independent, however, it may provide a useful measure for comparing dissimilar taxa. The authors use fractal measures to compare the movement pathways of individual beetles occupying semiarid shortgrass steppe in north-central Colorado. 20 refs., 1 fig., 1 tab.

Harvester Ant Foraging and Shrub-Steppe Seeds: Interactions of Seed Resources and Seed Use

Granivore—seed interactions involve a feedback between granivore seed selectivity and seed availability. We examined this feedback to determine how seed preferences by the western harvester ant, Pogonomyrmex occidentalis, related to seed availability and, in turn, affected the soil seed pool. Preferences were estimated from natural diets as well as from experiments that controlled seed size, relative availability, and distance from ant nests. Seed availability to ants varied with season and over 2 yr. Colony activity and seed intake rates were correlated with seed availability. Seed preference by ants was correlated with the seasonal availability of preferred species, but not with unpreferred seeds. From the soil seed pool, ants preferentially harvested small, sound seeds. They removed 9—26% of the potentially viable seed pool each year, and as much as 100% of available preferred species. Seed densities were lower 2—7 m from nests, where foraging activity was concentrated, than 7—12 m from nests. In contr...

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.

The Impact of Fungi on Soil Seeds: Implications for Plants and Granivores in a Semiarid Shrub‐Steppe

Fungi are ubiquitous in soil and may affect seed survivorship directly by decomposition or pathogenesis, or indirectly through the effects of fungi on seed preference of granivores. We studied the role of fungi in the persistence of soil seeds in a shrub-steppe ecosystem using two experimental approaches. In the first, we examined survivorship of 900 seeds of five shrub-steppe species in nylon bags over a 1 0-mo period and determined viable seed losses to germination, decomposition, and pathogens. The proportion of seeds representing a viable seed bank decreased from 84.4% in the May pre-experimental con- ditions to 4.4% in March. Decomposition and attack by fungi were responsible for the greatest decrease from fall to winter (0% in August, 32.0% in November, 35.2% in March) while germination accounted for the greatest decrease from winter to spring (18.5% in August, 21.9% in November, 55.0% in March). Small seeds were more likely to be lost to decomposition and pathogens, whereas larger seeds had greater losses to germination. A subset of 80 experimental seeds was cultured for seed-infecting fungi. We found fungi in virtually all seeds; the most common fungal isolates were cosmopolitan soil fungi, such as species of Penicillium and Mucor. These species are known producers of extracellular enzymes and mycotoxins. Two sterile (nonfruiting) forms of fungi were also frequently isolated, as were two pathogenic species of Fusarium. In a second experiment we placed seeds in fungal cultures obtained from soil seeds and presented moldy seeds along with control seeds to a shrub-steppe granivore, the western harvester ant (Pogonomyrmex oc- cidentalis). Ants harvested control seeds at nearly twice the rate of moldy seeds, suggesting that they avoided seeds infected with fungi. In this way, fungi may indirectly affect seed mortality due to predation by granivores, in addition to the direct effects fungi can have on seed survivorship. We suggest that fungi may play an important role in soil seed dynamics, but one that is poorly understood.