Jessica J. Hellmann

Counting the Uncountable: Statistical Approaches to Estimating Microbial Diversity

All biologists who sample natural communities are plagued with the problem of how well a sample reflects a communitys “true” diversity. New genetic techniques have revealed extensive microbial diversity that was previously undetected with culture-dependent methods and morphological

Climate change hastens population extinctions

Climate change is expected to alter the distribution and abundance of many species. Predictions of climate-induced population extinctions are supported by geographic range shifts that correspond to climatic warming, but few extinctions have been linked mechanistically to climate change. Here we show that extinctions of two populations of a checkerspot butterfly were hastened by increasing variability in precipitation, a phenomenon predicted by global climate models. We model checkerspot populations to show that changes in precipitation amplified population fluctuations, leading to rapid extinctions. As populations of checkerspots and other species become further isolated by habitat loss, climate change is likely to cause more extinctions, threatening both species diversity and critical ecosystem services.

BIAS, PRECISION, AND ACCURACY OF FOUR MEASURES OF SPECIES RICHNESS

Species richness is a widely used surrogate for the more complex concept of biological diversity. Because species richness is often central to ecological study and the establishment of conservation priorities, the biases and merits of richness measurements demand evaluation. The jackknife and bootstrap estimators can be used to compensate for the underestimation associated with simple richness estimation (or the sum of species counted in a sample). Using data from five forest communities, we analyzed the simple measure of richness, the first- and second-order jackknife, and the bootstrap estimators with simulation and resampling methods to examine the effects of sample size on estimator performance. Performance parameters examined were systematic under- or overestimation (bias), ability to estimate consistently (precision), and ability to estimate true species richness (accuracy). For small sample sizes in all studied communities (less than ∼25% of the total community), the least biased estimator was the ...

Translocation experiments with butterflies reveal limits to enhancement of poleward populations under climate change.

There is a pressing need to predict how species will change their geographic ranges under climate change. Projections typically assume that temperature is a primary fitness determinant and that populations near the poleward (and upward) range boundary are preadapted to warming. Thus, poleward, peripheral populations will increase with warming, and these increases facilitate poleward range expansions. We tested the assumption that poleward, peripheral populations are enhanced by warming using 2 butterflies (Erynnis propertius and Papilio zelicaon) that co-occur and have contrasting degrees of host specialization and interpopulation genetic differentiation. We performed a reciprocal translocation experiment between central and poleward, peripheral populations in the field and simulated a translocation experiment that included alternate host plants. We found that the performance of both central and peripheral populations of E. propertius were enhanced during the summer months by temperatures characteristic of the range center but that local adaptation of peripheral populations to winter conditions near the range edge could counteract that enhancement. Further, poleward range expansion in this species is prevented by a lack of host plants. In P. zelicaon, the fitness of central and peripheral populations decreased under extreme summer temperatures that occurred in the field at the range center. Performance in this species also was affected by an interaction of temperature and host plant such that host species strongly mediated the fitness of peripheral individuals under differing simulated temperatures. Altogether we have evidence that facilitation of poleward range shifts through enhancement of peripheral populations is unlikely in either study species.

The application of rarefaction techniques to molecular inventories of microbial diversity

With the growing capacity to inventory microbial community diversity, the need for statistical methods to compare community inventories is also growing. Several approaches have been proposed for comparing the diversity of microbial communities: some adapted from traditional ecology and others designed specifically for molecular inventories of microbes. Rarefaction is one statistical method that is commonly applied in microbial studies, and this chapter discusses the procedure and its advantages and disadvantages. Rarefaction compares observed taxon richness at a standardized sampling effort using confidence intervals. Special emphasis is placed here on the need for precise, rather than unbiased, estimation methods in microbial ecology, but precision can be judged only with a very large sample or with multiple samples drawn from a single community. With low sample sizes, rarefaction curves also have the potential to lead to incorrect rankings of relative species richness, but this chapter discusses a new method with the potential to address this problem. Finally, this chapter shows how rarefaction can be applied to the comparison of the taxonomic similarity of microbial communities.

The route to extinction: population dynamics of a threatened butterfly

Abstract. We compare results of field study and model analysis of two butterfly populations to evaluate the importance of alternative mechanisms causing changes in abundance. Although understanding and predicting population fluctuations is a central goal of population ecology, it is not often achieved because long-term abundance data are available for few populations in which mechanisms causing fluctuations also are known. Both kinds of information exist for two populations of the checkerspot butterfly, Euphydryas editha bayensis, which are matched in most ways except for habitat area and topography. We applied results from field study to make predictions about the dynamics of the two populations. Then we tested these predictions using nonlinear modeling of abundance data. Models included endogenous factors, exogenous effects of weather, or both. Results showed that the populations differed in variability and responses to endogenous and exogenous factors. The population in the more homogeneous habitat varied more widely, went extinct first, and fluctuated more severely with climate. Dynamics of the population occupying the topographically diverse habitat were more complex, containing damped oscillations and weaker influences of weather. We draw four main conclusions. First, the routes to extinction for E. e. bayensis populations in protected habitat were random walks driven by climatic variability. Climatic influences dominated both populations, but the timing and functional forms of climatic effects differed between populations. Second, topographic diversity reduced weather-induced population variability and increased persistence time. Third, one must explicitly consider both endogenous and exogenous components to fully understand population dynamics. Fourth, resolving the debate over population regulation requires integrating long-term population sampling, model analysis, and investigation of mechanisms in the field.

Population persistence in fragmented landscapes

Abstract Many ecologists believe that fragmenting habitats into discontinuous patches disrupts the reproduction, survivorship and movement of animals. It has seldom, however, been possible to measure all these processes in one study. Recent work by Lesley and Michael Brooker on an Australian songbird, the blue-breasted fairy wren Malurus pulcherrimus , has achieved this elusive goal. Their new paper in Wildlife Research demonstrates that reduced connectedness among habitat patches lowers population recruitment to below break-even levels.

Intra-individual variation allows an explicit test of the hygric hypothesis for discontinuous gas exchange in insects.

The hygric hypothesis postulates that insect discontinuous gas exchange cycles (DGCs) are an adaptation that reduces respiratory water loss (RWL), but evidence is lacking for reduction of water loss by insects expressing DGCs under normal ecological conditions. Larvae of Erynnis propertius (Lepidoptera: Hesperiidae) naturally switch between DGCs and continuous gas exchange (CGE), allowing flow-through respirometry comparisons of water loss between the two modes. Water loss was lower during DGCs than CGE, both between individuals using different patterns and within individuals using both patterns. The hygric cost of gas exchange (water loss associated with carbon dioxide release) and the contribution of respiratory to total water loss were lower during DGCs. Metabolic rate did not differ between DGCs and CGE. Thus, DGCs reduce RWL in E. propertius, which is consistent with the suggestion that water loss reduction could account for the evolutionary origin and/or maintenance of DGCs in insects.

Do hypotheses from short-term studies hold in the long-term? An empirical test

Abstract. 1.u2002A sequence of population estimates for two now‐extinct populations of Euphydryas editha bayensis is presented. After removing biased sampling days, estimates of demographic parameters from the long‐term data were used to test five hypotheses built from studies of shorter duration. Such tests of short‐term conclusions are rare.