Person:

Ellison, Aaron

Quantifying and assessing changes in biological diversity are central aspects of many ecological studies, yet accurate methods of estimating biological diversity from sampling data have been elusive. Hill numbers, or the effective number of species, are increasingly used to characterize the taxonomic, phylogenetic or functional diversity of an assemblage. However, empirical estimates of Hill numbers, including species richness, tend to be an increasing function of sampling effort and thus tend to increase with sample completeness. Integrated curves based on sampling theory that smoothly link rarefaction (interpolation) and prediction (extrapolation) standardize samples on the basis of sample size or sample completeness and facilitate the comparison of biodiversity data. Here we extend previous rarefaction and extrapolation models for species richness (Hill number (^q)D, where q = 0) to measures of taxon diversity incorporating relative abundance (i.e., for any Hill number (^q)D, q > 0) and present a unified approach for both individual-based (abundance) data and sample-based (incidence) data. Using this unified sampling framework, we derive both theoretical formulas and analytic estimators for seamless rarefaction and extrapolation based on Hill numbers. Detailed examples are provided for the first three Hill numbers: q = 0 (species richness), q = 1 (the exponential of Shannon's entropy index) and q = 2 (the inverse of Simpson's concentration index). We develop a bootstrap method for constructing confidence intervals around Hill numbers, facilitating the comparison of multiple assemblages of both rarefied and extrapolated samples. The proposed estimators are accurate for both rarefaction and short-range extrapolation. For long-range extrapolation, the performance of the estimators depends on both the value of q and on the extrapolation range. We tested our methods on simulated data generated from species abundance models and on data from large species inventories. We also illustrate the formulas and estimators using empirical datasets from biodiversity surveys of temperate forest spiders and tropical ants.

Foundation species have a major impact on biotic and abiotic processes and create a stable environment for many other species. Eastern hemlock (Tsuga canadensis), a foundation tree species native to North America, is currently declining due to infestation by an invasive insect, the hemlock woolly adelgid (Adelges tsugae). Loss of hemlock canopies can greatly alter the dark, cool, and damp microclimate of hemlock forests. We studied five years of microclimatic changes following logging or girdling (to simulate physical effects of adelgid) of hemlocks in a multi-hectare-scale experiment in a New England forest. Both logging and girdling of hemlocks caused large changes in light availability, air and soil temperature, and soil moisture. Even though the impact of logging was more rapid than the effect of gradual hemlock mortality after girdling, the microclimatic changes in these two canopy treatments converged over time. The microclimate in hardwood control plots, which represent the predicted forest composition 50 years after hemlock loss, was intermediate between the two canopy treatments and the hemlock control plots. Our fine-scale results were generally consistent with average microclimatic effects observed in comparative studies but revealed additional changes in variance and seasonal rhythms, and the importance of stochastic events such as ice storms. The variance in air temperature, but not in soil temperature, greatly increased after loss of hemlock.We also observed a striking saw-tooth pattern, consisting of a small peak before bud-break in temperature differentials between hemlock control and the two canopy treatments – likely due to the insulating hemlock canopy preventing snow from melting – followed by a larger difference in temperatures after bud-break. We expect the ongoing decline of eastern hemlock - due to both infestation and pre-emptive salvage logging - to greatly impact the microclimate of hemlock forests, as well as the many taxa that are associated with it.

Range expansion by native and exotic species will continue to be a major component of global change. Anticipating the potential effects of changes in species distributions requires models capable of forecasting population spread across realistic, heterogeneous landscapes and subject to spatiotemporal variability in habitat suitability. Several decades of theory and model development, as well as increased computing power and availability of fine-resolution GIS data, now make such models possible. Still unanswered, however, is the question of how well this new generation of dynamic models will anticipate range expansion. Here we develop a spatially explicit stochastic model that combines dynamic dispersal and population processes with fine-resolution maps characterizing spatiotemporal heterogeneity in climate and habitat to model range expansion of the hemlock woolly adelgid (HWA; Adelges tsugae). We parameterize this model using multiyear data sets describing population and dispersal dynamics of HWA and apply it to eastern North America over a 57-year period (1951–2008). To evaluate the model, the observed pattern of spread of HWA during this same period was compared to model predictions. Our model predicts considerable heterogeneity in the risk of HWA invasion across space and through time, and it suggests that spatiotemporal variation in winter temperature, rather than hemlock abundance, exerts a primary control on the spread of HWA. Although the simulations generally matched the observed current extent of the invasion of HWA and patterns of anisotropic spread, it did not correctly predict when HWA was observed to arrive in different geographic regions. We attribute differences between the modeled and observed dynamics to an inability to capture the timing and direction of long-distance dispersal events that substantially affected the ensuing pattern of spread.

Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators--larvae of the pitcher-plant mosquito--indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occur in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.

This first comprehensive assessment of the ant fauna of Nantucket Island, Massachusetts revealed that 43% of New England ant species and 70% of New England ant genera occur on an island occupying only 0.07% of New England’s land area. Ants collected by four different research groups between 2000 and 2009 included 32,158 individual ants (2,911 incidences) from 384 spatially and temporally distinct samples representing 14 different vegetation community types. The majority of the ant species were collected from anthropogenically-derived and maintained sandplain grasslands, sandplain heathlands, and scrub oak shrublands. These three communities are state-ranked S1 community types; the lower state-ranked communities of beaches and sand dunes, bogs, salt marshes, and forest fragments had distinct ant assemblages with much lower species richness. The large number of samples described here, from a wide range of vegetation community types, expands the known list of Nantucket ant species more than three-fold and provides a baseline for future assessment of the effects of ongoing, long-term ecosystem management on Nantucket.

The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them. We apply such an approach to 30–60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.

I review the distribution of ant genera in cold biomes of the northern hemisphere, and discuss opportunities and challenges in using ants as environmental, ecological, and biodiversity indicators in these biomes. I present five propositions that, if supported with future research,would allow ants to be used as biological indicators in north-temperate cold biomes: (1) distribution of individual species or species groups are leading (early-warning) indicators of climatic warming at tundra/taiga or taiga/broadleaf forest boundaries; (2) mound-building species in the Formica rufa LINNAEUS, 1758 group are ecological indicators for land-use changes in European taiga and broadleaf forests; (3) relative abundance (evenness) is a leading indicator of environmental changes whereas high species richness is an indicator of past or ongoing disturbance; (4) presence or social parasites and slave-making species are better indicators of ecological integrity than presence or abundance of their hosts alone; (5) occurrence of non-native or invasive species is an indicator of reduced ecological integrity. Important aspects of long-term sampling, surveying, monitoring, and experimenting on ants are discussed in light of future research needs to test these propositions and to further develop ants as indicators of changing environmental conditions in north-temperate cold biomes.

The carnivorous plant family Sarraceniaceae comprises three genera of wetland inhabiting pitcher plants: Darlingtonia in the northwestern United States, Sarracenia in eastern North America, and Heliamphora in northern South America. Hypotheses concerning the biogeographic history leading to this unusual disjunct distribution are controversial, in part because genus- and species-level phylogenies have not been clearly resolved. Here, we present a robust, species-rich phylogeny of Sarraceniaceae based on seven mitochondrial, nuclear, and plastid loci, which we use to illuminate this family's phylogenetic and biogeographic history. The family and genera are monophyletic: Darlingtonia is sister to a clade consisting of Heliamphora+Sarracenia. WithinSarracenia, two clades were strongly supported: one consisting of S. purpurea, its subspecies, and S. rosea; the other consisting of nine species endemic to the southeastern United States. Divergence time estimates revealed that stem group Sarraceniaceae likely originated in South America 44–53 million years ago (Mya) (highest posterior density [HPD] estimate = 47 Mya). By 25–44 (HPD = 35) Mya, crown-group Sarraceniaceae appears to have been widespread across North and South America, and Darlingtonia (western North America) had diverged from Heliamphora+Sarracenia (eastern North America+South America). This disjunction and apparent range contraction is consistent with late Eocene cooling and aridification, which may have severed the continuity of Sarraceniaceae across much of North America. Sarracenia and Heliamphora subsequently diverged in the late Oligocene, 14–32 (HPD = 23) Mya, perhaps when direct overland continuity between North and South America became reduced. Initial diversification of South American Heliamphora began at least 8 Mya, but diversification of Sarracenia was more recent (2–7, HPD = 4 Mya); the bulk of southeastern United States Sarraceniaoriginated co-incident with Pleistocene glaciation, < 3 Mya. Overall, these results suggest climatic change at different temporal and spatial scales in part shaped the distribution and diversity of this carnivorous plant clade.