Aaron M. Ellison

Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.

Determinants of Pattern in a New England Salt Marsh Plant Community

In New England salt marshes, Spartina alterniflora dominates the low–marsh habitat, which is covered daily by tides. The high–marsh habitat, which is not flooded daily, is dominated on its seaward border by Spartina patens, and on its terrestrial border by Juncus gerardi. Each of these vegetation zones has a characteristic suite of physical factors associated with differences in tidal inundation. In particular, substrate redox increases and salinity decreases with decreasing marsh elevation. Although correlations between physical factors and the occurrence of specific marsh plants have been suggested to be causal, a 5–mo transplant experiment suggested that the distribution of perennials across the marsh does not correspond to their potential performance across the marsh in the absence of surrounding vegetation. While the high–marsh perennials appear to be restricted to the high–marsh habitat by harsh physical conditions in the low–marsh habitat, the low–marsh dominant, S. alterniflora, is capable of vigorous growth across the entire marsh and appears to be excluded from the high–marsh habitat by the high–marsh perennials. Throughout the high marsh, two other plant species, Distichlis spicata and Salicornia europaea, are found associated with areas that have been disturbed recently. Physical disturbance, in the form of mats of dead plant material (wrack) rafted by tides onto the marsh, is most severe in the spring and early summer, and decreases with increasing marsh elevation. Differential plant mortality results from short–term disturbance events. D. spicata and S. alterniflora are more tolerant of wrack burial than are the other marsh plants, and short–term disturbance increases the relative abundance of these species in the community. Longer lasting disturbance events kill all the underlying vegetation, leaving discrete bare patches throughout the high marsh. D. spicata rapidly colonizes these patches with vegetative runners, while S. alterniflora and Sa. europaea recruit to these patches by seed. The relative abundance of these plants in recently created bare patches exceeds greatly their relative abundance in the surrounding vegetation. Over time, however, these early colonizers are overgrown and displaced in high–marsh patches by S. patens and J. gerardi, which grow slowly, as dense turfs of roots, rhizomes, and tillers. Physical disturbance and interspecific competition appear to be major determinants of the spatial pattern of marsh plant communities. These processes will need to be considered in relation to edaphic factors in elucidating the underlying mechanisms of salt marsh plant zonation.

Rarefaction and extrapolation with Hill numbers: a framework for sampling and estimation in species diversity studies

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 extended previous rarefaction and extrapolation models for species richness (Hill number q D, where q ¼ 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 ¼ 0 (species richness), q ¼ 1 (the exponential of Shannons entropy index), and q ¼ 2 (the inverse of Simpsons concentration index). We developed 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 data sets from biodiversity surveys of temperate forest spiders and tropical ants.

Methods for Detecting Early Warnings of Critical Transitions in Time Series Illustrated Using Simulated Ecological Data

Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called ‘early warning signals’, and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data.

Anthropogenic Disturbance of Caribbean Mangrove Ecosystems: Past Impacts, Present Trends, and Future Predictions

We review historical, current, and projected future impacts of four classes of anthropogenic disturbance-extraction, pollution, reclamation, and changing climate-on Caribbean mangrove ecosystems (mangal). These disturbances occur, respectively, at increasing spatial and temporal scales, and require increasing recovery time. Small-scale selective extraction has little system-wide effect, but regeneration is slow even on single hectare clear-cuts due to rapid soil acidification. Petroleum is the primary pollutant of Caribbean mangal, and results in tree defoliation, stand death, and loss of associated sessile and mobile animal species. Hydrocarbons persist in mangrove sediments for decades, and are correlated with increasing seedling mutation rates. Chemical, industrial, and urban wastes are associated with increased heavy metal content of seedlings, stand die-back, reduced system-wide species richness, and higher incidence of Vibrio spp. (shellfish poisoning). Mangal has been reclaimed for urbanization, industrialization, and increasingly, for tourism. Overall, the region is losing mangrove forests at 1 percent per yr, although the rate is much faster on the Caribbean mainland (- 1.7% yr-1) than it is on the islands (=0.2% yr-1). The regions fisheries are declining at a similar rate, as most commercial shellfish and finfish use mangal for nurseries and/or refugia. Few Caribbean states have legislation or enforcement capabilities to protect or manage mangal, although at least 11 international treaties and conventions could be applied to conserve or sustainably use these forests. These treaties may protect riverine and basin mangal, but are likely to be moot with respect to fringing mangal, which may vanish as a consequence of global climate change. Growth enhancements of mangroves resulting from increasing atmospheric CO2 probably will not compensate for negative effects of concomitant rises in regional sea level.

Climatic drivers of hemispheric asymmetry in global patterns of ant species richness.

Although many taxa show a latitudinal gradient in richness, the relationship between latitude and species richness is often asymmetrical between the northern and southern hemispheres. Here we examine the latitudinal pattern of species richness across 1003 local ant assemblages. We find latitudinal asymmetry, with southern hemisphere sites being more diverse than northern hemisphere sites. Most of this asymmetry could be explained statistically by differences in contemporary climate. Local ant species richness was positively associated with temperature, but negatively (although weakly) associated with temperature range and precipitation. After contemporary climate was accounted for, a modest difference in diversity between hemispheres persisted, suggesting that factors other than contemporary climate contributed to the hemispherical asymmetry. The most parsimonious explanation for this remaining asymmetry is that greater climate change since the Eocene in the northern than in the southern hemisphere has led to more extinctions in the northern hemisphere with consequent effects on local ant species richness.

Analysis of abrupt transitions in ecological systems

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.

Effects of Competition, Disturbance, and Herbivory on Salicornia Europaea

The effects of interspecific competition, physical disturbance, and insect herbivory on the distribution and abundance of Salicornia europaea in a New England salt marsh were studied using manipulative field experiments. In New England salt marsh plant communities, S. europaea is restricted to disturbance- generated patches. S. europaea was unable to colonize successfully low-marsh habitats that were fiooded daily by tides, because seedlings germinating in these areas could not anchor in soft substrate and were easily uprooted. In high-marsh habitats, S. europaea seeds reliably colonized recently created patches because the seeds hairy coat bound to the plant debris that created the patches. S. europaea was successful in naturally occurring and artificially maintained patches. However, it was rapidly outcompeted for light by perennials in un? disturbed areas and eventually was overgrown by perennials growing into the patches. When the perennial canopy was removed, S. europaea increased in biomass and fecundity relative to conspecifics that were beneath the perennial canopy. S. europaea, however, was attacked by insect herbivores more frequently in patches than beneath the perennial canopy and this herbivore pressure reduced survivorship in patches to approximately the same level as beneath the perennial canopy. Interactions among physical disturbance, seed dispersal patterns, interspecific compe? tition, and herbivory appear to be major factors controlling patterns of distribution and abundance of Salicornia in New England salt marsh plant communities.

BIOGEOGRAPHY AT A REGIONAL SCALE: DETERMINANTS OF ANT SPECIES DENSITY IN NEW ENGLAND BOGS AND FORESTS

We examined species density gradients of ants of New England in 22 om- brotrophic bogs and their surrounding forests. We tested the hypothesis that species density was correlated with large-scale geographic variables (latitude, longitude, elevation) and small-scale site variables (habitat area, vegetation composition, light availability). Species density was consistently higher in forests than in bogs. Ant species density measured in three other New World studies yielded similar results, with steeper diversity slopes in closed canopy vs. open habitats. In New England bogs and forests, latitude was the single most important predictor of species density, even though the latitudinal span of the entire study region was less than three degrees. Diversity patterns documented in our study of mid- latitude ant communities are similar to those seen in studies spanning tropical and temperate habitats. Species density of forest ants was associated strongly with latitude, elevation, light availability, and vegetation composition. Species density of bog ants was less pre- dictable and was correlated only with latitude and vegetation. Overall, our results suggest that species-energy relationships are important at regional spatial scales. Explanations for the latitudinal gradient in ant species density may not depend on unique differences between tropical and temperate communities, but could operate at all latitudes.

Scale‐Dependent Spatial and Temporal Variability in Biogeography of Mangrove Root Epibiont Communities

Studies across a range of spatial and temporal scales are needed to discern multiple forces structuring communities. Subtidal prop roots of red mangroves host diverse assemblages of sessile marine epibionts that provide a model system for examining community development and maintenance at a variety of discrete spatial scales. During 1991- 1992 we twice surveyed 11 sites at four cays in Belize, Central America, to quantify spatial variability and temporal change in distribution and abundance of root-fouling organisms at five sampling scales: (1) fronts and backs of roots (1-cm scale); (2) roots close to and extending away from peat bank (0.5-m scale); (3) along linear transects parallel to shore (1-50 m scale); (4) on leeward and windward shores of cays (0.5-km scale); and (5) among cays (1-10 km scale). Although epibiont community structure differed widely among sites, all cays surveyed had similar seasonal values of water salinity, pH, and temperature. Within cays, windward sites had higher dissolved oxygen levels and water flow rates than leeward sites. At still smaller scales, outer roots and fronts of roots received significantly more light and were subject to higher water flow rates than inner roots and backs of roots. Species richness, diversity, and mosaic diversity patterns indicated that epibiont assemblages were distributed non-randomly in space: leeward sites were more speciose than windward sites, and fronts of roots were more speciose than backs. Jaccards index of similarity, cluster analysis, and Kendalls coefficient of concordance showed hierarchical patterns of decreasing similarity with increasing sampling distance. Significant spatial autocorrelation among Jaccard values occurred at 2-3 m intervals, possibly reflecting mean larval dispersal distances. Analysis of mosaic diversity among sites indicated the absence of a clear environmental gradient and supported the hypothesis that species distributions may reflect patterns of dispersal from initial source populations. While precise identity of species was unpredictable among roots, species groups based on taxonomy, morphology, and life history showed very consistent distributions among sites that may reflect variability in local root environments: algae were most prevalent in well-lit areas and on windward sites, while sponges and ascidians predominated in leeward areas. Relative importance and dominance of both individual species and species groups changed substantially between 1991 and 1992. Representatives of four species groups were transplanted across three spatial scales to assess whether post-settlement dynamics limit distributions of these taxa. All transplants survived well for the first 6 wk of the experiment. After 6 mo, all transplants exhibited similarly high levels of mortality regardless of treatment. Overall, the results indicate that larval supply may shape epibiont community composition on short time scales and small and very large spatial scales, while variation in physical factors may influence distributions over the long term and at intermediate spatial scales.