Robert K. Booth

Ecology and the ratchet of events: Climate variability, niche dimensions, and species distributions

Climate change in the coming centuries will be characterized by interannual, decadal, and multidecadal fluctuations superimposed on anthropogenic trends. Predicting ecological and biogeographic responses to these changes constitutes an immense challenge for ecologists. Perspectives from climatic and ecological history indicate that responses will be laden with contingencies, resulting from episodic climatic events interacting with demographic and colonization events. This effect is compounded by the dependency of environmental sensitivity upon life-stage for many species. Climate variables often used in empirical niche models may become decoupled from the proximal variables that directly influence individuals and populations. Greater predictive capacity, and more-fundamental ecological and biogeographic understanding, will come from integration of correlational niche modeling with mechanistic niche modeling, dynamic ecological modeling, targeted experiments, and systematic observations of past and present patterns and dynamics.

A high-resolution record of late-Holocene moisture variability from a Michigan raised bog, USA

We reconstructed the late-Holocene surface-moisture history and vegetation dynamics of a raised bog in Michigan using testate amoebae, peat humification, pollen, stomata and plant macrofossils. Our primary objective was to compare bog palaeohydrology with the water-level history of Lake Michigan, and to regional and local vegetation changes. Hydrologic histories inferred from testate amoebae and humification show similar trends, and correspond with records of past water-level variability in Lake Michigan. The bog clearly shows effective-moisture increases during the Algoma highstand (∼3200–2300 cal. BP) and a later unnamed highstand (∼1900–1300 cal. BP). Some higher-frequency fluctuations are also similar. The good correspondence indicates that bog hydrology and the water levels of Lake Michigan have been driven by changes in large-scale atmospheric circulation patterns for at least the past 3500 years. Climate during the Algoma highstand may have been spatiotemporally complex, with increasing moisture occurring several hundred years earlier in the northern portion of the western Great Lakes basin. Locally, the expansion and contraction of Picea populations on the bog surface was probably directly related to moisture conditions. The well-documented decline in Fagus populations in the central Great Lakes region after 1000 cal. BP is contemporaneous with a major shift towards drier conditions, suggesting that relative dryness caused the decline.

Testate amoebae as paleoindicators of surface-moisture changes on Michigan peatlands: modern ecology and hydrological calibration

Peatland testate amoebae are sensitive indicators of local hydrology and have been used as proxies for surface moisture conditions in fossil studies. However, few regional calibration datasets exist in North America, and knowledge of testate amoeba ecology and distribution patterns are limited. The objectives of this study were to (1) investigate the relationship between testate amoebae, environment, and Sphagnum species in Michigan peatlands; (2) generate transfer functions from this dataset that can be applied to fossil data; and (3) describe vertical variation of testate amoebae inhabiting Sphagnum moss. Testate amoeba assemblages from 139 microsites within 11 peatlands in Michigan were compared to assess variability between and within peatlands. Most peatlands contained similar testate amoeba assemblages, although within individual peatlands the amount of assemblage variability is correlated to the amount of environmental heterogeneity. Of the measured environmental variables, depth to water table showed the strongest relationship with testate amoebae. Depth to water table can be reconstructed from fossil data with a mean error of ±7.5 cm, although predictive ability deteriorates in extremely dry environments (>30 cm water table depth). Vertical variation in testate amoeba assemblages was investigated at 89 Sphagnum-dominated microsites by directly comparing amoeba assemblages and the abundance and frequency of common taxa in upper and lower portions of the Sphagnum stem. Except for extremely dry microsites, considerable vertical variation in assemblage composition exists. Many agglutinate taxa are more abundant on lower portions of the Sphagnum stem, and taxa containing symbiotic zoochlorellae are more abundant on upper portions. Refinements in sampling procedures and analysis may improve the predictive ability of transfer functions.

ECOLOGY OF TESTATE AMOEBAE (PROTOZOA) IN TWO LAKE SUPERIOR COASTAL WETLANDS: IMPLICATIONS FOR PALEOECOLOGY AND ENVIRONMENTAL MONITORING

Testate amoebae are common inhabitants of moist soils, wetland, and lacustrine habitats. They produce a decay-resistant test, or shell, which can be identified to species in most cases and recovered from sediments in quantities sufficiently large to permit estimation of relative abundance. The objectives of this study were to assess the potential of testate amoeba assemblages as paleoenvironmental and environmental indicators in two Lake Superior coastal wetlands and to determine if morphological variation in four common taxa (Arcella spp., Assulina spp., Centropyxis cassis type, and Nebela tincta-parvula-collaris group) is related to microenvironment. Study localities included ridge-swale wetland systems adjacent to Grand Traverse Bay and Tahquamenon Bay in the Upper Peninsula of Michigan. Testate amoeba assemblages from 74 microsites were compared with percent moisture, depth to water table, pH, porosity, depth of living moss, and associated moss and vascular plant species. Morphometric analysis (e.g., test length and aperture diameter) was conducted on 25 individuals from at least 10 microsites for each of the four selected taxa. Gradient analysis indicated that testate amoeba assemblages are primarily controlled by substrate moisture and pH, consistent with results from other regions. Transfer functions for pH and substrate moisture were developed using ‘jackknifed’ validation procedures. Little relationship was found between microenvironmental parameters and morphological variation in the investigated taxa, except for the Nebela tincta-parvula-collaris group, where test size was significantly correlated with pH (r2=0.68). Results indicate that wetland testate amoeba assemblages in these coastal wetland systems are sensitive environmental and paleoenvironmental indicators that can be used to monitor and reconstruct water-level or pH changes.

Holocene carbon accumulation rates from three ombrotrophic peatlands in boreal Quebec, Canada: Impact of climate-driven ecohydrological change

Understanding the processes controlling peatland carbon (C) sequestration is critical to anticipate potential changes in the global C cycle in response to climate change. Although identification of these factors may be relatively straightforward on seasonal timescales, at centennial to millennial timescales complexities arise because of interactions between climate, vegetation, hydrology and long-term ecological processes. To better understand the factors controlling long-term C accumulation, Holocene rates of C sequestration were quantified from three pristine ombrotrophic peatlands in boreal Quebec, northeastern Canada (52°N, 75–76°W). Bulk density and loss-on-ignition analyses, combined with radiocarbon dating and age–depth modelling, were used to estimate long-term apparent rates of carbon accumulation. Past changes in vegetation and water-table depth were obtained from plant macrofossil and testate amoeba analysis. Earliest regional peat accumulation started ~7520 cal. BP, with long-term rates of C sequestration varying between 14.9 and 22.6 g/m2 per yr. High C sequestration rates occurred during the mid Holocene when relatively stable Sphagnum section Acutifolia communities were present, while low rates were found during the cooler late Holocene when Cyperaceae and ligneous vegetation were more dominant. However, C sequestration was highly variable among cores, implying that local topography, geomorphology and hydrology, or disturbance factors such as fire, mediate the influence of climate on C accumulation. Reconstructed water-table depths reveal several dry shifts since 3000 cal. BP, suggesting that episodic cold and dry conditions during the late Holocene may have contributed to lower C sequestration rates. Given the intensity of the water-table shifts at these times, we hypothesize that recurrent episodes of frozen subsurface peat might have intensified surface drying. As projected by climate scenarios, anticipated warmer and wetter conditions may lead to greater stability of hummock Sphagna cover and increased C sequestration potential in boreal peatlands.

Multi‐decadal drought and amplified moisture variability drove rapid forest community change in a humid region

Climate variability, particularly the frequency of extreme events, is likely to increase in the coming decades, with poorly understood consequences for terrestrial ecosystems. Hydroclimatic variations of the Medieval Climate Anomaly (MCA) provide a setting for studying ecological responses to recent climate variability at magnitudes and timescales comparable to expectations of coming centuries. We examined forest response to the MCA in the humid western Great Lakes region of North America, using proxy records of vegetation, fire, and hydroclimate. Multi-decadal moisture variability during the MCA was associated with a widespread, episodic decline in Fagus grandifolia (beech) populations. Spatial patterns of drought and forest changes were coherent, with beech declining only in areas where proxy-climate records indicate that severe MCA droughts occurred. The occurrence of widespread, drought-induced ecological changes in the Great Lakes region indicates that ecosystems in humid regions are vulnerable to rapid changes in drought magnitude and frequency.

Testate amoebae and δ13C of Sphagnum as surface-moisture proxies in Alaskan peatlands

Testate amoebae and stable isotopes have been used as surface-moisture proxies in peatlands. However, adequate modern calibration is critical to successful application and interpretation. Testate amoebae, δ13C of Sphagnum, and environmental conditions were examined at 126 sites within 12 peatlands of south-central and central Alaska to assess the potential of testate amoebae and δ 13C as surface-moisture proxies. Results indicate that water-table depth and pH were both correlated with testate amoeba community composition. However, the relative importance of these two variables varied, with pH more important in wetter habitats and water-table depth more important in drier habitats. Cross-validation of transfer functions indicated that water-table depth and pH can be inferred from testate amoeba communities with mean errors of ~8 cm and ~0.4 pH units, respectively, an improvement over previous calibration work in the region. However, application of these transfer functions should consider potential temporal variability in the relative importance of pH and water-table depth, and we applied our calibration data set to a subfossil testate amoeba record to highlight how knowledge of changes in the relative importance of these environmental variables can inform interpretation. In contrast with testate amoebae, δ13C of Sphagnum was found to be a relatively weak indicator of water-table depth. Variable carbon sources for Sphagnum photosynthesis, such as CO2 released by methanotrophic bacteria, likely complicate the relationship between δ 13C and moisture. Although more work is needed before δ13C of Sphagnum can be used as a proxy for water-table depth, testate amoebae should be useful in paleoenvironmental studies of peatlands in Alaska.

The potential influence of short-term environmental variability on the composition of testate amoeba communities in "Sphagnum" peatlands

Testate amoebae are a group of moisture-sensitive, shell-producing protozoa that have been widely used as indicators of changes in mean water-table depth within oligotrophic peatlands. However, short-term environmental variability (i.e., sub-annual) also probably influences community composition. The objective of this study was to assess the potential influence of short-term environmental variability on the composition of testate amoeba communities in Sphagnum-dominated peatlands. Testate amoebae and environmental conditions, including hourly measurements of relative humidity within the upper centimeter of the peatland surface, were examined throughout the 2008 growing season at 72 microsites within 11 peatlands of Pennsylvania and Wisconsin, USA. Relationships among testate amoeba communities, vegetation, depth to water table, pH, and an index of short-term environmental variability (EVI), were examined using nonmetric multidimensional scaling and correlation analysis. Results suggest that EVI influences testate amoeba communities, with some taxa more abundant under highly variable conditions (e.g., Arcella discoides, Difflugia pulex, and Hyalosphenia subflava) and others more abundant when environmental conditions at the peatland surface were relatively stable (e.g., Archerella flavum and Bullinularia indica). The magnitude of environmental variability experienced at the peatland surface appears to be primarily controlled by vegetation composition and density. In particular, sites with dense Sphagnum cover had lower EVI values than sites with loose-growing Sphagnum or vegetation dominated by vascular plants and/or non-Sphagnum bryophytes. Our results suggest that more environmental information may be inferred from testate amoebae than previously recognized. Knowledge of relationships between testate amoebae and short-term environmental variability should lead to more detailed and refined environmental inferences.

Ecology of testate amoebae in a North Carolina pocosin and their potential use as environmental and paleoenvironmental indicators

ABSTRACT Testate amoebae inhabiting Sphagnum peatlands are sensitive indicators of substrate-moisture content. However, ecological data from a range of peatland types is needed to assess their indicator-value in other peatland systems. We examined testate amoeba communities and species–environment relationships at 40 locations within a pocosin of North Carolina. Primary objectives were to provide baseline data on the ecology of testate amoebae in pocosins, compare communities with those of northern peatlands, and assess the potential of using testate amoebae as environmental indicators in these systems. Results indicated that pH, substrate-moisture content, and conductivity were important controls on community composition. Communities that were relatively similar and dissimilar to those of northern peatlands were encountered, and the ecology of species with respect to water-table depth was similar in both peatland types. Cross validation of transfer functions for pH and water-table depth suggests that pocosin testate amoebae can be used as indicators for these variables. However, water-table depth was poorly inferred from communities at several dry sites characterized by high bulk density. These sites may experience large seasonal or interannual variability in moisture conditions because of differences in peat structure, suggesting that research is needed on the influence of short-term variability in shaping community structure. Nomenclature: Radford, Ahles & Bell, 1969; Charman, Hendon & Woodland, 2000.

Testing peatland testate amoeba transfer functions: Appropriate methods for clustered training-sets

Transfer functions are widely used in palaeoecology to infer past environmental conditions from fossil remains of many groups of organisms. In contrast to traditional training-set design with one observation per site, some training-sets, including those for peatland testate amoeba-hydrology transfer functions, have a clustered structure with many observations from each site. Here we show that this clustered design causes standard performance statistics to be overly optimistic. Model performance when applied to independent data sets is considerably weaker than suggested by statistical cross-validation. We discuss the reasons for these problems and describe leave-one-site-out cross-validation and the cluster bootstrap as appropriate methods for clustered training-sets. Using these methods we show that the performance of most testate amoeba-hydrology transfer functions is worse than previously assumed and reconstructions are more uncertain.