[show abstract][hide abstract] ABSTRACT: As the earth system moves to a novel state, model systems (experimental, observational, paleoecological) are needed to assess and improve the predictive accuracy of ecological models under environments with no contemporary analog. In recent years, we have intensively studied the no-analog plant associations and climates in eastern North America during the last deglaciation to better constrain their spatiotemporal distribution, test hypotheses about climatic and megaherbivory controls, and assess the accuracy of species-and community-level models. The formation of no-analog plant associations was asynchronous, beginning first in the south-central United States; at sites in the north-central United States, it is linked to declining megafaunal abundances. Insolation and temperature were more seasonal than present, creating climates currently nonexistent in North America, and shifting species–climate relationships for some taxa. These shifts pose a common challenge to empirical paleoclimatic reconstructions, species distribution models (SDMs), and conservation–optimization models based on SDMs. Steps forward include combining recent and paleoecological data to more fully describe species' fundamental niches, employing community-level models to model shifts in species interactions under no-analog climates, and assimilating paleoecological data with mechanistic ecosystem models. Accurately modeling species interactions under novel environments remains a fundamental challenge for all forms of ecological models.
Annals of the New York Academy of Sciences 09/2013; 1297:29-43. · 4.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: Biotic interactions drive key ecological and evolutionary processes and mediate ecosystem responses to climate change. The direction, frequency, and intensity of biotic interactions can in turn be altered by climate change. Understanding the complex interplay between climate and biotic interactions is thus essential for fully anticipating how ecosystems will respond to the fast rates of current warming, which are unprecedented since the end of the last glacial period. We highlight episodes of climate change that have disrupted ecosystems and trophic interactions over time scales ranging from years to millennia by changing species' relative abundances and geographic ranges, causing extinctions, and creating transient and novel communities dominated by generalist species and interactions. These patterns emerge repeatedly across disparate temporal and spatial scales, suggesting the possibility of similar underlying processes. Based on these findings, we identify knowledge gaps and fruitful areas for research that will further our understanding of the effects of climate change on ecosystems.
[show abstract][hide abstract] ABSTRACT: "Space-for-time" substitution is widely used in biodiversity modeling to infer past or future trajectories of ecological systems from contemporary spatial patterns. However, the foundational assumption-that drivers of spatial gradients of species composition also drive temporal changes in diversity-rarely is tested. Here, we empirically test the space-for-time assumption by constructing orthogonal datasets of compositional turnover of plant taxa and climatic dissimilarity through time and across space from Late Quaternary pollen records in eastern North America, then modeling climate-driven compositional turnover. Predictions relying on space-for-time substitution were ∼72% as accurate as "time-for-time" predictions. However, space-for-time substitution performed poorly during the Holocene when temporal variation in climate was small relative to spatial variation and required subsampling to match the extent of spatial and temporal climatic gradients. Despite this caution, our results generally support the judicious use of space-for-time substitution in modeling community responses to climate change.
Proceedings of the National Academy of Sciences 05/2013; · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Data, whether images, measurements, counts, occurrences, or character codings, are a cornerstone of vertebrate paleontology. Every published paper,master’s thesis, and doctoral dissertation relies on these data to document patterns and processes in evolution, ecology, taphonomy, geography, geologic time, and functional morphology, to name just a few. In turn, the vertebrate paleontology community relies on published data in order to reproduce and verify others’ work, as well as to expand upon published analyses in new ways without having to reconstitute data sets that have been used by earlier authors and to accurately preserve data for future generations of researchers. Here, we review several databases that are of interest to vertebrate paleontologists and strongly advocate for more deposition of basic research data in publicly accessible databases by vertebrate paleontologists.
Journal of Vertebrate Paleontology 02/2013; 33(1):13-28. · 1.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: Age–depth relationships in sedimentary archives such as lakes, wetlands and bogs are non-linear with irregular probability distributions associated with calibrated radiocarbon dates. Bayesian approaches are thus well-suited to understanding relationships between age and depth for use in paleoecological studies. Bayesian models for the accumulation of sediment and organic matter within basins combine dated material from one or more records with prior information about the behavior of deposition times (yr/cm) based on expert knowledge. Well-informed priors are essential to good modeling of the age–depth relationship, but are particularly important in cases where data may be sparse (e.g., few radiocarbon dates), or unclear (e.g., age-reversals, coincident dates, age offsets, outliers and dates within a radiocarbon plateau).Here we assessed Holocene deposition times using 204 age–depth models obtained from the Neotoma Paleoecology Database (www.neotomadb.org) for both lacustrine and palustrine environments across the northeastern United States. These age–depth models were augmented using biostratigraphic events identifiable within pollen records from the northeastern United States during the Holocene and late-Pleistocene.Deposition times are significantly related to depositional environment (palustrine and lacustrine), sediment age, and sediment depth. Spatial variables had non-significant relationships with deposition time when site effects were considered. The best-fit model was a generalized additive mixed model that relates deposition time to age, stratified by depositional environment with site as a random factor. The best-fit model accounts for 63.3% of the total deviance in deposition times. The strongly increasing accumulation rates of the last 500–1000 years indicate that gamma distributions describing lacustrine deposition times (α = 1.08, β = 18.28) and palustrine deposition times (α = 1.23, β = 22.32) for the entire Holocene may be insufficient for Bayesian approaches since there is strong variation in the gamma parameters both in the most recent sediments and throughout the Holocene. Time-averaged gamma distributions for lacustrine (α = 1.35, β = 19.64) and palustrine samples (α = 1.40, β = 20.72) show lower overall deposition times, but variability remains. The variation in gamma parameters through time may require the use of multiple gamma distributions during the Holocene to generate accurate age–depth models. We present estimates of gamma parameters for deposition times at 1000 yr intervals. The parameters generated in this study can be used directly within Bacon to act as Bayesian priors for sedimentary age models.
[show abstract][hide abstract] ABSTRACT: Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth's climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO(2) and CH(4) to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.
Proceedings of the National Academy of Sciences 02/2012; 109(19):E1134-42. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Empirically derived species distributions models (SDMs) are increasingly relied upon to forecast species vulnerabilities to future climate change. However, many of the assumptions of SDMs may be violated when they are used to project species distributions across significant climate change events. In particular, SDM's in theory assume stable fundamental niches, but in practice, they assume stable realized niches. The assumption of a fixed realized niche relative to climate variables remains unlikely for various reasons, particularly if novel future climates open up currently unavailable portions of species fundamental niches. To demonstrate this effect, we compare the climate distributions for fossil-pollen data from 21 to 15 ka bp (relying on paleoclimate simulations) when communities and climates with no modern analog were common across North America to observed modern pollen assemblages. We test how well SDMs are able to project 20th century pollen-based taxon distributions with models calibrated using data from 21 to 15 ka. We find that taxa which were abundant in areas with no-analog late glacial climates, such as Fraxinus, Ostrya/Carpinus and Ulmus, substantially shifted their realized niches from the late glacial period to present. SDMs for these taxa had low predictive accuracy when projected to modern climates despite demonstrating high predictive accuracy for late glacial pollen distributions. For other taxa, e.g. Quercus, Picea, Pinus strobus, had relatively stable realized niches and models for these taxa tended to have higher predictive accuracy when projected to present. Our findings reinforce the point that a realized niche at any one time often represents only a subset of the climate conditions in which a taxon can persist. Projections from SDMs into future climate conditions that are based solely on contemporary realized distributions are potentially misleading for assessing the vulnerability of species to future climate change.
Global Change Biology 01/2012; 18(5):1698-1713. · 6.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: In Quaternary paleoecology and paleoclimatology, compositionally
dissimilar fossil assemblages usually indicate dissimilar environments;
this relationship underpins assemblage-level techniques for
paleoenvironmental reconstruction such as mutual climatic ranges or the
modern analog technique. However, there has been relatively little
investigation into the form of the relationship between compositional
dissimilarity and climatic dissimilarity. Here we apply generalized
dissimilarity modeling (GDM; Ferrier et al. 2007) as a tool for modeling
the expected non-linear relationships between compositional and climatic
dissimilarity. We use the CCSM3.0 transient paleoclimatic simulations
from the SynTrace working group (Liu et al. 2009) and a new generation
of fossil pollen maps from eastern North America (Blois et al. 2011) to
1) assess the spatial relationships between compositional dissimilarity
and climatic dissimilarity and 2) whether these spatial relationships
change over time. We used a taxonomic list of 106 genus-level pollen
types, six climatic variables (winter precipitation and mean
temperature, summer precipitation and temperature, seasonality of
precipitation, and seasonality of temperature) that were chosen to
minimize collinearity, and a cross-referenced pollen and climate dataset
mapped for time slices spaced 1000 years apart. When GDM was trained for
one time slice, the correlation between predicted and observed spatial
patterns of community dissimilarity for other times ranged between 0.3
and 0.73. The selection of climatic predictor variables changed over
time, as did the form of the relationship between compositional turnover
and climatic predictors. Summer temperature was the only variable
selected for all time periods. These results thus suggest that the
relationship between compositional dissimilarity in pollen assemblages
(and, by implication, beta diversity in plant communities) and climatic
dissimilarity can change over time, for reasons to be further studied.
[show abstract][hide abstract] ABSTRACT: PalEON is a multidisciplinary project that combines paleo and modern
ecological data with state-of-the-art statistical and modelling tools to
examine the interactions between climate, fire and vegetation during the
past two millennia in the northeastern United States. A fundamental
challenge for PalEON (and paleo research more broadly) is to improve age
modelling to yield more accurate sediment-core chronologies. To address
this challenge, we assessed sedimentation rates and their controls for
218 lakes and mires in the northeastern U.S. Sedimentation rates (yr/cm)
were calculated from age-depth models, which were obtained from the
Neotoma database (www.neotomadb.org) and other contributed pollen
records. The age models were recalibrated to IntCal09 and augmented in
some cases using biostratigraphic markers (Picea decline, 16 kcal BP -
10.5 kcal BP; Quercus rise, 12 - 9.1 kcal BP; and Alnus decline, 11.5 -
10.6 kcal BP) as described in Blois et al. (2011). Relationships between
sedimentation rates and sediment age, site longitude, and depositional
environment (lacustrine or mire) are significant but weak. There are
clear and significant links between variations in the NGRIP record of
δ18O and sedimentation in mires across the PalEON region, but no
links to lacustrine sedimentation rates. This result indicates that
super-regional climatic control of primary productivity, and thus
autochthonic sediment deposition, dominates in mires while deposition in
lacustrine basins may be driven primarily by local and regional factors
including watershed size, surficial materials,and regional vegetation.
The shape of the gamma probability functions that best describe
sedimentation rate distributions are calculated and presented here for
use as priors in Bayesian age modelling applications such as BACON
(Blaauw and Christen, in press). Future applications of this research
are also discussed.
[show abstract][hide abstract] ABSTRACT: Mapping past vegetation dynamics from heterogeneous databases of fossil-pollen records must face the challenge of temporal uncertainty. The growing collection of densely sampled fossil-pollen records with accurate and precise chronologies allows us to develop new methods to assess and reduce this uncertainty. Here, we test our methods in the context of vegetation changes in eastern North America during the abrupt climate changes of the last deglaciation. We use the network of fossil-pollen records in the Neotoma Paleoecology Database (www.neotomadb.org) and data contributed by individual investigators. Because many of these records were collected decades before the current generation of 14C and age-model technologies, we first developed a framework to assess the overall reliability of 14C chronologies by systematically evaluating individual 14C ages and associated chronologies. We developed a qualitative ranking scheme for individual 14C ages that combines information about their accuracy and precision. ‘Benchmark’ pollen records were defined to have at least one 14C age with an accuracy within 250 years and a precision less than 500 years that is within 1000 years of the time interval of interest, and at least five pollen samples per 1000 years across this time period. Only 22 of >350 late-Pleistocene pollen cores in eastern North America met the benchmark criteria.We then used Bayesian change-point analysis to identify widespread ecological events (Picea decline, Quercus rise, and Alnus decline), and interpolated the ages of these events from the benchmark sites to non-benchmark sites. Leave-one-out cross-validation analyses with the benchmark sites indicated that the spatial error associated with interpolation was less for inverse distance-weighting (IDW) than thin-plate splines (TPS) and was about 500 years for the three biotic events. By comparison, the difference between the original ages of events at poorly constrained sites and the biostratigraphic ages interpolated from the benchmark sites was close to 1000 years, suggesting that the use of biostratigraphic ages can significantly improve the age models for poorly constrained sites. Overall, these analyses suggest that the temporal resolution of multi-site syntheses of late-Pleistocene fossil-pollen data in eastern North America is about 500 years, a resolution that allows analysis of ecological responses to millennial-scale climate change during the last deglaciation.Highlights► We develop a conceptual framework for identifying ‘benchmark’ pollen records. ► We applied the framework to reduce uncertainty in late-Pleistocene pollen cores. ► The effective temporal limit to late-Pleistocene vegetation syntheses is ∼500 years. ► This approach reduces uncertainty within many pollen cores in eastern North America. ► Our knowledge of vegetation responses to past climate changes is greatly enhanced.
[show abstract][hide abstract] ABSTRACT: Summary1. Abrupt changes and regime shifts are common phenomena in terrestrial ecological records spanning centuries to millennia, thus offering a rich opportunity to study the patterns and drivers of abrupt ecological change.2. Because Quaternary climate changes also often were abrupt, a critical research question is to distinguish between extrinsic versus intrinsic abrupt ecological changes, i.e. those externally driven by abruptly changing climates, versus those resulting from thresholds, tipping points, and other nonlinear responses of ecological systems to progressive climate change. Extrinsic and intrinsic abrupt ecological changes can be distinguished in part by compiling and analysing regional networks of palaeoecological records.3. Abrupt ecological changes driven by spatially coherent and abrupt climate changes should manifest as approximately synchronous ecological responses, both among different taxa at a site and among sites. However, the magnitude and direction of response may vary among sites and taxa. Ecological responses to the rapid climatic changes accompanying the last deglaciation offer good model systems for studying extrinsic abrupt change.4. When abrupt ecological changes are intrinsically driven, the timing and rate of ecological response to climate change will be strongly governed by local biotic and abiotic processes and by stochastic processes such as disturbance events or localized climatic extremes. Consequently, at a regional scale, one should observe ‘temporal mosaics’ of abrupt ecological change, in which the timing and rate of ecological change will vary among species within sites and among sites. These temporal mosaics are analogous to the spatial mosaics observed in ecological systems prone to threshold switches between alternate stable states. The early Holocene aridification of the North American mid-continent and the middle-Holocene aridification of North Africa may be good examples of temporal mosaics.5. Synthesis. Past instances of extrinsic and intrinsic abrupt change are of direct relevance to global-change ecologists. The former allow study of the capacity of ecological systems to quickly adjust to abrupt climate changes, while the latter offer opportunities to understand the ecological processes causing abrupt local responses to regional climate change, to test tools for predicting critical thresholds, and to develop climate-adaptation strategies.
[show abstract][hide abstract] ABSTRACT: Neotoma Consortium Workshop; Madison, Wisconsin, 23-26 September 2010; Paleoecology can contribute much to global change science, as paleontological records provide rich information about species range shifts, changes in vegetation composition and productivity, aquatic and terrestrial ecosystem responses to abrupt climate change, and paleoclimate reconstruction, for example. However, while paleoecology is increasingly a multidisciplinary, multiproxy field focused on biotic responses to global change, most paleo databases focus on single-proxy groups. The Neotoma Paleoecology Database (http://www.neotomadb.org) aims to remedy this limitation by integrating discipline-specific databases to facilitate cross-community queries and analyses. In September, Neotoma consortium members and representatives from other databases and data communities met at the University of Wisconsin-Madison to launch the second development phase of Neotoma. The workshop brought together 54 international specialists, including Neotoma data stewards, users, and developers. Goals for the meeting were fourfold: (1) develop working plans for existing data communities; (2) identify new data types and sources; (3) enhance data access, visualization, and analysis on the Neotoma Web site; and (4) coordinate with other databases and cooperate in tool development and sharing.