Publications

2017
Pen˜a TS, Watson JR, Gonza´lez-Guzma´n LI, Keitt TH. Step-wise drops in modularity and the fragmentationof exploited marine metapopulations. Landscape Ecology [Internet]. 2017. Publisher's Version
2016
Lasky JR, Keitt TH, Weeks BC, Economo EP. A hierarchical model of whole assemblage island biogeography. Ecography [Internet]. 2016 :n/a–n/a. Publisher's VersionAbstract

Island systems have long played a central role in the development of ecology and evolutionary biology. However, while many empirical studies suggest species differ in vital biogeographic rates, such as dispersal abilities, quantitative methods have had difficulty incorporating such differences into analyses of whole-assemblages. In particular, differences in dispersal abilities among species can cause variation in the spatial clustering and localization of species distributions. Here, we develop a single, hierarchical Bayes, assemblage-wide model of 252 bird species distributions on the islands of Northern Melanesia and use it to investigate a) whether dispersal limitation structures bird assemblages across the archipelago, b) whether species differ in dispersal ability, and c) test the hypothesis that wing aspect ratio, a trait linked to flight efficiency, predicts differences inferred by the model. Consistent with island biogeographic theory, we found that individual species were more likely to occur on islands with greater area, and on islands near to other islands where the species also occurred. However, species showed wide variation in the importance and spatial scale of these clustering effects. The importance of clustering in distributions was greater for species with low wing aspect ratios, and the spatial scale of clustering was also smaller for low aspect ratio species. These findings suggest that the spatial configuration of islands interacts with species dispersal ability to affect contemporary distributions, and that these species differences are detectable in occurrence patterns. More generally, our study demonstrates a quantitative, hierarchical approach that can be used to model the influence of dispersal heterogeneity in diverse assemblages and test hypotheses for how traits drive dispersal differences, providing a framework for deconstructing ecological assemblages and their drivers.This article is protected by copyright. All rights reserved.

ecog2303.pdf
Levy O, Buckley LB, Keitt TH, Angilletta MJ. A dynamically downscaled projection of past and future microclimates. Ecology [Internet]. 2016 :n/a–n/a. Publisher's VersionAbstract
Ecological forecasting requires information about the climatic conditions experienced by organisms. Despite impressive methodological and computational advances, ecological forecasting still suffers from poor resolutions of environmental data. Published data comprise relatively few layers of surface climate and suffer from coarse temporal resolution. Hence, models using these data might underestimate heterogeneity of microclimates and miss biological consequences of climatic extremes. Moreover, we currently lack predictions about vegetation cover in future environments, a key factor for estimating the spatial heterogeneity of microclimates and hence the capacity for behavioral thermoregulation. Here, we describe microclimates and vegetation for the past and the future at spatial and temporal resolutions of 36 km (approximately 0.3°) and 1 h, respectively. We used the Weather Research and Forecasting model to downscale published, bias-corrected predictions of a global-circulation model from a resolution of 0.9° latitude and 1.25° (approximately 100 km in latitude and 130 km in longitude). Output from this model was used as input for a microclimate model, which generated temperatures and wind speeds for 1980–1999 and 2080–2099 at various heights, as well as soil temperatures at various depths and shade intensities. We also predicted the percentage of green vegetation and the percentage of shade given the angle of the sun. These data were evaluated using several criteria, each of which shed light on a different aspect of value to researchers. The metadata describe the modeling protocol, microclimate calculations, computer programs, and the evaluation process.This article is protected by copyright. All rights reserved.
Levy O, Buckley LB, Keitt TH, Angilletta MJ. Ontogeny constrains phenology: opportunities for activity and reproduction interact to dictate potential phenologies in a changing climate. Ecology Letters [Internet]. 2016 :n/a–n/a. Publisher's VersionAbstract

As global warming has lengthened the active seasons of many species, we need a framework for predicting how advances in phenology shape the life history and the resulting fitness of organisms. Using an individual-based model, we show how warming differently affects annual cycles of development, growth, reproduction and activity in a group of North American lizards. Populations in cold regions can grow and reproduce more when warming lengthens their active season. However, future warming of currently warm regions advances the reproductive season but reduces the survival of embryos and juveniles. Hence, stressful temperatures during summer can offset predicted gains from extended growth seasons and select for lizards that reproduce after the warm summer months. Understanding these cascading effects of climate change may be crucial to predict shifts in the life history and demography of species.

levy_et_al-2016-ecology_letters.pdf
Keitt TH, Addis C, Mitchell D, Salas A, Hawkes CV. Climate change, microbes, and soil carbon cycling. In: Marxsen J, Liebig J Climate Change and Microbial Ecology: Current and Future Trends. Norwich, UK: Caister Academic Press ; 2016. pp. 220.Abstract

Microbial responses to climate change will partly control the balance of soil carbon storage and loss under future temperature and precipitation conditions. We propose four classes of response mechanisms that can allow for a more general understanding of microbial climate responses. We further explore how a subset of these mechanisms results in microbial responses to climate change using simulation modeling. Specifically, we incorporate soil moisture sensitivity into two current enzyme-driven models of soil carbon cycling and find that moisture has large effects on predictions for soil carbon and microbial pools. Empirical efforts to distinguish among response mechanisms will facilitate our ability to further develop models with improved accuracy.

climate_change_1p_ch7.pdf
2015
Behrman KD, Juenger TE, Kiniry JR, Keitt TH. Spatial land use trade-offs for maintenance of biodiversity, biofuel, and agriculture. Landscape Ecology [Internet]. 2015;30 :1987–1999. Publisher's VersionAbstract

Expansion of bioenergy production is part of a global effort to reduce greenhouse gas emissions and mitigate climate change. Dedicated biomass crops will compete with other land uses as most high quality arable land is already used for agriculture, urban development, and biodiversity conservation.

art3a10.10072fs10980-015-0225-1.pdf
Hawkes CV, Keitt TH. Resilience vs. historical contingency in microbial responses to environmental change. Ecology Letters [Internet]. 2015;18 :612–625. Publisher's Version hawkes_et_al-2015-ecology_letters_1.pdf
Levy O, Buckley LB, Keitt TH, Smith CD, Boateng KO, Kumar DS, Angilletta, Michael J. J. Resolving the life cycle alters expected impacts of climate change. Proc R Soc B [Internet]. 2015;282 :20150837–. Publisher's VersionAbstract

Recent models predict contrasting impacts of climate change on tropical and temperate species, but these models ignore how environmental stress and organismal tolerance change during the life cycle. For example, geographical ranges and extinction risks have been inferred from thermal constraints on activity during the adult stage. Yet, most animals pass through a sessile embryonic stage before reaching adulthood, making them more susceptible to warming climates than current models would suggest. By projecting microclimates at high spatio-temporal resolution and measuring thermal tolerances of embryos, we developed a life cycle model of population dynamics for North American lizards. Our analyses show that previous models dramatically underestimate the demographic impacts of climate change. A predicted loss of fitness in 2% of the USA by 2100 became 35% when considering embryonic performance in response to hourly fluctuations in soil temperature. Most lethal events would have been overlooked if we had ignored thermal stress during embryonic development or had averaged temperatures over time. Therefore, accurate forecasts require detailed knowledge of environmental conditions and thermal tolerances throughout the life cycle.

20150837.pdf
2014
Levy O, Ball BA, Bond-Lamberty B, Cheruvelil KS, Finley AO, Lottig NR, Punyasena SW, Xiao J, Zhou J, Buckley LB, et al. Approaches to advance scientific understanding of macrosystems ecology. Frontiers in Ecology and the Environment [Internet]. 2014;12 :15–23. Publisher's VersionAbstract

The emergence of macrosystems ecology (MSE), which focuses on regional- to continental-scale ecological patterns and processes, builds upon a history of long-term and broad-scale studies in ecology. Scientists face the difficulty of integrating the many elements that make up macrosystems, which consist of hierarchical processes at interacting spatial and temporal scales. Researchers must also identify the most relevant scales and variables to be considered, the required data resources, and the appropriate study design to provide the proper inferences. The large volumes of multi-thematic data often associated with macrosystem studies typically require validation, standardization, and assimilation. Finally, analytical approaches need to describe how cross-scale and hierarchical dynamics and interactions relate to macroscale phenomena. Here, we elaborate on some key methodological challenges of MSE research and discuss existing and novel approaches to meet them.

130019.pdf
Heffernan JB, Soranno PA, Angilletta MJ, Buckley LB, Gruner DS, Keitt TH, Kellner JR, Kominoski JS, Rocha AV, Xiao J, et al. Macrosystems ecology: understanding ecological patterns and processes at continental scales. Frontiers in Ecology and the Environment [Internet]. 2014;12 :5–14. Publisher's VersionAbstract

Macrosystems ecology is the study of diverse ecological phenomena at the scale of regions to continents and their interactions with phenomena at other scales. This emerging subdiscipline addresses ecological questions and environmental problems at these broad scales. Here, we describe this new field, show how it relates to modern ecological study, and highlight opportunities that stem from taking a macrosystems perspective. We present a hierarchical framework for investigating macrosystems at any level of ecological organization and in relation to broader and finer scales. Building on well-established theory and concepts from other subdisciplines of ecology, we identify feedbacks, linkages among distant regions, and interactions that cross scales of space and time as the most likely sources of unexpected and novel behaviors in macrosystems. We present three examples that highlight the importance of this multiscaled systems perspective for understanding the ecology of regions to continents.

130017.pdf
Behrman KD, Keitt TH, Kiniry JR. Modeling Differential Growth in Switchgrass Cultivars Across the Central and Southern Great Plains. BioEnergy Research [Internet]. 2014;7 :1165–1173. Publisher's Version art3a10.10072fs12155-014-9450-8.pdf
Belaire AJ, Kreakie BJ, Keitt T, Minor E. Predicting and Mapping Potential Whooping Crane Stopover Habitat to Guide Site Selection for Wind Energy Projects. Conservation Biology [Internet]. 2014;28 :541–550. Publisher's VersionAbstract

Migratory stopover habitats are often not part of planning for conservation or new development projects. We identified potential stopover habitats within an avian migratory flyway and demonstrated how this information can guide the site-selection process for new development. We used the random forests modeling approach to map the distribution of predicted stopover habitat for the Whooping Crane (Grus americana), an endangered species whose migratory flyway overlaps with an area where wind energy development is expected to become increasingly important. We then used this information to identify areas for potential wind power development in a U.S. state within the flyway (Nebraska) that minimize conflicts between Whooping Crane stopover habitat and the development of clean, renewable energy sources. Up to 54% of our study area was predicted to be unsuitable as Whooping Crane stopover habitat and could be considered relatively low risk for conflicts between Whooping Cranes and wind energy development. We suggest that this type of analysis be incorporated into the habitat conservation planning process in areas where incidental take permits are being considered for Whooping Cranes or other species of concern. Field surveys should always be conducted prior to construction to verify model predictions and understand baseline conditions.

cobi12199.pdf
Lasky JR, Yang J, Zhang G, Cao M, Tang Y, Keitt TH. The role of functional traits and individual variation in the co-occurrence of Ficus species. Ecology [Internet]. 2014;95 :978–990. Publisher's Version 13-04372e1.pdf
2013
Lasky JR, Keitt TH. Reserve Size and Fragmentation Alter Community Assembly, Diversity, and Dynamics. The American Naturalist [Internet]. 2013;182 :pp. E142-E160. Publisher's VersionAbstract

Abstract Researchers have disputed whether a single large habitat reserve will support more species than many small reserves. However, relatively little is known from a theoretical perspective about how reserve size affects competitive communities structured by spatial abiotic gradients. We investigate how reserve size affects theoretical communities whose assembly is governed by dispersal limitation, abiotic niche differentiation, and source-sink dynamics. Simulations were conducted with varying scales of dispersal across landscapes with variable environmental spatial autocorrelation. Landscapes were inhabited by simulated trees with seedling and adult stages. For a fixed total area in reserves, we found that small reserve systems increased the distance between environments dominated by different species, diminishing the effects of source-sink dynamics. As reserve size decreased, environmental limitations to community assembly became stronger, α species richness decreased, and γ richness increased. When dispersal occurred across short distances, a large reserve strategy caused greater stochastic community variation, greater α richness, and lower γ richness than in small reserve systems. We found that reserve size variation trades off between preserving different aspects of natural communities, including α diversity versus γ diversity. Optimal reserve size will depend on the importance of source-sink dynamics and the value placed on different characteristics of natural communities. Anthropogenic changes to the size and separation of remnant habitats can have far-reaching effects on community structure and assembly.

673205.pdf
Levy O, Buckley LBB, Keitt TH, Angilletta MJ. Modeling the costs of thermoregulation in lizards: the interplay between competition, climate and vegetation cover in Sceloporus undulatus, in Integrative and Comparative Biology. Vol 53. OXFORD UNIV PRESS INC JOURNALS DEPT, 2001 EVANS RD, CARY, NC 27513 USA ; 2013 :E126––E126. Publisher's Version
Kiniry JR, Anderson LC, Johnson M-VV, Behrman KD, Brakie M, Burner D, Cordsiemon RL, a. Fay P, Fritschi FB, Houx JH, et al. Perennial Biomass Grasses and the Mason–Dixon Line: Comparative Productivity across Latitudes in the Southern Great Plains. BioEnergy Research [Internet]. 2013;6 :276–291. Publisher's Version art3a10.10072fs12155-012-9254-7.pdf
Behrman KD, Kiniry JR, Winchell M, Juenger TE, Keitt TH. Spatial forecasting of switchgrass productivity under current and future climate change scenarios. Ecological Applications [Internet]. 2013;23 :73–85. Publisher's Version 12-04362e1.pdf
Lasky JR, Sun I-F, Su S-H, Chen Z-S, Keitt TH. Trait-mediated effects of environmental filtering on tree community dynamics Canham C. Journal of Ecology [Internet]. 2013;101 :722–733. Publisher's VersionAbstract

* Individual performance is a function of an individual's traits and its environment. This function, known as an environmental filter, varies in space and affects community composition. However, filters are poorly characterized because dispersal patterns can obscure environmental effects, and few studies utilize longitudinal data linking individual performance to environment. * We model the effects of environmental filters on demographic rates of nearly all tree species (99) in a 25-ha subtropical rain forest plot. We develop a hierarchical Bayesian model of environmental filtering, drawing inspiration from classic studies of intraspecific natural selection. We characterize the specific environmental gradients and trait axes most important in filtering of demographic rates across species. * We found that stronger filtering along a given trait axis corresponded to less spatial variation in the value of favoured traits. * Environmental gradients associated with filtering were different for growth versus survivorship. * Species maximum height was under the strongest filtering for growth, with shorter species favoured on convex ridges. Shorter stature species may be favoured on ridges because trees on ridges experience higher wind damage and lower soil moisture. * Wood density filtering had the strongest effects on survival. Steep slopes and high available P in the soil favoured species with low-density wood. Such sites may be favourable for fast-growing species that exploit resource-rich environments. * Synthesis: We characterized trait-mediated environmental filters that may underlie spatial niche differentiation and life-history trade-offs, which can promote species coexistence. Filtering along trait axes with the strongest effects on local community composition, that is, traits with the strongest filtering, may necessarily have a weaker potential to promote species coexistence across the plot. The weak spatial variation in filters with strong effects on demography may result from long-term processes affecting the species pool that favour habitat generalist strategies.

lasky_et_al-2013-journal_of_ecology.pdf
2012
Pinto N, Keitt TH, Wainright M. LORACS: JAVA software for modeling landscape connectivity and matrix permeability. Ecography [Internet]. 2012;35 :388–392. Publisher's Version
Keitt TH. Meshpoly. [Internet]. 2012. Publisher's Version

Pages