Joshua J. Tewksbury

Impacts of climate warming on terrestrial ectotherms across latitude

The impact of anthropogenic climate change on terrestrial organisms is often predicted to increase with latitude, in parallel with the rate of warming. Yet the biological impact of rising temperatures also depends on the physiological sensitivity of organisms to temperature change. We integrate empirical fitness curves describing the thermal tolerance of terrestrial insects from around the world with the projected geographic distribution of climate change for the next century to estimate the direct impact of warming on insect fitness across latitude. The results show that warming in the tropics, although relatively small in magnitude, is likely to have the most deleterious consequences because tropical insects are relatively sensitive to temperature change and are currently living very close to their optimal temperature. In contrast, species at higher latitudes have broader thermal tolerance and are living in climates that are currently cooler than their physiological optima, so that warming may even enhance their fitness. Available thermal tolerance data for several vertebrate taxa exhibit similar patterns, suggesting that these results are general for terrestrial ectotherms. Our analyses imply that, in the absence of ameliorating factors such as migration and adaptation, the greatest extinction risks from global warming may be in the tropics, where biological diversity is also greatest.

A framework for community interactions under climate change.

Predicting the impacts of climate change on species is one of the biggest challenges that ecologists face. Predictions routinely focus on the direct effects of climate change on individual species, yet interactions between species can strongly influence how climate change affects organisms at every scale by altering their individual fitness, geographic ranges and the structure and dynamics of their community. Failure to incorporate these interactions limits the ability to predict responses of species to climate change. We propose a framework based on ideas from global-change biology, community ecology, and invasion biology that uses community modules to assess how species interactions shape responses to climate change.

Putting the Heat on Tropical Animals

I mpacts of climate warming in the tropics— the cradle of biodiversity—are often predicted to be small relative to those in temperate regions (1, 2), because the rate of climate warming in the tropics is lower than at higher latitudes (3). Yet, predictions based only on the magnitude of climate change may be misleading. Models that include organismal physiology suggest that impacts of climate warming may be more severe in the tropics than in temperate regions. The impacts of climate warming on organisms depend not only on the magnitude of the environmental temperature shift but also on the behavior, morphology, physiology, and ecology of the organisms in question (4–6). This added complexity is daunting, but some general principles are emerging from research focused mainly on ectothermal animals (such as insects, fish, reptiles, and amphibians), which cannot maintain a constant internal body temperature. Negative impacts should be greatest on animals that are physiologically specialized with respect to temperature (7) and have limited acclimation capacity (8). Further, species living in warm climates are likely to suffer disproportionately from small increases in temperature (9), and species that live in aseasonal environments may be particularly vulnerable to increases in temperature, because changes in behavior and physiology are less likely to provide relief from rising temperatures (10). Terrestrial ectotherms with these vulnerability traits are typically tropical (7, 11, 12). In the 1960s, Janzen (13) noted that tropical ectotherms should be thermal specialists (see the figure, top) and have limited acclimation capacities, relative to higher-latitude species, because they have evolved in relatively constant, aseasonal environments. These predictions have been largely validated for various terrestrial and aquatic ectotherms (7, 11, 12, 14–17), yet the implications of this pattern for species vulnerabilities to climate change have rarely been investigated (15, 17–19). Tropical ectotherms have other traits that increase vulnerability. Because tropical organisms experience far more warm weather throughout the year than do temperate organisms, tropical animals might be expected to have greater heat tolerance. Surprisingly, that is often not the case: Heat tolerance typically varies very little across latitude in terrestrial ectotherms (7, 12, 15). Thus, many tropical ectotherms live much of the year in environments where equilibrium body (“operative”) temperatures are near or above optimal temperatures for performance (15). Tropical forest species may be particularly vulnerable, because they live in constant shade, are not generally adapted to the high operative temperatures found in warmer open habitats, and have few behavioral options available to evade rising temperatures (10, 15). Any climateinduced increase in operative temperaturecould cause steep declines in thermal performance and Darwinian fitness (see the figure, top). To assess whether independent data support these assertions, longterm demographic data on tropical species are required. Such data are rare, but in the study of frogs and lizards in lowland Costa Rica, densities have declined by ~4% per year between 1970 and 2005 (20). These declines are explained by climatedriven declines in leaf litter on the forest floor over the study period. Theoretically, these patterns can cut both ways: The same factors that make tropical ectotherms vulnerable to changing climate may benefit some temperate ectotherms (15) (see the figure, bottom). Empirical data tell a more complex story. During the last rapid warming event, 50 million years ago, insect damage on temperate plants did increase sharply (21), but data on contemporary temperate-zone insects are mixed: Some species are expanding rapidly (22), occasionally causing large changes to ecosystems and economies (23), whereas others—often specialists relying on day-length cues and species living in disappearing high-elevation habitats—are predicted to decline (6). All these predictions are for terrestrial habitats, and patterns may differ elsewhere. In marine habitats, for example, thermal specialists occur both at low and high latitudes, and thermal generalists appear most common at mid-latitudes (9, 24). Yet this pattern tracks the seasonality of ocean surface temperatures—polar oceans are cold but show little temperature variation throughout the year, and the largest seasonality in ocean surface temperatures are seen at mid-latitudes. Therefore, both tropical and high-latitude species live at near-stressful temperatures and could be vulnerable to warming (24). In intertidal habitats, which Putting the Heat on Tropical Animals ECOLOGY

Why tropical forest lizards are vulnerable to climate warming

Biological impacts of climate warming are predicted to increase with latitude, paralleling increases in warming. However, the magnitude of impacts depends not only on the degree of warming but also on the number of species at risk, their physiological sensitivity to warming and their options for behavioural and physiological compensation. Lizards are useful for evaluating risks of warming because their thermal biology is well studied. We conducted macrophysiological analyses of diurnal lizards from diverse latitudes plus focal species analyses of Puerto Rican Anolis and Sphaerodactyus. Although tropical lowland lizards live in environments that are warm all year, macrophysiological analyses indicate that some tropical lineages (thermoconformers that live in forests) are active at low body temperature and are intolerant of warm temperatures. Focal species analyses show that some tropical forest lizards were already experiencing stressful body temperatures in summer when studied several decades ago. Simulations suggest that warming will not only further depress their physiological performance in summer, but will also enable warm-adapted, open-habitat competitors and predators to invade forests. Forest lizards are key components of tropical ecosystems, but appear vulnerable to the cascading physiological and ecological effects of climate warming, even though rates of tropical warming may be relatively low.

Corridors affect plants, animals, and their interactions in fragmented landscapes

Among the most popular strategies for maintaining populations of both plants and animals in fragmented landscapes is to connect isolated patches with thin strips of habitat, called corridors. Corridors are thought to increase the exchange of individuals between habitat patches, promoting genetic exchange and reducing population fluctuations. Empirical studies addressing the effects of corridors have either been small in scale or have ignored confounding effects of increased habitat area created by the presence of a corridor. These methodological difficulties, coupled with a paucity of studies examining the effects of corridors on plants and plant–animal interactions, have sparked debate over the purported value of corridors in conservation planning. We report results of a large-scale experiment that directly address this debate. In eight large-scale experimental landscapes that control for patch area and test alternative mechanisms of corridor function, we demonstrate that corridors not only increase the exchange of animals between patches, but also facilitate two key plant–animal interactions: pollination and seed dispersal. Our results show that the beneficial effects of corridors extend beyond the area they add, and suggest that increased plant and animal movement through corridors will have positive impacts on plant populations and community interactions in fragmented landscapes.

Big data and the future of ecology

The need for sound ecological science has escalated alongside the rise of the information age and “big data” across all sectors of society. Big data generally refer to massive volumes of data not readily handled by the usual data tools and practices and present unprecedented opportunities for advancing science and inform- ing resource management through data-intensive approaches. The era of big data need not be propelled only by “big science” – the term used to describe large-scale efforts that have had mixed success in the individual-driven culture of ecology. Collectively, ecologists already have big data to bolster the scientific effort – a large volume of distributed, high-value information – but many simply fail to contribute. We encourage ecologists to join the larger scientific community in global initiatives to address major scientific and societal problems by bringing their distributed data to the table and harnessing its collective power. The scientists who contribute such information will be at the forefront of socially relevant science – but will they be ecologists?

Corridors Increase Plant Species Richness at Large Scales

Habitat fragmentation is one of the largest threats to biodiversity. Landscape corridors, which are hypothesized to reduce the negative consequences of fragmentation, have become common features of ecological management plans worldwide. Despite their popularity, there is little evidence documenting the effectiveness of corridors in preserving biodiversity at large scales. Using a large-scale replicated experiment, we showed that habitat patches connected by corridors retain more native plant species than do isolated patches, that this difference increases over time, and that corridors do not promote invasion by exotic species. Our results support the use of corridors in biodiversity conservation.

BREEDING PRODUCTIVITY DOES NOT DECLINE WITH INCREASING FRAGMENTATION IN A WESTERN LANDSCAPE

Fragmentation of breeding habitat may cause declines in many bird populations. Our perception of the demographic effects of habitat fragmentation comes primarily from studies in the midwestern and eastern United States and Scandinavia. We know very little about the demographic effects of anthropogenically caused habitat fragmentation in habitats prone to natural disturbance, as is typical of most forest types in the western United States. We located and monitored 1916 nests on eight sites located in mostly forested landscapes and eight sites located in primarily agricultural landscapes to study the effects of landscape-level fragmentation on nest predation and brood parasitism in riparian areas in western Montana. Patterns of nest predation were opposite those documented from more eastern locales; predation rates were higher in forested landscapes than in fragmented landscapes dominated by agriculture. This pattern probably reflects the importance of forest predators in these landscapes: red squirrels (Ta...

Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size

Positive interactions often play an important role in structuring plant communities and increasing biological diversity. Using three scales of resolution, we examine the importance of a long-lived desert tree, ironwood (Olneya tesota), in structuring plant communities and promoting biological diversity in the Sonoran Desert. We examined the positive effects of Olneya canopies of different sizes on plant communities in mesic and xeric habitats throughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abundance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya canopies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefactor size also mediated positive interactions, with larger Olneya canopies supporting larger perennials in both xeric and mesic sites. Thus stress gradients and benefactor size both influenced the balance of facilitative and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative effects shape community structure.

Seed dispersal: Directed deterrence by capsaicin in chillies

The primary function of ripe, fleshy fruit is to facilitate seed dispersal by attracting consumers, yet many fruits contain unpleasant-tasting chemicals that deter consumption by vertebrates. Here we investigate this paradox in the chilli (Capsicum) and find that capsaicin, the chemical responsible for the fruits peppery heat, selectively discourages vertebrate predators without deterring more effective seed dispersers.