Eric Post

Ecological responses to recent climate change.

There is now ample evidence of the ecological impacts of recent climate change, from polar terrestrial to tropical marine environments. The responses of both flora and fauna span an array of ecosystems and organizational hierarchies, from the species to the community levels. Despite continued uncertainty as to community and ecosystem trajectories under global change, our review exposes a coherent pattern of ecological change across systems. Although we are only at an early stage in the projected trends of global warming, ecological responses to recent climate change are already clearly visible.

Ecological Dynamics Across the Arctic Associated with Recent Climate Change

Assessing the Arctic The Arctic is experiencing some of the most rapid climate change currently under way across the globe, but consequent ecological responses have not been widely reported. At the close of the Fourth International Polar Year, Post et al. (p. 1355) review observations on ecological impacts in this sensitive region. The widespread changes occurring in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering. At the close of the Fourth International Polar Year, we take stock of the ecological consequences of recent climate change in the Arctic, focusing on effects at population, community, and ecosystem scales. Despite the buffering effect of landscape heterogeneity, Arctic ecosystems and the trophic relationships that structure them have been severely perturbed. These rapid changes may be a bellwether of changes to come at lower latitudes and have the potential to affect ecosystem services related to natural resources, food production, climate regulation, and cultural integrity. We highlight areas of ecological research that deserve priority as the Arctic continues to warm.

Ecological effects of the North Atlantic Oscillation

Climatic oscillations as reflected in atmospheric modes such as the North Atlantic Oscillation (NAO) may be seen as a proxy for regulating forces in aquatic and terrestrial ecosystems. Our review highlights the variety of climate processes related to the NAO and the diversity in the type of ecological responses that different biological groups can display. Available evidence suggests that the NAO influences ecological dynamics in both marine and terrestrial systems, and its effects may be seen in variation at the individual, population and community levels. The ecological responses to the NAO encompass changes in timing of reproduction, population dynamics, abundance, spatial distribution and interspecific relationships such as competition and predator-prey relationships. This indicates that local responses to large-scale changes may be more subtle than previously suggested. We propose that the NAO effects may be classified as three types: direct, indirect and integrated. Such a classification will help the design and interpretation of analyses attempting to relate ecological changes to the NAO and, possibly, to climate in general.

CLIMATIC VARIABILITY, PLANT PHENOLOGY, AND NORTHERN UNGULATES

Models of climate change predict that global temperatures and precipitation will increase within the next century, with the most pronounced changes occurring in northern latitudes and during winter. A large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO), is a strong determinant of both interannual variation and decadal trends in temperatures and precipitation during winter in northern latitudes, and its recent persistence in one extreme phase may be a substantial component of increases in global temperatures. Hence, we investigated the influences of large-scale climatic variability on plant phenology and ungulate population ecology by incorporating the NAO in statistical analyses of previously published data on: (1) the timing of flowering by plants in Norway, and (2) phenotypic and demographic variation in populations of northern ungulates. We analyzed 137 time series on plant phenology for 13 species of plants in Norway spanning up to 50 yr (44 ± 0.5 yr, mean ± 1 se) and 39 time seri...

Filling key gaps in population and community ecology

We propose research to fill key gaps in the areas of population and community ecology, based on a National Science Foundation workshop identifying funding priorities for the next 5-10 years. Our vision for the near future of ecology focuses on three core areas: predicting the strength and context-dependence of species interactions across multiple scales; identifying the importance of feedbacks from individual interactions to ecosystem dynam- ics; and linking pattern with process to understand species coexistence. We outline a combination of theory devel- opment and explicit, realistic tests of hypotheses needed to advance population and community ecology.

Climate change reduces reproductive success of an Arctic herbivore through trophic mismatch

In highly seasonal environments, offspring production by vertebrates is timed to coincide with the annual peak of resource availability. For herbivores, this resource peak is represented by the annual onset and progression of the plant growth season. As plant phenology advances in response to climatic warming, there is potential for development of a mismatch between the peak of resource demands by reproducing herbivores and the peak of resource availability. For migratory herbivores, such as caribou, development of a trophic mismatch is particularly likely because the timing of their seasonal migration to summer ranges, where calves are born, is cued by changes in day length, while onset of the plant-growing season on the same ranges is cued by local temperatures. Using data collected since 1993 on timing of calving by caribou and timing of plant growth in West Greenland, we document the consequences for reproductive success of a developing trophic mismatch between caribou and their forage plants. As mean spring temperatures at our study site have risen by more than 4°C, caribou have not kept pace with advancement of the plant-growing season on their calving range. As a consequence, offspring mortality has risen and offspring production has dropped fourfold.

Opposing plant community responses to warming with and without herbivores

If controls over primary productivity and plant community composition are mainly environmental, as opposed to biological, then global change may result in large-scale alterations in ecosystem structure and function. This view appears to be favored among investigations of plant biomass and community responses to experimental and observed warming. In far northern and arctic ecosystems, such studies predict increasing dominance of woody shrubs with future warming and emphasize the carbon (C)-sequestration potential and consequent atmospheric feedback potential of such responses. In contrast to previous studies, we incorporated natural herbivory by muskoxen and caribou into a 5-year experimental investigation of arctic plant community response to warming. In accordance with other studies, warming increased total community biomass by promoting growth of deciduous shrubs (dwarf birch and gray willow). However, muskoxen and caribou reduced total community biomass response, and responses of birch and willow, to warming by 19%, 46%, and 11%, respectively. Furthermore, under warming alone, the plant community shifted after 5 years away from graminoid-dominated toward dwarf birch-dominated. In contrast, where herbivores grazed, plant community composition on warmed plots did not differ from that on ambient plots after 5 years. These results highlight the potentially important and overlooked influences of vertebrate herbivores on plant community response to warming and emphasize that conservation and management of large herbivores may be an important component of mitigating ecosystem response to climate change.

Ecosystem consequences of wolf behavioural response to climate

Because apex predators exert considerable influence on the structure and function of top-down ecosystems, their responses to climate may shape responses at lower trophic levels. Previous reports of trophic cascades and ecosystem dynamics induced by predators have focused on changes in their abundance, whereas we investigated whether changes in predator behaviour could precipitate cascades of similar ecological scale. Here we report the ecological consequences of predator behavioural response to global climatic variation using 40 years of data on wolf predation from Isle Royale, USA, where wolves limit abundance of moose, which limit productivity of fir trees. In response to increases in winter snow related to the North Atlantic Oscillation, wolves hunted in larger packs and, consequently, tripled the number of moose killed per day compared with less snowy years when they hunted in smaller packs. Following increased predation rates, moose abundance declined, and, following release from heavy browsing, growth of understory fir increased. Hence, cascading behavioural responses of apex predators may be a substantial link in the pathway from climatic change to ecosystem function.

Breeding phenology and climate

European amphibians and birds have been breeding consistently earlier over the past two to three decades,. These changes have been attributed to the observed trends in increasing average spring temperatures in Europe producing earlier growing seasons and increased forage availability. Here we show that variations in breeding of European amphibians and birds are influenced by variations in a natural, large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO). Our results support the proximate cause (that is, increase in spring temperatures) of the altered breeding phenology as suggested previously,, but by extending previous analyses as well as integrating data from other bird species, they also suggest that organisms with complex life histories respond to year-to-year variations in their abiotic environment.

The effects of phenological mismatches on demography

Climate change is altering the phenology of species across the world, but what are the consequences of these phenological changes for the demography and population dynamics of species? Time-sensitive relationships, such as migration, breeding and predation, may be disrupted or altered, which may in turn alter the rates of reproduction and survival, leading some populations to decline and others to increase in abundance. However, finding evidence for disrupted relationships, or lack thereof, and their demographic effects, is difficult because the necessary detailed observational data are rare. Moreover, we do not know how sensitive species will generally be to phenological mismatches when they occur. Existing long-term studies provide preliminary data for analysing the phenology and demography of species in several locations. In many instances, though, observational protocols may need to be optimized to characterize timing-based multi-trophic interactions. As a basis for future research, we outline some of the key questions and approaches to improving our understanding of the relationships among phenology, demography and climate in a multi-trophic context. There are many challenges associated with this line of research, not the least of which is the need for detailed, long-term data on many organisms in a single system. However, we identify key questions that can be addressed with data that already exist and propose approaches that could guide future research.