Nils Chr. Stenseth

Ecological mechanisms and landscape ecology

Landscape ecology deals with the effects of the spatial configuration of mosaics on a wide variety of ecological phenomena. Because problems in many areas of conservation biology and resource management are related to landscape use, development of a rigorous theoretical and empirical foundation for landscape ecology is essential. We present an approach to research that focuses on how individual-level mechanisms operating in a heterogeneous mosaic produce ecological patterns that are spatially dependent. The theoretical framework that we develop considers the density and distribution of a population among patches as a function of (a) within-patch movement patterns of individuals; (b) emigration from patches as a function of population density, patch configuration, patch context, and within-patch movement; and (c) loss of individuals as they disperse through landscape elements

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...

Climate, changing phenology, and other life history traits: Nonlinearity and match–mismatch to the environment

Climatologists tell us that Earths climate is changing (1): It currently seems clear that a warmer climate is developing in the northern hemisphere, and that the weather will become more variable (2, 3). As part of this global change, seasonal patterns are being altered to make spring conditions occur earlier in the year in the north (4), without necessarily corresponding changes in more southern latitudes (5).

Rapid Advance of Spring Arrival Dates in Long-Distance Migratory Birds

Several bird species have advanced the timing of their spring migration in response to recent climate change. European short-distance migrants, wintering in temperate areas, have been assumed to be more affected by change in the European climate than long-distance migrants wintering in the tropics. However, we show that long-distance migrants have advanced their spring arrival in Scandinavia more than short-distance migrants. By analyzing a long-term data set from southern Italy, we show that long-distance migrants also pass through the Mediterranean region earlier. We argue that this may reflect a climate-driven evolutionary change in the timing of spring migration.

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.

Wavelet analysis of ecological time series

Wavelet analysis is a powerful tool that is already in use throughout science and engineering. The versatility and attractiveness of the wavelet approach lie in its decomposition properties, principally its time-scale localization. It is especially relevant to the analysis of non-stationary systems, i.e., systems with short-lived transient components, like those observed in ecological systems. Here, we review the basic properties of the wavelet approach for time-series analysis from an ecological perspective. Wavelet decomposition offers several advantages that are discussed in this paper and illustrated by appropriate synthetic and ecological examples. Wavelet analysis is notably free from the assumption of stationarity that makes most methods unsuitable for many ecological time series. Wavelet analysis also permits analysis of the relationships between two signals, and it is especially appropriate for following gradual change in forcing by exogenous variables.

Plague: Past, Present, and Future

The authors argue that plague should be taken much more seriously by the international health community.

Linking climate change to lemming cycles

The population cycles of rodents at northern latitudes have puzzled people for centuries, and their impact is manifest throughout the alpine ecosystem. Climate change is known to be able to drive animal population dynamics between stable and cyclic phases, and has been suggested to cause the recent changes in cyclic dynamics of rodents and their predators. But although predator–rodent interactions are commonly argued to be the cause of the Fennoscandian rodent cycles, the role of the environment in the modulation of such dynamics is often poorly understood in natural systems. Hence, quantitative links between climate-driven processes and rodent dynamics have so far been lacking. Here we show that winter weather and snow conditions, together with density dependence in the net population growth rate, account for the observed population dynamics of the rodent community dominated by lemmings (Lemmus lemmus) in an alpine Norwegian core habitat between 1970 and 1997, and predict the observed absence of rodent peak years after 1994. These local rodent dynamics are coherent with alpine bird dynamics both locally and over all of southern Norway, consistent with the influence of large-scale fluctuations in winter conditions. The relationship between commonly available meteorological data and snow conditions indicates that changes in temperature and humidity, and thus conditions in the subnivean space, seem to markedly affect the dynamics of alpine rodents and their linked groups. The pattern of less regular rodent peaks, and corresponding changes in the overall dynamics of the alpine ecosystem, thus seems likely to prevail over a growing area under projected climate change.

The Response of Terrestrial Ecosystems to Climate Variability Associated with the North Atlantic Oscillation

Climatic factors influence a variety of ecological processes determining patterns of species density and distribution in a wide range of terrestrial ecosystems. We review the effects of the NAO on processes and patterns of terrestrial ecosystems, including both plants and animals. In plants, the NAO index correlates with date of first flowering, tree ring growth and with quality of agricultural crops (wheat and wine grapes). Also, breeding dates are earlier after high NAO index winters for amphibians and birds in Europe. Population dynamical consequences of the NAO have also been reported for birds, and the differential impact of the NAO on two similar species may prevent competitive exclusion. Different effects of the NAO on large herbivore populations have been reported for different regions, depending on limiting factors and the correlation with local weather parameters. The NAO synchronizes population dynamics of lynx and some other carnivore populations in the eastern U.S. Most effects are on an ecological time scale; the evolutionary consequences of long term trends in the NAO are poorly documented. Important for predator and prey dynamics is (1) the disruption of phenology (the match-mismatch hypothesis), (2) that there may be delayed effects (cohort-effects), and (3) that effects of the NAO may interact with other factors such as density. We discuss the challenges related to nonlinearity, of using different climate indices, and how we can progress using these pattern-oriented NAO studies at coarse scales to conduct better process-oriented small-scale experiments.