Atle Mysterud

Review article. Studying climate effects on ecology through the use of climate indices: the North Atlantic Oscillation, El Niño Southern Oscillation and beyond

Whereas the El Niño Southern Oscillation (ENSO) affects weather and climate variability worldwide, the North Atlantic Oscillation (NAO) represents the dominant climate pattern in the North Atlantic region. Both climate systems have been demonstrated to considerably influence ecological processes. Several other large–scale climate patterns also exist. Although less well known outside the field of climatology, these patterns are also likely to be of ecological interest. We provide an overview of these climate patterns within the context of the ecological effects of climate variability. The application of climate indices by definition reduces complex space and time variability into simple measures, ‘packages of weather’. The disadvantages of using global climate indices are all related to the fact that another level of problems are added to the ecology–climate interface, namely the link between global climate indices and local climate. We identify issues related to: (i) spatial variation; (ii) seasonality; (iii) non–stationarity; (iv) nonlinearity; and (v) lack of correlation in the relationship between global and local climate. The main advantages of using global climate indices are: (i) biological effects may be related more strongly to global indices than to any single local climate variable; (ii) it helps to avoid problems of model selection; (iii) it opens the possibility for ecologists to make predictions; and (iv) they are typically readily available on Internet.

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).

FUNCTIONAL RESPONSES IN HABITAT USE: AVAILABILITY INFLUENCES RELATIVE USE IN TRADE-OFF SITUATIONS

Current methods for evaluating habitat selection from animal space-use ob- servations ignore possible interactions between time allocation patterns relative to different resources, their relative abundance, and their spatial arrangements. Habitat selection may occur in situations in which animals experience a trade-off, e.g., between time used foraging in areas with abundant forage but poor protective cover, and time used for resting in areas with good protective cover but low forage abundance. We show how functional responses in habitat use (i.e., change in preference with availability of one of two main habitat types) may be tested. Given radio-telemetry data for a sample of individuals, binomial logit models can be used to regress proportionate use of a habitat type P(u) against the proportion of that habitat available, P(a). Given an appropriate fit to the data by a linear predictor on a logit scale, functional response will be indicated by a estimated slope parameter ? 1, while a slope = 0 will indicate a consistent use as availability changes. Habitat preference is inferred from the logit regression parameters when the fitted value of the proportion of use at a specified proportion of availability, is significantly greater than the proportional availability.

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.

Nonlinear effects of large-scale climatic variability on wild and domestic herbivores.

Large-scale climatic fluctuations, such as the North Atlantic Oscillation (NAO), have been shown to affect many ecological processes. Such effects have been typically assumed to be linear. Only one study has reported a nonlinear relation; however, that nonlinear relation was monotonic (that is, no reversal). Here we show that there is a strong nonlinear and non-monotonic (that is, reversed) effect of the NAO on body weight during the subsequent autumn for 23,838 individual wild red deer (Cervus elaphus) and 139,485 individual domestic sheep (Ovis aries) sampled over several decades on the west coast of Norway. These relationships are, at least in part, explained by comparable nonlinear and non-monotonic relations between the NAO and local climatic variables (temperature, precipitation and snow depth). The similar patterns observed for red deer and sheep, the latter of which live indoors during winter and so experience a stable energy supply in winter, suggest that the (winter) climatic variability (for which the index is a proxy) must influence the summer foraging conditions directly or indirectly.

Temporal scales, trade-offs, and functional responses in red deer habitat selection

Animals selecting habitats often have to consider many factors, e.g., food and cover for safety. However, each habitat type often lacks an adequate mixture of these factors. Analyses of habitat selection using resource selection functions (RSFs) for animal radiotelemetry data typically ignore trade-offs, and the fact that these may change during an animals daily foraging and resting rhythm on a short-term basis. This may lead to changes in the relative use of habitat types if availability differs among individual home ranges, called functional responses in habitat selection. Here, we identify such functional responses and their underlying behavioral mechanisms by estimating RSFs through mixed-effects logistic regression of telemetry data on 62 female red deer (Cervus elaphus) in Norway. Habitat selection changed with time of day and activity, suggesting a trade-off in habitat selection related to forage quantity or quality vs. shelter. Red deer frequently used pastures offering abundant forage and little canopy cover during nighttime when actively foraging, while spending much of their time in forested habitats with less forage but more cover during daytime when they are more often inactive. Selection for pastures was higher when availability was low and decreased with increasing availability. Moreover, we show for the first time that in the real world with forest habitats also containing some forage, there was both increasing selection of pastures (i.e., not proportional use) and reduced time spent in pastures (i.e., not constant time use) with lowered availability of pastures within the home range. Our study demonstrates that landscape-level habitat composition modifies the trade-off between food and cover for large herbivorous mammals. Consequently, landscapes are likely to differ in their vulnerability to crop damage and threat to biodiversity from grazing.

Age- and density-dependent reproductive effort in male red deer.

Reproductive effort in female ungulates originates from gestation and lactation and has been studied extensively; however, no comparable studies of reproductive effort in males (due to fighting for access to mates) have, to our knowledge, previously been reported. Here, we report on weight loss of male red deer during the annual mating season—a direct measure of male reproductive effort (or somatic reproductive costs). The ‘terminal investmen’ hypothesis predicts that reproductive effort should increase with age, given that costs remain stable. We also propose the ‘mating strategy–effort’ hypothesis, which predicts that reproductive effort peaks in prime–aged males, since they are most often the harem holders. Consistent with the mating strategy–effort hypothesis, relative weight loss during the rutting season peaked at prime age and was lower in younger and senescent males. Weight loss during the rut was relatively smaller as density increased and more so for older males. This is probably primarily due to males (particularly senescent males) starting their rut in poorer condition at high density. The pattern of reproductive effort in males with regard to age and density therefore differs markedly from the pattern reported for females.

The relationship between ecological segregation and sexual body size dimorphism in large herbivores

Abstract Ecological segregation (sexual differences in diet or habitat use) in large herbivores has been intimately linked to sexual body size dimorphism, and may affect both performance and survival of the sexes. However, no one has tested comparatively whether segregation occurs at a higher frequency among more dimorphic species. To test this comparatively, data on sex-specific diet, habitat use and body size of 40 species of large herbivores were extracted from the literature. The frequency of ecological segregation was higher among more dimorphic herbivores; however, this was only significant for browsers. This provides the first evidence that segregation is more common among more dimorphic species. The comparative evidence supported the nutritional-needs hypothesis over the incisor breadth hypothesis, as there was no difference in frequency of segregation between seasons with high and low resource levels, and since segregation was also evident among browsers. Whether the absence of a correlation between ecological segregation and level of sexual body size dimorphism for intermediate feeders and grazers is due to biological differences relative to browsers or to the fact that the monomorphic species included in the analysis were all browsers is discussed.

The concept of overgrazing and its role in management of large herbivores

Abstract Increasing populations of cervids in Europe and North America have made the issue of overgrazing relevant outside areas with domestic or semi-domestic herbivores. Over grazing is defined depending on management objectives. I focus on challenges related to implementing a ‘range ecologist’ baseline, defining overgrazing as situations when ‘forage species are not able to maintain themselves over time due to an excess of herbivory or related processes’. Herbivores may be naturally regulated at ecological carrying capacity (K) with no overgrazing, but overgrazing may occur below K. Rare, preferred plant species can decline in density due to a ‘herbivore pit’ created by generalist herbivores, without having much effect on K. The concept of overgrazing is almost meaningless unless the issue of spatial scale is considered, and the extent to which preferred plant species decline in coverage. Herbivore population instability increases with increased population growth rate, but overgrazing depends also on the tolerance to grazing of the forage used by a given herbivore, which is closely related to functional plant traits. Ecosystem factors such as soil quality and slope also affect the likelihood that overgrazing will occur. Currently we can only qualitatively identify some important factors to consider. A better understanding of the sequence of events happening to performance of both animals and plants over time when a herbivore population increases provides a very useful approach until tools are developed to measure overgrazing quantitatively. More detailed knowledge about grazing effects on biodiversity is necessary to implement a broader ecosystem perspective of overgrazing.

Importance of climatological downscaling and plant phenology for red deer in heterogeneous landscapes

Understanding how climate influences ecosystems represents a challenge in ecology and natural resource management. Although we know that climate affects plant phenology and herbivore performances at any single site, no study has directly coupled the topography–climate interaction (i.e. the climatological downscaling process) with large-scale vegetation dynamics and animal performances. Here we show how climatic variability (measured by the North Atlantic oscillation ‘NAO’) interacts with local topography in determining the vegetative greenness (as measured by the normalized difference vegetation index ‘NDVI’) and the body masses and seasonal movements of red deer (Cervus elaphus) in Norway. Warm springs induced an earlier onset of vegetation, resulting in earlier migration and higher body masses. Increasing values of the winter-NAO corresponded to less snow at low altitude (warmer, more precipitation results in more rain), but more snow at high altitude (colder, more precipitation corresponds to more snow) relative to winters with low winter-NAO. An increasing NAO thus results in a spatially more variable phenology, offering migrating deer an extended period with access to high-quality forage leading to increased body mass. Our results emphasize the importance of incorporating spring as well as the interaction between winter climate and topography when aiming at understanding how plant and animal respond to climate change.