Are responses of herbivores to environmental variability spatially consistent in alpine ecosystems?

Abstract

Animal responses to global climate variation might be spatially inconsistent. This may arise from spatial variation in factors limiting populations' growth or from differences in the links between global climate patterns and ecologically relevant local climate variation. For example, the North Atlantic Oscillation (NAO) has a spatially consistent relation to temperature, but inconsistent spatial relation to snow depth in Scandinavia. Furthermore, there are multiple mechanistic ways by which climate may limit animal populations, involving both direct effects through thermoregulation and indirect pathways through trophic interactions. It is conceptually appealing to directly model the predicted mechanistic links. This includes the use of climate variables mimicking such interactions, for example, to use growing degree days (GDD) as a proxy for plant growth rather than average monthly temperature. Using a unique database of autumn body mass of 83331 domestic lambs from the period 1992-2007 in four alpine ranges in Norway, we demonstrate the utility of hierarchical, mechanistic path models fitted using a Bayesian approach to analyse explicitly predicted relationships among environmental variables and between lamb body mass and the environmental variables. We found large spatial variation in strength of responses of autumn lamb body mass to the NAO, to a proxy for plant growth in spring (the Normalized Difference Vegetation Index, NDVI) and effects even differed in direction to local summer climate. Average local temperature outperformed GDD as a predictor of the NDVI, whereas the NAO index in two areas outperformed local weather variables as a predictor of lamb body mass, despite the weaker mechanistic link. Our study highlights that spatial variation in strength of herbivore responses may arise from several processes. Furthermore, mechanistically more appealing measures do not always increase predictive power due to scale of measurement and since global measures may provide more relevant "weather packages" for larger scales.

Livestock management at northern latitudes

“…In particular, winter conditions affect lamb autumn weights indirectly through snow melt effects on the vegetation (Mysterud et al 2011, Nielsen et al 2012, while spring temperature and precipitation has an indirect effect through their effect on plant spring phenology (Nielsen et al 2012). After the grazing season, the animals are mustered and the wool is shorn.…”

“…It has previously been shown that local weather conditions during winter, in spring (before the animals are released to the mountains) and during summer (the outfield grazing season) affect lamb weights (Nielsen et al 2012). However, which weather variable (snow depth the previous winter, precipitation or temperature in spring or summer) that is most significant varies among Norwegian mountain ranges; not only in strength, but also in direction.…”

“…Throughout the outdoor grazing season, lamb growth is affected by climate conditions, both directly, but also indirectly through climate effects on the vegetation (Nielsen et al 2012). …”

“…These projections indicate that Nordic sheep farmers will face novel climate conditions in the future. Nielsen et al (2012) showed that in southern Norway increased spring temperature would have contrasting effects on lamb autumn body mass, depending on the location of the areas where the animals are kept during the outdoor grazing season. This indicates that any attempt to include weather conditions and climate change in optimization models for individual farmers has to be site specific.…”

“…Second, along with empirical findings, we consider increased spring temperature as having a positive or negative effect on lamb slaughter weights depending on the specific site of consideration; that is, the spatial pattern and the location of the farm play a role. We focus on two mountain ranges and two scenarios; the Northern scenario, exemplified by Forollhogna in Trøndelag and the Southwestern scenario, exemplified by the western side of Hardangervidda, where increased spring temperature has been shown to have a positive and negative effect, respectively, on lamb growth over summer (Nielsen et al 2012). See Figure 1.…”