Date Published: May 21, 2019
Publisher: Public Library of Science
Author(s): James D. M. Speed, Gunnar Austrheim, Anders Lorentzen Kolstad, Erling J. Solberg, Emmanuel Serrano.
Herbivores have important impacts on ecological and ecosystem dynamics. Population density and species composition are both important determinants of these impacts. Large herbivore communities are shifting in many parts of the world driven by changes in livestock management and exploitation of wild populations. In this study, we analyse changes in large herbivore community structure over 66 years in Norway, with a focus on the contribution of wildlife and livestock. We calculate metabolic biomass of all large-herbivore species across the whole region between 1949 and 2015. Temporal and spatial patterns in herbivore community change are investigated and we test hypotheses that changes in wildlife biomass are driven by competition with livestock. We find that total herbivore biomass decreased from 1949 to a minimum in 1969 due to decreases in livestock biomass. Increasing wild herbivore populations lead to an increase in total herbivore biomass by 2009. Herbivore communities have thus reverted from a livestock dominated state in 1949 (2% of large herbivore metabolic biomass comprised of wildlife species) to a state with roughly equal wildlife and livestock (48% of metabolic biomass comprised of wildlife species). Declines in livestock biomass were a modest predictor of wildlife increases, suggesting that competition with livestock has not been a major limiting factor of wild herbivore populations over the past decades. Instead there was strong geographic variation in herbivore community change, with milder lowland regions becoming more dominated by wild species, but colder mountain and northern regions remaining dominated by livestock. Our findings indicate that there has been notable rewilding of herbivore communities and herbivore-ecosystem interactions in Norway, particularly in milder lowland regions. However, Norwegian herbivores remain mostly regulated by management, and our findings call for integrated management of wild and domestic herbivores.
Large herbivores drive ecological and ecosystem dynamics in many terrestrial ecosystems [1–3]. Individual herbivore species have unique effects on their habitat, depending largely on species characteristics such as body and group size, feeding strategies and other life history traits [4–6]. Large herbivore communities are undergoing rapid changes at global and local scales, including a biased loss of the largest species [1, 7], as well as a loss of trophic interactions and complexity [8–10]. Consequent shifts in the impacts of herbivores on ecosystems are thus expected, but to predict these shifts we first need to understand how the structure of herbivore communities is changing.
The study system used here is unenclosed land within Norway (utmark in Norwegian). Species of large herbivores (here defined as species >10 kg) using unenclosed land comprise both wild species and rangeland livestock. The wild species are the three forest cervids: moose (European elk, Alces alces), roe deer (Capreolus capreolus) and red deer (Cervus elaphus) and a population of musk oxen (Ovibos moschatus), reintroduced in the 1940s. Reindeer (Rangifer tarandus) include both wild and semi-domestic populations that were assessed separately. Four species of livestock use unenclosed land in Norway: domestic sheep (Ovis aries), cattle (Bos taurus), goat (Capra aegagrus hircus) and horse (Equus ferus caballus). The wild boar (Sus scrofa) has recently re-established in south-eastern Norway. Since its current distribution is highly limited, and abundance low , it is not included here. The herbivore species of Norway vary from species which are predominantly grazers (livestock) to browsers (wild ungulates), with red deer, and goat being intermediate feeders in this classification . The herbivores also vary in their use of outlying land , with some being associated mostly with forests (moose, roe deer, red deer) and others with mountains (sheep, reindeer, musk ox). Goats, cattle and horses have traditionally used all but the most alpine areas.
Total large herbivore metabolic biomass across Norway decreased from 127 kg km-2 (unenclosed land) in 1949 to a minimum of 72 kg km-2 in 1969. After this it increased again to 113 kg km-2 in 2009 before declining to 108 kg km-2 in 2015 (Fig 1). The decrease between 1949 and 1969 was driven by declining livestock metabolic biomass (in particular cattle, Fig 1), highest in 1949 at 120 and lowest during 1999 at 62 kg km-2. An increase in wild herbivore metabolic biomass (from a minimum of 6 kg km2 in 1949 to a peak of 47 kg km-2 in 2009, Fig 1) was behind the rise in total herbivore biomass from 1969 to 2009, and this was largely due to increases in moose and red deer biomass (Fig 1). The rate of change in herbivore community was greatest during 1949 to 1979, with lower rates of change during later periods (Fig 1 and Fig C in S1 File).
Shifts in herbivore community composition have profound impacts on community and ecosystem dynamics. Livestock abundances have increased to dominate herbivore assemblages in most parts of the world . Here we have shown a reversal of this process, where herbivore communities in Norway have reverted from a livestock dominated state around 1949 (associated with widespread use of unenclosed land for grazing during summer months and low cervid densities due to heavy hunting pressure), to a wild herbivore dominated state today. We found that the increase in wildlife biomass was highest in warmer and wetter regions of Norway. Declines in livestock biomass were a modest predictor of wildlife increases, suggesting that direct competition with livestock has not been a major limiting factor of wild herbivore populations over the past decades.