Induced Shift in Ecosystem Productivity? Extensive Scale Effects of Abundant Large Herbivores

Abstract

Abundant large herbivores can strongly alter vegetation composition, shifting the ecosystem into a lasting state of changed productivity. Previous studies of the effects of abundant reindeer on alpine and arctic vegetation have yielded equivocal results, probably due to differing environmental contexts. To overcome context dependency we devised a large-scale survey in the region of Finnmark, northern Norway, possessing some of the most densely stocked reindeer herds in the world. The effects of reindeer abundance on summer pasture vegetation were assessed by employing a quasi-experimental design, including site fertility as a potential modifier of the reindeer–vegetation interaction. The study design comprised ten pairs of neighboring management districts (encompassing 18,003 km2), where over the two last decades a high-density district on average had reindeer densities more than twice as high and calf weights consistently lower than the low-density district. The abundance of different plant functional groups, ranging from those having facilitating to retarding effects on ecosystem productivity, were quantified by the point intercept method on plots selected according to a hierarchical, stratified random sampling design. Species with strong retarding effects on ecosystem productivity (for example, ericoids) were by far the most abundant. However, we found no consistent effects of reindeer density on their abundance. The most consistent differences between high- and low-density districts were found in plant functional groups with facilitating to neutral effects on ecosystem productivity. In particular, the abundance of N-facilitators, large dicotyledons and grasses were substantially reduced in the high-density districts. However, this reduction was restricted to fertile sites. Thus, reindeer when present at high densities have homogenized the biomass of palatable plants across environmental productivity gradients according to predictions from exploitation ecosystem models. Such reduction of plants with facilitating to neutral effects on ecosystem productivity indicates a reduced state of ecosystem productivity in high-density districts.

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Acknowledgments

We are grateful to Annika Hofgaard, Ingibjörg Svala Jónsdóttir and René van der Wal for discussions, Johan Ingvald Hætta and Anders Aarthun Ims for information about reindeer herding districts, Torstein Engelskjøn for providing a flora database, Bernt Johansen for providing satellite images, Norwegian Coast Guard and Jan Kåre Amundsen for transportation during field work, Sunna Pentha for field assistance, and to Marianne Iversen and Siw Killengreen for leadership during field work. This study, which is a contribution from the “Ecosystem Finnmark” project, was financed by Norwegian Research Council.

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Correspondence to Kari Anne Bråthen.

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Bråthen, K.A., Ims, R.A., Yoccoz, N.G. et al. Induced Shift in Ecosystem Productivity? Extensive Scale Effects of Abundant Large Herbivores. Ecosystems 10, 773–789 (2007). https://doi.org/10.1007/s10021-007-9058-3

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Keywords

  • Alpine and arctic tundra
  • plant abundance
  • flower abundance
  • functional groups
  • density-dependence
  • exploitation ecosystems
  • ecosystem state
  • herbivore overabundance
  • grazing
  • ungulate management