Reindeer grazing inhibits climate-driven biodiversity in the
Fennoscandian tundra
Abstract
Herbivores in the tundra interact with vegetation through several
mechanisms, especially defoliation, trampling and nutrient addition
through urine and faeces. Through these mechanisms, herbivores drive
shifts in plant species composition, richness and diversity. As reindeer
effects on vegetation accumulate over time, they might cascade to other
trophic levels, but how and when this happens is poorly understood.
Since it is methodologically demanding to measure biodiversity across
spatial gradients, an alternative approach is to assess it indirectly
via biodiversity indices of vascular plants. Values from the Index of
Biodiversity Relevance were coupled with vegetation data from a network
of 96 fenced and paired grazed plots across Fennoscandia. We analysed
the role herbivory has on plant richness and diversity, and on the
number of organisms that depend on the vegetation according to the index
values. We also explored how herbivores affect the competitive effects
of shrubs on other plants since the dominance of a vegetation type links
directly to biodiversity. Vegetation richness and diversity did not
present any differences between treatments, yet reindeer had an
increasing effect on plant diversity when testing the interaction
between grazing and herbaceous vegetation. Three out of six biodiversity
indexes were higher in fenced plots indicating a higher number of
interactions between plants and organisms from other trophic levels.
Finally, herb abundance was negatively related to shrubs in both
treatments but with a faster decline in the absence of herbivores,
suggesting that herbivory increases plant diversity and decreases the
diversity of other taxa by reducing shrub abundance. This study
highlights the importance of maintaining herbivore populations in the
Arctic to prevent the expansion of climate-driven biodiversity into the
tundra. The effect of herbivores on ecological communities is not merely
a product of plant diversity but can be quantitatively and qualitatively
different.