Postglacial expansion of trees and declining nitrogen availability linked to plant-plant interactions, not mega-herbivores
In a study published this week in Ecology Letters, Dr Elizabeth Jeffers and colleagues report that plants had greater impacts on terrestrial ecosystems than mega-herbivores prior to their extinction during the Late Pleistocene and early Holocene in Britain and Ireland.
The Late Quaternary extinction event removed more than half of the large and charismatic herbivore species from most continents. Previous studies suggested that these extinctions imposed significant and persistent effects on ecosystem structure and biogeochemistry. This consumer-centric view of Quaternary ecosystem change provides a basis for trophic rewilding, i.e. the use of herbivore species (re)introductions to restore top-down trophic interactions. This includes, in the extreme case, controversial attempts to restore the woolly mammoth, or a genetically modified Asian elephant, to the Arctic tundra. It is hoped that an ‘Arctic elephant’ could help recreate the once productive steppe-tundra habitat of the Late Pleistocene period. Yet to date, the strength of these claims has not been tested against alternative hypotheses of ecosystem change during this period.
The multi-disciplinary team, led by Dr Jeffers, produced an unprecedented amount of ecological and climatic information for five study sites in England, Scotland and Ireland. Their dataset included proxy measurements of plant and large herbivore biomass, nitrogen availability, growing season temperatures and fire activity spanning the transition from the Late Pleistocene to the Holocene period (16,000 to 4,800 years ago). According to fossil bone data, one-third of the region’s mega-herbivore species became extinct during this time. The authors applied statistical modelling to their data in order to investigate the relative impacts of mega-herbivores, plants, fire and summer temperatures on ecosystem structure and function across the extinction event. Contrary to the prevailing view, they found that plants had a greater impact on Late Quaternary terrestrial ecosystems than mega-herbivores.
“Our results challenge the ecological argument underpinning trophic rewilding by showing that the fauna living in these regions at the end of the last glacial period were unable to stem the expansion of woody plants across the northern hemisphere”, stated Jeffers.
The team found that shrubs were consistently one of the strongest predictors of ecosystem change, equaling or exceeding the effects of the other biotic and abiotic factors considered in the study. Increasing shrub biomass reduced ecosystem-scale nitrogen availability and promoted the growth and expansion of trees. Burning, not herbivory, was important for reversing these effects; however declining fire activity in the early Holocene enabled shrubs (and ultimately trees) to dominate terrestrial ecosystems.
The findings provide new empirical evidence for the long-term ecosystem engineering effects of woody plants and demonstrate the importance of burning for maintaining the structure and function of open ecosystems in northern biomes. According to Jeffers, “rewilding projects that rely on herbivores to provide this important service should consider reintroducing native fire regimes in order to ensure the sustainability of open, fertile habitats.”