Integrated macroecological-paleoecological perspectives on large-herbivore effects on ecosystems: Implications for conservation and restoration

Jens-Christian Svenning and Rasmus Østergaard Pedersen, Department of Biology, Section for Ecoinformatics and Biodiversity, Aarhus University, Aarhus, Denmark, Jens-Christian Svenning and Rasmus Østergaard Pedersen, Center for Biodiversity Dynamics in a Changing World (BIOCHANGE), Aarhus University, Aarhus, Denmark, Camilla Fløjgaard and Rasmus Ejrnæs, Department of Bioscience, Aarhus University, Rønde, Denmark, Emilio Berti, German Centre for Integrative Biodiversity Research (iDiv), Leipzig, Germany, Søren Faurby, Gothenburg Global Biodiversity Centre, Gothenburg, Sweden, Søren Faurby, Department of Biological and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden, Pil B.M. Pedersen, Centre for Landscape and Climate, University of Leicester, Leicester, United Kingdom, Christopher J. Sandom, School of Life Sciences, University of Sussex, Brighton, United Kingdom

Background/Question/Methods
Large herbivores are potential ecosystem engineers via their vegetation feeding activities, non-trophic disturbances and movements of propagules and nutrients. Hereby, they promote key factors for biotic community assembly and the maintenance of species richness, namely environmental heterogeneity and dispersal. However, the importance of large herbivores for biodiversity conservation and ecosystem restoration is often underappreciated or misunderstood due to near-ubiquitous functional simplification of large-herbivore assemblages worldwide across both recent decades and centuries and on deeper time scales, starting at least 50,000 years ago. These losses have left natural and semi-natural ecosystem with large-herbivore assemblages that are highly down-sized and simplified relative to the mid- and late Cenozoic norm. Using macroecological approaches and allometry- and phylogeny-based modelling, we here assess intact levels of three factors of key importance for understanding the role of large herbivores in natural ecosystems, i.e., in the absence of past and present faunal declines: 1) Total biomass of large herbivores, 2) vegetation consumption rates, and 3) animal-mediated movement rates.
Results/Conclusions
Comparing large-herbivore biomass across natural ecosystems world-wide and assessing the relationship to primary productivity, we estimate that biomass levels are often much reduced due to past defaunation and current anthropogenic pressures. Notably, our results suggest that natural biomass levels at medium primary productivity would be 100-200 kg/ha or higher, while the realized biomass levels are often tenfold less or lower. Concerning vegetation consumption, we find that that consumption rates by wild mammals in current natural areas worldwide are only 56% (mean) of their level in the absence of late-Quaternary extinctions and extirpations, based on a conservative estimation procedure, not accounting for abundance reductions in extant populations. We furthermore find that 74% of average and 83% of maximum movement capacity of mammal assemblages, estimated from home ranges, has been lost due to late-Quaternary extinctions and extirpations. Our results highlight that the past and current pressures on large-herbivore assemblages have resulted in strong reductions in total biomass and key processes for the ecosystem and biotic community structure, namely vegetation consumption and dispersal rates, affecting natural areas throughout the world. These findings highlight the need to carefully reconsider assumptions and management goals for wild large herbivores in conservation and restoration efforts.