Herbivore diversity effects on Arctic tundra ecosystems: a systematic review

Background Northern ecosystems are strongly infuenced by herbivores that difer in their impacts on the ecosystem. Yet the role of herbivore diversity in shaping the structure and functioning of tundra ecosystems has been overlooked. With climate and land-use changes causing rapid shifts in Arctic species assemblages, a better understanding of the consequences of herbivore diversity changes for tundra ecosystem functioning is urgently needed. This systematic review synthesizes available evidence on the efects of herbivore diversity on diferent processes,
functions, and properties of tundra ecosystems. Methods Following a published protocol, our systematic review combined primary feld studies retrieved from bib‑
liographic databases, search engines and specialist websites that compared tundra ecosystem responses to diferent levels of vertebrate and invertebrate herbivore diversity. We used the number of functional groups of herbivores (i.e.,
functional group richness) as a measure of the diversity of the herbivore assemblage. We screened titles, abstracts, and full texts of studies using pre-defned eligibility criteria. We critically appraised the validity of the studies, tested
the infuence of diferent moderators, and conducted sensitivity analyses. Quantitative synthesis (i.e., calculation of efect sizes) was performed for ecosystem responses reported by at least fve articles and meta-regressions including the efects of potential modifers for those reported by at least 10 articles. Review fndings The literature searches retrieved 5944 articles. After screening titles, abstracts, and full texts, 201 articles including 3713 studies (i.e., individual comparisons) were deemed relevant for the systematic review, with 2844 of these studies included in quantitative syntheses. The available evidence base on the efects of herbivore diversity on tundra ecosystems is concentrated around well-established research locations and focuses mainly on the impacts of vertebrate herbivores on vegetation. Overall, greater herbivore diversity led to increased abundance of feeding marks by herbivores and soil temperature, and to reduced total abundance of plants, graminoids, forbs, and litter, plant leaf size, plant height, and moss depth, but the efects of herbivore diversity were difcult to tease apart from those of excluding vertebrate herbivores. The efects of diferent functional groups of herbivores on graminoid and lichen abundance compensated each other, leading to no net efects when herbivore efects were combined. In turn, smaller herbivores and large-bodied herbivores only reduced plant height when occurring together but not when occurring separately. Greater herbivore diversity increased plant diversity in graminoid tundra but not in other habitat types.
Conclusions This systematic review underscores the importance of herbivore diversity in shaping the structure and function of Arctic ecosystems, with diferent functional groups of herbivores exerting additive or compensatory efects that can be modulated by environmental conditions. Still, many challenges remain to fully understand the complex impacts of herbivore diversity on tundra ecosystems. Future studies should explicitly address the role of herbivore diversity beyond presence-absence, targeting a broader range of ecosystem responses and explicitly including invertebrate herbivores. A better understanding of the role of herbivore diversity will enhance our ability to predict whether and where shifts in herbivore assemblages might mitigate or further amplify the impacts of environmental change on Arctic ecosystems.