Remote sensing reveals that wild herbivores limit senescent vegetation accumulation on dryland conservation reserves

Wild herbivores threaten vegetation recovery on dryland conservation reserves globally. Monitoring herbivore impacts in remote drylands is difficult because vegetation biomass transitions between living and senescent states in response to irregular precipitation events. However, land managers need a detailed understanding of the impacts that wild herbivores have on vegetation to develop and refine herbivore management strategies. Remote sensing provides the ability to assess grazing impacts on living and senescent vegetation with high temporal and spatial resolution. Here, we use Sentinel-2 satellite imagery to investigate how grazing by kangaroos and rabbits impacted the fractional cover of photosynthetic (PV) and non-photosynthetic (NPV) vegetation over 7 years on three dryland reserves with experimental herbivore exclusion plots. We compared PV and NPV cover in plots that were accessible to all herbivores, accessible to kangaroos only, and inaccessible to both rabbits and kangaroos. Generalized linear mixed models were used to determine if the grazing impacts of rabbits and kangaroos varied from each other, between reserves, and in response to variable rainfall patterns. Grazing impacts varied between each herbivore, conservation reserve, and between PV and NPV. PV was only weakly limited by kangaroos across all reserves and antecedent rainfall totals. NPV was limited by rabbits and kangaroos, with grazing having stronger impacts on NPV than PV. The grazing impacts of rabbits and kangaroos varied spatially, with evidence that NPV was limited by kangaroos only, by rabbits only, and by both species across different reserves. Both herbivores had stronger impacts on NPV as antecedent rainfall decreased. Our results show that the impact of herbivores on vegetation biomass is greatest during periods of dry climatic conditions. These findings contribute to a growing body of evidence showing that grazing by wild herbivores can have detrimental impacts on dryland ecosystems by disrupting ecological processes supported by NPV. Our results highlight the importance of herbivore management during productive periods to ensure NPV is retained during periods of low rainfall.