Wild herbivores enhance resistance to invasion by exotic cacti in an African savanna

Abstract

1. Whether wild herbivores confer biotic resistance to invasion by exotic plants remains a key question in ecology. There is evidence that wild herbivores can impede invasion by exotic plants, but it is unclear whether and how this generalises across ecosystems with varying wild herbivore diversity and functional groups of plants, particularly over long-term (decadal) time frames.2. Using data from three long-term (13-to 26-year) exclosure experiments in central Kenya, we tested the effects of wild herbivores on the density of exotic invasive cacti, Opuntia stricta and O. ficus-indica (collectively, Opuntia), which are among the worst invasive species globally. We also examined relationships between wild herbivore richness and elephant occurrence probability with the probability of O. stricta presence at the landscape level (6150 km 2 ).3. Opuntia densities were 74% to 99% lower in almost all plots accessible to wild herbivores compared to exclosure plots. Opuntia densities also increased more rapidly across time in plots excluding wild herbivores. These effects were largely driven by megaherbivores (≥1000 kg), particularly elephants. 4. At the landscape level, modelled Opuntia stricta occurrence probability was negatively correlated with estimated species richness of wild herbivores and

Litter accumulation suppresses grass production but facilitates shrub expansion in a long‐term fenced grassland

“…Yet, the likelihood of successful ecosystem recovery remains controversial, as many ecosystems have internal thresholds between alternative states and exhibit hysteretic responses, making regime shifts difficult to reverse (Kleinhesselink, 2020). Recently, reintroducing herbivores has shown promise as a means to restore grassland biodiversity and revive biotic resistance to shrub invasion (Guyton et al, 2020; Wells et al, 2023). Our conclusion that litter accumulation in long‐term exclosure suppresses grass production but favors shrub expansion by promoting its growth and recruitment echoes a similar effect of herbivore loss on vegetation communities and suggests that disturbance is a premise for maintaining grassy biomes in semiarid grasslands.…”

“…Our results showed that for two native shrubs, A. sacrorum and C. brachypoda , abundance increased in exclosure areas compared with mowing disturbance. Some experimental and observational data indicate that herbivore disturbance confers biotic resistance against shrub encroachment, and this ecosystem function can be lost due to reduction or extinction of herbivores (Guyton et al, 2020; Wells et al, 2023). Therefore, disturbance exerts a strong top‐down regulating role in shaping grassland community structure, especially shrub–grass interactions.…”

“…These responses have been corroborated by a broad range of evidence, including in situ observations, precipitation manipulations, dendroecology, time‐series aerial photography, and satellite remote sensing (Gherardi & Sala, 2015; Jobbágy & Sala, 2000; Winkler et al, 2019). However, herbivore exclusion experiments and documentation of historical and contemporary monitoring data provide evidence that anthropogenic disturbance has resulted in widespread encroachment of shrub into grasslands (Bakker et al, 2016; Guyton et al, 2020; Wells et al, 2023). The entanglement of these climatic and disturbance factors presents further complication that inhibits our understanding of shrub expansion.…”

Timing of invasive species removal influences nonnative biotic resistance and trajectories of community reassembly

“…Historically, invasion science has focused primarily on interactions established between native and nonnative species, particularly competition and trophic interactions (e.g. Wells et al, 2023; Zhang & van Kleunen, 2019). In recent decades, given the increasing accumulation of nonnative species in plant communities worldwide (Van Kleunen et al, 2015), interactions among nonnative species are gaining attention as an important factor determining the assembly of communities (Golivets & Wallin, 2018; Kuebbing & Nuñez, 2015, 2016).…”