Generalist indigenous herbivores resist alien tree invasion: Rhabdomys pumilio limits establishment of Acacia cyclops

Regulation of alien invasive species by indigenous generalist herbivores is a strong component of ecosystem biotic resistance. Globally, Australian Acacia species are highly invasive trees that are regulated by specialist invertebrate herbivores, granivorous birds and consumption by indigenous and exotic vertebrates in their native ranges. Australian Acacias have become highly invasive in the Cape Floristic Region (CFR) of South Africa where seeds are consumed by generalist bird, ant, and rodent species, but there is as yet no evidence that generalist herbivores consume the seedlings. Despite this, indigenous mice are common in the CFR and we consequently hypothesised that seedling herbivory by indigenous herbivores may limit survival of Acacia cyclops in relatively undisturbed indigenous ecosystems. Acacia cyclops seedlings were transplanted into indigenous CFR coastal strandveld vegetation at two distinct study sites and monitored for eight days in one of three caged treatments (i) no cage, (ii) partially caged excluding large mammalian herbivores or (iii) fully caged excluding all mammalian herbivores. Herbivory-induced seedling mortality without a cage and for partially caged plants was significantly greater than for fully caged plants (all contrasts P < 0.001). Rapid herbivory of A. cyclops seedlings in the uncaged and partially caged treatments was largely attributable to highly abundant striped mice, Rhabodmys pumilio. Seedling herbivory was not dependent on rodent density, which was consistently high (200-300 mice ha(-1)). However, seedling herbivory was higher in areas with dense vegetation and lower in open-canopy areas, probably because R. pumilio is known to avoid foraging in open areas. These generalist herbivores reduced A. cyclops recruitment in relatively undisturbed indigenous ecosystems, contributing to a degree of ecosystem resistance.

Automated summary

The regulation of invasive species by indigenous generalist herbivores plays a significant role in ecosystem resistance. This study found that herbivory by striped mice is a major factor affecting the survival of Acacia cyclops seedlings, and it is influenced by vegetation density and canopy openness.