Unintended consequences of invasive predator control in an Australian forest: overabundant wallabies and vegetation change

Abstract

Over-abundance of native herbivores is a problem in many forests worldwide. The abundance of native macropod wallabies is extremely high at Booderee National Park (BNP) in south-eastern Australia. This has occurred because of the reduction of exotic predators through an intensive baiting program, coupled with the absence of other predators. The high density of wallabies at BNP may be inhibiting the recruitment of many plant species following fire-induced recruitment events. We experimentally examined the post-fire response of a range of plant species to browsing by wallabies in a forest heavily infested with the invasive species, bitou bush Chrysanthemoides monilifera. We recorded the abundance and size of a range of plant species in 18 unfenced (browsed) and 16 fenced (unbrowsed) plots. We found the abundance and size of bitou bush was suppressed in browsed plots compared to unbrowsed plots. Regenerating seedlings of the canopy or middle storey tree species Eucalyptus pilularis, Acacia implexa, Allocasuarina littoralis, Breynia oblongifolia and Banksia integrifolia were either smaller or fewer in number in grazed plots than treatment plots as were the vines Kennedia rubicunda, Glycine tabacina and Glycine clandestina. In contrast, the understorey fern, Pteridium esculentum increased in abundance in the browsed plots relative to unbrowsed plots probably because of reduced competition with more palatable angiosperms. Twelve months after plots were installed the community structure of the browsed and unbrowsed plots was significantly different (P = 0.023, Global R = 0.091). The relative abundance of C. monilifera and P. esculentum contributed most to the differences. We discuss the possible development of a low diversity bracken fern parkland in Booderee National Park through a trophic cascade, similar to that caused by overabundant deer in the northern hemisphere. We also discuss its implications for broad scale fox control in southern Australian forests.

Figure 1. Conceptual model of impact of wallaby browsing on flora of Booderee National Park.

Figure 2. Mean numbers of 12 plant species in (3×3 m) browsed by macropods (n = 18) and (3×3 m) unbrowsed plots(n = 16) (±1SE) from Booderee National Park, south-eastern Australia.

Figure 3. Mean size of 12 plant species in (3×3 m) browsed by macropods (n = 18) and (3×3 m) unbrowsed plots(n = 16) (±1SE) from Booderee National Park, south-eastern Australia.

Figure 4. Percentage cover of C. monilifera and P. esculentum in browsed and unbrowsed plots.