Damage caused by low-density exotic herbivore populations: the impact of introduced European rabbits on marsupial herbivores and Allocasuarina and Bursaria seedling survival in Australian coastal shrubland

Abstract

The impact of over-abundant exotic herbivores is well recognised, but their impact at low population densities is poorly understood. This study examined interactions between European rabbits and native herbivores, and their impact on seedling recruitment in coastal South Australia, 2 years after rabbit haemorrhagic disease (RHD) had reduced rabbit density to 4.48 rabbits ha -1 . Rabbit density was further reduced to 0.44 rabbits ha -1 in replicated experimental treatments. Rabbit control reduced total grazing pressure by 39% despite compensatory grazing increases of [100% for both western grey kangaroos and common wombats. Rabbit control slowed the rate of grazing and mortality for planted drooping sheoak and sweet bursaria seedlings, but few survived for 12 months: 0 and 3% of sheoak, in untreated areas and rabbit control treatments, respectively, and 3 and 11% of bursaria, respectively. Planted sheoaks survived well if protected by rabbit-proof netting (60%). Within treatments, seedling grazing and survival rates were negatively correlated with rabbit density but kangaroo and wombat density had no measurable effect. We conclude that RHD may briefly have reduced rabbit densities enough to allow recruitment of bursaria but that sheoak require much lower rabbit densities than those provided by existing biological control agents. If left unaddressed, rabbit grazing could ultimately lead to the loss of sheoaks throughout most of their current range, irrespective of other attempts to conserve them. More generally, these data show how species-specific damage caused by low-density exotic herbivore populations may occur in the presence of more abundant but less-damaging native herbivores.

Rabbits: manageable environmental pests or participants in new Australian ecosystems?

“…However, wombats did not increase in two adjacent areas where rabbits remained (Cooke 1998). In a later experiment, Bird et al (2012) reconfirmed that wombats repopulated areas following rabbit removal because the perennial grasses they needed were more readily available even though total pasture biomass changed little.…”

“…Rabbits inhibit the recruitment of many other tree and shrub species as well, including white pines (Callitris glaucophylla; Johnston 1968), sheoaks (Allocasuarina verticillata; Cooke 1987), bulokes (A. luehmannii;Murdoch 2005), silver cassias (Senna artemesiodes; Silander 1983), snow gums (Eucalyptus pauciflora; Wimbush and Forrester 1988), rosewood (Alectryon oleifolius ;Sandell 2006) and sweet bursaria (Bursaria spinosa; Bird et al 2012). Not all native tree seedlings are highly susceptible to rabbit browsing, however.…”

The ability of magnetic field sensors to monitor feeding in three domestic herbivores

“…Previous studies investigating what animals are feeding on, and in what quantities, with a wider aim of investigating of animals on the environment have included methods such as stomach content analysis and scat sampling (e.g., Bird et al, 2012). Here, we suggest magnetic field sensors can be used to distinguish various food items being eaten based on biting and chew rates and chew:bite ratios.…”

Invasive species and their impacts on agri-ecosystems: issues and solutions for restoring ecosystem processes

“…For example, the regeneration of drooping she-oak, Allocasuarina verticillata, and sweet bursaria, Bursaria spinosa, in arid ecosystems in South Australia cannot occur while rabbits occur at densities of 0.5 ha -1 (Bird et al 2012). Without recruitment of the structural backbones of the ecosystem, it is destined for degradation and local extinction of dependent native fauna.…”