Threatened stick-nest rats preferentially eat invasive boxthorn rather than native vegetation on Australia’s Reevesby Island

Abstract

Context

The incorporation of invasive plants into novel ecosystems often has negative effects, but it can also sometimes enhance ecosystem function. The threatened native rodent species Leporillus conditor (greater stick-nest rat) is extinct on the Australian mainland and now lives primarily on small islands off the coast of southern Australia. Many of these are degraded novel ecosystems invaded by African boxthorn (Lycium ferocissimum), a weed of national significance. However, L. conditor does not appear to be negatively affected by the presence of boxthorn, raising the question of how the two species co-exist.

Aims

To understand how L. conditor uses African boxthorn, we evaluated dietary composition of L. conditor on parts of Reevesby Island by comparing consumption of invasive boxthorn with that of native vegetation.

Methods

We identified three key vegetation types on the centre of the island and used point-intercept vegetation surveys to estimate relative availability of plant species in each. We then used micro-histological faecal analysis to estimate the proportions of each species in the diet of L. conditor, and quantified plant species selection by using selection ratios (use/availability).

Key results

Qualitative evidence of L. conditor activity suggested that it was mostly confined to vegetation with greater abundance of boxthorn than the other vegetation types (13.5%, compared with 5.7% total sampled vegetation). Furthermore, 51.7% of the faecal plant content and 11.8% of total sampled vegetation was African boxthorn, resulting in a selection ration for boxthorn of 4.4. Native species that appeared to be favoured food sources of L. conditor included Olearia axillaris, Myoporum insulare and Enchylaena tomentosa.

Conclusions

Stick-nest rats of Reevesby Island demonstrate a clear selection for African boxthorn, both in terms of diet (tested quantitatively) and nesting (from previous research and our field observations).

Implications

The strong selection of stick-nest rats for a declared noxious weed as its main food source and persistence of stick-nest rats on Reevesby Island require consideration with regards to vegetation management on islands where L. conditor occurs. More broadly, it highlights that some elements of novel ecosystems may have unexpected positive impacts on parts of original ecosystems.