Movement Behavior of Native and Invasive Small Mammals Shows Logging May Facilitate Invasion in a Tropical Rain Forest

Abstract

Invasive species pose one of the greatest threats to biodiversity. This study investigates the extent to which human disturbance to natural ecosystems facilitates the spread of non‐native species, focusing on a small mammal community in selectively logged rain forest, Sabah, Borneo. The microhabitat preferences of the invasive Rattus rattus and three native species of small mammal were examined in three‐dimensional space by combining the spool‐and‐line technique with a novel method for quantifying fine‐scale habitat selection. These methods allowed the detection of significant differences for each species between the microhabitats used compared with alternative, available microhabitats that were avoided. Rattus rattus showed the greatest preference for heavily disturbed habitats, and in contrast to two native small mammals of the genus Maxomys, R. rattus showed high levels of arboreal behavior, frequently leaving the forest floor and traveling through the understory and midstory forest strata. This behavior may enable R. rattus to effectively utilize the complex three‐dimensional space of the lower strata in degraded forests, which is characterized by dense vegetation. The behavioral flexibility of R. rattus to operate in both terrestrial and arboreal space may facilitate its invasion into degraded forests. Human activities that generate heavily disturbed habitats preferred by R. rattus may promote the establishment of this invasive species in tropical forests in Borneo, and possibly elsewhere. We present this as an example of a synergistic effect, whereby forest disturbance directly threatens biodiversity and indirectly increases the threat posed by invasive species, creating habitat conditions that facilitate the establishment of non‐native fauna.

Minimal Spillover of Native Small Mammals From Bornean Tropical Forests Into Adjacent Oil Palm Plantations

“…Although we focussed on the distribution and movement of native small mammal species at the forest-plantation edge, we also found no evidence that forest harbours elevated abundance of the invasive R. exulans and R. rattus. Both species were present across all our forest trap lines, which supports previous evidence that they can competitively invade logged forest (Cusack et al, 2015;Loveridge et al, 2016;Wearn et al, 2016Wearn et al, , 2017. However, capture rates of neither species differed from the plantation control site at any distance.…”

“…We set traps at dusk, and processed captures between 21:00 and 22:00, remaining >50 m away from the grid during the intervening period to minimise disturbance. To track released small mammals, we used a spool-and-line device following Loveridge et al (2016). We covered an industrial nylon cocoon bobbin (Danfield Ltd, Lancashire, UK) with a cling film wrapping to ensure that it unravels and detaches cleanly, and then wrapped it with cloth tape and attached it to the back of the small mammal using cyanoacrylate glue.…”

“…We assumed that small mammal movement trajectories can be simplified to a series of step lengths in centimetres, connected by relative turning angles in degrees (following e.g., Almeida et al, 2015;Loveridge et al, 2016). To simulate random paths and generate null crossing proportions, we extracted species-specific distributions of these step lengths and turning angles from Loveridge et al (2016). These authors made detailed measurements of small mammal paths in continuous selectively logged forest within our study landscape.…”

“…These authors made detailed measurements of small mammal paths in continuous selectively logged forest within our study landscape. This forest was >1 km away from the nearest forest-plantation edge, but was analogous in logging intensity and vegetation structure to the forest at our study edges (Loveridge et al, 2016). We then iteratively constructed null paths by sampling randomly from these distributions of step lengths and turning angles.…”

“…The benefits of crossing the edge and foraging within oil palm plantations relative to the costs should depend on the microhabitat preferences of the forest small mammal species concerned, and on the characteristics of the plantation itself. Although Southeast Asian forest small mammals inhabit a variety of foraging niches (Wells et al, 2006;Loveridge et al, 2016), most species prefer sheltered environments (Bernard, 2004). Younger plantations generally exhibit denser herbaceous ground vegetation (Luskin and Potts, 2011) and lower, more accessible fruiting crowns (Puan et al, 2011), and appear to support higher densities of invasive Rattus spp.…”