Indirect impacts of invaders: A case study of the Pacific sheath-tailed bat (Emballonura semicaudata rotensis)

Highlights

• Invasive species can indirectly decrease resources suitable for native taxa.

• Our study demonstrates indirect effects of invasion on a threatened species of bat.

• Bat activity was low in lantana habitat owing to high wind speed and too few prey.

• Bat activity was not correlated with goat feeding preference, but with stem density.

• Future research should test if goats facilitate the spread of lantana.

Abstract

Although many indirect consequences of biological invasions are plausible, few studies test hypotheses for management of threatened taxa. A case study of the endangered Pacific sheath-tailed bat (Emballonura semicaudata rotensis) illustrates the importance of investigating indirect effects of invasion on species of conservation concern. We hypothesized that two invaders, feral goats and Lantana camara, would indirectly affect the bat by decreasing availability of suitable resources. Specifically, that microclimate and bat prey abundances in lantana shrub differ from native forest habitat, and that preferential browsing by goats structures forests to be less suitable for bats. Our results suggest that bats avoid lantana shrub. However, we found no evidence that preferential goat browsing influenced bat activity. Our research implies that the impact of lantana on the persistence of the bat has been underestimated and that it is unclear how goats alter bat habitat aside from reducing understory vegetation. Future managers should prioritize efforts that restore native forest and reforest areas currently dominated by lantana. We urge conservation scientists to evaluate indirect effects of invasive species and publish findings that elucidate the consequences for native populations.

Introduction

Global conservation for bats is needed: 15% of species are listed as extinct or threatened, 17% do not have a threat status owing to insufficient data, and over half have unknown population trends (IUCN, 2015). Moreover, bats are an oddity among mammals in that roughly a quarter of species are island endemics (Jones et al., 2009), making them important members of island communities. Given uncertainty regarding population viability and their significant contribution to island diversity, research on threats to bats is a priority for conservation (Mickleburgh et al., 2002).

Endangerment and extinction occur disproportionately on islands, in part owing to island systems vulnerability to invaders (Gimeno et al., 2006). Biological invasion can have profound ecological and socioeconomic impacts of isolated ecosystems (reviewed in Reaser et al., 2007). Studies on how biological invasions affect bats have focused on direct interactions, such as predation and epidemiology of introduced pathogens (e.g., Fritts and Rodda, 1993, Rodriquez-Duran et al., 2010). Few have evaluated indirect effects — how interactions between coexisting species are affected by another species (Strauss, 1991) — on bats.

The Pacific sheath-tailed bat (Emballonura semicaudata) is a small insectivore designated “endangered” because its geographic area is < 5000 km² and highly fragmented (Bonaccorso and Allison, 2008). Emballonura semicaudata rotensis is a subspecies formerly distributed across limestone islands of Guam and the Commonwealth of the Northern Mariana Islands (CNMI) but now present only on the uninhabited island of Aguiguan (Wiles et al., 2011). The estimated 500 individuals of E. s. rotensis are commonly detected in forests and roost primarily in three caves (Wiles et al., 2011). Consequently, E. s. rotensis is highly vulnerable to stochastic demographic and environmental events.

Unconfirmed threats to E. s. rotensis include invasive species (Berger et al., 2005). Shrub covers at least 20% of Aguiguan (150 ha) and is dominated by lantana (Lantana camara) (Amidon, 2009), a “top 100” invasive species (IUCN, 2001). Many studies have shown that lantana can greatly alter native community structure (see review by Sharma et al., 2005), though its impacts on Aguiguan remain unstudied. Feral goats (Capra hircus) were introduced to Aguiguan during the mid-1800s (Butler, 1992), and their density has reached over 200 individuals per square kilometer (Esselstyn et al., 2002), earning Aguiguan the nickname “Goat Island”. The detrimental effects of goats on Aguiguan's native species are largely anecdotal.

Here, we consider the conservation of E. s. rotensis to illustrate indirect impacts of invasion. We hypothesized that goat browsing and lantana spread indirectly impact E. s. rotensis by altering habitat structure and prey abundances. To evaluate our hypotheses, we ask: (1) Do bats behave differently in lantana shrub than in native forest, and what factors account for this pattern? and (2) Does native understory cover indicate goat browsing, and is bat activity related to the structural complexity of native forest?