Toad on the road: Use of roads as dispersal corridors by cane toads (Bufo marinus) at an invasion front in tropical Australia
Introduction
The invasion of native ecosystems by introduced organisms is considered to be one of the most serious threats to ecosystem functioning and global biodiversity (Lodge, 1993, Vitousek et al., 1996). Because the spread of introduced invasive species has significant ecological, social and economic costs (Mack et al., 2000, Pimental et al., 2000), identifying the factors that influence the rate of spread of invasive species across the landscape is an important goal for ecologists. From a practical perspective, identifying the types of habitats that alien invaders use as dispersal corridors may render it possible to manipulate the landscape in ways that limit the rate of expansion of an invasive species (Fagan et al., 2002, With, 2002).
Recently, there has been a growing awareness that roads may play an important role in the invasion process (Spellerberg, 1998, Trombulak and Frissell, 2000, Gelbard and Belnap, 2003). Most work in this respect has focused on alien plants, many of which thrive in open or disturbed roadside verge habitats and seldom colonize relatively undisturbed natural environments (Milton et al., 1999, Pauchard and Alaback, 2004). However, roads may play a more general role earlier in the invasion process by providing long continuous and linear corridors of open habitat that are ideally suited to long-distance movement. Roads may therefore facilitate rapid dispersal, allowing the invasive organism to penetrate more rapidly into previously unoccupied areas than would otherwise have been the case (Jules et al., 2002). Presumably, invasive species that actively prefer open habitats will frequently use such corridors in the initial phase of their population expansion (Macdonald, 1979, Newsome and Noble, 1986). However, actual empirical verification of this prediction among invasive species is rare, because few studies have obtained data at the invasion front. Instead, researchers have documented preferential use of open habitats, including roads, in populations of introduced species living in areas long after their initial colonization (Mack et al., 2000). Thus, the inference that such corridors facilitated the initial colonization process largely remains untested for most organisms (Jules et al., 2002). The current study provides such data for an invasive anuran in tropical Australia.
Section snippets
Study species and area
Cane toads (Bufo marinus) are large (up to 24 cm snout-urostyle length and 2.8 kg in mass) anurans with an extensive native range in Central and South America (Lever, 2001). Females mature at approximately one year of age and produce one or more large clutches (up to 30 000 eggs per clutch) of small aquatic eggs per year (Zug and Zug, 1979). After a brief (3–6 week) tadpole phase, the young toads are primarily terrestrial in habits. Eggs, tadpoles and terrestrial-stage cane toads all possess potent
Radio-tracking
The map of displacements by radio-tracked toads (Fig. 1) reveals highly non-random locations. First, all radio-tracked toads moved consistently in a northwest direction. Second, dispersal routes overlapped strongly. Third, toads were never recorded on large areas of the study site, instead concentrating their activity in a few linear segments. Thus, 99% of toad locations occurred in only about half of the area within the 100% minimum convex polygon containing all the toad locations (1400 ha vs.
Discussion
Many authors have reported that cane toads actively select relatively open habitats rather than densely vegetated areas, both within the native range in South and Central America (Zug and Zug, 1979) and in regions to which toads have been introduced (Lever, 2001) including Australia (Seabrook and Dettmann, 1996). The current study shows the same pattern at an invasion front in tropical Australia, confirming that toads utilize roads as dispersal corridors into previously unoccupied areas.
Acknowledgements
We thank the Australian Research Council for financial support, and Beatrice Hill Farm for logistical help. Matt Greenlees and Cathy Shilton assisted with measurements of toad locomotor speeds and bearings.
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