A Toad More Traveled: The Heterogeneous Invasion Dynamics of Cane Toads in Australia

Abstract

To predict the spread of invasive species, we need to understand the mechanisms that underlie their range expansion. Assuming random diffusion through homogeneous environments, invasions are expected to progress at a constant rate. However, environmental heterogeneity is expected to alter diffusion rates, especially by slowing invasions as populations encounter suboptimal environmental conditions. Here, we examine how environmental and landscape factors affect the local invasion speeds of cane toads (Chaunus [Bufo] marinus) in Australia. Using high-resolution cane toad data, we demonstrate heterogeneous regional invasion dynamics that include both decelerating and accelerating range expansions. Toad invasion speed increased in regions characterized by high temperatures, heterogeneous topography, low elevations, dense road networks, and high patch connectivity. Regional increases in the toad invasion rate might be caused by environmental conditions that facilitate toad reproduction and movement, by the evolution of long-distance dispersal ability, or by some combination of these factors. In any case, theoretical predictions that neglect environmental influences on dispersal at multiple spatial scales may prove to be inaccurate. Early predictions of cane toad range expansion rates that assumed constant diffusion across homogeneous landscapes already have been proved wrong. Future attempts to predict range dynamics for invasive species should consider heterogeneity in (1) the environmental factors that determine dispersal rates and (2) the mobility of invasive populations because dispersal-relevant traits can evolve in exotic habitats. As an invasive species spreads, it is likely to encounter conditions that influence dispersal rates via one or both of these mechanisms.

Ecological genetics of invasive alien species

“…Although awareness of the crucial role that dispersal plays in biological invasions is increasing (Kokko and Lopez-Sepulcre 2006;Ronce 2007), so far few studies have actually tried to measure the dispersal ability of IAS, except in the context of biological control (but see below and Heimpel and Asplen 2011). This information is crucial for understanding and predicting spread of invasive species and biological control agents (Heimpel and Asplen 2011), as well as consequences of other global environmental change (Urban et al 2008;Niitepo˜ld et al 2009). Since there is a direct, causal relationship between dispersal, gene flow and population struc-ture, detailed analyses of genetic structure can be used to quantify ''effective'' dispersal (i.e.…”

“…Urban et al 2008). The simplest model of range expansion is a random-diffusion process, often referred to as Fisher's ''wave of advance'' model, in which the range of an invading species is predicted to increase linearly with time (Fisher 1937;Skellam 1951 Reeves and Usher 1989), other cases demand more complex range expansion models that include, for example, probability of long-distance dispersal (LDD) events (either by wind or human transport), which can accelerate the rate of range expansion as the length of the invasion front increases (Shigesada et al 1995;Ciosi et al 2010).…”

“…Environmental heterogeneity is expected to be an important determinant of range expansion, with invasions accelerating as individuals encounter favourable conditions, and decelerating as they reach less favourable environments (Urban et al 2008). Heterogeneous environmental conditions can now be incorporated 7 into theoretical frameworks for predicting expansion (Hastings et al 2005) and ecological and landscape variables (including spatial structure and metapopulation dynamics) can be explicitly linked to invasion rates (e.g.…”

“…Heterogeneous environmental conditions can now be incorporated 7 into theoretical frameworks for predicting expansion (Hastings et al 2005) and ecological and landscape variables (including spatial structure and metapopulation dynamics) can be explicitly linked to invasion rates (e.g. Facon and David 2006;Urban et al 2008). For example, Urban et al (2008) analysed invasion trajectories in invasive cane toads, Bufo marinus (L.) (Anura: Bufonidae), in Australia to determine whether range expansion accelerated, decelerated or was linear, and if certain environmental conditions influenced population growth.…”

Knowing the past to predict the future: land‐use change and the distribution of invasive bullfrogs

“…Environmental features change in time, species can quickly evolve in the new environment and these modifications can influence the invasion dynamics (Urban et al, 2008). Temporal change of climate is probably the dynamic feature most often integrated in models of biological invasions.…”