To quantify body condition (mass relative to length) of toads, we used residual scores from the general linear regression of ln body mass against ln SUL. The toads were placed under shelters in opposing corners of a 60×36 cm enclosure to allow them to recover from manipulation. After 2 min, the shelters were lifted off both shelters at the same time and a single cricket was placed in the center of the enclosure. The trials began when the shelters were lifted and ended when the cricket was eaten (one toad wins one toad loses, so lack of independence is not an issue). We recorded the time elapsed before the cricket was eaten, which toad ate the cricket and the number of feeding attempts made by each toad. In this experiment we used the 67 individuals tested on the personality trials plus 23 collected from our field trials.
Evolution driven by the invader’s impact on interactions among native species
- With all the cane toads roaming Australia, it makes sense that researchers in the area are curious to discover whether cane toad toxins can be used as medicine.
- In its native range—from the southern United States to northern South America—the cane toad is, well, just a big, ordinary toad.
- The coloured areas represent the areas made available to toads by natural sources of water (blue) and artificial sources of water (red).
Intense public interest in toads and their impacts has spawned vigorous (and competing) community-based attempts to combat toad invasion, creating a political firestorm in which scientific approaches to toad biology must confront populist opinions. The cane toad invasion of Australia provides an unusually well-documented case of the ways in which a single introduced species has substantial direct effects, and generates adaptive responses via multiple pathways, both in native fauna and human society. Biological invasions can massively disrupt ecosystems, but evolutionary and ecological adjustments may modify the magnitude of that impact through time. We quantified the abundance of two species of giant monitor lizards, and of the availability of their mammalian prey, across 45 sites distributed across the entire invasion trajectory of the cane toad (Rhinella marina) in Australia.
Fauna surveys
To compare the relative abundances of small mammals across invasion categories, we used GLMM to examine the effect of invasion stage on the number of small mammals within each transect. Based on pilot studies, we used a 30-min event period to quantify relative abundances of varanids from camera footage. Events were considered independent if they were separated by 30 min without a trigger during the intervening period.
Habitat variables
Subsequent laboratory trials showed correlated personality differences (behavioural syndromes) between these two groups of toads. For example, toads that approached already-occupied rather than unoccupied feeding sites in the field, took longer to emerge from a shelter-site in standardized trials, suggesting these individuals are ‘shy’ (whereas toads that approached unoccupied feeding stations tended to be ‘bold’). In feeding trials, a bold toad typically outcompeted a shy toad under conditions of low prey availability, but the outcome was reversed when multiple alcohol effects in the brain prey items were present. This invasive population of toads contains individuals that exhibit a range of personalities, hinting at the existence of a wide range of social dynamics in taxa traditionally considered to be asocial. Here we assess the morphological niche overlap between cane toads and Australian frog species in order to discriminate between the empty niche and competitive exclusion hypotheses. Under the empty niche hypothesis, we would expect cane toads to fill a unique morphological niche not occupied by Australian native frog species.
The scientific neglect of the toad’s impacts at the southern invasion front stands in contrast to the situation in tropical Australia. Many topics related to toad biology and ecological impacts have been investigated in remote tropical regions (review by Shine 2010). Between 1963 and 2014, 102 scientific publications assessed the ecological impacts of cane toads on the native fauna of northern Australia (i.e., north of Brisbane) compared to 14 articles on cane toad impacts in southern Australian (Shine 2010, 2014). Of those 14 articles, the most recent was completed more than 20 years ago (Seabrook 1993). This disparity in research effort is perplexing in light of the logistical difficulties that have compromised experimental designs in these tropical studies. We surveyed habitat characteristics and fauna at 16 campgrounds and picnic areas surrounded by bushland (from 28°22′S, 153°14′E to 29°57′S, 153°15′E) between October 2013 and February 2014.
Thus, morphological traits could be used as a proxy for a species’ ecological niche in a community, especially when those morphological traits are correlated with functional traits, such as performance capacity (Azzurro et al., 2014; Ricklefs & Miles, 1994). A number of morphological traits have been used previously in several taxa as a way to determine niche overlap among species (Gatz, 1979; Losos, 1990). As morphological plasticity broadens the range of environmental conditions under which a species could thrive, understanding the body size and shape patterns of a species vanderburgh house and their plasticity would capture its niche breadth (Whitlock, 1996). Here we exploit one of the best‐known biological invaders to discriminate between the two competing hypotheses of empty niche and niche competition. The destabilizing effects of biological invasions on host–parasite relationships remain a substantial challenge for future research. Other parasites may benefit from the invader’s arrival, for example, myxosporidians that occupy anuran bladders have increased in frequency among Australian frogs since the cane toad’s arrival (Hartigan et al. 2010).
Dams, therefore, provide a network of refuge habitats or “invasion hubs” in which toads congregate during dry seasons and visit on an almost daily basis. The cane toad hatches from an egg laid in water, begins its life as a tadpole, and eats pesky insects. For these trials we used larger enclosures (115×115 cm floor area), but followed the same methodology as for Experiment 1. Following 2 min acclimation of toads beneath shelters, 10 crickets were released at the center of the enclosure, and we recorded the number of crickets that each toad consumed.
The traits that enhance dispersal rate are system specific – the features that enable a seed to drift through the air are very different from those that enable it to cling to a mobile bird or mammal, and from the ones enabling that host organism to move further than its conspecifics. One interesting set of traits involves host–pathogen interactions; if pathogens vary in the degree to which is mixing cymbalta and alcohol safe they impede host dispersal, we expect to see the evolution of lower-impact pathogens in invasion-front populations of the host (Phillips et al. 2010a). Impacts of cane toad presence on (A) mean abundance per site (± SE) and (B) mean species richness per site (± SE) of native fauna encountered in campgrounds and surrounding bushland areas in northeastern New South Wales, Australia.
Accordingly, we need to assess invader impacts in the context of other changes that interact synergistically or antagonistically over time [13], or affect ecosystem or species resilience [14]. For example, a warming climate may present new opportunities for non-native species to invade areas that were previously outside their thermal tolerances, increasing their impact [15]. Excluding toads from water could be conducted strategically to prevent their spread. Fencing the thousands of dams that exist across Australia to exclude cane toads is a daunting and prohibitively expensive prospect. However, using simulations we have shown that a practical and cost-effective approach to control cane toads would be to strategically create “toad breaks” to disrupt the network of refuge habitats available for toads. Control of toads at invasion hubs could be conducted reactively to control established populations or prevent the spread of toads by rendering invasion hubs unsuitable for colonization ahead of the invasion front.
A total of 50 toads (25 pairs) out of the 67 tested for personality were used during this experiment. To test whether the presence of a feeding toad facilitates recruitment of more toads to a feeding station, we modified a protocol previously used to assess foraging-site choice [22]. Four smooth white rubber mats (3.6-mm thick, 80×80 cm) were laid on the ground, 10 m apart, near known toad foraging sites. On top of each mat we placed a rectangular wire-mesh enclosure (50×40×24 cm) in which a toad could be detained. The white wire mesh formed 5×3 cm openings, with monofilament fishing line bisecting each opening to prevent toads from escaping. On top of each enclosure we placed a 250-mm fluorescent tube bulb (12 V, 8 W) to attract insects (Figure 1).