Diurnal activity in cane toads (Rhinella marina) is geographically widespread

Increased rates of dispersal of free-ranging cane toads (Rhinella marina) during their global invasion

Invasions often accelerate through time, as dispersal-enhancing traits accumulate at the expanding range edge. How does the dispersal behaviour of individual organisms shift to increase rates of population spread? We collate data from 44 radio-tracking studies (in total, of 650 animals) of cane toads (Rhinella marina) to quantify distances moved per day, and the frequency of displacement in their native range (French Guiana) and two invaded areas (Hawai’i and Australia). We show that toads in their native-range, Hawai’i and eastern Australia are relatively sedentary, while toads dispersing across tropical Australia increased their daily distances travelled from 20 to 200 m per day. That increase reflects an increasing propensity to change diurnal retreat sites every day, as well as to move further during each nocturnal displacement. Daily changes in retreat site evolved earlier than did changes in distances moved per night, indicating a breakdown in philopatry before other movement behaviours were optimised to maximise dispersal.

Wildfires modify the parasite loads of invasive cane toads

The frequency and severity of wildfires are increasing due to anthropogenic modifications to habitats and to climate. Post-fire landscapes may advantage invasive species via multiple mechanisms, including changes to host-parasite interactions. We surveyed the incidence of endoparasitic lungworms (Rhabdias pseudosphaerocephala) in invasive cane toads (Rhinella marina) in near-coastal sites of eastern Australia, a year after extensive fires in this region. Both the prevalence of infection and number of worms in infected toads increased with toad body size in unburned areas. By contrast, parasite load decreased with toad body size in burned areas. By killing moisture-dependent free-living lungworm larvae, the intense fires may have liberated adult cane toads from a parasite that can substantially reduce the viability of its host. Smaller toads, which are restricted to moist environments, did not receive this benefit from fires.

Spatial ecology of cane toads (Rhinella marina) in their native range: a radiotelemetric study from French Guiana

Like most invasive species, cane toads have attracted less research in their native range than in invaded areas. We radio-tracked 34 free-ranging toads in French Guiana, a source region for most invasive populations, across two coastal and two rainforest sites. Coastal toads generally sheltered in pools of fresh or brackish water but nocturnally foraged on beaches, whereas rainforest toads sheltered in forested habitats, moving into open areas at night. Over five days of monitoring, native toads frequently re-used shelters and moved little between days (means = 10-63 m/site) compared to invasion-front toads from Australia (~ 250 m). Larger toads moved less between days, but displaced in more consistent directions. At night, foraging toads travelled up to 200 m before returning to shelters. Foraging distance was related to body condition at coastal sites, with toads in poorer body condition travelling farther. Rain increased the probability of coastal toads sheltering in the dry habitats where they foraged. Dispersal and rainfall were lower at coastal sites, and the strategies utilized by coastal toads to minimize water loss resembled those of invasive toads in semi-desert habitats. This global invader already exhibits a broad environmental niche and substantial behavioural flexibility within its native range.

Adult-Juvenile interactions and temporal niche partitioning between life-stages in a tropical amphibian

Divergence in ecological niche offers organisms the opportunity of exploiting different food and habitat resources, scaling down competition and predation both among species, and within different age or size-classes of the same species. In harsh environments, where abiotic factors determine a clustering of resources during short timespans, competition and predation between organisms is likely to be enhanced. This is the case in tropical dry forests, where amphibians have limited opportunities to feed, their activity being restricted to the short rainy season. One way to maximize resource exploitation while avoiding predation risk is by adopting different diel activity patterns. We tested this hypothesis by comparing activity patterns in adults and recently metamorphosed juveniles of Pacific horned frogs (Ceratophrys stolzmanni) during field surveys and in an experimental study. Field surveys showed that the adults are strictly nocturnal, whereas freshly metamorphosed juveniles can be found active above ground at all hours, with a peak activity during daytime. The average body condition index of juveniles found active during the night was higher than that of juveniles found active during the day, suggesting that the weaker individuals may be constrained to being active during the day. On the other hand, in a laboratory experiment, juveniles that were visually exposed to adults moved less than those in the absence of adults. Both field and experimental observations indicate a temporal niche divergence between life stages. The results of the experiment offer support to the hypothesis that the juveniles in this species display an inverse activity pattern compared to adults, which can reduce competitive interactions and predation pressure from the larger conspecifics.