An examination of cane toad (Rhinella marina) behaviour: how can we use this knowledge to refine trapping regimes?

Muller, Benjamin John (2018) An examination of cane toad (Rhinella marina) behaviour: how can we use this knowledge to refine trapping regimes? PhD thesis, James Cook University.

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Invasive species are of major concern to ecologists, because of their impacts on native fauna, communities, and ecosystems. Invasive species may alter the evolutionary pathways of native species by competitive exclusion, niche displacement, hybridisation, introgression, and predation, at times ultimately causing extinction. Further, the economic cost associated with invasive species, through losses in agriculture, forestry, and tourism, as well as the costs of preventing and controlling these species, are of major concern to land managers and governments. Specifically, the management of vertebrate invasive species is a crucial component of biosecurity, ecology, and land management. There are a range of control methods for invasive vertebrates, including hand-capture, trapping, baiting, shooting, and biological and genetic control methods. These control strategies vary in efficacy, depending on the life history and behaviour of the target species, the area over which removal occurs, and the method of delivery of the control. Understanding these factors assists with designing targeted control strategies, in which the chance of removal of each individual, or the impact of each capture, or both, is increased. The success of control methods for some invasive vertebrates has improved considerably over the last several decades, due to the ever-increasing body of research about the behaviour and life history of certain invasive species, and the refinement of control regimes in relation to new information.

The invasive capabilities and impacts of amphibians generally receive less attention than other invasive vertebrates; as such, control methods for invasive amphibians are rare. Some invasive amphibians are generalist feeders, have high reproductive rates, and attain large population sizes; however, specific behavioural and life history traits are varied, and are often unknown. Further, abiotic factors, such as atmospheric temperature and moisture, effect the behaviour and activity of many amphibians. Current control strategies for invasive amphibians (e.g., hand-capture, exclusion fencing, and habitat modification) are often non-targeted, under-researched, and ineffective.

Cane toads (Rhinella marina) are highly invasive anurans, native to south and central America. Their invaded range extends through many tropical areas worldwide, including Australia. Cane toad paratoid (shoulder) glands secrete powerful bufotoxins that are lethal to some native predators, and domestic pets. The impact pathways of cane toads on native species include poisoning after ingestion (both at larval and adult stages), and competition with other anurans. Further, the presence of cane toad tadpoles may affect growth rates of native tadpoles, while the presence of adults may affect calling behaviour of some native anurans.

Potential control strategies for cane toads within their invaded range include hand-capture, tadpole traps, and biological and genetic control methods. These strategies are often non-targeted (e.g., tadpole traps, biological and genetic control methods), have been ineffective at suppressing toads for long periods, on a large scale, and in some cases require extremely high effort (e.g., hand-capture events). Trapping adult individuals using a solar-powered light and acoustic lure that automatically plays a cane toad call to attract toads into traps may be a viable control method that is easily refined to increase captures by exploiting behavioural characteristics of the cane toad. The success of any control method is dependent on the ability to refine it by targeting specific demographics of the invasive population, and increasing the number of captures per unit effort spatially, and temporally.

Many control methods for cane toads are ineffective because they do not consider the activity patterns of toads in response to abiotic factors; however, understanding and exploiting these patterns could allay wasted effort. For example, land managers could augment captures by understanding the environmental conditions that drive activity, at different times of year, and focusing trapping effort on periods when toads are most active. I examined cane toad activity (numbers of captures) in response to several environmental variables (humidity, temperature, rainfall, wind speed, and moon luminosity) over eleven months of trapping. Captures were highest (i.e., toads were most active) in the wet season (Dec – Feb), and lowest in the dry season (Jun – Aug). In the wet season, wind speed and minimum temperature effected activity (toads were most active on warm, still nights), while rainfall was the strongest predictor of activity in the dry season. I suggest that land managers could allay wasted trapping effort by focussing on nights with conditions conducive to toad activity (e.g., wet nights during the dry season).

It is important to determine the area over which toads are attracted to the call used as a lure in traps (the active space of the call), to aid in trap placement and the design of large scale trapping regimes. A vocalisation's active space is the area within which a receiver responds to it, while its maximum extent occurs when a receiver stops responding. I mapped behavioural responses of male and female cane toads to advertisement calls by conducting experimental playbacks to quantify the active space of calls for both sexes, separately. Both sexes displayed positive phonotaxis 20 – 70 m from calls. Males also displayed positive phonotaxis 70 – 120 m from calls, whereas females' movement preferences were random >70 m from a call. Differences between male and female responses were likely driven by differences in their use of information provided by calls. I suggest that traps should be placed m apart, such that a female toad can never be more than 70 from a trap, but effort is not wasted by 'over-trapping' in the target area.

Targeting reproductively active females is the best strategy for reducing recruitment into the next generation, and is a common control technique for vertebrate pests with high reproductive rates. Female cane toads can lay over 10 000 eggs per clutch, and should be targeted, however current control regimes do not focus on the removal of females. The lures used in adult cane toad traps play an advertisement call used by male toads to attract females. In many anurans, females select mates based on the structural parameters of advertisement calls (e.g., dominant frequency and pulse rate), therefore modifying the parameters of calls used as lures in cane toad traps, to create especially attractive calls, may augment gravid female captures. I altered the frequency and pulse rate of artificial calls used as lures, and conducted several trapping regimes in and around the Townsville region in northern Australia, to determine which calls were most attractive to gravid females. Overall, gravid females preferred a 'combination' call with a low dominant frequency, and high pulse rate (relative to the population median for these parameters). Approximately 91% of the females trapped using a low frequency and high pulse rate combination call were gravid, whereas in traps using a call with population median parameters only approximately 75% of captured females were gravid. Calls that indicated large-bodied males (low frequency) with high energy reserves (high pulse rate) are often attractive to female anurans, and were effective lures for gravid female toads in my study.

Often, advertisement calls differ among populations. In this case, the attractive 'combination' call I identified in the Townsville cane toad population may be less attractive to gravid females in other populations. I sampled calls from 4 cane toad populations across Australia (south east Queensland, north Queensland, Western Australia, and the Northern Territory), and constructed artificial vocalisations based on the median parameters of the sampled calls. I conducted trapping at each population, using calls tailored to each population, to determine which call was most attractive to gravid females in those populations. I created 'median' calls based on median call parameters of each population. I also manipulated the frequency and pulse rate of tailored calls from the population median by the same percentages as the altered parameters of the Townsville combination call (an artificially manipulated call with a low frequency and a high pulse rate, relative to the median values of these parameters in Townsville) to create a unique combination call for each population. Median calls, from Townsville or the local population, were always less attractive to females than combination calls. In south east Queensland, Western Australia, and the Northern Territory, there was no significant difference in mean nightly female captures between traps producing the Townsville combination call, and traps producing tailored combination calls for each population. In north Queensland, traps producing the Townsville combination call caught significantly fewer females than traps producing the tailored combination call for that region. I suggest that calls used as lures in traps should have tailored parameters derived from vocalisations in the area in which trapping occurs, to maximise gravid female captures.

Cane toad management strategies should increase the chance of removal of every individual, by exploiting behavioural characteristics, and by increasing the period over which removal occurs. The lures in cane toad traps start and stop automatically, and operate all night, thus managers need only be on-site to remove trapped toads. Conversely, 'toad-busting' hand-capture events require participants to be on-site to find and remove toads, and may therefore be less efficient, in terms of captures per person-hour, than trapping. I used capture-mark-recapture analysis to compare the efficacy of trapping, and hand capturing cane toads, over 10 weeks, in Townsville, Australia. I trapped 7.1% - 22.4% of the estimated population per week, and hand-captured 1.7% - 6% of the estimated population per week. Trapping was more efficient than hand-capture in my regime; overall, more toads were caught per trapping person-hour than per hand-capture hour. Traps attract toads and maximise the period over which removal occurs, thus the probability of removal for each toad was higher than by hand-capture. Also, many toads caught in traps were not encountered during active searches, and vice versa, so the use of both methods, together, may be beneficial. I conclude the thesis by placing my research into an applied context, and exploring future directions for cane toad management.

Item ID: 54308
Item Type: Thesis (PhD)
Keywords: acoustic attractant, acoustic communication, acoustic lure, active distance, animal activity, anuran communication, auditory threshold, calling, cane toad, invasive anuran, invasive, mate attraction, model selection, pest management, phonotaxis, quantile count model, quantile regression, Rhinella marina, signal-to-noise ratio, sound production, trapping
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Muller, Benjamin J., Cade, Brian S., and Schwarzkopf, Lin (2018) Effects of environmental variables on invasive amphibian activity: using model selection on quantiles for counts. Ecosphere, 9 (1).

Chapter 3: Muller, Benjamin J., Pike, David A., and Schwarzkopf, Lin (2016) Defining the active space of cane toad (Rhinella marina) advertisement calls: males respond from further than females. Behaviour, 153 (15). pp. 1951-1969.

Chapter 4: Muller, Benjamin J., and Schwarzkopf, Lin (2017) Success of capture of toads improved by manipulating acoustic characteristics of lures. Pest Management Science, 73 (11). pp. 2372-2378.

Chapter 6: Muller, Benjamin J., and Schwarzkopf, Lin (2018) Relative effectiveness of trapping and hand-capture for controlling invasive cane toads (Rhinella marina). International Journal of Pest Management, 64 (2). pp. 185-192.

Date Deposited: 25 Jun 2018 05:08
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 25%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 25%
SEO Codes: 96 ENVIRONMENT > 9604 Control of Pests, Diseases and Exotic Species > 960404 Control of Animal Pests, Diseases and Exotic Species in Forest and Woodlands Environments @ 75%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960505 Ecosystem Assessment and Management of Forest and Woodlands Environments @ 25%
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