The diet of invasive toads in Mauritius has some rare species on the menu



The invasive guttural toad.
Author supplied.

James Baxter-Gilbert, Stellenbosch University

The guttural toad (Sclerophrys gutturalis) is a common amphibian found in much of sub-Saharan Africa, from Angola to Kenya and down to eastern South Africa. With such a wide geographic range, and a liking for living in human-disturbed areas, it’s often seen in people’s backyards. Around gardens it can be thought of as a helpful neighbour, as it is a keen predator of insects and other invertebrates that may try to eat plants. Yet it also has the potential to be ecologically hazardous outside its native range – and this toad is an accomplished invader.

In the Mascarene Archipelago in the Indian Ocean, far from mainland Africa, these toads have been an established invasive species for almost 100 years. In 1922, the director of dock management in Port Louis, Mauritius, deliberately released guttural toads in an attempt to control cane beetles – a pest of the country’s major crop, sugar cane. This attempt at biocontrol failed, but the toads appeared to thrive and rapidly spread across the island.

Mauritius had no native amphibian species for it to compete with, and no native predators with a recent evolutionary history with toads. In mainland Africa these toads would have to divide resources, like food, with a host of native amphibians and deal with an array of native birds, mammals and snakes that evolved feeding on them. But without these challenges on Mauritius, the toads colonised the entire island rapidly.

Most toads are generalist predators and hunt a wide variety of prey, more or less eating whatever they can fit in their mouth. So as the guttural toad’s population numbers grew through the decades, so too did the concerns from Mauritian ecologists about the impact on native fauna. Anecdotal accounts as early as the 1930s suggest that the toads were having a negative impact on endemic invertebrate populations. In fact it has been suggested that the toads may have been a driver in the decline, and possible extinction, of endemic carabid beetles and snails.

But it’s only recently that the toad’s diet in Mauritius has been examined closely. In our new study we examined the stomach contents of 361 toads collected in some of the last remaining native forests of Mauritius.

By knowing more about what species the toads are eating, and which groups they favour, our research may help inform toad control actions to protect areas with known sensitive species.

In the belly of the beast

Through our research we were able to identify almost 3,000 individual prey items, encompassing a wide variety of invertebrates like insects, woodlice, snails, spiders, millipedes and earthworms.

This research also went one step further to examine the prey preference of the toads. In general, they seemed to favour, some of the more abundant and common prey species. These included ants and woodlice, which made up about two-thirds of their overall diet.

These findings may suggest that the toads were able to identify a readily available food source, and this may have fuelled their invasive population growth. Yet they are also eating prey that represents a more serious conservation concern.

Inside the toads we found 13 different species of native snail, most of which were island endemics. Four species are listed as being vulnerable to extinction and one, Omphalotropis plicosa, being critically endangered – having been presumed extinct until it was rediscovered in 2002. Understandably, we found it very troubling to find a “Lazarus species” within the stomach of an invasive predator.

Unanswered questions

These early insights into the native species now being hunted by a widespread and voracious predator raise new research questions. To understand the greater impact the toads are having on native species much more work is required to understand their prey’s population dynamics so we can determine if the toad’s invertebrate “harvest” is contributing to declines.

Furthermore, how does the toad’s invasive diet in Mauritius compare with that of other invasive populations, like those in Réunion or Cape Town – is their invasive success linked to a common prey type? And how does it compare with their diet in their own native species range?

Our study could only examine what they are eating currently, but Mauritius has seen numerous species decline over the past 100 years. What role did the toad play in these losses? Perhaps they historically fed more readily on creatures that were more abundant in the past, but had to switch their favour to ants and woodlice when the populations of other species dropped. We may never know.

What is clear is that there is much to learn about the habits of this far-from-home amphibian and its impact on the ecosystems it has invaded.The Conversation

James Baxter-Gilbert, Postdoctoral Fellow, Centre for Invasion Biology (C·I·B), Department of Botany & Zoology, Stellenbosch University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Think taxing electric vehicle use is a backward step? Here’s why it’s an important policy advance


Jago Dodson, RMIT University and Tiebei (Terry) Li, RMIT University

The South Australian and Victorian governments have announced, and New South Wales is considering, road user charges on electric vehicles. This policy has drawn scorn from environmental advocates and motor vehicle lobbyists who fear it will slow the uptake of less-polluting vehicles. But, from a longer-term transport policy perspective, a distance-based road user charge on electric vehicles is an important step forward.

Superficially, a charge on electric vehicle use seems misguided. Road sector emissions are the worst contributors to climate change. Electric vehicles powered by clean energy offer the promise of near-zero emissions.




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As electric vehicle and renewable energy costs decline we can expect a shift to full electrification of urban vehicles over the next 30 years. Surely accelerating this transition is an urgent climate task?

The downside lies not in the carbon benefits of these vehicles, but in their use as private passenger transport in congested urban areas and the costs this use imposes on cities. As renewable energy becomes cheaper, the marginal cost of every kilometre driven is likely to decline. As driving becomes cheaper, more of it is likely to occur.

More driving means more congestion. Inevitably, that increases demand for increasingly expensive road projects, such as Sydney’s WestConnex, or Melbourne’s Westgate Tunnel and North East Link. It certainly will run against the recognition in urban plans such as Plan Melbourne that we must shift to alternative transport modes.

If we don’t have a pricing regime that accounts for the cost of car use in cities, the transition to electric vehicles is likely to work against the wider goals of urban and transport policy.




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How would distance-based charging work?

Many urban transport policy advocates have called for distance-based road-user charging to be imposed on all vehicles in cities. This sounds great in theory, but in practice is difficult for technical and political reasons of privacy and surveillance. Such concerns will diminish over time as cars increasingly incorporate automated telematics that necessarily track their movement.

Distance-based road-user charging efficiently matches road use to its costs – of infrastructure, congestion, noise, pollution and deaths. It improves on fuel excise, which drivers can nearly completely evade by using a highly efficient vehicle. It also goes beyond tolling to fund major roads, which typically apply only to specific links.

Second, road-user charging can be varied in response to demand that exceeds road capacities. Higher rates can be applied at peak times to ensure free-flowing traffic and shift travel to other times and modes. Various taxation reviews, including the 2009 Henry Taxation Review and Productivity Commission reports, have promoted such policies.




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Exactly how big would the disincentive be?

Would imposing such charges on electric vehicles retard their uptake?

Based on our work with ABS Census journey-to-work data, in Melbourne the average daily round-trip commuting distance by car is about 25 kilometres. The proposed Victorian charge is 2.5 cents per kilometre. Thus, in Melbourne the average daily commuter’s road user charge is likely to be 63 cents – $3.13 for a typical five-day working week. Over a 48-week working year that totals A$150, hardly a large sum for most people.

By comparison, a commuter in a conventional vehicle with the average current fuel efficiency of 10.9 L/100km will use about 2.73 litres of fuel on which they pay 42.3 cents per litre in fuel excise. That’s about $1.15 a day, or $5.75 a week.

The average tax saving for electric vehicles compared to conventional vehicles will be about 2.1 cents per kilometre. Electric vehicle drivers will be taxed about 53 cents a day, or $2.64 a week, less for their car work travel. They’ll be about $126 a year better off.

Commuting trips make up about 25% of car use, so electric car users’ overall savings are likely to be even greater.

It is difficult to see how such savings on excise tax are a disincentive to electric vehicle uptake. Fears of a “great big new tax”, as the Australia Institute puts it, seem unfounded, as are concerns that road-user charges would “slam the brakes on sales”.

Let’s be clear, the big barrier is the upfront cost of electric vehicles, about $10,000 more than their conventional equivalents. Advocates for electric vehicles should focus on that difference, and the failures in Australian government policy, not state road-user charges.




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Why taxing actual road use matters

It needs to be recognised that, with lower marginal costs, electric vehicles are likely to be used more than conventional cars. That would increase pressure on urban road capacity. So while the new road-user charge of 2.5 cents per kilometre is flat across the time of day or the route driven, this will likely need to change.

Distance-based road-user charges have been politically controversial. Imposing a tiny charge on a minority vehicle type is an expedient way of introducing a needed reform. Fewer than 1.8% of vehicles in Australia are currently electric or hybrid. But as all cars become electric, distance-based road charges will become an increasingly powerful policy tool.

Thanks to advancing telematics, transport planners will eventually be able to impose variable road-user charging by time of day and route, similar to ride-hailing companies’ “surge” pricing. We could then apply novel approaches such as a cap-and-trade system. A city could allocate its motorists an annual kilometres quota, which is then traded to create a market for excess urban road use.

The private car could also be integrated into mobility-as-a-service models.

Road-user charges could be regressive for people with few alternatives to the car. But telematic tracking could allow for lower charges for less affluent households in dispersed outer suburbs with few other options.

Beyond fuel, private cars have high environmental costs in steel, plastic, aluminium, glass and rubber use. And about one-third of our increasingly valuable urban space is given over to cars in the form of roads and parking.




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To reduce this demand on resources and space, car use could be priced to shift travel to, and fund, more sustainable and city-friendly modes such as public transport, walking and cycling. We could even price the car out of cities completely. The most environmentally sustainable car, after all, is no car at all.The Conversation

Jago Dodson, Professor of Urban Policy and Director, Centre for Urban Research, RMIT University and Tiebei (Terry) Li, Research Fellow, School of Global, Urban and Social Studies, RMIT University

This article is republished from The Conversation under a Creative Commons license. Read the original article.