Friday, March 18, 2011

Hop to It

Did you ever see the film "Rabbit-Proof Fence"? The one that wasn't actually about rabbits and only sort of about the fence? Back at the turn of the 20th century, the Australian government commissioned a fence stretching from the north end of the continent to the south in a marginally successful attempt to halt the terrible onslaught of voracious rabbits marching and munching their way westward.

Thomas Austin was to blame for the bunny invasion. In 1859, he ordered two dozen rabbits from England, destination Victoria. (If you're not familiar with your Australian geography, Victoria is in the southeast, just across the water from Tasmania). As he put it, "The introduction of a few rabbits could do little harm and might provide a touch of home, in addition to a spot of hunting."

Well done, Tom. Your desire to bring a few homey touches to your adopted land touched off an ecological disaster.

As the long-eared transplants bred like, well, rabbits, they overwhelmed Australia's unprepared ecosystem. Predators down under were accustomed to much larger jumping things (you know… kangaroos) and couldn't keep pace with the ever-proliferating cottontails.

But that's old news. These days, Australia is under siege from a different hopping creature, and these critters are considerably less cute. The new menace? Cane toads.

Cane toads (Bufo marinus) are lumpy, bumpy, nocturnal amphibians with slimy-looking, mottled brown skin. The males grow to about 6", and the hefty females can reach over 9". For scale, here's an Aussie with his racing toad:

To add to their charm, they have toxic glands in their shoulders, so invasivory isn't a feasible option for dealing with their invasion; even their eggs are poisonous. Cane toads themselves aren't picky eaters, gulping down everything from birds' eggs to native invertebrates to young members of their own species.

Like rabbits, the toads are an introduced species, though in this case, they were brought in in 1935 from Hawaii as a way to combat sugar cane pests in northeastern Australia. Like rabbits, they reproduce at an alarming rate.

And like rabbits, they're wreaking ecological destruction in that part of the continent by virtue of their appetites and their toxic nature. Cane toad toxins, called bufogenins, are structurally different from those of native amphibians, so the physiological defenses of Australian predators aren't effective against them.

Fish that eat the toads' eggs get sick or go belly-up, and snakes, crocodiles, and the adorable northern quoll (Dasyurus hallucatus, below) are all highly sensitive to cane toad toxin. Many have seen their populations decline precipitously following the arrival of the toads.

Some animals learn to steer clear of cane toads and their eggs after a bout of toxin-induced nausea (if they're lucky enough to survive it), much the same way as North American birds learn to avoid eating monarch butterflies.

Last year, researchers honed in on this behavior, known as aversion learning. They succeeded in teaching quolls, which have been especially hard hit by the hopping invaders, that cane toads do not make good eatin'.

To do this, the scientists fed a group of captive-bred quolls a dead toad spiked with a compound that causes nausea. The selected toads were too small to actually kill the quolls, but the combo of toad and chemical gave the quolls a nasty upset stomach.

In order to test whether the mammals were still interested in eating cane toads post-recovery, the team offered each quoll a live specimen. Most turned their furry noses up at the offering. When released, these quolls were five times more likely to survive than the unlearned control group.

Aversion learning is useful, but people can't catch and train every vulnerable animal in NE Australia. And it still leaves the problem of cane toads eating native wildlife and bullying out the local amphibians. The toads have a weakness, though: like all anurans (frogs and toads), they need water to survive.

Unlike Australia's endemic amphibians, which have evolved to withstand the brutal dry season (by burrowing underground or forming cocoons, for instance), cane toads must frequently quench their thirst, or rather their skin, to avoid desiccation. They cluster in and around water sources--often human-made ones like irrigation bore-holes--giving a new meaning to hanging around the water cooler.

A study recently published in the biological journal Proceedings of the Royal Society B documented an attempt to exploit this weakness. Scientists erected cloth fences about two feet high around artificial water sources in Australia's Northern Territory.

The fences kept cane toads out--they were designed such that shrewd toads couldn't just burrow underneath--while trapping those remaining inside. Researchers removed the trapped toads each morning.

Compared with fence-free control water sources and procedural controls (fences with the cloth raised several inches off the ground to allow the toads access to the water), toads in fenced areas dropped like flies. There were ~20 radio-tagged toads in each group: only one of the control toads was dead after three days (it was eaten by a bird… no word on whether or not the bird is still flapping its wings), whereas every single one of the fenced toads died within three days.

(Modified from Figure 2 in Florance et al., 2011. I can't understand why they bothered log+1 transforming the data, since it's not like their study groups were unmanageably large.)

Sounds like a pretty effective control method to me! The authors observed that other animals like birds, large snakes, and large mammals that depend on the bore-holes can still access them. Native anurans were not adversely affected by the fences either.

Back during the rabbit crisis, the government appointed Alexander Crawford as Acting Chief Inspector of Rabbits. Mr. Crawford was in charge of fence maintenance. I wonder... has Australia has appointed a Chief Inspector of Cane Toads yet? He or she could be in charge of these new fences!

This all brings to mind yet another Australian invader: Tropical Race Four. Haven't heard of it? It's not big news yet outside of agricultural circles, but it spells death for the banana cultivation trade. If you can find the Jan. 10, 2011 edition of The New Yorker, read Mike Peed's report entitled "We Have No Bananas" (unfortunately, it's not available for free online). Otherwise, stay tuned; I'll write about it at some point! For now, since I don't have any cane toad recipes, here's a recipe for banana bread. Make it while you can still find bananas at the grocery store...

Banana Bread
Makes one loaf
Adapted from a recipe in Martha Rose Shulman's "Recipes for Health" series

2 large eggs
1/2 cup brown sugar (packed) or turbinado sugar
1 cup whole wheat flour (preferably pastry flour)
1 cup unbleached all-purpose flour
1 tsp baking soda
1 tsp salt
1 tsp cinnamon
1/2 tsp nutmeg
1/4 tsp cardamom
Dash of allspice
1/3 cup canola oil
1/4 cup plain yogurt or buttermilk*
1 cup mashed ripe bananas (2 bananas)
1 tsp vanilla
1 cup chopped walnuts (optional)

*To make your own buttermilk, combine 1/4 cup milk with 1/4 Tbsp white vinegar or lemon juice and let stand 10min. I hear this also works with soymilk, but I haven't actually tried it.

1.   Preheat your oven to 350˚F.

2.   Crack the eggs and dump the sugar in a mixing bowl and beat for about 5 minutes. If you have a standing mixer, great. If you're using a handheld version, hold it with one hand and play with your iPhone in the other or something. You're going to be there for a while.

3.   Sift or whisk together the dry ingredients (except walnuts) in a small bowl and set aside.

4.   Smoosh those bananas with a fork or your fingers, preferably in a bowl.

5.   When the eggs and sugar are sufficiently mixed, beat in the oil, yogurt or buttermilk, mashed bananas, and vanilla. Slowly beat in the flour mixture (don't add it all at once). Stir in the walnuts if you're one of those people who likes nuts in your bread. I don't understand you.

6. Plop the batter in a greased 5x9" loaf pan and bake for 50min or so. To test if it's done, take a butter knife and stab it in the middle. If the knife comes out clean, the bread is done. If it comes out goopy, stick it back in the oven for a few minutes and test again. Let the bread cool in the pan for 10 or 20 minutes before taking it out to cool.

(I meant to take a picture of the bread before we devoured it all… but suddenly it was gone. No final picture this time. Whoops.)

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