Invasive Species
I was walking through Cleland Wildlife Park in Adelaide, South Australia when I came across a most peculiar bird. It was unlike any I had seen before, standing over a foot tall on long, spindly legs. It had white downy feathers streaked with an eye-catching dark brown. Its plumage was a combination of whites, brown, and black, each feather with a base of one color was streaked with bits of the others. At its head, it had a strong beak and bright greenish-yellow eyes. It walked with long legs awkwardly bent at an angle. I had found the bush stone-curlew.
The most unusual thing about the bird wasn’t its appearance or even its name, but its behavior. There was a group of them, and I must have surprised them because when they saw me they reacted immediately. They quickly jerked their heads with one eye facing the sky, staring intently. And then they froze, just standing in that position.
“What are you doing?” I wondered out loud. “Shouldn’t you be running for cover? Or sounding some high-pitched alarm call?” I took a step closer and they just remained there. From their frozen stance it seemed they were hoping to go unnoticed, despite my being only feet away and talking to them. I stepped closer and took the picture above, wondering when they would switch to plan B, but still, they remained motionless. They were insistent in their futile strategy that left them so defenseless that I felt almost awkward standing next to them as if were we playing hide-and-seek and I was supposed to pretend I couldn't see them. Was this their defense strategy, to make predators feel uncomfortable by such a pathetic defense strategy? Probably not. So what is the strategy?
Their strategy does make sense though, in the right context. Historically, bush stone-curlews were preyed upon from the sky - eagles, hawks, kites, and owls would all have been eager to make their acquaintance, especially of the younger stone-curlews. Raptors rely on their keen eyesight and being able to see their prey from above, and any movement makes this easy. So keeping a camouflaged body still and silent makes a lot of sense when death flies overhead.
But times have changed. With the arrival of humans came cats and foxes. Against these, standing still is a truly awful ploy. First, they hunt from the ground, so looking up isn’t doing anything. Secondly, eagles don’t use scents in hunting, but foxes sure do. No amount of standing still is going to convince a fox that can smell you that you are not a tasty meal. Bush-stone curlews can fly but rarely choose to. Instead, they prefer to casually scamper away. Again, this works in many cases, but they are not particularly fast, so they can’t outrun cats and foxes. Bush stone-curlews also like to make an awful screaming sound at night, and there’s not much an eagle can do about it. But to a cat, this is the sound of a particularly obnoxious dinner bell.
These are the curlew's instincts. They have evolved over millions of years and can’t be changed overnight. How long will it take for them to adapt? Who knows? There's certainly no guarantee that it will happen fast enough. Against a radically different threat - such as a ground-based predator - change might not come quickly enough to avoid extinction.
When facing the predators they have historically, the fittest survived, which in that case were the ones that could stand perfectly still in the face of danger. But with the introduction of invasive species such as cats and foxes, their ecosystem, in particular their predators in this case, have changed way too fast for natural selection to prepare these birds. If an olfactory-based carnivore evolved with them, they would have had time to adapt. But the introduction of 15 million feral cats in just over a couple hundred years has left the bush-stone curlews, and many other animals, in an ecosystem they are not prepared for.
Islands, whether they be a small rock cropping or a large landmass, have ecosystems with one thing in common - they are highly susceptible to devastation by invasive species. The plants and animals adapt to the world around them. And if that world suddenly changes. They are in trouble. If that world only hunts them in a particular way, or doesn’t hunt them at all, they get used to it.
Among the most devastating invasive species is the house cat, or, in many cases, the feral cats that arise when house cats escape. A study in 2011 found that feral cats have contributed to 33 extinctions on islands. A study looking at the effect of cats in the United States concluded that free-ranging cats kill billions of birds and billions more animals per year, and “are likely the single greatest source of anthropogenic mortality for US birds and mammals”.
Due to its size, Australia is more robust to invasive species than, say, the Galapagos Islands, where the fauna appear to lack all fear. There are rough marsupial counterparts to the placental mammals that roam the rest of the world. Australia has a quoll, which evolved to fill a niche in Australia similar to cats elsewhere. But if the quoll is the marsupial counterpart to the cat, then why didn’t it “teach” the bush-stone curlew and other prey how to avoid them? Well, the similarity between quolls and cats is not perfect. They both enjoy smaller prey, rat-sized animals and below. Thus many of Australia’s smaller prey, like small lizards and desert mice, are doing just fine with the new invasive species. But only cats will feast on the larger animals. This has left animals like the bush-stone curlew in a tough spot.
The other carnivores are different too. Goannas and snakes would certainly target the young curlews, but they are exothermic and so eat infrequently. They might find one bird and be satisfied for a week, while a cat might kill multiple in a single night. Even though raptors are also endothermic like cats, their impact is still manageable. An eagle coming along a colony of birds will grab one before heading off to eat it. But a fox will kill as many as it can, even beyond what it will eat, and store the leftovers in hiding. Bush-stone curlews have never experienced this type of predation. It’s no surprise that this type of hunting tactic is far more damaging to the population than that of any of the native predators.
Australia has a long and tangled history with invasive species. The recent history began where all things in recent Australian history do - with the adventures of Captain Cook. As you might imagine, the vessels in his fleet were covered with nasty lice and rats. So, with the forethought and concern typical for European colonization, he tied his ship up to the shore overnight hoping they would leave his ship and invade this new land. He got his wish. The subsequent history of European colonialization and invasive species hasn’t been much better since.
Despite all the damage they do, most invasive species are brought intentionally. The most infamous invasive species in Australia, the cane toad, falls into this category. When I was in Darwin, seeing a frill-necked lizard was at the top of my list. But their numbers have fallen significantly because they eat cane toads, which are toxic to them. So, instead of seeing a frill-necked lizard, I saw cane toads. Lots and lots of cane toads. They are ubiquitous throughout any grassy area. Walking through the grass of the Esplanade in the middle of the city after sunset is an exercise in trying to avoid stepping on them.