Invasion of the Fire Ants

Small, herbivorous mammals have a hard time of it when it comes to being attacked by larger carnivorous ones. Life expectancies are generally short in the wild, counter-balanced by the ability of most animals to breed like... well, rabbits. Many of their behavioural traits are also shaped by the need to avoid being eaten, including, not just general watchfulness, but trying to do things like foraging where (or when) predators are least likely to be a problem.

On the other hand, predators naturally have an interest in getting round this problem, and the result is a never-ending battle between the habits and skills of predators and prey alike. Over the course of thousands of years, this settles into a sort of equilibrium, changing perhaps only as new species evolve and as the geography or climate change on even longer time scales. Unless, of course, we humans get involved.

Humans can mess up the balance of life among wild animals in quite a number of ways, but the one we're concerned with today is that of invasive species. We purposefully carry species from one part of the world to others, especially when it comes to domesticated animals. Pre-Columbian America had no cows, no domestic sheep (although, of course, two native wild species), no pigs, and no horses or cats. There were dogs, but only because humans had brought them across much earlier. The introduction of all of these animals, and the changes in land use required to support them, have had significant effects on the local wildlife in at least some parts of America, and that's before we include animals deliberately introduced for other reasons, such as fur-farming or hunting, never mind those that we didn't introduce purposefully at all, such as rats.

Apart from the cats, most of these animals aren't predators, and cause problems by competition for land or other resources, rather than by directly attacking the native fauna. But, in fact, you don't need to be a large predator to physically attack, and causes problems for, local herbivores. Because there are some tiny invertebrates that can be a real pain.

One of the world's worst invasive species, insofar as it's possible to draw up an objective list of such things, is the red imported fire ant (Solenopsis invicta). Fire ants, in general, are fairly common thing, and are found in many parts of the world, although not all such animals with the name are necessarily close relatives. At any rate, most of them aren't really a problem, so long as you leave them alone, and, just like predators, the local wildlife is usually well adapted to their presence. The red imported fire ant is a bit different.

The place that the red imported fire ant was imported from is probably northern Argentina, but they are also found wild throughout much of central South America. Here, they are not a problem to the wildlife, although local humans might have a different opinion about them, but the real issue is where they have been introduced. At some point in the 1930s, some of them arrived at the port of Mobile, Alabama, presumably having stowed away on a cargo ship. Slowly, they began to march across the US, with the population undergoing a particularly dramatic expansion in the 1950s. They are currently found in 13 of the United States, and climate change is only aiding their advance northwards.

In the 1980s, they also began to invade a number of Caribbean nations, and, in the following decade, they reached China. They were first spotted in Australia in 2001, but have so far failed to gain a foothold there, despite several further incidents.

Fire ants are most notable for their painful bites, which they inflict in self-defence, but they also, being herbivores, cause massive damage to crops. Back in 2000, the state of Texas estimated that it was losing $300 million of agricultural income and health costs every year to the ants, and spent a further $200 million in attempts to get rid of them. And that's just Texas... in prices 16 years ago.

So, yeah, from a human perspective, they're not to be sniffed at. But they also affect the local wildlife. Which is where we get back to mammals. (You may have been wondering...)

It should come as no surprise to discover that the presence of red imported fire ants does, indeed, affect the local wildlife. The ants can give humans and livestock nasty bites, so they can certainly do some serious damage to smaller mammals. We know, for instance, that the ants affect the foraging habits of local wild mice, some of which apparently find signs of the insects more scary then they do the scent of predator urine. This sort of thing can have significant effects on the local ecology. Even if the ants don't kill the mice outright - and they probably can - the fact that the mice won't be able to gather as much food for fear of getting bitten will affect the mammals' survival, and hence the diet of other animals further up the foodchain.

It isn't jut mice; rats are affected, too. At the site of one experimental station in Georgia, the most common small mammals, at least at some times of the year, are reported to be hispid cotton rats (Sigmodon hispidus). These are slightly smaller than the familiar brown rat so often seen in urban areas, but at up to 250 g (9 oz.) in weight, they are definitely "rats", rather than "mice". They live throughout the southeastern United States, broadly between the Rio Grande and Platte Rivers in the west, across to the Atlantic coast from the southern tip of Florida to somewhere in Virginia. Like most wild rats, they feed on a mixture of seeds and foliage, and they live in open grassy environments or patches of light woodland. Ecologically, they are significant as a major source of food for local bobcats, red wolves, hawks, and owls.

So how do you find out whether cotton rats adjust their behaviour, depending on the presence of fire ants? Sitting out in the wild and trying to watch them would be difficult at best, and, besides, the ants are everywhere in the vicinity, so it wouldn't help much if you could. Instead, researchers placed bowls of sand with mixed-in millet seed at random locations across the grassland, covered in netting to stop birds getting at them. Most of these were placed in areas previously treated with insecticide to keep down the fire ants, but a few were not, allowing the researchers to compare one with the other.

Four days later, there was, on average, over twice as much edible food remaining in the bowls placed where there were fire ants than in the areas where the insects had been killed off. Tracks and dung pellets around the bowls, along with a general sampling of the local small mammal population, demonstrated that the great majority of the animals feeding on the bowls appeared to be cotton rats, rather than, say, mice.

This is likely to be bad news for the rats. If they are less able to forage in any given area, they probably have to spend more time doing it, and more time travelling between potential food sources. That burns energy, so you get less overall benefit from the food, but it also exposes you to more risk of being spotted by a predator. This is may well be why an earlier study found that cotton rats living in areas with fire ants are more likely to die early than those that do not.

And, if there are less cotton rats, that's unfortunate for the bobcats, birds of prey, and other animals that rely on them for their own food. The entire ecological web can be affected - not catastrophically, perhaps, but enough to do general harm. Everything is interconnected in ecology, and damage to one part of the web has knock-on effects elsewhere.

All because of one tiny little nuisance with a nasty bite.

[Photo by CDC/ James Gathany, in the public domain].