Stiltgrass and the American Toad
Ecological Ripple Effects
Invasion of an area by Japanese Stiltgrass (Microstegium vimineum) leads to some surprising developments, beyond the displacement of the native vegetation. Most native insects don’t eat this grass, but wolf spiders, voracious predators, do well in it. They eat the few insects that exist there (and the spiders will even eat each other). The resulting lack of insects spells bad news for the American toad. Not only do the toads find few insects to consume, but the young ones fall prey to the wolf spiders, so the toad population plummets.
Spiders, toads, and grass. These three organisms are kind of strange when placed together in a single sentence. It would seem that the presence of each would have, if anything, marginal effects on the other. In healthy forests full of native species, this is the case. However, when new players enter the game, things are bound to change. As John Muir once said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.” As we grow to understand the natural world that we live in, the reality of this statement only becomes more apparent.
The new player in this case is a grass. Microstegium vimineum, commonly referred to as Japanese stiltgrass, was introduced to the US sometime around 1919. Since then it has spread to over 16 states and is especially abundant in the southeast. It invades disturbed habitats and forms dense mats, which completely displace native vegetation. It quickly rises to monoculture status and it is on the move. It is only a matter of time before it spreads well into the north.
It’s not just vegetation that gets displaced either, most native insects don’t feed on M. vimineum. A monoculture of this grass is almost devoid of an insect community. However, there is one group of creepy crawlies that seems to have benefited from M. vimineum invasions. Wolf spiders are voracious predators. They eat a wide variety of insects and are certainly not above cannibalism. The dense carpets formed by M. vimineum offer security for wolf spiders. They can avoid one another and thus rise to abundance wherever this invasive grass grows. What few insects live in these stands quickly get gobbled up by the spiders. This is bad for yet another member of the forest community, the American toad.
It has been noticed that, in forests where M. vimineum dominates, toads are on the decline. It was long thought that the lack of prey insects was the cause but recent research has pointed to a different culprit, the wolf spiders. Aside from eating what little food can be found in the carpet of grass, they are also dining on young toads. Spider depredation on toads seems to be rather routine amongst the grass, so much so that toad survival decreased by 65% in these areas. To make matters worse, the effects of the invasive grass seem to be at their worst in areas that were once the best forests for toad survival.
These findings are startling but by no means unique. The researchers are now going to look to see if this is happening to other amphibian species as well. At face value, it is not apparent how an invasive grass could affect toads but it is likely that instances like this are far more common than we even realize.
Further Reading: http://www.esajournals.org/doi/abs/10.1890/13-1715.1