Is Stiltgrass Killing our Toads? Yes, but indirectly.

Japanese stiltgrass infestation

Invasive plant impacts on native plant species and on community structure is well documented.  Invasive plants direct-impacts on native wildlife through reduction in forage availability is also fairly straightforward.  What is less understood is the more complex indirect-impacts on wildlife from invasion by exotic plant species.

We've discussed these types of indirect-impacts to wildlife from invasive plants here on this blog several times.  Buckthorn infestations alter the distribution and movement patterns of predators, contributing to the decline of amphibians in the upper Midwest LINK.  Bush honeysuckle can change the competition balance in amphibians, leading to dominance by one native species, to the detriment of many other species LINK.  Invasive plants of prairies, such as knapweed and leafy spurge, alter the habitat structure in such a way that promotes increased web building by native spiders, leading to a trophic cascade that impacts native flora and fauna by throwing everything out of balance LINK.  In a presentation at this year's Illinois Invasive Species Symposium, Dr. Matt Allender indicated that the reduction in habitat quality from invasive plant species could be contributing to the decline in health and wellness for box turtles, which makes them more susceptible to diseases LINK.

Of course, sometimes our native wildlife can facilitate invasive plants, as is the case with deer and garlic mustard LINK.

A recent article published in the journal Ecology brought to light evidence that Japanes stiltgrass is indirectly leading to increased mortality of young toads.  Worse yet, this indirect impact is greatest felt in forests, a preferred habitat for young toads and traditionally a stronghold for survivorship.  Now habitat that once was a source, could be a sink.

So how and why is this happening?  Basically stiltgrass infestations are superb habitat for wolf spiders and wolf spiders are super predators and they love to eat young toads.  So more stiltgrass = more wolf spiders = less toads.  A press release published in Science Daily (LINK) elaborates:

Spiders are incredible predators, Maerz explained, and they eat everything -- even other spiders. That typically keeps spider populations in check, Maerz said, but Japanese stiltgrass is "kind of like a tall shag carpet," and it provides the cannibalistic spiders refuge from one another. The accumulation of large, predatory spiders in these invaded habitats then results in higher mortality for small toads that have recently emerged from wetlands... spider densities were 33 percent higher and toad survival decreased by 65 percent... with the presence of stiltgrass. The presence of stiltgrass alone, in the absence of spiders, did not affect toad survival.  "Spiders are actually tremendously important and incredibly abundant predators on the forest floor, and they will eat many of the small species that live there, so this effect is unlikely to only influence toads,"

This research made the cover of the journal Ecology and for good reason!  Just to notice and speculate about this type of interaction is impressive enough, but then to design a study that teases out other impacts and clearly demonstrates this important impact and result from invasion is admirable. 

The citation for the research is:

Jayna L. DeVore, John C. Maerz. Grass invasion increases top-down pressure on an amphibian via structurally mediated effects on an intraguild predator. Ecology, 2014; 95 (7): 1724

Amphibians, as a suite of species, are in global decline. This trend holds true here in Illinois as well. Water pollution, climate change, and habitat loss are all contributing to this decline and now we are starting to understand that invasive plants not only impact forage availablity for wildlife but, in addition, can cause much more complex changes to invaded areas that are not as easy to understand or mitigate.  We need more research like this.  We need to understand these impacts, just to allow us to be able to prioritize our control efforts and develop practices and protocols to better manage our native wildlife.