Saturday 4 October 2014

How an invasive Grass species makes American Toads more vulnerable to Wolf Spiders.

Invasive plants can significantly modify local habitats, particularly if they lack herbivores in the environments they are colonising, by both modifying the structure of the environment and excluding native plants which provide food and homes to native animals. One such plant is the Japanese Stilt Grass (Microstegium vimineum), which has invaded moist forest-floor environments across the eastern United States. Japanese Stilt Grass has been shown to modify soil chemistry and Arthropod diversity and abundance in areas where it invades, and lacks any significant herbivores in the United States, allowing it to spread with little impediment.

Japanese Stilt Grass, Microstegium vimineum. Theresa Yednock/National Park Service/Wikimedia Commons.

One Vertebrate species is also apparently affected by the presence of Japanese Stilt Grass, the American Toad (Anaxyrus americanus), which appears to decline where the Grass becomes established. This could potentially be for a number of reasons. Firstly, like all Amphibians, American Toads have permeable skin through which they exchange fluids and gasses with the environment, which may make them vulnerable to changes in soil chemistry. Secondly they feed almost exclusively on Arthropod prey, notably Mites and Ticks (Acari), Spiders (Araneae), Beetles (Coleoptera), Springtails (Collembola), Flies (Diptera), Bugs (Hemiptera), and Ants, Bees and Wasps (Hymenoptera) and changes in the abundance of these prey items may affect the survival of the Toads. Finally the Toads themselves are vulnerable to predation by larger Arthropods, as they metamorphose (change from tadpoles into adult Toads) very young, achieving most of their growth in the terrestrial environment.

American Toad, Anaxyrus americanus. Brad Glorioso/National Wetlands Research Center/USGS.

In a paper published in the journal Ecology in July 2014, Jayna DeVore of the Warnell School of Forestry and Natural Resources at the University of Georgia and the School of Biological Sciences at the University of Sydney, and John Maerz, also of the Warnell School of Forestry and Natural Resources, describe the results of a series of experiments designed to determine exactly how the presence of Japanese Stilt Grass affects the American Toad.

Firstly DeVore & Maerz created enclosures in four areas of woodland in Georgia State that were enclosed in a way that prevented the entrance or exit of Toads but allowed Arthropods free movement. Within these enclosures the numbers of both Toads and the various Arthropod groups were monitored. No correlation between prey species numbers and Toads was found, but Toad numbers did decline in numbers in pens where Wolf Spiders (Lycopsidae) were present in large numbers.

Wolf Spiders are large predatory Spiders that hunt by waiting in ambush for mobile prey, then running it down. Small Toads, which actively seek out prey, moving about as they forage, are particularly vulnerable to this hunting technique, as they cannot feed it they remain immobile (the Spiders do not themselves forage, and will not find stationary prey) and have no defence against the Spiders.

The Carolina Wolf Spider, Hogna carolinensis. Patrick Edwin Moran/Wikimedia Commons.

DeVore & Maerz then carried out a series of cage experiments where Toads were placed in cages with or without Japanese Stilt Grass, and Wolf Spiders (Hogna carolinensis or Hogna helluo). In cages lacking Spiders the Toads survived well, regardless of the presence of the Stilt Grass, suggesting that it does not affect the soil chemistry in a way that harms the Toads. However in cages with Wolf Spiders the Toads suffered significant mortality, and this rose sharply in cages where both Spiders and Grass were present, suggesting that the Grass modifies the environment in a way that makes it easier for the Spiders to hunt the Toads.

Conceptual diagram of explored mechanisms through which Microstegium vimineum invasion can affect the survival and growth of metamorphic Toads, indicating both direct (solid line) and indirect (dashed line) effects. Black arrows are used to portray connections that were altered following invasion, resulting in significant changes in depicted parameters (e.g., densities, survival rates), whereas those that were unaffected are depicted in gray (i.e., growth rates). Significant changes are also annotated with a sign indicating whether the indicated parameter was positively (+) or negatively (-) affected (as compared with adjacent, uninvaded habitats; see Appendix C for effect sizes). In summary, invasion amplified top-down pressure on toads by increasing structural complexity, which dampened the strength of a pre-existing intraguild (IGP)/cannibalistic trophic linkage among Lycosid Spiders, resulting in higher spider densities and, subsequently, lower Toad survival within invaded habitats. The potential for bottom-up effects occurred via post-invasion changes in detrital food webs, which ultimately decreased the availability of edible invertebrates, but Toad growth was unaffected by these reductions. Although we also documented significant changes in abiotic habitat parameters following invasion, these effects alone did not significantly influence Toad survival (linkage not pictured). DeVore & Maerz (2014).

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