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.
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.
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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