Welcome to Invasive Plants

MMajor invasive plants that threaten natural areas by replacing diverse native plants communities with nonnative monocultures. Purple loosestrife invasion (left photo), nonnative Phragmites australis invasion (center photo), and Japanese knotweed invasion of old field (right photo; knotweed is white flowering plant visible in background)

“Foraging bucket” in native vegetation (upper photo) and knotweed invaded (middle photo) areas. Note dense layer of dead knotweed stalks but lack of other vegetation or litter. Foraging buckets consist of a standard 5 gal bucket with the bottom cut off and three 15 cm tall X 10 cm wide “windows” cut in the sides. The windows in are covered by hardware cloth small enough to keep frogs but large enough to allow insects to pass through captive ( lower photo). Buckets are recessed into the ground 5 cm and held in place with sod stakes. Buckets were covered with no-see-um cloth fastened with an elastic band.

Worldwide declines of amphibian populations have drawn increasing attention to the factors that regulate amphibian populations and the complex causes of declines (Alford and Richards 1999). Most research into the causes of declines has focused on the aquatic; egg or larval phases of amphibians (Alford and Richards 1999, Kiesecker et al. 2001), and research on the contributions of introduced species to amphibian population declines has been limited to the impacts of introduced predators or competitors (Kupferberg 1997, Kiesecker and Blaustein 1998, Knapp and Matthews 2000). Limited attention has been given to the contributions of nonnative plant invasions to amphibian population declines (see Brown 2002), and to impacts on amphibian populations during later life stages and away from aquatic habitats. Several studies suggest that amphibian population viability is linked to the availability of vegetated terrestrial habitats such as forests and meadows around breeding ponds (Pope et al. 2000). These terrestrial habitats are critical to frogs because they provide overwintering habitat (and Madison 1999) and are critical areas where frogs go to forage and replenish depleted energy reserves before winter (Lamourex et al. 2002).

The terrestrial habitats surrounding many wetlands are under considerable pressure from invasive, nonnative plants like purple loosestrife (Lythrum salicaria), nonnative races of the common reed (Phragmites australis), and Japanese knotweed (Fallopia japonica). These aggressive species can rapidly exclude diverse native plant communities, creating near monocultures of the invasive plant (Blossey 1999). These plants are generally unpalatable to herbivorous insects. These plants also decompose differently than the native plants they displace, so they can also have an effect on the availability of detritivorous insects. For example, nonnative Phragmites remains standing and non-decomposed longer than the native Phragmites, and Japanese knotweed stalks form thick layers that decompose very slowly (see related links). Because they can significantly alter the distribution and abundance of invertebrates in habitats critical to frog foraging, nonnative plant invasions of terrestrial habitats surrounding wetlands may pose a significant threat to amphibian population viability.

We used pitfall traps and “foraging buckets” in invaded and non-invaded areas of old fields to determine whether Japanese knotweed invasions were affecting invertebrate abundance and green frog (Rana clamitans) foraging success. We hypothesized that, compared to adjacent areas with diverse native vegetation, invertebrate abundance in pitfall traps would be lower in areas of old fields invaded by Japanese knotweed, and frogs in Japanese knotweed would gain less mass and have less food in their stomachs. We conducted our study in two old fields that surround Hawkins Pond in southeastern Broome Co., NY. At both sites, we ran two parallel transects starting 25 m into non-invaded portions of old field, running through the “invasion front”, and going 25 m deep into the Japanese knotweed stand. At 5 m intervals we installed 3 pitfall traps and a “foraging bucket”. Soil cores from installing pitfall traps were preserved to measure the abundance of soil dwelling invertebrates that might be important prey for frogs (e.g., earthworms and slugs). Pitfall traps were opened for 24 h on three different nights between 10 August and 31 August 2002 to collect insects. On 23 September, during the natural period when frog foraging forays into terrestrial habitats peek (Lamoureux et al. 2002), we put one green frog (7-20 g wet mass) into each bucket and covered it with no-see-um netting and an elastic band. Before we put the frogs into the buckets, we fasted them for 3 d and we weighed them shortly before release. Frogs were put into buckets in the later afternoon during a rain when they would naturally disperse from ponds to foraging areas (Lamoureux et al. 2002). After 48 h (36 h of rain), we collected frogs from buckets, placed them in containers with spring water for 2 h to make sure they were fully hydrated, and weighed them. Then we flushed each frog to check its stomach contents, and we retained the frogs in captivity for an additional 24 h and recorded which frogs defecated.

We found that frogs in native vegetation gained significantly more weight than frogs in Japanese knotweed. All but one frog in knotweed lost mass, while more than half of frogs in native vegetation gained mass. That some frogs in the native vegetation lost mass suggests that not all frogs in the native vegetation fed. One explanation might be that frogs were forced to into unfamiliar locations of unknown prey availability rather than having the opportunity to go to familiar areas known to be rich in prey (Lamoureux et al. 2002). Another explanation is that immediately prior to this study the region experienced a prolonged drought. The drought may have reduced invertebrate abundances below normal levels, limiting prey availability for frogs.

Our research shows that Japanese knotweed invasions degrade terrestrial foraging habitats of frogs. It is likely that the effects of knotweed invasions on food availability extend to other vertebrates that also use fields, such as small mammals and birds. Japanese knotweed invasions so reduce the foraging success of frogs, that essentially, Japanese knotweed invasions eliminate terrestrial habitat for frog foraging. This adds invasive plants to the list of causes of habitat loss, like roads and development, that reduce the viability of wildlife populations.

References
Alford, R. A. and S. J. Richards. 1999. Global amphibian declines: a problem in applied ecology. Annual Review of Ecology and Systematics 30:133-165.

Blossey, B. 1999. Before, during, and after: the need for long-term monitoring in invasive species management. Biological Invasions 1:301-311.

Brown, C. J. 2002. Impacts of a purple loosestrife (Lythrum salicaria) invasion on American toad (Bufo americanus) tadpoles and associated food webs. Unpublished Master’s thesis, Department of Natural Resources, Cornell University, Ithaca, NY.

Kiesecker, J. M. and A. R. Blaustein. 1998. Effects of introduced bullfrogs and smallmouth bass on microhabitat use, growth, and survival of native red-legged frogs (Rana aurora). Conservation Biology 12:776-787.

Kiesecker, J. M., A. R. Blaustein, and L. K. Belden. 2001. Complex causes of amphibian population declines. Nature 410:681-684.

Knapp, R. A. and K. R. Matthews. 2000. Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas. Conservation Biology 14:428-438.

Kupferberg, S. J. 1997. Bullfrog (Rana catesbeiana) invasion of a California river: the role of larval competition. Ecology 78:1736-1751.

Lamoureux, V. S. and D. M. Madison. 1999. Overwintering habitats of radio-implanted green frogs, Rana clamitans. Journal of Herpetology 33:430-435.

Lamourexu, V. S., J. C. Maerz, and D. M. Madison. 2002. Pre-migratory autumn foraging forays in the green frog, Rana clamitans. Journal of Herpetology 36:245-254.

Pope, S. E., L. Fahrig, and H. G. Merriam. 2000. Landscape complementation and metapopulation effects on leopard frog populatons. Ecology 81:2498-2508.