Comparative Analysis of Plant and Ground Dwelling Arthropod Communities in Lacustrine Dune Areas with and without Centaurea Biebersteinii (Asteraceae)

Article excerpt

ABSTRACT.-

Open dune systems are being degraded through human development and exotic species invasions. The Grand Sable Dunes, Pictured Rocks National Lakeshore, Michigan, are protected from development but not from the spread of exotic species. The invasive exotic spotted knapweed, Centaurea biebersteinii, has invaded significant portions of this dune system. Areas without spotted knapweed had higher native plant diversity than areas with spotted knapweed, as well as lower exotic plant diversity. Of native plant species occurring in the highest frequencies along transects in both spotted knapweed infested and non-spotted knapweed infested areas, four out of the five species were more likely to be encountered along transects in non-spotted knapweed areas than along transects in spotted knapweed areas. Insect families Curculionidae and Formicidae were captured more often in areas with spotted knapweed. Increased dune stabilization increased the ability of exotic plant species not adapted to the actively shifting sand dunes to invade and alter the plant communities. Differences in Curculionidae and Formicidae abundance were due to the changes in plant communities. Also, increased stabilization of sand dunes as a result of spotted knapweed invasion increased the abundance of Formicidae by increasing the stability of nest sites. Limiting the range of spotted knapweed in dune systems could maintain natural insect distribution and native plant diversity.

INTRODUCTION

Exotic species introductions can alter many ecological processes, including dune succession, which depend on native plant species and local successional patterns (Walker and Vitousek, 1991; Leege and Murphy, 2001). Although sand dunes stabilize naturally as a result of native plant succession, rapid stabilization initiated by exotic species invasions can exclude organisms adapted to the movement of sand in highly dynamic portions of dunes (Garcia-Mora et al., 2000). Sand dune systems are especially vulnerable to exotic species invasion because of limited competition by native plants due to low amounts of plant cover and frequent, high intensity disturbances (Crawley, 1987). In other systems, higher levels of native biodiversity may exclude exotic species invasion, but dynamic systems like sand dunes, which repeatedly return to early successional stages, tend to have inherently lower diversity levels (Morrison and Yarranton, 1973; Kennedy et al., 2002).

Dynamics of coastal sand dune systems direct the potential success of establishing organisms. This allows specialized communities to form, composed of organisms adapted to exploit limited resources and survive periodic burial by sand (Carter, 1991; Maun, 1998; Maun and Perumal, 1999; Bach, 2001). Over time, as succession occurs, portions of a sand dune become more stable (Cowles, 1899; Olson, 1958; Johnson, 1997; Lichter, 1998). The retention of sand by pioneering plant species allows organisms that are less adapted to burial and excavation by sand movement to colonize these areas (Moreno-Casasola, 1986; Bach, 2001). With this rapid immigration, increases in species diversity can be expected as the surface of a dune is stabilized, over several hundreds of years. Through competition and other successional processes, herbaceous sand dune plant community diversity will subsequently plateau and decrease over time (Morrison and Yarranton, 1973).

Spotted knapweed, Centaurea biebersteinii de Candolle (Asteraceae) (syn C. maculosa de la Marck), is an exotic plant that has destructively invaded millions of hectares across most of the United States and Canada (Watson and Renney, 1974; Story, 2002). Since its introduction to British Columbia, Canada, in the late 1800s (Harris and Cranston, 1979), spotted knapweed has invaded rangelands, roadsides and other disturbed areas in all of the contiguous United States, Alaska, Hawaii and all Canadian provinces except the Northwest Territories and Nunavut Its effects in rangelands include increased soil runoff and sedimentation, reduced plant biodiversity and decreased wild and domestic grazer production (Lacey et al. …