Persistence of floating pennywort patches (Hydrocotyle ranunculoides, Araliaceae) in a canal in its native temperate range: effect of its natural enemies.
Hydrocotyle ranunculoides is a perennial macrophyte that forms dense interwoven mats on stagnant or slow moving waters. It is native to the Americas, but has become invasive in many countries. Its rapid and thick growth can block watercourses interfering with their economic and ecological functions. The ecology of H. ranunculoides and of some of its natural enemies from Argentina was studied, oriented to the potential for biological control of the plant. A monthly sampling plan was designed to obtain data on natural enemy diversity and damage levels, presence and development of competitor plants, and biomass variations of individual H. ranunculoides patches. Results suggest competitor plants and climatic factors have little bearing on the survival of the plants. Two key natural enemies, the weevil Listronotus elongatus and the fungal pathogen Cercospora sp., may have an important role in the demise of individual H. ranunculoides patches. This is inferred from the fact that patch recovery was only observed when the densities of these natural enemies were at their lowest, and because stratified sampling indicated a negative correlation between plant biomass and pathogen and feeding lesions. In its natural environment of the lower Parana Delta, H. ranunculoides seems to behave as a fugitive species, because although its presence in the ecosystem is permanent, the individual patches were comparatively short-lived, surviving 6 months or less.