AN ECOSYSTEM-LEVEL PERSPECTIVE OF ALLELOPATHY

Allelopathy is an interference mechanism by which plants release chemi cals which affect other plants; while it has often been proposed as a mechanism for influencing plant populations and communities, its accep tance by plant ecologists has been limited because of methodological p roblems as well as difficulties of relating the results of bioassays u sed for testing allelopathy to vegetation patterns in the field. Here we argue that the concept of allelopathy is more appropriately applied at the ecosystem-level, rather than the traditional population/commun ity level of resolution. Firstly, we consider the wide ranging effects of secondary metabolites (widely regarded as allelochemicals) on orga nisms and processes which regulate ecosystem function, including herbi vory, decomposition and nutrient mineralization. It is apparent that p lants with allelopathic potential against other organisms induce net c hanges in ecosystem properties, which may in turn impact upon the plan t community in the longer term. We then illustrate these concepts usin g two contrasting examples of how invasive plant species with allelopa thic: potential may alter ecosystem properties through the production of secondary metabolites, i.e. Carduus nutans (nodding thistle) in New Zealand pastures and Empetrum hermaphroditum (crowberry) in Swedish b oreal forests. In both cases the production of secondary metabolites b y the invasive species induces important effects on other organisms an d key processes, which help determine how the ecosystem functions and ultimately the structure of the plant community. These examples help d emonstrate that the concept of allelopathy is most effectively applied at the ecosystem-level of resolution, rather than at the population-l evel (i.e, plant-plant interference).