Theory of Seed Plant Invasiveness

Some biological and ecological traits are known to be associated with invasiveness in plants. These include high fecundity, efficient dispersal, the ability to utilize generalist mutualists, the ability to evade specific natural enemies, small genome size, high relative growth rate, or high specific leaf area. Although such traits have some value in prediction, defining a syndrome associated with invasive-ness that is applicable to all vascular plants is unrealistic. Components of invasiveness are more realistically sought at finer taxonomic scales or for particular life forms. Pine trees (genus Pinus, with >100 species) have proved a useful group to explore this in detail. Differences in invasiveness among pine species can be explained using only three traits (seed mass, length of juvenile period, and interval between seed mast years), and further precision (with proven value in prediction, not only for pines and other conifers, but also for other woody species) is achieved by adding considerations relating to dispersal by vertebrates and characteristics of fruits.

Some theories have taken an overarching approach to plant invasions by integrating the concepts of species invasiveness and community invasibility. Marcel Rejmanek's theory of 'seed plant invasiveness' synthesized available knowledge into a unified scheme. It highlights a low nuclear amount of DNA as a result of selection for the short generation time, membership of alien genera, and size of the primary latitudinal range as vital factors contributing to the invasiveness of seed plants. Large geographical range is a good predictor of invasion success. Widespread species are more likely to be dispersed because they occur in more locations and have higher chances of being dispersed, and they are more likely to be adapted to a wider range of conditions. The same traits that allow a species to be widespread in the native range are also favorable for a successful invasion.

Some additional predictions from the emerging theory of plant invasiveness are as follows: (1) Fitness homoeos-tasis, that is, the ability of an individual or population to maintain relatively constant fitness over a range of environments, promotes invasiveness. (2) Characters favoring passive dispersal by humans greatly improve a chance of becoming invasive. (3) Vegetative reproduction is responsible for many plant invasions, especially in aquatic and wetland environments. (4) The ability to utilize generalist mutualists greatly improves an alien taxon's chances of becoming invasive. (5) Efficient competitors for limiting resources are likely to be the best invaders in natural and seminatural ecosystems.

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