ABSTRACT
Similar to the empty niche hypothesis, the “fluctuating resource” hypothesis states that as resources become available within a given area, that area will become more susceptible to invasion (Davis et al. 2000). Excessive nutrients can become available in two primary ways (Davis et al. 2000): (1) The first is when native plants decrease their uptake because of decreased populations following a disturbance or predatory outbreak. (2) The second occurs when more nutrients become available from external or internal sources, such as increased precipitation or accelerated mineralization. Because invasive plants are often more successful in capturing excessive nutrients compared to native species, the community becomes vulnerable to invasion. Using Lantana camara L. (lantana) as an example, Duggin and Gentle (1998) illustrated this concept in a series of experiments conducted in New South Wales, Australia. Native to Central and South America, L. camara is an invasive shrub that has been introduced in over 60 countries around the world. It can grow in a variety of soil types and habitats, but it generally does not occur in undisturbed forests (Sharma et al. 2005). Duggin and Gentle (1998) tested the impacts of fertilization, biomass removal, and fire on L. camara germination, survival, and growth. These authors reported that whereas fertilization alone had little effect on plant development, fertilization combined with biomass removal or burning significantly increased L. camara germination, survival, and growth (Figure 2.1). This increase in invasion success was correlated with increased light, water, and nutrient availability, and it explains why the species performs better in disturbed areas. Similar results have been observed for Imperata cylindrica (L.) Beauv. (cogongrass), an invasive C4 perennial grass species native to Asia. The species often exists only in sparse patches in undisturbed forests (MacDonald 2004), but it becomes dominant following fire or overstory removal (Holzmueller and Jose 2011).
