Invasion, competitive dominance, and resource use by exotic and native California grassland species

Abstract

The dynamics of invasive species may depend on their abilities to compete for resources and exploit disturbances relative to the abilities of native species. We test this hypothesis and explore its implications for the restoration of native ecosystems in one of the most dramatic ecological invasions worldwide, the replacement of native perennial grasses by exotic annual grasses and forbs in 9.2 million hectares of California grasslands. The long-term persistence of these exotic annuals has been thought to imply that the exotics are superior competitors. However, seed-addition experiments in a southern California grassland revealed that native perennial species, which had lower requirements for deep soil water, soil nitrate, and light, were strong competitors, and they markedly depressed the abundance and fecundity of exotic annuals after overcoming recruitment limitations. Native species reinvaded exotic grasslands across experimentally imposed nitrogen, water, and disturbance gradients. Thus, exotic annuals are not superior competitors but rather may dominate because of prior disturbance and the low dispersal abilities and extreme current rarity of native perennials. If our results prove to be general, it may be feasible to restore native California grassland flora to at least parts of its former range.

Fig. 1.

Exotic annual species (▴) are less effective at depleting soil water at the 60-cm depth (b) than are native perennial species (○) (P = 0.050) across all dates. Water use at the 15-cm depth (a) is similar between community types (n = 16).

Fig. 2.

Exotic annual species (▴) are less effective than native perennial grasses (○) at extracting soil nitrate at all depths in May (a) and at 0-15 cm in June (b). Nitrate concentration by depth was recorded in May (a). Monthly sampling was conducted at the 0- to 15-cm depth (b). (Error bars = 1 SEM; n = 32.)

Fig. 3.

Cover of native perennial grasses (○) was higher than exotic annual species (▴) 5 years (2002) after adding native perennial seed in the restoration experiment. Plots were subjected to factorial combination of pocket gophers (No Gopher; +Gopher) and nitrogen and burning (Control, +N, and +Fire). (Error bars = 1 SEM; n = 36.)

Fig. 4.

Production of exotic annual (▴) and native perennial (○) species is nearly identical 1 year after mowing. Dominance of perennial grasses increased after cessation of mowing. (Error bars = 1 SEM; n = 17.)

Fig. 5.

(a) The addition of seeds of perennial species to an annual community resulted in higher production of perennials (P < 0.001) and a corresponding decline in annuals (P = 0.021) 2 years after the initial seed introductions. (b) The addition of annual seeds to a perennial-dominated community had no significant effect on perennial (P = 0.167) or annual (P = 0.358) species. Means are shown from plots receiving no resource additions. (Error bars = 1 SEM; n = 17.)