Plant invasions and extinction debts

Abstract

Whether introduced species invasions pose a major threat to biodiversity is hotly debated. Much of this debate is fueled by recent findings that competition from introduced organisms has driven remarkably few plant species to extinction. Instead, native plant species in invaded ecosystems are often found in refugia: patchy, marginal habitats unsuitable to their nonnative competitors. However, whether the colonization and extinction dynamics of these refugia allow long-term native persistence is uncertain. Of particular concern is the possibility that invasive plants may induce an extinction debt in the native flora, where persistence over the short term masks deterministic extinction trajectories. We examined how invader impacts on landscape structure influence native plant persistence by combining recently developed quantitative techniques for evaluating metapopulation persistence with field measurements of an invaded plant community. We found that European grass invasion of an edaphically heterogeneous California landscape has greatly decreased the likelihood of the persistence of native metapopulations. It does so via two main pathways: (i) decreasing the size of native refugia, which reduces seed production and increases local extinction, and (ii) eroding the dispersal permeability of the matrix between refugia, which reduces their connectivity. Even when native plant extinction is the deterministic outcome of invasion, the time to extinction can be on the order of hundreds of years. We conclude that the relatively short time since invasion in many parts of the world is insufficient to observe the full impact of plant invasions on native biodiversity.

Fig. 1.

Metapopulation viability in the study system following invasion. (A) Metapopulation dynamics preinvasion and postinvasion, with the width of arrows signifying the strength of the process. Invaders reduce colonization rates through decreased seed production and by altering competitor composition between patches (matrix permeability). Reduced local population sizes also increase local extinction rates. (B) Spatial layout of the study system. Black lines represent present-day distributions, and dashed gray lines represent one preinvasion scenario in which the habitat of native annuals was double the present-day area.

Fig. 2.

Impact of invasion on local population sizes and dispersal. (A) When competing only against other native annual species, the focal annuals have significantly higher finite rates of increase in the area now occupied by invasive grasses than in their current refugia. Bars show mean ratio ± SE, with ratios greater than 1 indicating that areas now dominated by invasive species are optimal for the native plants. (B) Native annuals had higher finite rates of increase among native bunchgrasses than among invasive grasses (mean R ± SE) in the matrix habitat. Data are not presented as ratios because the finite rate of increase of Chaenactis among exotic grasses was zero. (C) Lower finite rates of increase of native annuals in invaded habitat greatly reduce connectivity by decreasing multigenerational dispersal through the matrix. Curves show the effect on the probability of dispersal of lowering species’ finite rates of increase in the matrix after invasion, given a mean dispersal distance of σ = 1. The effect of dispersal through the matrix is calculated assuming that offspring from a parent plant could not persist for more than 30 y in the matrix (i.e., n_max = 30 in Eq. S5). Cg, Chaenactis galibriuscula; Cp, Chorizanthe palmerii; Lc, Lasthenia californica; Lw, Lotus wrangelianus; Mc, Micropus californicus; Pe, Plantago erecta; Sc, Salvia columberiae.

Fig. 3.

Impact of loss of seed production (blue line) and reduction in habitat connectivity (red line) on metapopulation viability (black line; joint contribution) for the seven focal species. Seed production alters both colonization and extinction dynamics, whereas loss in connectivity results from a reduction in habitat size and alteration of dispersal through the matrix. Panels correspond to the species listed by genus (full names are given in Fig. 2).

Fig. 4.

Extinction thresholds and times to extinction for species in an extinction debt. (A) Extinction threshold is the minimum proportion of habitat that a species must occupy before invasion to avoid falling into an extinction debt following invasion. Estimates are based on incorporating both species-specific responses to invaders (black lines in Fig. 3) and the level of habitat loss shown into Eq. 5. Species abbreviations are given in Fig. 2. (B) Median time to extinction for an average species with a given mean density and proportion of habitat lost to invaders. Estimates are based on simulations for an “average” species containing the mean trait values of the seven focal species.