Spreading to a limit: the time required for a neophyte to reach its maximum range

Abstract

Williamson et al. worked with range sizes on a logit scale of grid squares and estimated, using reduced major axis (RMA) analyses, the time for neophytes (alien plants introduced after 1500 ad) to achieve the same average range size as natives. They assumed that all neophytes would eventually reach a maximum range even though none of their data sets showed a clear asymptote and argued that the neophyte average maximum would, in time, match the native average. They found times of 151, 177, 145 and 141 years for Ireland, Britain, Germany and the Czech Republic, respectively. It should perhaps be noted that this process can be modelled in various ways and that different models are needed for different species (Williamson et al., 2009b).Here we note that applying the same logit transformation used in Williamson et al. (2009a) to the range sizes in Gassó et al. (2009) gives a graph (Fig. 1) which shows unambiguously that neophytes in Spain, on average, reach a maximum range size at 143 years (the peak point in the loess line in Fig. 1). Figure 1 uses an upper limit of the area in Spain from which neophytes have been recorded. Using a different limit such all Spain, or even an infinite area (which implies a logarithm rather than a logit) does not affect the time to a maximum, the graph is scarcely changed. In general, it is best to use the actual area surveyed when transforming ranges by logits (Williamson & Gaston, 1999 Williamson et al., 2009a) with data points for 106 neophyte Spanish plants. The residence time is the time since the first publication of a record or the first dated herbarium specimen for a species. The current range size is on a logit scale using 2590 hectads (i.e. total number of hectads in Spain in which neophytes have been recorded out of 5096 total) as the upper limit. The data are a transformation of the data in Fig. 3 of Gassó et al. (2009).

Widespread plant species: natives versus aliens in our changing world

“…2), probably for the same reasons. It has been recently shown that it takes on average about 150 years for an alien plant species in Europe to achieve a full distribution according to Williamson et al (2009) and Gassó et al (2010). Those authors also showed that, although many aliens are still in the process of extending their ranges, it may be assumed that some have made more progress than others.…”

Impacts of biological invasions: what's what and the way forward

“…Concern is also heightened because many introduced populations remain innocuous for extended periods before spreading and becoming invasive [30,31]. For instance, Brazilian pepper remained restricted in Florida for a century before rapidly expanding across a wide area [30], whereas plants introduced to Europe might take 150-400 years to reach their fullest areal extent [32].…”

How common are invasion-induced ecosystem impacts?

“…The reason for such lags is often mysterious, but their existence cannot be doubted. Further, even introduced species that do not experience a pronounced lag often take a long time to reach their fullest extent; for plants introduced in Europe, this time appears to average 150 years (Gassó et al 2010). Simply by the subtle initial nature of many invasions that ultimately have substantial ecosystem consequences, it is likely that many introduced species whose impact has not yet occurred or been recognized will turn out to have ecosystem impacts.…”