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Abstract
When introduced to new ecosystems, species' populations often grow immediately postrelease. Some introduced species, however, maintain a low population size for years or decades before sudden, rapid population growth is observed. Because exponential population growth always starts slowly, it can be difficult to distinguish species experiencing the early phases of slow exponential population growth (inherent lags) from those with actively delayed growth rates (prolonged lags). Introduced ungulates provide an excellent system in which to examine lags, because some introduced ungulate populations have demonstrated rapid population growth immediately postintroduction, while others have not. Using studies from the literature, we investigated which exotic ungulate species and populations (n = 36) showed prolonged population growth lags by comparing the doubling time of real ungulate populations to those predicted from exponential growth models for theoretical populations. Having identified the specific populations that displayed prolonged lags, we examined the impacts of several environmental and biological variables likely to influence the length of lag period. We found that seventeen populations (47%) showed significant prolonged population growth lags. We could not, however, determine the specific factors that contributed to the length of these lag phases, suggesting that these ungulate populations' growth is idiosyncratic and difficult to predict. Introduced species that exhibit delayed growth should be closely monitored by managers, who must be proactive in controlling their growth to minimize the impact such populations may have on their environment.
Highlights
Non-native species have been introduced worldwide and often demonstrate rapid population growth postrelease (Chollet et al, 2015; Froese et al, 2017; Ikagawa, 2013)
Empirical population growth was determined using the initial and final population sizes found in our literature search (Table 1). 95% confidence intervals were calculated for the empirical growth curve and populations were classified as having exhibited a prolonged lag phase when the slope of a population's exponential curve fell outside 95% confidence intervals of the empirical model
After determining which populations showed significant prolonged lags (n = 17), we examined possible causes of variation in population growth rates using generalized linear models, and a range of different explanatory variables associated with climate and life history (Table 3)
Summary
Non-native species have been introduced worldwide and often demonstrate rapid population growth postrelease (Chollet et al, 2015; Froese et al, 2017; Ikagawa, 2013). To determine whether a lag is ‘prolonged’ (i.e., an extended period of even slower growth than that predicted from exponential growth, which occurs prior to a marked increase in the rate of growthAagaard & Lockwood, 2014; Crooks, 2005), it is necessary to determine whether the observed lag is longer than the inherent lag We follow these definitions and statistically distinguish between natural exponential population growth and prolonged lags (Aikio et al, 2010; Crooks, 2005; Crooks & Soule, 1999; Sakai et al, 2001) for ungulate species introduced into non-native ranges. We expected species with shorter gestation periods, introduced to areas with no native competitors, and consistent rainfall throughout the year would exhibit earlier, faster population growth
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