Effects of an invasive plant on the population dynamics of common and rare native species explain patterns of invasions on biodiversity

Background/Question/Methods

Invasive plants often decrease diversity at small but not broad spatial scales. To date, no studies have examined if these scale-dependent changes in diversity could be explained by population-level mechanisms. To address this, our study combines community-level surveys of invaded and native communities with detailed demographic studies on seven native plant species. We surveyed communities affected by the invasive shrub, Lonicera maackii, and found that declines in plant diversity are explained by larger negative effects of L. maackii on the population size of locally common species relative to locally rare species. Changes in native species’ population size could be due to multiple factors, such as dispersal, longevity, or the effect of L. maackii on species’ population growth. Thus, we used integral projection models to explore the effect of L. maackii on the population dynamics of pairs of locally common and locally rare native plant species. To parameterize the demographic models, we collected data from seven native plant species, each growing in high-density L. maackii habitat and low-density L. maackii habitat. We evaluated declines in the population growth of rare versus common species, differences among species in vital rate sensitivities, and vital rate contributions to changes in population growth.

Results/Conclusions

Results reveal that L. maackii decreases the population growth (λ) of common species more than rare species. Despite differences in species’ abundances and life history strategies, Life Table Response Experiments (LTREs) reveal that vital rates involving reproductive stages (i.e. growth into a reproductive stage, stasis in a reproductive stage, and seed production) contribute most to declines in population growth. However, there is no consistent explanation for why rare species experience smaller changes in population growth in the presence of L. maackii. For some rare species, L. maackii causes large changes in reproductive vital rates, countered by low sensitivity of the population growth to changes in those vital rates. For other rare species, L. maackii causes small changes in reproductive vital rates that have large, negative consequences for population growth. These results suggest that plant invasions can alter complex population dynamics that underlie overall changes in native species’ population size and diversity patterns at the community level.