Although plant introductions are often performed and evaluated in an idiosyncratic manner, experimental introductions can be coupled with careful evaluation to provide tests of specific hypotheses related to introduction methods or ecological concepts. Here, we adopt a comparative method to evaluate the success of introductions in relation to wild populations, comparing vital rates of introduced to wild plants for three species with a range of life histories. Cirsium pitcheri is a semelparous forb native to dynamic sand dune ecosystems in the Great Lakes, Lomatium cookii is a long-lived forb native to Pacific Northwest grasslands, and Ziziphus celatais a long-lived clonal shrub of sandhill ecosystems in central Florida. For each species, long-term demographic monitoring (> 10 years) provides context for evaluating multiple experimental introductions over many years. We perform analyses of vital rates (annual survival, annual growth, transitions to reproduction or reproductive size) in relation to introduction status (introduced vs. wild), year, and life history stage. We hypothesize that introduced plants may have greater vital rates due to targeting of suitable planting sites and initial care, but that this advantage will be minimal in longer-lived species that cannot reach reproductive size until well after outplanting.
Results/Conclusions
Introduced plants had higher survival than wild plants for Cirsium and similar survival for Ziziphus. For Cirsium, survival decreased with successional stage. Lomatium survival interacted with stage such that only smaller introduced plants had higher survival than wild plants. Annual growth had different patterns; it was relatively low for introduced Cirsium plants and interacted with life history stage for Ziziphus. Transitions to reproductive size were more frequent for introduced than wild plants for Cirsium but the opposite was true for Ziziphus and Lomatium. After nearly a decade in some introduced populations, reproduction is still rare for Ziziphus, in contrast to the two forbs, which rapidly completed life cycles and produced a new generation of recruits. For all species, survival increased and growth decreased with size or stage; both varied among years. Our comparisons of vital rates of introduced plants with wild plants show that these introductions have been fairly successful, especially in producing comparative survival, but that growth and reproduction have often lagged. Constraints on ex situ plant development as well as habitat quality at the reintroduction site may slow plant and population growth after introductions. These results give context to experimental introductions and suggest ways of improving the success of future introductions.