Abstract
Climatic niche models based on native-range climatic data accurately predict invasive-range distributions in the majority of species. However, these models often do not account for ecological and evolutionary processes, which limit the ability to predict future range expansion. This might be particularly problematic in the case of invaders that occupy environments that would be considered marginal relative to the climatic niche in the native range of the species. Here, we assess the potential for future range expansion in the shrub Chromolaena odorata that is currently invading mesic savannas (>650 mm MAP) in South Africa that are colder and drier than most habitats in its native range. In a greenhouse experiment we tested whether its current distribution in South Africa can be explained by increased competitive ability and/or differentiation in drought tolerance relative to the native population. We compared aboveground biomass, biomass allocation, water use efficiency and relative yields of native and invasive C. odorata and the resident grass Panicum maximum in wet and dry conditions. Surprisingly, we found little differentiation between ranges. Invasive C. odorata showed no increased competitive ability or superior drought tolerance compared to native C. odorata. Moreover we found that P. maximum was a better competitor than either native or invasive C. odorata. These results imply that C. odorata is unlikely to expand its future range towards more extreme, drier, habitats beyond the limits of its current climatic niche and that the species’ invasiveness most likely depends on superior light interception when temporarily released from competition by disturbance. Our study highlights the fact that species can successfully invade habitats that are at the extreme end of their ranges and thereby contributes towards a better understanding of range expansion during species invasions.
Highlights
Understanding the mechanisms that determine the success of invasive species is of fundamental importance to limit their negative impact on biodiversity and ecosystem functioning [1]
We aim to identify factors that allow C. odorata to invade the mesic savannas of southern Africa; habitat which is considered marginal habitat based on the native climatic niche of the species, and explore its potential for further range expansion into more extreme, drier, habitats
Allocation to leaf and stem biomass was higher in the wet treatments than in the dry treatments for both C. odorata populations (LWR: F1,16 = 5.0, p = 0.04, Fig. 4b, SWR: F1,16 = 8.0, p = 0.01, Fig. 4c), whereas allocation to root biomass was lower in the wet than in the dry treatments (RWR: F1,16 = 7.7, p = 0.01, Fig. 4d)
Summary
Understanding the mechanisms that determine the success of invasive species is of fundamental importance to limit their negative impact on biodiversity and ecosystem functioning [1]. A common rule-of-thumb in invasion biology is that for a species to be able to establish, persist and spread in a new environment, the set of ecological conditions in the new environment must approximately match the ecological conditions in their native environment, a phenomenon known as niche conservatism [2]. This view is used widely to predict the distribution of invasive species in climate matching models [3,4] and ecological niche modeling [5,6]. In many cases introduced species can overcome these constraints on range margins, for example through enemy or competitor release, long-distance dispersal, release from gene flow from the native range and/or admixture of multiple introduced populations [11]
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