Mariska te Beest

Linking functional traits to impacts of invasive plant species: a case study

Our understanding of the link between plant functional traits and ecological impact of invasive alien plant species is fragmentary and the mechanisms leading to impacts are poorly understood. Moreover, current knowledge is heavily biased to the temperate regions of the world and we know much less about traits and impacts of invaders in tropical and subtropical ecosystems. We studied two leaf traits of the invasive alien shrub Chromolaena odorata and the impacts of its invasion on native vegetation in savannas. We compared specific leaf area (SLA) and leaf area index (LAI) between C. odorata and native species and assessed how C. odorata differentially affects canopy light interception, soil moisture, soil nutrients, and litter accumulation compared to native species. We found that C. odorata has higher SLA and LAI than native species, lower light and moisture levels below its canopy, but higher nutrient levels and a higher litter accumulation rate. Because of its higher SLA, C. odorata grows faster, resulting in more biomass, increased litter accumulation and higher nutrient availability. Due to its high SLA and LAI, C. odorata intercepts more light and reduces available moisture more than do native trees due to higher transpiration rates, reducing the biomass of native understory vegetation. This study provides empirical evidence for strong links between plant functional traits and ecological impact of invasive plant species, highlighting the importance of traits in predicting ecosystem-level impacts of invasive plant species.

Invasion success in a marginal habitat: An experimental test of competitive ability and drought tolerance in Chromolaena odorata

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.

Trophic rewilding as a climate change mitigation strategy

The loss of megafauna at the terminal Pleistocene has been linked to a wide range of Earth-system-level changes, such as altered greenhouse gas budgets, fire regimes and biome-level vegetation changes. Given these influences and feedbacks, might part of the solution for mitigating anthropogenic climate change lie in the restoration of extant megafauna to ecosystems? Here, we explore the potential role of trophic rewilding on Earths climate system. We first provide a novel synthesis of the various ways that megafauna interact with the major drivers of anthropogenic climate change, including greenhouse gas storage and emission, aerosols and albedo. We then explore the role of rewilding as a mitigation tool at two scales: (i) current and near-future opportunities for national or regional climate change mitigation portfolios, and (ii) more radical opportunities at the global scale. Finally, we identify major knowledge gaps that complicate the complete characterization of rewilding as a climate change mitigation strategy. Our perspective is urgent since we are losing the Earths last remaining megafauna, and with it a potential option to address climate change. This article is part of the theme issue ‘Trophic rewilding: consequences for ecosystems under global change’.