Long-term dynamics of plant invasions
One of the most consistent findings in invasion biology is the fact that establishment success and range sizes of invaders depend on residence time. Yet, few studies have considered temporal dynamics such as the potential effects of eco-evolutionary changes during the invasion process. By colonising a new region and thereby being exposed to novel biotic and abiotic conditions, alien species have a high potential for rapid evolution. Conversely, while native species may initially be highly impacted by alien species due to a lack of shared evolutionary history, with time they may adapt in response to invasions, which could result in increased biotic resistance of native communities. In a project funded by the German Research Foundation DFG (SH 924/1-1), I hence investigated such long-term invasion dynamics using an alien-native species continuum as a study system, which includes Asteraceae species of continuously increasing residence time in Germany (from recent neophyte over archaeophyte
to native). I combined multi-species common garden experiments (picture above, taken by V. Ferenc), field surveys and macroecological analyses. We found a unimodal relationship between residence time and range size of immigrant plant species in Germany, and showed that an increase in biotic resistance by native communities may contribute to limiting performance and range size of immigrant plant species over long time‐scales (Sheppard & Schurr 2019 Glob. Ecol. Biogeogr.). We did not find any evidence that negative effects of climatic mismatches on population growth of invaders weaken with residence time, as might be expected if introduced species over time adapt to new environments. However, trait mismatches acted as important constraints on population growth (Brendel et al. 2021 Glob. Ecol. Biogeogr.). We also found limited evidence of native target species tolerating neighbours with longer potential co-existence times better (Sheppard & Brendel 2021 NeoBiota). In a follow-up project, I plan to further elucidate the role of length of co-existence time and traits, focusing on the long-term dynamics of biotic interactions between alien plant species and native organisms including plants, soil biota, herbivores and pathogens (DFG SH 924/1-2, started May 2022).
Interactions among alien species
As the number of biological invasions continues to increase globally, conservation managers commonly have to deal with several invasive species that co-occur in native ecosystems, yet research to date has concentrated on the study of single highly invasive species. When multiple invaders co-occur, they may either reduce each other’s performance (‘invasional interference’) or they may benefit from each other’s presence, which could lead to ‘invasional meltdown’ (i.e. increased rate of establishment or impacts of alien species). Consequently, in an ongoing project funded within the Eliteprogramme for Postdocs of the Baden-Württemberg Stiftung, I investigate in what context interactions among multiple co-occurring alien plants may be competitive or facilitative, using annual alien plant species in Germany. To generalise results beyond the
study system, this project aims to determine how mechanisms of competition or facilitation relate to patterns of phylogenetic relatedness and similarity in functional traits. Inspired by the patterns found in a macroecological analysis on how established alien plants affect invasion success of new invaders in French grasslands (Sheppard et al. 2018 Divers. Distrib.), I here use multi-species experiments to gain further insights into alien-alien interactions (Sheppard 2019 Biol. Invasions; Ferenc & Sheppard 2020 Oikos). A better understanding on interactions among multiple plant invaders is crucial to determine appropriate control measures that ensure the conservation of native species and contributes to ecological theory of community assembly in general.
Effects of climate change on plant invasions
Climate change and plant invasions have been studied extensively as individual factors, but few studies have considered their combined and potentially synergistic impacts. My PhD thesis aimed to test if climate change may provide opportunities for alien plants to expand into regions where they previously could not survive and reproduce. Using three recently naturalised plants in New Zealand from warmer native ranges as model species (Archontophoenix cunninghamiana, Psidium guajava and Schefflera actinophylla), I assessed their potential invasiveness under climate change. I used species distribution models and validated them with a transplant experiment across New Zealand (Sheppard et al. 2014 Glob. Change Biol.), as well as conducting experiments in controlled environments to study growth responses to elevated temperature, CO2 and drought, and investigated effects of competition on closely related native species. The combined results from the models, field trials and
controlled environment experiments provided strong evidence of the potential invasiveness of these plants. By having higher confidence in the potential risk of new weeds, cost-effective management actions can be taken to control alien plants at an earlier stage of their naturalisation (Sheppard et al. 2016 New Zeal. J. Ecol.).
Biodiversity in agricultural landscapes
To combat biodiversity loss, it is crucial that biodiversity conservation is not only attempted in protected areas, but also implemented in anthropogenic landscapes, particularly agricultural lands. Intensive agriculture is one of the main drivers of biodiversity loss. In a group project conducted by our lab (landscape ecology and vegetation science) together with another lab (ecology of tropical agricultural systems) we hence aim to investigate the effectiveness of various measures to increase biodiversity in intensively used agricultural landscapes. We set up a multi-year experiment whereby we test the effect of measures that can be implemented in a narrow strip along arable fields, including a perennial flower mix, tilling at two different times to create open soil, adding sand as a substrate for soil-nesting wild bees or plants intolerant of high competition, and targeted planting of rare agricultural weeds. Thereby we particularly aim to increase the heterogeneity of the resource supply for various taxonomic groups. To investigate the effects on biodiversity, we monitor the vegetation and insect composition regularly.
Besides my main areas of research, I am also involved in a range of other projects generally addressing biodiversity dynamics under global change. For instance, in a three-year experiment we tested the potential for Mediterranean rock plants to establish in Central Europe depending on the climatic distance to their origin, in light of climate change induced range contractions and discussions of assisted migration as a conservation strategy. I was involved in a workshop discussing problems and solutions on how to integrate biotic interactions into species distribution models (Dormann et al. 2018 Glob. Ecol. Biogeogr.). In context of InDyNet (Invasion Dynamics Network), involving a group of invasion ecologists covering broad taxonomic and methodological perspectives, we work on conceptual papers addressing temporal changes in biological invasions and their impacts.