ECR Feature: Nicky Lustenhouwer on niche shifts in invasive plants

Nicky Lustenhouwer is a postdoc at the University of Aberdeen. She is an evolutionary ecologist interested in range expansions and invasive organisms. Nicky shares her recent work on the relative roles of climate change tracking versus niche evolution in the spread of an invasive weed.

Nicky Lustenhouwer with a particularly large individual of Dittrichia graveolens during fieldwork collecting seeds in California. (photo credit: Nicky Lustenhouwer)

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Institute. School of Biological Sciences, University of Aberdeen, UK

Academic life stage. Postdoc

Research themes. Evolutionary ecology, range shifts, invasions, population spread

Current study system. I am currently studying the blue-tailed damselfly, Ischnura elegans, which is rapidly shifting its native range northward with climate change. A decade of previous work has shown all kinds of interesting evolutionary changes that have happened during this range shift in Sweden and Scotland, including changes in thermal tolerance, dispersal, and female morph frequencies. The aim of my current postdoc is to expand on this work with two new transects in Norway and Finland, so we can study how evolution varies along four parallel transects. I just returned from 7 weeks of exciting fieldwork travelling from pond to pond in Scandinavia.

Recent paper in JBI. Lustenhouwer, N. & Parker, I.M. 2022. Beyond tracking climate: Niche shifts during native range expansion and their implications for novel invasions. Journal of Biogeography. 10.1111/jbi.14395

Motivation behind this paper. This paper was part of my previous postdoc at the University of California, Santa Cruz, where I was studying the Mediterranean annual plant Dittrichia graveolens. This is a fascinating species because it is currently both expanding its European native range very rapidly, and invading several other continents, including California where it is considered a noxious weed. I had previously found that the native range shift of D. graveolens was promoted by rapid evolution of earlier flowering time (the species produces seeds in autumn and is constrained by the end of the growing season in the north). I was therefore really interested to ask whether this range shift was simply tracking climate change, or if the climate niche had shifted along the way, allowing for further range expansion. And if that was the case, how would this change our risk assessment of climatic areas that may be invaded elsewhere?

[left] Dittrichia graveolens is a typical ruderal plant and can be found in any kind of disturbed environment, like this crack in the pavement in its native range in southern France. [right] Highways are key range expansion corridors for Dittrichia graveolens, as shown here at the northern range limit in the Netherlands. (photo credit: Nicky Lustenhouwer)

Key methods. We collected occurrence data for Dittrichia graveolens across its native range in Eurasia and in two invaded ranges in California and Australia. We used these data to quantify the climate niche in both environmental space (using the COUE framework of niche centroid shift, overlap, unfilling and expansion) and in geographic space (using MaxEnt). A key step was to reconstruct the historic native range limit pre-expansion (1901-1930) using old maps and records. This allowed us to model the climate niche based on the historic range and climate, and then project it forward to the present (1990-2019) to see what the expected range shift would be when tracking climate change, and how this matched the observed current distribution of the species. We also modelled how D. graveolens’ climate niche changed over the course of the native range expansion. Finally, we used this information to ask which areas of California and Australia may be at risk of invasion if similar niche shifts were to occur in the exotic range.

Major results. We found that the native range expansion of D. graveolens in Europe went well beyond expectations based on tracking climate change alone, accompanied by a 5% niche expansion into more temperate climates over the course of this range shift. In contrast, the two invasions were still confined to climatic areas predicted by the historic native niche only, showing niche conservatism. This was especially surprising in Australia, where D. graveolens has been present for nearly 150 years. Our results highlight that niche shifts are not necessarily most common during invasions, which is where they have historically received most attention in the literature. They can also play an important role during native range shifts induced by climate change, with important consequences for the location of the new range limit.

Dittrichia graveolens growing at the edge of a salt marsh where the species was first observed in California (Alviso). (photo credit: Nicky Lustenhouwer)

Unexpected challenges. Our biggest challenge was dealing with the geographic bias in species occurrence records that are readily available, a very familiar problem in niche modelling. One way we mitigated this issue (apart from accounting for it in our models as much as possible) was by investing a lot of time in collecting extra occurrence records in under-sampled areas, from all kinds of data sources such as small botanical journals in local languages, old floras that the library had shipped over for me, and local databases. In the end about a quarter of our data points came from outside the Global Biodiversity Information Facility (GBIF – the major database for open biodiversity data). What I enjoyed most was directly contacting local experts in countries where we had minimal data, and these local experts provided a wealth of interesting anecdotes about their experience with this species in its recently expanded range.

Next steps. I will continue to collaborate with my colleagues in Santa Cruz, as we are writing up a series of greenhouse experiments and genomics work to investigate evolutionary changes during both the native range expansion of D. graveolens and the invasion in California. I am very excited to be able to compare a native and exotic range expansion scenario in the same species but against different backgrounds of genetic diversity, gene flow from the historic native range, and environmental gradients. We are for example studying how plant height, seed traits, and phenology shift from core to edge in each range.

If you could study any organism on Earth, what would it be? A plant that grows somewhere beautiful, like an arctic species – everyone always makes fun of me for picking study organisms that thrive in the most human-disturbed environments (both Dittrichia and Ischnura elegans do). During my PhD in Switzerland, I spent all my time on highway parking lots instead of in alpine meadows!

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