Marc Riera, Center for Ecological Research and Forestry Applications (CREAF), discusses his article: Climatic niche conservatism in non-native plants is largely dependent on their climatic niche breadth in the native range
Setting the scene: niche conservatism in the context of biological invasions
The Biosphere is on the move. Thousands of species are jumping from one continent to another due to human activity, giving rise to a global wave of biological invasions that have pervasive negative impacts on local plant communities. A crucial tool to forecast an invasive species’ spread are Ecological Niche Models (ENMs), which approximate a species’ niche (i.e. set of suitable conditions) by matching a species’ occurrence to environmental conditions. However, the predictions of these models are most reliable when a species occupies similar conditions in their native and invaded ranges, a pattern that we call “niche conservatism”. Despite the importance of understanding the variability in niche conservatism, the underlying causes of this pattern remained poorly known.
In our study, we assessed niche conservatism using 158 non-native plants invading Spain as a study system. In short, we built a two-dimensional ordination space, onto which we projected the rainfall and temperature values extracted on georeferenced occurrences. After applying a kernel smoother, we then divided the whole niche into four niche components: unfilling (conditions available in both ranges but occupied only in the native); stability, which indicated niche conservatism (conditions available and occupied in both ranges); expansion (conditions available in both ranges but only occupied in the introduced range); and pioneering (conditions available only in the introduced range).
We found that non-native plants invading Spain tended to occupy similar climates in their native and introduced ranges, although this coexisted with frequent and mostly small niche shifts. In other words, the general trend was that non-native plants tended to be “at home” while being “far away from home”, at least in terms of climate.
The niche was mostly conserved: but why?
We tested two interrelated types of factors that could potentially explain the variation in niche conservatism. On one hand, we have biological attributes, which have been shaped by natural selection, and comprise a plant’s life history or its physiological tolerance. On the other hand, we have features of the introduction process, that encapsulate the human aspect of biological invasions, such as when, where and how often a plant has been introduced.
Our analyses supported that the most important predictor of niche conservatism was a biological attribute: plants with wide climatic niches in their native range often had more conserved niches, possibly because there is less climatic space left to occupy. Plant growth form was a biological attribute of secondary importance: herbs had more conserved niches than shrubs and trees, a finding that contradicted the pre-conceived idea that herbs would conserve their niches less because of faster evolution rates on account of shorter life cycles.
The time since introduction was an important feature of the introduction process, which showed that plants introduced a long time ago had more conserved niches. This suggests that as time goes by, and plants spread geographically, they tend to do so in climatic conditions they are pre-adapted to. In contrast, we found that niche conservatism was not strongly related to a plant’s introduction pathway: whether the plant was introduced intentionally or unintentionally.
Take away
Our research suggests that most non-native plants are “at home far away from home”, at least from a climatic perspective. This pattern of niche conservatism was most strongly influenced by the breadth of climatic conditions occupied in the native range, a biological attribute that arose from natural selection. The features of the introduction process were of secondary importance, indicating that in the context of niche conservatism among non-native plants, the effect of biological attributes is not completely overturned by human influence.
