A species that is locally common can be globally rare and vice versa. But why? Turns out that tolerance of climatic conditions drives plant species commonness towards global spatial scales, while at finer local scales, competitive ability is relatively more decisive. Accounting for this scale dependence in species occupancy is important when anticipating the effects of climate change or invasive species at local vs. broader scales.
Above: Arctic vegetation, like on the slopes of this mount Saana in North-Finland, is threatened by climate change. Species that are specialized to cold environments are directly affected by warming climate, while those not being strong in competition with other species are threatened also locally by the spread of boreal species northwards. Photo by Miska Luoto.
Why are some species common while others are rare? Trying to answer this question has a long history in biogeography, but despite the decades of studies and suggested and supported reasons, there is no ultimate answer. The quest for the answer is even more topical now, when climate change and invasive species together with other human related actions alter the environment. Indeed, maybe one should rather ask why and which species will become more common or rarer in the future?
While reading through examples of studies investigating the reasons behind species commonness vs. rarity – a feature called ‘occupancy’ in ecology – I stumbled on a study by Heino and Tolonen (2018). They made a short note that the used spatial scale might have affected the outcome of their study, where they found that habitat availability was the most important driver of occupancy while species’ traits or taxonomy played only minor role.
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Mod, H. K., Rissanen, T., Niittynen, P., Soininen, J., & Luoto, M. (2023). The relationships of plant species occupancy to niches and traits vary with spatial scale. Journal of Biogeography, 00, 1– 13. https://doi.org/10.1111/jbi.14608
Having studied spatial scale and its effect on decisiveness of different drivers behind ecological phenomena, our immediate thought then was that habitat availability, as representing species preferences of environmental conditions, could be more decisive at broader spatial scales where environmental conditions are thought to determine species growth and survival. Instead, at more local scales, where organisms are close enough to each other to compete, biotic interactions would dictate which species get along in a specific location. As species’ ability to compete can be deduced from some of its traits (such as size, effectiveness of resource usage, reproductive capacity), what the species is like, in comparison to other species, would appear important for occupancy only at very fine spatial scales. Species preference and tolerance of environmental conditions are called abiotic ‘niche marginality’ and ‘niche breadth’, respectively, describing how specialist or generalist a species is in terms of environmental conditions, while from species traits one can derive a measure of how varying the species itself can be, i.e., ‘intraspecific trait variability’ (ITV), and a measure of how much its traits deviates from the traits of other species (‘trait distinctiveness’). High niche breadth and low niche marginality should thus lead to high occupancy through availability of potential habitats, while high ITV and trait distinctiveness would lead to high occupancy due to being assets in competition with other species.
In our study we thus wanted to investigate whether spatial scale affects the role of these niche and trait measures in driving species occupancy. For this we needed information on how often a species is encountered at study areas of different scales and information of niches and traits of these species. For this we chose four arctic study areas that varied in size from a few square kilometers to all of terrestrial non-glaciated area north of Arctic Circle and in studied plot sizes from 0.04 m2 to 4 km2. The analyses were done for 106 plant species occurring in the study areas with varying occupancies and niche and trait metrics.
The four study areas of different scales are located north of the Arctic Circle. From each study area we mapped in the field or derived from open databases species occurrences to calculate occupancy, i.e., how often species are encountered, per spatial scale. The location of the study area at the finest scale and the extent of the study area at the next finest scale are marked with yellow and orange squares, respectively. The study area at the second coarsest scale covers mainland Finland, Sweden, and Norway north of Arctic Circle (in purple) and the study area at the coarsest scale covers all terrestrial non-glaciated area north of Arctic Circle (in blue).
The results supported our hypothesis. At the two largest study areas species occupancy was most related to their niche breadth: the species that tolerate range of climatic conditions are more common than those that can only stand certain type of conditions. In contrast, at the more local and finer scales, species that are strong competitors, such as those that can adjust their resource use effectiveness, were found to be more common than those that have traits indicating lower competitive ability.
So, coming back to the burning question: ‘why and which species will become more common or rarer in the future?’, the answer according to our results is that it depends on the spatial scale. Towards the global scale, the species that tolerate varying environmental conditions are likely to remain as common as they are now under a changing climate, while those specializing to certain environmental conditions are at risk of becoming even rarer. On a local scale, in turn, tolerating varying climatic conditions is not much of an asset in being or becoming common, whereas a good ability to compete with other species drives commonness and those that don’t have the properties to compete with other species are at risk to become even locally extent. This can mean that the Arctic is in double jeopardy: cold-adapted arctic vegetation as a whole is under the threat of warming climate while the spread of competitive boreal species northwards threatens the arctic plants also locally.
However, our study do not provide the ultimate answers to the question of underlying mechanisms of species occupancy nor to the question of the future of the Arctic. It still remains to be tested if our findings hold for other species groups than plants and for other niche and trait metrics than those we used. Also, there are additional factors influencing the future of Arctic environment and vegetation than those included in our study. Thus, the quest continues!
Heidi Mod, university lecturer, Department of Geosciences and Geography, University of Helsinki, Finland
Tuuli Rissanen, PhD candidate, Department of Geosciences and Geography, University of Helsinki, Finland
Pekka Niittynen, Post doc, Department of Geosciences and Geography, University of Helsinki, Finland & Department of Biological and Environmental Science, University of Jyväskylä, Jyväskylä, Finland
Janne Soininen, professor, Department of Geosciences and Geography, University of Helsinki, Finland Miska Luoto, professor, Department of Geosciences and Geography, University of Helsinki, Finland