Many studies report that mountain plant species are shifting upward in elevation. However, the majority of these reports focus on shifts of upper limits. Here, we expand the focus and simultaneously analyze changes of both range limits, optima, and abundances of 183 mountain plant species. We therefore resurveyed 1,576 vegetation plots first recorded before 1970 in the European Alps. We found that both range limits and optima shifted upward in elevation, but the most pronounced trend was a mean increase in species abundance. Despite huge species-specific variation, range dynamics showed a consistent trend along the elevational gradient: Both range limits and optima shifted upslope faster the lower they were situated historically, and species' abundance increased more for species from lower elevations. Traits affecting the species' dispersal and persistence capacity were not related to their range dynamics. Using indicator values to stratify species by their thermal and nutrient demands revealed that elevational ranges of thermophilic species tended to expand, while those of cold-adapted species tended to contract. Abundance increases were strongest for nutriphilous species. These results suggest that recent climate warming interacted with airborne nitrogen deposition in driving the observed dynamics. So far, the majority of species appear as "winners" of recent changes, yet "losers" are overrepresented among high-elevation, cold-adapted species with low nutrient demands. In the decades to come, high-alpine species may hence face the double pressure of climatic changes and novel, superior competitors that move up faster than they themselves can escape to even higher elevations.
Withstanding extinction while facing rapid climate change depends on a species' ability to track its ecological niche or to evolve a new one. Current methods that predict climate-driven species' range shifts use ecological modelling without eco-evolutionary dynamics. Here we present an eco-evolutionary forecasting framework that combines niche modelling with individual-based demographic and genetic simulations. Applying our approach to four endemic perennial plant species of the Austrian Alps, we show that accounting for eco-evolutionary dynamics when predicting species' responses to climate change is crucial. Perennial species persist in unsuitable habitats longer than predicted by niche modelling, causing delayed range losses; however, their evolutionary responses are constrained because long-lived adults produce increasingly maladapted offspring. Decreasing population size due to maladaptation occurs faster than the contraction of the species range, especially for the most abundant species. Monitoring of species' local abundance rather than their range may likely better inform on species' extinction risks under climate change.
AimPlant invasions often follow initial introduction with a considerable delay. The current non‐native flora of a region may hence contain species that are not yet naturalized but may become so in the future, especially if climate change lifts limitations on species spread. In Europe, non‐native garden plants represent a huge pool of potential future invaders. Here, we evaluate the naturalization risk from this species pool and how it may change under a warmer climate.LocationEurope.MethodsWe selected all species naturalized anywhere in the world but not yet in Europe from the set of non‐native European garden plants. For this subset of 783 species, we used species distribution models to assess their potential European ranges under different scenarios of climate change. Moreover, we defined geographical hotspots of naturalization risk from those species by combining projections of climatic suitability with maps of the area available for ornamental plant cultivation.ResultsUnder current climate, 165 species would already find suitable conditions in > 5% of Europe. Although climate change substantially increases the potential range of many species, there are also some that are predicted to lose climatically suitable area under a changing climate, particularly species native to boreal and Mediterranean biomes. Overall, hotspots of naturalization risk defined by climatic suitability alone, or by a combination of climatic suitability and appropriate land cover, are projected to increase by up to 102% or 64%, respectively.Main conclusionsOur results suggest that the risk of naturalization of European garden plants will increase with warming climate, and thus it is very likely that the risk of negative impacts from invasion by these plants will also grow. It is therefore crucial to increase awareness of the possibility of biological invasions among horticulturalists, particularly in the face of a warming climate.
Biotic resistance represents an important natural barrier to potential invaders throughout the world, yet the underlying mechanisms that drive such resistance are still debated. In theory, native communities should repel both functionally similar invaders which compete for the same resources, and invaders which possess less competitive traits. However, environmental stress, trade‐offs across vital rates and competition‐induced plastic trait shifts may modify expected competitive outcomes, thereby influencing invasion dynamics. In order to test these theoretical links between trait distributions and biotic resistance, we performed a mesocosm experiment with 25 non‐native ornamental species invading native plant communities. Each non‐native species was grown with and without the native community under two watering treatments (regular and reduced). We measured biotic resistance as the difference in performance of non‐native individuals grown with and without the community in terms of their survival, growth and reproduction. We quantified overall functional dissimilarity between non‐native ornamental individuals and native communities based on the combination of plant height, specific leaf area and seed mass. Then, assuming each of these traits is also potentially linked to competitive ability, we measured the position of non‐natives on trait hierarchies. While height is positively correlated with competitive ability for light interception, conservative leaf and seed characteristics provide greater tolerance to competition for other resources. Finally, we quantified plastic trait shifts of non‐native individuals induced by competition. Indeed, the native community repelled functionally similar individuals by lowering the invader's survival rate. Simultaneously, shorter ornamental individuals with larger specific leaf areas were less tolerant to biotic resistance from the community across vital rates, although the effect of trait hierarchies often depended on watering conditions. Finally, non‐natives responded to competition by shifting their traits. Most importantly, individuals with more competitive traits were able to overcome biotic resistance also through competition‐induced plastic trait shifts. Synthesis. Our results highlight that both functional dissimilarity and trait hierarchies mediate biotic resistance to ornamental plant invaders. Nevertheless, environmental stress as well as opposing trends across vital rates are also influential. Furthermore, plastic trait shifts can reinforce potential invaders’ competitive superiority, determining a positive feedback.
Mountain plant species shift their elevational ranges in response to climate change. However, to what degree these shifts lag behind current climate change, and to what extent delayed extinctions and colonizations contribute to these shifts, are under debate. Here, we calculate extinction debt and colonization credit of 135 species from the European Alps by comparing species distribution models with 1576 re-surveyed plots. We find extinction debt in 60% and colonization credit in 38% of the species, and at least one of the two in 93%. This suggests that the realized niche of very few of the 135 species fully tracks climate change. As expected, extinction debts occur below and colonization credits occur above the optimum elevation of species. Colonization credits are more frequent in warmth-demanding species from lower elevations with lower dispersal capability, and extinction debts are more frequent in cold-adapted species from the highest elevations. Local extinctions hence appear to be already pending for those species which have the least opportunity to escape climate warming.
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