Despite the restricted vulnerability evaluation method used here, our assessment revealed that the flora of alpine ecosystems in the Canary Islands is at significant risk due to climate change. According to the vulnerability index applied here, even at present 16% of the species are at risk (high mean vulnerability), and this percentage will increase to 32% in the high-risk category in 2040–2060 (future I) and to 60% in 2061–2080 (future II), including also the critical category. Species vulnerability depends on a trade-off between the F.N. size and the A.C.C. of the species. These two drivers vary according to the rarity group under assessment. The specific drivers associated with global warming (F.N. loss and persistence index) were not the main components explaining species vulnerability, although their significance increases in the future scenarios for all groups and islands (Fig. 4). The A.C.C. hindering the species responses is an important driver in all scenarios on both islands (Fig. 4) and for rare and restricted species. Rare species show significantly higher vulnerability than common and restricted species. However, this difference is not due to a stronger significant effect of climate change drivers, but rather to the current state of the populations and their threats. The vulnerability pattern during all three scenarios was different on the two islands (Fig. 4). While on La Palma it was notably dependent on F.N. size, on Tenerife it depended mainly on the constraints and rarity. Tenerife, where the alpine ecosystem occupies a larger zone, shows a delay in climate change impact compared to La Palma, resulting in lower vulnerability at present (intermediate on Tenerife with an index of 0.34 and high on La Palma, 0.47). However, this vulnerability will probably increase according to the future scenarios, due to shrinkage in fundamental niche area attributable to island topography and in the persistence of the currently suitable areas for plants to survive.
In this study we are working with very small areas (mean F.N. size for all species was 18,306ha at present, 15,215ha in future I and 11,998ha in future II). The F.N. size of the species showed a significant negative correlation with species vulnerability (p < 0.0001) throughout our assessment periods, but no correlation was found with the percentage area occupied in this niche (p > 0.05). In fact, rare species showed no significant differences from common species in F.N. size (p > 0.05), which along with their significant higher percentage of constraints points to anthropogenic rarity. Thus, constraints are limiting the area of the realized niches within the fundamental niches (Gaston 2003; Svening & Skov 2004) and the effectivity of species dispersal capacity, an important trait in their response to climate change (Estrada et al. 2015). These results are clearly related to the high degree of anthropogenic disturbance of the alpine ecosystem on both islands (Rodríguez-Delgado & Elena-Roselló, 2006; Garzón et al. 2010; Irl et al. 2012). Human impacts are significantly more intense on islands than in mainland areas (Kier et al. 2009), which might explain both, the large fundamental niches of rare species and the non-correlation with the selected drivers (F.N. loss or the persistence index). Mostly due to invasive herbivores, many rare species occupy only a small area within the fundamental niche. Consequently, their ecological requirements are unknown, since reduced populations do not always show the true ecological requirements of the species. Sometimes, they are even preserved in areas where they are doomed to extinction, because the climatic conditions there are not the most suitable for them (Marrero-Gómez et al. 2007). For these species, experimental translocations, close monitoring and modelling are essential to estimate the fundamental and realized niche size necessary to manage them in the face of climate change (Bellis et al. 2021).
Fundamental niche size was positively correlated with drivers directly related to climate change: F.N. loss (p < 0.0001) and persistence index (p < 0.0001), and with the persistence of the number of occurrence cells (p < 0.0001). Therefore, we should expect that vulnerability of common species (those occupying a greater proportion of the fundamental niche), would be more dependent on the above drivers. However, F.N. size prevents establishing this relationship, since the common species are also highly vulnerable in very small habitats that are disappearing with global warming. Small habitat size is itself a notable risk (Enquist et al. 2019; Horváth et al. 2019), and vulnerability of the studied alpine ecosystem is probably strongly underestimated due to habitat loss. Moreover, the characteristic isolation of insular alpine ecosystems, islands within islands (Fernández-Palacios et al. 2014) makes them more vulnerable because of limitations to spatial processes (Rybicki & Hanski 2013; Horváth et al. 2019).
The emerging picture is that species with large ranges may have small populations, and that there are common species in small sized fundamental niches. Thus, two types of rarity are combined in the species of this alpine ecosystem: habitat rarity (dependent on F.N. size) and species rarity, mostly related to stressor factors (R.N.). This appears to reduce the ecological significance of F.N. size to climate change and highlights the importance of identifying and understanding the drivers that most constrain the realized niche. In fact, the widely established close relationship between rarity and extinction risk (e.g. Schwartz et al. 2006) has been long questioned because of the existence of natural rarity in small habitats (Gaston 1994; Martín 2009). Indeed, the use of the UICN absolute presence thresholds to evaluate the risk species face has been discouraged on small oceanic islands (Martín et al. 2009; González-Mancebo et al. 2012), since it can lead to overestimating the threat in smaller habitats. Nevertheless, current climate change has arrived as a factor that increases the risk level of small populations, whether the rarity is due to natural or anthropic causes. Thus, small natural habitats may be also in risk even for common species, because climatic stability is particularly essential to species survival in small areas and populations (Morueta-Holme et al. 2013). This is of special concern regarding geographically isolated habitats subjected to sustained stress over time, as occurs on oceanic islands with undergoing introduced herbivore damage (Caujapé-Castells et al. 2010).
Although rare species show significantly higher vulnerability than common and restricted species, these latter two groups include species under high risk at present and even critical in the future. Common species may play an important role in ecosystem functioning and their decline can even lead towards ecosystem collapse, as recently detected for Cytisus supranubius on Tenerife (Cubas et al. 2022). Although common species may have a greater evolutive potential under the pressure of abiotic and biotic changes, the rapidity of current climate change may cancel advantages for this species group, depending on the constraints to their adaptability and loss of their fundamental niches. Other common species at high risk at present on La Palma include Arrhenatherum calderae, the dominant member of poaceae currently present in this ecosystem, Genista benehoavensis and Echium gentianoides. The latter two are threatened species managed for the last 30 years by the Caldera de Taburiente National Park. They now occupy respectively 89.15% and 83.03% of their fundamental niche, which however is currently very small (4708 ha and 6109 ha respectively). The near future also holds high risk for some common species on Tenerife: A. calderae, Argyranthemum teneriffae, Descurainia bourgeauana, Nepeta teydea and Tolpis webbii. Some of these species are currently in expansion: A. teneriffae, T. webbii and A. calderae (Martín et al. 2021), which supports a delay in their reaching high risk in the alpine zone of this island. A greater area and elevation (more than 1000 meters higher than La Palma) mean without doubt more time to extend their survival of climate change in this zone. Although some species are already starting to recoil in southern areas, such as D. bourgeauana, N. teydea or C. supranubius (Martin et al. 2020, 2021, Cubas et al. 2022).
Rare species with large F.N. size included some with a high dispersal potential, but highly threatened, like Cicer canariense (Tenerife and La Palma). This small legume can grow at from 445 m to 2000m in elevation, is also quite drought tolerant and regenerates quickly but is highly palatable to invasive herbivores. Its fundamental niche is 21,290 ha with 12% occupied on La Palma, 50741ha on Tenerife (1% occupied). Sixty-one % of the species we classified as rare occupy a maximum 5% of their fundamental niche and are threatened on their native island, while those that occupy more are expanding currently, such as Cheirolophus teydis on Tenerife, or threatened species with new populations resulting from National Parks management. Knowledge is greatly lacking about the fundamental and realized niches of these species, even though some are well studied. However, there are also species whose rarity derives from their small F.N. size, as for instance Viola palmensis (64% of 648 ha occupied, in 46 occurrence cells).
Climate change is affecting plant communities and ecosystems around the world. Managing communities so that they can resist requires specific measures with precise methods for assessing vulnerability. Such supportive aids can prioritize actions within the framework of comprehensive management. Our results indicate that conservation management nowadays must be widely distributed among all native species, and not only focus on threatened or restricted ones. Drivers of vulnerability vary strongly between species, and therefore understanding and including them in climate change vulnerability assessments is essential for efficient allocation of finite management resources (Beever et al. 2015). Our results highlight the need for urgent management of rare and restricted species, to gauge the possibilities of enlarging their realized niches to enhance and prolong their adaptive capacity (Thurman et al. 2021). In other words, to reduce the constraints on the adaptability of these species through the removal or mitigation of threats, and to strengthen and better understand the realized niche by means of experimental translocations. Common species also need management since are inhabiting habitats that are already disappearing around the summits of the islands, so management must be addressed to preserving them in the best possible conditions. Notably, removal or effective control of invasive herbivores will help to achieve this, since they are the strongest constraint on all the groups analyzed. This study emphasizes the need to further assess the climate change vulnerability of species and the drivers of their responses, to achieve better management of these unique ecosystems.