Vascular Plant Extinction in Macaronesia: Biogeographical and Biological Drivers of Loss

Abstract

Global extinction rates have accelerated due to increasing anthropogenic pressures (Pimm et al. 2014), with plant species facing particularly high risks. Current estimates suggest that approximately 39% of plant species are at risk of extinction (Nic Lughadha et al. 2020), with habitat destruction, climate change, biological invasions, and pollution contributing significantly to the decline of plant diversity. One critical but often overlooked aspect of extinction dynamics is dark extinction, which refers to the loss of species before they are scientifically recognized, leading to an underestimation of biodiversity loss (Boehm and Cronk 2021). Understanding dark extinction is crucial, as it can provide insights into silent biodiversity erosion occurring at local and global scales. In this scenario, research studies on plant biodiversity loss that focus on pre-Linnean (dating to times before the modern classification of species) and post-Linnean literature, alternative sources of data (e.g., herbarium and carpological collections) and ecological and genetic plant dynamics are needed to fully understand how biodiversity has changed over the centuries, what role the human species has played, and what policies should be adopted to improve the conservation of biodiversity as a whole (specific, ecological and genetic).

Concerning extinction, another significant form is local extinction, which involves the disappearance of plant populations within specific areas while the species persists elsewhere. This type of extinction is particularly alarming as it results in the loss of ecological functions, alters species interactions, and reduces genetic diversity (Sax and Gaines 2003). Consequently, like a stone thrown into the water, the concentric wave of local extinction widens, threatening ecosystem resilience and the ability of natural communities to recover from environmental disturbances (Donaldson et al. 2019). In this metaphorical wave motion, islands are among the most vulnerable ecosystems to extinction events due to their isolated nature, high endemism, and susceptibility to invasive species (Fernández-Palacios et al. 2021).

In addition to extrinsic environmental conditions, these trends are strongly interlinked with certain plant biological traits which were found to correlate with higher extinction probabilities (Gray 2019). Species with specialized ecological requirements, such as those reliant on particular pollinators or seed dispersers, face increased extinction risk when their mutualistic partners decline. Plants with low reproductive rates and slow growth struggle to recover from population losses, making them particularly vulnerable in rapidly changing environments. Additionally, limited dispersal ability restricts a species' capacity to colonize new habitats, further increasing susceptibility to habitat fragmentation and environmental disturbances (Chichorro et al. 2019). High genetic variability is generally associated with greater adaptability, whereas populations with high inbreeding depression and low genetic diversity face higher extinction risk (Frankham 2005), for example, due to reduced resilience to stressors like climate change and disease. These factors once more highlight the importance of considering multiple factors—including plant functional traits—in conservation efforts to avoid underestimation of the risk extent and unsuccessful recovery strategies.

In the research article presented in Global Change Biology by Orihuela-Rivero et al. (2025), the authors investigate how biogeographical features and some selected biological plant traits influence plant extinction dynamics, particularly in the context of oceanic islands (Macaronesia archipelagos), where species loss is expected to be pronounced. The study aims to identify the level of dark extinction, the magnitude of global and local extinction relative to the background extinction rate and its drivers, and assess how these factors interact with anthropogenic impacts to shape extinction patterns. By assembling three datasets (the first comprehensive database of global and local vascular plant events in Macaronesia, a dataset of island attributes related to extinction risk on islands, and a dataset of nine biological traits related to plant extinction risk on islands), the authors analyzed plant extinction patterns across the archipelagos, focusing on the historical factors and key biological traits that contribute to species extinction. Remarkably, a total of 126 species were found to be extinct, of which 13 were extinct at the global level. The results showed a concerning, heterogeneous pattern of extinctions across the Macaronesia archipelagos, with extinction rates exceeding background levels by orders of magnitude—well above the global average for vascular plants, especially in the Canary Islands and the Azores. Results reveal that extinction risk is not uniform but varies significantly based on geography, elevation, and anthropogenic pressures. Human population growth and tourism have emerged as major drivers of extinction, particularly in recent decades. The introduction of herbivorous mammals has also had a significant impact, with animals like rabbits, goats, and rats threatening native plants through grazing, seed predation, and flower consumption. Moreover, rather than island size or isolation, it is the age and elevation of islands that play a crucial role. Plant populations in lowland coastal areas are particularly threatened due to urban expansion and habitat conversion. Anthropogenic factors significantly impacted the archipelagos plant diversity, and the local extinction events found are worryingly high (70, or 99 if pre-Linnaean extinctions are included) and significantly impact ecological resilience and ecosystem functioning. Endemism emerges as a key predictor of extinction, though with an interesting twist—Macaronesian endemics actually show lower extinction rates than non-endemic or archipelago-endemic species. This counter-intuitive finding may be explained by how local extinctions can actually create new endemics by reducing species ranges, as illustrated by Dracunculus canariensis Kunth, which was once found in both the Canary Islands and Madeira but is now restricted to the Canaries.

Pollination mechanisms also seem to play a key role as a predictor of extinction rate, with pollination by vertebrates and water-dependent (hydrophilous) species showing particular vulnerability. This finding about vertebrate pollination seems to contradict previous research that identifies insect-pollinated (entomophilous) species as more vulnerable (Gray 2019), as bird pollination typically provides more effective gene flow than insect pollination. The researchers suggest this might be explained by other factors, such as these plants' nitrogen-fixing abilities making them more appealing to invasive herbivores.

Woody plants show higher extinction vulnerability compared to herbaceous species, which is particularly significant given Macaronesia's history of deforestation. This vulnerability arises from their smaller populations, slower reproduction, and less robust seed banks. This finding is especially noteworthy because insular woodiness—a characteristic feature of island plants—should theoretically help with competition and drought resistance, yet these advantages appear overwhelmed by the pressures of introduced herbivores and exploitation. Other traits also influence extinction risk, including zoochory (animal-based seed dispersal) and nitrogen-fixing abilities. The vulnerability of nitrogen-fixing plants, particularly members of the Fabaceae family, poses special concerns for ecosystem stability and biodiversity, especially in the summit scrublands of the Canary Islands where these plants play a crucial structural role.

Overall, these findings have particular relevance for conservationists and policymakers, especially regarding coastal urbanization's impact on biodiversity. This research reveals several concerning patterns, particularly in coastal areas following the tourism boom of the 1960s. To address these challenges, the researchers recommend targeted monitoring of vulnerable species, island-level Red Lists, and sustainable tourism practices. The study emphasizes protecting coastal and summit ecosystems, suggesting species propagation and reintroduction programs. The use of multidisciplinary research approaches, from taxonomy to genetic analysis, is crucial for effective conservation. Finally, emphasizing the negative impacts of coastal urbanization and the key biological traits that increase extinction risk would be especially supportive to promote more effective conservation policies and plan effective actions, especially when engaging policymakers, land managers, and the general public in conservation efforts.

Giulia Albani Rocchetti: conceptualization, writing – original draft.

The author declares no conflicts of interest.

This article is a Invited Commentary on Orihuela-Rivero et al., https://doi.org.10.1111/gcb.70072.