The fate of terrestrial biodiversity during an oceanic island volcanic eruption

To our knowledge, this is the only work carried out on the state of terrestrial biodiversity in the direct vicinity of a volcanic eruption of limited duration. In this contribution, we document and assess the impact on major plant and animal groups within ecosystems during a volcanic eruption (Table 1). While some groups were clearly disadvantaged: ferns and herbaceous plants as well as invertebrates and saurians (lizards and geckos); other groups such as conifers and woody shrubs showed better resilience, as did birds.

This study is particularly important because of its location in a Mediterranean biodiversity hotspot.13.14, hosting a unique ecosystem of oceanic island organisms (38% of the endemicity of the Canary Islands). The islands indeed feature a disproportionate amount of the world’s biodiversity, but unfortunately a high number of extinctions have also occurred there.14. The biodiversity of the south of the island is poorer than that of the north. This is probably partly explained by the relatively frequent volcanic activity with seven major eruptions since 1585, including this one in 2021 (see15), which led to an alternation of processes of destruction and neo-colonization.

In terms of flora, the Canarian pine forest was the most affected ecosystem and vegetation type, as it is dominant near new volcanic vents. The southern slopes of this forest were the most disturbed area due to the location of the volcano, combined with the prevailing northeast trade winds (Fig. 1). Tephra fallout and sulphurous gases were the main factors that affected the pine forest, over a wide area. In addition, local xerophytic and thermophilic habitats have also lost a large part of their area. Unlike the pine forest, this drastic reduction was caused by the gradual downward expansion of lava flows.

The Canary Islands pine was thus particularly affected by the fall of tephra, an aerosol of sulfuric acid12, and short acid rain episodes. However, this conifer shows great resistance to temperature, confirming its great adaptation to volcanic events.16which is probably also one of the keys to its resistance to more frequent current fires17. This species of pine has been evolving among volcanoes for 13 Ma16 and successfully adapted to high temperatures. In addition, thunderstorms with lightning occur in the Canary Islands with heavy rains; therefore, wildfires probably should not have been so frequent in the island’s past, before human settlement. In this habitat, it is also notable that epiphytic lichens (U. articulated) apparently survived on pines until the 12th week, given their high susceptibility to anthropogenic pollution18.

The life cycle of flowering plants has been significantly disrupted due to all of the above factors, with significant impact on foliage, photosynthesis and growth. However, soil changes due to tephra deposition and its leaching by rain are one of the most dramatic factors affecting plants and a long-term impact of volcanic eruptions.19. Individuals closest to the crater were most directly affected by the intense tephra falls and concentrated volcanic gases (SO2HCl, HF, CO2). However, plants located within 200 m of the lava flows but more than 2 km from the crater were likely to have been more disturbed by the high temperature of the slowly cooling lava and its lower gas emissions.

Large woody plants showed a better frequency of survival than smaller ones in the face of this extreme stress (Table S1 and19). In the Hekla region (Iceland), most trees have thickened trunks, indicating that the surviving trees had a long life subject to frequent volcanic damage.19. Secondary woodland of island plants (sensu20) is traditionally linked to drought20.21ecological shift22 or counter-selection for inbred depression in founding island populations23. However, this adaptation also promotes the resistance of many shrubby plants to high temperatures close to craters and lava flows, but above all their resistance to intense tephra falls which affect a much larger area. In addition, the height of plants and stems plays a fundamental role in overcoming deep layers of deposits. This last effect was particularly important up to 2.5 km from the crater (tephra thickness > 30 cm) (Figs. 1 and 2), because the herbaceous plants were completely buried, sometimes more than 1.5 m deep. Consequently, the seed bank was probably rendered largely non-functional as well. However, deposits were recorded over almost the entire island, indicating that longer lasting or more intense eruptions would severely affect an even larger area. Such events have so far been ignored in the intensely debated debate on “island afforestation”21,23,24,25,26.27. We found surviving populations of endemic woody taxa strongly impacted by tephra deposits near lava flows, across a wide range of genera such as Rumex (R.Lunaria), Echium (E. breviram), Euphorbia (E. lamarckii, E. canariensis and E. balsamifera), Aeonium (A. davidbramwellii), Rubia (R. fruiticosa), Schizogynous (S. sericea), Carlina (C.falcata) Where sonchus (S. hierrensis) (Table S2), which coincide with the general list of woody plants of the Canaries20. Most members of these genera in other ecosystems on continents are primarily herbaceous. As such eruptions and their impacts due to ash deposition are frequent occurrences on volcanic islands, for example several times in a century at La Palma, this is a “frequent” selective process on evolutionary time scales .

Regarding the fauna, the invertebrate community collapsed in the first two weeks (Table S2), probably due to the rapidly deteriorating plant growth status. These changes in invertebrates were caused by contact of tephra with the cuticular lipid layer28 and water loss due to abrasion of seed coats29. During this period, many insect pests (especially whitefly pupae) in banana plantations (farmers’ observations) were significantly reduced. This sudden decline in insect populations affected the entire food web and likely caused part of the ecological collapse of saurian and some passerine communities.30. In the case of lizards, small individuals seem to withstand adverse conditions better than large ones, as observed in other flares3. This could be related to their lower food requirements and greater ease in finding shelters. Loss of body condition of lizards after eruption has been recorded and negatively affects reproductive quality31. However, some lizards have shown a good ability to find food in the tephra substrate.32. We found abundant tephra particles in some vertebrate droppings (lizards, birds and mammals) during the eruption, probably ingested involuntarily. At least in bats, ingestion during feeding produces physiological stress that is likely related to baldness, high loads of ectoparasites, or possible mineral deficiencies.33.

As described in the Canary Islands, some passerines show great fidelity to their territory (see34). During the eruption, Sardinian warblers (Curruca melanocephala) maintained their territories until the imminent arrival of lava flows. Large birds (kestrels F. tinnituscrows C. corax and the nozzles B.buteo) were able to continue flying in the areas surrounding the crater. Also, some cases like F. tinnitus showed great food plasticity during the first two weeks. At least six times, kestrels attempted to catch birds (especially small passerines and doves), contrary to their usual diet based on an abundance of lizards and insects.35. Expanding trophic niches in island organisms have traditionally been interpreted to be related to discord in island ecosystems36,37.38. However, this plasticity is extremely beneficial in ecological disasters, where food becomes exceptionally scarce. In the case of bats, their flight is limited by the delicate structure of their patagium, which can be damaged by the frequent fall of pyroclastic tephras. In addition, the scarcity of insects in the first kilometers of the crater probably led to their displacement to other areas with more distant and richer food resources.

As we have learned from the movement capacity of vertebrate animals that still inhabited the affected area, those with greater mobility, birds and bats, withstood the eruptive process much better than those with less mobility. mobility, for example the saurians.

Finally, during this destructive event on La Palma, we had the opportunity to deepen our knowledge of how eco-evolutionary adaptations favored the survival of island organisms. Such responses are traditionally discussed in the context of island biology. As already mentioned, one of the most interesting findings verifies the remarkable adaptation of the Canary Islands pines (P. canariensis) to volcanism (see16), including extremely harsh ecological conditions. Other island trends related to the prevalence of woodiness in island flowering plants20.21or the great trophic plasticity of certain oceanic island vertebrates36, have not previously been associated with their potential evolution with volcanic processes. However, these evolutionary adaptations most likely played an important role in the survival of plants and animals affected by the volcano. For this reason, it is worth considering and debating whether these previously mentioned evolutionary processes are in fact also related to repeated volcanic episodes on oceanic islands.