Fate of terrestrial biodiversity during an ocean island volcanic eruption

To our knowledge, this is the only work done on the state of terrestrial biodiversity in the direct vicinity of a time-limited volcanic eruption. In this contribution, we document and assess the impact of volcanic eruptions on ecosystems on major groups of plants and animals (Table 1). Although some groups are clearly disadvantaged: ferns and herbivores, including invertebrates and saurians (lizards and geckos); other groups, such as conifers and tree shrubs, showed better resilience, as did birds.

This study is particularly important because it is located in a Mediterranean biodiversity hotspot13,14, harbors a unique ecosystem of oceanic island organisms (38% of the endemicity of the Canary archipelago). Islands indeed display a disproportionate amount of the world’s biodiversity, but unfortunately there have also been large numbers of extinctions.14. Biodiversity is poorer in the south of the island than in the north. This is probably partly explained by relatively frequent volcanic activity, with seven major eruptions since 1585, including one in 2021 (see15), which led to alternating processes of destruction and neocolonization.

Regarding the flora, the Canary pine forest was the most affected ecosystem and plant type, as it dominates near the new volcanic vents. The southern slopes of this forest were the most disturbed area due to the location of the volcano with the prevailing northeast trade winds (Fig. 1). Tephra waste and sulfurous gases were the main factors affecting the pine forest over a large surface area. In addition, native xerophytic and thermophilic habitats have also lost much of their surface area. In contrast to the pine forest, this sharp decline is due to the progressive downslope expansion of the lava flows.

Thus, Canary Island pine was particularly affected by tephra fall, sulfuric acid aerosol12, and brief episodes of acid rain. However, this coniferous plant is highly resistant to temperature, confirming its great adaptability to volcanic phenomena16this is probably one of the keys to its resistance to modern wildfires17. This type of pine has evolved over the past 13 years among volcanoes16 and successfully adapted to high temperatures. Moreover, in the Canary Islands there are thunderstorms with heavy rain; therefore, in the island’s past, before human colonization, wildfires probably should not have occurred so frequently. In this habitat, epiphytic lichens (U. articulata) showed resistance in pine trees until the 12th week, considering their high sensitivity to anthropogenic pollution.18.

The life cycle of flowering plants has been drastically disrupted due to all of the above factors, greatly affecting foliage, photosynthesis and growth. However, soil changes due to the deposition of tephra and its washing by rain are among the most dramatic factors affecting vegetation and are the long-term effects of volcanic eruptions.19. Individuals closest to the crater were most directly affected by intense tephra falls and concentrated volcanic gases (SO).2HCl, HF, CO2). However, plants located within 200 m of the lava flows, but more than 2 km from the crater, were probably more disturbed by the higher temperature of the slowly cooling lava and its lower gas emissions.

Large tree plants exhibited a better survival rate than small plants 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 lived a long life, often subject to volcanic damage.19. Secondary woodiness of island plants (sensu20) has traditionally been associated with drought20,21environmental change22 or counterselection of inbreeding depression in founder island populations23. However, this adaptation also favors the resistance of many shrubs to high temperatures near craters and lava flows, but primarily to their resistance to intense tephra fall affecting a larger area. In addition, plant and stem height play a key role in overcoming deep layers of sediments. This latter effect was particularly significant within 2.5 km of the crater (tephra thickness > 30 cm) (Figs. 1 and 2), as herbaceous plants were completely buried, sometimes to a depth of more than 1.5 m. Therefore, the seed bank is also probably not operational. However, precipitation was recorded almost all over the island, suggesting that prolonged or more intense eruptions would have a significant impact on an even larger area. Such phenomena have so far not been taken into account in the intensively debated “forestation of the island”.21,23,24,25,26,27. We found surviving populations of heavily impacted endemic woody taxa from tephra deposits close to lava flows, such as a wide variety of Rumex (R. lunaria), Echium (E. brevirame), Euphorbia (E. lamarckii, E. canariensis and E. balsamifera), Aeonium (A. davidbramwellii), Ruby (R. fruticosa), Shizojin (S. sericea), Carlina (C. falcata) or Sonchus (S. hierrensis) (Table S2), which coincides with the general list of woody Canary plants20. In other ecosystems on the continents, most representatives of this genus are mainly herbivores. Since such eruptions and their effects on ash deposits are frequent events on volcanic islands, such as several times per century at La Palma, this is a “frequent” selective process on evolutionary time scales.

Regarding the fauna, the invertebrate community collapsed during the first two weeks (Table S2), probably due to the rapid deterioration of the plant growth condition. These changes in invertebrates were caused by contact of tephra with the cuticular lipid layer28 and water loss through tegument erosion29. During this period, many insect pests (especially whitefly pupae) decreased dramatically in banana plantations (farmers’ observations). This sudden decline in insect populations affected the entire food web and probably caused some of the ecological collapse of saurian and some passerine communities.30. In the case of lizards, small individuals resist adverse conditions better than large ones, as observed in other eruptions.3. This could be attributed to their lower food requirements and easier finding of shelter. Loss of body condition after molting of lizards has been recorded and has a negative effect on reproductive quality.31. However, some lizards showed a good ability to find food in the tephra substrate32. We found abundant tephra particles in the litter of some vertebrates (lizards, birds, and mammals) that were probably ingested involuntarily during the eruption. At least in bats, undernutrition induces physiological stress associated with baldness, high ectoparasite loads, or possible mineral deficiency.33.

As described in the Canary Islands, some roadrunners show high fidelity to their territory (see34). Sardinian nightingales during eruption (Curruca melanocephala) held their territory until the approach of the lava flow. Larger birds (kestrels F. tinnunculuscrows C. corax and corns B. buteo) were able to continue flying in the areas surrounding the crater. In addition, as some cases F. tinnunculus showed great nutritional plasticity in the first few weeks. At least six times, the lizards tried to catch birds (especially small passerines and pigeons) in contrast to the usual diet based on abundant lizards and insects.35. The expansion of trophic niches in island organisms is traditionally associated with disharmony in island ecosystems.36,37,38. However, this plasticity is very useful in environmental disasters, where food is extremely scarce. In the case of bats, their flight is often limited by the delicate structure of the patagium, which can be damaged by falling pyroclastic tephra. In addition, the scarcity of insects in the first few kilometers from the crater probably led to their migration to other more distant and richer food resource zones.

As we learned from the mobility of vertebrates still inhabiting the affected area, more mobile birds and bats withstood the eruption better than less mobile ones, such as saurians.

Finally, during this devastating event in La Palma, we had the opportunity to increase our knowledge of how ecological-evolutionary adaptations contribute to the survival of insular organisms. Such responses are traditionally noted in the context of island biology. As already mentioned, one of the most interesting findings confirms the excellent adaptation of Canary Island pines (P. canariensis) volcanism (see16), including extremely harsh environmental conditions. Other insular trends related to the distribution of woody plants in insular flowering plants20,21or the high trophic plasticity of some vertebrates on oceanic islands36, have not previously been linked to their potential evolution together with volcanic processes. However, such evolutionary adaptations likely played an important role in the survival of volcano-affected plants and animals. For this reason, it is worth considering and discussing whether these previously mentioned evolutionary processes are actually related to repeated volcanic episodes on oceanic islands.

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