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The Mediterranean region under climate change

 | 
Jean-Paul Moatti
, 
Stéphane Thiébault

Part 2. Vulnerability and impacts

Sub-chapter 2.4.3. Plant biodiversity and vegetation on Mediterranean islands in the face of global change

Frédéric Médail

Texte intégral

Introduction

1The profound environmental heterogeneities and the complex historical biogeography explain the high diversity of landscapes and vegetation types in the Mediterranean Basin, one of the world’s biodiversity hot-spots (Médail & Myers, 2004). The associated high levels of biodiversity and ecological complexity have favored the emergence of functional uniqueness in several ecosystems and plant communities that occur nowhere else. With about 10,000 islands and islets (approx. 250 inhabited by humans), the Mediterranean Sea can be considered as one of the largest archipelagos in the world. Its islands contain a significant component of Mediterranean biodiversity, notably a number of range-restricted species and peculiar vegetation types (e.g. Vogiatzakis et al. 2008; Médail, 2013, in press). The richness of Mediterranean insular nature is linked to the long-lasting influence of humans as ‘designers’ of landscapes, who have shaped vegetation dynamics through burning, cutting, grazing and plowing (Blondel, 2008). More recent threats linked to global environmental change (climatic and sea-level changes, biological invasions) weaken the biodiversity and functioning of these ecosystems.

2The diversity of Mediterranean islands Mediterranean islands are a kaleidoscope of environmental and biotic conditions. This is due to the wide range of sizes (from the largest island of Sicily covering 25,700 km2 to small islets a few dozen square meters in size), in altitude (from Mt. Etna, 3,342 m a.s.l. to flat islets only one meter a.s.l) and in remoteness, all of which represent key parameters in island biogeography. There are a total of 157 large islands exceeding 10 km2 (1,000 ha) in size in the Mediterranean Sea, of which 86 (55%) are located in Greece (Médail, in press). Forty nine islands cover more than 100 km2, of which 36 cover more than 200 km2. Small islands (less than 10 km2, www.initiative-pim.org) are the most common, and there are several thousand of such islands (Photos 1). Most islands belong to the Greek archipelago with ca. 7,600 islands and islets in the Aegean Sea, more than 90% of which cover less than 10 km2 (Triantis & Mylonas, 2009). In the Ionian Sea, there are ca. 300 Greek islands and islets. Croatia is the second country in terms of the number of islands (n = 1,246), including 79 islands covering more than 1 km2 and 653 small islands and islets with vascular vegetation (Nikolić et al., 2008).

Photos 1
Example of small rocky islands. Left: Capense island in N. Corsica (Cap Corse, Centuri), V. 2015 Right: Es Vedrà island, a small (640 ha) island with an impressive peak (384 m) emerging from the sea SE of Ibiza island, IV. 2014 (F. Médail).

3Nearly all Mediterranean island ecosystems are under the influence of the Mediterranean climate (Figure 1), except some parts of the high mountains of Corsica that contain subalpine and alpine vegetation belts with Euro-Siberian and even arctic-alpine plants. The insular vegetation types usually considered as ‘typically Mediterranean’ are the evergreen and sclerophyllous shrublands and forests under semi-arid bioclimatic conditions. Deciduous trees were widespread during the postglacial period, notably in the northern Mediterranean, but these forests were severely impacted by humans and their livestock.

Figure 1
Schematic distribution of major vegetation types in the Mediterranean region, according to phytoecological and bioclimatical criteria, notably the mean minimum temperatures of the coldest month (m) of the year (see Médail, 2008).

4The existence of endemic or range-limited plant species can determine specific patterns of vegetation types and landscapes. On large islands, plant endemism ranges from 8% to 17%, whereas the total number of taxa is greater than expected, usually between 1,600 and 2,800 species and subspecies (Table 1). In insular mountain ranges, the level of endemism is higher: above 1700 m a.s.l. endemics represent 35%-40% of the vascular flora in Corsica and in Crete. Each island has its own vegetation specificities even if major physiognomic structures may appear similar from one island to another. These include vegetation typical of low and medium altitudes, whereas the upper vegetation levels are restricted to the summits of the largest islands, notably Corsica, Sicily, Crete, Cyprus and Samos.

Table 1. Indigenous plant richness and endemism level of the six largest Mediterranean islands (Médail unpublished data, various sources).

Table 1. Indigenous plant richness and endemism level of the six largest Mediterranean islands (Médail unpublished data, various sources).

Insular plant biodiversity facing current global environmental change

5The Mediterranean climate, in particular the drastic und sometimes unpredictable nature of climatic patterns and resource availability, puts severe and contrasted stress on species and communities. The impacts of climate change in the 20th century are less well documented than direct human impacts, but the situation is now changing rapidly and severe impacts on island ecosystems and biodiversity are expected.

Widespread human impacts and land use changes

6Most changes in Mediterranean ecosystems are linked with human land use and its dynamics over time and these will likely continue to play a major role (Blondel & Médail, 2009). Beginning in the 19th century and with a constant acceleration throughout the 20th century, major land-use changes on large and medium islands were characterized by the widespread collapse of the ‘traditional Mediterranean tryptic’ ager-saltus-sylva (agriculture, pastoralism, forestry) that shaped insular landscapes for millennia. Until the middle of the 20th century, natural resources were crucial for island populations, notably for food, livestock, nutraceutical and healing compounds. For example, in a single year (1867) more than 7,000 metric tonnes of wood and coal were consumed in Palma de Mallorca for domestic and artisanal uses (Mayol, 1995). The end of subsistence economies on most islands led to reduced pressures on natural resources and therefore a general increase in matorral and forest areas, as well as to the abandonment of cultivated terraces that were formerly the rivet of agricultural landscapes on steep and mountainous islands (e.g. Cerabolini et al., 1996, for Elba; Petanidou et al. 2008, for Nisyros).

7These trends cannot be generalized to all islands. For example, the relation between successional processes and increased grazing pressure after land abandonment is still uncertain in many Mediterranean islands (e.g. Rühl & Pasta, 2008; Schaich et al. 2015). In some cases, the end of traditional agricultural activities has led to severe soil erosion or to the rapid extinction of many ruderal endemics and several particular archaeophytes. Throughout the Aegean archipelago, the collapse of cultivation practices on terraces in the early 20th century led to major landscape changes, and has resulted in the near disappearance of traditional crops since the 1960s: lentils on Samos and Lesvos, wheat on Chios, beans on the Cyclades. The unique terraced landscapes have disappeared on most of these islands. In western Crete, human emigration from the arid mountains reduced agricultural land area by almost 40% between 1945 and 1990, thereby favoring the recovery of forest ecosystems dominated by the coniferous Cupressus sempervirens and Pinus brutia. The same process has taken place on the Balearic islands, for example in Mallorca, where the extent of Aleppo pine forests has multiplied more than fivefold in the last century (Marull et al. 2015). Similarly, holm oak forests have increased from 5,000 ha to more than 10,000 ha in Mallorca, and from 900 ha to 2,600 ha in Menorca since 1860 (Mayol, 1995). In contrast, the tall shrublands and natural forests of eastern Sardinia decreased by 35% between 1955 and 1996, whereas pastures, burned low shrublands and afforestation are progressing. Landscape dynamics are more contrasted in Corsica. Since the beginning of the 20th century, land abandonment has led to an increase in shrublands and forested areas. Forest cover, which was only 17.6% in 1866 (153,775 ha), is currently 58%, an increase of 3.3% over a period of 150 years (Panaïotis et al. 2015). This increase can be locally counterbalanced by frequent fires, often linked to illegal pastoral practices. Rural areas in Corsica still account for 80% of the land and traditional summer transhumance herds use more 130,000 ha throughout the mountain range (source: Parc Naturel Régional de Corse, PNRC). Grazing is an obstacle to forest expansion and regeneration, it has been practiced on Mediterranean islands for millennia, shaping both landscapes and their biodiversity. Grazing is intensive on most Greek islands. Uncontrolled practices can lead to overgrazing and even to land degradation under arid and semi-arid climates (e.g. Papanastasis et al. 2002).

8Today, there is a slight increase in permanent human population on the bigger islands, whereas the medium-sized islands – except some hot-spots of tourism such as Corfu and Djerba – are undergoing a population decline. Even on large islands, disparities are strong between densely populated islands like Malta (1,330 people/km2) and less-populated ones like Corsica (36 people/km2). Since the 1960s, island tourism has increased everywhere, with a paroxysm on two Balearic Islands (Mallorca and Ibiza) where a peak was reached in 2000-2001 with 11 million tourists. This pressure has led to major urban development along the coasts, threatening fragile ecosystems such as sand dunes, wetlands and, to a lesser extent, rocky habitats. For example, on the Greek island of Skiathos (N. Sporades), the development of tourism since the 1970s has led to an 80% reduction in these coastal ecosystems (Economidou, 1995).

Impacts of climatic change

9With an expected temperature increase of 3 ° to 5 ° C in the Mediterranean over 21st century, potential evapotranspiration is expected to reach an average of 200 mm annually, which is equivalent to a loss of 50 mm in annual rainfall (Le Houérou, 1990). The expected shifts in vegetation belts resulting from increased aridity and a 3 ° C increase in temperature will be an upward shift of approx. 545 m (almost the amplitude of a vegetation belt) and a 50–80 km northwards shift in latitude (Médail & Quézel, 2003). These impacts will be exacerbated on islands where no (or insufficient) areas are available for such shifts. The flora and vegetation of the alpine areas (i.e. mostly the oro-and alti-Mediterranean belts) and the spatially restricted summit areas of mountain ranges will probably be the most threatened, as in the Lefka Ori massif in Crete (Kazakis et al., 2007). In Corsica, this is particularly the case of arctic-alpine species (ca. 25 taxa) in the alpine vegetation belt (Contandriopoulos & Gamisans, 1974).

10It is too simplistic to consider a single range shift of plant communities in response to global warming. As suggested for Mediterranean mammals, the effects of climate change on species distribution and communities may consist of changes in community structure (Maiorano et al., 2011). The extent to which many organisms will be able to cope with climate change is still largely an open question, especially because climate change is now taking place at an unprecedented rate. Microevolutionary changes may occur rapidly in fitness related traits such as the flowering time in plants (Peñuelas et al., 2002). Differential responses of organisms interacting in complex food chains or symbiotic associations may also disrupt interactions that are essential for ecosystem functioning such as pollination or seed dispersal.

11Climatic change is also a threat for insular plant populations and communities linked to wet habitats or mesophilous conditions, including some endemics that constitute the cornerstone of Mediterranean plant diversity. This is the case of two endemic species of the Apiaceae family restricted to the Balearic Islands. Apium bermejoi, a narrow endemic of Menorca located in a single area of 50 m2 where the ca. 100 individuals occupy only one square meter. As this critically endangered plant is vulnerable to prolonged droughts, its present decline is probably related in part to a series of dry summers (Moragues & Mayol, 2013). The narrow ecological niche of the palaeoendemic Naufraga balearica, only distributed along a short section (ca. 15 km) of the north facing slopes of the northern Majorcan coast (Figure 2), explains its current extreme rarity (Fernández-Mazuecos et al. 2014)-rapid climatic shifts could jeopardize its survival. In Cyprus, several populations of narrow endemics (Onosma caespitosa, Salvia veneris, Sideritis cypria) are also threatened by rainfall reduced by 20% to 40%, and warming, both of which could modify their germination window in the fall (Kadis & Georghiou, 2010).

Figure 2
The highly threatened narrow endemic Naufraga balearica (Apiaceae) located in the north-western part of the island of Majorca, with the distribution of the three haplotypes across the five sampled populations (a), the phylogenetic consensus tree (b) and the statistical parsimony network (c) of plastid DNA haplotypes showing the ancestral haplotype A (Fernández-Mazuecos et al. 2014).

12So far, there is only little robust evidence for direct depletion or extinction of populations due to climate change. About 10 endemic species are known to have become extinct on the Mediterranean islands, of the ca. 40 Mediterranean plants presumed to be totally extinct in this region (Blondel & Médail, 2009). Recent studies focused on the Cretan endemic tree Zelkova abelicea (Fazan et al. in prep.), and on some plant communities (e.g. Henne et al. 2015), showed that the ecological amplitude of many so-called ‘temperate’ species that thrive on Mediterranean mountains may have been underestimated. Local persistence of Mediterranean plants, notably perennials, in diverse microhabitats may be due to multiple demographic strategies of persistence by longevity or regeneration, depending on the local or regional conditions (García & Zamora, 2003). This is corroborated by the important role played by glacial refugia on islands (Médail & Diadema, 2009). However, most plants interact with bioclimatic characteristics at a physiological rather than macro-climatic level (Curtis et al. 2016). Differences in small-scale habitats in a landscape may therefore explain species’ ability to cope with drastic and changing climate. The high habitat heterogeneity of Mediterranean-type ecosystems may thus represent an ‘ecological insurance’ for the future persistence of plant species at local scale, allowing species to migrate locally in more favorable ecological niches. Nevertheless, at the scale of small islands, this may not be sufficient to ensure the survival of highly specialized plants. The future persistence of ‘islet specialists’ such as the small annual Nananthea perpusilla (Asteraceae) in some shady and humid patches of islets around Corsica and Sardinia may well be at risk (photos 2).

13Photos 2
The large granite rocks on some small islands surrounding Corsica and Sardinia form a highly specialized ecological niche hosting the palaeoendemic Asteraceae Nananthea perpusilla in temporary humid and granitic soils with shaded exposure; Cavallo island (Lavezzi archipelago, S. Corsica) III. 2014 (F. Médail).

Impact of sea level rise

14Sea level rise (SLR) is another important component of climate change. At the end of the last glacial period, a major consequence of climatic oscillation from cold to warm conditions was the melt of the Northern Hemisphere ice sheets causing a continuous eustatic sea level rise worldwide. In the Mediterranean Sea, the main part of this marine transgression occurred before ca. 60,00 cal. yr B.P, with major disparities between areas. Some intra-island phylogeographies–such as for the narrow Balearic endemic Senecio rodriguezii (Molins et al., 2009)–indicate an “island beneath island syndrome”, i.e. a split of populations into several isolated and genetically divergent lineages that are explained by the repeated cycles of sea level changes during the Quaternary.

15A rapid rise in global sea levels is expected in the coming decades. Regional rates of sea level change may increase by a factor of 1 to 6 relative to the observed long term rates (Galassi & Spada, 2014). Coastal ecosystems and small islands are especially threatened. Effects include the exacerbation of coastal erosion, the submersion of low elevation islands and flat coasts, and the salinization of coastal wetlands (Nicholls et al. 2016). Island biodiversity generally depends not only on immigration-extinction dynamics, but also on changes in insular area, isolation and connectivity (Weigelt et al. 2016). On a small Mediterranean island (Cavallo, S. Corsica) Holocene sea level changes played a significant role in loss of wetland biodiversity and ecosystem changes because of the increase in salinity caused by marine intrusions (Poher et al. submitted). Analysis of coleopteran fossils preserved in a 7,000 year sedimentary record showed that 60% of past wetland beetle fauna became locally extinct as a result of regime shift in this freshwater pond. The largest impoverishment occurred 3,700 years ago when the relative Mediterranean sea- level rose more than –1.5± 0.3 m.

16The Gulf of Gabes, in south-eastern Tunisia, is one of the Mediterranean areas that is most threatened by sea level rise, estimated locally at 5.7 mm per year. Geo-archaeological studies mention significant flooding of coastal lands and antique remains, up to two meters 2,000 years ago (Slim et al. 2004). This had profound impacts on the biodiversity of the flat and erodible islands of Djerba, Kneiss and Kerkennah (Médail et al., 2015) (Photos 3). On the latter, land salinization has led to a sharp increase in sebkhas (+ 20% between 1984 and 2011) and a 27% decline in palm groves, equivalent to 26 km2 (Etienne et al., 2012).

Photos 3
Typical vegetation of the large satellite islands of the Kerkennah archipelago (E. Tunisia). Left: open steppe with Lygeum spartum (Poaceae); Right: halophilous shruby vegetation on sebkhras with Arthrocnemum macrostachyum and Sarcocornia fruticosa (Amaranthaceae) (F. Médail).

17The effects of SLR might be less harmful for plant biodiversity on rocky island or coasts, because a slight altitudinal rise of halophilous communities and species towards the salt-tolerant habitats present just above could take place. Nevertheless, this shift may be limited by disturbed habitats on land. In “pocket beaches” in Provence (France), Brunel and Sabatier (2007) found an 12.1 ± 3.5 m retreat in the shoreline between 1896 and 1998, of which 5.8 ± 3.5 m was caused by SLR. On small islands like Porquerolles (Port-Cros National Park) pocket beaches could almost completely disappear (from 75% to 97% regression of their present surface area) by 2100. Because of frequent coastal cliffs or rocky slopes blocking landward migration, typical plant communities and species of fixed maritime sands (e.g. the psammophytes Eryngium maritimum, Otanthus maritimus, Pancratium maritimum of the grey dunes) could become locally extinct on various islands.

Increasing forest fires

18Like grazing, forest fire has been a major driving force of Mediterranean ecosystem dynamics since the emergence of the Mediterranean climate, 3.2 Ma ago. For the emblematic Corsican pine (Pinus nigra subsp. laricio), fires have played a key role in the functioning of these mountainous woodlands since this tree has survived mean fire-return intervals of 80 years over the last 13,200 years (Leys et al. 2014). Fires contributed to the frequent dominance of shrubland on islands during the mid-Holocene (ca. 8,000–7,000 cal. years BP) under dry conditions, as is the case of the Pistacia matorrals in Sicily and Malta (Djamali et al. 2013), and the dense Erica scoparia and E. arborea stands in north-eastern Sardinia (Beffa et al. 2016).

19Fires can slow down the expansion of forest cover explained by the general collapse of traditional human practices on islands, in particular on persisting terraces that promote fast vegetation recovery. Thus, mature stages of woodlands are scarce because of recurrent fires, and matorral, and sometimes xerophytic grasslands, still often dominate insular landscapes. In Greece, the situation has been of particular concern for the last few decades given the increase in frequent intense wildfires. On Thasos Island, for example, a series of wildfires since 1984 has reduced forest cover from 61.6% to almost 20% (Ranis et al., 2015). Furthermore, there often is spatial congruence between the most fire-affected micro-regions and the main cattle-rearing regions, like in Corsica. On this fire-prone island, 28,000 starting fires occurred between 1973-2004 (i.e. 1000 starting fires /year) and a third of the total surface area of Corsica was burned in a period of 30 years.

20Fire regime changes and the occurrence of extreme fire events (or “megafires”) are related to both land use change and climate change, and involve multiple biotic and socio-economic drivers (e.g. Pausas & Keeley, 2014). It is difficult to disentangle their relative importance because the management of Mediterranean landscapes plays a major role. In the western Mediterranean basin, a fire regime shift has occurred and fires are now less fuel limited and more drought-driven than before the 1970s (Pausas & Fernández-Muñoz, 2012). In the future, increased drought could increase fire activity on most Mediterranean islands, with secondary effects of land degradation and erosion.

21The need to combine a multi-factorial approach is illustrated by the case of Abies cephalonica on the Greek island of Kefalonia (Politi et al., 2011). The decrease of these fir populations has been attributed to different causes such as root damage, infestation by mistletoe, pathogens or insects, and more intense and more frequent extreme drought events probably in relation with the climatic warming trend,. Extreme drought events contribute to the recent increase in fire episodes spreading at high altitudes and are threatening non-fire resilient species whose future persistence is jeopardized in such insular situations.

Conclusion

22Phylogenetical and phylogeographical studies have demonstrated the complex historical biogeography of the Mediterranean Basin and also the importance of islands as reservoirs of unique genetic lineages, notably for most endemics and narrowly distributed plants (Médail & Diadema, 2009). Nevertheless, the time frame and evolutionary consequences of biogeographical events linked to repeated cycles of island connections and isolation, in relation to marine regressions-transgressions, remain largely unknown (Mansion et al., 2008). This is of particularly concern for efficient evolutionary conservation of these heterogeneous insular floras.

23With the biome crisis of the Mediterranean basin (Hoekstra et al. 2005), islands constitute key ecological systems to ensure the preservation of coastal plant biodiversity. And while insular systems still represent fascinating ecological systems, they are also key entities to disentangle the role of environmental versus human pressures in the long-term preservation of these biodiversity hot-spots. Because of increased threats across the Mediterranean region and the complex consequences of climate change (Klausmeyer & Shaw, 2009), it is crucial to observe, monitor, and analyze changes in vegetation and plant biodiversity across ecological and biogeographical gradients. Mediterranean islands, notably the small ones, are favorable sites for such long-term observations as well as for monitoring at various spatial scales. These “natural insular microcosms” are indeed appropriate systems to study adaptation to climate change by species or communities, and the functional biogeography approach (Violle et al. 2014) is undoubtedly an interesting topic that needs further research.

24The diversity of situations facing Mediterranean islands should facilitate their integration as laboratories or testing grounds of extinction in relation to global change and human pressures. To this end, it would be useful to combine reactive approaches on the most threatened (often largest) islands, and proactive approaches on relatively less threatened islands (notably small islands and islets). Multi-disciplinary collaboration among prehistorians, archaeologists, palaeoecologists, historians, socio-economists, soil scientists, ecologists and biogeographers is needed to disentangle the complex interactions between past human societies and insular environments. Enhancing these interdisciplinary research efforts is a prerequisite for the design of sound policies and practices concerning the conservation of these unique and fragile insular floras and plant communities (Médail, 2013). The smallest islands should not be neglected, as they are often isolated territories where micro-speciation processes occurs, offering modern refuge-areas for diversity that is put at risk by the impacts of human activity on the coasts of the adjacent mainland.

25Owing to their high biotic originality and vulnerability to global change, Mediterranean islands and islets urgently require integrated and ambitious conservation planning aimed at the long-term preservation of their outstanding biodiversity and cultural heritage.

Acknowledgments

26Some data concerning the small islands off eastern Tunisia and Corsica were obtained during field missions funded by the PIM Initiative (Small Mediterranean Islands Initiative) of the French Conservatoire du Littoral and by the Tunisian agency APAL (Agence de protection et d’aménagement du littoral) in 2014-2015. The Inititiative d’excellence Amidex of Aix-Marseille University also participated in the funding of some of these field trips through the MedNet project (2013-2015).

27I thank my colleague Pr. Wolfgang Cramer (IMBE) for his invitation to write this short review and for his useful comments on the manuscript.

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Table des illustrations

Légende Photos 1Example of small rocky islands. Left: Capense island in N. Corsica (Cap Corse, Centuri), V. 2015 Right: Es Vedrà island, a small (640 ha) island with an impressive peak (384 m) emerging from the sea SE of Ibiza island, IV. 2014 (F. Médail).
URL http://books.openedition.org/irdeditions/docannexe/image/23658/img-1.jpg
Fichier image/jpeg, 165k
Légende Figure 1Schematic distribution of major vegetation types in the Mediterranean region, according to phytoecological and bioclimatical criteria, notably the mean minimum temperatures of the coldest month (m) of the year (see Médail, 2008).
URL http://books.openedition.org/irdeditions/docannexe/image/23658/img-2.jpg
Fichier image/jpeg, 207k
Titre Table 1. Indigenous plant richness and endemism level of the six largest Mediterranean islands (Médail unpublished data, various sources).
URL http://books.openedition.org/irdeditions/docannexe/image/23658/img-3.jpg
Fichier image/jpeg, 155k
Légende Figure 2The highly threatened narrow endemic Naufraga balearica (Apiaceae) located in the north-western part of the island of Majorca, with the distribution of the three haplotypes across the five sampled populations (a), the phylogenetic consensus tree (b) and the statistical parsimony network (c) of plastid DNA haplotypes showing the ancestral haplotype A (Fernández-Mazuecos et al. 2014).
URL http://books.openedition.org/irdeditions/docannexe/image/23658/img-4.jpg
Fichier image/jpeg, 191k
URL http://books.openedition.org/irdeditions/docannexe/image/23658/img-5.jpg
Fichier image/jpeg, 228k
Légende Photos 3Typical vegetation of the large satellite islands of the Kerkennah archipelago (E. Tunisia). Left: open steppe with Lygeum spartum (Poaceae); Right: halophilous shruby vegetation on sebkhras with Arthrocnemum macrostachyum and Sarcocornia fruticosa (Amaranthaceae) (F. Médail).
URL http://books.openedition.org/irdeditions/docannexe/image/23658/img-6.jpg
Fichier image/jpeg, 187k

Auteur

IMBE, Aix-Marseille University, France
Plant ecologist, conservationist and biogeographer, Institut méditerranéen de biodiversité et d’écologie marine et continentale (IMBE), Aix-Marseille Université, CNRS, IRD, Université d’Avignon
frederic.medail@imbe.fr

© IRD Éditions, 2016

Conditions d’utilisation : http://www.openedition.org/6540