Loss or gain? Invasive aliens and biodiversity in the Mediterranean Sea

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Abstract

More than 500 alien species were listed from the Mediterranean Sea. Though no extinction of a native species is known, sudden decline in abundance, and even local extirpations, concurrent with proliferation of aliens, had been recorded. Examination of the profound ecological impacts of some of the most conspicuous invasive alien species underscores their role, among multiple anthropogenic stressors, in altering the infralittoral communities. Local population losses and niche contraction of native species may not induce immediate extirpation, but they augur reduction of genetic diversity, loss of functions, processes, and habitat structure, increase the risk of decline and extinction, and lead to biotic homogenization. The relevant environmental policy and management framework is discussed.

Introduction

Invasive alien species are increasingly seen by scientists and policy makers as a major threat: “Invasions … are now widely recognized as one of the most significant components of global change, with far reaching and often harmful effects on biodiversity” (www.eupolitix.com). The “Jakarta Mandate on Marine and Coastal Biological Diversity”, adopted by the Parties to the “Convention on Biological Diversity” (CBD), cites “invasion of exotic species” as one of the five main categories of the anthropogenic impact on marine and coastal biota (www.biodiv.org). Marine invasions are recognized as imperiling global “biodiversity, marine industries (including fishing and tourism) and human health” (Bax et al., 2003). Biodiversity in this context is understood as more than the number of species in an area, it is a conglomeration including genetic variability, species interactions and ecosystem processes. The CBD defines biodiversity as “the variability among living organisms from all sources… and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.” (www.biodiv.org). However, it has been suggested recently that whereas invasive species are indeed “altering many communities and ecosystems… Evidence supporting a general and primary role for invasive aliens in extinctions remains limited.” (Gurevitch and Padilla, 2004).

Alien macrophytes, invertebrates and fish – over 500 species have been recorded thus far – are prominent in most coastal habitats in the Mediterranean (Galil, in press). It is assumed this is only a partial inventory, as our ignorance of the marine biota leads to “massive underreporting and thus understatement of … the altered distributions of nonindigenous species” (Carlton, 2000): a distinct size bias is apparent in the Mediterranean records, and data is entirely absent for many of the small-sized invertebrate taxa. With few exceptions, the ecological impact of invasive alien species on the native Mediterranean biota is poorly known (Zibrowius, 1992, Boudouresque, 2004), though it is believed that keystone invasive species may cause major shifts in community composition. In this paper, the reports on the ecological impacts of invasive marine alien species in the Mediterranean are reviewed, and biodiversity loss is discussed.

Section snippets

Impacts of invasive alien species

The loss of marine biodiversity in the Mediterranean is of major concern, though no extinction of native species has been recorded (Boudouresque, 2004). Little is known about the kaleidoscopic inter-relationships of native and alien biota in the Mediterranean Sea, hindering thorough evaluation and direct tests of competition leading to niche limitation, displacement or extirpation (local extinction). The documented instances of sudden concurrent changes in abundance, where populations of native

Loss or gain?

Confronted by adversity, a common reaction is seeking refuge in denial. Faced with the preponderance of Erythrean biota in the Southeastern Mediterranean, the risks of invasion were initially dismissed – “Other than ….. Asterina gibbosa there is no known case in which a Lessepsian migrant species has completely replaced a local one” (Por, 1978, p. 149), or entirely denied “The Lessepsian migrants may be considered, in a figurative sense “welcome guests” in the impoverished, subtropical

Ou Allons Nous?

The Conference of Parties to the Convention on Biological Diversity called upon governments to act ‘to prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species’ (CBD 1992, Article 8h), but preventing alien species introductions is a task which needs scientific, administrative and political coordination at the regional, if not international, level. The results thus far have been disheartening.

The International Maritime Organization

Acknowledgements

A. Crosnier, Museum national d’Histoire naturelle, Paris, and P. Clark, the Natural History Museum, London, hosted me with the greatest kindness. I am grateful to G. Ruiz for his remarks on an earlier version of this manuscript. Special thanks to the librarians of the American Museum of Natural History, New York, and Israel Oceanographic & Limnological Research, Haifa. This review has been supported by the European Commission’s 6th Framework Programme DAISIE (contract SSPI-CT-2003-511202).

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