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Marine invasive species

What are they?

Alien species — sometimes termed exotic, introduced or
non-native species — are plants and animals that have
been intentionally or unintentionally introduced, have
established populations and have spread into the wild in the
new host region (IUCN, 2002). In their home ranges, these
species live in balance with their local native environment,
and populations are controlled by ecosystem interactions
such as predation, parasitism and disease. However, once
they arrive in a new environment, they may become
established and invasive.

Following the IUCN definition, also adopted by the
Convention on Biological Diversity, ‘invasive alien species’
(IAS, often abbreviated to ‘invasive species’) are those alien
species which become established in natural or semi-natural
ecosystems or habitats and become an agent of change,
increasing in abundance and distribution and threatening
native biological diversity (IUCN, revised 2012). IAS are
introduced outside their natural range by human action, Codium fragile subsp. fragile. Photo: J.C. Calvin - OCEANA
either direct or indirect, and can cause harm to biodiversity
or ecosystem services by competing with and on some
occasions replacing native species, and causing complex contain. These characteristics include the capacity to thrive
changes within the structure and function of the new hosting in different environments and tolerate a wide range of
ecosystem (Galil, 2007, 2009). Invasive species often owe environmental conditions, high growth and reproduction
their success in colonizing new ecosystems to certain rates, a lack of natural predators and an ability to exploit a
characteristics that make them more difficult to control and variety of food sources.

Fig. 1. Schematic representation of major barriers limiting the spread of introduced alien species. The barriers are: (A) geographical
barrier(s), (B) Captivity or cultivation (for those species), (C) Survival and reproduction barriers, (D) Local/regional dispersal barriers; and
E) Environmental barrier(s). Arrows A through E indicates the paths followed by different species to reach different states from
introduced alien to invasive species. From Blackburn et al., 2011.

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