Marine ecosystems contain many species of plants, animals and microorganisms that have evolved in isolation, separated by natural barriers. Human transportation activities such as shipping and air travel have allowed these species to move beyond their natural ranges into new areas. An invasive species is defined as a species that is non-native (alien) to an ecosystem and/or whose introduction causes, or is likely to cause social, economic, or environmental harm. Native species may also become invasive if they cause harm by dominant colonization of an ecosystem due to loss of natural controls (i.e., loss of predators or herbivores).
Marine invasive species include:
Eighty-four percent of the world’s marine ecosystems are already impacted by invasive species. ref Biogeographically isolated communities, such as Hawaii’s ecosystems, are more susceptible to the introduction of invasive species.
Not all non-native species will become invasive. While marine invasive species vary considerably, a number of general characteristics apply. ref Invasive species have:
- Capability of adapting to and thriving in different habitats and a wide range of conditions
- Rapid growth rates of individuals, thus making them able to displace other plants or animals
- Characteristics that make them easily dispersible to new localities
- Reproductive characteristics that allow for rapid population growth
Invasive species are often able to establish and overtake new environments quickly. This is because their populations are unlikely to be kept in check by natural processes such as control by predators, parasites, or disease.
Pathways of Introductions of Marine Invasive Species
Invasive species can be introduced into the marine environment a number of ways: ref
- Commercial shipping — Cargo vessels, fishing boats, towed platforms
- Ballast water: Seawater that is used to stabilize cargo ships. If a ship has no cargo, it is filled with water to stabilize it. When it has cargo, the ballast tanks are emptied. Water loaded at one port (with many of marine species in it) may be unloaded at another port (dumping these species into a new environment). Some of these species become invasive. ref
- Vessel hulls: A hull is the underside of a vessel, which provides a surface that species are able to settle on and attach to. When the vessels move, these species are transported to other areas of the world. This applies to all vessels, including cargo ships, sailing ships and fishing vessels. ref
- Live holding and bait wells (containers to keep fish alive on boats)
- Fisheries gear and debris
- Recreational boating — Hull fouling and fouling through other factors (e.g., outboard motors, live wells, water lines)
- Aquaculture, aquarium, water gardens
- Accidental release of target organisms from culture/grow-out facilities (some species are intentionally introduced in capacity for food and then escape into the wild and become established)
- Accidental release of non-target organisms
- Intentional release of organisms (usually through consumers or hobbyists)
- Government programs and research — Authorized release (biocontrol) and unauthorized/unintentional release
- Private sector
- Live seafood shipments (e.g., the Live Reef Food Fish Trade – non-native marine species can be bought and intentionally or accidentally released into local waters)
- Aquarium release
- Release for cultural practices
- Illegal or accidental imports
- Marine debris — Fouling organisms on abandoned nets and floats
Invasive species are considered one of the greatest threats to our marine ecosystems. Marine invasives have had significant impacts on biodiversity, ecosystems, fisheries, and mariculture (breeding and farming marine organisms for human consumption), human health, industrial development and infrastructure.
Climate change may exacerbate the spread of invasive marine species. For example, climate change may result in changes in patterns of human transport (longer shipping seasons, new routes), changes in climatic restraints which favor invasive species or increase the possibility of their survival, and altered species distributions (for example, range shifts in response to temperature increases). ref Range shifts in response to increasing temperature are likely because studies have determined that some invasives have superior ability to tolerate increased temperatures compared to native species. ref This ability may partly explain why the number of new invasions is increasing.
Managing invasive species can be a major challenge. Prevention is the most cost-effective approach for managers, for once invasive species are established in marine ecosystems, it can be nearly impossible to eliminate them. ref