When native species turn alien

In Australia we’re used to the idea of plants and animals bought from overseas causing havoc in our local environment. Carp and cane toads “spring” to mind.  What receives less attention are the local species that become pests or “native invaders”*.

Invasive species on land, waterways or oceans are species that establish in a new area and have a measured impact on the ecology and economy of the recipient ecosystem. However, human activities such as wildfire suppression, urbanization, predator removal, stocking for recreational opportunities, and climate change can change how ecosystems work. This is when native species can become native invaders.

The US Pacific Northwest provides a good example of the complexities in dealing with native invaders. In the first place it is not straightforward to define when a species has become a native invader or to predict which one will be next. This is often due to a lack of data.

The population of Caspian terns (a species of seabird), for example, has doubled since 1980. Dam construction and waterway dredging have created artificial islands which provide excellent nesting habitat free from native predators. Local salmon hatcheries also provide abundant food for the terns.

A Caspian tern (Hydroprogne caspia)

A Caspian tern (Hydroprogne caspia). Photo by Robert Steffens

The Northern pikeminnow (a local fish) has also experienced a boom in numbers. This is caused by the construction of dams and the creation of reservoirs which provide more rearing habitat for the pikeminnow.

For these species the ecological impact of concern is on endangered salmon species. For example, through high investment in research, scientists were able to establish that Caspian terns consume up to 15% of juvenile endangered salmon populations in the Columbia River estuary.

Managers dealing with native invasions often only address the symptoms rather than the cause. With the northern pikeminnow, anglers have been offered a reward of $4 – $8 per fish. Between 1998 and 2009 more than 2.2 million fish were harvested. However, stream flow regulation for hydropower generation still creates the artificial conditions that promote high reproduction in pikeminnow.

The northern pikeminnow (Ptychocheilus oregonensis)

The northern pikeminnow (Ptychocheilus oregonensis)

Likewise policy makers often need to juggle competing interests and the different values society has for species. For example, while the native northern pikeminnow are aggressively removed, non-native bass species, with impacts on salmon similar to those of pikeminnow, are maintained or even promoted for recreational fisheries.

Ultimately we need to decide as a society what is socially and ecologically acceptable and have the data available to inform our decisions.

*The full article on which this piece is based is: Native invaders – challenges for science, management, policy, and society. By Michael P Carey, Beth L Sanderson, Katie A Barnas, and Julian D Olden. Frontiers in Ecology and the Environment 2012; 10(7): 373-381

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