A review of the impacts from invasion by the introduced kelp Undaria pinnatifida

Abstract

Invasive species are those which have evolved in one location and are introduced, often by human activities, to another location where they successfully establish, spread and cause harm to the receiving Environment. Invasive species pose one of the foremost threats to marine ecosystems. An increasing number of marine ecosystems, primarily near shorelines, are being altered or even destroyed by invasive species. Undaria pinnatifida is a highly successful invasive kelp species. It is regarded as one of the worst invasive species in the world. Undaria is native to the northwest Pacific and is now invasive to more than fourteen countries around the globe. Undaria has a set of attributes common to highly invasive species; it is easily spread and transported, can colonise a wide range of substrata, has a broad ecological niche and a typically weedy life history. Under suitable conditions Undaria can spread rapidly and form dense monospecific stands, with lifecycle and growth strategies varying from winter annual to year round dependant on local temperature regimes. Undaria can substantially alter natural habitats and change community structure and trophic interactions. Undaria invasion often represents an addition to canopy cover at invaded sites, or invades sites naturally devoid of native canopy-forming species, where it can create a completely novel biogenic habitat. Undaria invasion can significantly increase local primary productivity as the high biomass turnover associated with the short life span of Undaria sporophytes increases carbon export to the surrounding ecosystem. Where Undaria forms dense stands it can reduce the presence and diversity of smaller understory algal species and can in certain circumstances out-compete some canopy-forming species. Undaria invasion increases food and habitat availability for some species, including epibiotic communities. The epibionts associated with Undaria can be different and less diverse than those associated with native algal species and this has the potential to significantly affect the flux of materials to higher trophic levels. Macrofaunal assemblages found in association with Undaria habitat can also be different from those associated with un-invaded sites. Undaria invasion can result in an increase in the abundance of grazers, including gastropods, urchins and crabs, as well as fish. Such changes can further affect local food chains by increasing the abundance of food for predators. The winter annual growth cycle of Undaria at many sites means carbon inputs and interactions with higher trophic levels will be temporally variable. An increase in fauna associated with Undaria habitat during its growth season could result in a lack of food or habitat for increased numbers of associated fauna when Undaria dies off during summer. This may in turn lead to effects from grazing pressure on native algal species post Undaria season. Whilst there is evidence to illustrate how Undaria causes changes to community structure and function, quantifying ecological impacts can be difficult where there is a lack of baseline ecological data. Furthermore, separating changes caused by Undaria invasion from natural variation, over space and time, is a challenging task. In order to more effectively quantify impacts from Undaria invasion more manipulative experiments, including those which assess trophic interactions, are needed. However, the effects currently reported from surveys and small scale experiments illustrate how Undaria invasion can have significant ecological impacts. In addition, responses of Undaria to climate change, and the interactions of climate change with other stressors, will further alter ecological impacts from Undaria. The presence and impacts from Undaria invasion undermine efforts to conserve biodiversity and representative native ecosystems in particular. Efforts should remain in place to protect important high value areas, such as marine protected areas, from Undaria invasion.