Marine Invasions in New Zealand: A History of Complex Supply-Side Dynamics

New Zealand's recent ecological history is often held up as a textbook example of the havoc that can be wrought by non-native species (Clout and Lowe 2000). The first human inhabitants of New Zealand arrived (by boat) just 800 years ago, and brought with them food crops and dogs. They arrived in a country where, already, many elements of the endemic fauna were in serious decline; an apparent legacy of the introduction of the Polynesian rat (Rattus exulans: ‘kiore’) by transient human visitors, some 1000 years before (Holdaway et al. 2002). In the eighteenth and nineteenth centuries, British immigrants to New Zealand brought with them a wave of new predators, plant pests and grazing animals. When Charles Darwin stopped in NZ on his Beagle voyage in 1835, the settled European population in NZ numbered fewer than 2000 but Darwin lamented the rampant spread of “very troublesome” weeds which had already “overrun whole districts” and the loss of native flightless birds, “annihilated” sic by introduced Norway rats (Rattus norvegicus) (Darwin 1889). Now, 170 years later, there are more than 4 million human inhabitants and 25,000 introduced plant species in New Zealand, with established exotics outnumbering native species (Beston 2005; NZ Plant Conservation Network 2006).

Introduced species have significantly altered the natural landscape and ecological functioning of New Zealand's environments. Deliberate and accidental introductions of organisms continue to occur at an alarming rate. In this chapter we discuss the status of marine invasions in NZ, some of the impediments to accurately defining that status and the importance of taking account of “supplyside” dynamics when assessing the risks of new introductions. “Supply-side” ecology is the term introduced into marine ecology in the late 1980s to describe the study of the processes of arrival of new members of populations (see also Johnston et al. this volume). Its importance was to re-emphasise the consequences of variability in the supply of recruits to adult populations. While not new, the concept served to refocus attention on the dynamics of reproductive success, oceanographic influences on dispersal, larval behaviour, the process of settlement, and features of the receiving environment that cause variations in numbers of recruits from place to place and time to time (Underwood and Fairweather 1989). Supply-side dynamics are also an important part of marine invasions because establishment of new populations of introduced species is contingent on variability in the same processes described above that influence the success of new recruits. However, because human activities spread marine invaders, the supply-side of the equation also encompasses variability in the transport pathways in which the species are carried to new environments. We use data on changing trade patterns in New Zealand to demonstrate the importance of including that aspect of supply-side dynamics into assessments of incursion risk. Other equally important processes that form part of supply-side ecology, such as dispersal of propagules and settlement success, are not discussed in this chapter.