Prospects for marine invasions by non-indigenous species

This page is part of the topic Marine biology over the next 100 years

Establishment of non-indigenous species (NIS) is widely considered one of the greatest threats to biodiversity and endemic species. The threat these invaders pose once they have arrived in a new location, established themselves, and begun successfully to reproduce, is to outcompete native species for food or space, eat them or even hybridise with some. This has happened all over the planet, on all continents and most islands (even around Antarctica), across land, freshwater and marine habitats. The result is that over long time periods species have increased, decreased or otherwise altered the geography of their distributions. In the last few centuries, humans have radically altered organismal transport vectors, frequencies, journey times and survival prospects. Of the thousands of species travelling, probably only a small fraction survive and, of those, again only few establish themselves and yet fewer become ‘invasive pests’. Even in these few aggressive invasive NIS, there is often a long lag phase between arrival and becoming a pest. Once established in terrestrial habitats, NIS have proved very difficult, often practically impossible to remove – except in the case of large mammals from small islands. In the sea, though, there is not a single case of an invading NIS being successfully removed. As a result, the global fauna is becoming more homogeneous as these few winners spread from port to port and the many losers will be native species with restricted distributions (endemics).

The potential for NIS arrivals to affect Southern Ocean biodiversity in the coming century is considerable because of: 1) historic Southern Ocean isolation and its domination by endemic species 2) lack of established NIS and the ‘pristine’ nature of the environment; 3) the slow response time of native organisms because of their extended generation turnover times; 4) the lack of durophagus predators (consuming prey with hard shells and bones); and 5) accelerating transport opportunities for NIS in a region of intense warming. These points are considered in turn below.

Firstly the Southern Ocean is the largest marine environment to be ‘semi-isolated’ for long periods of time - by surrounding deep water, the ACC and the Polar Frontal Zone (PFZ) (see Clarke et al., 2005). As a result most marine species and many genera are endemic to one or more regions within the Southern Ocean (Arntz et al., 1997). Their loss regionally thus would also mean a global loss. Experience of NIS arrival in regions with high endemism is that invasion tends to lead to drastic reduction and extinction of many endemic species, at least in terrestrial and fresh water environments. No NIS fauna have yet been found to be established in the Southern Ocean, although several species of assumed NIS algae now grow inside the caldera of Deception Island (Clayton et al., 1997; C. Wienke pers. com.). Recently the North Atlantic spider crab (Hyas araneus) has been recorded from the Antarctic Peninsula (Tavares and Melo, 2004), as have larvae of related subantarctic species (Thatje and Fuentes, 2003), but to date none are believed to be established. However, species have been found travelling in or close to the Southern Ocean in ballast water (Lewis et al., 2003) or fouling ship hulls (Lewis et al., 2006) or marine debris (Barnes and Fraser, 2003). Most recently, Lee and Chown (2007) found that mussels (Mytilus galloprovincialis) had survived a journey to and from the Southern Ocean on the ship Agulhas. This is an extremely aggressive invader that can smother coastal life in the absence of predation and that has been found to successfully breed at just 1ºC (Lewis, unpublished data).

Macro-organisms native to the Southern Ocean tend to be characterised by slow development, growth and generational turn over (Arntz et al., 1994). The few species for which we have ample age spectra grow very old (over many decades) and many do not reach the age of first breeding for many years. This means that their ability to respond ecologically to competition for space or food is poor. Their long generation time also drastically slows rates of adaptation compared with potential invaders from temperate regions. Even the long lag times taken by many NIS take before they spread aggressively are well within the span of a single generation of some common Antarctic species. A closely linked (and fourth) point is the potential influence NIS durophagus (crushing) predators would have on Antarctic benthos. Southern Ocean shelf communities show many resemblances to community structure in Tertiary times, for example in lacking durophagus predators and having many shallow echinoderm suspension feeders (see Aronson et al., 2007). Invasion of even a few crushing predators could cause major changes to communities not adapted to such predation. Both the survival of such NIS and the ability of Antarctica’s indigenous species to respond depend on the extent of regional warming.

The fifth point about the vulnerability of Antarctic marine biodiversity to NIS concerns increasing transport opportunities offered by rapid warming, both in the Scotia Arc and West Antarctic Peninsula region. This is the area most visited by tourist and scientific ships – which are amongst the most likely vectors for marine NIS (Lewis et al., 2003, 2006). It is also the area of most intense warming, and the only region within the Antarctic to show physiologically meaningful sea temperature increases to date (Meredith and King, 2005). Even the slight surface warming projected to occur in the next century, at least regionally, may decrease the ability of some native Southern Ocean fauna to function – e.g. to avoid predators (Peck et al., 2004). Conversely, rises in temperature might significantly raise the survival chances and competitiveness of temperate NIS. Crucially some crushing predators, like brachyuran crabs require slightly higher temperatures than those currently prevalent. Rising CO₂ levels in the ocean may also result in secretion of aragonite skeletons (shells) becoming more difficult, which would be more of a problem if NIS durophagus predators were to become established.

Potential invasions of NIS must be considered in the context of past species transport on various time scales. Species have moved into and out of the Southern Ocean both on evolutionary and ecological time scales (see Barnes et al., 2006). This will have occurred to some species even without them physically moving, simply because the Polar Front will have moved back and forth across their habitats between glacial and interglacial cycles. For example benthos on the shelf around the Kerguelen Islands will have been inside and outside the Polar Front multiple times (see Moore et al., 1999). It is likely that considerable movements of species occurred in response to glacial-interglacial cycles. It is therefore possible that there may be many species currently outside the Polar Front that will return, not as NIS, but as natives expelled during glacial maxima. The poor fossilisation conditions and destruction of potential fossils by advancing ice sheets during the onsets of glaciations makes it hard for us to know definitively which species are native. Another potential problem with recognising invading NIS is the patchy level of knowledge of Southern Ocean biodiversity. Arguably the most likely arrival areas of NIS do, however, coincide with the best-known regions (the shallow shelf, the Scotia arc, and the western Antarctic Peninsula). Despite the deep water and oceanographic barriers between Antarctic and temperate environments, there are many mechanisms for transport (Clarke et al., 2005; Barnes et al., 2006). Organisms may travel on floating debris, such as floating volcanic rock - pumice and driftwood. Some organisms may hitch-hike on megafauna (such as fur seals) and diseases can be introduced especially by highly mobile animals - such as avian influenza by albatrosses. Shipping has undoubtedly drastically increased opportunities for NIS, because of rapid travel from temperate ports (hotspots of invasive NIS) and across oceanographic barriers. Both Lewis et al. (2006) and Lee and Chown (2007) have found known invasive NIS associated with ships in the region – it seems that it is only a matter of time before the first established invading animal is found. How the native fauna will respond to such an invasion will depend on the chance nature of the invader identity, the area it arrives at (isolated island or Western Antarctic Peninsula) and the pace of climate change.