ABSTRACT
In Earth’s history, having two frozen polar regions is unusual, but icy coasts are not marginal habitats. Coastlines currently experiencing freezing sea temperatures or floating ice constitute 30% globe and 50% during the last glacial maximum (Bond et al. 1992). The two polar regions not only experience freezing sea temperatures but also seasonally intense UV irradiation and most obviously extreme light climates and associated primary productivity (see Dayton 1990). At these high polar latitudes severe wind speeds, wave action, ice scour and anchor ice (as well as massive fresh water runoff and localised anoxia in the Arctic) make the nearshore environment the most disturbed anywhere (Arntz et al. 1994, Gutt & Pipenburg 2003). On land, in fresh water and in the intertidal zone there are few colonist species but just a few metres deeper in the sea there can be rich, diverse and abundant benthos even in shallow water (Dick & Ross 1988, Barnes 1995a, Barnes & Kuklinksi 2003). In comparison with other major regions of the globe the Arctic does appear to be species poor, though this
may simply be due to its youth and being still in the process of invasion (Dunton 1992). In contrast there is no evidence that the Antarctic benthic environment is impoverished compared with other latitudes (Clarke 1992) and some shelf areas are highly rich (Galeron et al. 1992, Brey et al. 1994). Amongst the most abundant shallow water benthos are lithophyllic polychaetes, cheilostome and cyclostome bryozoans, sponges and, in the arctic, barnacles. Although these taxa occur elsewhere at lower latitudes there is a very distinct polar fauna in many ways. Firstly, to the casual observer life between the tides seems to have disappeared and that below the tides seems very still: most animals are sessile or sedentary – even predators are mainly sluggish (Arntz et al. 1994). Secondly, some unusual animal types such as brachiopods and pycnogonans are very well represented, others such as the gastropods and especially the decapod and balanomorph crustaceans are not (Clarke & Johnston 2003). Thirdly, most Antarctic (though not Arctic) species are endemic, nowhere-else are levels of endemism as high. Fourthly, representatives of some taxa are giant compared to elsewhere – due to the
David K.A. Barnes British Antarctic Survey, NERC, Cambridge, United Kingdom
ABSTRACT: Nearshore animals living in the two polar regions have much in common, they experience extreme light climates and associated primary productivity, freezing sea temperatures and seasonally intense ice disturbance and UV irradiation. Arctic and Antarctic shallow water environments and their communities have important differences from each other, though, as well as from those in low latitude. Life is tough on land and on the shore, the fauna is small and scarce but not so in the sea. Bryozoans are highly ecologically successful in shallow waters in terms of species richness, abundance and biomass. In this paper critical influences on life and death of polar bryozoans are reviewed as well as the importance of competition. Closer inspection of seemingly barren boulders reveals a bustle of pioneer species fighting for their undersurfaces. The organisation of these battles is extremely hierarchical: at any given locality one species is overgrown by all others and one species overgrows all others – everyone else occupies a rigid rank in between. With no keystone predators to remove competitive dominant species only the catastrophically destructive power of ice and waves prevents monoculture of certain species. In ice-sheletered areas, such as crevices the end point of classically envisaged ‘succession’ can be seen. In these shallow water environments many animal populations display exactly the converse of characters typically associated with the polar regions. The most abundant species of many clades are the rarer broadcast spawners with pelagic larvae, that grow and reproduce fast (for polar animals) are small and have but brief lifespans. Many of these contrasts can be seen in the representatives of just one phylum – the Bryozoa. Rather than the predicted K selected species of deeper waters the shallows are ruled by lightly calcified pioneers. Here ecological and evolutionary success have become very much decoupled. A 2°C rise, predicted in polar waters, could be enough to transform this unique zone in our lifetimes.
