Life, death and fighting at high latitudes: a review

In earth's history, having two frozen polar regions is unusual. Not only do these regions experience extreme light climates and associated primary productivity, but freezing sea temperatures and seasonally intense UV irradiation. 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. On land, in fresh water and in the intertidal zone there are few colonist species but just a few meters deeper in the sea there can be rich, diverse and abundant benthos even in shallow water. The severity of the physical environment is reflected in the interactions in the biological sphere. Amongst the most abundant shallow water benthos are lithophyllic polychaetes, bryozoans and sponges and in these communities its overgrow or be overgrown. The organisation of sessile animals is extremely hierarchical: at any given locality one species is overgrown by all others and one species overgrows all others - everyone else occupies a 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.

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