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Arachnid Apartheid

What South Africa did to New Zealand's All Blacks and Silver Ferns -- displace them from dominance -- it's also doing to a tiny, native all black, Latrodectus katipo.

The katipo -- New Zealand's best known poisonous spider and its only native one -- seems to be losing some of its traditional habitats to a South African immigrant, Steatoda capensis.

Lincoln University arachnologist Cor Vink says that many well known local katipo habitats are now virtually bereft of the glossy black spiders. In their place is Steatoda capensis.

"At Brighton there used to be three to four katipo per square metre, but on searches today it's hard to find a single specimen.

"Brighton is experiencing what has been noted in North Island areas such as from Paekakariki to Waikanae -- the disappearance of the katipo and its replacement by another species."

But while Vink has found an abundance of S. capensis in the old katipo habitats at Brighton, and North Island researchers report the same phenomenon, spider specialists cannot explain the precise mechanism of displacement.

One theory, explored but exploded, suggested that the katipo was competitively inferior to S. capensis. A more likely explanation lies in the superior ability of S. capensis to recolonise habitats after storm damage or human interference. S. capensis seems able to recolonise vacant habitats more rapidly than the katipo for a number of reasons -- there is a source of "immigrants" available from inland, which is not true of the katipo; the reproductive rate of S. capensis is higher than that of the katipo; and S. capensis can reproduce year round whereas katipo produce few egg sacs in winter.

From this it is argued that through its ability to colonise habitats more rapidly, S. capensis becomes the dominant established species and thus displaces the katipo.

Furthermore, says Vink, as S. capensis is an introduced species this phenomenon may be relatively new and, combined with increasing human interference along coastal areas, it is likely that the katipo will continue to decline in areas in which it was once common.

While the disappearance of a poisonous spider might not be lamented in all quarters, it saddens Vink.

"The loss of any species, and particularly a native one, is a blow to biodiversity," he says.