11 August 1995 Migrant Spider In Katipo Takeover

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

The katipo – New Zealand's best known, and until recently only, poisonous spider, seems to be losing some of its traditional habitats to a migrant South African spider, Steatoda capensis.

Lincoln University arachnologist Cor Vink, of the Department of Entomology and Animal Ecology, notes that many well-known local katipo habitats are now virtually bereft of the glossy black spiders. And in their place is Steatoda capensis!

"For example at Brighton (across Christchurch city from Lincoln University) there used to be three to four katipo per square metre, but on searches today it's hard to find a single specimen," he says.

"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 Mr Vink has found an abundance of Steatoda 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 re-colonise habitats after storm damage or human interference, says an earlier researcher.

S. capensis seems able to re-colonise vacant habitats more rapidly than L. 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 L. katipo; and S. capensis can reproduce year round whereas L. katipo produce very few egg sacs in the winter months.

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

Furthermore, says the researcher, 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 and finally be totally displaced by S. capensis.

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

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

"Ideally you want to keep as many species alive as possible because all species have evolved to fill a particular niche and of course extinction is irreversible!"

Ian Collins, Journalist, Lincoln University, Canterbury, New Zealand.