Web site characteristics, dispersal and species status of New Zealand's katipo spiders, Latrodectus katipo and L. atritus

Abstract

Over the last 30 years, Latrodectus katipo Powell 1870 and L. atritus Urquhart 1890 numbers have declined and these species are now absent from many dune systems where they were once common. The cause of the decline in L. katipo and L. atritus numbers is not certain and little is known about their ecology. This thesis examined L. katipo and L. atritus web site characteristics, dispersal, species status and the probable implications of habitat modification on their ecology. The thesis focused on these aspects of L. katipo and L.atritus in order to provide information that might be used to conserve these species.

Results showed that L. katipo and L. atritus web sites are defined by similar characteristics. Both species were commonly found in Muehlenbeckia complexa A. Cunn., 1838, Coprosma acerosa A. Cunn., 1839, and Desmoschoenus spiralis (A. Rich.), but were also found in driftwood, Spinifex sericeus (R. Br. 1810) and Ammophila arenaria Link 1827. Web sites were most commonly associated with dune regions defined by 33-66 % ground cover, northerly, easterly or westerly aspect, sloping ground and the absence of detritus.

Using habitat classification trees, a model was constructed that was useful for predicting L. katipo and L. atritus presence and absence. The model accurately predicted L. katipo or L. atritus absence at > 90% of points sampled, but less accurately predicted L. katipo or L. atritus presence at 33 - 38 % of points sampled. It is argued that model was less able to predict L. katipo or L. atritus presence because intra-specific competition among the two species or inter-specific competition with Steatoda capensis Hann 1990 might prevent greater use of potential web sites in optimal habitat. Alternatively, the model may not accurately define web sites. This assertion is supported by results that suggest web sites might be associated with patches of open sand rather than ground cover per se and that the web site association with northerly, easterly or westerly aspect might be related to temperature rather than aspect.

The association between web sites and patches of open sand and northerly, easterly or westerly aspect was examined in the laboratory. Results showed that all catching-webs built by L. katipo and L. Atritus were positioned over open sand and that no catching-webs were built within dense A. Arenaria clumps. These results suggests the structural requirements of the catching-web are not met by dense dune grasses, which would explain why L. katipo and L. atritus are rarely present in dune regions dominated by dense A. arenari, or other dune grasses, such as Pennisetum clandeestinum Chiou 1903 and Stenotaphrum secundatum Kuntze 1891 that also have a dense growth habit. That the majority of spiders included in the experiment favoured web sites at the warmer well-lit end of a temperature and light gradient supported the hypothesis that the association between web sites and aspect may be linked to temperature.

L. katipo and L. atritus spiderlings were found to disperse by 'ballooning' and adult females are able to tolerate exposure to salt water for up to nine days, indicating that they may be capable of travelling substantial distances at sea on driftwood. These results suggest that both species are good dispersers, as evidenced by their current distributions, which span numerous geographic barriers, such as open sea, estuaries and large rivers. This assertion was further supported by molecular analysis that revealed low intra-specific pairwise distances between L. katipo and L. atritus populations in the ND1 gene region, indicating that intra-specific gene flow has recently occurred, and may still occur throughout the distributions of these species. Molecular analysis also revealed low inter-specific pairwise distances between L. hasselti and New Zealand's widow spiders suggesting that gene flow has recently occurred between the endemic Australian and New Zealand widow fauna and raises questions about the accuracy of their current taxonomy.

Overall, my results suggest that the likely cause of L. katipo and L. atritus decline is the increasing dominance of dense exotic dune grasses in New Zealand's coastal dune systems, as these do not appear to meet the structural requirements of either species' webs. Moreover, that L. katipo and L. atritus are rarely recorded from dunes modified by urban or rural development indicates that these types of development may also fail to provide suitable habitat for these species. Although results showed that L. katipo and L. atritus are good dispersers, the introduction of exotic plants and/or urban or rural development may result in the reduction and fragmentation of these species' habitat, inhibiting dispersal. This would reduce both species' capacity to re-colonise dune regions from which they have been displaced. Consequently, both species may be vulnerable to local extinction in areas where their habitat has been extensively modified.