Tuatara: Volume 24, Issue 1, October 1979
The Relative Effectiveness of the Six Factors
The Relative Effectiveness of the Six Factors
An attempt has been made to rank these factors in order to compare their effects on the survival of P. antipodiana. All of the information was taken from actual field observations. In some cases assumptions had to be made as to the effectiveness of a factor and these are noted on the chart:
|Tactic||Assumptions||No. Survivals Observed|
|a. Non-response to wasp on sheet web||If spider had come out to sheet web it would have been captured||100|
|b. Silk cover at tunnel entrance Aestivation||Had wasp entered, a capture would have been likely||15|
|c. Aestivation||Non-capture was due to wasps failing to take interest in old webs||20|
|d. Presence of a side tunnel||This protected spider when wasp was seen entering and searching the tunnel||10|
|e. Rapid retreat of spider from web||If spider had remained it would have been captured||8|
|f. Spider gave fight||This caused wasp to retreat||12|
These observations certainly support the non-response category as the most significant factor in the survival of the spider during the wasp season.
Some General Considerations of the Salius/Porrhothele Relationship
The selective action of the wasps in removing mainly the young-to-recently-mature members of a Porrhothele population must have some effect on the population structure of the spiders. Reduction of the number of spiders would certainly lower the pressure on food page 19 supplies in a particular locality and, furthermore, it should benefit the remaining spiders, especially over winter when food supplies taper off. Examination of a P. antipodiana population in Palmerston North, where intermittent observation over several years had failed to show the presence of Salius wasps, revealed large numbers of spiders in the 13-18 mm body length range. This is the type of observation that could be expected if the hypothesis that the wasps' activities reduce intraspecific competition among the spiders is a valid one. However, it is obvious that a more detailed investigation of growth rates and age structures would be necessary before this hypothesis could be confirmed.
Continuing the theme of the selective behaviour of the wasps, it does seem necessary to assume learning behaviour on the part of the wasp. The avoidance of large spiders, and the prevalence of medium sized spiders as prey indicate to the author that the wasps may learn preferences for certain sized spiders. Such learning ability would be of no great surprise to those familiar with the work of Tinbergen on the bee-wasp Philanthus; or that of Baerends on the sand wasp Ammophila. For a discussion of the learning abilities of these and other Hymenoptera, Thorpe (1963) should be consulted. Another source of information on the intricacies of hunting-wasp behaviour is the very readable work by Rau and Rau (1918).
Relating the selection theme to Part I of this study, it can be seen as a partial explanation for the reduction of the Johnsonville crib-wall population almost to zero. This particular population was a young one with few large spiders; as such it was particularly susceptible to wasp activity, and the results of several years predation by the wasps proved this to be the case.
It has already been mentioned that certain individuals in a Porrhothele population survive year after year despite the activities of Salius wasps. There may well be long-term genetic implications arising from this; for the individuals surviving and breeding for many years are contributing large numbers of their genes to the gene pool. If they survive longer because of certain characteristics they possess then it is likely these characteristics will be spread through the gene pool. It is known that any one mature P. antipodiana female can produce up to 300 offspring in one year. A spider which survives to breed for six years could contribute her genetic material to 1800 offspring; whereas those individuals that are captured by wasps at the end of their first breeding season will have contributed to a maximum of 300 offspring each.
While it is relatively easy to discover the number of offspring produced by the spider, it is more difficult to say with any certainty how many offspring each wasp is likely to leave each season. From a knowledge of the number of wasps active in a given area, and utilising information on how many spiders have been captured in that area, it was possible to give an estimation of the number of wasp offspring page 20 likely to have been produced. The figure arrived at by the author was that 10 larvae per wasp could be possible. Mr A. C. Harris, in a personal communication, has confirmed that a figure of this order would not be far wrong, although his research indicates the maximum number of larvae per wasp might be even lower — at 7 or 8. What is clear, is that the reproductive potential of the spider far exceeds that of the wasp, and is probably in the order of thirty times greater when comparing a mature female of each type. This disparity is the ultimate reason why P. antipodiana populations are unlikely to be seriously threatened by wasp activity in the long term.
If a broader perspective is taken and the Pompilidae are considered in terms of trophic levels, then the fact that they procure food for their larvae from the third trophic level would mean that they must always be relatively insignificant in terms of biomass. This is characteristic of a predator which preys on other predators.
I would like to thank Dr R. R. Forster of the Otago Museum for the encouragement he gave me in the early years of this study. For his help on the biology and taxonomy of the New Zealand Pompilidae, I would like to thank Mr Tony Harris, also of the Otago Museum. Finally, my thanks to Mr R. Ordish of the National Museum for his help in resolving the problem of the current common-name of Salius.
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