Tuatara: Volume 23, Issue 2, July 1978
Age Structure in P. Antipoidiana Populations
Age Structure in P. Antipoidiana Populations
The method of assessing age structures for these studies was to use body length as a crude indicator of age. It has been found, from records kept over the past six years, that P. antipodiana will grow to 12 mm long by the end of year one (see Fig. 1 for details). By the end of year two, the spiders may be upwards of 20 mm long, and the females may be found in possession of egg-sacs at this age. These statistics give three reasonably natural groups which were used in the age structure surveys:
|1.||‘Small’ spiders — less than 12 mm body length, and in their first year.|
|2.||‘Medium’ spiders — between 12 - 20 mm body length, and in their second year.|
|3.||‘Large’ spiders — in excess of 20 mm body length and more than two years of age.|
Fig. 1: Growth rate of P. antipodiana. Points plotted on the graph are mean, and the upper and lower ends of the range found in each age group that was measured. Numbers of spiders measured were as follows: 3 months, 110; 6 months, 80; 9 months, 68; 12 months, 45; 24 months, 31. The upper growth limits at 12 and 24 months are the ones referred to in the text for determining age structures of populations. Note how the growth curve flattens during the winter months of the first year. After sexual maturity is reached by the end of year two, the growth rate appears to slow down but not enough measurements were available to reliably plot the curve beyond this point.
There are several methods of assessing body length in spiders for age structure studies. They are: (1) by capture and direct measurement; but this usually results in damage to the web and so was used infrequently. (2) Inspection at night by torch light and visual estimation of body length. (3) Measurement of the tunnel that the spider lives in. This method proved to be a reliable estimate of body length. It involves the assumption that the width of the tunnel opening is equal to three-quarters of the body length of the spider living there.
Fig. 2: Age structure of a Johnsonville population of P. antipodiana, as at January. Small spiders (S) numbered 167, medium spiders (M) numbered 18, large spiders (L) numbered 18.
If, however, a count was taken in winter or spring, then many of these juvenile spiders would have died and the resulting figures show a bias toward the mature spiders. In the Johnsonville broken rock bank population, shown in figures 2 and 3, the decrease in the number of small spiders present from January to August was 73 per cent. Winter mortality of young spiders is due to several factors. Starvation and disease must play a part, but predation by planarian worms accounts for a good number of the deaths. Predation by other spiders — sometimes their own species and sometimes from other species — certainly takes place. The brown grass spider Miturga has been observed feeding on young tunnel web spiders several times during the course of this survey. See Table 1 for detailed mortality figures.page 71
Age structure can be modified substantially by the nature of the habitat. For example, a population in a brick wall at Northland, Wellington is always characterised by having more individuals in the large spider group than in any of the other groups. This is due in there being a lack of web sites on the wall; a row of drainage holes at the bottom, and a few cracks in the wall being the only sites. Because of this, there is strong competition for sites, and they are usually filled by large spiders, giving the unusual age structure as shown in Figure 4.
Causes of mortality in Porrhothele antipodiana. Figures were derived from observations on several populations, 1971-76.
|N. first||N. all|
|year spiders||other spiders|
|Brown grass spider (Miturga)||8||1|
|Complications at moulting||-||2|
|Cause of death unknown||2||3|
Similar effects have been noticed in populations in more natural habitats, such as rock faces and hillsides with accumulations of loose rocks. In many such areas a normal age structure may not be found. It seems that the maturing smaller spiders, and the medium spiders, are forced to migrate in search of suitable sites. This hypothesis gains support from the observation that most of the spiders found moving at night do fall into the medium size groupings.
Note: While the small and medium groups are composed of single year groups, the large group is made up of third, fourth and possibly older age groups. For this reason, the large group does take up a disproportionate amount of space on the age structure diagrams. The problem could not be avoided because it was not possible to accurately determine the age of spiders older than two years.
Similar factors appear to govern density on habitats such as hillsides covered by logs or stones; density will be limited by the same combination of suitable sites and food supply.
Where density has been measured, figures have ranged from 4.4 spiders per square metre (clay/rock bank) to one spider per 150 square metres (regenerating broadleaf bush). If density is considered for a small area such as around the web of a mature female after the young have dispersed, then densities of 20 spiders per square metre are not uncommon.
Below are further measurements of density, based on the number of medium and large spiders per 20 x 20 metre sample area, 10 samples having been taken where possible:
|20 x 20 metre sample|
|Podocarp/broadleaf bush, Wilton, Wellington||4|
|Sand dunes, Spinifex clumps, Waitarere ….||8|
|Beech dominated bush, Eastbourne, Wellington||4|
|Loose rocks, Hillside, Paekakariki ….||….||200|
|Broken rock face, Paremata ….||….||40|
|Beech forest, Lake Rotoiti ….||….||….||3|
|Rocks, logs in paddock, Martinborough||….||15|
|Suburban Wellington (Northland)||….||….||20|
These figures reinforce the general impression held by this writer — that P. antipodiana is most successful in the open habitat rather than in bush areas.page 73
It has already been mentioned that the young spiders disperse up to 1.5 metres from the parental web. This is not the end of the migratory behaviour in P. antipodiana, for although the majority of spiders do not move very often, and indeed may occupy the same site for a year or more, some of the medium spiders, in particular, move several times from site to site until a suitable site is found. The distance of these site to site movements is commonly 1-4 metres, and movements may be made as often as every 1-3 weeks. During the course of a year, it is possible that highly mobile individuals could disperse for distances of 20-100 metres from their original web sites.
In addition to these short site to site migrations, longer migrations also take place. In these, the individuals concerned may cover distances of 10-20 metres in one move. During the breeding season (late spring), the males may wander considerable distances. In some of the populations studied by this writer, males were often found up to 30 metres away from their known web sites.
When this migratory activity is considered, it is apparent that the clumpings of spiders — as in the Mt. Kaukau example, Fig. 5 — are not too far apart for gene flow between them to take place. P. antipodiana population units seem to be similar to the deme page 75 concept as outlined by Savage (1969), in that they are semi-isolated, yet permit some gene flow to take place. This gives the sub-units (clumps, demes) the chance to develop small local variations. Whether such is the case for the P. antipodiana populations is not clear, but Forster (pers. comm.) has noted that some populations of this species show a very pronounced chevron pattern on the abdomen, while other populations possess almost uniformly dark abdomens.page 74
Keeping track of the movements of these spiders sometimes takes up a considerable amount of time. It involves regular checking of all the known webs in the locality. Then, when a spider is found to have left its web, a search of the area must be made to find a newly established web. Checking on the size and colour of the inhabitant is usually sufficient to establish whether it is the individual from the empty web or not. When dealing with these relatively small groups, it is possible to become familiar with them as individuals. Where spiders happen to be very similar in body length and colour, they can be separated by a dab of white acrylic paint on the cephalothorax or the abdomen.
Mapping of the web sites in the locality being studied proved to be a definite advantage in keeping track of spider movements. The example in Fig. 6 was used to help keep track of the spiders in part of population 2 on the crib wall at Johnsonville.
Population 1, Paremata
In 1971, a broken rock/clay cliff face at Paremata measuring approximately 20 metres wide and 40 metres high was surveyed for numbers of P. antipodiana in excess of 20 mm body length. Counts were made at two-monthly intervals and the results were graphed (Fig. 7). The number of mature spiders was relatively stable over most of the year, except for the fall off in numbers over the summer. The peak number recorded was 60 spiders, in July-September. This decreased by 30 per cent, to 42 spiders, by December.
During late October-December, several large black wasps were seen searching the cliff face, running into P. antipodiana webs, and occasionally dragging paralysed spiders along. These Pompilid wasps (species: Salius monachus) are known hunters of the larger New Zealand spiders, so it was thought they might be the cause of the summer decrease in the spider numbers. Other factors investigated were: predators, but none were observed working in the area; dehydration due to summer drought, but the remaining spiders appeared to be in good condition so this factor was not regarded as being responsible for the population loss. Starvation/summer drought effects can be identified by the considerable shrinkage of the spider's abdomen that takes place under these conditions.page 76
Population 2, Johnsonville
With the Paremata study in mind, a longer term study was begun at Johnsonville on a more accessible population, located on a concrete crib wall. The wall was six cribs high, the spaces between cribs being 14 cm high by 60 cm wide. The assumption was that in the web sites it provided, this crib wall was not dissimilar to a broken cliff face. Methods of study used were: spiders assessed for body length on the basis of width of tunnel opening; the state of their web was taken as the main indicator of whether the spider was still resident in the site. If the spider does not continually add new silk to the web it soon deteriorates, and this is a reliable sign that the spider has either migrated or has been captured.
Wasps were captured, marked on the thorax with white acrylic paint and released. Particular attention was to be paid to spider numbers in September (prior to the onset of wasp activity) and late in February (after wasp activity ceases in this locality). Two wasp species were active in the area: Salius monachus (black) and Salius fugax (red).
Since the last survey, the crib wall has been planted in thick creepers and is no longer suitable for observation or for counting numbers of spiders present. Unfortunately, this study had to be terminated with the spider population consisting of a few very young individuals. It would have been interesting to plot the populations for another two or three years to see if the trends displayed were repeated; that is, a second rise in the spider population followed by a rise in the wasp numbers. As it is, the trends are reminiscent of classical prey-predator cycles.
The spider population at Johnsonville appears to have been a young one, undergoing expansion during 1971-74, by which stage it began to support an unduly large wasp population. No doubt the wasps were also taking spiders from adjacent populations, for it is doubtful that a peak population of 27 medium and large spiders could provide for the larvae of up to 30 wasps. The wasp activity was intense on the crib wall; sometimes as many as 10 wasps at a time were running over the bank, investigating webs and searching for spiders. Several spiders were observed being captured, and others were seen to escape the wasps by vacating their tunnels and leaving the wall on the run.
A Chronology of the Johnsonville Study
|TIME||SPIDER NUMBERS||WASP NUMBERS|
|Sept. 1971||Fewer than 10 spiders in excess of 12 mm length||Occasional red wasp visits|
|Sept. 1972||15 spiders present||As for 1971|
|Sept. 1973||24 spiders present||First black wasps seen in area|
|Feb. 1974||18 spiders present}|
|Sept. 1974||27 spiders present||9 red wasps, 7 black wasps active on wall|
|Feb. 1975||12 spiders present}|
|Sept. 1975||24 spiders present||18 red wasps, 12 black wasps active on wall|
|Feb. 1976||5 spiders present}|
|Sept. 1976||10 spiders present||14 red wasps, 10 black wasps active|
|Feb. 1977||No spiders in excess of 12 mm length present}|
Why this spider population attracted so many wasps is not clear, but the effect of such activity was to reduce the spider population down to a few very young individuals. This is, in all probability, a rare situation.
Population 3, Johnsonville
This was another crib wall population, less than 100 metres distant from population 2. Occasional visits by red wasps were noted, but black wasps were not found active in this area. This population acted as a valuable control, for the influence of other factors such as summer drought/food shortage would have shown up in this population if they had been involved in the decline of population 2. However, as population 3 did not exhibit undue mortality over summer, then drought and food shortage were ruled out as contributors to the decline of population 2. The figures taken for population 3 in the years 1975-77 are as follows:
|Sept. 1975||38 medium/large spiders|
|Feb. 1976||31 medium/large spiders|
|Sept. 1976||44 medium/large spiders|
|Feb. 1977||36 medium/large spiders|
Plate 2: A rock-strewn gully, Johnston's Hill, Wellington. This is the type of habitat that P. antipodiana is very successful in — the looser rocks, particularly those associated with clumps of grass or low shrubs, are the most favoured ones. The slaters, millipedes and beetles which form the bulk of the spider's food are plentiful around the rocks. In addition, the drainage is good and this appears to be an important factor in determining whether P. antipodiana populations will attain high densities or not.
Plate 3: The male tunnel web spider has more than one use. After fertilising the female, he may be utilised as food if his withdrawal from her web is not rapid. In this particular example, the remains of the male are on the left with the characteristic, modified front leg clearly evident. The tunnel opening (T) of the female is the dark area to the right. Photographed in September, Wellington.
Population 4, Northland, Wellington
A fourth small population on a broken greywacke bank at Northland was observed for a year from July, 1976, to July, 1977. The familiar pattern of a summer population decrease was evident (Fig. 9). Again, in this area small numbers of Pompilid wasps were seen searching for spiders during the summer.
From these four population studies, it appears that populations of P. antipodiana can usually maintain stable numbers of mature indivudals — that is except when the Pompilid wasps are active during the summer months. Under these conditions the decrease in numbers is usually made up, partly from the pool of maturing younger spiders in the population, and probably in part from immigration of mature spiders from adjacent populations.
In part two of this study, detailed consideration will be given to the effects of wasp activity in modifying the P. antipodiana populations. Attention will also be given to the mechanisms by which the spider populations survive wasp activity — without suffering undue mortality rates.
Forster, R. R., and Forster, L. M., 1973: New Zealand Spiders — An Introduction. Auckland, Collins.
Laing, D. J., 1973: Prey and Prey Capture in the Tunnel Web Spider Porrhothele antipodiana. Tuatara 20:2 57-64.
——, 1975: The Postures of the Tunnel Web Spider Porrhothele antipodiana: A Behavioural Study. Tuatara 21:3 108-120.
Savage, J. M., 1969: Evolution. Holt, Rinehart and Winston (second ed.).