CLIMATE AND ECOLOGY STUDIES AND MANAGEMENT AT WAITOMO CAVES
The first detailed study of the New Zealand glowworm (Arachnocampa luminosa in caves at Waitomo, was done in 1955 by A. Richards. In that study the insect was described and observed over a full annual season and changes in the population structure noted, also recorded was the climate inside and outside the cave. This early paper provided a good reference to compare later observations, particularly after modification to one of the two entrances to the cave was suspected of bringing about changes to the climate and consequently to the glowworm population. In 1974 the Waitomo Caves Scientific Research Programme was begun which instigated research into lampenflora, carbon dioxide in tourist caves, the hydrology of the Waitomo Catchment, and later the ecology of glowworms in caves at Waitomo, and also the climate of Glowworm Cave.
This paper discusses the management implications resulting from the two recent studies of glowworm ecology and cave climate in light of the earlier study and most recent observations.
In 1955 about 40,000 people visited the Glowworm Cave at Waitomo. During that year A. Richards' study was done on glowworms in the cave. The life cycle of eggs, larvae, pupae and adults was found to be continuous throughout the cycle, with only a slight seasonality as more individuals were in the non-larvae stages in the winter months. The time intervals for each stage of the life cycle were recorded, as was the food supply of the glowworms. Climatic recordings showed a stable cave climate, similar to other caves in the area and consistent with a cave with only one significant entrance.
The upper entrance, at this time, was sealed by an airtight wood and stone construction, with a door to allow for the passage of visitors. Temperature and humidity recordings showed only slight annual variations, both showing a slight summer maximum. Diurnal variations were almost absent. Air movements through the cave were difficult to detect with a candle and no seasonal or diurnal pattern was recorded. Looking back, it appears that this stable cave climate and abundance of food provided the optimum ecological conditions for a large population of glowworms in the cave.
By 1974 the Waitomo Caves Research Programme was underway, examining various problems in the caves. One of these research projects examined the carbon dioxide levels in the cave and found that the CO2 breathed out by people was accumulating at concentrations much higher that found in other cave atmospheres. In one dead end section of Glowworm Cave, called the Organ Loft, it was found that there was enough atmospheric CO2 to reverse the chemical growth of speleothems and cause corrosion. There may even have been a health risk to cave visitors.
It was decided to improve the ventilation in the cave by modifying the upper entrance, so in March 1975 this entrance was greatly enlarged by the removal of the solid door and surroundings which were replaced with an iron grille. This modification greatly improved the ventilation.
In 1977, C. Pugsley began a three year doctorate study of the ecology of glowworms in the caves at Waitomo. It had already been noticed, by guides and management staff, that the glowworm display was not as spectacular or regular as in previous years, and that the cave was much drier, particularly in winter. As the glowworm ecology study progressed it was found the life cycle was now very seasonal, with the large majority of individuals pupating from May to August. This, in part, accounted for the reduced luminescent display in the winter months. Observations suggested that there had also been a considerable decline in glowworms over recent years. Early winter months saw a significant number of older glowworms not producing light. Earlier it was found a pathogenic fungus was taking a heavy toll of glowworms in the summer months. This fungus was recorded in the earlier studies, but this too seemed to be increasing and was much more abundant in the Glowworm Cave than other caves in the Waitomo area. Early in 1979 the number of glowworm lights in the Grotto, the main display area, began to decline and by mid-April it was decided to close the cave to the public. Only 4% of the total population were producing light, an indication that the Grotto colony was under some physiological stress. Change in the cave climate following the modification to the upper entrance was suspected. Laboratory trials had shown that glowworms were very vulnerable to desiccation and it appeared that the drying of the cave and its inhabitants was the main cause of the decline in glowworm numbers and luminescence. Initial cave climate recordings showed that the outside climate was now greatly influencing the interior climate, with pronounced seasonal and diurnal variations. In July of 1979 the emergence of young larvae in the Grotto allowed the cave to be opened again. A difficult decision had to be made to leave the cave exactly as it was until more climatic studies could be made.
In October 1979, R. Littlejohn began a Masters study of the climate of the Glowworm cave. Detailed recordings of temperature, humidity, evaporation, condensation, air density and air movement were made over a thirteen month period, and from these a thermodynamic model was produced to explain the physical processes governing the cave climate. A distinct seasonal pattern emerged. Mean cave air temperature and humidity showed a summer maximum. This coincided with a flow of air through the upper entrance downward to the lower entrance. In winter, air flows reversed and became stronger; evaporation increased five fold and cave humidity dropped dramatically. Cold, dense, dry air would enter the cave through the lower entrance. This air would be warmed by the warmer cave walls; convection would occur and the air would leave unhindered through the upper entrance taking with it moisture absorbed from all suitable surfaces.
During the study the upper entrance was sealed completely with polythene. The results were dramatic. Within thirty minutes temperature, humidity and air movements responded, and over a ten day period stabilised. The climate study was completed in November 1980, and a clearer understanding of the climate was now available. As expected, one of the main recommendations was to control the rate of air exchange by installing an airtight door on the upper entrance. This was done in February 1981.
So far, during winter 1981, the cave climate has become closer to the 1955 situation than more recent years. The cave is noticeably damper and no evaporation has been recorded. Temperatures have also stabilised, with a much reduced range. Air flows are only noticeable when the door is left open. The number of glowworm lights did decline early in the year before the solid door was installed. This was in response to two cold nights in early February, but as winter progressed it appeared that the glowworms now found the winter cave environment more acceptable, and the display remained adequate and much improved on the previous two winters. The emergence of young larvae from the eggs began on schedule in mid-June. It is hoped that from this generation overall numbers will increase until an optimum population size is reached .
With the return of the restricted ventilation, the original carbon dioxide problem returns. The number of visitors to the Organ Loft, the main problem area, has been severely restricted. A gas analyser monitor continually reads the CO2 and without much difficulty, visitor numbers to the Organ Loft are restricted accordingly.
Since 1976 the number of visitors to the cave have declined, again relieving the problem of CO2. The increased dampness in the cave benefits some of the less desirable life forms, also. There has been a noticeable increase in lampenflora in areas previously too dry to support plant life, but again this problem is not insurmountable.
The various research projects at Waitomo and events in recent years, all point to the need to understand fully all components that make up the cave environment. Cave climate, often overlooked in the management of tourist caves becomes crucially important when the inhabitants of the cave are its prime attraction.
BIBLIOGRAPHY
LITTLEJOHN, R. 1981. Climate of the Glowworm Cave, Waitomo.
PUGSLEY, C.W. 1980. Ecology of the New Zealand Glowworm in caves at Waitomo.
RICHARDS, A.M. 1960. Observations on the New Zealand Glowworm Arachnocampa luminosa (skuse) 1890. Trans. Roy. Soc. N.Z. 88: 559 - 574.
WAITOMO CAVES SCIENTIFIC RESEARCH PROGRAMME. Minutes of meetings, 1975-1981.