Cave Monitoring and Management: The Glowworm Cave, New Zealand
A typical cave is a low energy, stable environment and one that is potentially highly sensitive to change by human beings. The presence in a cave of just a few people can change its energy regime in terms of heat, humidity and moisture. This impact directly affects cave air, but a range of other impacts are associated with the human presence and their effects are cumulative and often synergistic. This innate sensitivity of caves to the human presence led Aley (1976) to remark "the carrying capacity of a cave is zero." As far as tourist caves are concerned, however, the presence of people is clearly not optional.
Though widely used in management theory, the concept of 'carrying capacity' hangs on the assumption that there is an upper limit to use that a resource that an area or resource can stand. But this rarely applies in the case of tourist caves as the resource base is not fixed, and the pattern of such factors as timing and intensity of use are constantly changing. Also, impacts are not linear; for example, the effect of a group of 15 people may be more than three times the impact of a group of five. Furthermore, as Gillieson (1996) points out, the concept of maximum usage does not take into account the possible irreversibility of many ecosystem changes. For instance, cave fauna are frequently obligate species and habitat specialists that are often vulnerable to minor changes of light, moisture and heat, and populations may not recover from a short term or longer term stress. Rather than being a matter of usage levels or carrying capacity, it is more one of determining environmental management techniques that are appropriate to a particular cave condition, or environmental state that should prevail. The real issue, therefore, is one of visitor impact management.
With the above in mind the cave manager is concerned, firstly, with defining the desired or optimal level or range of environmental conditions that should prevail and, secondly, with maintaining them. To do this requires an appropriate and reliable monitoring system. It involves selecting key indicators (say air temperature or vapour pressure deficit) to be monitored and setting target standards (say a given range of temperature and humidity). By this monitoring, cave managers can assess the consequences of change and modify management strategies accordingly. Selection of an appropriate monitoring system, however, relies on having a good understanding of the processes that affect the cave environment and ecosystems — basically how they work and what upsets them.
According to some estimates, over 20 million people a year visit tourist caves (Gillieson 1996), yet there are few well documented studies of visitor impact management, and even fewer dealing with appropriate theory and management concepts. The Waitomo Glowworm Cave, however, has been the focus of a variety of studies and it continues to be the focus of research work.
The Glowworm Cave is a small tourist cave located near the centre of the North Island of New Zealand in the heart of a limestone region called Waitomo. The Glowworm Cave is probably the best known tourist cave in the world where cave fauna is the prime attraction. In New Zealand, it is a major tourist attraction which has played, and continues to play, a vital part in the development of the nation's tourist industry. To large numbers of tourists from both New Zealand and overseas, a visit to the Glowworm Cave is a high point of their holiday experience. The Cave, along with the geothermal areas in and around Rotorua, have come to symbolise the North Island tourist encounter. For this reason, the value of the Glowworm Cave to New Zealand tourism extends beyond its great commercial importance. It is a natural resource of great significance for which the Government of New Zealand through its Department of Conservation has a major custodial responsibility.
Tourist caves, like most other natural tourist attractions, are sensitive to change or damage caused by the presence of visitors. The Glowworm Cave, however, is potentially more sensitive to human impact than most others. This is because of its small size and the presence of cave fauna crucial to its tourist appeal.
As tourist caves go, the Glowworm Cave is tiny, yet it has had to cope with significant numbers of visitors, especially in recent years. More people visit the Glowworm Cave than any other in Australia or New Zealand. The next most visited cave is the much larger Lucas Cave in New South Wales in Australia which has an annual visitor rate that is less than a quarter of that for the Glowworm Cave (120,000 vs 416,000 visitors per year). Between 1979 and 1994 there was a doubling of the number of people visiting the Glowworm Cave each year. This trend has continued. On some days visitor numbers have exceeded 2,700, and in February, 1996 a record 66,593 people visited the Cave giving a staggering daily average 2,296. Clearly, with this level of usage and rate of increase, there is growing potential for conflict to arise between the dual requirements of protecting and presenting the resource.
The Glowworm Cave has been used continuously as a tourist cave since late last century. Over this time several lessons have been learned. Most notably, during the 1970s, it was recognised that conditions in the Glowworm Cave were rapidly deteriorating. There was concern that many changes occurring would be irreversible but, at that time, little was understood about the cave environment and factors that controlled conditions in the cave. The problem peaked in April 1979 when the Glowworm Cave was closed because only four percent of the glowworms had their lights on. On occasions such as this the cost to the region in lost revenue can be considerable. The cave re-opened in the following July. Later that year, in recognition of the fact that the cave atmosphere is a fundamental element of a cave ecosystem, an intensive study of the microclimate of the Glowworm Cave began. This coincided with detailed in situ studies of glowworms and sedimentation processes in the stream passing through the cave. The work culminated in a series of publications both in the form of university postgraduate theses and papers in international scientific journal literature. Results of this research were subsequently taken into account in setting out cave management guidelines.
everal major decisions on cave management came from this early work, but the main recommendation was that the cave ecosystem, especially the cave air or microclimate, should be carefully monitored. This monitoring should provide long term, high quality data on the atmospheric and other environmental processes that affect the cave ecosystem in general, and the health of the glowworm population in particular.
The Waitomo Caves Research Committee, reporting in 1982, emphasised the need to establish sustainable resource management guidelines to protect the cave environment in terms of the glowworm ecology and speleothems, and at the same time, guarantee visitor safety. The protection mechanism should ensure that changes to the cave microclimate and low glowworm numbers experienced in the late 1970s are avoided in future. This requires maintenance of a cave microclimate and ecology monitoring programme that provides data suited to scientific investigations over time.
In the case of a tourist cave, the concept of 'cave monitoring' embraces measurement, observation and recording in the broadest sense and includes physical and biological (i.e. environmental) and social (i.e. visitor) variables. An essential part of identifying and selecting appropriate variables to be monitored is an understanding of physical and biological processes that comprise the cave system; basically, how it works and what upsets it. Key reference criteria are concerned with defining optimal conditions and maintaining them. The main questions and problems are summarised below.
- purpose of environmental monitoring
- purpose of visitor monitoring
- What to monitor?
- Where to monitor?
- How to monitor?
- feasibility and cost of monitoring
- choice and representativeness of key indicators
- replication and frequency of measurement
- quality control
- plan for data analysis
- management standards and indicators of impact
THE CASE OF THE GLOWWORM CAVE
- Purpose of environmental monitoring:
- to assess the impact of human activity in the cave;
- to expand knowledge of the cave resource by adding a long term dimension to the data collected during initial intensive research;
- to identify environmental seasons, cycles, changes and trends that may impact on the cave or the glowworms;
- to assess the impact on the cave and glowworms of management practices such as cave microclimate control, desilting, lampenflora removal, etc.; and
- to assess the impact of human activity outside the cave such as changes in land use or to the catchment.
- Purpose of visitor monitoring:
- to provide an information/data base to assess the impact of people on the cave and glowworms;
- to identify visitor patterns; and
- to provide information for auditing and planning.
- What to monitor?
- Microclimate (cave and outside air and related processes)
is the key element. Need to understand processes operating so decisions
can be made on what to measure and where to measure it.
- cave air temperature
- outside air temperature
- cave air humidity (specific humidity and relative humidity)
- outside air humidity (specific humidity and relative humidity)
- air flow rate
- air flow direction (upwards and out through top entrance, or downwards and out through lower entrance
- rock temperature
- carbon dioxide
- Biological systems
- glowworm numbers (quadrat)
- special features (diseased glowworms, presence/absence of predators, fungus, etc)
- Continuous streamflow recording flow recorder at a point along the stream that runs through the cave.
- Record of the number of days when the boats cannot operate through the Glowworm Grotto because of high water, and number of occurrences and water level of floods.
- Microclimate (cave and outside air and related processes) is the key element. Need to understand processes operating so decisions can be made on what to measure and where to measure it.
- Where to monitor?
- Microclimate: measurements at three key (indicator) sites inside the cave and one site outside the cave.
- Glowworms: large monitoring quadrat on outer edge of main display colony located in the Glowworm Grotto.
- How to monitor?
- Automated systems using electronic sensors and data loggers.
- Instruments to suit harsh cave conditions.
- Sensors to suit range of conditions encountered (i.e. appropriate sensitivity).
- Data collected and stored in electronic form to enable:
- real-time display of conditions (data) being monitored
- short term diagnosis of conditions in the cave
- analysis of trends over many years
- Indicators of impact
- Increase in air temperature
- Increase or decrease in humidity
- Increase in carbon dioxide concentration
- Increased vapour pressure deficit
Monitoring of conditions within the Glowworm Cave has been done off and on for approximately 16 years. Initially, monitoring was developed as a follow-on from detailed research instigated and supervised by the Waitomo Caves Scientific Research Group which was established in 1974. A relatively large amount of microclimate data has been collected since 1983 using standardised procedures. However, collection and assembly of data relied on cave guides and administrative staff taking readings and maintaining instruments themselves. Gaps in the data and poor equipment maintenance reduced the quality of the data. Moreover, as the data set was assembled manually, processing and analysis were difficult and time consuming. The accumulated microclimate data gathered in this way was transferred from paper records to a computer compatible database and analysed (de Freitas, 1990). The results showed that there are many large gaps in the data record and that reliability of measurements at certain times and for certain extended periods is suspect due mainly to lack of equipment maintenance and instrument failure.
In the latter part of 1993 a scheme was proposed for improving the quality and quantity of cave climate data. Continuous monitoring was recommended employing remote automated systems using electronic sensors and data loggers. Data loggers allow for the collection of large amounts of data from a variety of sensors at a relative low cost. Also, problems of observer error are removed, and data are presented in a form amenable to computer analysis. By the start of 1994 a computerised, electronic monitoring system was installed in the cave at four different sites to measure rock temperature at different depths, air temperature, humidity and air movement and direction. The system is currently in place and operated under contract by the National Institute for Water and Atmosphere Research (NIWA), Hamilton. Information is accumulated continuously by a datalogger and then downloaded onto a laptop computer by NIWA staff. These data are then loaded onto a NIWA computer in Hamilton and processed to produce quarterly reports.
This undertaking has already proved to be worthwhile. Although there are problems that need attention, the information collected will ultimately contribute to a substantial database essential for overseeing the well being of the cave environment. In addition to being important for day to day monitoring, the data will provide a vital retrospective record should conditions change or problems arise in the future. By the end of 1995, the results of two years of automated monitoring of the Glowworm Cave microclimate had been assembled. The data were assessed in a detailed report (de Freitas 1996).
Instrumentation and Monitoring
Temperature, Humidity and Airflow
The cave microclimate, which includes all cave and outside-air related processes, is the key element of the Glowworm Cave system. A cave climate in a condition of stable high humidity, little or no evaporation and only moderate air movement, is essential for the well being of the glowworms. When there is a 2 or 3°C difference between the cave temperature and the outside temperature, air movements to and from the cave are generated which, if strong and prolonged, can increase evaporation and stress the glowworms (de Freitas & Littlejohn 1987; de Freitas et al. 1982). Monitoring the cave and outside air indicates when it is necessary to control air movements and evaporation by opening and closing the upper entrance door, or in extreme cases using a humidifier near the stream entrance to the Grotto.
An automated monitoring system has been in place since the start of 1994. It is dependent on one electrically powered Campbell CR10 datalogger which can store up to two months worth of data. Loss of data can occur from power failure, electronic malfunction, vandalism, or from human error in downloading data. For example, a drop in power resulted in the loss of most the data for January 1994. Extended gaps in the record of outside air temperature and humidity also occurred during March through to November 1994. It has been recommended that more frequent routine checks are made to ensure that the logger and sensors are working correctly. In addition, a duplicate data logger and set of sensors should be run in parallel to provide an on-going comparative record and backup system.
Outside and just above the upper entrance of the cave, air temperature and humidity are measured by a shielded Skye electronic sensing unit. Problems with this unit resulted in data being lost during the first quarter of 1994. This highlighted the need for a backup system. It is vitally important that a reliable uninterrupted record of outside air temperature and humidity be maintained. This information is essential for analysing thermal and moisture fluctuations inside the cave that may be linked to air circulation through the cave.
Wet bulb and dry bulb air temperatures are measured at a site called the Tomo. The results of earlier research work on the Glowworm Cave suggested that the Tomo is an important indicator site for the cave as a whole (de Freitas & Littlejohn 1987; de Freitas et al. 1982). At this site there are also two rock temperature thermistors at depths of 2cm and 8cm. Nearby, there is a sensitive cup anemometer and an airflow direction sensor intended to give vital information on cave ventilation. Airflow sensors should be carefully located in positions known to give reliable indications of airflow rates through the cave.
Wet bulb and dry bulb air temperatures are also measured at a site in the Banquet Chamber. This site is representative of the spacious central regions of the cave where visitors frequently congregate prior to boarding tour boats taking them through the Glowworm Grotto. Rock temperatures within the cave at depths of 2cm and 8cm from the rock surface are measured at two sites. In theory, internal rock temperatures give an indication of trends in the longer term thermal state of the cave, as well as the direction of heat flow, to or from the rock surface.
Wet and dry bulb air temperature and rock temperature at a depth of 2cm are measured in the Glowworm Grotto. This site is important because of its proximity to the main population of glowworms, the scenic high point of the cave. This instrument is carefully watched and maintained. Clearly the health of the glowworm population is one of the prime objectives of good cave management. In terms of microclimate, the aim is for a thermally stable, moist, stress-free environment.
Just before the top of the steps leading down to the Banquet Chamber a sensor with an infrared beam is positioned to count visitor numbers. However, the counts recorded were found to be unreliable. The device has a tendency to over-count owing to people milling around during talks by guides and walking back and forth across the beam. Ticket sales records provide convenient, accurate, visitor data, at hourly resolution if required, and it has been recommended that these be used instead. Ideally they should be fed electronically from the point of sale to a central datalogger.
A stress free and safe cave environment for tourists is a primary concern for cave managers. Also, cave air quality should be maintained at a level which ensures that cave features are not affected. People exhale air that is slightly depleted in oxygen and enriched in carbon dioxide (approximately 4% CO2 ). Because of this, concentrations of CO2 can build up in the Glowworm Cave to over 5,000 ppm, depending on visitor numbers and ventilation rates through the cave. One person exhales CO2 at approximately 17 l/hr (Marion 1979), thus a tour group of 200 visitors expels about 3,360 l/hr. The 5,000 ppm threshold is considered to be maximum allowable for tourist caves (Osborne 1981); however, the allowable level that should be specified in cave management guidelines is open to debate (Dragovitch & Grose 1990). Added to this is the concern that when CO2 concentrations exceed about 2,400 ppm, water can absorb the CO2 and calcite, of which the cave is formed, dissolves, leading to deterioration of limestone features of the cave. There is also the indirect effect of increased (high) base level of CO2 in cave microbial respiration due to shedding of hair and skin which provides a food source for aerobic bacteria.
Carbon dioxide is not routinely measured but should be monitored at key sites in the cave at regular intervals, especially during periods of heavy visitor traffic so that patterns of concentrations in the cave may be identified and used in analyses of environmental conditions. This would assist in decisions related to controlling cave visitor traffic. Visitor numbers could be restricted when carbon dioxide concentrations approach a predetermined, upper-acceptable threshold level. An upper level of 2,600 ppm has been suggested by Kermode (1974, 1980). Measurements could be made less frequently at other times. The ideal, of course, is continuous measurement and the feasibility of implementing this is being investigated. An elevated cul-de-sac passage called the Organ Loft has very poor ventilation and carbon dioxide accumulates here when visitor numbers exceed 90 people per hour. It has been necessary to close this part of the cave every day between 1030 and 1500 hours.
Every two weeks detailed counts are made. The number of glowworm lights in the quadrat are used as a measure of the larvae population. Also counted are the pupae and adult flies. Both species of predatory cave harvestmen are also counted, but only up to a metre or so above eye level. Above that level they are too small to see. The number of insects caught in the lines of ten glowworms are counted and graded into three sizes. The use of automated methods using video cameras and computer-based digital counting systems is being explored.
Quality Control and Data Presentation
Quality checks and review should follow the setting up of long term monitoring programmes. Calibration is normal procedure and essential to establish the reliability of the data being collected. All of these things need to be taken into account in assessments of the data record. How data are presented is also important, but may vary depending on whether: a) data are being used by cave managers on an ongoing, regular, short term basis to watch conditions and, if necessary, make short term operational adjustments; b) records are being used for longer term, retrospective analyses of cave microclimate variability, or for post mortems of ecological crises that may occur; or c) data presentations are to be provided as appealing information displays for cave visitors.
Significant drying within the cave can occur at any time of year, also evaporation rates can vary considerably over relatively short periods of time, and between sites. A major cause of this is high rates of air exchange between the cave and atmosphere outside, but other factors may also play a part. Cave managers should monitor conditions throughout the year and pay close attention to any signs of drying in the cave.
Guidelines for ventilation and microclimate control have been proposed earlier based on studies of the cave atmosphere. The microclimate data reviewed here suggest that these guidelines are effective. The aim is to maintain optimal conditions in the cave for both glowworms and tourists, but without causing damage to physical features of the cave itself or affecting sustainable use of the cave. To accomplish this, several factors have to be controlled simultaneously. Rates of evaporation have to be kept low or even negative (i.e. condensation). At the same time, adequate ventilation is required to prevent the build up of excessive CO2 levels within the cave, but not at the expense of desiccation of the cave milieu or large temperature variation inside. To a large extent this can be achieved by carefully controlling air exchange with the outside. Guidelines are summarised below.
Close door to upper entrance when:
- external air temperature is below 10°C, regardless of humidity level outside; and
- external specific humidity levels are low (this usually occurs in the cool period of the year, typically between 1700 and 1000 hours).
Open door to upper entrance during:
- 'summer' airflow conditions (i.e. when airflow is downward through the cave), thus allowing for condensation in the cave as well as maximum ventilation at times usually associated with high visitor numbers.
- 'winter' airflow conditions, when the cave-to-outside-air thermal gradient is weakest; for example, from mid-morning to mid-afternoon, to permit ventilation without excessive drying of the cave.
The terms 'winter' and 'summer' airflow relate to the so-called 'chimney effect' brought about by changing thermal and moisture conditions outside the cave. They are not confined solely to winter and summer seasons. 'Summer' airflow conditions occur when air moves in through the upper entrance, down through the cave, and out the lower entrance. 'Winter' airflow conditions occur when air moves in through the lower entrance and out the upper entrance (de Freitas et al. 1982).
It is important to keep in mind the dual effects of ventilation controls, namely, cave moisture and heat on the one hand and carbon dioxide concentration on the other. Should visitation rates increase during the cooler parts of the year, then door-closing routines may need to be re-assessed. Reduced ventilation at these times may control desiccation of the cave environment, but may also reduce ventilation to the point where carbon dioxide concentrations rise to undesirable levels.
To stabilise cave microclimate, in 1980 a recommendation was made to the cave operators to seal the upper entrance and install an airtight door. As a result, the microclimate of the cave appears to have become more stable. However, subsequent data showed that the door may have been inadvertently left open at times when airflow through the cave is unwanted. The tour guides lead tourist groups into the cave and rely on the last member of the group to shut the door. For a variety of reasons the door may often be left open. To ensure that the door remains shut when required, in 1995 it was recommended that an automatic door closing device be installed, but managed according to ventilation guidelines outlined above.
Control of Visitor Numbers
From the studies of cave microclimate that followed the dramatic decline of glowworm numbers in 1979, it was recommended that visitation rates should not exceed 200 per hour. This limit has since been informally relaxed. There is evidence, however, that the presence of large numbers of visitors (>1000 per day) does have a noticeable effect on temperature and humidity in the cave during both the cool and the warm parts of the year (de Freitas 1990). So far, however, the effect does not appear to be large enough, nor of sufficient duration to have a discernible effect of cave microclimate generally. Nevertheless, as with carbon dioxide concentrations, management guidelines are that the effects of increasingly large numbers of visitors should be carefully watched.
A stress-free and safe cave environment for tourists is a primary concern for cave managers. Carbon dioxide is not routinely measured in the cave so concentrations are not known. Ideally, carbon dioxide concentrations should be measured in the cave at regular intervals, especially during periods of heavy visitor traffic. This would assist in decisions related to controlling cave visitor traffic and demonstrate responsible resource management. Visitor numbers could be restricted when carbon dioxide concentrations approach a predetermined, upper-acceptable threshold level. Measurements could be made less frequently at other times. Decisions about this need to be carefully considered as carbon dioxide levels in the cave can remain high for extended periods. For example, after a day of high visitor numbers, concentrations can remain high overnight, so that levels are abnormally high the next morning when the first group of visitors enter the cave.
It has been recommended that a study be commissioned to examine patterns of CO2 concentrations in the cave during periods of high visitor numbers. This information could later be used in analyses of cave air quality and in regulating visitor numbers in various parts of the cave, if necessary, during particular conditions of cave ventilation.
On-going Observation and Monitoring
Environmental conditions in the cave, especially those related to visitor traffic and flooding, need to be carefully watched and events noted for the record. Ecological data relating directly to glowworms should be routinely collected from sample populations, such as: number of glowworm lights; number of adults and pupae; number of fungal cadavers; samples of number of insects caught in glowworm lines; and number of harvestmen.
Disproportionately low minimum air temperatures relative to corresponding maximum temperatures may show up periodically. Likewise, elevated maximum air temperatures out of phase with minimum temperatures may occur at times. These may be due to several factors including: a) instrument malfunction; b) the thermal effect of increased visitor traffic; and c) the effect of increased rates of air exchange with the outside during periods of the day when the cave entrance door is left open. The occurrence of these and possible effects on the cave environment need to be carefully watched.
Formal Reporting Procedures
Quarterly data reports should be regularly scrutinised by cave management staff to check for continuity of the data record and instrument performance. Every six months these quarterly reports should be examined by an appropriate scientific expert and any evidence of environmental change discussed with the cave manager and Department of Conservation personnel. Every 12 months cave microclimate data should be formally reviewed by a qualified specialist to assess conditions in the cave and effectiveness of relevant management procedures. Periodically, but at less frequent intervals, detailed analyses of cave microclimate and related environmental conditions should be carried out.
Management and Decision making
Periodically, depending on a variety of factors, established management strategies may have to be re-assessed and guidelines modified accordingly. It is important that there are clear procedures in place to handle this. The nature and timing of communications between cave staff, the Department of Conservation and the Waitomo Cave Management Committee need to be explicitly defined. Important decisions need to have the formal approval of the Management Committee, or some other group or person charged with ultimate decision making authority and with overall responsibility for the cave. Depending on the nature and seriousness of the situation, specialist scientific advice may also be required, sometimes at short notice. To facilitate the decision making procedure it has been recommended that a standing, advisory committee of four people be retained. This group would offer advice on important decisions relating to management of the cave environment and those affecting the well-being of the cave ecosystem. The make-up of such a scientific and management advisory committee would be:
a) cave manager
b) Department of Conservation representative
c) cave climatologist
d) glowworm entomologist.
Reporting lines should be well defined, and areas of responsibility in decision making relating to cave management and development planning should be clearly specified.
Adequate environmental monitoring is vitally important to the proper management of the Glowworm Cave. But measurement alone is not sufficient. Regular, detailed, formal scientific appraisals of data by qualified personnel are essential. Casual or informal assessments and reliance on low cost options for monitoring are hard to justify for managing such an important national resource. An essential part of identifying and selecting appropriate variables to be monitored is an understanding of physical and biological processes that comprise the cave system; basically, how it works and what upsets it. Key reference criteria can be used in defining optimal conditions and maintaining them.
Conscientious cave management is concerned with defining acceptable environmental conditions and maintaining them. It involves selecting appropriate indicators, setting standards to be maintained, and monitoring to allow comparison to that standard. If necessary, operators will modify management strategies if standards cannot be consistently met. The choice of indicator-variables must take into account their representativeness and the feasibility of monitoring them. In the case of the Glowworm Cave, the undertaking to continuously monitor conditions in the cave using automated data collection systems has proved to be worthwhile. However, the quality of microclimate and environmental data collected using automated systems must meet acceptable standards. This is of the utmost importance if the data are to be of value for future analysis of the cave environment and for assessing the effectiveness of cave management techniques. Frequent monitoring at a few sites is usually preferable to occasional monitoring at many sites. The monitoring system should take into account the possibility of interference by visitors or vandalism, and intrusiveness of the monitoring equipment. In many cases, the presence of equipment may be built into site interpretation and commentary used during tours of the cave. Monitoring of the same key variables at the same sites should be maintained to give long term comparative data. Identification and analysis of many aspects of ecological well-being or change can best be achieved by considering medium to long term trends in environmental and associated data. The information collected will ultimately contribute to a substantial database essential for overseeing the well-being of the cave environment. In addition to being important for short term monitoring of conditions, the data will provide a vital retrospective record should conditions change or problems arise in the future.
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