Cave fauna conservation in a Golden Bay cave
Abstract
Recent new initiatives by the Department of Conservation to increase activity nationally in invertebrate conservation have included the commencement of work on what was thought to be a unique and at-risk cave community confined to a small limestone outcrop in Golden Bay, New Zealand. Effort is being divided between intensive survey of this fauna, with a view to assessing conservation status of key species, and extensive surveys of neighbouring karst areas for the various species concerned.
The intensive programme has proven the most difficult to make progress in, especially with respect to establishing meaningful sampling protocols. However, progress is being made in the case of the gradungulid cave spider, Spelungula cavernicola. The extensive programme has already turned up what appear to be two of the main species concerned in two other karst areas.
Introduction
Council Cave (also known as Motupipi Cave) lies in a short section of hog's-back ridge formed in Tertiary limestone south of Dry River, two kilometres south of the village of Motupipi in the lower Takaka valley (Fig. 1). North from Dry River the limestone continues for over 7km as a series of similar hog's-back ridges and cuestas, cut through at intervals by streams arising on the Pikikiruna Range to the east; to the south it dips below softer strata and is lost from sight. This ridge feature appears to have been caused by bending up of a comparatively thin limestone bed as a result of uplift along the adjacent Pikikiruna fault. The limestone re-emerges at several places to the west of the ridge system, having passed through a synclinal structure, to form a series of limestone hills. The Council Cave limestone is therefore connected, normally below local water tables, with a similar hog's-back ridge north of Dry River. It is also possibly connected, via syncline, with the outcrop to the southwest at Payne's Ford Scenic Reserve. It has no known direct connections to the massive Ordovician marble outcrops forming the fault scarp less than 500m to the east.
The Council Cave hog's-back has almost certainly been burnt in the past and now has a vegetation cover largely consisting of dense stands of exotic invaders such as barberry and hawthorn. A few native species and individual larger trees persist. The cave lies partly under a tiny outlier of the nearby Dry River Scenic Reserve, partly under unformed legal road and partly under private land.
The cave consists of two parallel passages which converge to form the 20m of passage leading to the main entrance (Fig. 2). The floor of the easternmost passage is slightly higher in altitude than the other, with the height difference averaging 0.5-1m along their length. The lower passage is frequently water-filled and usually accessible for only one or two months after a summer drought. It lies close to the western edge of the limestone and has a series of low-lying entrances along this edge. This passage has been explored and mapped for approximately 165m beyond its convergence with the upper passage.
The upper passage is usually accessible, unless the main entrance area is flooded. This part of the cave is well-known locally and frequently visited. It has been heavily vandalised (graffiti, damaged formations) and a large proportion of the floor sediments, important as habitat for many of the species, have been compressed by visitor traffic. The passage extends for approximately 150m beyond its junction with the lower passage and ends in a rockfall chamber. In heavy rain it appears that this passage takes flood overflows.
The limestone collects substantial water runoff from pasture and grazing lands on either side, channelling it towards Dry River via the cave. The loss of the original vegetation cover in the catchment area, along with a degree of blocking of the outflow stream which seems to have occurred as a result of road construction some years ago, has probably led to the deep silt deposits which are a permanent feature of the lower passage. This may have led, also, to a greater frequency of flood overflows entering the upper passage.
Council Cave harbours an apparently unique community of cave-dwelling invertebrate species. For a number of these species this cave was, until recently, the only known habitat. On the basis of its fauna Johns (1991) ranked it as one of the most important caves in New Zealand. There is no obvious reason why this cave community should be confined to the Council Cave limestone, but searches have failed to find it, and some of its individual species, elsewhere.
Recent new initiatives by the Department of Conservation to increase its activities nationally in the area of invertebrate conservation have included the commencement of work on this fauna. The priority is to establish the conservation status of the rarer elements of this fauna, to assess from this what management, if any, is required for these species, and to determine whether this community is truly unique or whether the Council Cave habitat simply makes it more accessible than elsewhere.
THE CAVE COMMUNITY
At the start of this project Council Cave was the only known locality for the following species:
Confuga persephone
C. persephone is a cixiid bug which feeds on tree roots which penetrate the cave from the surface. It is the only known species of troglobitic cixiid in New Zealand (Johns 1991) and the sole member of its genus (Fennah 1975). It is considered to be potentially the most at-risk of the Council Cave species because of its reliance on the continued presence of plants of palatable species on the surface. What its preferred host species is/are is unknown. Also unknown is whether the roots of the exotic adventive species which have established over its habitats are acceptable hosts.
This species is rarely encountered in the cave, despite the large numbers of plant roots evident on the cave floor in several places in the upper passage. Most sightings have occurred in one area in the upper passage about 140m in from the entrance.
Burmjapyx sp.
This species belongs to the family Japygidae in the primitive insect order Diplura. As with other members of the family the cerci at the rear of the abdomen are developed into a pair of sclerotised pincers, making the animal look rather like an anaemic earwig. Although all species within this order are eyeless, Johns (1991) considered this one to be troglobitic.
This is the most regularly encountered of the troglobitic species in this cave. It can be found in association with detritus through much of the upper passage, beginning about 60m into the cave. The adults reach in the order of 30mm length and are a pale yellow-white colour, making them relatively conspicuous.
Cryptops sp.
This is a centipede species which Johns (1991) considered likely to be troglobitic. A species assumed to be this one is encountered occasionally in the cave. It has been seen in at least three sites, usually with detritus and other fauna in evidence nearby. A positive identification of this species has yet to be made.
Haasiella sp.
A second centipede species, apparently known only from Council Cave. Johns (1991) does not indicate whether or not he considered this species to be troglobitic. This species is not known to have been encountered in the course of this work.
Species which were known to occur elsewhere are:
Duvaliomimus pluto
As presently defined this troglobitic carabid has a widespread, anomalous distribution which includes the Oparara caves on the West Coast, caves on coastal hills north of Westhaven Inlet and Council Cave. It is likely that this species will be split in a forthcoming revision, making the populations in the Motupipi area a distinct species (J I Townsend, pers. comm.).
This species has not yet been encountered in Council Cave in the course of this work. The beetles of this genus are usually associated with wetter areas of caves, such as active streamways.
Pholeodytes cerberus
As presently defined this troglobitic carabid beetle is also problematic. It is also found in the caves of the Oparara valley, as well as Takaka Hill south into the marble adjacent to Council Cave, and elsewhere in the Takaka valley. Within this complex, two distinct forms of this species have been recognised. Some specimens collected from Council Cave were significantly smaller than specimens from elsewhere (R Emberson, pers. comm.). This species is also in need of revision (J I Townsend, pers. comm.).
This species is encountered quite regularly in Council Cave, usually in areas with some detritus nearby. In general beetles of this genus seem to prefer drier situations than those favoured by Duvaliomimus, and they are often amongst the most widespread of troglobitic species in caves.
Hendea takaka
This is one of the short-legged harvestmen of the sub-order Laniatores. Council Cave is the type locality for this species, which has also been collected from caves in the Canaan area on the Pikikiruna Range (Forster 1965). This species is troglobitic. It has only occasionally been encountered in the course of the present work.
Nuncia marchanti
Members of this genus are generally bulkier than their relatives in the genus Hendea. This species appears to have a very widespread distribution, including caves at Paturau and in the Buller Gorge, as well as Council Cave. Nonetheless it is considered to be troglobitic. It is regularly encountered within the cave, usually in areas with detritus and other species present.
Spelungula cavernicola
This spider, the largest New Zealand species in the ancient family Gradungulidae, has the distinction of being the only New Zealand spider species which is protected under the Wildlife Act. Its strongholds are the caves of the Oparara and lower Heaphy Valleys in Buller District, some 60km southeast of Council Cave. It is unknown from caves between the two areas, including those of the Aorere Valley and the marble caves on the western side of the Takaka Valley. S. cavernicola is the sole member of its genus (Forster et al 1987). The biology of the Oparara populations has been investigated by McLachlan (1993).
Spelungula has functional eyes and none of the troglomorphies normally associated with troglobitic species. It appears to be confined to caves and is usually found in dark zones not very far from entrances where cave weta (cave crickets - Rhaphidophoridae), its major prey, may also be found. This species appears to fit the definition of a "second level troglophile" (Eberhard & Spate 1995). It is a "sit and wait" predator, which does not utilise a web for snaring prey. It produces a large (2.5cm long by 2cm wide) egg sac which is suspended from a thick silken thread up to 9 or more centimetres long. These sacs often remain long after they have hatched: sometimes for well over a year in favourable conditions.
In Council Cave Spelungula is found to occur predominantly from the junction of the two passages to a point about 70m into the upper passage from the main entrance. One or two animals are also found regularly in a rockfall chamber at the termination of this passage, 170m from the main entrance. Neither spiders nor any sign of them (exuviae, egg sacs) have been seen in the intervening passage in the course of this programme. The species also occurs in higher parts of the roof in the lower passage in a limited area adjacent to the main spider habitat in the upper passage. These high roofs are generally above flood level.
Recently, Spelungula has been found in a second cave in the Motupipi area, in a limestone block 6km northeast from the Council Cave limestone. The species and its sign are only occasionally encountered in this cave, suggesting that it may be utilising more extensive habitat which is inaccessible to humans.
Of these two groups of species, conservation interest centred primarily on those species which were known only from Council Cave and on Spelungula, for which Council Cave was one of only two known habitats within Golden Bay. The Council Cave population is also the most accessible of the Golden Bay Spelungula populations.
Figure 1
Figure 2
Surveys within Council Cave
A series of surveys within Council Cave was undertaken to establish distributions and relative numbers of the species of interest. In the course of these it rapidly became obvious that some species could not be found with any consistency. This was especially the case for Confuga and the two species of centipedes. For these species, setting up meaningful monitoring protocols will be difficult at best.
Confuga persephone has proven to be the most elusive of the target species, having been found on only five occasions in the cave during searches over the past eight years. The type series from which this species was first described by Fennah (1975) consists of 11 adults and eight nymphs collected on four separate days in 1973 and 1974. These collections were made by from one to three people. This rate of collecting suggests that this species may have always been uncommon within the cave, but possibly less so 25 years ago than now.
Howarth (1983) has shown that for many obligate cave-dwelling species (troglobites) the primary habitat is not the large open cave passages penetrable by humans (macrocaverns), but smaller, associated voids of the order of 1mm - 20cm in diameter, which he refers to as mesocaverns. As a utiliser of tree roots which penetrate the cave from the surface through such voids, it seems logical that Confuga should fit this pattern. Interestingly enough most of the roots which are evident in Council Cave appear out of the floor sediments, rather than directly from cracks in the cave walls or ceiling, suggesting that quite circuitous routes are taken into the cave passage.
A survey has been made of plant species growing on the limestone above the site in the cave where most sightings of Confuga have occurred. The list of species includes a number of both indigenous and adventive species which would be capable of sending roots deep into the limestone. Identification of roots penetrating the cave passage has yet to be attempted.
The dipluran Burmjapyx sp. is found with some consistency through a major part of the upper passage. On present knowledge this appears to be the most numerous and frequently encountered of the target species. No assessments have yet been attempted of the numbers or population structure of this species but such a programme may be initiated in the future.
The spider Spelungula is likely to prove to be a suitable subject for monitoring. Not only are the animals relatively conspicuous because of their size, but they often leave other evidence of their presence in the cave, such as their egg sacs and exuvia after moulting.
Individual adult or large juvenile animals seem to occupy certain areas within the cave for perhaps several weeks at a time before either moving or being displaced. These "residence" periods are not long enough to provide really useful information on such things as species numbers etc. and it is clear that a more invasive method such as marking individuals will be required to gain this information. The major concern with such methods is that the individual animals are easily stressed by too much light and activity close to them and will often take flight when touched. This could in turn affect movements and displacements within the population. A limited duration marking study will be undertaken to assess population size and stability, after which the requirements for longer term monitoring can be assessed.
As surveys have progressed it has become clear that much of the floor of the upper passage is inundated with water regularly during the wetter times of the year, presumably from its close association with the slightly lower streamway. From this, detritus and muds seem to be replenished within this passage at a rate sufficient to maintain a rich food supply but low enough not to build up excessive deposits. In effect the passage provides an apparently ideal environment for troglobitic fauna, with comparatively high stability combined with nutrient replenishment. The larger detrital deposits in this passage have been found to contain a range of fauna, including both cave-limited fauna and surface species, such as earthworms. These are potential foods for the larger, predatory troglobites. For example, the large opilione Nuncia marchanti has been observed carrying pieces of freshly excised earthworm.
Surveys in other karst areas
At the time of this conference surveys have concentrated on two other limestone karsts in the lower Takaka valley - Motupipi area.
Four kilometres northeast of Motupipi, at Pohara, a large limestone outcrop on the western arm of the syncline underlies the Hanson-Winter Scenic Reserve and associated private land. Two caves within this block, Murray's Cave and Water-supply Cave, have been searched for fauna. None of the species associated with Council Cave have yet been found in Murray's Cave.
Water-supply Cave, in Tasman District Council's water reserve in the Pohara valley, became only the second known habitat in Golden Bay for Spelungula cavernicola when this species was found there by S Courtney (pers. comm.) in 1995. With this link the cave became an obvious place to look for further elements of the Council Cave fauna.
Searches of this cave yielded only the harpaline carabid beetle Pholeodytes cerberus which is widespread in caves in the Takaka area, so baited pitfall traps were placed in the cave in September 1996. These yielded more specimens of Pholeodytes and a single specimen of Duvaliomimus pluto, which is likely to be referable to the same species as that from Council Cave when the revision of this group is completed (J I Townsend, pers. comm.). In addition a single specimen of a late juvenile stage japygid dipluran was taken. This specimen has yet to be compared with specimens of Burmjapyx sp.
The second site of interest is the limestone block underlying Payne's Ford Scenic Reserve, about three kilometres southwest from Council Cave. This block may be connected to the Council Cave hog's-back below ground level through a synclinal structure, although separation of the two through hidden faults is possible. There are at least two caves known in this small karst unit: Irvine's Cave, a surprisingly complex cave developed on three levels within the limestone, and Irvine's Tomo, formerly a part of the Irvine's Cave system, which is entered by a short vertical shaft. Irvine's Cave, despite its length and complexity, does not appear to be a good cave fauna site (personal observations; J I Townsend, pers. comm.).
Irvine's Tomo also has passage remnants at several levels in the limestone. In late 1996 N. Mountfort and H. Dixon noticed five Confuga-like bugs on tree roots in a breakdown alcove off the main passage of this cave, not far beneath the land surface. More specimens were found in February 1997 and these appeared to be identical with Confuga. However the only specimen collected on this occasion was a large nymph, and mature males are required for certainty of species identification in this group (M-C Lariviere, pers. comm.). Locating a male from this site for species determination is a major priority. No other members of the Council Cave fauna have been found in this limestone to date.
Conclusions
The intensive fauna surveys in Council Cave have shown that while some of the species found in this community may be suitable subjects for population assessments and monitoring, others are likely to be more difficult to access using available techniques. For these latter species some lateral thinking in the approach to assessing conservation status is required.
There is insufficient information so far to determine the full significance of the Council Cave fauna community. However in the interim it is desirable that this site be protected as far as possible to preserve this fauna. An important consideration must be control of the more aggressively invasive elements in the adventive flora on the surface and their replacement with indigenous species whose roots may host the cixiid C. persephone. There is also a need to identify actual host species for this bug.
The more extensive survey programme has shown that either elements of the Council Cave fauna or closely related species may be found in other lowland limestones in the area. This part of the study should be continued by extending it to other areas and searching more intensively in habitats which hold some of these species.
A clear difficulty in this project has been the "taxonomic impediment" — the lack of formal descriptions of a number of the species involved. This makes it impossible for staff undertaking the work to be certain, in some cases, which species they are dealing with, and whether or not similar animals found in other sites are of the same species. With both these and some of the named species a significant input from specialists will be required to complete this work, making close comparisons of apparently similar fauna from different sites to determine exact relationships.
Update
Since giving this paper a more intensive programme of population monitoring of Spelungula cavernicola has been initiated, involving marking of individual animals. Results of this programme will be reported on in the future. Also, a third Golden Bay site for this species has recently been located: an unnamed cave on the north side of the lower Pohara Valley, in the limestone block which has been intensively quarried in the past by the Tarakohe cement works.
Acknowledgements
I thank my colleague Nigel Mountfort from the Golden Bay Area Office, DOC, for assistance and company in the field and his continuing interest in this programme. Garry Holz prepared Figure 1 for this text.
References
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