Caves and Swallets - Gower - page 286

more and sometimes larger eggs. Like lacewings, some spiders place their egg sacs on stalks to protect
them from predators. The cave spider makes an exceptionally large sac with a small stalk.
The wild rock dove Columba livia has been extinct as a breeding species in Glamorgan since 1925,
butitsdescendenttheferalrockdove,theresultofcenturiesofinterbreedingwithferalpigeons,isacom-
mon breeding resident and regularly nests in a number of the coastal caves close to the entrance. Badgers
also use many of the smaller caves as ready-made setts and can penetrate considerable distances into the
systems.

ANIMALS OF THE DARK

There are two significant factors in the ecology of the dark zone of caves, the absence of light and a re-
markably constant environment. Temperature, for instance, usually stays constant at 8 or 9 degrees Celsi-
us, although it may vary in the vicinity of active stream passages. The lack of light means that the cave
community does not have a direct source of energy and that the food chain must generally start outside
the cave, and it is noticeable that animals tend to be more numerous in parts of caves which are near the
surface. This is particularly true of the fly Heleomyza serrata , which is common in many Gower caves,
usually where the rock cover is thin. There are considerable populations of animals in parts of caves far
from the surface, however, and where there are no obvious signs of any organic matter coming from the
surface. In these cases the animals appear to be feeding on a floccular material present on the surfaces of
stalagmites, which seems to consist largely of filamentous chlamydobacteria.
Jefferson found 30 species of invertebrates in the eight caves he explored, the richest cave being
Llethrid Swallet with 16 species (Table 15). Cavers too have noted a 'healthy fauna such as freshwater
shrimps and springtails' in the cave. The shrimps include the well-shrimp Niphargus fontanus , which has
also been collected from the entrance of Ram Grove Exsurgence where a strong spring of fresh water
rises near the foot of the cliff. This aquatic crustacean is a translucent eyeless animal totally adapted to
underground life, whose main areas of distribution are the Mendips and South Wales. A related species,
N. aquilex , has been recorded from the Red Chamber. Generally this species is only occasionally repor-
ted from caves, being found mainly in the gravel of riverbeds. Another shrimp found in Llethrid Cave
and which is often abundant in caves is the freshwater shrimp Gammarus pulex , which is actually not
a shrimp, but an amphipod. Some populations have reduced body pigmentation, showing an adaptation
to cave life. The shrimp feeds on decaying organic debris and is known sometimes to be predatory; it is
itself a major source of food for brown trout and other fish. The distribution of aquatic cave fauna such
as the well-shrimp is affected by a variety of factors including sediment type, food supply, water currents
and water temperature. The total population of some cave species is very small and a few may even be
restricted to one or two individuals in a single cave. As a result they are very vulnerable to extinction and
collecting is not to be encouraged.

TABLE 15. The invertebrates of Llethrid Swallet. (Adapted from Ford, 1989)

COMMON NAME SCIENTIFIC NAME

Enchytrachid worm Henlea nasuta

Copepod Acanthocyclops vernalis

Copepod Acanthocyclops viridis

Shrimp Gammarus pulex

Springtail Anurida granaria

Springtail Isotoma notabilis