by B.A. Foster THE EFFECT OF ASPECT ON POPULATION COMPOSITION

Transcription

1 TANE (1966) 12 : THE EFFECTS OF WAVE EXPOSURE AND ASPECT ON THE VERTICAL SHORE DISTRIBUTION AND POPULATION COMPOSITION OF MELARHAPHA OLIVERI. by B.A. Foster INTRODUCTION The periwinkle Melarhapha oliveri Finlay characteristically inhabits the supra-littoral fringe on rocky shores of New Zealand. The variation in vertical extent and composition of the populations can be correlated with essentially local conditions. This paper demonstrates this variation on the Auckland shores. THE EFFECT OF THE DEGREE OF WAVE EXPOSURE ON THE VERTICAL DISTRIBUTION Fig. 1 shows the vertical distribution of M. oliveri at two localities on the lower Auckland Peninsula. The slope of the rock has not been taken into account, except that where measurements were made the slope was reasonably uniform and greater than 45. Sites were chosen to present as few irregularities as possible to influence the effect due to wave action, but such irregularities are inherent in field work of this nature. Silvester (1963) states that for an open coast area at Leigh, M. oliveri extends up to 10 feet above E.H.W.S. on southern faces (i.e. a vertical elevation of about 11 feet), but "on the north and east facing sides, where exposure is greatest, the wide storm platforms and the drying effects of the sun tend to depress the upper limit as much as 3 feet". The vertical distribution of M. oliveri at Otata Island in the Hauraki Gulf is 10 feet (Haughey 1963). On Narrow Neck reef, M. oliveri has a vertical extent of 5 feet. M. oliveri is not a characteristic species of harbour or estuarine shores, but isolated populations can be found on such shores where long fetch distances are offered across the harbour water. Woods (1963) records finding Af. oliveri from 900 to 1200 yards up the Marakopa Estuary. In such situations the vertical distribution is rarely in excess of two feet. viz. THE EFFECT OF ASPECT ON POPULATION COMPOSITION To compare populations of M. oliveri, (i) density; numbers per given area; three parameters have been used: (ii) mean frequency; 50% on a cumulative graph based on numbers per size group; (iii) mean biomass; 50% on a cumulative graph based on weight per size group. To obtain these parameters, all snails longer than 3m.m. in length were collected from a suitably wide strip of the shore, i.e. over the entire vertical distribution, the density calculated, the snails sorted into millimetre size groups and each size group weighed. Data was collected for two localities; A. Lion Rock, Piha, for northern and southern aspects:

2 THE VERTICAL EXTENT (IN FEET) OF POPULATIONS OF MELARHAPHA O L I V E R I A T T w LOCALITIES ON THE LOWER AUCKLAND PENINSULA.

3 39 and B. Sentinel Rock, Mangawhai, for northern, southeastern and southern aspects. Graphs representing the composition of the populations at these localities are given in Fig. 2. The population parameters have been extracted and are given with the densities in Tables 1 and 2. A population from a crevice on the north of Sentinel Rock had a density of 25 snails per square foot, but had a mean frequency of 4-5m.m. and a mean biomass of 5-6m.m.. DISCUSSION The extent that populations of M. oliveri reach above the actual high water level of the tides depends very effectively on the amount of sea water reaching these levels in the form of splash or spray for at least part of the time. The upper limit of the distribution is probably set by some factor attributable to the lack of sea water, e.g. desiccation, starvation because of restricted feeding time, or the accumulation of toxic waste products that cannot be voided. M. oliveri is very much a marine animal, depending on the sea for dispersal of eggs and larvae, but is able to tolerate wide ranges of such factors as temperature and salinity that are characteristic of the intertidal environment. Speculation as to the causes that set the lower limits of the intertidal distribution is not so easy. The distribution of juvenile snails can be correlated with the amount of physical shelter available (Fig. 3). On the south side of Sentinel Rock, empty barnacle (Chamaesipho brunnea) provide shelter for snails up to 3.5m.m. long. On one occasion 15 empty barnacle shells contained on the average 6 to 7 snails. With 612 empty barnacle shells per square foot, it is estimated that there was a density of 28 juvenile snails per square inch. Adult snails invariably exhibit a tendency to crawl out of sea water and congregate about and above the water level, as shown by laboratory tests (Fig. 4), and by shore experiments (Fig. 5). Adult snails (2-12m.m. long) occur among the barnacles but in general are more prolific above the barnacle line. The distribution of the adults may be associated with grazing space as well as with shelter. The causes and mechanisms for the maintenance of the shore zonation of M. oliveri, and the physiological and behavioural adaptations that permit M. oliveri to survive in the upper shore provide scope for intensive study. It is tempting to establish the correlation of the vertical extent of M. oliveri and wave exposure as a biological index of wave exposure; i e. a scale from 1 to 50 (being the vertical extent in feet) representing very sheltered to very exposed conditions. But to be strictly applicable, every factor but wave exposure and its effects would need to be constant, a condition which is never realized. Such a scheme might have limited use, however, in comparing nearby shores, e.g. those within the Hauraki Gulf. A representative sample of snails over 3.5m.m. in length can easily be made over a portion of a shore. The size distribution of 1118 snails collected from open coast areas at Mangawhai is shown in Fib. 6. Considerable mortality occurs among the various size groups of Af. oliveri, a snail with no known predators and of which very few empty shells may be found. The European "ecological equivalent" of M. oliveri, Littorina neritoides, suffers some predation from rock pipits (Gibb 1956); and may experience some effects of trematode parasites (Lysaght 1941). Dislodgement by wave action seems a likely source of mortality among all size groups. In view of the domination influence of juvenile snails in any

4 4 S C 7 S 9 lo 11 SOUTH mm mtm gro«i FIG. S. TO SHOW THE VARIATION IN POPULATION COMPOSITION OF MELARHAPHA OLIVER! WITH RESPECT TO VARYING ASPECT ON THE SAME ROCK MASS

5 I t i i i i i i i i i r O */ barnacle cover snails per sq. inch FIG MINUTES FIG. 4

6 42 population, and of the difficulties of taking a sample to include a significant number of the larger snails, the comparison of populations based on frequency of size groups is likely to be somewhat erroneous. In Fig. 2 the graphs for cumulative percentage frequency is true only for snails over 3m.m. in length. The influence of the juvenile size groups on the total biomass of a population, however, is slight, as is the comparatively rare occurrence of snails over 10m.m. long. Consequently the cumulative percentage weight graphs provide fairly accurate parameters of populations. Comparison of the populations on the north and south aspects of the large rock masses indicates relatively more larger snails on the southern faces. On the average, snails attain a greater size on the shaded aspects, i.e. the environmental conditions there are more favourable to survival. Responses of snails to various environmental stimuli lead to aggregation in more favourable microhabitats such as cracks and crevices, and this is shown in the two populations sampled on the northern aspect of Sentinel Rock and which are of the same population. The density could be taken to represent the favourability of a particular set of environmental factors, including time. The difference in the density of the northern and southern populations does appear to have some permanent reality however, even though only snails over 3m.m. in length are taken into account. A reduced settling density on the southern aspects may result in less intraspecific competition for grazing. Also, the generally damper aspects will offer better growing conditions for algal and lichen sporelings (on which the snails are thought to feed), and longer grazing time for the snails. If these speculations are only in part true, then the problem would reside in the differing rates of settling of juveniles on shores of differing aspect, or on the present evidence, on the mortality in snails less than 3m.m. long. If the latter is the case then predation or any other operative mortality factor, has its effect modified according to the aspect. Although the correlation of certain features of the environment with variation in the distribution or composition of populations may be demonstrated, explanations of the causes requires an approach utilizing the techniques of experimental ecology. DESCRIPTION OF DIAGRAMS Fig. 3. Correlation of density of juvenile Melarhapha oliveri ( 2m.m.) with the density of barnacles (Chamaesipho) on the shore at Bream Tail. Fig. 4. The emergence of groups of 10 snails after being placed in fresh sea water, in the laboratory. Specimens collected from: A sunbaked supralittoral fringe rock surface; B shallow supralittoral fringe pool; C sundried rock amongst barnacles; D moist crevices amongst barnacles. Fig. 5. Diagrammatic representation of a shore experiment on a vertical rock face at Sentinel Rock, Mangawhai. Groups of snails were displaced from zones A and B to lower levels. After 24 hours the snails had returned to the individual levels as shown by the arrows. The percentage survival of the groups of displaced snails is shown. The effective high tide level was raised about 2 feet above the predicted high tide level by wave action. Fig. 6. Frequency of 0.4m.m. size groups among 1118 Melarhapha oliveri collected from exposed coastal regions at Mangawhai.

7 FIG. 6

8 44 SUMMARY The vertical extent of populations of Melarhapha oliveri can be correlated with the degree of wave exposure. A maximum vertical range of 42 feet was found for an exposed locality. Southerly facing shores support a relatively sparser population of snails greater than 3m.m. in length than do northern shores, but the snails tend to attain a greater size on the average. Gibb, J. Haughey A. Lysaght A.M. REFERENCES 1956 Food, feeding habits and territory of the rock pipit Anthus spinoletta. Ibis, 98, An observation on the tides at Otata Island. Tane, 9, The biology and trematode parasites of the gasteropod Littorina neritoides (L.) on the Plymouth breakwater. J. mar. bio. Ass. U.K., 25, Silvester W.B An introductory study of the marine algal ecology of an open coast area at Leigh. Tane, 9, Wood D.H A study of the macrofauna of an exposed "iron sand" beach, and a nearby estuary. Tane, 9, North South Density nos./sq.ft. Mean frequency m.m. mean biomass m.m Table 1. Population parameters for M. oliveri at Lion Rock, Piha. North South East South Density nos./sq.ft Mean frequency m.m Mean biomass m.m Table 2. Population parameters for M. oliveri at Sentinel Rock, Mangawhai.