Relation of the Species to Tidal Factors
The vertical zonations of the species studied are summarized in Fig. 5. This represents the basic pattern of zonation as found in a fairly sheltered situation. In determining the upper and lower limits an attempt has been made to eliminate as far as possible the effect of wave action. The effect of the breaking of the waves is to form a “wash zone” (Bokenham, Neugebauer and Stephenson,
1938), which raises the tide marks above the levels predicted in tide tables or recorded by tide gauges. The height of this “wash zone” is extremely variable, depending on the amount of shelter, the state of the weather, and the configuration of the rock. The influence of exposure on the vertical zonation will be discussed later.
Fig. 6.—Comparison of the vertical zonation of the same species at Narrow Neck, Piha and Taylor's Mistake. The vertical ranges of the species at the latter localities are reduced to the Taylor's Mistake tidal scale.
A comparison of the vertical zonation of some of the species at Taylor's Mistake with that of the same species at Stanmore Bay (Chapman, 1950), Piha (Beveridge and Chapman, 1950), and Narrow Neck (Dellow, 1950), near Auckland, has been made. Fig. 6 compares the zonation at Taylor's Mistake with that of Piha and Narrow Neck, the levels at the latter localities being reduced to the Taylor's Mistake tidal scale. The following table compares the vertical heights of the zones occupied by the same species in the three localities. (The vertical heights of the zones are given in feet.)
|Narrow Neck||Stanmore Bay||Piha||Taylor's Mistake||Taylor's Mistake corrected to tidal range of 10ft.|
|Tidal Range M.H.W.S.-M.L.W.S.||10.3||10.3||10.3||7.7||10.0|
From the figure and the table the following points are worth noting:—
In nearly all cases the vertical height of the zones occupied are much greater at Taylor's Mistake, although the tidal range is considerably less. The single exception is Modiolus neozelanicus, and an explanation for this discrepancy is offered below.
The upper limit of the majority of the species is higher at Taylor's Mistake, and generally, the lower the relative position an organism occupies in the intertidal zone the greater the elevation.
The lower limits of the majority of the species are lower at Taylor's Mistake, in some cases the difference being very marked. One notable exception is Modiolus neozelamcus. Under the conditions of exposures where the zonation was determined, the lower limit of this species is fixed by the presence of the encrusting growth of Pomatoceros coeruleus, which prevents the downward extension of the Modiolus zone. Where the encrustations are absent Modiolus extends down to M.L.W.N., having a vertical range of 6.0 feet.
The narrow band occupied by Pomatoceros coeruleus at Auckland is very interesting. The lower limit appears to be determined by the presence of a Sabellid, Sabellaria occurring immediately below; Dellow states that Vermilia (Pomatoceros) nearly always occurs above Hermella (Sabellaria), although the two species occasionally inter-mix. However, the combined vertical range is less than three feet; whereas the vertical height of the Pomatoceros zone at Taylor's Mistake is nearly five feet. Chapman and Beveridge state that at Piha the Hermella-Vermilia (Sabellaria-Pomatoceros) association is characterized by a distinctly restricted vertical range (about 18 inches). Elminius plicatus has a similarly restricted vertical range, Dellow stating that this species is locally dominant just above Mean Sea Level, where it may form a closed community, usually not exceeding one foot in vertical extent. At Taylor's Mistake the same species has a vertical range of five feet.
The elevation of the upper limits of the species at Taylor's Mistake when compared with the Auckland localities could be explained on the basis of temperature differences between the two regions. Gislén (1944) in an investigation of the littoral region of the Pacific coast of North America, found a depression of the levels of the communities when proceeding from a northern latitude to a
southern one; this would correspond to an elevation when proceeding from north to south in the Southern Hemisphere. The depression of the lower limits of the dominant sessile animals, Chamaesipho, Elminius and Pomatoceros, at Taylor's Mistake is more difficult to explain on the basis of temperature differences. It appears that in Auckland region the lower half of the midlittoral zone is occupied by algae to the exclusion of the fixed animal species. Dellow states that below the level of low water neap tides there is an abrupt change in the type of community; animals become of secondary ecological importance, and algae of one kind or another are physiognomic. No such marked change occurs at Taylor's Mistake, although Codium and Corallina may have a restricted local dominance. The lower half of the midlittoral zone down to the upper limit of the brown kelp is occupied by barnacles, mussels and Pomatoceros. The variations in the levels of the same species at the four localities serves to emphasise the complexity of the factors that control the zonation of intertidal organisms.
Exposure and Submergence.
The primary causal factor of intertidal zonation is the effect of the tide on the periods of exposure and submergence to which the plants and animals are subjected. As shown in the graph of percentage annual exposure to air (Fig. 2) marked changes in the amount of exposure occur between M. H. W.S. and M.H.W.N. and between M. L.W.N. and M.L.W.S All levels between E.H W.N and E. L.W.N are submerged and exposed twice daily, and as pointed out below this is the least critical region of the shore for intertidal organisms Levels above E. H. W.N. are subject to periods of continuous exposure—i.e., periods of more than 12 hours when no tide covers the area; while levels below E. L. W.N. are subject to periods of continuous submergence.
The percentage of monthly submergences and exposures at each level are shown in Fig. 7. There is a significant change in the percentage of submergences between 5 feet and 6 feet C. D.—i. e., between M.H.WN and E.H.W.N. From Fig. 5 it can be seen that a number of species have their upper limits between these levels. From the 5ft. level up the plants and animals are subject to increasing periods of continuous exposure. Above this level, therefore, the upper limits of the animals and particularly of the plants will be determined by the degree of desiccation they can tolerate. For the filter feeders a limiting factor is also the availability of food, which depends on the amount of submergence. The majority of suspension feeders, barnacles, bivalves and Pomatoceros have their upper limits between 6ft. and 5ft. C.D.—i.e, between M.H.W.N. and E.H.W.N.
A significant change in the percentage of exposures occurs between 0.5ft. and .0.5ft. C.D.—i.e., between ML.W.N. and M.L.W.S. From M.L.W.N. down the plants and animals are subject to increasing periods of continuous submergence, and it is significant that a large number reach their upper limits between M. L.W.N. and M.L.W.S.
The exposure factor is of great importance during the critical settling period of the life history of the plants and animals. As shown in Fig. 7, the percentage of monthly submergences shows a marked decrease from the 5.5ft. level up during August-October, the levels from 6ft. up being rarely covered. Periods of continuous exposure above this level must result in the death of newly settled larvae and sporelings. The percentage of monthly exposures also shows a marked
increase from the -0.5ft. level down during the same period. The lowest spring tides of the year are recorded during September and October, the times of the low tides occurring between 10 a.m. and 4 p.m. These low tides may also result in the death of the sporelings of the young plants of the brown kelps.
Colman (1933), Chapman (1941), Evans (1947a, 1947b), Beveridge and Chapman (1950) and Dellow (1950) have discussed the significance of critical levels on the shore and the possible factors that may account for them. Such levels have been recognised in the present survey, and a comparison of the results obtained with those of the above workers has proved most interesting.
From Fig. 5 the number of upper and lower limits and the total number of species occurring between -2.0ft. and -0.5ft., -1.5ft. and 0.0ft. etc., can be obtained as Colman (1933, p. 463) has described. These are shown graphically in Fig. 8.
Fig. 8.—Number of upper and lower limits, total number of limits and total number of species present at different shore levels.
From the graphs the following may be noted:—
There is a maximum number of lower limits lying between 0 0ft. and -1.5ft., C.D.—i.e., between M.L.W.S. and E.L.W.S. This marks the lower limits of Elminius modestus, Rizellopsis varia, Patelloida corticata and Cystophora scalaris.
Another maximum in the number of lower limits occurs between 1.0ft. and -0.5ft. C.D., or between M.L.W.N. and M.L.W.S. This level marks the lower limits of Elminius plicatus, Chamaesipho columna, Pomatoceros coeruleus, Codium adhaerens, Colpomenia sinuosa and Mytilus planulatus.
There occurs another smaller, though well defined maximum of lower limits between 6.0ft. and 4.5ft., C.D.—i.e., between M.H.W.N. and
E.H.W.N. The lower limits of Bostrychia arbuscula, Melaraphe cincta, Melaraphe oliveri and Cellana ornata occur here.
On the graph for upper limits there is a maximum marking the upper limit of the species of the infralittoral fringe between 1.5ft. and 0.0ft. C.D. or between E.L.W.N. and M.L.W.N. The brown kelps Carpophyllum maschalocarpum, Cystophora scalaris and Durvillea willana end here.
Another very prominent maximum of upper limits occurs between 6.5ft. and 6.0ft. C.D.—i.e., at M.H.W.N. This marks the upper limits of the majority of filter feeders, the barnacles, bivalves and Pomatoceros.
Between 7.5ft. and 6.0ft. C.D. there is a less well defined maximum of upper limits. This lies below M.H.W.S. and marks the upper limits of Cellana ornata, Notoacmea parviconoidea, Sypharochiton pelliserpentis and Chamaesipho columna.
The minima of the graphs for both upper and lower limits occur at about 3.5ft., C.D., or approximately at M.T.L. This marks the least critical level on the shore for the species studied.
In the region studied then there appear to be six critical levels for the species under investigation. These are:—
Between M.L.W.S. and E.L.W.S.
Between M.L.W.N. and M.L.W.S.
Between M.H.W.N. and E.H.W.N.
Just above M.L.W.N.
A comparison of the findings here with those of Coleman (1933), Evans (1947), Beveridge and Chapman (1950), and Dellow (1950) has yielded some interesting results. If the graphs in Fig. 7 are compared with those of Evans a marked similarity in the general trend of the curves and the positions of the maxima are noted. Evans, however, did not find a marked maximum at the point numbered 3—i.e., between M.H.W.N. and E.H.W.N.
There appears to be a good deal of similarity between Taylor's Mistake and the locality Evans studied at Cardigan Bay, Wales. Evans summarizes the climatic conditions at the latter locality in the following words: “The climate is typically northern cold temperature then, and the intertidal region is not subject to great annual variation; generally speaking conditions may be described as mild.” If the word northern were to be changed to southern the above description might well be a summary of the climate of the area studied in the present investigation. The mean annual range of sea temperature is about 20° F. in both localities, while the mean annual range of air temperature is 21° F. (11° C.) at Christchurch and 25° F. (14° C.) at Aberstyth, Wales. Also, the species studied by Evans, although in most cases belonging to different genera, are members of the same family or order. It would appear that under similar conditions these species have developed the same tolerances as regards exposure to air and wave action.
Beveridge and Chapman also recognise six critical levels for 36 species, of which 7 are animals and 29 are plants. Corresponding numbers for the present survey are 34 species, 21 animals and 13 plants, for that of Evans (1947) 28
species, 15 animals and 13 plants, and for that of Dellow (1950) 25 species, 8 animals and 17 plants. In view of the different proportions of animals and plants in the four surveys the correspondence in the relative positions of the critical levels as shown in the following table is very marked. As the critical levels given by Beveridge and Chapman (1950) were calculated by a different method to that outlined above, the date for Piha, as depicted by them (p. 200, Fig. 13), has been graphed and the critical levels determined as for the other surveys.
|Cardigan Bay||Taylor's Mistake||Narrow Neck||Piha|
|4. M.L.W.N.–||M.L.W.N.||M.L.W.N.||M.L.W.N.–1 0ft.|
Comparing the critical levels at Taylor's Mistake with those at Piha, it will be seen that level (2) is lower with reference to the tidal levels at the former locality; while levels (4) and (5) are higher and levels (1), (3), (6) and (7) are the same. At Narrow Neck level (1) is absent and level (6) is higher than at Taylor's Mistake. At all four localities the least critical level on the shore is in the region of M.T.L.
Graphs of the total number of limits at each level on the shore were compared for the four localities. In all cases the graphs showed two pronounced maxima, one in the lower and one in the upper region of the intertidal zone. The positions of these maxima are shown below:
|Lower Maximum||Upper Maximum|
|Taylor's Mistake||Just below M.L.W.N.||Between M.H.W.N. and E.H.W.N.|
|Narrow Neck||Between M.L.W.N. and M.L.W.S.||Between M.H.W.S. and M.H.W.N.|
|Piha||Between M.L.W.N. and M.L.W.S.||M.H.W.S.|
|Cardigan Bay||Between M.L.W.N. and M.L.W.S.||Between M.H.W.N. and E.H.W.N.|
In addition, at Cardigan Bay there was another lower maximum between M.L.W.S. and E.L.W.S. and a less pronounced second upper maximum of total limits between E.H.W.S. and M.H.W.S. However, when the varying proportions of plants and animals and the differences in the total number of species studied at the four localities are taken into consideration, the agreement in the relative positions of the principal critical levels is very marked. The results of all four investigations indicate that the two most critical regions which determine the vertical zonation of the plants and animals are between M.L.W.N. and M.L.W.S. and between M.H.W.S. and E.H.W.N. Between these levels there must occur a change in one or several of the factors that determine vertical zonation. The most obvious of these is the exposure factor. As pointed out above significant changes in the percentage of monthly submergences occurs between M.H.W.N. and E.H.W.N. and in the percentage of exposures between M.L.W.N. and M.L.W.S. The operation of this exposure factor is highly complex, depending on the nature of the tidal system. As Beveridge and Chapman (1950) point out, the tidal factor may be considered under three headings:
Hours of submergence and air exposure.
Periods of continuous air exposure or submergence.
Number of submergences and exposures.
The operation of these factors varies throughout the year, from month to month, as well as from week to week. Also they operate with different intensities at different stages in the life histories of the intertidal organisms. A period of continuous exposure that an adult plant or animal could easily tolerate might well be fatal to newly settled sporeling or larvae. More information on the tolerances of intertidal organisms at different periods of the life history is required.
As mentioned by Evans (1947b), the results obtained by any arbitrary method of graphing specific limits and deriving critical levels from the graphs will vary with the number and types of species studied, the degree of exposure to wave action, the presence of shading influences, rock slope, etc. In spite of these factors, the results of investigations on coasts in different parts of the world indicate that there are certain levels that can be regarded as being more critical than others in determining the vertical zonation of intertidal organisms. Results of the present survey support the following generalizations arrived at by Evans (1947b).
The regions between M.L.W.N. and M.L.W.S. is critical in the sense that the majority of intertidal plants and animals reach their lower limits here.
While certain sublittoral species penetrate into the littoral region, they generally extend only as high as the vicinity of M.L.W.N.
The region between M.H.W.S. and E.H.W.N. is critical for a large number of species in that they reach their upper limit here.
The least critical level on the shore is in the vicinity of M.T.L.
The most critical regions lie between M.H.W.S. and E. H.W.N. and between M.L.W.N. and M.L.W.S. It is in these regions that marked changes in the amounts of exposure and submergence occur.