of exposure as are provided by the tide factors. The tide factors are considered to be the primary factors controlling abrupt variations in secondary factors that, in turn, have a physiological, or at times physical, vertical range limiting effect on intertidal organisms.”

The above two theories are not necessarily antagonistic, but rather complementary. Granted that the three primary zones are due to the existence of an interface between air and water plus the influence of wave action, there still exists within the intertidal zone the often sharp upper and lower limits of different species of plants and animals. As discussed above it appears from the results of the present work and that of Coleman (1933), Grubb (1936), Zanefeld (1937), Beveridge and Chapman (1950) and Dellow (1950) that the action of the tidal factor does cause abrupt variations in environmental factors, and that the levels where these changes occur are related to the upper and lower limits of intertidal organisms. These tidal factors, as pointed out by Doty and Archer (1950), result in a large number of secondary factors that act on intertidal organisms with varying intensities and in differing combinations. The action of these factors is subject to modification primarily by wave action and the configuration of the rock.

It must be emphasized here that the results of tidal analyses and observations on the levels of intertidal organisms are approximate only, since, as Colman (1933) states, the ecological tide level is not that shown on the recorder, but that reached by the surf. Also a large amount of personal judgment enters into the determination of the upper and lower limits, as these are affected to varying extents by wave action, rock configuration and aspect. However, if a sufficient number of observations are carried out a reasonable estimate of the limits as they would be in the absence of these modifying factors, can be made.

The vertical range of the tide has little effect on the basic zonation, which is just as marked in Tasmania where the mean tidal range is only 1 foot 10 inches, as it is at Taylor's Mistake, where it is 7 feet, or on shores where it may be 30 or 40 feet. Intertidal organisms are still subjected to the same conditions of exposure and submergence no matter what the range of the tide may be, although conditions will differ according to the nature of the tides—e.g., in the region studied by Doty (1946) there occurs mixed semi-diurnal tides of such a nature that successive high and low tides are often of appreciably different heights.

The principal factors determining the upper and lower limits of plants and animals on the shore are:

The above factors interact in varying combinations and proportions for different localities and for different species in the same locality. Different factors may also operate in determining the upper and lower limits—e.g., the upper limit

of the barnacles C. columna and E. plicatus is determined largely by the amount of exposure and the degree of wave action while the lower limit is, in many localities, determined by competition with algae.

Rigg and Miller (1950) regard zonation as the resultant of two primary factors:

The conditions which Rigg and Miller list as vertical range limiting factors are secondary factors due to combinations of the principal factors listed above. A secondary factor limiting the upper levels reached by plankton feeders, such as C. columna, E. plicatus and Modiolus is food supply. The level reached depends on the minimum coverage by water that is necessary for the animals to obtain an adequate food supply, which in turn depends on the amount of submergence and exposure and the degree of wave action. Desiccation is also an important secondary limiting factor due to a combination of several of the principal factors, including submergence and exposure, wave action, aspect, angle of slope, temperature, sunshine and wind. The influence of sand in elevating the upper limits of the species studied at Taylor's Mistake has been noted above. Notable examples are the Littorinid snails M. oliveri and M. cincta, and the algae Bostrychia arbuscula, Ralfsia verrucosa and Codium adhaerens.

Chapman (1943) has proposed a division of the ecological factors affecting the vertical zonation of intertidal organisms into three groups:

Any of the primary factors mentioned above may, according to the species and the combination of other factors involved, be classified in one, two, or all three of the above categories. Wave action may act as a presence or absence factor, determining the presence of a species not adapted to withstand strong wave action—e.g., Bostrychia, or as a modifying factor, increasing the breadth of the zone occupied by many species, especially barnacles. Competition acts as a presence or absence factor where the dense Pomatoceros encrustations prevent the development of Codium, Colpomenia, Scytothamnus and Ralfsia, and as a modifying factor where the same encrustations considerably shorten the vertical heights of the Modiolus and Chamaesipho zones by restricting the lower level of these species to M.H.W.N.

As previously mentioned, the sandy beach at Taylor's Mistake is subject to considerable scour, the sand level on the inshore rocks varying up to two feet

in vertical height. In such localities the proximity of sand often acts as a presence or absence factor eliminating completely the infralittoral fringe with its characteristic algae. Laurencia is one species that can tolerate considerable coverage by sand. It also acts as a modifying factor raising the upper limit of the typical species of the midlittoral zone. The fixed animals of this zone have different ranges of tolerance towards coverage by sand, and can be arranged in the following series of increasing tolerance, E. plicatus, C. columna, M. planulatus, E. modestus, P. coeruleus and M. neozelanicus. The latter species is definitely the most tolerant and on the rocks in the centre of the beach there is often a band of Modiolus, up to a foot in width below the abrupt lower limit of M. planulatus and C. columna.

Wave action is one of the most important factors causing variation in the pattern of distribution of intertidal organisms, and its effect will be discussed in more detail.

At Taylor's Mistake, as in other parts of the world, there is a definite correlation between the intensity of wave action and the distribution of the algae. However, the majority of the algae are confined to the infralittoral fringe, and there does not occur an invasion of the mid-beach barnacle zone by fucoids with decreasing intensity of surf, as described by Evans (1947b) for the Plymouth region, Great Britain. There the invasion proceeds from low water and from high water towards the centre of the beach, until finally the barnacle belt is more or less obliterated. Apart from restricting the lower limit of the barnacles under certain conditions, the algae at Taylor's Mistake have little influence on their distribution.

The only species of algae that occurs in quantity in the upper littoral is Bostrychia arbuscula. For this species wave action acts as a presence or absence factor, the species being absent in exposed situations and the vertical height of the zone shows little variation. Wave action also acts as a presence or absence factor for the majority of the algal species of the midlittoral zone, Scytothamnus, Colpomenia, Splachnidium and Codium occurring only in semi-exposed to sheltered situations.

For the algae of the infralittoral fringe wave action acts both as a presence and absence and as a modifying factor. A gradual change in the species of algae can be seen from the outer, exposed headlands to the sheltered areas near the beach. In situations of extreme exposure the single dominant algal species is Durvillea willana or Durvillea antarctica, with an underflora consisting almost entirely of “Lithothamnion.” Corallina may replace the Lithothamnion where the wave action is slightly less intense. With increase in shelter Macrocystis appears and Durvillea is replaced by Carpophyllum, with the addition of Cystophora with further increase in shelter.

As a modifying factor the action of surf may elevate the upper limit of the infralittoral algae several feet. On steep faces subject to considerable wave shock the upper limit is not affected, while in localities where the actual wave shock is lessened by broken outer reefs, but where there is still considerable surge, the upper limit may be considerably raised. This effect is also noted on steep faces at right angles to the wave front. Such faces, while protected from direct shock, are still subject to considerable swell.

For the animals studied at Taylor's Mistake wave action, as for the plants, acts as both a presence and absence factor, and as a modifying factor. The animals