Variation in Rocky Shore Populations

Variation in abundance and distribution of intertidal rocky shore populations in relation to the extremities of the environment

Characterized by patchiness in the distributions of abundance of species in space and time, intertidal zones have been studied for decades in order to isolate the several important ecological interactions that cause these diverse assemblages of plants and animals (Underwood & Chapman,2000; Underwood & Chapman, 1998; Underwood, 2000). In order to recognize these environmental factors, two sampling techniques were implemented to obtain the vertical abundance and distribution of selected biota. By applying basic principles of methodology in both of the two ecological sampling practices, a comparison of patterns of biota was formed, thus the ability to quantify distribution and abundance of species on a rocky shore. This resulted in patterns depicting strong evidence for environmental pressures as a defining element of distribution and abundance.

Marked by the upper and lower limits of the tide, the shores intertidal zone is exposed at low tide and immersed at high tide. Sampling took place on a rocky platform of Flinders beach, Victoria, and had a vertical range of between 0 – 95 meters, with 0m being the point furthest from the shore, known as the high intertidal zone, and 110m being the point closest to the ocean, otherwise known as the low intertidal zone. Two sampling methods were employed, each covering at least 95 meters. In the type I technique the area of 0-95 was divided vertically in 5m points, each of which sampling took place. At each transect, quadrats were placed unsystematically on a horizontal axis 3 times over. For each quadrat both selected flora and fauna were identified and tallied either individually or for area cover. In the type II, the area of 0-110m was divided at 10m intervals, where at each sampling took place. At each transect, quadrats were again placed arbitrarily on a horizontal axis, however, this was repeated 6 times as opposed to 3. As in the type I, both selected flora and fauna were accounted for; either as individuals or in area coverage.

As a general pattern, organisms appeared to be larger and more complex in the lower parts of the shore. In particular, this pattern was seen true for most mollusc species. As seen in Figure 1, Austrocochlea constricta population density was highest between 20-90m, and seemed to not appear before the 10m transect. The distribution of the species seemed to be fairly constant throughout these 70m. In addition to Austrocochlea constricta, the Siphonaria sp was also not accounted for before 15m transect however did show varying results between the sampling methods in where the abundance was at its peak (Figure 2). Straying from this general pattern, the Notilittorina sp was accounted for only between the 2-40m area, with the abundance most dense at the higher parts of the shore. Most flora seen were accumulated in the latter parts of the shore, with Coralline Red occurring between 20-90m. Coralline Red abundance percentage steadily increased as we moved to the lower parts of the shore.

Intertidal habitats are of extensive interest due to the varying microhabitats formed across a shore. The environment of each microhabitat differs, however harsh extremes are experienced at both low-shore and high-shore, and all in between. For an average rocky shore, there are usually two high tides and two low tides per day, and depending on the time of day, some areas of the intertidal zone may be wet or dry (Underwood & Chapman, 2000). This can potentially be problematic for the inhabitants of such an extreme environment. Water supply is essential for semi-marine and marine organisms. However, water supply is intermittent. Water is also delivered in the form of waves, which, if applied with enough force, is capable of washing away or dislodge poorly adapted organisms. Due to the formation of these zones, the salt water trapped in the rock pools within does evaporate, elevating salt concentration. However this concentration can alter with rainfall. In addition, the intertidal zone is highly exposed to the sun, hence the temperature can range from very hot to near freezing in frigid climates (Underwood & Chapman 2000). The interaction of these four factors presents the intertidal zone as an extreme environment in which to live.

In order to survive an intertidal zone, inhabitants must be able to withstand harsh abiotic and biotic stress. As height above sea level increases, the conditions of the intertidal zone strengthens in harshness; there is less moisture, daily changes in pH and salinity and temperature, and isolation is increased. (Womersley & King, 1990). As expected, as harshness increased, species diversity generally declined, with fewer species able to withstand such intense stresses (Underwood & Jernakoff, 1981). Abiding by this general rule, Austrocochlea constricta provided the perfect example with majority of the sample abundance in both type I and II occurring after 30m. Part of this may be attributed to the feeding patterns of Austrocochlea constricta, which tend to scrape the algal film coating off rocks (Parsons & Ward, 1994). However, in the type I sampling, more Austrocochlea constricta were found before 30m than those observed in the type II sampling (Figure 1 & 5). This could be attributed to many environmental factors, but ultimately the elevation of the rocks and the impact at which waves from the last tide hit those rocks would be a very significant factor. If the area surveyed in the type I had rocks in a closer vicinity to each other, there would be less chance of a high impact wave. Desiccation was also of particular concern on the day of sampling due to the very high temperature of 30 degrees Celsius. With this environmental pressure, Austrocochlea constricta have been known to cluster to conserve moisture (Underwood. &. Chapman 2007), therefore we can assume for the higher abundance transects, the molluscs would have been clustered highly rocky creviced area.

Going against the norm, there are those species that adapt to the harsh extreme of the high-shore, obtaining the least moisture and risking desiccation. The organisms that locate themselves in the high- mid intertidal zone, such as the Nodilittorina species, common name periwinkles, use their operculum as a ‘trap-door’ like structure to prevent desiccation in the low tide (Reid, 1989). As seen in both the type I and type II sampling in figure 2 & 6, the majority of the Nodilittorina species was found before 40m, however the peak abundance was seen to differ between type I and type II techniques by 30m. Due to the diverse range of altitudes of rocks found along the rocky shore of Flinders, this discrepancy could easily be accounted as a result of a high elevation of rock within a semi-moist area in the type I sampling, which portrayed the highest abundance of periwinkles at 0m. However, in light of this, this variation could also be a result of a type 1 error, in which the sample gathered was misconstrued or reported incorrectly.

Although most species followed a pattern of some kind, samples from Siphonaria species were mirror opposites in the different sample techniques. In type I, as seen in figure 3, peak abundance occurred at the 20m transect and populations were rare after 45m. Opposing this pattern, type II samples indicated that populations were sparse before 60m with a peak abundance t 80m (figure 7). As with the Notilittorina species, this again can be attributed to the extremes of the environment taking place at varying zones in the intertidal region. The rocky shore can ensure that some discrepancy will occur due to elevated rocks, resident rock pools and therefore increased salinity in nearby areas, and the exposure to the sun. In most environments, random sampling is considered to be a true representation of a selected population. However, due to the combination of haphazard sampling and the varying microhabitats within an intertidal shore, it cannot be confirmed that the same conditions of each quadrat were used in both type I and type II techniques, hence a true representation of Siphonaria may not be possible.

When focusing on the flora of rocky shores, it is important to note that while the same environmental stresses act upon plants, the respect in which they affect the plants differs from fauna. For ideal growing conditions, algal organisms require decreased emersion, increased moisture, and decreased temperature on a low tide (Underwood & Jernakoff, 1981). However while the low tide only occurs twice daily, wave action associated with tide is essential for the existence of the algal. Wave action, while can result in the uprooting of the plant from the substrata, is also important in driving distribution, as it can alleviate desiccation (Lubchenco & Menge,1978). Biotic factors also are imperative and influence species abundance significantly. As shown in previous studies of the east Australian coast, particular zones of the intertidal shore are occupied by distinct groups of organisms. In the lowest areas, foliose algae are seen to be copious. (Underwood & Kennelly, 1990) However, in the mid-shore zone, encrusting algae, and the presence of gastropod grazers frequent the region (Underwood, 1981). This is reflected the results from Corraline Red in both the type I and type II sampling techniques (figure 4 & 8). Steadily increasing form 25m, the abundance percentage of coralline red was at its peak when the abundance of the mollusc grazers, Austrocochlea constricta, Notilittorina sp, Siphonaria sp, were at their lowest.

Patterns of intertidal species are complex on both minute and larger scales. The patterns, caused from a range of environmental pressures, can change unpredictably and predictability over various time scales. The study at Flinders rocky shore indicated that generally, with decreasing height above sea level, there is increasing species diversity. The presence of algal grazers and physical stresses of living at intertidal regions are dominant causes of variation in the vertical distribution of floral and faunal species. Some environmental pressures particular for that day, such at temperature, may have skewed our results, as it accounts for a strong selective pressure as to which zones species reside. Although the sampling techniques covered a generous area and gave a somewhat true interpretation, the nature and physics of intertidal zones requires repeated surveying to ensure each quadrat for each equal metered transect has similar conditions