The Cobble Beach is divided into two regions (exposed to Haro Strait and slightly sheltered in Mitchell Bay) and organized around a tidal gradient that influences the location of species throughout the intertidal zone. For example, during low tides the porcelain crabs ( Petrolisthes) are only found near the water, and periwinkle snails (Littorina) are only found high above the water. Why? One way to understand this zonation pattern is to consider protection strategies used by various organisms.

Zone 1: Lowest intertidal (with many algae)

• Near the low tide mark one would expect to find the animals that could not survive long exposure, such as the sponges, bryozoans, and echinoderms (which all lack any kind of protective outer layer that retains water), and the porcelain crab Petrolisthes. One might also expect to find the whelks, such as Nucella, and the scavenger Amphissa , which also must stay close to the water. Polychaetes and Pagurus hermit crabs should also be here. Lastly, one might expect to find the animals that were least tolerant of the low salinity levels often found in the high intertidal areas after a heavy rainfall, such as Hemigrapsus shore crabs.

Zone 2: Middle intertidal (barnacles)

• As one heads up the beach into the areas dominated by barnacles, one would expect to find animals that can tolerate changes in temperature and moisture associated with the tide cycles. These animals include the limpets and the periwinkles (Littorina). One might also expect to find some Hemigrapsus here and some Pagurus, as they are less susceptible to desiccation. The carapace of crabs includes a waxy cuticle, which slows desiccation. However, the crab will dry out eventually, and is therefore found under the rocks in this zone. When the tide comes back up the beach, or when the temperature drops at night, the crabs emerge to forage. One would also expect to find some Littorina in this zone, as these snails are adept at using empty barnacle shells to protect themselves from both desiccation and wave action.

Zone 3: Highest intertidal (bare rocks)

• In the highest zone, one would expect to find some barnacles, Littorina, and limpets, all of which can withstand relatively long periods of emersion.

In summary, the species gradient might look something like the following:

low: Bryozoans/Sponges/Echinoderms/Hemigrapsus/Petrolisthes/Polychaetes/ Pagarus
middle: Limpets/Littorina/Hemigrapsus
high: Limpets/Littorina


Biotic Factors (Predation)
However, if one were to consider the community structure of Cobble Beach based on the need for protection from predation, one might predict the following:

Zone 1: Lowest intertidal (algae)

• Near the low tide mark one would expect to find the animals that eat the sponges, bryozoans, echinoderms, and Petrolisthes , all of which are constrained to this zone by their physiological requirements described above. In addition, Petrolisthes has developed a thin carapace shape that allows it to squeeze between tight rock crevices in this zone, perhaps further avoiding predation. As a suspension feeder, this crab's disproportionately big claws are actually quite useless in defending it against a predator. It has developed the remarkable ability to drop its claws and other appendages under stress, but dropping the claws does not appear to enable it to move with any greater speed or agility. In addition to Petrolisthes , one would also expect to find the scavenging Pagarus and the omnivorous Hemigrapsus, which preys on Petrolisthes. Pagarus is present here too; hermit crabs are generally well-protected from predation by their shells, as long as there is an adequate supply of shells of appropriate sizes. On Cobble Beach, there is no shortage of gastropod shells available for use by these crabs. The sessile animals in this zone have adopted other means of protection. Many sponges are protected mechanically by their spicules, and some are protected biochemically by the production of toxins that are distasteful and even harmful to predators. Some of the echinoderms have modified pinching spines (pedicellaria) that defend them against predators.
  

Zone 2: Middle intertidal (barnacles)

• Here one might expect to find fewer limpets, as space is more limited by the presence of barnacles. Without limpets, one would expect to find fewer predatory Hemigrapsus. In the absence of that predator, one might expect to find significant numbers of Littorina and Petrolisthes, as well as some whelks, such as Nucella, that feed on barnacles.
  

Zone 3: Highest intertidal (bare rocks)

• In the third zone, one would expect to find limpets and perhaps their predators, Hemigrapsus. Limpets are defended by their shells and the ability to suction themselves against surfaces.

In summary, under this scenario, the species gradient might look something like the following:

Zone 1: Bryozoans/Sponges/Echinoderms/Hemigrapsus/Petrolisthes/Polychaetes/ Pagarus
Zone 2: Littorina
Zone 3: Limpets/Hemigrapsus


So what is the true story at Cobble Beach?

The tables below show abundance data for the nine most commonly observed taxa in our transects. Quadrat 1 was closest to the low water mark; Quadrat 5 was near the high water mark, and Quadrat 3 was within the barnacle zone, midway up the transect. Transects A-C are from the exposed beach; transects D-F are from the sheltered beach.

Note the very clear vertical zonation patterns of Petrolisthes and Hemigrapsus, as well as the nearly complete division between the Littorina and Hemigrapsus. Interestingly, many of the predictions one might make with a narrow set of assumptions about the habitat are not accurate. For example, the divide between the limpets and Hemigrapsus is not as clean as one might predict. One possible explanation for their apparent overlap is that all Hemigrapsus are considered equally in the data, although the majority of Hemigrapsus higher in the transect (among the smaller rocks) are juveniles. Perhaps not surprisingly, the majority of limpets at the higher quadrat, without predation pressure, are considerably bigger than the ones found lower in the intertidal. Note too that, contrary to prediction, Hemigrapsus isn't found along the low water line or in great numbers high on the beach, even though it thrives both in and out of seawater for extended periods of time, and its prey are found in both zones. Why?

In addition to these observations and questions, it is interesting to consider in general why there is greater species diversity at the exposed site. Are the species at the exposed site in less need of protection from predation? Why would a less protected site have more animals? One theory proposed by the intertidal ecologist Joseph Connell is that certain disturbance regimes actually lead to higher species numbers because they prevent one or a few species from dominating the habitat (so that poorer competitors can hang on in the habitat), or open up types of microhabitat that only exist shortly after some kind of environmental disruption (so that weedy, colonizing species can also persist in the habitat by jumping into newly disturbed patches). This idea is illlustrated graphically in the drawing from Connell's paper reproduced below: highest species numbers in habitats that are disturbed occasionally and not too extensively. The exposed region of Cobble Beach may be just such a habitat, with its cobbles of various sizes being rolled by the waves at different rates, and drift logs crashing onto the rocky surface in some storms: such occasional (but not devastating) disruption of the beach might enable a relatively high number of species to exist in the same habitat.

Finally, it is also interesting to consider which species are not found at Cobble Beach. For example, there were very few Mytilus mussels found in the transects. One possible explanation might be that mussels are at greater risk of predation and overheating if they settle in isolation. Thus it is possible that for some reason, this habitat is not favorable to them and they never occurred here in great numbers. Why?

Sorting out the answers to questions like the ones above requires both a knowledge of the individual species' requirements as well as a strong knowledge of the habitat and ecological characteristics. Protection provides one framework for thinking about these characteristics, but Feeding , Reproduction and Locomotion are other important aspects to consider.


Reference:
Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science 199:1302-1310.

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