Factors contribute to the zonation of Sri lankan rocky shores

1.       Physical factors

2.       Biological factors

3.       Combination of factors



Physical factors

Desiccation, temperature changes, effect of wave action

These factors strongly influence the distribution of organisms.

At the upper inter tidal zone.


Biological factors

Are Grazing , competition & predation

Grazing: sea urchins graze on algae

Competition: for space, food & for algae light.

For space Eg: Mytilus can litoraly out grow & brush aside the Barnacles.


Predation:  Whelks feed on Acron barnacles the distribution of barnacle is set by predation.

Whelks are found only in the lower part of eulittoral zone because lack of tolerance to the exposure,

Since barnacles are more tolerant to tidal exposure they flourish only above the level of whelks can tolerate.

AS sea star preys on mussels ,mussel population is reduced in the zone of abundant sea stars.

Factors in combination

Zonation can be due to the combination of both physical & biological factors.

In seashores 2 types of barnacles smaller Chthamalus  and larger Balanus are zoned on the shore one above the other .

Chthamalus is more tolerant to tidal exposure so it lives higher in the tide zone then Balanus.

When Balanus zone is cleared Chthamalus was able to extend their distribution downwards.

But when the Chthalamus zone was cleared Balanus could not extend upwards.

Downward distribution of Chthamalus was restricted due to larger Balanus smothering it.

the distribution of Balanus upwards arrested  Due to desiccation.


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