Short Essay on Heterotrophic Succession (592 Words)

Winston (1956) observed that an acorn supports a tiny parade of life from the time it drops from the tree until it becomes part of the humus. During the pioneer stage of the heterotrophic succession certain insects like the acorn weevil, Curctilio rectus, invade the acorn by burrowing through the pericarp and its female deposits the eggs in the embryo of acorn.

Upon hatching, the larvae tunnel through to the embryo and consume about half of it. Some fungi like Penicillium and Fusarium also invade the acorn, either simult­aneously with the weevil or alone, and utilize the material. The embryo of acorn then turns brown and leathery and the weevil larvae become stunted and fail to develop.

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When the acorn’s embryo is destroyed, partially or completely, by the pioneering organisms, other animals and fungi enter the acorn. Weevil larvae leave the acorn through an exit hole, which they cut through the outer shell. Through these whole fungi feeders and scavengers enter. Most important is the moth Valentinia glandcnella, which lays its eggs on or in the exit hole, mostly during the fall.

Upon hatching, the larvae enter the acorn, spin a tough over the opening and proceed to feed on the remainder of the embryo and the faces of the previous occupant.

At the same time numerous species of fungi enter and grow inside the acorn, only to be utilized by another set of occupants, the cheese mites, Tryophagus and Rhyzozhyphus. By the time the remaining acorn’s embryo tissues are reduced to faeces and the acorn is occupied by cellulose- consuming fungi.

The fruiting bodies of these fungi, as well as the surface of acorn, are eaten by mites and Collembola and if moist, by cheese mites too. At this time predaceous mites enter the acorn. Outside on the acorn, cellulose and lignin-consuming fungi soften the outer shell and bind the acorn to twigs and leaves on the forest floor.

As the acorn shell becomes more fragile, new holes appear in it. Through these holes enter large animals such as centipedes, millipedes, ants, collembolan and earthworms. Due to their activities the amount of soil in the cavity increases and greatly softened shell eventually collapses into a mound and gradually becomes in-corporated into the humus.

Similarly, the heterotrophic succession of fungi on plant rema­ins in the soil (i.e., dead roots) involves the primary colonizers or pioneers in the form of weak parasites and saprophytes of sugai fungi group (mucoraceous phycomycetes) which feed on sugars and other carbon compounds.

The ascomycetes and their imperfect forms (cellulose decomposers) are the secondary colonizers. Finally, the basidiomycetes (lignin decomposers) appear in the succession. Thus, chemical make-up of the substrate determines the qualitative characteristics of fungal flora appearing at a particular stage of decomposition.

Likewise, the primary colonizers of heterotrophic succession on dungs and similar excreta of herbivorous animas include phycomyccte (mucorales). These are followed by secondary colonisers, the ascomycetes and deuteromycetes and in turn, being succeeded finally by the basidiomycetes.