Which of the following is a pioneer species in xerarch succession on bare rock…

Concept

Xerarch succession begins in an extremely dry habitat. Its bare-rock form is called lithosere.

In a lithosere, the pioneer must tolerate desiccation, live without developed soil and help create the first substrate for later organisms.

Application

Lichens can colonize bare rock where soil and available water are minimal. Their weathering action and trapped organic matter begin soil formation, so lichen serves as the pioneer species in this xerarch sequence.

Contrast

• Human: a consumer and ecosystem modifier, not the natural first colonizer of bare rock.

• Herb: a vascular plant that generally needs an already-developed soil layer and therefore appears in a later seral stage.

• More than one of the above: the other listed biological categories do not share the bare-rock pioneer role.

• None of the above: lichen is present among the listed choices.

Cross-check and result

The usual lithosere sequence begins with crustose lichens, followed by foliose lichens, mosses, herbs, shrubs and trees. Lichen occurs before herbs in this sequence, confirming the result: lichen.