A. General Introduction
Ecology is the science that deals with the habitats of plants and their response to the environment of climate or of substratum. Ecology in the lichen kingdom is habitat “writ large,” and though it will not be possible in so wide a field to enter into much detail, even a short examination of lichens in this aspect should yield interesting results, especially as lichens have never, at any time, been described without reference to their habitat. In very early days, medicinal Usneas were supposed to possess peculiar virtues according to the trees on which they grew and which are therefore carefully recorded, and all down the pages of lichen literature, no diagnosis has been drawn up without definite reference to the nature of the substratum. Not only rocks and trees are recorded, but the kind of rock and the kind of tree are often specified. The important part played by rock lichens in preparing soil for other plants has also received much attention[1116].
Several comprehensive works on Ecology have been published in recent times and though they deal mainly with the higher vegetation, the general plan of study of land plants is well adapted to lichens. A series of definitions and explanations of the terms used will be of service:
Thus in a work by Moss[1117] we read “The flora is composed of the individual species: the vegetation comprises the groupings of these species into ensembles termed vegetation units or plant communities.” And again:
1. “A plant formation is the whole of the vegetation which occurs on a definite and essentially uniform habitat.”—All kinds of plants are included in the formation, so that strictly speaking a lichen formation is one in which lichens are the dominant plants. [Cf. p. 394.] The term however is very loosely used in the literature. A uniform habitat, as regards lichens, would be that of the different kinds of soil, of rock, of tree, etc.
2. “A plant association is of lower rank than a formation, and is characterized by minor differences within the generally uniform habitat.”—It represents a more limited community within the formation.
3. “A plant society is of lower rank than an association, and is marked by still less fundamental differences of the habitat.”—The last-named term represents chiefly aggregations of single species. Moss adds that: “plant community is a convenient and general term used for a vegetation unit of any rank.”
Climatic conditions and geographical position are included in any consideration of habitat, as lichens like other plants are susceptible to external influences.
Ecological plant-geography has been well defined by Macmillan[1118] as “the science which treats of the reciprocal relation between physiographic conditions and life requirements of organisms in so far as such relations manifest themselves in choice of habitats and method of establishment upon them ... resulting in the origin and development of plant formations.”