The Comparative Biology of Closely Related Species Living in the same Area: IX. Rumex: The Nature of Adaptation to a Sea-Shore Habitat

Abstract

The maritime form of Rumex crispus (R. crispus var. littoreus (Hardy)) characteristically occupies a zone on shingle beaches just above the line of high spring tides. This variety is characterized by the possession of well-developed tubercles on all three perianth segments and a denser inflorescence than the inland form. The varietal characteristics are maintained after transplanting into inland sites. Many stringent demands are placed on the plants that live on shingle beaches. They must be able to germinate, grow and reproduce in spite of periodic flooding by high tides, buffeting by high winds and salt spray, and repeated drought in the surface layer of shingle. R. obtusifolius and the inland form of R. crispus are common weeds of agricultural and waste ground inland, but are not found on shingle beaches. The spatial isolation of plants of var. littoreus from the 'weedy' inland form of R. crispus is normally clearly defined and contrasts very strongly with the relatively weak interspecific differentiation in the ecologies of R. crispus and R. obtusifolius. The maritime shingle beach is the only habitat in which plants of R. crispus var. littoreus form an important part of the vegetation. In inland habitats occasional plants are found possessing some but seldom all characteristics of the maritime taxon. We have examined many shingle beaches in North Wales and at only one site were plants of the inland and maritime forms found to be growing together; this was disturbed waste land within 300 ft (100 m) of the sea at Penmaenmawr, Caerns. Ecologically precise distributions of this type pose the question of the manner in which distribution is determined. The distribution of a taxon may be restricted because: (a) there is failure of seed dispersal, (b) there are highly specialized environmental requirements for growth which are only locally satisfied, or (c) interference between species occurs which leads to the elimination of less adapted types. In a previous paper (Cavers & Harper 1967) it was shown that 'seed' (achene) of R. crispus var. littoreus (henceforth called 'the maritime form') germinates freely when sown in a wide range of inland habitats from which it is normally absent. The experiments described in the present paper were made to elucidate further the factors responsible for the ecological separation of the maritime and inland taxa.