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Stages in Invasion and Replacement Demonstrated by Species of Melandrium

H. G. Baker
Journal of Ecology
Vol. 36, No. 1 (Jul., 1948), pp. 96-119
DOI: 10.2307/2256649
Stable URL: http://www.jstor.org/stable/2256649
Page Count: 24
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Stages in Invasion and Replacement Demonstrated by Species of Melandrium
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Abstract

1. A theoretical analysis of the invasion of an area already containing one form by another, closely related form is presented. Four distinct stages are envisaged: (i) The original condition (Stage 0). (ii) If the forms are inter-fertile, the introduction of characters which are neutral as far as the ecological distinction between the forms is concerned (Stage 1). (iii) Following a modification of the environment, the introduction of characters excluded previously on ecological grounds (Stage 2). (iv) The disappearance of the original form except for neutral characters (Stage 3). Such a sequence may occur in one place through a period of time or at the same time over a wider area. A xenocline is defined as a cline of hybridization following the spread of the immigrant form from its place of introduction. Xenoclines may exist in forms corresponding with topoclines and ecoclines, respectively. 2. The general relationships of Melandrium dioicum and M. album are discussed. M. album is shown to have arisen by ecogeographical divergence from an ancestor probably resembling M. dioicum quite closely, and the two species form relatively fertile hybrids. 3. The fossil and historical records show that M. dioicum colonized northern Europe and the British Isles by natural means and that, subsequently, M. album has spread as a follower of agricultural man. 4. Suitable criteria of hybridity for the campions are enumerated. 5. Using the characters determined to be suitable, field observations made in Wales south-eastern England and East Anglia are described. (i) In western Wales, populations of M. dioicum are still mostly in Stage 0. (ii) In south-eastern England, all stages may be found and populations of M. dioicum appear as island relics largely surrounded and invaded by a network of M. album. (iii) In East Anglia, the opposite extreme to the Welsh condition has been reached. Over most of Cambridgeshire and Huntingdonshire forest-clearance followed by agronomy and coupled with unfavourable climatic and edaphic conditions has resulted in the almost complete disappearance of M. dioicum. Only one certain and one probable relict population are known in the two counties. M. album has found extremely favourable conditions, has become well established and has helped to speed the disappearance of M. dioicum. 6. The probable future of the two species in the British Isles is discussed. 7. Conditions in Great Britain are contrasted with those in America, where it appears that hybrid material was introduced along with M. album and the populations of campions are heterogeneous from the start.

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