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The evolutionary consequences of interactions between plants in permanent pastures Mehrhoff, Loyal Archie


The evolutionary consequences of interactions between neighbouring plants of Trifolium repens and three selected grasses were studied in a series of different-aged pastures. Experiments were conducted at three different levels of interaction; between species, between populations, and between individuals. The primary objective was to examine under natural conditions the relevance of two revolutionary theories of species coexistence - niche differentiation and competitive equivalence. Additional studies focused on the differentiation of T. repens subpopulations in response to neighbouring grasses, on the presence of carry-over effects, and the validity of extrapolating the results of greenhouse studies to predictions of natural pasture processes. Plants collected from different pastures showed different patterns of growth in a common garden. These patterns appeared to be related to the age of the pasture from which the plants were originally collected. Occasionally, species which were poorer competitors in 3'ounger pastures became superior competitors in mixtures from older pastures. Plants grown under natural field conditions were affected by both competitive and non-competitive forces. Non-competitive factors such as grazing, disturbance, and/or abiotic conditions, killed more individuals and accounted for more variation in growth than did competitive factors. However, plant interactions were important, including both competitive effects on neighbours and competitive responses to neighbours. These competitive effects and responses were not symmetrical and each appeared to behave independently. The balance between the intensity of intra- and inter-specific competition was critical. Two pairs of species which were more adversely affected by interspecific competition showed evidence of increased niche separation in older compared to younger pastures. A third pair of species was more affected by intraspecific competition and showed a decrease in niche separation in older pastures. These results support the theory that competition leads to changes in the pattern of resource use. The ability to predict the growth of species under natural pasture conditions from either greenhouse or controlled garden conditions was poor. This lack of predictability, when coupled with the observed rapid evolution of a species' competitive ability, suggests that caution should be exercised in developing theories based upon greenhouse growth characteristics or on "species" characteristics. Unlike previous studies in a Welsh pasture, no evidence of long-term specialization of Trifolium repens to patches of neighbouring grasses was found. Differences in the age of the pastures, and the average patch size of the neighbouring grasses (environmental grain) may explain these differences.

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