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文章很长,怀疑G把它高压缩了.有好几段都是废话.估计被G删了...求指点迷津
寂静里面高亮词都有.希望狗主确认.
如果不是,大家就当背景看吧..原理是一样的
Neighborhoods and Gaps
In the groundsel-bluegrass system, competition portrays a race in which small differences in emergence produce large differences in performance, and seeds respond vigorously to clues that indicate the presence of competitors.
Apparently, the threat of competition shapes the activity of these seeds. That raises broader questions: To what extent does competition influence the population dynamics of competitors, and to what extent does community context influence competitive interactions? Although biologists commonly assume that competition affects the dynamics of plant communities, we are just beginning to explore the interplay between individual performance, which responds to competition, and population and community-level factors. Such research provides an important arena for testing our understanding of the forces that structure plant communities.
At this interface between individuals and communities of plants, spatial patterning receives a disproportionate share of attention. Two distinct theories have been developed that predict that spatial patterning can exert a strong influence on competitive interactions. Since plant populations exhibit very patchy arrangements, as opposed to random distributions, the question arises as to whether patterning drives the dynamics of plant populations in nature.
Stephen Pacala of Princeton University and John Silander of the University of Connecticut champion the neighborhood-competition theory. That theory assumes that plant growth and reproduction depend on the density of nearby competitors and that competitors beyond a designated distance exert no impact on a particular plant. Combining that assumption with an elaborate mathematical theory suggests that patchiness in a distribution of competitors can profoundly alter competition at the population level. Intuitively, this theory can be understood by recognizing that patchiness leads to considerable variation in the amount of competition experienced by any individual plant. Some plants will experience lots of competition and experience little reproduction; other plants will be virtually free from competition and contribute a disproportionately large share to the next generation. In this way, the neighborhood-competition theory suggests that the spatial pattern of plants influences competition through interactions between contemporaneous plants.
An alternative theory for patchiness, gap colonization, considers the competitive impact of one generation on another. Traditionally, this theory has been applied to the dynamics of forests, where the germination and growth of seedlings depends on gaps left after trees fall. Nevertheless, it might be equally relevant to the dynamics of grassland plants. In nearly all cases, established vegetation can overpower seedlings. In fact, seedling emergence and survival can be totally inhibited by the presence of adult plants, or even by the litter left from previous generations. This type of inhibition can arise from several factors: chemicals, shading or structural interference produced by the established plants. In some cases, these factors may restrict seedling survival to areas that lack established vegetation. Moreover, the gaps in a patchy spatial pattern promote the persistence of competitively inferior plants. Gap-colonization theory, then, suggests that an area’s spatial pattern will influence competitive interactions because previous generations affect the survival of present seedlings.
Competition between common groundsel and annual bluegrass also depends on spatial patterns. Both of these species produce two generations each year-one in the spring and another in the fall. This rather unusual life cycle proves tremendously convenient for testing the neighborhood-competition and gap-colonization theories. In each fall generation, newly emerging seedlings compete with each other, which provides an opportunity for neighborhood competition, and they also interact with remnants from the spring generation, which could lead to gap colonization. |
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发表于 2012-4-11 05:04:27
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