Interest in why some plants are well defended and others are not has led to development of numerous hypotheses (Table 1). Collectively the body of work for these hypotheses and research related to them is referred to as the plant defense theory. Four of these hypotheses have been especially important as frameworks for research. The optimal defense hypothesis focuses on how defensive needs of plants leads to the evolution of secondary metabolites, with the cost of that defense maximizing fitness. The growth rate hypothesis focuses on genotypic variation in plant defenses, with constraints on plant defense due to resources shaping inherent growth rate. The carbon: nutrient balance hypothesis addresses the pheno-typic variation in secondary metabolites in terms of the carbon: nutrient (usually nitrogen) ratio in plants. The growth-differentiation balance (GDB) hypothesis addresses how plants balance allocation between growth and differentiation (such as plant defenses). These hypotheses are not mutually exclusive. The GDB hypothesis subsumes the others, but it is also the most difficult to test. Using these hypotheses as a framework for research continues to be useful when there is recognition of the limitations of each. However, the hypotheses cannot be tested directly; rather subhypotheses can be tested, which requires clearly articulating the assumptions and domain of the test.
woody shrubs, and early-successional (fast-growing) deciduous trees, such as aspen, willow, poplar, and birch (which have a relatively low lignin:nitrogen ratio). These plants have defenses. Moose favor less-defended individuals or parts. The pioneer tree species are succeeded by shade-tolerant, evergreen conifers, such as spruce and balsam fir. Spruce, which has a high lignin:nitrogen ratio and high concentrations of resins containing terpenoids, is seldom eaten by moose. Balsam fir is also less palatable than the pioneer species, but used by moose when other food is unavailable, especially in winter. Consequently, moose browsing reduces competition for the spruce trees, directly by reducing growth rate and, in turn, seed production of other plant species, and indirectly by changing the quantity and quality of the plant litter and, in turn, soil microbial processes that results in less nitrogen availability, which favors plant species, such as spruce, which is more tolerant of nutrient-poor conditions. Wolves, by reducing moose numbers, slow down this process of spruce taking over. Fire resets the successional process to the pioneer stage; the conifers are not fire-resistant and the pioneer species colonize quickly. Beavers also reset the successional process by flooding forest. The pioneer species benefit by increased light due to die back ofthe conifers which are less tolerant of such wet soil conditions and also by the increased nitrogen available under those conditions. Like moose, beaver prefer the pioneer species for food. Thus, response by the mammalian herbivores to plant defenses is a central feature in shaping the ecosystem.
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