520 REVIEWS Ecology, Vol. 70, No. 2 For certain time and space scales, limitations of seed dispersal can restrict plant distributions. Migration rates and patterns can therefore depend upon dispersal rates and mechanisms, and Sauer cites examples. Despite his inductive approach, Sauer includes a model for the forces controlling plant distribution. It emphasizes dis- persal as a centrifugal force for range expansion and environ- mental selection as a centripetal force holding back migration. His model seems too limited to me, because it does not make explicit enough the potential expansive effects of environ- mental change on species ranges, especially for time scales of 1000 years or longer. On these time scales, migration of taxa has been key to their survival. The long-term existence of species is evidence that the species have moved fast enough to stay in dynamic equilibrium with the major past changes in the environment. Had species not done so, they would have lost contact with the environment favoring their growth and reproduction and hence gone extinct. A key problem raised by the potential rapidity of greenhouse-gas-induced climatic change, however, is that the speed of climatic changes during the next century may outstrip the ability of many current plant taxa to migrate. Study of the many case histories in Sauer's book should help us better appreciate how and where these difficulties may arise. Sauer has written a timely review that deserves immediate and broad attention. THOMPSON WEBB III UNIVERSITY OF COLORADO Visiting Fellow, 1988-89, CIRES Boulder, Colorado 80309-0449 and BROWN UNIVERSITY Department of Geological Science Providence, Rhode Island 02912 Ecology. 70(2), 1989, pp. 520-521 (? 1989 by the Ecological Society of America A THEORY OF PLANT COMMUNITIES: THE SECOND APPROXIMATION Tilman, David. 1988. Plant strategies and the dynamics and structure of plant communities. Monographs in Population Biology. Volume 26. Princeton University Press, Princeton, New Jersey. xi + 360 p. $45.00 (cloth), ISBN: 0-691- 08422-2 (alk. paper); $15.95 (paper), ISBN: 0-691-08489-0 (alk. paper). "Although the natural world is complex we, as ecologists, cannot afford to revel in that complexity, but must seek explanations-the simplest viable explanations-for the patterns we observe." Tilman 1988 Much of the fascination and excitement engendered by ecol- ogy derives from the diversity and complexity of natural eco- systems. However, this complexity has also been the greatest impediment to the development of general theories of com- munity structure and dynamics. David Tilman's approach is to ignore much of this complexity as gimcrackery that merely obscures the important underlying structure of nature. While his cavalier disregard of what many of us appreciate most in ecology may well alienate some readers, the step is a necessary one if we are to understand the fundamentals of how com- munities work. Tilman's purpose in this and his previous book (1982. Re- source competition and community structure. Princeton Uni- versity Press) is to explore the extent to which a few simple mechanisms of plant competition might explain the major patterns we see in nature. The theory elaborated in his first book sought to explain patterns in species composition and diversity by means of a simple graphical model based on ratios of resources. Because Tilman, unlike many theorists, has been willing to test his theories with extensive field experiments, he has sometimes discovered them to fall short of the level of generality with which he earlier credited them. The role of light as a resource was under-emphasized relative to multiple soil nutrients in his first book, and Tilman soon published a series of papers appropriately shifting emphasis to light and nitrogen as critical resources. However, this change in em- phasis was not sufficient to explain many of the patterns he observed in his experiments. Unlike the single-celled algae for which Tilman first developed his resource-ratio model, higher plants have conspicuous morphological diversity which contributes significantly to plant fitness. Tilman's second book summarizes the structure and ecological implications of a new model (ALLOCATE) that incorporates competitive interac- tions stemming from differential allocation to roots, leaves, and stems. ALLOCATE is a simple model with a minimum of rules and variables. An individual plant has leaves only at the top of the stem and has invariant, species-specific allocation rates for roots, stems, and leaves. When a plant reaches maximum height, all allocation is directed to seeds. The death rate is constant for a particular species and cohort, and for most analyses does not vary across cohorts. Members of a cohort are all identical. There is no spatial structure, nor are season- ality or disturbance included. Among published models of plant interactions, ALLO- CATE is undistinguished. What sets the model apart is the conceptual context. Tilman wants to explain as much as pos- sible with as few rules as possible, and he is remarkably suc- cessful. He demonstrates how ALLOCATE recreates the es- sentials of his resource-ratio theory, explains differences in the successful allocation strategies for different levels of soil fertility and plant mortality, and maps community trajectories through the early phases of succession. Results of the model are shown to be consistent with many examples drawn from the world literature as well as numerous results from Tilman's extensive field work in Minnesota. While this is an important and stimulating book, it is also a frustrating one. The text is long and excessively wordy. Despite the length, the crucial description of the model AL- LOCATE is highly abbreviated, largely relegated to an ap- pendix, and lacks key details. Literature citations are used to This content downloaded from 152.2.15.137 on Sat, 28 Feb 2015 23:48:18 UTC All use subject to JSTOR Terms and Conditions