Ecology is an old word that came into vogue with the environmental and conservation movements of the 60's and 70's. Ehrlich gives the word new luster in this fine exposition of the science and of the particular field studies he and his colleagues and students out of Stanford have been conducting for the last 25 years. Using the definition of machinery as ""related elements in a system that operates in a definable manner,"" Ehrlich explores the linkages that tie living organisms and features of the environment together into a complex network of interdependencies. One goal for ecologists is to pin down the significant variables that will enable them to model nature; predict the growth or decline of populations; the development of new species or the extinction of others. Thus, ecologists discover nature's tricks-or treats; the ways living forms have evolved to cope with vicissitude to survive and thrive. For starters, Ehrlich the lepidopterist describes the painstaking work of capturing and marking checkerspot butterflies in California in order to track their numbers, movements, feeding and mating habits. Distinct and independent demographic units were found within narrow ranges of a ridge. These units showed surprising differences in size from year to year, some flourishing, others going extinct. The reason one group was able to survive well in spite of poor weather turned out to be the existence of a secondary food source that allowed larvae to survive after their primary source had dried up. All this close-in analysis led to studies farther afield, with the conclusion that there is great variety in adaptation even among the same species. Ehrlich moves from studies of individuals surviving in the physical environment to population ecology and evolution (""The Domains of Malthus and Darwin""); behavioral ecology (sexual selection, territoritality, dominance, etc.); predator-prey relations, cooperation and competition; biogeography; community ecology, and ""Ecosystem Ecology"" (life support systems). Each chapter is a huge canvas that Ehrlich covers with much current lore and theory: from the extinction of dinosaurs to why dandelions have flowers but produce asexually; from why penguins never invaded the northern hemisphere to why cattle have been a major element in desertification in Africa. The chapter lengths work against Ehrlich because too many examples lead to too many tangents. However, the style is winning: Ehrlich rarely assumes a polemical posture here. Instead, he is writing about a subject he knows and loves--and that is quite enough to win nature lovers everywhere.