  The introduction and rapidly growing popularity of the term “ecosystem,” especially during the late 1950s and 1960s, was marked by two major cognitive and one sociological shift in the practice of ecology: (a) coming at the end of the so-called golden age of theoretical population ecology of the late 1920s and 1930s (see  Section 2),  turning to ecosystems helped shift emphasis from populations with interacting individuals to much larger and more inclusive systems. In this sense it was a deliberate anti-“reductionist”  move.[76]  Ecosystem enthusiasts follow a long holistic tradition in natural history that tends to deify complexity and deny the possibility of explaining wholes in terms of their  parts.[77]  “Systems thinking” was supposed to replace reductionism, the decomposition of wholes into parts for the sake of analysis (see  Section 5);  (b) a second cognitive shift is that ecosystem studies involve models based at least partly on non-biological  variables.[78]  For instance, instead of tracking individuals or even species in communities, models may track energy or matter flow in food webs as a whole; and (c) at the sociological level, the expansion of ecosystem studies led to what one historian has called the invention of “big biology” in the 1960s, chiefly in the  US.[79]  These studies, for instance the massive Hubbard Brook Ecosystem  Study,[80]  required more than just many biologists working together.  They also demanded that other specialists, including geochemists and soil scientists, be brought in so that all the relevant physical parameters of ecosystems, besides the biological ones, could be tracked simultaneously. This study constituted the biologists' attempt to engage in publicly-funded Big Science, initiated by the physicists during the Manhattan Project, and subsequently profitably exploited by social scientists since the 1950s.