"One physical system": Tansley's ecosystem as Earth's critical zone
A remarkable congruence now exists in core concepts of two scientific disciplines: ecology’s ecosystem and Earth science’s critical zone. This congruence becomes evident from the perspectives of Tansley (1935), Lindeman (1942), and Hutchinson (1948), who defined an ecosystem to be “one physical system” (quoting Tansley) and as involving the study of “the living matter of the whole earth … a unit of higher order than the biome” (Hutchinson, 1940). Here we observe how these early ideas of the ecosystem are congruent with the Earth’s critical zone, defined by Jordan et al. in 2001 to be the integrated and life-supporting systems of Earth’s terrestrial processes.
Ecosystems and critical zones are congruent across spatial-temporal scales from vegetation-clad weathering profiles and hillslopes, small catchments, landscapes, river basins, continents, to Earth’s whole terrestrial surface. What may be less obvious is congruence in vertical dimension. We use ecosystem metabolism to demonstrate how a full accounting of photosynthetically-fixed carbon must include respiratory CO2 and associated carbonic acid that propagate to the base of the weathering profile, i.e., the base of the critical zone itself. Though a small fraction of ecosystem respiration, downward diffusion of CO2 helps determine rates of soil formation and, ultimately, ecosystem evolution and resilience. Because plant life in the upper portions of terrestrial ecosystems significantly affects biogeochemistry throughout weathering profiles, lower boundaries of most terrestrial ecosystems have historically been demarcated at depths too shallow to permit complete understanding of ecosystem structure and function, a consequence exacerbated by the historical subdivision of terrestrial ecosystem science into above- and belowground branches. This perspective of ecosystem metabolism helps bridge above and belowground ecology and better connect the hydrology and biogeochemistry of the surficial ecosystem and its soil, with groundwater, streams, lakes, and rivers. Given the congruence of ecosystems and critical zones, biogeoscientists can reap major scientific breakthroughs from coordinated investigations that engage the disciplinary sciences of hydrology, climatology, biology and microbiology, geochemistry and biogeochemistry, pedology, ecology, and geophysics. Opportunities abound to explore connections between upper and lower components of ecosystems and critical zones, between soils and streams in watersheds, and between plant-derived CO2 and deep microbial communities and mineral weathering. Such co-investigations will lead to more quantitative assessments of the evolution and resilience of critical-zone ecosystems, and satisfy the concerns and scope of Tansley's ecosystem and the recently conceived critical zone of Jordan et al. (2001).