The philosophical tension between the worldviews of holism and reductionism persists in today's ecology classroom. This debate traces roots to the "individualistic" versus "organismal" debate at the beginning of the twentieth century between the population and community ecology schools . The chief actors in this debate were Henry Gleason, proponent of the individualistic view, and Frederic Clements, who argued that plant communities function as "complex organisms." Though not cast in the same terms, Gleason's point of view has come to be associated with reductionism while Clements' point of view, including his analogy relating plant communities to the human body, was soon attached to the doctrine of holism.
Odenbaugh (2007) provides an excellent summary of the debate between Gleason and Clements:
Suppose a set of species in a particular place and time is disturbed by some exogenous process like a forest fire from a lightening strike. Clements argued that communities in response to such disturbances follow a very specific sequence of stages called "seres" and that there is a single self‐perpetuating and tightly integrated climax community. Clements considered communities to be "superorganisms"...Gleason considered Clements' views to be without empirical support and argued that succession results from individual species' physiological requirements and local meteorological conditions.
In general, holism is the view that an integrated whole has a reality independent of and greater than the sum of its parts. It is marked, particularly, by the belief in "emergent properties" which are only observed at the system-level. Reductionism, in contrast, posits that all phenomena are at all times physically realized and therefore system-level behavior is determined by and can be derived from the constituent parts .
Reductionism is clearly useful as a foundation on which concepts in ecosystem ecology can easily be built. For example, in examining ecosystems as functional units of nature, biochemical pathways in photosynthesis are often discussed as determining the spatial distribution of certain plant communities. We observe that C4 plants have higher photosynthetic and water use efficiency and are stimulated by higher temperatures than C3 plants. We also observe that C3 and C4 plants are found in different patterns on the landscape, with C4 plants found predominantly in drier ecoregions than C3 plants (and CAM plants in ecoregions drier still). The reductionist argument is that these individual differences in plant life histories determine their spatial distribution and, thus, the plant communities associated with a particular ecosystem.
There are also problems with Clements' view. His perspective was too broad-scale to appreciate real, fine-grained changes in species turnover along environmental gradients. However, rejecting Clement's view does not necessitate rejecting holism. There are very real examples of emergent phenomena at work. Conway's oft-cited Game of Life is the premier example but there are examples from the physical world as well (beyond cellular automata). While the spatial distribution of an animal species might be modeled effectively in a vacuum, when we consider multiple species interactions it becomes increasingly difficult if not impossible to predict their spatial distributions with any appreciable accuracy while restricting ourselves to thinking of them as linear combinations.
But is this merely "pragmatic anti-reductionism?" . I think not. As Joe Faith (1998) argues, there are two problems with the reductionist view of some physical systems. One is that our understanding of some physicals systems is limited to conceptual and mathematical models (e.g., ideal bodies in physics). Another is that properties of the constituent elements of a system are often determined by system-level behavior (while an insistence that it is exclusively the reverse is reductionism at its purest). An example of this from physics, provided by Faith, is the compression of an ideal gas, which results in an increased mean momentum per unit volume (and increased momentum of the constiuent particles) .
Many ecologists seem to agree there are merits to both views. My favorite postulation comes from Sierra et al. (2015):
Where these ecosystem manipulation experiments have included the interaction with one or two additional factors, results suggest that effects are not additive or predictable from individual variables alone.
While Currie (2011) and others push back on this view by distinguishing between holism and complexity, I would argue that the defense of reductionism by appealing to complexity is only as tenable as pragmatic anti-reductionism. In the end, the reality of complexity, seen either way, has led to more ideological defensiveness rather than scientific advancement. That is, the debate tends to generate more noise than heat (or more heat than work).
- Dalgaard, T., N.J. Hutchings, J.R. Porter. 2003. "Agroecology, scaling and interdisciplinarity." Agriculture, Ecosystems & Environment. 100 (1):39-51.
- Odenbaugh, J. 2007. Seeing the Forest and the Trees: Realism about Communities and Ecosystems. Philosophy of Science 74 (5):628–641.
- Faith, J. 1998. Why gliders don’t exist: Anti-reductionism and emergence. In Artificial Life VI: Proceedings of the Sixth International Conference on Artificial Life, eds. C. Adami, R. K. Belew, H. Kitano, and C. Taylor.
- Sierra, C. A., S. E. Trumbore, E. A. Davidson, S. Vicca, and I. Janssens. 2015. Sensitivity of decomposition rates of soil organic matter with respect to simultaneous changes in temperature and moisture. Journal of Advances in Modeling Earth Systems 7:355–356.
- Currie, W. S. 2011. Units of nature or processes across scales? The ecosystem concept at age 75. New Phytologist 190 (1):21–34.