The vigorous growth of ecology from its origins in the late 19th century and early 20th century has been accompanied by its gradual fission into several distinct subdisciplines. The unified view of ecology that was present in a book like Lotka’s Elements of Physical Biology (1925), which introduced many of the theoretical approaches that are still followed today, has given way to more specialized research programs. Although specialization is to some extent inevitable to make science more precise and predictive, it also creates problems. The conceptual frameworks of the various areas tend to become increasingly divergent over time, hampering communication across the discipline as a whole. This divergence is nowhere more apparent than between two of the major subdisciplines of ecology, i.e., community ecology and ecosystem ecology. These two subdisciplines have grown largely independently, each having its own concepts, theories, and methodologies. Community ecology is to a large extent an outgrowth of population ecology. It is mainly concerned with the dynamics, evolution, diversity, and complexity of the biological components of ecosystems; its starting point is the population and its interactions with other populations. Ecosystem ecology is mainly concerned with the functioning of the overall system composed of biological organisms and their abiotic environment; its starting point is the flow of matter or energy among functional compartments.
The separation of these subdisciplines is understandable insofar as they partly address issues at different hierarchical levels and different spatial and temporal scales. But it is harmful insofar as it is an obstacle to their unity and mutual enrichment. In the real world, populations and communities do not exist in isolation; they are parts of ecosystems, and, as such, they are subjected to constraints arising from ecosystem functioning, such as energy dissipation and nutrient cycling. These constraints can deeply alter species interactions and community properties, as we shall see in this book. On the other hand, ecosystems do not exist without their biological components; the latter impose their own constraints on ecosystem processes, as the disruptions generated by some biological invasions attest.
In a way, community ecology and ecosystem ecology provide two different perspectives on the same material reality. Real ecological systems are