Tag Archives: model

Bridge building ecological theory

A new book from my former McGill colleague, Michel Loreau is lying on my desk.  I haven’t read From Populations to Ecosystems: Theoretical Foundations for a New Ecological Synthesis yet, but Tadashi Fukami has, and his review is in Science.  He writes:

… Michel Loreau argues that an effective way forward is to give up building a single unified theory of ecology altogether. Loreau (a theoretical ecologist at McGill University) believes that “a monolithic unified theory of ecology is neither feasible nor desirable.” As an alternative approach, he advocates theoretical merging of closely related, yet separately developed subdisciplines.

The merging (or bridge-laying) Loreau advocates involves translating different “languages” used in the mathematical models developed separately in various subdisciplines into a common language so that the subfields can talk to one another. Although this approach does not yield a truly unified theory, it helps, Loreau argues, to “generate new principles, perspectives, and questions at the interface between different subdisciplines and thereby contribute to the emergence of a new ecological synthesis that transcends traditional boundaries.” Taking this tack, one gets a sense that the problem with specialization in subdisciplines can be solved by theoretical bridging without having to trade specificity for generality.

An elegant example of the author’s approach can be seen in the work conducted by him and his colleagues over the past decade or so that merges two major subdisciplines of ecology, community ecology and ecosystem ecology. Loreau devotes much of the book to recounting this body of research. He starts by summarizing essential elements of the mathematical models developed in the two subdisciplines. He then discusses how the two sets of models, though developed separately and with apparently distinct sets of equations, can be merged by basing the two on a common currency: the mass and energy budgets of individual organisms. Once this translation is accomplished, new models that simultaneously consider the composition of coexisting species (the focus of traditional community ecology) and the flow of materials through functional compartments of ecosystems (the focus of traditional ecosystem ecology) can be built and analyzed. These allow one to study reciprocal influences between species composition and material flows in the ecosystem.

As Loreau acknowledges, his is not the first book to advocate this type of theoretical merging. In particular, the approach he presents resembles that laid out in an influential 1992 book by Donald DeAngelis (3). What makes Loreau’s contribution novel and creative is his successful application of the merging approach to understanding the functional consequences of biodiversity loss, the topic that has received perhaps greater attention than any other ecological issue over the past two decades because of its broad social implications.

Maping global virtual waters flows

Fig. 4. World map of virtual water exports. (a) Total virtual water exports (flows exceeding 10 km3 yr−1 are shown); (b) flows of virtual water exports originating from blue (irrigation) water (flows exceeding 1.0 km3 yr−1 are shown); and (c) virtual water exports originating from nonrenewable and nonlocal blue water (flows exceeding 0.5 km3 yr−1 are shown).
Fig. 4. World map of virtual water exports.
(a) Total virtual water exports (flows exceeding 10 km3 yr−1 are shown);
(b) flows of virtual water exports originating from blue (irrigation) water (flows exceeding 1.0 km3 yr−1 are shown); and
(c) virtual water exports originating from nonrenewable and nonlocal blue water (flows exceeding 0.5 km3 yr−1 are shown).

Figure is from Hanasaki and others paper An estimation of global virtual water flow and sources of water withdrawal for major crops and livestock products using a global hydrological model (2009 Journal of Hydrology) doi:10.1016/j.jhydrol.2009.09.028.

They explain the figure:

The estimated flows of virtual water exports and imports in 2000 by nation were aggregated into 22 regions worldwide (Table 9; Fig. 4) to show net exports between regions.

Fig. 4a shows the virtual water export flows for all water sources. The figure indicates that North and South America were major regions from which virtual water export flows originate; East Asia, Europe, Central America, and West Asia were the major destinations. This pattern of flows agrees with the studies of (Oki and Kanae, 2004), (Yang et al., 2006) and (Hoekstra and Hung, 2005).

Fig. 4b shows the virtual water exports of blue water (withdrawn from streamflow, medium-size reservoirs, and NNBW sources), and

Fig. 4c shows the virtual water exports of NNBW. Most major flows of blue water and NNBW originated from North America and South Asia.

Interestingly, South America was the major total virtual water exporter but a minor blue water exporter because less cropland is irrigated on this continent.

Notably, South Asia, which is densely populated and where demand results in water scarcity (Oki and Kanae, 2006 and Hanasaki et al., 2008b), showed blue and NNBW virtual water export flows. [note: NNBW - is non-renewable and non-local blue water.]