Urban soil habitats

Musings on:
Byrne LB, 2007. Habitat structure: A fundamental concept and framework for urban soil ecology. Urban Ecosystems, 10:255-274.

The title's claim of a "fundamental concept and framework" are ambitious, and this paper has a few shortcomings that leave it falling somewhat short of such lofty goals. Despite this, the stated overall objective to ". . .stimulate interdisciplinary interest in, and research about . . . urbanized ecosystems" does seem to be achieved by a paper that, refreshingly, emphasises ideas over activity. And there's nothing wrong with an article having shortcomings, especially if it encourages further thought and debate.

The article itself is an odd hybrid of original empirical data and review-based analysis and conceptualisation (but perhaps it is only an odd hybrid for soil science, a discipline whose journals seldom publish conceptual articles). The original data are used to support the concept of habitat structure in urban soil systems, by measuring various soil parameters under four treatments, which are all (lawn, old field, bark mulch, gravel mulch) manipulations of the soil surface. This is all very well . . . IF these four treatments are really types of urban habitat structure. The challenge is then to describe and preferably quantify the "habitat structure" sufficiently to be able to relate it to habitat conditions, such as soil physical and chemical properties, or to biological responses, such as species abundances or biotic fluxes such as soil respiration. Later in the article it is suggested that surface are to volume ratio is ". . .can be an appropriate description of habitat complexity", but this seems simplistic; the assumption appears to be that "complexity" is a key component of "structure". I wonder if fractal geometry might have a role in more quantitatively describing habitat complexity?

Loren Byrne makes some useful observations in this article. Like Tim Low, the point is made that ". . .human-designed plant communities may have unique and perhaps unexpected effects on urban soil biodiversity." It would certainly be interesting to test this hypothesis across a range of contrasting soils. On the other hand, there are some less-than-useful implications; the paragraph at the bottom of page 264 implies that soils are separate from "habitat structure", being ". . .beneath the different types of habitat structure. . .". A more useful approach might have been to consider soils as part of the habitat structure continuum, as soils contain spatially arranged entities of their own.

The definition of habitat structure itself, although given quite comprehensively in Table 1, may also need some work. The phrase "patterns of habitat structure", seemingly containing another level of abstraction, appears to be used synonymously with "habitat structure" in the paragraph at the bottom of p.265.

Reading back through this, I seem too critical. There is great value in this article's presentation and application of the habitat structure concept to urban soil ecosystems. There is also considerable benefit to readers, particularly postgraduate students in soil science, in examining the way the author develops and justifies arguments to present a coherent concept. As I have maintained before, the discipline of soil science and its practitioners have much to gain from a true dialogue with disciplines such as ecology and geography.

Image from "Jack and The Beanstalk" by Richard Walker and Niamh Sharkey; Barefoot Paperbacks