9780521861724htl 1..2

Body-size patterns along environmental gradients and their

utility for biomonitoring

In the earlier analysis of the relationship between disturbance regime and body

size, Townsendet al.(1997) quantified size as the maximum achievable by

individuals in each taxon, and found that smaller invertebrates were more

prevalent in the more intensely and frequently disturbed sites. In the current

study of a subset of these sites, where body sizes were individually measured, no

such relationship was seen. While this might be an artefact related to our

selection of sites, it seems more likely to be a function of the long period of

stable flow preceding our study (at least 90 days without a significant flow

event). In the earlier study, sites had been disturbed much more recently. It

may be that the stronger representation in streams with extreme disturbance

regimes of species having small maximal sizes is the result of natural selection.

On the other hand, few individuals survive to maximal size and the results for

(a) (b)

(c) (d)

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0 50 100 150 200 0 50 100 200

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Disturbance intensity

Invertebrate body size (mm)

Algal production (mg(C)/m^2 /hr)

Mean substrate size (mm) Substrate standard deviation (mm)

0 20 40 60 80 100 0 20 40 60 80 100

Figure 5.3Relationships between body size of all macroinvertebrates combined and

(a) disturbance intensity, (b) algal productivity, (c) mean substrate size and (d) standard

deviation of substrate size. Mean lengths are shown, with bars extending to upper and

lower quartiles. Each relationship is for 18 stream sites, except for disturbance for which

only 10 sites were available. The humped shape relationship in (d) is described by the

regression equationy¼0.0003x^2 þ0.0355xþ2.7723 (R^2 ¼0.37).

90 C. R. TOWNSEND AND R. M. THOMPSON