9780521861724htl 1..2

than before, suggesting recolonization was mainly via ovipositing adults. In a

finer scale study in a New Zealand stream, Matthaei and Townsend (2000 )com-

pared recovery after high discharge events in bed patches where scouring or

deposition had occurred. SmallerDeleatidiummayflies andHydorabeetles charac-

terized patches of scour, and larger Chironomidae but smaller Simuliidae were

associated with depositional areas, indicating both that recolonization may be

size related but also taxon specific.

The size structure of the streambed may have a more direct influence on trait

representation too. Bourassa and Morin ( 1995 ) compared, for nine Canadian

streams, the relationship between invertebrate body size (minimum size 63mm)

and substratum particle size, by sampling relatively uniform patches of stream

bed consisting of sand, fine gravel, course gravel, cobble or small boulder.

Coarser substrata, with their correspondingly larger interstitial spaces, might

be expected, on average, to support larger invertebrates. Bourassa and Morin

(1995) found a significant, but weak, trend towards larger average size in coarser

substrata. In a related context, the presence of large wood (derived from forested

riparian zones) has been shown in North American streams to be positively

associated with large-bodied insects (Richardset al., 1997; Johnson, Breneman &

Richards,2003).

Gayraud and Philippe (2001 ) focused on the amount of interstitial space in the

streambed rather than the bed particles themselves. Their French study sites all

contained pebbles and cobbles, but the less porous beds were associated with a

greater preponderance of fine sediment. The results supported the hypothesis

that the relative abundance of small invertebrates (maximal size 1–5mm), which

are capable of penetrating the substratum, should be greater in beds with low

porosity, whereas larger invertebrates (5–10mm) would be better represented in

more porous streambeds that provide considerable interstitial space.

(a) (b)

% of Individuals

Intensity of disturbance Intensity of disturbance

100

80

60

40

100

80

60

40

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

Figure 5.1Relationship between a bed disturbance index in 54 stream sites and

percentage of individual macroinvertebrates whose maximal length was less than

10 mm (a) in the 27 sites where refugium availability was lowest and (b) in the 27 sites

where refugium availability was highest. The index measures the average proportion

of bed particles that are moved by high discharge events in each stream. Intensity

and frequency of disturbance are highly correlated in this case. (Based on data from

Townsendet al., 1997.)

80 C. R. TOWNSEND AND R. M. THOMPSON