gravels are of an appropriate size to reduce fish foraging efficiency on the
invertebrate carnivores, and is in accord with observations that fish modify
stream invertebrate behaviour and the amount of foraging time invertebrates
spend away from refuges (e.g. Kohler & McPeek, 1989 ; McIntosh & Townsend,
1994 ). In an experiment involving an invertebrate predator (the caddisfly larva
Plectrocnemia conspersa) and its stonefly prey (Leuctra nigra), Hildrew and
Townsend (1977) noted that larger prey were more vulnerable than smaller
ones in a substratum of small stones whose interstices were too small to shelter
the large individuals.
While the impact on stream communities of exotic chemicals, including
pesticides and metals, has yet to be systematically assessed, laboratory studies
have highlighted the likely importance of body size.
Vulnerability to dissolved contaminants can be expected to be greater in
insects that use dissolved oxygen, obtaining much of it through exchange
epithelial surface, than those that breath air. Buchwalter, Jenkins and Curtis
(2002) tested in the laboratory the hypothesis that stream insects with relatively
large areas of exchange epithelium would be more likely to accumulate the
insecticide chlorpyfiros from their environment. They estimated relative differ-
ences in exchange epithelium by measuring permeability to labelled water and
found, for both the dissolved-oxygen breathers and the air breathers, that water
permeability was strongly related to pesticide uptake rate. However, as pre-
dicted the dissolved-oxygen breathers had, in general, much higher uptake
rates. In turn, water permeability was positively related to body size, reflecting
higher surface-area-to-volume ratios in small individuals.
Across a wide range of aquatic taxa, Hendriks and Heikins (2001 )havedocu-
mented a general inverse relationship between body weight and rate constants
for the uptake of inorganic substances, as found for chlorpyfiros (a class of
organophosphate insecticides). However, sensitivity to contaminants depends
also on metabolic differences and detoxification mechanisms that vary across
taxa. In a study involving representatives of the Ephemeroptera, Trichoptera and
Plecoptera, uptake of cadmium and zinc was generally not related to size, but
rather to the relative numbers of ionoregulatory cells present (Buchwalter &
Luoma,2005 ). On the other hand, Kiffney and Clements (1996 )directlyexplored
the relationship between size and toxicity (of a mining-related mixture of cad-
mium, copper and zinc) in four stream invertebrates (Ephemeroptera and
Plecoptera) and found that the mean size of individuals at the end of the experi-
ment was higher in the metal treatment than the control because smaller indi-
viduals were significantly more likely to die. Even high concentrations of nitrate
in stream water (characteristic of agriculturally polluted streams) can be toxic to
stream invertebrates, due tothe conversion of oxygen-carrying pigments toforms
incapable of carrying oxygen: once again, the strongest effects are generally felt
by the smallest individuals (Camargo, Alonso & Salamanca,2005 ).84 C. R. TOWNSEND AND R. M. THOMPSON