Do macroconsumers affect insect responses to
a natural stream phosphorus gradient?
Abstract: We assessed the role
of macroconsumers (e.g. fishes and shrimps) in affecting the response of
insect assemblages to a natural phosphorus gradient formed by six streams
with different phosphorus levels (range 12-350 µg/L). We hypothesized
that insect responses to the phosphorus gradient would be strongest in
the absence of macroconsumers. Within each stream, macroconsumers
were allowed access to, or were excluded from, leaf packs using electric
"fences". Macroconsumers did not have significant effects on insects,
but there was a significant phosphorus effect. Insect assemblages
in high-phosphorus streams had 3- to 12- fold greater biomass and 3- to
11- fold greater abundance than assemblages in low-phosphorus streams.
The relationship between insects and phosphorus was not linear: abundance
and biomass increased with phosphorus concentration to an asymptote.
This suggests that insects were responding to a food resource gradient
(e.g. fungi and bacteria). The Michaelis-Menten model provided a
good fit for the relationship between insects and phosphorus concentrations,
with half-saturation constants ranging from 12-60 µg SRP/L.
The asymptotic relationship observed between phosphorus and insects suggests
that phosphorus saturation occurred above a threshold of ca. 100 µg
SRP/L. Our results provide support for the hypothesis that detritus-based
food webs are mainly controlled by bottom-up forces.
Ramirez, A. and C. M. Pringle. 2004. Do macroconsumers affect insect responses to a natural stream phosphorus gradient? Hydrobiologia 515: 235-246
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