Environmental Engineering Reference
In-Depth Information
In many aquatic and terrestrial ecosystems there are invertebrates particularly adapted
to ingesting large litter particles, thereby increasing overall mass loss. In streams, this
functional group of invertebrates (mostly aquatic insects) has been described as “shred-
ders” because of their important role in breaking apart leaf litter. Generation of small par-
ticles (or loss of large particles) is accelerated in the presence of these animals. When
pesticides were used to reduce numbers of aquatic insects in a North Carolina stream, the
quantity of fine particles exported decreased dramatically (
Figure 4.6
).
Hieber and Gessner
(2002)
used litter bags with different mesh sizes to estimate that macroinvertebrates were
roughly equal to microbes in accounting for loss of leaf mass. As discussed earlier, due to
both inefficient feeding and a relatively low assimilation efficiency the fine particulate
organic matter generated by shredder activity can be quantitatively significant and is
suitable as food for a diversity of filtering and collecting organisms. In soils, earthworms,
collembolans, and isopods play similar roles in large particle fragmentation.
In both terrestrial and aquatic ecosystems a significant fraction of macroconsumer sec-
ondary production is supported by consumption of detritus (e.g.,
Cummins and Klug
1979
). There have been substantial shifts in our understanding of the trophic connections
of detritivores (
Findlay 2010
). Early in the study of connections among detritus, microbes,
and macroconsumers it was clear that the timing of increases in microbial abundance or
(b)
2.0
Reference stream
(mean ± 95% CL)
Treated stream
(mean ± 95% CL)
Reference stream
r
2
= 0.79,
n
= 45
Y
= 0.277 + 1.537
X
1.8
(a)
1.6
3.4
3.2
1.4
3.0
± 95% confidence
belts of regression
line
2.8
1.2
2.6
2.4
1.0
2.2
2.0
1.8
0.8
1.6
1.4
0.6
1.2
1.0
0.4
0.8
0.6
Treated stream
r
0.2
0.4
2
= 0.81,
n
= 47
Y
= 0.027 + 0.179
X
0.2
O
ND J FMAMJ J ASOND J F
0.2
0.6
1.0
1.4
1.0
2.2
2.6
1980
1981
Discharge liters/sec
Date
FIGURE 4.6
(a) Plot of FPOM export from an insecticide-treated stream vs. an untreated reference stream at
the Coweeta Hydrologic Lab in North Carolina. Higher losses of FPOM from the reference stream relative to the
insect-free stream at comparable water discharge rates (b) shows the positive effect of stream insects on release of
FPOM from large particulate detritus. (
From
Wallace et al. 1982
.)
