J. N. Am. Benthol. Soc., 2008, 27(2):321–331
Ó 2008 by The North American Benthological Society
DOI: 10.1899/07-054.1
Published online: 25 March 2008
Leaf decomposition and invertebrate colonization responses to
manipulated litter quantity in streams
S. D. Tiegs1
AND
F. D. Peter2
Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, and
Institute of Integrative Biology (IBZ), ETH Zurich, 6047 Kastanienbaum, Switzerland
C. T. Robinson3
AND
U. Uehlinger4
Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, and
Institute of Integrative Biology (IBZ), ETH Zurich, 8600 Dübendorf, Switzerland
M. O. Gessner5
Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, and
Institute of Integrative Biology (IBZ), ETH Zurich, 6047 Kastanienbaum, Switzerland
Abstract. Resource availability is an important ecosystem attribute that can influence species distributions
and ecosystem processes. We manipulated the quantity of leaf litter, a critical resource in streams, in a
replicated field experiment to test whether: 1) greater litter quantity promotes microbial leaf decomposition
(through greater microbial inoculum potential), and 2) reduced litter quantity enhances decomposition by
leaf-shredding invertebrates (because shredders aggregate on rare resource patches). In each of 3 streams,
we identified reaches in which litter quantity was either: 1) augmented, 2) depleted, or 3) left unchanged. We
determined decomposition rates and macroinvertebrate colonization of alder leaves placed in coarse- and
fine-mesh litter bags, an approach intended to allow or prevent access to leaves by leaf-shredding
macroinvertebrates. Responses to litter manipulations were complex. In 2 streams, litter quantities differed
among treatments, but high quantities of litter in the control reach of the 3rd stream produced an overall
variable pattern. Microbial decomposition was similar across litter treatments. In contrast, in the 2 streams
where litter manipulation was successful, decomposition in coarse-mesh bags tended to be faster where
litter was scarce than where it was abundant. Abundances of total and leaf-shredding macroinvertebrates in
litter bags did not differ among litter manipulations in these 2 streams. However, a litter-consuming
amphipod (Gammarus fossarum) tended to be most abundant in bags placed in litter-depleted reaches in the 2
streams, indicating that this large and highly mobile shredder might have been instrumental in causing
differences in decomposition in response to litter manipulations. Overall, the effects caused by alteration of
litter quantities on leaf decomposition and macroinvertebrate colonization were relatively weak.
Nevertheless, results from 2 of the 3 streams where litter manipulation was successful were consistent
with the hypothesis that short-term changes in resource availability might influence ecosystem processes by
determining the spatial distribution of key consumers.
Key words: leaf retention, invertebrate aggregation, benthic organic matter, leaf breakdown, shredders, Gammarus, ecosystem process.
The quantity of resources available in ecosystems is
a key factor that determines the spatial distribution of
organisms, which, in turn, might govern ecosystem
processes. A fundamental resource in many streams is
leaf litter derived from terrestrial vegetation (Wallace
et al. 1999). Leaves in the canopies of streamside trees
cast shade that strongly limits instream primary
1
Current address: Department of Biological Sciences,
University of Notre Dame, Notre Dame, Indiana 46556
USA. E-mail: [email protected]
2
E-mail addresses: [email protected]
3
[email protected]
4
[email protected]
5
[email protected]
321
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S. D. TIEGS
production (Sabater et al. 2000). When shed and
retained in stream channels, these leaves provide
habitat and food or substrate to benthic invertebrates
and microorganisms (Baldy et al. 2007, Greenwood et
al. 2007). As a result, terrestrially derived leaves
constitute the major basal resource of food webs in
small forest streams (Wallace et al. 1999), and the
quantity of leaf litter present is likely to be a critical
factor governing stream ecosystem structure and
function.
Quantities of litter present at a given time and
location hinge on 3 processes: input, retention, and
decomposition. Litter input depends on the density,
composition, and productivity of riparian vegetation
(Benfield 1997) and on the hillslope transport (i.e.,
lateral input) that delivers material to the stream from
the forest floor (Webster et al. 1995). Retention of litter
in streams is determined by interactions between
hydrologic and geomorphic features, such as channel
depth, grain size of the substratum, and abundance of
large wood (Jones 1997, Hoover et al. 2006). Because
these factors vary markedly across time and space,
litter retention is also highly variable (Webster et al.
1999, Larranaga et al. 2003). Litter retention might be
as important as input in determining quantities of
benthic litter. For example, in a multiple regression
analysis with data from 19 streams located throughout
the USA, variables related to channel retentiveness
explained more variability in benthic litter quantities
across streams than did variables related to input
(Jones 1997). When retained in stream channels, litter is
colonized and used by detritivorous macroinvertebrates (shredders) and by microbial decomposers,
particularly fungi (Anderson and Sedell 1979, Hieber
and Gessner 2002). The interplay of these organisms
determines the biological decomposition of leaf litter in
streams (Gessner et al. 1999).
Experimental manipulation of litter quantity in
streamside channels has shown that shredders can
track litter resource patches. Such resource tracking
can lead to aggregation of shredders on leaf packs and
consequent acceleration of decomposition (Rowe and
Richardson 2001). Results from this small-scale and
short-term experiment have been corroborated by data
from resource-depleted streams, such as those that
drain clear-cut catchments (Benfield et al. 2001) or
those located above tree line (Robinson et al. 1998). In
this and other situations where litter resources in
streams are rare, aggregation of shredders on experimental leaf packs can be massive and can dramatically accelerate decomposition beyond the rates
caused by microbial activity alone (e.g., Baldy and
Gessner 1997, Robinson et al. 1998).
Effects of shredder aggregation on decomposition
ET AL.
[Volume 27
have been difficult to demonstrate by manipulating
litter quantity in whole-stream experiments. Reice
(1991) altered litter quantities in a series of 30-m
stream reaches and found no evidence for changes in
decomposition rates in either litter-augmented or
litter-depleted reaches. Leaf decomposition rate did
not differ between a headwater stream from which
litter was experimentally excluded and a reference
stream during the 1st year after exclusion (Eggert and
Wallace 2003). The absence of immediate responses to
litter manipulations in these 2 studies is surprising in
view of numerous field observations of shredder
aggregation, corresponding rapid decomposition, and
clear effects on decomposition in experimental stream
channels. Thus, the extent to which shredder aggregation responses to altered quantities of benthic litter
determine leaf litter colonization and decomposition is
unclear.
Microbial decomposers also might respond to
changes in litter availability, which would have
consequences for decomposition. Experimentally introduced litter might immobilize nutrients and slow
microbial decomposition when dissolved nutrient
concentrations in stream water are low (Eggert and
Wallace 2003). In contrast, abundant benthic litter
might promote, rather than curb, microbial decomposition when nutrient supply is less critical. Such an
effect could arise as a consequence of the life-cycle
characteristics of aquatic hyphomycete fungi, the key
microbial decomposers of leaf litter in streams (Gessner et al. 2007). Aquatic hyphomycetes are characterized by rapid and dense sporulation following initial
establishment and growth in decomposing litter
(Gessner and Chauvet 1994). As a result, spore
densities in stream water should be greater when
benthic litter is abundant than when it is sparse, and
the difference is likely to be large, as indicated by
often-dramatic increases in spore concentrations following autumn leaffall (Bärlocher 2000) and greater
spore concentrations following experimental enhancement of litter retention (Laitung et al. 2002). Given that
leaf decomposition in microcosms is considerably
faster when spore densities are high than when they
are low (Treton et al. 2004), greater fungal colonization
and decomposition of leaves would be expected when
quantities of benthic litter are high.
We present results from an experimental manipulation of benthic litter quantity at the scale of the stream
reach (sensu Bisson and Montogomery 2006) in 3
streams. We designed this replicated experiment to test
2 hypotheses related to the effect of benthic litter
quantity on macroinvertebrates colonization and
decomposition. First, we hypothesized that, in the
absence of pronounced nutrient limitation, augmenta-
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LEAF DECOMPOSITION
AND
LITTER QUANTITY
323
TABLE 1. Physical and chemical characteristics of the 3 streams at low-flow conditions. Temperature data refer to daily means
during the study period. Other measurements were taken on a single sampling date during the study. Values in parentheses are
standard deviations; n/a indicates data were not available.
Stream
Elevation
(m asl)
Width
(m)
Discharge
(L/s)
Temperature
(8C)
Turbidity
(NTU)
Alkalinity
(mmol/L)
Conductivity
(lS/cm)
PO4-P
(lg/L)
NH4-N
(lg/L)
NO3-N
(lg/L)
Andelsbach
Fohrenbach
Mühlebach
530
740
420
2.3 (0.53)
3.8 (0.71)
2.9 (0.65)
30
65
33
5.5 (1.5)
5.3 (1.6)
6.9 (1.2)
2.3
n/a
1.3
0.67
0.47
1.08
108
115
151
3.5
81.1
5.3
5.0
4.3
3.0
961
649
2500
tion of benthic litter would increase microbial decomposition relative to controls. We addressed this
hypothesis by testing whether rates of leaf decomposition in fine-mesh litter bags, which restrict shredder
access, varied across stream reaches in response to
experimental augmentation or depletion of the quantity of benthic litter. Second, we hypothesized that
shredding invertebrates would promote decomposition, particularly in stream reaches where benthic litter
was scarce. We tested this hypothesis by comparing
shredder colonization and decomposition of leaf packs
in coarse-mesh litter bags placed in reaches with
experimentally augmented, depleted, or unmanipulated litter.
Methods
Study sites
Three streams (Andelsbach, Fohrenbach, Mühlebach) were selected in the southern Black Forest,
Germany. Streams had similar watershed geology
(granitic), land cover (primarily managed forests),
and size (3rd-order). Channels were characterized by
pool and riffle sequences, substrata were dominated
by coarse gravel and small cobble (sensu Wentworth
1922), and similar volumes of large wood were
present, as judged by visual inspection along the
study sections. Riparian vegetation consisted of a
closed-canopy mixed community, including alder,
maple, ash, and beech. Table 1 summarizes the major
chemical and physical attributes of the streams.
Experimental design
Three reaches were delineated in each stream.
Reaches were 40 m long and homogeneous in terms
of morphological characteristics and riparian vegetation. One of 3 treatments was assigned to each reach in
each stream: 1) litter augmentation above background
levels, 2) litter depletion, and 3) unmanipulated
control. Potentially confounding upstream–downstream effects were mitigated by constraining random
assignment of treatments to reaches using the criterion
that each treatment was replicated in an upstream,
middle, and downstream reach, and each treatment
was replicated only once within a stream. Reaches
were separated by .100 m to minimize influences of
upstream treatments on treatments in middle or
downstream reaches. Thus, the result was a constrained complete block design using the stream as the
blocking factor (Fig. 1).
Litter augmentation was achieved by means of litter
traps (Dobson and Hildrew 1992, Dobson 2005). Traps
consisted of plastic mesh (20 3 20 cm, 1-cm mesh size)
held vertically in the stream by 2 rebars hammered
into the streambed and oriented perpendicular to
stream flow. In each augmented reach, 105 to 140 traps
were added (average trap density ¼ 1/m2) just before
leaffall (Dobson 2005).
Litter depletion was accomplished by removing all
visible leaf material from the reach by hand at weekly
intervals. Each handful of leaf material was rinsed
gently in the stream to minimize removal of resident
invertebrates. The first depletion campaign occurred
the same day that litter traps were installed. Litter
volume removed during each depletion campaign was
quantified by placing the collected leaf material into a
rigid plastic bin (volume ¼ 0.20 m3) and measuring the
height of leaf material in the bin. Numerous holes (2cm diameter) drilled into the walls of the bin allowed
water to drain. The content of the bin was mixed
thoroughly by hand, and three 1.6-L subsamples were
taken to the laboratory where they were oven-dried
(1058C, 5 d) and weighed. Dry mass of subsamples
was extrapolated to the volume of leaf material in the
bin to estimate the total mass of litter collected in
reaches during each removal campaign.
Effectiveness of litter augmentation and depletion in
modifying the quantity of benthic litter on the
streambed was determined with a cylindrical sampler
(area ¼ 0.071 m2) 58 d after initiation of the experiment.
The sampler was placed at 20 locations that were
randomly selected within a grid along each stream
reach. The enclosed litter on the streambed was
gathered, placed in plastic bags, transported to the
laboratory, oven-dried (1058C, 5 d), and weighed.
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S. D. TIEGS
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[Volume 27
FIG. 1. Schematic diagram of the randomized block design used to test the effect of benthic litter quantity on leaf decomposition
and macroinvertebrate abundance. Litter quantity was manipulated in 3 reaches (litter depletion, litter augmentation, and
unmanipulated control) in each of 3 study streams. Random assignment of litter manipulations to reaches was constrained to
ensure that each treatment was replicated in an upstream, middle, and downstream reach, and each treatment was replicated only
once within a stream. Stream-flow direction is from the top of the figure downward. Minimum distance between reaches was 100
m.
Litter-bag preparation, installation, and sample processing
A litter-bag approach was used to determine
decomposition rates. Bags were constructed of either
coarse-mesh (10-mm mesh size) or fine-mesh (0.5-mm
mesh size) plastic netting to allow or prevent,
respectively, macroinvertebrate access to enclosed
leaves. Recently senesced leaves of alder (Alnus
glutinosa [L.] Gaertn.), a common riparian species
throughout most of Europe, were collected, air-dried,
and weighed into batches of 5.00 6 0.25 g. After
weighing, each batch was remoistened to render the
leaves pliant, and the leaves were placed into the mesh
bags. Five randomly selected litter bags were used to
estimate initial leaf moisture content by drying (1058C,
24 h) and reweighing the material.
Litter bags were taken to the field the next day, and 6
coarse-mesh and 6 fine-mesh bags were placed in the
middle of each of the 9 study reaches. Steel rods were
hammered into the streambed, and a coarse- and finemesh bag was attached to each rod with nylon cord.
Flat cobbles were placed on the cord immediately
upstream of each bag to prevent bags from moving in
the current and to ensure contact with the sediment.
After 41 d of exposure in the streams, all leaf bags were
retrieved and placed in plastic bags. Bags were
returned to the laboratory in a cooler and frozen for
later processing.
In the laboratory, the contents of each plastic bag
were emptied into a shallow tray with a small amount
of water and allowed to thaw. Each leaf was cleaned
individually with a soft-bristled paint brush to remove
adhering debris and macroinvertebrates, placed in an
aluminum tray, oven-dried (1058C, 48 h), and weighed
to the nearest 0.01 g. Invertebrates were collected on a
500-lm mesh screen, preserved in 70% ethanol,
identified to the lowest practicable taxon, counted,
and assigned to functional feeding groups (Gessner
and Dobson 1993, Merritt and Cummins 1996).
Statistical analysis
Differences in log10(x þ 1)-transformed quantities of
benthic litter on the streambed were tested with
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AND
LITTER QUANTITY
325
TABLE 2. Results of analysis of variance for the effect of
litter manipulation and stream on benthic litter quantity.
Stream is treated as a blocking factor according to Newman
et al. (1997). – indicates F was not calculated because the
stream 3 litter manipulation term was significant.
Source of variation
Stream
Litter manipulation
Litter manipulation
3 stream
Error
FIG. 2. Mean (61 SE) quantity of benthic litter on the
streambed in litter-depleted, unmanipulated control, and
litter-augmented reaches in 3 streams; n ¼ 20 samples in each
reach. Note that the y-axis begins at 0.1 g.
analysis of variance (ANOVA) using the stream as a
blocking factor. When the effect of litter manipulation
was not consistent across streams, the litter manipulation 3 stream interaction term was included in the
ANOVA model, following the rationale of Newman et
al. (1997) and Quinn and Keough (2002). Differences in
the percentage of leaf dry mass remaining in litter bags
also were tested with ANOVA (using litter manipulation and mesh size as the main factors of interest, with
stream as a blocking factor), and when streams
differed in their responses, the interaction terms with
stream were included in the model as well (Newman
et al. 1997, Quinn and Keough 2002). Differences in
log10(x þ 1)-transformed invertebrate abundance in
coarse-mesh litter bags were tested with ANOVA
(using litter manipulation as the main factor of interest
and stream as a blocking factor). In this case, the litter
manipulation 3 stream interaction was never significant, so all p-values reported for invertebrates refer to
ANOVA models without this interaction term. When
significant (i.e., p , 0.05) differences were observed
among litter manipulations, Tukey’s post-hoc tests
were used to identify the means that differed.
Results
Sum of
squares
(%)
df
Mean
square
F
p
8.6
15.1
2
2
6.4
11.2
–
5.44
–
0.07
5.5
70.8
4
171
2.1
0.6
3.35
0.011
indicating an inconsistent effect of litter manipulation
among streams. This effect was caused by large
quantities of naturally accumulated benthic litter in
the control reach of Mühlebach (Fig. 2). When
Mühlebach was excluded from analysis, the litter
manipulation 3 stream interaction term was not
significant (F2,114 ¼ 1.08, p ¼ 0.34), litter manipulation
had a marginally significant effect on the quantity of
benthic litter on the streambed (F2,2 ¼ 16.9, p ¼ 0.056),
and the quantity of benthic litter was greater in
augmented than in control and depleted reaches
(Tukey’s post-hoc comparisons, p , 0.001) but did
not differ significantly between control and depleted
reaches (p ¼ 0.08).
Benthic litter removed from depletion reaches
The quantity of benthic litter removed from depleted
reaches varied among streams. A total of 7.2 kg dry
mass was collected in Andelsbach, 7.7 kg in Fohrenbach, and 21.8 kg in Mühlebach, corresponding to a
depletion of 72, 51, and 189 g/m2 in each stream
channel. The mass of benthic litter removed from
depleted reaches declined over the first month after
litter traps were installed and subsequently remained
low (Fig. 3), except in Mühlebach in late November,
when the quantity of benthic litter removed increased
at a time that coincided with a minor rainfall and
stream-flow event that occurred in this watershed but
not the 2 others.
Quantity of benthic litter on the streambed
Leaf decomposition
The quantity of benthic litter on the streambed
differed significantly among streams (F2,175 ¼ 8.6, p ,
0.001) and among litter manipulations (F2,175 ¼ 8.0, p ,
0.001). Depleted reaches had consistently less benthic
litter than control or augmented reaches (Fig. 2).
However, the litter manipulation 3 stream interaction
term was significant and was included in the ANOVA
model (model 1 of Newman et al. 1997; Table 2),
Leaf decomposition was significantly faster in
coarse-mesh than in fine-mesh litter bags (F1,100 ¼
31.4, p , 0.001) and differed significantly among
streams (F2,100 ¼ 8.1, p , 0.001; Fig. 4A, B). In contrast,
litter manipulation had no significant effect on leaf
decomposition (F2,100 ¼ 1.9, p ¼ 0.15), nor was the litter
manipulation 3 mesh size interaction term significant
(F2,100 ¼ 0.74, p ¼ 0.48). Leaf decomposition was more
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S. D. TIEGS
ET AL.
[Volume 27
FIG. 3. Dry mass of benthic litter removed from each
litter-depleted reach through time. The abrupt increase
observed in Mühlebach on 25 November coincided with a
rainfall event in this watershed and not the others.
variable across streams and litter manipulations in
coarse-mesh (Fig. 4A) than in fine-mesh litter bags
(Fig. 4B). Patterns of leaf decomposition were similar
among litter manipulations in 2 streams (Andelsbach
and Fohrenbach), whereas the pattern in Mühlebach
differed (Fig. 4A). When Mühlebach was excluded
from the analysis, the litter manipulation did not
significantly affect leaf decomposition (F2,65 ¼ 2.6, p ¼
0.08), but the litter manipulation 3 mesh size interaction term was significant (F2,65 ¼ 3.2, p ¼ 0.045). In
Andelsbach and Fohrenbach, leaf decomposition rate
in coarse-mesh bags decreased with the quantity of
benthic litter in the reach (augmented , control ,
depleted; Fig. 4A). Leaf decomposition in fine-mesh
bags varied little overall (Fig. 4B); the difference
between mean leaf decomposition across all streams
and litter manipulations was ,4% (Fig. 4B).
Macroinvertebrate colonization
Thirty-two taxa of macroinvertebrates were identified from coarse-mesh bags. Shredders accounted for
46% of all macroinvertebrates and consisted of stoneflies (Amphinemura, Leuctra, Nemoura, Protonemura,
Taeniopteryx), limnephilid caddisflies, and the amphipod Gammarus fossarum. Stonefly shredders accounted
for 77%, and the genus Nemoura accounted for 60% of
all shredders.
Large macroinvertebrates were almost never encountered in fine-mesh bags, indicating that fine-mesh
bags effectively excluded the most important shredders. Very small nemourids were found in fine-mesh
bags (449 individuals in fine-mesh bags; 1732 individuals in coarse- and fine-mesh bags combined). This
suggests that an appreciable fraction of nemourids in
FIG. 4. Mean (61 SE) leaf dry mass remaining in coarsemesh (A) and fine-mesh (B) litter bags after 41 d of
decomposition in litter-depleted, litter-augmented, and
unmanipulated control reaches of three streams; n ¼ 6 litter
bags for each mesh size in each reach. Note that the y-axes
begin at 40%.
fine-mesh bags and, by inference, also in coarse-mesh
bags, were very early instars, small enough to pass
through a 0.5-mm mesh. In contrast, only 2 individuals
of Gammarus (,0.5% of the total number) were
observed in fine-mesh bags, indicating that Gammarus
were typically larger than other abundant shredders.
Most of the remaining individuals in fine-mesh bags
(55% of all macroinvertebrates) were chironomids.
Numbers of other invertebrates (collector–gatherers,
collector–filterers, scrapers, and nonchironomid predators) were consistently low across streams and litter
manipulations.
The total number of invertebrates per coarse-mesh
litter bag and number of shredders per coarse-mesh
litter bag did not differ significantly among streams
(total invertebrates: F2,49 ¼ 0.73, p ¼ 0.49; shredders:
F2,49 ¼ 0.25, p ¼ 0.78) or litter manipulations (total
invertebrates: F2,49 ¼ 0.24, p ¼ 0.79; shredders: F2,49 ¼
0.20, p ¼ 0.81) (Fig. 5A, B). Numbers of individuals in
coarse-mesh litter bags from other functional feeding
groups did not differ significantly among litter
2008]
LEAF DECOMPOSITION
AND
LITTER QUANTITY
327
litter manipulations were not significant (F2,49 ¼ 2.86, p
¼ 0.067). However, when data from Mühlebach (where
only a few Gammarus colonized litter bags) were
excluded from the analysis, the number of Gammarus
per coarse-mesh litter bag also differed significantly
among litter manipulations (F2,32 ¼ 3.47, p ¼ 0.043; Fig.
5D). In Andelsbach and Fohrenbach, the number of
Gammarus per coarse-mesh litter bag was significantly
lower in augmented than in depleted litter manipulations (Tukey’s post-hoc comparison, p ¼ 0.033).
Discussion
Test of the shredder aggregation hypothesis
FIG. 5. Mean (61 SE) number of total macroinvertebrates
(A), total shredders (B), nemourid stoneflies (C), and
Gammarus (D) in coarse-mesh litter bags after 41 d of leaf
decomposition in litter-depleted, litter-augmented, and
unmanipulated control reaches of 3 streams; n ¼ 6 coarsemesh litter bags in each reach.
manipulations (F2,49 , 1.29, p . 0.28). The number of
nemourid stoneflies per coarse-mesh litter bag did not
differ significantly among streams (F2,49 ¼ 2.38, p ¼
0.10) or litter manipulations (F2,49 ¼ 1.78, p ¼ 0.18), even
though nemourids were rare in Fohrenbach (Fig. 5C).
In contrast, the number of Gammarus individuals per
coarse-mesh litter bag differed strongly among streams
(F2,49 ¼ 35.3, p , 0.001), although differences among
Previous studies on ecosystem effects of litter
depletion on decomposition have yielded equivocal
responses. Results from observational field studies and
small-scale experiments in streamside channels have
indicated an acceleration of decomposition when litter
is scarce (e.g., Benfield et al. 1991, 2001, Robinson et al.
1998, Rowe and Richardson 2001), but this response
has not been evident in field experiments (e.g., Reice
1991). The overall effects of benthic litter manipulations in our experiment were subtle; however, results
from coarse-mesh bags in 2 of the 3 study streams
were consistent with the predicted pattern: the leaf
decomposition rate tended to be fastest in depleted
reaches and slowest in augmented reaches. Differences
in shredder colonization of experimental litter bags
have been proposed as the mechanism underlying
varying rates of leaf decomposition in response to litter
quantity. This hypothesis proposes that shredders
would aggregate most in litter bags exposed in
resource-depleted environments (e.g., Benfield et al.
2001, Rowe and Richardson 2001), whereas shredders
would be distributed across a larger number of
resource islands in control reaches and, especially, in
resource-augmented reaches.
Data consistent with these hypotheses must show
that: 1) shredders cause significant litter mass loss, and
2) shredders aggregate in experimental leaf bags in
reaches where benthic litter is scarce, and they do not
aggregate in experimental litter bags in reaches where
benthic litter is abundant. The first requirement was
met in our study streams. Leaf decomposition was
significantly faster in coarse-mesh than in fine-mesh
bags. More rapid decomposition in coarse-mesh than
in fine-mesh litter bags can be caused by factors other
than shedders (Boulton and Boon 1991), but such
effects were unlikely to be important in our study
because no indications of mechanical fragmentation in
coarse-mesh bags or O2 depletion in fine-mesh bags
were observed. Moreover, controlled experiments
under various hydraulic conditions in experimental
328
S. D. TIEGS
stream channels have shown that litter mass loss does
not differ between coarse-mesh and fine-mesh litter
bags when shredders are absent, but it does differ
significantly between the 2 types of litter bags when
shredders are present (Ferreira et al. 2006).
The 2nd requirement, aggregation of shredders in
coarse-mesh bags placed in litter-depleted reaches,
was less well met in our study because total numbers
of macroinvertebrates, shredders, and nemourid stoneflies did not follow the predicted pattern. However, the
distribution of Gammarus across reaches supports the
idea—this shredder did converge on resource islands
provided by our experimental litter bags. Specifically,
Gammarus was more abundant in litter bags in
depleted reaches and rarer in litter bags in augmented
reaches. This pattern matched the patterns in benthic
litter quantities and leaf decomposition rate in the 2
study streams where litter manipulations were successful (i.e., in Andelsbach and Fohrenbach).
Gammarus are very effective leaf shredders (e.g.,
Groom and Hildrew 1989, Baldy and Gessner 1997,
Dangles et al. 2004) that feed very selectively
(Bärlocher and Kendrick 1973, Arsuffi and Suberkropp
1989, Graça et al. 2001). Moreover, they are extremely
mobile relative to other invertebrates in our study, and,
thus, they are capable of seeking out and making use
of resource islands. Gammarus were almost never
encountered in fine-mesh litter bags (0.5-mm mesh
size), suggesting that specimens were larger and more
effective at consuming leaves than other shredder
species in our study streams. For example, the
numerically abundant nemourid stoneflies often were
found in fine-mesh litter bags, indicating that a large
proportion of them were early instars with low
biomass (2-mm length, ,0.05 mg). These early instars
would have had a limited shredding capacity, even if
leaves were their main diet.
Mass loss also was significantly faster in coarsemesh litter bags than in fine-mesh bags in the stream
where Gammarus was rare. This result suggests that
other shredders contributed to litter mass loss as well.
However, the lower mobility of those taxa might
require greater differences in benthic litter quantities
than those achieved in our study to demonstrate
aggregation effects on leaf decomposition in field
situations. Collectively, this evidence indicates that
aggregation in experimental litter bags of highly
mobile Gammarus, but not other shredders, could have
been instrumental in causing the decomposition
patterns observed across litter-manipulated stream
reaches. The general implication is that resource
availability might influence ecosystem functioning by
modulating aggregation of key consumer species.
In the long term, shredder-mediated changes of
ET AL.
[Volume 27
decomposition rates also might occur as a result of
changes in shredder production, an idea that is
supported by data from a whole-stream litter-exclusion experiment (Eggert and Wallace 2003). No
immediate response to litter exclusion was observed
in that study, but decomposition of red maple leaves 1
and 2 y later was much slower in the litter-exclusion
stream than in the reference stream, which received
normal litter inputs (k ’ 0.010/d compared to
0.017/d). The suggested mechanism was severe food
limitation, which restrained recruitment of shredders
in the years following litter depletion. This mechanism
is in accordance with data from another study, which
showed that production of some large shredder taxa
was markedly lower in the litter-exclusion stream than
in the reference stream 1 y after litter inputs were
prevented (Wallace et al. 1999, Eggert and Wallace
2003). Such an effect, while possible, would not have
been captured by our single-season experiment.
In contrast to litter-depletion experiments done at a
large scale, litter-augmentation experiments typically
have been conducted at smaller spatial scales. Furthermore, most litter-augmentation experiments have
considered macroinvertebrate responses, rather than
responses of litter decomposition or other processes, to
altered litter availability. Shredders have shown
positive responses when benthic litter quantities have
been experimentally elevated (Dobson and Hildrew
1992). For example, in streamside experimental channels with varied quantities of benthic leaf litter, density
and biomass of shredders increased in response to
greater litter availability (Richardson 1991). Furthermore, abundances of invertebrates increased relative to
controls when boulders or litter traps installed in
stream channels increased litter quantity relative to
controls (Dobson and Hildrew 1992, Negishi and
Richardson 2003). However, shredder abundance
remained unchanged in a similar boulder-introduction
experiment that enhanced litter retention (Lepori et al.
2005, see also Wallace et al. 1995), and leaf decomposition also remained unchanged (Lepori et al. 2005).
The results of these augmentation studies lend indirect
support to the hypothesis that shredders accelerate leaf
decomposition in resource-limited environments.
Test of the microbial decomposition hypothesis
Our hypothesis that larger quantities of benthic litter
would lead to faster microbial decomposition in
augmented than in control or depleted reaches was
not supported by our results. The rationale behind this
hypothesis was that larger quantities of decomposing
benthic litter should lead to higher concentrations of
fungal spores in stream water. A greater fungal
2008]
LEAF DECOMPOSITION
inoculum would accelerate fungal colonization of fresh
litter and, therefore, microbial decomposition. Such an
outcome has been observed in a microcosm experiment (Treton et al. 2004). In one field study, fungal
spore concentrations in stream water were greater in
stream reaches to which logs or litter traps (identical to
those used in our study) had been added as retention
structures (Laitung et al. 2002). Some of the control
and depleted reaches in our study could have been
exposed to elevated spore concentrations from augmented upstream reaches, but our experiment was
designed to prevent the same systematic upstream–
downstream effect in all 3 streams (Fig. 1). Furthermore, even if spore concentrations in our experimental
reaches were influenced by upstream litter manipulations, the effect on decomposition was negligible
because mass loss of leaves in fine-mesh litter bags
was highly consistent across litter manipulations in all
3 streams.
Effect of differences among streams on results of litter
manipulations
Macroinvertebrate responses to litter manipulation
were different in Mühlebach than in the other 2
streams. The control reach of this stream flowed along
the base of a steep hillslope that delivered large lateral
inputs of litter to the channel, so quantities of benthic
litter were higher in the control than in the augmented
reach. As a consequence, relative quantities of benthic
litter in the 3 reaches of this stream differed from those
intended by our manipulations. However, even if a
less extreme control reach had been chosen, the
response pattern to our litter manipulation probably
would have been different from the responses in the
other 2 streams because Gammarus was rare in all 3
reaches of Mühlebach. If our conclusion regarding the
critical role of Gammarus in leaf decomposition is
correct, then the low abundance of this species in the
Mühlebach probably explains why decomposition
patterns across reaches did not reflect shredder
abundances.
The deviating pattern among streams in our study
illustrates the importance of replicating manipulative
ecosystem experiments and of exercising great care
when extrapolating results to other ecosystems, even
when they appear to be similar. Whole-ecosystem
manipulations are among the best means to assess
effects of environmental or biotic factors on ecosystem
processes and properties (Carpenter et al. 1989, 1995).
A drawback of the approach is that practical constraints often preclude replication of treatments (sensu
Hurlbert 1984). A suite of methods has been proposed
to alleviate this difficulty (e.g., Carpenter et al. 1989,
AND
LITTER QUANTITY
329
1995, Wallace et al. 1999), but none of the methods
fully resolves the problem (e.g., Murtaugh 2002).
However, although not always practical, use of
replicated designs often is possible, even in manipulative ecosystem experiments (e.g., Maron et al. 2006,
Entrekin et al. 2008). Headwater streams are prime
candidates for this approach because of their relatively
small size.
In summary, we observed that the quantity of leaf
litter in stream channels influenced the colonization of
experimental litter bags by Gammarus, and the
abundance of Gammarus in litter bags, in turn, might
have influenced leaf decomposition rate. In litterdepleted reaches, Gammarus appeared to aggregate in
litter bags and to accelerate decomposition. However,
these results were not consistent among all streams
examined, which illustrates the importance of treatment replication when conducting manipulative ecosystem experiments.
Acknowledgements
We thank Markus Schindler, Simone Graute, Torsten
Diem, Catherine Hoyle, Lucia Klauser, Caroline Joris,
Angelika Rohrbacher, Michael Siegrist, and Michael
Vock for their help in the field. Andrew Boulton, Alan
Covich, Pamela Silver, and anonymous referees provided many useful comments on previous drafts of the
paper. This research was funded by the Swiss State
Secretariat of Education and Research (SBF No.
01.0087) as part of the European Union project
RivFunction (contract no. EVK1-CT-2001–00088).
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Received: 11 June 2007
Accepted: 5 February 2008