Do installed steam logjams increase macroinvertebrate
richness and abundance?
Seyeon Kim and Ong Xiong with faculty mentor Dr. Todd Wellnitz
Biology Department | University of Wisconsin - Eau Claire
Results
Plecoptera
2%
Before-After-Control-Impact (BACI) experimental design:
1. Survey the initial condition of the control & impact site.
2. Apply treatment (logjam installation) to the impact site.
3. Allow time for the impact to occur.
4. Re-survey the control & impact sites.
Trichoptera
17%
Ephemeroptera
28%
Diptera
39%
600
Control
*
400
200
0
Logjam
CNTRL
LOGJAM
CNTRL
2011
LOGJAM
2012
Figure 1. Community composition at control and impact sites. Pie charts (left) show percent abundance of the
dominant macroinvertebrate orders one year after logjam installation. Graph (right) shows the total number
(mean ± 1 SE) of benthic invertebrates compared to the number of Chironomidae (Diptera). Chironomids made
up a smaller fraction of the community in 2012, the year after logjam installation (MANOVA, p < 0.05).
Abundance m-2
100
1.5
*
A
Trichoptera
Ephemeroptera
80
Plecoptera
*
60
40
20
0
Seyeon Kim
identifying stream
macroinvertebrate
samples in the lab.
All Inverterates
Chironomidae
Plecoptera
16%
Diptera
70%
Hypothesis
Methods
800
2012
Ephemeroptera
5%
120
Installing a “natural” stream logjam will increase macroinvertebrate richness and
abundance above those found on the streambed.
Coleoptera
10%
Abundance m-2
Stream logjams are fixed accumulations of wood that can increase the heterogeneity of
stream habitats by:
1. Creating hydraulic roughness.
2. Retaining organic matter such as leaves and sticks.
3. Inducing pool and riffle formation, thereby altering stream depth and velocity.
4. Accumulating fine sediments immediately above the logjam, exposing coarse bed
immediately below.
5. Providing substrate for algal colonization, which serves as food for many
macroinvertebrates.
By increasing habitat heterogeneity in this manner, we predicted that logjams would
enhance habitat for aquatic macroinvertebrate and increase their numbers.
Trichoptera
7%
Coleoptera
6%
CNTRL
LOGJAM
CNTRL
LOGJAM
2011
2011
2012
2012
Species richness per sample
Introduction
1.3
*
B
Trichoptera
Ephemeroptera
Plecoptera
1.0
*
0.8
0.5
0.3
0.0
CNTRL
LOGJAM
CNTRL
LOGJAM
2011
2011
2012
2012
Figure 2. Effect of logjam installation on the number of individuals (A) and species richness (B) within the
insect orders Trichoptera, Ephemeroptera and Plecoptera, i.e., caddisflies, mayflies and stoneflies. There was
a significant Time x Treatment interaction (MANOVA, p < 0.05) indicating that mayfly and stonefly abundance
and richness both increased at the logjam site 1 yr after logjam installation. Error bars represent ± 1 SE.
Discussion
We found partial support for our hypothesis. Although taxonomic richness and abundance did not
change for the macroinvertebrate community as a whole, it did for three important indicator taxa, the
Ephemeroptera (mayflies), Plecoptera (stoneflies) and the Chironomidae (midges; Diptera). Mayfly
and stonefly nymphs increased at the logjam whereas midge larvae decreased in number. We
speculate that the logjam provided mayfly and stonefly nymphs with refuge from fish predators and
also increased surface area for their colonization. Midge density, by contrast, may have decreased
because the logjam concentrated predacious stoneflies that commonly feed on chironomids.
The artificial logjam one year after installation. Note the
accumulation of woody debris on the upstream face.
Field Methods:
We used a BACI approach to examine the impact of an
installed logjam on a small stream in northern
Minnesota’s Superior National Forest. The control site
was located 50 m upstream of the impact (logjam) site.
We sampled the macroinvertebrate community at both
sites prior to logjam installation, and then assembled the
logjam at the impact site. The macroinvertebrate
community was sampled again one year later.
Our results lend support to the idea that logjams have an important role in structuring the biological as
well as the physical environment of streams. Mayflies and stoneflies are used as indicators of stream
health and ecosystem function, and consequently, installed logjams may provide resources managers a
tool for enhancing habitat for these sensitive taxa.
Team Logjam in the field. From left to
right are Zach Snobl, Aaron Devoe,
Brennean Dow and Ong Xiong.
Acknowledgements
Funding and support for this project was provided by the National Science Foundation, the US Forest Service, the
UWEC Center of Excellence for Faculty and Undergraduate Student Research Collaboration, and the UWEC Diversity
Mentoring Program. Special thanks goes to all the members of Team Logjam – Aaron Devoe, Zach Snobl, Brennen
Dow & Sarina Rutter and others -- who helped set up the study, sample the stream, and process the samples.