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The
Effects of Stream Fragmentation on Fish Communities
Robert
Fischer and Karen Popp
Department of Biological Sciences, Eastern Illinois University
INTRODUCTION
No single factor
has been a greater cause of population decline in wildlife communities than loss
of habitat due to fragmentation. Habitat fragmentation is usually associated
with terrestrial environments and is defined as the breaking up of contiguous
areas of land into smaller, more isolated patches. However, one of the major
questions facing stream biologists is to
determine if habitat fragmentation can occur within a stream.
What
is stream habitat fragmentation?
A
lack of connectivity between upstream and downstream populations which occurs
when the
longitudinal continuum is disrupted and/or when lateral
connections are severed between the stream channel and adjacent wetlands
or riparian zones.
How
are streams fragmented?
Through land-use changes such as deforestation of riparian and floodplain
areas, urban development, livestock grazing and conversion of
floodplain and riparian areas into cropland for agricultural use. In Illinois,
agricultural practices have deforested over 70% of native vegetation that have
resulted in the loss or simplification of near-stream vegetation and a reduction
in the physical environment of streams.
Physical changes caused by riparian fragmentation include increased
sedimentation, increased water temperatures, decreased dissolved oxygen
levels, increased pollution levels and decreased
habitat diversity within streams. The combined
effect of riparian zone fragmentation and associated changes in stream abiotic
factors may be detrimental to stream fish communities.
Possible changes in fish populations include reduced
diversity of fish, decreased
complexity in size structure of fish populations, decline in
fish specialists, increased
abundance of omnivores and detritivores and greater
temporal variability in fish abundance.
OBJECTIVES
Since land use
practices can cause stream habitat fragmentation along both a lateral and a
longitudinal gradient, one would expect that fragmentation might lead to the
creation of distinct patches within a stream. To date few studies have
investigated the response of stream fish communities to in-stream patch
formation caused by habitat fragmentation.
Therefore the
objectives of the study were to:
1)
Determine
if habitat fragmentation causes distinct in-stream patch formation.
2)
Determine
the effects of patch formation on stream fish communities.
METHODS
Sites
Seven sites in
various stages of riparian zone disturbance were sampled along Polecat Creek in
Coles County, Illinois during 1998 and 1999.
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Sampled sites along Polecat Creek |
Reaches
Reaches ranged
from 360-650 ft and contained at least one riffle/pool sequence when possible.
Habitat
assessment (In Stream Patch Formation):
At each site the
Stream Habitat Assessment Procedure (SHAP) was used to evaluate stream habitat
based on the following 15 metrics:
|
METRIC |
|
Excellent |
Good |
Fair |
Poor |
|
Substrate and instream cover |
|
|
|
|
|
|
Bottom substrate |
16-20 |
11-15 |
6-10 |
1-5 |
|
|
Deposition |
10-12 |
7-9 |
4-6 |
1-3 |
|
|
Substrate stability |
13-16 |
9-12 |
5-8 |
1-4 |
|
|
Instream cover |
10-12 |
7-9 |
4-6 |
1-3 |
|
|
Pool substrate |
16-20 |
11-15 |
6-10 |
1-5 |
|
Channel morphology and
hydrology |
|
|
|
|
|
|
Pool quality |
13-16 |
9-12 |
5-8 |
1-4 |
|
|
Pool variability |
13-16 |
9-12 |
5-8 |
1-4 |
|
|
Channel alteration |
7-8 |
5-6 |
3-4 |
1-2 |
|
|
Channel sinuosity |
10-12 |
7-9 |
4-6 |
1-3 |
|
|
Width/depth |
13-16 |
9-12 |
5-8 |
1-4 |
|
|
Hydrolic diversity |
10-12 |
7-9 |
4-6 |
1-3 |
|
Riparian and bank features |
|
|
|
|
|
|
Canopy cover |
10-12 |
7-9 |
4-6 |
1-3 |
|
|
Bank vegetation |
13-16 |
9-12 |
5-8 |
1-4 |
|
|
Immediate land use |
7-8 |
5-6 |
3-4 |
1-2 |
|
|
Flow-related refugia |
10-12 |
7-9 |
4-6 |
1-3 |
Six individuals
subjectively assessed each metric along each stream reach and assigned metrics
to one of four categories using guidelines established by the Illinois
Environmental Protection Agency. The total score of 15 metrics forms the basis
of overall habitat quality for the reach.
Fish Community
Assessment
To evaluate the
effects of patch formation on stream fish communities, the seven sites
previously identified on Polecat Creek were sampled using electroshocking
techniques. Upon shocking, fish were collected with drift nets and placed into
buckets. After fish identification, stream quality within each site based on the
fish community was determined through the following techniques:
1)
Index of Biotic Integrity (IBI) using the following metrics
|
Category |
Metric |
|
Species richness and |
Total number of fish species |
|
composition |
Number and identity of darter
species |
|
|
Number and identity of sunfish
species |
|
|
Number and identity of sucker
species |
|
|
Number and identity of intolerant
species |
|
|
Proportion of individuals as green
sunfish |
|
|
|
|
Trophic composition |
Proportion of individuals as
omnivores |
|
|
Proportion of individuals as
insectivorous cyprinids |
|
|
Proportion of individuals as
piscivores (top carnivores) |
|
|
|
|
Fish abundance and |
Number of individuals in sample |
|
condition |
Proportion of individuals as
hybrids |
|
|
Proportion of individuals with
disease, tumors, fin |
|
|
damage, and skeletal anomalies |
2) Species
richness
Number of
species collected at each site.
RESULTS
Habitat
assessment:
An analysis of
variance showed a significant difference in SHAP scores between sites
(p<0.001). To determine where the differences occurred a Tukey’s HSD was
performed. The results indicated that the seven sites could be placed into two
distinct groups; sites with good quality habitat (PCG 1,2 and 3) and sites with
poor quality habitat (PCB 1,2,3,and 4).
|
 |
 |
 |
| Good
quality habitat |
Poor quality habitat |
Poor sites vs. Good sites |
In poor quality habitats the loss in overall habitat quality was due to an increase in
deposition and top of bank land use and a loss in
bottom substrate stability, in-stream cover, bank vegetation and pool
quality.
Fish community
assessment:
General
results:
A total of
4,273 fish were collected from the seven sites. Fish from 32
species representing eight families were collected. Cyprinidae
(minnow and carp) and Percidae (darters and perch) were the most common
families with eleven and six species respectively. Eleven
cyprinid species composed 3,334 individuals or 78% of the total number
collected.
Index of
Biotic Integrity:
Reasons for
differences in mean IBI scores
Decreased
number of intolerant species in poor sites.
Good sites
had ten intolerants, poor sites had only six.
Examples:
Northern hogsucker and
Silver redhorse
Increased proportion of omnivores in poor sites.
12.55% in
good sites, 31.65% in poor sites.
Examples:
Bluntnose minnow and
Common shiner
Reduction in
carnivore proportion in poor sites.
1% in good
sites, 0.05% in poor sites.
Examples:
Spotted bass and
Largemouth bass
Reduction in
the number of trophic levels present in poor sites
Two major
types of fish were lost:
Top
carnivore populations were reduced in poor patches
Loss of
Spotted bass and
Largemouth bass
Decrease in
the number of intolerant species present
Loss of
Northern hogsucker and Fantail darter
CONCLUSIONS
1. SHAP scores
varied in association with land use, with increased land use leading to a
decline in habitat quality.
2. Fragmentation
of riparian zone vegetation can cause the formation of distinct patches
within a stream system.
3. Poor patches
had reduced in-stream cover and pool quality as well as increased deposition
and top of bank land use.
4. Poor patches
had reduced biotic integrity and species richness as well as a reduction in
the number of trophic levels.
These results
suggest that restoring native riparian zone vegetation is an essential element
for the management of stream ecosystems.
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