3. Results
3.1 Short-term study
3.1.1 Sediment mass and organic matter content: GLAS
sites
Mean deposited sediment levels in the pre-fencing period ranged from
19.96± 5.29 gm-2 (SE) at the GLAS site 11 control
point to 538.15± 473.74 gm-2 (SE) at the GLAS site 3
pressure point. In the post fencing period, mean deposited sediment
levels ranged from 40.48± 11.94 gm-2 (SE) at the GLAS
site 7 control point to 458.76± 197.88 gm-2 (SE) at
the GLAS site 1 control point.
PERMDISP analysis detected a marginally significant difference in
homogeneity of dispersions among samples for levels of deposited
sediment mass (z scores) at sampling points prior to fencing
compared to sampling points following fencing (F15,188=
P=0.06*), but none in relation to %OM. No reductions were found,
however a pattern of reductions in dispersions (z scores) following
fencing was observed at several sites (Figure 3).
A significant interaction effect was
identified between Fencing and Treatment in relation to
deposited sediment mass (F1,28= 5.17, P=0.03). Pairwise
results demonstrated significantly greater masses of deposited sediment
at pressure points compared to control points, prior to fencing
(t1,14=2.38, P=0.04) but no significant difference
following fencing. It should be noted however, that there was also a
significant increase in levels of deposited sediment at control points
in year 2 compared to year 1 (t1,14=3.09, P=0.01)
(Figure 4).
There were no significant effects in relation to %OM data for either
PERMDISP or PERMANOVA analyses.
3.1.2 Macroinvertebrate
A significant difference was detected in the magnitude of multivariate
dispersions among samples (across control and pressure points) between
pre-fencing and post-fencing periods (F15,176=3.81,
P=0.01). Pairwise comparisons indicated that at Site 1
(t1,10=2.34, P=0.04) and Site 3
(t1,10=3.58, P=0.01) (both on the Blacklion stream) the
magnitude of multivariate dispersion between samples was significantly
less following fencing. Additionally, a marginally significant decrease
in sample dispersions was found in relation to Site 4
(t1,10=2.07, P=0.06) (Figure 5).
Principle coordinate ordination (PCO) plots demonstrate (in most cases)
closer grouping of macroinvertebrate data between control and pressure
points post fencing (Figure 6).
SIMPER analysis highlighted twelve taxa that consistently contributed to
dissimilarities between the Treatment/Fencing levels (Table 1).A. fluviatilis, R. semicolorata, S. pallipes and Agapetussp. had higher abundance at control points than at pressures points in
the pre-sampling period (Pre-Pressure v Pre-Control) and greater in the
post-fencing period compared to the pre-fencing period at both control
(Pre-Control v Post-Control) and pressure (Pre-Pressure v Post-Pressure)
points. The grazing riffle beetles, Elmis aenea and Limnius
volckmari occurred in higher abundance at pressure points following
fencing. Three taxa (B. rhodani/atlaniticus , Simuliidae and
Chironomidae) were less abundant in the post-fencing period at both
control and pressure points, and less abundant at control points in both
the pre-fencing and post-fencing periods.
In terms of the univariate metrics, there was a significant interaction
effect (Fencing/Treatment x Site ) in relation to total
richness (F21,157=5.10, P=0.01) and EPT richness
(F21,157=5.96, P=0.01). Pairwise results highlighted
significantly higher values at the pre-fencing control point relative to
the pre-fencing pressure point at Site 3 (t1,10=5.17,
P=0.01), Site 6 (t1,10=2.63, P=0.04) and Site 7
(t1,10=4.15, P=0.01), while at Site 1 total richness
values were greater at the pressure point (t1,10=3.92,
P=0.01) (Figure 7). Following fencing none of these significant
differences in total richness persisted and there were significant
increases in total richness at the pressure points following fencing
compared to prior to fencing at Site 2 (t1,10=4.13,
P=0.01), Site 3 (t1,10=6.29, P=0.01) and Site 6
(t1,10=2.79, P=0.03)
In relation to EPT richness (Figure 8), there were higher values at the
control points relative to the pressure points at Site 3
(t1,10=6.76, P=0.01) and Site 7
(t1,10=3.80, P=0.01) in the pre-fencing period. At Site
1, EPT richness values were greater at the pressure point in the
pre-fencing period (t1,10=3.39, P=0.02). Here again,
none of these differences persisted following fencing and there were
also significant increases in EPT richness at pressure points following
fencing at Site 2 (t1,10=5.26, P=0.01) and Site 3
(t1,10=5.26, P=0.01).
3.1.3 Habitat assessment: short-term
study
Univariate PERMANOVA analysis of
habitat index scores (THI and RHI) did not highlight any difference
between pre- and post-fencing periods. Multivariate analysis of habitat
score sub-indices and stream substrate cover, geomorphic unit
representation, physico-chemical measurements and stream dimensions,
similarly did not detect any differences. However, RHI scores (available
in supplementary information) did show an increase at pressure points
following fencing, but RHI scores were typically higher at control
points.
In relation to the univariate analyses, the ground coversub-index of the RHI and THI was the only habitat parameter that showed
a difference between pre- and post-fencing periods. A significantFencing and Treatment interaction was observed
(F1,28=6.44, P=0.02) and pairwise results highlighted a
significantly (t1,14=2.65, P = 0.05) higher metric value
at post-fencing pressure points (9.75 ±0.66 SE) compared to pre-fencing
pressure points (7±0.2.65 SE). Significant main term Treatmenteffects with no Fencing interaction were detected for
longitudinal connectivity (F1,28=9.29, P=0.01), canopy
cover (F1,28=7.58, P=0.01) and shrub layer cover
(F1,28=8.82, P=0.01). For each of these RHI sub-indices,
mean values at control points were greater than for those at pressure
points.
3.2 Long term study
There was a significant interaction between the factors Fencingand Catchment in relation to community structure data in the
Milltown Lake catchment (F1,11=2.78, P = 0.02).
Significant differences were detected in community structure in both the
fenced (t1,6=3.09, P=0.01) and the control
(t1,5=2.51, P = 0.01) catchments between pre- and
post-fencing periods. Pairwise results also showed that prior to fencing
there was a significant difference in community structure between the
‘to be fenced’ catchment and control catchment
(t1,11=2.31, P = 0.01) that did not persist following
the fencing period.
Ordination based on principle components (PCO) illustrated a clear
separation between pre- and post-fencing sites in the fenced catchment
along the first PCO axes which accounted for 34.8% of total variation
in the community structure (Figure 9). There is also a separation
between pre-and post-fencing samples in the control catchment, although
this appears not to be as pronounced as in the fenced catchment.
SIMPER analysis on the fenced catchment data showed that increased
abundances of Simuliidae, Ancylus fluviatilis, Glossomatidae,
Elmidae and Baetidae, and reduced abundance of Asellidae, in the post
fencing period accounted for 41% of the dissimilarity in community
structure between the pre- and post-fencing period. In the control
catchment 48% of the dissimilarity in community structure between the
two periods was due to increased abundances of Gammaridae and Simuliidae
in the post-fencing period, and reduced abundances of Baetidae,Hydropsyche siltalai and Elmidae.
In relation to univariate metrics, there was a significant interaction
effect (Fencing x Catchment ) for % ephemeropteran
abundance (%E) (F1,11=19.62, P=0.01) and EPT abundance
(F1,11=14.69, P=0.01). Pairwise results for both, showed
significant differences in values between pre- and post-fencing periods
in both the fenced and control catchments. In the fenced catchment
values for both metrics generally increased following fencing while in
the control catchment values for both generally decreased (Figure 10).