Cattle exclusion over the short-term impacted instream
deposited sediment and aquatic macroinvertebrate communities
Significantly greater mean masses of deposited sediment were observed at
pressure points (relative to control points) prior to fencing, however,
following fencing there was no difference between control and pressure
points, despite a general increase in background levels of deposited
sediment. Patch scale deposits of fine sediment (as assessed here)
reflect stream bank erosion in the immediate upstream vicinity of the
affected site (Larsen et al., 2009). Habitat assessment results here did
not highlight any improvement in bank stability following exclusion of
cattle access, although increased ground cover in riparian areas was
observed. Reductions in the extent of bare ground following cattle
exclusion via fencing were also observed in Alberta, Canada by (Miller
et al., 2010a) and in the north of Ireland by Rice et al. (2021) and
were linked to improved water quality (Miller et al., 2010a). These
authors also found no improvement in stream bank stability following
four years of cattle exclusion but did suggest that increased ground
cover could also contribute to reduced rates of stream bank erosion.
Collins et al. (2010) concluded that stream bank erosion was
significantly reduced in all of six catchments studied in the UK
following fencing and eight years of exclusion, leading to reduced
siltation of salmonid spawning gravels. Hansen and Budy (2011) however
did observe significant improvement in stream bank stability after only
one to two years of exclusion of cattle via fencing in Utah. The
time-frame of the current study may have been too short to allow for
improvements in bank stability following fencing.
Elevated levels of deposited sediment have been shown to drive changes
in macroinvertebrate communities affected by cattle access (Braccia and
Voshell, 2006; Conroy et al. 2018). The results here have highlighted
the elimination of pre-fencing disparities in masses of deposited
sediment between control and pressure points as a result of one year of
exclusion via fencing. Additionally, reductions in multivariate
dispersions (macroinvertebrate data) between control and pressure
samples following fencing have been highlighted at several sites.
Warwick and Clarke (1993) found that multivariate dispersion among
marine community samples collected from impacted areas may be greater
than from un-impacted areas, thus, here, higher dispersion among
(control and pressure) samples prior to fencing compared to post-fencing
may indicate reduced community stress following restoration via fencing.
Other authors (see White et al., 2017) have shown increases in the
magnitude of multivariate dispersion following restoration of streams
and instream habitats due to improved habitat heterogeneity. It should
be noted however that multivariate dispersions in the current study were
measured across the gradient of impact (i.e. upstream and downstream of
cattle access points) and thus reductions in dispersion here represent a
convergence of communities towards control conditions following
the removal of the cattle access impact. Results for EPT richness and
total richness also suggested that improvements in ecological quality
had occurred following one year of cattle exclusion at certain sites.