Fencing over the long-term results in improvements in
macroinvertebrate communities
In the present study, significant improvements in stream
macroinvertebrate community structure were seen in response to the
long-term fencing of a sub-catchment of the Milltown Lake in Co.
Monaghan. This was attributed to increases in the abundances of taxa
such as A. fluviatilis and Glossomatidae (taxa that
responded positively to fencing in the short term) and Elmidae (a taxon
that is shown to be sensitive to cattle access (Braccia and Voshell,
2007) and was more abundant downstream of access points following short
term fencing) in the post fencing sampling period of the fenced
catchment. These results are more emphatic given the apparent decrease
in ecological water quality in the control stream, where cattle access
was still permitted over the nine-year period. The lower ecological
quality scores in the post-fencing period were driven by increases in
taxa such as Simuliidae and Gammaridae and reductions in Baetidae,Hydropsyche siltalai and Elmidae compared to the pre-fencing
period. Despite Simuliidae and Gammaridae being highly sensitive and
moderately sensitive to sediment respectively, they are also moderately
tolerant of organic pollution and increases in their abundances may
reflect such pollution (Herbst et al., 2012; Burdon et al., 2013).
Furthermore, previous work by O’Sullivan et al. (2019a) demonstrated
that deposited sediment levels were unaffected by cattle access at sites
located in the control catchment, while a detailed characterisation of
the catchment by Wynne and Linnane (2008) concluded that bare, coarse
substrates were dominant in the catchment. Also work by Bragina et al.
(2017) in 2012-2013 found significantly higher Escherichia colilevels in the stream sediments for the unfenced (control) tributary (TH)
compared to the fenced tributary (TV), suggesting higher loading with
organic matter from animal and/or human wastes.
The results from the univariate analyses also highlighted ecological
improvements in the fenced catchment and deteriorations in the control
catchment following the fencing period. The scope for inferences in
relation to the impact of fencing however, are limited due to the lack
of temporal replication over the course of the nine years between
fencing and the post-fencing sampling period.
Limitations in the study design are acknowledged by the authors,
particularly in relation to the failure of the analyses to account for
fluctuations in environmental conditions (e.g. sediment) and resultant
macroinvertebrate responses over time. Replication of sampling in
relation to the Fencing factor (i.e. replicate sampling of
sediment and macroinvertebrate populations at multiple times prior to
fencing and after fencing, both at control and pressure points) would
provide a more robust basis on which conclusions could be drawn
(Stewart-Oaten et al., 1986; Miller et al., 2010b). Macroinvertebrate
populations, however, respond to the prevailing environmental conditions
and as such changes in macroinvertebrate metrics here are considered to
reflect changes in sediment deposition in the longer term.