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.