Cattle exclusion had a significant impact on grazing ephemeropteran, plecopteran and trichopteran taxa
SIMPER results highlighted consistency in the taxa that contributed to dissimilarities between the communities in the pre and post fencing periods. The grazing EPT taxa, R. semicolorata, B. rhodani, Silo pallipes , Agapetus sp. all contributed significantly to these dissimilarities and these taxa generally showed consistent responses (except B. rhodani ). During the post-fencing sampling season,R. semicolorata , S. pallipes and Agapetus sp. increased in abundances at both control and pressure points (across all sites) compared to the pre-fencing period. Pre-fencing, these taxa occurred in greater abundances at control than pressure points. Interestingly however, this pattern was reversed in the post-fencing period with abundances at pressure points increasing beyond those seen at control points. Such a response may indicate that affected streams are in a state of intermediate disturbance (Connell, 1978), with community niches developing due to an abundance of food resources (algae) for grazing taxa following a reduction of the main community stressor, sediment. Cattle access points have been shown to negatively affect grazers and EPT fauna such as Agapetus sp. and R. semicolorata with sediment highlighted as the most likely stressor (O’Sullivan unpublished). Similar impacts have also been highlighted by Braccia and Voshell (2006) and Burdon et al. (2013). Elevated deposited sediment can affect periphytic algal biomass and reduce its palatability (Wood and Armitage, 1997; Schofield et al., 2004; Jones et al., 2012). Higher fine sediment deposits downstream of cattle access points have been detected in first and second order streams in the Irish setting (O’Sullivan et al., 2019a) and were demonstrated here in the pre-fencing sampling period. Such deposits however are transient in nature and can be removed by high flows (Gomi et al., 2005). Nonetheless, macroinvertebrate community composition reflects prevailing stream conditions (Metcalfe, 1989) and sediment inputs from recurrent entry of cattle to streams may represent a continuous stressor for these communities.
Recovery time in macroinvertebrate communities from stressor effects is variable and dependent on the specific stressor or stressors shaping affected communities and the aspects of the communities assessed (Laasonen et al., 1998; Muotka et al., 2002). Several authors have highlighted similarly rapid responses in macroinvertebrate communities to restoration efforts although not specifically relating to the exclusion of cattle via fencing. Miller et al. (2010b) concluded from a meta-analysis of a range of restoration projects that improved macroinvertebrate richness can occur in communities within one year of the commencement of restorative works. Contrastingly, Laasonen et al. (1998) demonstrated a delayed recovery of shredder macroinvertebrates in streams channelized for logging activities, relating the delay to poor retention of coarse particulate organic matter (CPOM) in such streams. In relation to cattle exclusion studies, recovery times are equally variable with this variability reflecting, again, the main stressor influencing communities and the scale of exclusionary measures. Herbst et al. (2012) reported a relatively rapid recovery of macroinvertebrate communities (in streams where bank erosion and sediment deposition were significant drivers) following fencing (4 years), however with active restoration (involving stream channel adjustment), even quicker responses (2 years) were observed (Herbst and Kane, 2009). Where significant depletion of riparian vegetation occurs however, greater recovery times can be expected, owing to the substantial time required for woody vegetation to re-establish and grow (Belsky et al., 1999; Braccia and Voshell, 2007; Ranganath et al., 2009; Herbst et al., 2012).
Rapid responses of macroinvertebrates to restorative efforts are indicative of communities that possess good elasticity (Halpern, 1988). Greathouse et al. (2005) state that, recovery is driven by nearby sources of macroinvertebrates for colonisation and the mobility of dominant taxa. This point is reiterated by Díaz Villanueva et al. (2017) who state that the distribution of invertebrates in disturbed environments is shaped by the dispersal capabilities of the resident taxa and the changes in environmental conditions along a spatial gradient. Wallace (1990) cites the importance of drifting taxa and aerially dispersing taxa (particularly in headwater streams) for ecological recovery. Here eight out of the twelve taxa listed in Table 1 have an adult life stage that disperses aerially. Also, the downstream extent of impacts of cattle access points in headwater streams is limited (O’Sullivan et al., 2019a), thus the rapid restoration of communities observed here is likely aided by both drifting taxa from upstream control sites and from aerial dispersal of adults from un-impacted downstream sites.