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(O’Sullivan unpublished).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.