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Providing evidence to improve practice

Action: Install and maintain gates at mine entrances to restrict public access Bat Conservation

Key messages

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  • Nine studies evaluated the effects of installing gates at mine entrances on bat populations. Eight studies were in the USA and one in Australia.

COMMUNITY RESPONSE (1 STUDY)

POPULATION RESPONSE (3 STUDIES)

BEHAVIOUR (6 STUDIES)     

OTHER (2 STUDIES)

Supporting evidence from individual studies

1 

A before-and-after study in 1991–2004 at 47 gated abandoned mines in forested areas of Colorado, USA (Navo & Krabacher 2005) found that 43 of 47 mines with gates of various designs continued to be used by eight bat species up to 12 years after installation. None of 43 mines with full gates with or without culverts were abandoned by bats. Three mines with ladder gates and one mine with a culvert ladder gate were abandoned by bats. Four types of gate were evaluated, all with bar spacings of 150 mm. Traditional gates allowed access to bats across the whole gate, ladder gates allowed access to bats at the centre only, and both types of gate were also constructed in metal culverts where mine entrances were too unstable to anchor the gate itself. Each of 47 mines were surveyed 2–10 times in 1991–2004 using multiple methods (catching, visual counts and infra-red motion detectors).

2 

A replicated, controlled, before-and-after and site comparison study in 2003 at 28 mine and cave sites between Ontario, Canada and Tennessee, USA (Spanjer 2006) found that at mine and cave entrances with gates, bats circled, retreated more and passed through less often than at ungated entrances.  Bats circled and retreated significantly more and passed through less at entrances with existing mine or cave gates (37% of bats circled and retreated, 50% passed through) or newly installed mock gates (60% circled and retreated, 25% passed through) than at ungated entrances (23% circled and retreated, 68% passed through). Separate results for mines and caves are not provided. Seven mines or caves had existing gates (of various designs), twelve mines or caves were ungated and had mock wooden gates installed (horizontal bars 25 mm diameter with 146 mm spacing). Ungated entrances were surveyed before and after mock gates were installed. At each of 28 sites, observations of behaviour were made during 3–4 x 5 minute periods during 1–2 nights in July–October 2003.

3 

A replicated, site comparison study in 2002 of 24 gated and 23 ungated abandoned mines in West Virginia, USA (Johnson et al. 2006) found that mines with gates had fewer bats captured of nine species than ungated mines, but other mine features were more important than gates for predicting bat presence. The number of bats captured was lower for nine bat species at mine entrances with gates than at mine entrances without gates (data reported as statistical model results). However, mine entrance size, shape and distance to other entrances were more important than gates for predicting the presence of bats (see original paper for detailed results). Twenty-four mine entrances were gated (one had a ‘bat-friendly’ angle-iron design, 23 had a round-bar design with 1.5 cm bars spaced 500 cm horizontally and 200 cm vertically). Twenty-three mine entrances had no gates installed. Bats were captured with harp traps and/or mist nets for one night at 36 of 47 mines in June–July 2002 and at all 47 mines in August–September 2002.

4 

A replicated, controlled, before-and-after study in 2003 at four derelict mines in a forested area of south-eastern Australia (Slade & Law 2008) found that installing gates with 125 mm horizontal spacing resulted in fewer eastern horseshoe bats Rhinolophus megaphyllus and Schreiber’s bats Miniopterus schreibersii using the mines and more bats aborted exit and entry flights, whereas gates with horizontal spacings of 450 mm and 300 mm did not affect bat numbers or behaviour. Fewer bats used two mines after gates with a 125 mm horizontal spacing were installed (before: 120 and 540 bats; after: 30 and 290 bats). The number of bats aborting exit and entry flights also significantly increased (data reported as standardized results). Gates with horizontal spacings of 450 mm and 300 mm did not significantly affect bat numbers or behaviour. Bat numbers at two similar control mines either remained constant or increased. Two mines were fitted with gates (made from 20 mm plastic tubing), and two were left ungated (controls). In March–April 2003, bat activity at the two experimental mines was observed in four stages of 11 days each: before gating followed by the successive addition of horizontal gate bars to reduce the spacing size (to 400, 300 and 125 mm). Bats were logged automatically using infra-red beams, and night-vision video cameras recorded flight behaviour for 30 minutes at dusk and dawn.

5 

A replicated, before-and-after study in 2002–2004 at five pairs of abandoned mines in northern Idaho, USA (Derusseau & Huntly 2012) found that installing gates resulted in fewer bats and fewer bat species entering the mines. Significantly fewer bats entered mines after gates were installed with an overall decrease of 65% across all gated mines (before: average 29 bat entries; after: 10 bat entries). The number of bats entering five ungated mines increased by 45% over the same period (‘before’: 20 bat entries; ‘after’ 32 bat entries). Significantly fewer bat species entered the mines after gates were installed (before: average 2.3 bat species; after: 1 bat species), but no change was observed at ungated mines (‘before’: 2 bat species; ‘after’: 1.8 bat species). Gates were installed at five of 10 mines in 2002 and 2003. Gates had vertical supports (10 x 10 x 1 cm iron) and horizontal bars (10 x 10 cm angle iron) with gaps of <14.6 cm. Each of five pairs of mines was surveyed twice in July–August in two consecutive years in 2002–2004 (before and after gating). One mist net survey and one video survey were carried out at the mine entrance of each site/year.

6 

A replicated, controlled, before-and-after study in 2003–2004 at four abandoned mines in western Utah, USA (Diamond & Diamond 2014) found that gated mines had more Townsend’s big-eared bats Corynorhinus townsendii circling at entrances than entering or exiting them, and 2–7% of bats flying through the entrances collided with the gates. More Townsend’s big-eared bats circled at gated mine entrances than flew through them (data not reported). However, there was no difference in the number of bats circling and entering/exiting at ungated mines. Bats were observed colliding with gates at all four gated mines (2–7 % of bats entering or exiting/night, total <5–50 bats/gate). All of four mines had maternity colonies of Townsend’s big-eared bats (average 84–112 bats). Two mines were gated before the study in 1998 and 2000 and two had gates installed during the study in 2004. All gate designs were ‘bat-compatible’ (round steel bars with horizontal bars spaced 10–14 cm apart). Each of the four mines was surveyed with infrared video cameras at the entrances during two consecutive mornings and a single night each month in May–July 2003 (before gating) and in May and July–September 2004 (after gating).

7 

A replicated, controlled, before-and-after study in 2014–2015 at 11 abandoned mines in southern Arizona and New Mexico, USA (Tobin et al. 2018) found that after gates were installed bat activity levels remained stable or increased at five of seven gated mines and three of four ungated control mines. After gating, bat activity levels decreased at two of seven gated mines and one of four ungated control mines (data reported as bat logger voltage measures). Seven bat species were recorded within the mines (data not reported for individual species). Eleven mines (4–200 m long) with similar characteristics (bat use, mine features, number of entrances) were surveyed. Seven mines had gates (standard square-tube bar gates or corrugated metal culverts with rectangle-tube bar gates, both with 14.6 cm horizontal spacing) installed in winter 2014 or spring 2015. Four control mines were left ungated. Visual observations and bat logger surveys were carried out in June–September 2014 (before gating) and 2015 (after gating).

8 

A replicated, before-and-after study in 2015 at two abandoned mines in Arizona, USA (Tobin et al. 2018) found that bats performed more flight manoeuvres at mine entrances after mock gates were installed than before, but gate material and height had no effect on bat behaviour. Bats performed more energetically demanding flight manoeuvres at mine entrances after mock gates were installed (data not reported). There was no significant difference in bat behaviour between two types of gate material (corrugated metal and non-corrugated high-density polyethylene) or two gate heights (0.15 m and 1.15 m above the ground). Both mines (60–80 m long) had single ungated entrances and were occupied by winter colonies (>100 individuals) of California leaf-nosed bats Macrotus californicus. Round bar gates (14.6 cm horizontal bar spacing) were installed within culverts (76 cm diameter, 1.2 m length) at each of two mine entrances. In March–April 2015, bats were filmed with infrared cameras for three nights before gates were installed, followed by three nights with one randomly chosen gate material/height installed and three nights with the other.

9 

A replicated study in 2015 at 41 abandoned gated mines in Arizona, Colorado, Nevada, New Mexico and Utah, USA (Tobin et al. 2018) found that gate age and design had varied effects on the presence of four bat species, but mine features were more important than gates for predicting presence. Townsend’s big-eared bats Corynorhinus townsendii were found more often in mines with narrower horizontal bar spacing (12–15 cm) than wider spacing (18 cm; data reported as statistical model results). California myotis Myotis californicus and western small-footed myotis Myotis ciliolabrum were found more often in mines with older gates (>10 years old) and less often in mines with angle-iron bar gates than mines with four other gate designs. Cave myotis Myotis velifer were found more often in mines with newer gates (<9 years old) and less often in mines with culvert gates than mines with four other gate designs. Fringed myotis Myotis thysanodes were found more often in mines with gates closer to the entrance (<2 m) with smaller gate areas (<2.5 m2) and wider vertical bar spacing (>0.9 m). Mine features (e.g. elevation, number of levels or entrances) were more important than gate age, location or design for predicting the presence of all four bat species. Each of 41 mines had one of five gate designs installed: standard round bar (8 mines); standard angle-iron bar (15 mines); standard square-tube bar (7 mines); corrugated metal culvert with square-tube bar (7 mines); ladder gate (4 mines). Fresh guano samples were collected from the mines in June–December 2015 for DNA analysis, and mine features were recorded.

Referenced papers

Please cite as:

Berthinussen, A., Richardson O.C. and Altringham J.D. (2019) Bat Conservation. Pages 67-140 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.