Bat boxes are not a silver bullet conservation tool

Creating wildlife habitat using artificial structures: a review of their efficacy and potential use in solar farms

The biodiversity crisis is exacerbated by a growing human population modifying nearly three-quarters of the Earth's land surface area for anthropogenic uses. Habitat loss and modification represent the largest threat to biodiversity and finding ways to offset species decline has been a significant undertaking for conservation. Landscape planning and conservation strategies can enhance habitat suitability for biodiversity in human-modified landscapes. Artificial habitat structures such as artificial reefs, nest boxes, chainsaw hollows, artificial burrows, and artificial hibernacula have all been successfully implemented to improve species survival in human-modified and fragmented landscapes. As the global shift towards renewable energy sources continues to rise, the development of photovoltaic systems is growing exponentially. Large-scale renewable projects, such as photovoltaic solar farms have large space requirements and thus have the potential to displace local wildlife. We discuss the feasibility of 'conservoltaic systems' - photovoltaic systems that incorporate elements tailored specifically to enhance wildlife habitat suitability and species conservation. Artificial habitat structures can potentially lessen the impacts of industrial development (e.g., photovoltaic solar farms) through strategic landscape planning and an understanding of local biodiversity requirements to facilitate recolonization.

Improving the science and practice of using artificial roosts for bats

Worldwide, artificial bat roosts (e.g., bat boxes, bark mimics, bat condos) are routinely deployed for conservation, mitigation, and community engagement. However, scant attention has been paid to developing best practices for the use of artificial roosts as conservation tools. Although bats readily occupy artificial roosts, occupancy and abundance data are misleading indicators of habitat quality. Lacking information on bat behavior, health, and fitness in artificial roosts, their conservation efficacy cannot be adequately validated. We considered the proximal and ultimate factors, such as evolutionarily reliable cues, that may prompt bats to preferentially use and show fidelity to suboptimal artificial roosts even when high-quality alternatives are available. Possible negative health and fitness consequences for artificial roost inhabitants include exposure to unstable and extreme microclimates in poorly designed roosts, and vulnerability to larger numbers of ectoparasites in longer lasting artificial roosts that house larger bat colonies than in natural roosts. Bats using artificial roosts may have lower survival rates if predators have easy access to roosts placed in conspicuous locations. Bats may be lured into occupying low-quality habitats if attractive artificial roosts are deployed on polluted urban and agricultural landscapes. To advance the science behind artificial bat roosts, we present testable research hypotheses and suggestions to improve the quality of artificial roosts for bats and decrease risks to occupants. Because continued loss of natural roosts may increase reliance on alternatives, such as artificial roosts, it is imperative that this conservation practice be improved.

Size at Birth, Postnatal Growth, and Reproductive Timing in an Australian Microbat

Reproductive phenology, size at birth, and postnatal growth are important life history traits that reflect parental investment. The ability to document detailed changes in these traits can be a valuable tool in the identification and management of at-risk wildlife populations. We examined reproductive traits in a common, widespread Australian microbat, Chalinolobus gouldii, at two sites over two years and derived growth curves and age estimation equations which will be useful in the study of how intrinsic and extrinsic factors alter parental investment strategies. We found that male and female offspring did not differ significantly in their size at birth or their postnatal growth rates. Bats born in 2018 were smaller at birth but grew at a faster rate than those born in 2017. When date of birth was compared across sites and years, we found bats born in 2018 had a later median birthdate (by 18 days) and births were more widespread than those born in 2017. Cooler and wetter weather during late gestation (Nov) in 2018 may have prolonged gestation and delayed births. With many bats facing threatening processes it is important to study reproductive plasticity in common and widespread “model” species, which may assist in the conservation and management of threatened microbats with similar reproductive traits.

The common noctule bat (Nyctalus noctula): population trends from artificial roosts and the effect of biotic and abiotic parameters on the probability of occupation

As urbanization and intense forestry management are increasing, natural bat roosting opportunities, e.g. tree cavities, are constantly declining. A common management practice, especially in urban environments, is the installation of bat boxes. We assessed (1) abundance trends inside artificial bat boxes of the noctule bat (Nyctalus noctula) over 11 years in Neu-Ulm, Germany, and identified (2) parameters affecting the occupation rate. Further, we compared (3) abundance of noctule bats in bat boxes between areas with large availability of natural roosts (forests) and urban areas with primarily artificial roosting structures (parks). Our results revealed a severe decline of noctule bats over 11 years. Further, our results indicate that bat boxes cannot fully replace natural tree cavities. Nonetheless, they support roost availability in areas already altered and providing only low amounts of natural bat roosts. The findings of our study have important implications for the conservation of bats in urbanized landscapes and underline the importance of old and intact forests for local biodiversity.

Artificial refuges for wildlife conservation: what is the state of the science?

Artificial refuges are human-made structures that aim to create safe places for animals to breed, hibernate, or take shelter in lieu of natural refuges. Artificial refuges are used across the globe to mitigate the impacts of a variety of threats on wildlife, such as habitat loss and degradation. However, there is little understanding of the science underpinning artificial refuges, and what comprises best practice for artificial refuge design and implementation for wildlife conservation. We address this gap by undertaking a systematic review of the current state of artificial refuge research for the conservation of wildlife. We identified 224 studies of artificial refuges being implemented in the field to conserve wildlife species. The current literature on artificial refuges is dominated by studies of arboreal species, primarily birds and bats. Threatening processes addressed by artificial refuges were biological resource use (26%), invasive or problematic species (20%), and agriculture (15%), yet few studies examined artificial refuges specifically for threatened (Vulnerable, Endangered, or Critically Endangered) species (7%). Studies often reported the characteristics of artificial refuges (i.e. refuge size, construction materials; 87%) and surrounding vegetation (35%), but fewer studies measured the thermal properties of artificial refuges (18%), predator activity (17%), or food availability (3%). Almost all studies measured occupancy of the artificial refuges by target species (98%), and over half measured breeding activity (54%), whereas fewer included more detailed measures of fitness, such as breeding productivity (34%) or animal body condition (4%). Evaluating the benefits and impacts of artificial refuges requires sound experimental design, but only 39% of studies compared artificial refuges to experimental controls, and only 10% of studies used a before-after-control-impact (BACI) design. As a consequence, few studies of artificial refuges can determine their overall effect on individuals or populations. We outline a series of key steps in the design, implementation, and monitoring of artificial refuges that are required to avoid perverse outcomes and maximise the chances of achieving conservation objectives. This review highlights a clear need for increased rigour in studies of artificial refuges if they are to play an important role in wildlife conservation.

Using mounting, orientation, and design to improve bat box thermodynamics in a northern temperate environment

Wildlife managers design artificial structures, such as bird houses and bat boxes, to provide alternative nesting and roosting sites that aid wildlife conservation. However, artificial structures for wildlife may not be equally efficient at all sites due to varying climate or habitat characteristics influencing thermal properties. For example, bat boxes are a popular measure employed to provide compensatory or supplementary roost sites for bats and educate the public. Yet, bat boxes are often thermally unstable or too cold to fulfill reproductive females needs in northern temperate environments. To help improve the thermodynamics of bat boxes, we tested the effect of (1) three mountings, (2) four orientations, and (3) twelve bat box designs on the internal temperature of bat boxes. We recorded temperatures in bat boxes across a climate gradient at seven sites in Quebec, Canada. Bat boxes mounted on buildings had warmer microclimates at night than those on poles and those facing east warmed sooner in the morning than those facing west or south. Our best new model based on passive solar architecture (Ncube PH1) increased the time in the optimal temperature range (22–40 °C) of targeted species by up to 13% compared to the most commonly used model (Classic 4-chamber) when mounted on a building with an east orientation (other designs presented in the Supplementary Information). Based on bioenergetic models, we estimated that bats saved up to 8% of their daily energy using the Ncube PH1 compared to the Classic 4-chamber when mounted on a building with an east orientation. We demonstrate that the use of energy-saving concepts from architecture can improve the thermal performance of bat boxes and potentially other wildlife structures as well.

Validation of quantitative magnetic resonance as a non-invasive measure of body composition in an Australian microbat

Body composition (the total amount of fat mass, lean mass, minerals and water that constitute the body) is an important measure for understanding an animal’s physiology, ecology and behaviour. Traditional measures of body composition require the animal to either be placed under anaesthetic, which is invasive and can be high-risk, or be euthanised, preventing the ability to perform repeated measures on the same individual. We aimed to validate quantitative magnetic resonance (QMR) as a non-invasive measure of body composition by comparing QMR scans with chemical carcass analysis (CCA) in Gould’s wattled bats (Chalinolobus gouldii). In addition, we compared a commonly used microbat body condition index (residuals of mass by forearm length) to CCA. We found that QMR is an accurate method of estimating body condition in Gould’s wattled bats after calibration with regression equations, and the condition index could accurately predict lean and water mass but was a poor predictor of fat mass. Using accurate, non-invasive, repeatable measures of body condition may have important implications for ecological research in the face of changing environments.

Nest boxes do not cause a shift in bat community composition in an urbanised landscape

Nest boxes are often used to provide supplementary roosts for cavity-dependent wildlife, but little is known about if they influence faunal community composition. Long-term monitoring of bat boxes in south-eastern Australia indicated that their use was dominated by one generalist species (Chalinolobus gouldii), causing concern that installing bat boxes could cause a shift toward less diverse bat communities. To test this, we conducted a large-scale before-after control-impact experiment at 18 sites, over five years. Sites were either: (1) those with existing bat boxes, (2) those where boxes were added during the study, or (3) controls without boxes. We used echolocation call data from 9035 bat detector nights to compare community composition, diversity, and species' relative activity between the sites. Chalinolobus gouldii continued to dominate the use of existing boxes, but we found little difference in community composition between sites based on the presence, absence, or addition of boxes. Our study is the first to explore the influence installing artificial hollows has on localized faunal assemblages over spatio-temporal scales relevant to management. We conclude that there is cause for optimism that bat boxes might not have perverse outcomes on local community composition in the short- to medium-term, as we had feared.

Long-term monitoring suggests bat boxes may alter local bat community structure

Bat boxes are often used to provide supplementary roosting habitats; however, little is known of their impacts on community composition. Data collected from a 25-year box-monitoring and 31-year harp trapping case study provides preliminary evidence that the installation of boxes may have contributed to one species, Gould’s wattled bat (Chalinolobus gouldii), dominating the bat community of a periurban park in Melbourne. This highlights the need for systematic monitoring and empirical assessment of conservation-focused bat box programs.

The influence of meal size on the digestive energetics of Gould’s wattled bat, Chalinolobus gouldii

Although variation in meal size is known to have an impact on digestive energetics, there is limited information on how it influences metabolic rate and energy assimilation in insectivorous bats. We investigated the influence of meal size, representing 10% or 20% of an individual’s weight, on the digestive energetics of Gould’s wattled bat, Chalinolobus gouldii (n = 61 bats). Using open-flow respirometry, we recorded a median resting metabolic rate of 2.0 mL g–1 h–1 (n = 51, range = 0.4–4.8) at an air temperature of 32°C. Median postprandial metabolic rate peaked at 6.5 (range = 3.4–11.6, n = 4) and 8.2 (range = 3.8–10.6, n = 7), representing 3.3- and 4.1-fold increases from resting metabolic rate for the two meal sizes. Using bomb calorimetry, we calculated the calorific value of the two meal sizes, and the calories lost during digestion. Following gut passage times of 120 min (range = 103–172, n = 15) and 124 min (range = 106–147, n = 12), C. gouldii assimilated 88.0% (range = 84.6–93.8, n = 5) and 93.3% (range = 84.0–99.4, n = 10) of the kilojoules available from the 10% and 20% meal sizes, respectively. When fed ad libitum, C. gouldii consumed a mean of 23.2% of their body weight during a single sitting (n = 18, range = 6.3–34.1%). Overall, digestive energetics were not significantly different between 10% or 20% meal sizes. The ability to ingest small and large meals, without compromising the rate or efficiency of calorie intake, indicates that free-ranging C. gouldii are likely limited by food available in the environment, rather than the ability to assimilate energy.

Flexible roost selection by Gould’s wattled bats (Chalinolobus gouldii) using bat boxes in an urban landscape

Bat boxes are often used as a conservation tool in human-disturbed landscapes across Australia; however, to assess their effectiveness we need to understand the factors influencing their occupancy by insectivorous bats. We investigated roost selection by Gould’s wattled bat (Chalinolobus gouldii) using 76 bat boxes, comprising six designs, across three sites in suburban Melbourne, Australia. We conducted monthly surveys for a year and recorded the physical characteristics of each box. Five species of bats were recorded but Gould’s wattled bats dominated box occupancy year-round at all three sites. Group sizes ranged from 1 to 58 individuals, with maternity colonies forming over summer. There was little consistency in the use of selection criteria by Gould’s wattled bats when choosing a bat box as a day roost, with considerable variability across sites and seasons, highlighting the flexibility in roost site selection by this widespread, adaptable species. Our findings show that bat boxes can be an effective tool for providing supplementary roosts for Gould’s wattled bats in urbanised landscapes. However, little is known about the impact on the whole bat community, especially disturbance-sensitive taxa, of artificially increasing roosting resources for common species.

Roost selection in concrete culverts by the large-footed myotis (Myotis macropus) is limited by the availability of microhabitat

The large-footed myotis (Myotis macropus) is a specialist trawling bat with flexible roosting behaviour, being able to switch between caves, tree hollows and artificial roosts such as bridges, tunnels and culverts. However, little is known about how this species selects culvert roost sites in urban landscapes where hollows may be limited or absent. We surveyed 57 concrete culverts and found 21 M. macropus roosts comprising day and maternity roost sites; 305 bats were captured. Colony sizes averaged 20.6 ± 17.7 (range = 4–49) for maternity roosts and 2.0 ± 0.8 (range = 1–3) for day roosts. Roost culverts differed significantly from available culverts predominantly in terms of availability of microhabitat (lift holes and crevices). Roost culverts had lift holes that had greater cavity dimensions than available culverts and crevices were found only at roost culverts. Culverts containing microhabitat were a limited resource in this urban landscape and so increasing their availability may provide more urban roost sites for this specialist species.

Physical and microclimate characteristics of Nyctophilus gouldi and Vespadelus vulturnus maternity-roost cavities

Context Tree cavities suitable to rear young are a key resource in managed landscapes to support viable populations of tree cavity-roosting bats. Little is known about the selection of cavities for maternity roosts, presumably because of the difficulty in accessing such roosts. Aims Our study investigated physical and microclimate characteristics of maternity roosts of two species, namely, Gould’s long-eared bat (Nyctophilus gouldi) and little forest bat (Vespadelus vulturnus). Methods Maternity-roost cavities were identified in a timber-production landscape in south-eastern Australia. Roost trees (V. vulturnus n=5; N. gouldi n=9) and a subsample of available cavity-bearing trees (n=16) were climbed to obtain cavity characteristics. Key results Vespadelus vulturnus used tree hollows exclusively, whereas N. gouldi used both tree hollows (n=7) and thick loose bark (n=2). No significant difference in roost-cavity characteristics was detected between the species. However, V. vulturnus selected significantly narrower cavity entrances (mean: 16±3mm) than those of the available cavities. Temperature did not differ between maternity roosts and available cavities when investigated after the maternity season. However, a V. vulturnus maternity roost occupied for 33 consecutive days was warmer than mean roost and available cavities, suggesting that long-term roost use may be influenced by the thermal property of a cavity. Conclusions Our study has provided the first detailed tree-cavity description of maternity roosts of N. gouldi and V. vulturnus. The nightly roost switching and the large variation of tree-cavity characteristics used by N. gouldi suggest that this species requires a high density of non-specific tree cavities that are large enough for colony formation, whereas preferred roost cavities for V. vulturnus are likely to be hollows comprising narrow entrances that facilitate long-term use. Implications Our results highlighted the likely importance of narrow roost entrances for V. vulturnus, presumably for predator protection, and the conservation of tree cavities large enough for colonies to congregate. The formation of such hollows is likely to take many decades. Poor silvicultural practices, land clearing and inappropriate management of veteran trees have the potential to have an impact on this hollow resource. Our study also highlighted the need to further assess the importance of cavity microclimate for maternity-roost selection in warm climates.