Análisis 5

1.8.1 Study design

From the studies discussed here we can see that in many cases there are strong effects of habitat conversion and fragmentation on bats. Yet it is still hard to state conclusively what effect habitat fragmentation has on bats, as many studies show conflicting

results. This is part due to the varied interpretations of the term ‘fragmentation’, and the magnitude and nature of the fragmentation studied. Future studies could compare landscapes with different degrees of fragmentation in the same geographical region, to try and see how fragmented a habitat can get and still maintain diversity, and which landscape variables are important.

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Almost all the work that has been done on the responses of bats to tropical habitat fragmentation has been done in the neotropics. This creates a huge bias in our understanding; more palaeotropical research is urgently needed to identify species and areas at risk, and to develop evidence based strategies for their conservation, especially as bat communities in the neotropics and palaeotropics are not

ecomorphologically analogous (Heller and Volleth 1995).

Many studies were able to analyse how different species respond to changes in landscape factors, but were unable to do this with rarer species due to low sample sizes. These are the species of greatest conservation concern; perhaps with sustained use of a variety of catching and ultrasound detection methods, we would be able to discover more about what affects rare and threatened species.

1.8.2 Catching and acoustic studies

There are currently no long-term monitoring programmes for bats in the tropics, and that have not been included in established multi-taxa monitoring programmes such as Conservation International’s Tropical Ecology, Assessment and Monitoring (TEAM) network (http://www.teamnetwork.org). If monitoring protocols are to be established it is important to understand the biases inherent in different sampling methods and how they affect bats in different habitats.

Most of the tropical studies discussed used only ground level mist nets to survey for bats. This completely excludes higher flying bats from analysis, and species that can more easily detect and/or avoid mist nets. Using nets at greater heights and

introducing methods such as harp traps and tunnel traps, shown to catch some species that avoid mist nets, will give us a truer picture of bat assemblages – as will the use of ultrasound detectors (Sedlock et al. 2008). Also, in some studies species accumulation curve didn’t reach asymptotes, indicating that even with the limited methods used more species could have been caught with more replication; this again makes interpretation of data, particularly on rare species, difficult.

This is important because for many bat assemblages we lack even the most basic information on the abundance of different species, their distribution or habitat requirements. The development of comprehensive survey and monitoring methods is critical for understanding the current status of bats and allowing future monitoring of

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populations (MacSwiney et al. 2008). Ideally the relative roles of different capture and acoustic methods need to be tested in a range of different habitats and different environmental conditions, as different methods may be more or less effective under different conditions (MacSwiney et al. 2008).

All methods for studying bats have potential advantages and disadvantages. Catching bats often allows better species level identification than acoustic methods (although some cryptic species can only be separated by calls), and allows the collection of biometric data, data on sex, reproductive status and the collection of genetic material. However it is also expensive, time consuming, requires skilled labour and disturbs bats, possibly altering behaviour. It can also lead to biases in sampling as some species fly high above nets, are more agile or are better at detecting nets than other species. Harp trapping is less disturbing to bats, but traps are less versatile than nets. Capture success is also likely to decrease on consecutive nights at the same location as bats learn where the nets are and avoid them(Kunz and Brock 1975). Habitats such as open fields, large water bodies or high in the canopy cannot be easily or effectively sampled using mist nets or traps (Kunz and Brock 1975). Nets and traps also sample a very small percentage of the aerospace used by bats, and alert, foraging bats are often able to detect and avoid structures intended to capture them (O’Farrell and Gannon 1999). These drawbacks have in recent years been compensated for in temperate zones by the use of ultrasound detectors, which detect the high frequency echolocation calls made by bats. Over time, these are often cheaper than catching methods and they are non-invasive. They can help achieve a more complete species list for the area – after decades of intense catching on Barro Colorado Island, Panama, five new species were added to the inventory by using acoustic methods – all species that roost in difficult to reach areas and fly especially high (Kalko et al. 1996). In another study in the

neotropics, aerial insectivores that often fly above the forest canopy were only ever recorded using acoustic methods (MacSwiney et al. 2008). They may also give a better idea of bat activity than mist nets do; in one study bat activity data from ultrasound sampling were correlated with insect abundance, and capture data were not

(MacSwiney et al. 2008). Ultrasound detectors can sample areas difficult to sample with mist nets such as open areas where bats fly high.

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However they are not without issues; some bat species cannot be separated using echolocation calls alone, and again there can be biases in the sample; low intensity echolocators and non-echolocating bats are under sampled or not sampled at all (O’Farrell and Gannon 1999). Sites with dense vegetation may muffle ultrasound calls, high levels of insect noise may mask calls (MacSwiney et al. 2008). Still, in most bat assemblages studied, the use of ultrasound detectors and catching together gave the greatest number of species recorded (Murray et al. 1999, O’Farrell and Gannon 1999, MacSwiney et al. 2008, Furey et al. 2009).