Discussion

2.5.1. Methods used to measure bat diets

The sensitivity of molecular methods compared to other methods (figure 2.1) strongly supports the use of them for measuring bat diets.

Furthermore, as databases of sequence data are developed and

expanded, molecular methods would be expected to become increasingly sensitive.

2.5.2. Guild structure and niche partitioning

Where diets overlap, this suggests that there is potential for dietary competition for the species for prey, where the ranges also overlap. However, as there are around 3,000 lepidopteran species in Great Britain (Bradley and Bradley, 2000), it is necessary to have greater information about individual species consumed before one could attempt to quantify this potential competition.

The first guild, comprised of B. barbastellus, P. austriacus and P. auritus,

feed primarily on Lepidoptera, and have very similar, quiet, echolocation calls (table 1.2) (Stebbings, 1986). However, their emergence times vary significantly (table 1.1), with B. barbastellus emerging at ~19.5 minutes

after sunset (Russo et al., 2007), P. austriacus emerging at ~30 mins

after sunset (Middleton et al., 2014), and P. auritus emerging at ~54 mins

after sunset (Jones and Rydell, 1994). As a result, direct competition between the bats will be avoided. At these different times, it is likely that the prey species available will be different, which is further reflected in their different feeding styles (table 1.1): B. barbastellus is an aerial hawker (Holderied and Von Helversen, 2003), whereas P. auritus feeds through gleaning (Coles et al., 1989). They appear to have a narrow dietary diversity (figure 2.3), which may be an artefact of the fact that many studies do not distinguish between Lepidoptera species.

The second guild, which is comprised of the three pipistrelle species, M. daubentonii, and N. leisleri, which feeds predominantly on Diptera, has a wider range of echolocation call types than seen in guild one. Guild two

does have a broad range of emergence times. P. nathusii emerges

between 11 and 55 minutes (Gelhaus and Zahn, 2010), N. leisleri at ~18

minutes (Jones and Rydell, 1994), P. pygmaeus at ~24 minutes

(Davidson‐Watts and Jones, 2006), P. pipistrellus at 32 minutes (Jones and Rydell, 1994), and M. daubentonii at 84 minutes (Jones and Rydell,

1994). Again, this is likely a mechanism by which competition is avoided. The third guild includes M. alcathoe, M. bechsteinii, M. brandtii, M.

mystacinus, M. nattereri, N. noctula, and R. hipposideros. This guild has the highest average dietary diversity, with all species relying on a range of Lepidoptera, Diptera, and other orders.

M. alcathoe and R. ferrumequinum do not cluster well into guilds as

defined by Pianka’s indexes, however they both have high proportions of Coleoptera in their diets, which is not as heavily represented in the other bat diets. However, as the rest of their dietary preferences are dissimilar, they do not cluster with each other.

2.5.3. Niche partitioning in cryptic sympatric species P. pipistrellus and P. pygmaeus

These data do not distinguish between the two species until they were formally identified as separate species in 1999 (Jones and Barratt, 1999), with diets before this time typically being assigned to P. pipistrellus. In

order to study the true differences between the species, it was necessary to look at the data produced since 1999 in isolation.

Using the all of the data the overlap (Pianka’s overlap index) between the

species was 0.926, however, when only using the data collected after 1999, the overlap was 0.806. This suggests that the diets of P. pipistrellus

and P. pygmaeus may be less similar than previously thought. This may be a result of misassignment of diets to P. pipistrellus before the

description of P. pygmaeus as a separate species. This further highlights the need for a rigorous comparative study of bat diets.

2.5.4. Mechanisms of trophic partitioning

There are numerous mechanisms by which trophic partitioning may occur. Species may be partitioned by habitat preference: where the primary foraging habitats are species specific, species are able to co- exist within the same ecosystem (Arlettaz, 1999). Additionally, one

foraging habitat may be partitioned temporally: the emergence time of the different species of bats, plays an important part in the resource

partitioning between the species (Adams and Thibault, 2006). 2.5.4.1. Feeding style and prey availability

The presence of non-volant (ametabolous) arthropods within the diet, such as Arachnida, Chilopoda, Entognatha, and Opiliones, is used to confirm the use of gleaning as a feeding style, as non-volant arthropods are unlikely have been captured in the air (Swift and Racey, 2002). In the studies surveyed, a number of bats fed heavily upon Arachnida (M. alcathoe,M. bechsteinii, and M. nattereri have Arachnida as >10% of their diets). This suggests that these are gleaning bats, unless arachnids

have been caught whilst ‘on-the-silk’. Both M. bechsteinii (Fenton and

Bogdanowicz, 2002, Petrov, 2006, Wolz, 1993) and M. nattereri (Arlettaz

et al., 1997, Geisler and Dietz, 1999, Jones, 1993, Shiel et al., 1991, Siemers and Schnitzler, 2000, Vaughan, 1997) have previously been reported as gleaning (table 1.1), whereas, M. alcathoe did not have any

data on feeding styles. In contrast to previous literature, the rhinolophids (Ahmim and Moali, 2013, Fenton, 1997, Jones et al., 1995, Jones and Rayner, 1989) and P. auritus (Coles et al., 1989) only had a small

proportion of their diets attributed to gleaned prey. McAney et al. reported the non-volant Siphonaptera in the diets of E. serotinus (McAney, 1991) which were likely ingested during grooming rather than gleaning (Shiel et al., 1998).

2.5.4.2. Bat morphology

The morphology of the bat species will impact greatly the bat’s prey

preference (Andreas et al., 2013, Freeman, 1979). Bat size, as well as durophagy, is correlated with prey hardness, with larger bats able to feed

on harder prey (Freeman, 1981, Freeman and Lemen, 2007, Ghazali and Dzeverin, 2013). The average Coleoptera is 3.2 times harder than moths of the same size (Freeman and Lemen, 2007), making Coleoptera

amongst the hardest of the bat prey. The large bat species E. serotinus, R. ferrumequinum, and N. noctula (these bats have wing spans of about

330-450mm (Stebbings, 1986)), consume the highest proportion of Coleoptera: 42%, 31% and 24% of their diets respectively. This will be discussed in greater detail in chapter seven.

2.5.5. Trophic breadth and extinction risk

In terms of dietary specialisation, it would be expected that M. bechsteinii, being the greatest generalist, would be the most robust against variations

in prey availability due to it’s diverse diet and therefore be more

successful than other species (Boyles and Storm, 2007). However, it’s

vulnerable status, and decreasing population belies this (I.U.C.N., 2013), showing that factors other than diet do affect population declines.

Conversely, P. auritus, which is common in Europe, with a stable

population, has a far lower prey species richness and dietary breadth, despite being predicted to have a broader dietary diversity (Battersby, 2005, Greenwood et al., 1996). This is further confounded by the limited number of studies carried out in Great Britain; diets of bats vary within species across different countries (Shiel et al., 1998). Measuring the diets of bats from guano collected from within Great Britain will be directly of use for conservation in Great Britain.