Situación profesional de evaluación número 1

Issues related to spatial and temporal variation in observer effort were a recurring theme throughout this work, and the results reported in each of the data chapters should be interpreted carefully against this backdrop.

The association of colonising species with PAs as both sites of first reported breeding (Table 2.1) and sites where populations establish (Fig 2.1) could potentially be explained by the differences that exist between protected and unprotected land in terms of (1) intensity of observer effort (in this case bird-watching time) and (2) the probability of incidences of breeding being reported to the relevant bodies. Given that bird-watchers are more attracted to protected areas (Ferrer et al., 2016) and therefore focus attention in PAs, and that wardens within PAs are perhaps more likely to confirm breeding than casual observers outside of PAs, it is plausible that the true percentage of ‘landing pads’ (sites of first reported breeding for colonising species) that have occurred in PAs is lower than the figures that we report in Chapters 2 and 3.

Furthermore, the declining association of colonising species with PAs over time could also be counfounded by this observer effort issue. Firstly, it could be argued that observers within PAs are more likely to identify ‘new’ species. As public awareness of colonising species grows, observers outside of PAs become better equipped to identify, and then report

incidences of breeding. Secondly, under the assumptions that observer intensity is higher in PAs and that colonising species breed at the same site year-after-year, a declining trend of association with PAs would be reported even if it was not happening in reality, as incidents of breeding in unprotected land are gradually discovered. Figure 6.1 illustrates this issue under

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the assumption of 100% observer coverage in PAs, and imperfect coverage in unprotected land.

Figure 6.1 Potential influence of variation in observer effort on reported association of

colonising species with PAs over time. Open circles are breeding pairs which have not been reported, closed circles are breeding pairs which have been reported. This model assumes 100% observer coverage in PAs, and imperfect detection in unprotected land.

Figure 6.1, while illustrative of the potential issues presented by variation in observer effort, is an extreme scenario, and is unlikely to reflect reality for many species. This is because (1) the discrepancy in observer coverage is unlikely to be as high as shown in Fig 6.1 (in the UK at least the popularity of ‘patch’ birding ensures coverage in unprotected land), (2) for some species, PAs contain a large proportion of the suitable breeding habitat within the wider landscape and so by default, percentages breeding in PA must be high, and (3) many of the colonising species that we focus on e.g. Little Egret and, more recently, Spoonbill, are

Number of breeding pairs: 12 Reported breeding pairs: 7 True percentage in PA: 50% Reported percentage in PA: 86%

Number of breeding pairs: 12 Reported breeding pairs: 8 True percentage in PA: 50% Reported percentage in PA: 75%

Number of breeding pairs: 12 Reported breeding pairs: 9 True percentage in PA: 50% Reported percentage in PA: 67% PA PA PA Non-PA Non-PA Non-PA Non-PA Non-PA Non-PA

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large-bodied, conspicuous species and thus highly visible and likely to be reported whether or not the breeding event is in a PA. Furthermore, we present contrasting trends for colonising non-native species in Chapter 3, indicating that a declining association in PAs is not a ‘given’ for species undergoing range-expansions.

Analysis of the spatial distribution of bird records submitted to online portals such as BirdTrack (UK), or eBird (globally) could allow researchers to quantify the true extent of the difference in observer intensity between PAs and unprotected land, and would be a

worthwhile avenue of future study. The use of null models based on estimates of observer effort in/out of PAs and probabilities of detection of species would allow trends in PA use over time to be verified.

In Chapter 4, we used four ten-minute point counts for the bird surveys in each square. This time limit was used for logistical reasons, but it might have influenced the reported species richness of different locations on the basis that rates of species accumulation potentially vary between bioclimatic zones, modification categories and levels of protection. Longer counts increase the probability of detection of inconspicuous species, and also the likelihood of detecting mobile species which have moved into the study area during the point count (Fuller & Langslow, 1984). Analysis of species accumulation curves in different

bioclimatic zones/modification categories/protection levels would allow us to set an objective time limit in order to take representative samples of true local species richness at each site.

Issues related to observer effort are further discussed in sections 1.4.2, 2.5, 3.5 and 5.5.1

At least 24 presence-absence measures of beta-diversity have been employed by researchers (Koleff et al., 2003), as well as several more which use species abundance data (Barwell et al. 2015). The relative performance of these metrics can be evaluated in different ways, including independence from alpha-diversity and sensitivity to sample size. The βJaccard measure, which was used in Chapters 4 and 5 to measure pairwise similarity of squares and entities, performs well for several measures, including independence from alpha-diversity when the pairs of assemblages compared are equal in size, and independence from differences in abundance. When comparing assemblages which vary in species richness, however, the βsim metric may be favoured as it is less sensitive to nestedness (Barwell et al., 2015). Thus

attempts to compare similarity of assemblages should consider the structure and underlying assumptions of the data before deciding on the most appropriate metrics. Although the use of

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different metrics of beta-diversity in this thesis might have affected the size of any reported change, both the patterns observed here (the presence of habitat generalists in modified areas in several different vegetation zones in Chapter 4, and the presence of non-native/naturally colonising species in several ‘new’ entities), and the implications of these issues for

conservationists, remain relevant.