Changes in PA associations between Periods 1 and 2 suggest that several introduced species may have showed increasing associations with PAs over time (Figure 3.3). However, the possible difference in temporal trends in PA association between introduced and natural colonists was not significant (t-test, t7,5=-2.045, P=0.076).
3.4.5 Additional species
One additional (outside Anatidae) introduced wetland bird species became established in the UK after 1940 (Black-crowned Night Heron Nycticorax nycticorax) but died out again as a breeding species in 2003. All sites occupied by N. nycticorax during Landing Pad, consolidation and establishment phases were outside PA land. Hence, inclusion of this species in the
analyses resulted in increased significance of the differences between introduced species and natural colonists at all stages of arrival and establishment (Appendix 2F). We extended the analysis to include three non-wetland species, which were the Firecrest Regulus ignicapillus, Collared Dove Streptopelia decaocto (both natural colonists), and Rose-ringed Parakeet
Psittacula krameri (introduced). The results remained significant with these species included,
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Figure 3.3 The difference in PA association (measured in percentage points) between Periods 1
and 2 for introduced species (black) and natural colonists (white). The abbreviations for species names are shown in Figure 3.1.
3.5 Discussion
Introduced species can have negative environmental and economic impacts, particularly within PA networks, which might harbour protected native species. Although the vulnerability of PAs to colonisation by introduced species is theoretically low (e.g. Tansley, 1939), empirical evidence suggests that invasions of reserves are common (e.g. Usher, 1988; McDonald et al., 1989; GISP, 2007; Allen et al., 2009; Burfeind et al., 2013). Nonetheless, we hypothesised that introduced wetland birds in the UK would show weaker PA associations than natural colonists; they have different habitat requirements, and are more likely to be released away from reserves.
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When colonising a new county, bird species can first breed on either protected or unprotected land; protected land attracts colonising species (Hiley et al., 2013), whereas the majority of first breeding events by introduced species were outside PAs (Figure 3.1). The benchmark for associations with protected areas – for these wetland-associated species – is the percentage of wetland under protection (Appendices 2G, 2H), which is around 30% (compared to 12% of the total land area (jncc.defra.gov.uk)). Using coarse metrics of the availability of protected and unprotected wetland in each county, introduced species colonised PAs and non-PA land approximately in proportion to their availability, whereas natural
colonists still favoured them (Appendix 2I). Further data are required to test this conclusion more robustly because the land cover data for this analysis were too coarse to accurately calculate species-specific levels of protected and unprotected habitat availability (i.e. land cover layers did not adequately describe species-specific associations with open water, reed, grazing marsh, freshwater/saline conditions).
Given the different habitat requirements of the two groups, this result is expected. Many PA management actions are specifically targeted at habitat improvement for some of the subject species (reedbed maintenance for Great Bittern and water level control for Avocet
Recurvirostra avosetta). On the other hand, management would not be directed towards the
requirements of the introduced wetland birds, such as Anatidae, which are often successful ‘invaders’ (Lever, 2005). The Canada Goose Branta canadensis, for example, breeds on manicured lawns, such as those found in parks, airports and golf courses, in both their natural and introduced ranges (Cabot, 2009). The invasive species which showed the highest
association with PAs, Ruddy Duck Oxyura jamaicensis (Figure 3.1), was also the species whose habitat-requirements most closely match British PA priority habitats.
In this study, the group of introduced species was unavoidably taxonomically narrower than the natural colonists. We do not believe, however, that this affects either the conclusions or the implications of this study because invaders frequently belong to a non-random subset of potential taxa that could be introduced (e.g. Schmidt & Drake, 2011). Furthermore, our introduced species are representative of introduced wetland bird species in general. For example, of the 31 introduced wetland bird species that have become established in Africa and Eurasia, 27 (87%) are from the family Anatidae (Banks et al., 2008).
Nevertheless, we carried out three further analyses to test the robustness of our findings. Other introduced wetland species (outside Anatidae), both in continental Europe and historically in the UK, appear to follow patterns in PA use similar to the Anatidae (Appendices
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2F, 2J). The inclusion of those non-wetland bird species (both introduced and natural colonists) that have recently colonised the UK also validated our results (Appendix 2F), although the Collared Dove, which is an invasive species in North America (e.g. Hengeveld, 1993) but a natural colonist to the UK, was an interesting exception.
Whereas most natural colonists showed a declining association with PAs as they became established (Hiley et al., 2013), there is a trend for introduced species to show the opposite pattern (Figure 3.3). Although the difference in temporal trends between natural colonists and introduced species was not statistically significant using species as replicates, we did observe that there was an increase in the total percentage of pairs of introduced species that were in PAs: only about a third (38%) of ‘first breeding’ events was in PAs (Figure 3.1), whereas over a half (55%) of all pairs was in PAs 14-16 years after their first arrival (Figure 3.2). Such a pattern might reflect either a natural process of introduced species moving away from points of release in non-PAs and into the nearest available habitat or, alternatively, a growing preference for a PA network which can potentially offer invaders, as well as natural colonists, high quality habitat in which to breed, and a safe haven from potential threats. Whatever the reason, large numbers of individuals of non-native species now regularly appear on reserves. Increasing incidences of invasions are being noted across a broad range of groups (e.g. Huang
et al., 2011), potentially leading to an increased need for control measures (one of our subject
species, Ruddy Duck, has subsequently been reduced by an eradication programme to help prevent global extinction, by hybridisation, of the European native White-headed Duck Oxyura
leucocephalus).
This study used data from county bird reports, which provide an important historical record of bird sightings on a regional scale in the UK. Such a dataset, which relies on
contributions from casual observers, is particularly important when analysing distribution patterns of rare birds, or ‘new’ birds within a county, which attract more interest from bird watchers and recorders (e.g. Pithon & Dytham, 2002). Although sightings data in general may show a bias towards protected areas, it has been previously demonstrated, for a wide range of wetland bird species, that there was no systematic trend in observer effort either towards or away from PAs (Hiley et al., 2013). Thus, the differing trends in PA use presented here appear robust.
Wetland bird species that have been introduced to the UK in the past have been significantly less associated with PAs than natural colonists during the initial stages of their colonisation. However, with time, their propensity to use PAs increases. Whilst not
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specifically favoured for initial colonisation, PAs might unintentionally subsequently facilitate the establishment of persistent populations. The patterns of first arrival, establishment and consolidation observed for these bird species suggest that PAs are more effective at facilitating the range expansion of naturally colonising species from nearby regions than they are at assisting the establishment of introduced species from other parts of the world.
3.6 Acknowledgements
We are grateful to all of those involved in the collation of records for, and the production of, county bird reports. We give thanks to Natural England, Scottish Natural Heritage and the Countryside Council for Wales for the use of their online software and particular thanks to the Rare Breeding Birds Panel for their continued support throughout this work.
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Chapter 4: Impacts of habitat change and protected areas on the alpha and beta diversity of