9a. Analysis Conclusions
In order to effectively manage estuarine environments under the complex system of conservation priorities and provision of goods and services (and the consequent possible conflicts) which are usually in place on these areas, managers should be clearly informed about the relationships occurring between the species and the environment (natural and anthropogenic), in order to properly address possible changes in the latter.
This study investigated these relationships in three TIDE estuaries (Weser, Elbe and Humber), by focusing on bird use of estuarine habitats. The results highlighted that it is usually a combination of different factors that determines bird use, with the spatial factors (i.e. those affecting the distribution among different areas within an estuary) showing an overall higher influence compared to temporal ones (i.e. those factors accounting for inter-annual changes in bird abundances).
In general, the outer zones of estuaries are likely to support more diverse and dense bird assemblages, as a result of the higher availability of suitable estuarine habitats (e.g. in the Humber wider, more extensive, intertidal mudflats and marsh areas are available in the polyhaline sectors) as well as the proximity of other suitable habitats along the adjacent coastal area. However, locally, inner (oligohaline) areas may also be relevant in supporting abundant bird populations of some species, for instance Lapwing, Golden Plover Teal, Wigeon and Mallard in the Humber.
The analysis described in this report identified the extent of habitats (in particular intertidal mudflats) to be highly important in affecting bird distribution within estuarine areas. This strong association between bird distribution and habitat areas is likely mediated by the food availability (in quantitative and qualitative terms), a factor that is usually considered as the major determinant of bird distribution within estuaries. Although a higher carrying capacity is usually associated with wider, more extensive estuarine habitats, the positive relationship with density of several bird species observed here suggests that the possible higher diversity of resources associated with more extensive habitats allows higher concentrations of species that are able to take advantage of a wider range of food prey, as in the case of Dunlin and Redshank. In turn, specialist feeders (such as Bar-tailed Godwit) are more likely to depend on the distribution of specific prey, a factor that might be more relevant at a smaller spatial scale (i.e., within a mudflat) hence resulting in contrasting relationship with the total intertidal habitat area. Water quality characteristics (e.g. nutrients, water oxygenation) also showed a relative importance in affecting species distribution, although this effect was only particularly evident in the Elbe.
The application of habitat distribution models has allowed the identification of a series of habitat requirements for several waterbird species, resulting in the potential for the derivation of guidelines on the provision of important environmental characteristics of a habitat in terms of combinations allowing the maximisation of occurrence and abundance of a species. A summary of these characteristics is reported for each of the species investigated in Table 8.
Except for the Elbe, where the results are likely to be driven by the very high density of species observed in the outer estuary, where small counting units were present around some islands in the Wadden Sea, Dunlin showed a general preference for wider, more extensive intertidal areas, where larger mudflats, but also marshes and shallow subtidal areas occur.
These conditions are usually associated with the outer parts of the estuary and the analysis identified the most suitable habitats for Redshank and Shelduck in the Humber also to be associated with the wider, more extensive intertidal areas (>7 km2), although higher densities of additional species are expected where littoral sands are present. Intertidal areas dominated by littoral sand adjacent to intermediate supralittoral areas and with higher total densities of benthic invertebrates are likely to support higher densities of Redshank. Intertidal areas dominated by littoral mud and adjacent to extensive shallow subtidal zones and relatively wide supralittoral areas (>0.6 km2) were identified as being preferred by Shelduck.
Golden Plover density distribution in the Humber was mostly associated with sectors where the area is dominated by wider marsh and intertidal habitats with subtidal area being smaller. The presence of sandy substrata in the littoral zone was also characteristic, but with lower intertidal benthic densities present. In contrast, higher densities of Bar-tailed Godwit can be expected on the Humber in areas featuring smaller intertidal mudflats (<5 km2), where lower total densities of benthic invertebrates occur, as well as intermediate supralittoral areas, these characteristics being possibly related to the higher selectivity of this species towards specific prey, as explained above.
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