To identify the species responsible for these changes, the most abundant species were analysed separately (Tables 2 and 3). Some showed no changes in numbers during the study period or changed in a similar way at CG and CE, such as Little Egret (Egretta garzetta), Common Shelduck (Tadorna tadorna), Eurasian Wigeon (Anas penelope), Mallard (Anas plathyrhynchos), Common Pochard (Aythya ferina), Ringed Plover (Charadrius hiaticula), Sanderling (Calidris alba) and Little Stint (Calidris minuta). Another group of species showed marked and consistent differences between the salinas. This was the case of diving species, such as the Little Grebe (Tachybaptus ruficollis), Black-necked Grebe (Podiceps nigricollis), White-headed Duck (Oxyura leucocephala) and Common Coot, whose numbers remained stable before and after 1988 at CG, but showed a significant increase at CE (Tables 2 and 3). The opposite was found for species of wading birds and shorebirds, such as Greater Flamingo (Phoenicopterus ruber), Black-winged Stilt (Himantopus himantopus), Avocet (Recurvirostra avosetta), Kentish Plover (Charadrius alexandrinus), Curlew Sandpiper (Calidris ferruginea), Dunlin (Calidris alpina), Black-tailed Godwit (Limosa limosa) and Common Redshank (Tringa totanus), whose numbers remained nearly constant or increased at CG, but decreased or were stable at CE (Tables 2 and 3). A unique case was that of Northern Shoveler (Anas clypeata). Although its numbers increased significantly from one period to the other, it occurred at CG exclusively during summer and only at CE during the winter (Tables 2 and 3).

Table 3. Average abundance (± SD) during summer of the main waterbird species at Salinas de Cabo de Gata and Salinas de Cerrillos before and after 1988. Statistical differences (Mann-Whitney U test) between both intervals are also indicated: n.s., not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001. N, sample size (number of years). Feeding guilds: o, wading birds; ●, shorebirds; +, dabblers; □, divers.

On analysing the relationship between the change in numbers of the main species of shorebirds at both sites from before and after 1988 and the average length of their tarsi and bills, a significant relationship was found at CE during winter (Fig. 3; for both sites in summer and also for CG in winter, N = 10 and 8 respectively: r_s > -0.7, n.s.).

DISCUSSION

Consistent changes over time in the waterbird community at both CG and CE could not be due to changes occurring in the study areas, and could be the result of regional or extraregional trends at larger scales (see Dolz and García 1992; Tucker and Heath 1994; Rose and Scott 1997; Delany et al. 1999). However, changes in the bird community occurring at CE but not at CG, are likely to be related to the changes at the former site following the abandonment of salt production. Thus, the observed increase in the species richness at CE after the cessation of salt extraction was probably due to deeper, brackish, more productive water and with a greater cover of helophytic vegetation in concrete enclaves within the abandoned saltpans. This favored the increase of bird species diversity because the original species could still persist in the remaining shallow waterbodies while other new species, feeding principally by diving, used the new deeper and brackish pools. These pools provided suitable requirements for this guild of divers (Nilsson 1972a b 1978; Hobaugh and Teer 1981; Ydenberg 1988; Owen and Black 1990; Halse et al. 1993; Castro et al. 1994; Boyd 1997).

The decline in abundance of Greater Flamingo and certain species of shorebirds at CE following abandonment was probably due to the reduced size of shallow and salty waterbodies suitable for feeding (Finlayson et al. 1992; Pérez-Hurtado and Hortas 1993; Pérez-Hurtado et al. 1993; Barbosa 1997). However, this change did not affect all the species in the same way, and we found a relationship between the change in numbers and the lengths of the tarsus and bill in the shorebird species. This relationship is probably explained by the relationships between the depth of water in habitats used by each species and their body dimensions (i. e. species with longer bills and tarsi use deeper water; Rubio 1986; Wiens 1992; Barbosa 1993 1997; Pérez-Hurtado and Hortas 1992; Pérez-Hurtado et al. 1993). Thus, as the size of shallow and salty lagoons decreased in CE after abandonment, those larger species adapted to feed in the interior and deeper zones (Greater Flamingo, Avocet and Black-tailed Godwit) were more affected because they had proportionally less remaining habitat than the smaller species (especially Sanderling and Little Stint), which used neighboring beaches as feeding habitat (Rubio 1986; Pérez-Hurtado and Hortas 1992; Barbosa 1993 1997). This relationship was not evident during the summer, probably because of the large variance in body dimensions of individuals, owing to the presence of adults and young of various sizes.

Fig. 3. Relationship between the percentage change in abundance (%) form before to after 1988 of the main species of shorebirds at Salinas de Cerrillos during winter and the average sizes (mm) of the tarsus (Spearman rank correlation; r_s = -0.7 , P < 0.05 , N = 8) and bill (Spearman rank correlation; r_s = -0.9 , P < 0.01 , N = 8 ). Hh, Black-winged Stilt (Himantopus himantopus); Ra, Avocet (Recurvirostra avosetta); Cx, Kentish Plover (Charadrius alexandrinus); Cb, Sanderling (Calidris alba); Cm, Little Stint (Calidris minuta); Cp, Dunlin (Calidris alpina); Ll, Black-tailed Godwit (Limosa limosa); Tt, Common Redshank (Tringa totanus).

MANAGEMENT IMPLICATIONS

 In some cases, the abandonment of saltpans is likely to have favourable repercussions on species richness, with the appearance of species of a high conservation importance, such as the White-headed Duck, listed as globally threatened (BirdLife International 2000). In order to improve the conditions for the Greater Flamingo and certain shorebirds following the abandonment of saltpans, the controlled reflooding of the lagoons with seawater would be advisable. Moreover, with the object of not altering the habitat of the White-headed Duck, it would also be recommended that this reflooding shouldn’t affect the pools most related to underground water supplies (deeper and brackish pools).

ACKNOWLEDGEMENTS

We are grateful to A. J. Green for his comments on earlier versions of the manuscript, to P. Kramer, J. G. Martínez, A. J. Green and J. C. Coulson for the translation into English, as well as to J. J. Soler and J. G. Martínez for their comments related to data analysis. The counting of birds was partly financed by the Consejería de Medio Ambiente (Junta de Andalucía, Spain).

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