SEARCH

SEARCH BY CITATION

In a paper in this issue (Beresford et al., 2011), we set out to quantify the extent to which site-based conservation initiatives overlapped with the ranges of globally threatened bird species in Africa, using a GIS approach. We considered Protected Areas and Important Bird Areas (IBAs), both of which cover c. 7% of the land surface of Africa. The former, as noted by both Brooks & Matiku (2011) and Rodrigues (2011), carry significant governmental weight despite not necessarily being identified for their conservation value, while the latter (which are of conservation value to other taxa in addition to birds –Pain et al., 2005) have no legal standing, but are identified using objective criteria that include their value for globally threatened species (Fishpool & Evans, 2001). We find that Protected Areas perform poorly at protecting the ranges of Africa's most threatened birds, covering just 13.9% on average, and even where a Protected Area overlaps part of an IBA, the part excluded from the Protected Area has higher value for these species than the protected part.

As Rodrigues (2011) notes, the limitations of GIS studies that utilize Extent of Occurrence (EOO) maps are well documented. We therefore attempted to reduce the commission errors inherent in the use of EOO maps by analysing extent of suitable habitat (ESH: the area of potentially suitable vegetation types within the altitudinal preferences of the species (Rondinini, Stuart & Boitani, 2005; Buchanan et al., 2008)). While it is desirable to move beyond interpolation approaches based on EOO or ESH and utilize only presence/absence data derived from recent complete field surveys, these are not available for the majority of species. Even in the most well-watched countries, such as the United Kingdom, the most reliable information on the distribution of birds comes from systematic atlas surveys in which ranges are interpolated from sampled field counts which themselves record only a tiny proportion of the individuals present. At least for the foreseeable future therefore, we will have to continue to assess geographical distributions using interpolative approaches, whilst acknowledging the limitations of these methods.

ESH estimates were considerably smaller than EOOs for most species, with a concomitant reduction in commission errors and increase in omission errors. Reducing commission errors reduces the likelihood that a species is assumed to be conserved elsewhere (Rodrigues & Gaston, 2001), and hence in our study, the danger of overestimating the coverage of species' ranges by Protected Areas or IBAs. Omission errors, on the other hand, reduce the estimated overlap between sites and species' ranges.

Considering the information we presented, Rodrigues (2011) suggests that the ESH approach was not well able to distinguish between occupied and unoccupied areas of the EOO, because both approaches have similar ratios of true presences to total predicted presences. We agree that by focusing on other aspects of the analysis and by presenting only a broad overview of the accuracy of the ESH approach compared with EOOs (table 1; Beresford et al., 2011), we did not show the full advantage of the ESH method.

A species-level analysis indicates that although the ESH comprises, on average, 28% of the EOO – hence reducing the chance of the species being assigned to areas where it did not occur – this did not make any difference to the accuracy of the estimated distribution for 74 species [47%, assessed by comparing the total numbers of errors (commission plus omission) using ESH vs. EOO]. Importantly, however, it did improve accuracy for 58 species (37%), decreasing commission errors by, on average, 11.8 IBAs per species. This was at the expense of a reduction in the accuracy of maps for 25 species, due to an increase in omission errors, although these averaged just 1.8 sites per species (Fig. 1).

image

Figure 1.  Increases in omission errors versus decreases in commission errors observed when using ESH instead of EOO. Each point represents one species. For all species falling on the right of the line, the ESH reduced commission errors more than it introduced omissions. ESH, extent of suitable habitat; EOO, Extent of Occurrence.

Download figure to PowerPoint

While considerable inaccuracies remain, especially in commission errors (table 1; Beresford et al., 2011), we suggest that the increase in accuracy for over a third of species, with no effect on almost half of the species, justifies the approach. It is also important to note that commission errors are likely to be overestimated in our analysis, as the data used only recorded species present in sufficient abundance to trigger IBA designation of sites (Fishpool & Evans, 2001), rather than being true presence/absence data based on complete surveys.

We do however share the concerns of Rodrigues about the weaknesses of range maps. Even systematic atlases are estimates of broad-scale distributions based on small amounts of field data, and the accuracy of them will depend on the size of sampling units (Gibbons et al., 2007). Therefore, we emphasize that EOO or ESH methods are not a substitute for dedicated, site-based field-collected data. Until such complete field survey data are available (perhaps guided partially, but not exclusively, by ESH), we think that ESH remains useful as long as its limitations are acknowledged. Brooks & Matiku (2011) suggest that the ESH approach could be used to inform planning policies and to guide research priorities for environmental assessments to minimize the impact of developments. This could in turn lead to targeted field surveys which would, over time, increase our knowledge of species' distributions and reduce our reliance on broad EOO maps or GIS-based estimates of ranges such as ESH.

While it is important to consider the methods used to assess species ranges, we believe that any errors in the range maps are likely to have affected the analysis for Protected Areas and IBAs in the same manner. The substantial differences in coverage between the two site networks and between species of differing conservation status should not be taken as absolute values, due to these uncertainties in the range maps, but the direction and approximate magnitude of the differences are likely to be robust. These results indicate that the Protected Area network needs considerable targeted expansion to improve coverage of globally threatened species.

As Brooks & Matiku (2011) point out, expansion of the Protected Area network should be targeted to the areas of highest importance, and we hope that our paper is used to support arguments for the expansion to cover sites of objectively assessed, recognized importance (such as IBAs and other key biodiversity areas). While the Protected Area network may be the most effective strategy for protecting tropical biodiversity (Brooks, Wright & Sheil, 2009), Protected Areas may not be the most or sole appropriate conservation action for all species. Many species have extensive ranges, occur at low densities and/or cannot be adequately protected by site-based conservation, and may therefore require alternative or additional approaches (Boyd et al., 2008). In other cases, community-based management, rather than formal designation, may be the most appropriate means of ensuring protection, for example, for eight of the nine most threatened unprotected IBAs in Uganda (BirdLife International, 2008). There is increasing evidence that legal protection is effective at reducing, though not necessarily halting, land cover change (e.g. Leverington et al., 2010). Consequently, monitoring is required not just of the total area of land or sea designated as protected, but also of the effectiveness of these designations at reducing biodiversity loss. Such monitoring will be a key area of development for the interface between pure and practical conservation.

Acknowledgement

  1. Top of page
  2. Acknowledgement
  3. References

We would like to thank Tom Brooks, Paul Matiku and Ana Rodrigues for their comments. We appreciate the opportunity to clarify our findings, and to discuss our results in a broader context.

References

  1. Top of page
  2. Acknowledgement
  3. References