Reconciling ‘irreplaceability’ and ‘importance’

Quantifying the relative irreplaceability of important bird and biodiversity areas

  • Priority sites for biodiversity have in the past been based on thresholds or complementarity
  • A complementarity-based analysis of irreplaceability in Important Bird Areas seeks to combine these two approaches
  • It is important to complement the threshold-based identification of important biodiversity sites with the systematic identification of irreplaceable sites

The orange-bellied parrot is endemic to Australia and critically endangered. Eighteen IBAs have been identified in Australia for the presence of this species. (Photo: © Jeremy Ringma.)

The orange-bellied parrot is endemic to Australia and critically endangered. Eighteen IBAs have been identified in Australia for the presence of this species. (Photo: © Jeremy Ringma.)

The expansion of the world’s protected area network is often held up as a measure of global progress towards effective biodiversity conservation. However, having more protected areas does not necessarily mean better biodiversity outcomes. In the past, two main approaches have been used to identify priority sites for biodiversity conservation: one based on thresholds, the other on complementarity. We recently combined both approaches to guide conservation planning.

Threshold-based approaches build largely on the Important Bird and biodiversity Areas (IBAs) program of Birdlife International. IBAs are a set of more than 12,000 sites around the world identified using four criteria that relate to the occurrence of: -globally threatened species, -restricted-range species, -species assemblages confined to a single biome, and -congregations of one or more species.

In order for a site to meet one or more of these criteria and qualify as an IBA, it needs to have more than a minimum threshold number of bird species and/or individuals (for example, a congregation of over 20,000 water birds).

Complementarity-based approaches identify sites for protection that complement, rather than replicate, each other. These approaches are typically associated with the measure of ‘irreplaceability’, defined as the contribution of a site in achieving biodiversity conservation targets or the extent to which the options for achieving the targets are compromised if the site is lost.

After decades of parallel development between threshold- and complementarity-based approaches, and in light of a pressing need to find shared strategies for an efficient expansion of protected areas, we brought together these approaches by performing a complementarity-based analysis of irreplaceability in IBAs (Di Marco et al, 2016). Our objectives were to determine: 1) whether irreplaceability values are higher inside than outside IBAs, and 2) whether any observed difference in irreplaceability depended on known characteristics of the IBAs.

We focused on three regions with comprehensive IBA inventories and bird distribution atlases: Australia, southern Africa, and Europe. These regions represent a broad spectrum of conditions. For example, Australia and southern Africa have more endemic birds than Europe, whereas Europe has smaller and more numerous IBAs. In each of the study regions we used the software Marxan to measure irreplaceability inside and outside of IBAs.

We found that irreplaceability values were significantly higher inside IBAs than outside of them. The differences were much larger in Australia and much smaller in Europe. This is likely because Australia has more restricted-range birds and fewer IBAs than Europe. In fact, in all regions we found that higher irreplaceability values in IBAs were associated with the presence and number of restricted-range species. The relationship between higher irreplaceability and presence of restricted-range species is not surprising: representation targets for widespread species can typically be met under many different spatial solutions; whereas, there are relatively few options for meeting targets for small-range species.

The irreplaceability of a site depends on its characteristics and its regional context, while the IBA status is an absolute, rather than relative, attribute (it only depends on the characteristics of the site). In some cases, the two approaches are expected to produce convergent results: a site that includes the only occurrence of one or more endemic bird species is totally irreplaceable and is also an IBA. The relationship between the two approaches in other situations is context dependent and requires testing.

The correspondence between threshold-based and complementarity-based approaches can be complex. Sites with species below IBA-threshold levels could still be important to efficiently achieve species’ representation targets, especially when they have high levels of complementarity with other sites. On the other hand, a site may be of particular importance for a given species (eg, if the species congregates there) and be identified as an IBA even if it has low irreplaceability. We stress the importance of complementing the threshold-based identification of important biodiversity sites with the systematic identification of irreplaceable sites. Failing to do this may create significant gaps in any reserve system built on these sites.

Recently the IUCN has undertaken a process to consolidate global standards for the identification of Key Biodiversity Areas (KBAs). This builds on existing approaches such as IBAs. Our results informed this process and in particular a proposed criterion for irreplaceability that will allow the new KBA standard to draw on the strengths of both threshold-based and complementarity-based approaches.

More info: by Moreno Di Marco


Di Marco M, T Brooks, A Cuttelod, LDC Fishpool, C Rondinini, RJ Smith, L Bennun, S Butchart, S Ferrier, RPB Foppen, L Joppa, D Juffe-Bignoli, AT Knight, JF Lamoreux, PF Langhammer, I May, HP Possingham, P Visconti, JEM Watson & S Woodley (2016). Quantifying the relative irreplaceability of important bird and biodiversity areas. Conservation Biology 30: 392-402.


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