IUCN Guidance when considering habitats with sub-habitats in different conditions/
threat assessments
Guidance from the IUCN has been provided in order to help deal with habitat assessments that are
problematic. The main concern is how to assess habitats using the information provided from
several sub-habitats that have had different threat categories assigned to them. Using the Worst
Case scenario, i.e. assigning the EUNIS level 4 habitats with the highest threat category from all the
sub-habitats is not considered appropriate. The following guidance should help when dealing with
this situation.
Look at all the data/information in the sub-habitat assessments, including the criteria used for
assessment and the information on the threats (type, severity, quantifiable, time scales) and
construct a picture of the general habitat. Expert judgement can be used to make assumptions, e.g.
that threats and impacts in some parts of the range are similar in other parts of the range. The
assumptions need to be justified thoroughly in the assessment text boxes of the online platform. For
example, if 60% of the extent is EN and 40% is unknown, you could perhaps list it as NT or even VU,
depending on the threats, and how and how much they have impacted the sub-habitats we know
about.
Criteria A: Reduction in Quantity
You should be able to apply criteria A even with only presence/absence data. Data such as maps of
extent at different time periods, and presence/absence is sufficient to determine if the extent has
reduced by a certain %. If the habitat has collapsed in some areas, then you should consider it
absent there, and use this to calculate % reduction over time. If there has not been a decline in
extent, a criterion A is not applicable. However, there may have been a decline in quality of habitat
with time (see Criteria C/D).
Criteria B: Restricted Geographic Distribution
For criterion B, you should be able to measure EOO and AOO with presence/absence data. the
evidence you have to indicate some sub-habitats are threatened should be sufficient to evaluate
whether the habitat would meet the decline and threatening process sub-criteria – Ba(i), Ba(ii),
Ba(iii) and Bb – and to measure the number of locations (based on presence/absence data) (Bc and
B3).
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For Ba(i), Ba(ii), and Ba(iii) you need to determine if there are continuing declines in spatial
extent, abiotic or biotic quality in order to trigger those sub-criteria. For these sub-criteria,
the declines do not have to be taking place uniformly across the entire habitat; however the
idea is to distinguish restricted habitat types exposed to an appreciable risk of collapse from
those that are essentially stable and not exposed to identifiable threats. Although there are
no formal thresholds, expert judgement should be used. For example, if there are localized
declines taking place in only a few parts of the habitat type, it may not be considered
sufficient to trigger the sub-criteria. However, if declines are localized but taking place is
critical areas for the habitat or having severe impacts/ likely to expand in future, experts
may consider this to be sufficient to use this criterion.
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An important point for sub-criteria Ba(i), Ba(ii), and Ba(iii) is that the declines must be
continuing. A single decline (e.g. a decline in extent caused by an oil spill or trawling of the
sea floor destroying a seagrass meadow) that caused a one-off decline is not enough to
trigger the continuing decline sub-criteria.
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For Bb, the threatening processes that you know of/have data for can be used to determine
if this sub-criterion is met. The declines in quantity and/or quality caused by the threatening
process(es) do not have to be uniform across the habitat type but must be continuing. Again,
expert judgement should be used here. If there is an expectation of even a small continuing
decline caused by the threatening processes; this might be considered enough to trigger the
sub-criterion. Given that this criterion relates to habitats with restricted geographic
distributions, relatively small declines can imply larger impacts across the restricted type.
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You should be able to calculate the number of locations for Bc and B3 given the information
you have on threatening activities.
Criteria C/D (or C and D): Reduction in Quality
Using expert judgement and the information from the sub-habitat assessments, develop an overall
picture of the habitat status. The types of threatening processes and degree of threat (CR, EN, VU,
NT) of the threatened sub-habitats will give a good idea of the processes and interruptions to
functionality taking place in parts of the habitat’s distribution.
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If the non-threatened sub-habitats are LC and/or you know there are only limited/no
threatening processes taking place in those sub-habitats, you can assume there is no decline
in quality in those areas.
However for example, if 5 sub-habitats when considered together may result in an overall
Red List habitat type, mapping of the sub-habitats can be used to determine the % extent of
each habitat.
From this, the degree of decline in quality needs to be established, either across the whole
range or – more likely – in different parts of the range.
When large areas are unknown, the Red List status can either be extrapolated or these areas can be
left “blank” when calculating % extent. Information for 100% of the range/extent is not required for
this criteria, for example, if you know that there has been a severe decline in over 30% of the total
range, that is enough to identify that the habitat type qualifies for VU (even if you do not know the
status in the remaining 70% of the range).