[Syrphidae] IUCN Red list Hoverflies: calculation AOO

[Marija Miličić]

Hi Wout,

AOO is calculated using a standard 2 × 2 km grid overlaid on the 
species' distribution points. In some cases, a different grid cell size 
may be more appropriate depending on the biology and ecology of the 
species or taxonomic group, but 2 × 2 km is the standard recommended by 
IUCN, and the one used in IUCN Red List of hoverflies. Each grid cell in 
which the species is present counts as 4 km², regardless of the number 
of records within it or the actual size of the occupied habitat.

So, in your example, if the 10 habitat patches fall within 10 different 
grid cells, the AOO would be 40 km², even if each patch is smaller than 
5 ha.

There are also online tools available for calculating AOO, such as this 
one: https://geocat.iucnredlist.org/editor. You can simply upload your 
species occurrence records as a CSV file with coordinates, and the tool 
will calculate the AOO automatically.

Hope this is helpful.

Best,

Marija

On 11/6/2026 11:58 AM, wout.opdekamp at gmail.com wrote:
> Hi,
>
> I'm trying to find more detailed information on how the Area of 
> Occupancy has been assessed in the IUCN Red List. Unless I misread 
> this information is lacking. In Version 3.1 of the IUCN RED LIST 
> CATEGORIES AND CRITERIA attention is drawn to the fact that assesment 
> scale impacts the outcome but without giving general guidelines.
>
> More specifically I would like to know which AOO would be attributed 
> to a species known from for instance 10 fragmented (>5km distance 
> between patches) patches of suitable habitat. Let's say individual 
> patch vary in size but never exceed 5ha.
>
> Could anyone clarify this?
>
>
> Regards,
>
> Wout
>
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-- 
dr Marija Miličić
Senior Research Associate
University of Novi Sad
BioSense Institute
dr Zorana Đinđića 1
21000 Novi Sad, Serbia
e-mail: marija.milicic at biosense.rs
         marija.milicic at dbe.uns.ac.rs
         mmmarija88 at gmail.com
tel: +381 (0)64 48 49 132