Graduated circle size based on an attribute value can be achieved by current QGIS styling by selecting graduated style of type ‘size’. For example here is a map of ladybird species richness by tetrad for Shropshire:
Here’s an example of the a styling for that:
Trouble is, this styling is for a point dataset, so for this to work for tetrad maps created from the plugin biological recording tool, the circles or squares produced from that tool first had to be converted to points using the centroids tool. I think it would be useful to produce a third output option from the biological recording tool - in addition to the current circles or squares - for atlas maps of ‘points’. Style files could be applied at the point of creation, as they can now (options tab) to achieve the desired effect.
My first thoughts are that this would be a better solution than adding a considerable layer of complexity to the biological recording tool itself to allow users to style graduated circle/square sizes. Better to add this third output option to facilitate existing QGIS functionality.
The plugin does not distinguish species, or any other taxonomic rank - it only recognises different taxa. The taxon field doesn’t have to be a species - it can be any taxon. Sometimes people think that the plugin ‘understands’ genus and sub-species because of the ‘scientific names’ checkbox - but all that does is split taxon names, using spaces as separators, and uses the first word as a group in the taxon tree on the taxa tab. So adding logic to recognise species and subspecies would be a significant amount of work.
Grouping taxa defined as separate sub-species to create a single map for a species can be done by selecting all sub-species in a species and generating a single map. That won’t work for batch generation however. In that case, the best solution would be to pre-process the input file as alluded to in the feature request.
So to undertake the development work required for this, we’d have to be satisfied that this would be a useful feature for a significant number of users. I don’t have that impression at this stage (I think that this is the first request for it) and given that there are ways around it - even if not perfect - I think that this one would have a lower priority than the first. But it is a reasonable suggestion and one that should stay on the wish list.