What causes fluctuations in Painted Woman numbers?

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The abundance of Painted Woman butterflies seen within the UK varies massively from yr to yr, extra so than for another butterfly species, and that these fluctuations happen synchronously in different close by international locations demonstrating that Europe is recolonised yearly by this migratory insect. Subsequently, particulars of the butterfly’s extraordinary, multi-generational c.12,000km (7500 miles) annual migratory circuit, from sub-Saharan Africa to Scandinavia and again, have been pieced collectively.

It had lengthy been assumed that the fluctuations in Painted Woman numbers, which can fluctuate by greater than two orders of magnitude from one yr to the subsequent within the UK, was in the end pushed by winter climate circumstances in North Africa by impacts on the butterfly’s larval hostplants. Nevertheless, a latest examine, making use of knowledge from the UK Butterfly Monitoring Scheme and comparable knowledge from Catalonia (Spain) and the Netherlands, has revealed a extra complicated image.

The analysis correlated environmental circumstances in numerous components of the Painted Woman’s migratory cycle with abundance. The Normalized Distinction Vegetation Index (NDVI) proved to be probably the most informative environmental variable. NDVI is a measure of vegetation well being and density derived from satellite tv for pc measurements evaluating the reflectance of purple and near-infrared gentle – lively photosynthesis absorbs a lot of the purple gentle so areas with wholesome, dense vegetation progress have a excessive NDVI worth.

Painted Woman, copyright Glyn Sellors, from the surfbirds galleries

Modelling confirmed that there was certainly a optimistic correlation between the NDVI worth of the North African Maghreb area (Morocco, Algeria and Tunisia) in March and April and the variety of Painted Women arriving in Catalonia in spring. Extra stunning, there was additionally a transparent relationship between the winter (January and February) NDVI values of the world south of the Sahara, compromising tropical savannah and semi-arid scrubby grasslands, and European Painted Woman abundance.

It appears that evidently the very giant influxes of butterflies which can be termed ‘Painted Woman years’ rely on excessive vegetation productiveness through the winter dry season in sub-Saharan West Africa. The profitable breeding of the butterfly in these areas, primarily in Mali and Burkina Faso, in beneficial winters is the catalyst for subsequent booms within the Maghreb after which Mediterranean Europe, culminating within the mass arrivals of Painted Women within the UK in early summer time. Alternatively, poor winter breeding circumstances for the butterfly on this identical area might be the primary limitation on European abundance in poorer years.

Whereas earlier research had demonstrated that Painted Women migrated throughout the Sahara, that is the primary to indicate the significance of this desert crossing as a part of the annual cycle due to the pivotal position of winter breeding within the savannahs of West Africa.

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