On Madeira, the success of the speckled wood butterfly (Pararge aegeria) has coincided with declining populations of the Madeiran speckled wood (Pararge xiphia): is the colonist to blame?

Abstract

The endemic Madeiran speckled wood butterfly (Pararge xiphia) was once abundant and widely distributed on the island of Madeira. Declining populations and a range contraction have coincided with the colonisation of Madeira by the speckled wood (Pararge aegeria). The colonist has expanded its range and increased in abundance, whereas the opposite is true for P. xiphia, where a decline in occurrence and abundance resulted in its current endangered designation. During 3 weeks in July and August of 2018, we assessed the relative abundance, distribution and habitat preferences of the Pararge species on Madeira on ten transects, at a range of locations and altitudes, recording all individuals of both species. In addition, we scored percentage cover of several habitat variables per 5-min periods of recording on each transect. Our findings support an ongoing decline in relative abundance of P. xiphia, which accounted for 25% of Pararge individuals in 2018 compared with historical date from 1986, when P. xiphia represented 78% of the Pararge population. The endemic species was associated with the native laurel forest and P. aegeria with non-native planted forests and agriculture. In addition, we found evidence for an altitudinal range-shift ‘uphill’ which was particularly evident in P. aegeria. Causal reasons for the decline of P. xiphia are difficult to pinpoint; however, we surmise that one or more, or a combination of factors ranging from interspecific competition, habitat loss and disturbance resulting from recent environmental events and parasitism may be accountable.