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.

Appendix 1: Summary of transect information and abundance of Pararge species

Transect	Dates surveyed	Altitude (m)	Length (km)	Repeats	Time (min)	Mean number per 5-min period
						P. xiphia	P. aegeria
1. Ribeiro Frio: The transect begins at the EN103 road and follows the Levado do Furado for approximately 1 km to the ‘Balcões’ viewpoint. This is a mid-altitude area with both native laurel forest and some agriculture and non-native ornamental planting along the path	25/07 29/07	800	1.0	2	100	1.3	1.45
2. Fajã do Noguiera: Starting approximately 1 km along the road leading from the EN103 road to the Fajã do Nogueira hydroelectric power station; the transect follows the road past the power station and onto the Levado do Pico Ruivo. Primarily laurel forest and tree heather with some non-native vegetation	15/07	500–800	4.5	3	160	1.44	5.59
3. Queimadas: This 7 km transect begins at the Casa das Queimadas and follows the path alongside the Levado do Caldeirão Verde to the Caldeirão Verde waterfall. This is a relatively high-altitude transect of predominantly laurel forest and tree heather but with substantial amounts of pine	19/07 20/07	900	7.0	3	150	0.2	0.1
4. Santa: This is the only transect on the western side of the island, despite being a low altitude area this site is surrounded by native laurel forest. The transect starts at the Levada da Ribeira da Janela above Porto Moniz and follows the levada through pine, eucalyptus and laurel forest	16/07	400	4.2	1	75	0.93	4.06
5. Boca do Risco: Starting at the beginning of the Vereda da Boca do Risco walk and leading through the Ribeira Seca Valley to the Boca do Risco viewpoint. This area is dominated by agriculture and abandoned terraces; the transect proceeds through planted pine forest with some laurel and tree heather	13/07 17/07	300	2.1	4	170	0.15	4.03
6. Camacha: Close to the town of Camacha, on the southern side of the island the transect follows the final section of the Levada dos Tornos towards Gaula. The transect passes through areas of agriculture, as well as planted pine and eucalyptus plantations	18/07	600	6.2	2	115	0.09	5.39
7. Portela: The beginning of the transect is close to the bus stop at Ribeiro Frio. Starting at the ‘PR10 Portela’ signpost, the transect follows the Levada do Furado towards the Lamaceiros water-house. The dominant vegetation is the native laurisilva forest with some planted pine	29/07	800	3	1	80	0.19	0.25
8. Poiso: This high altitude transect begins at the Casa de Abrigo do Poiso and follows the path towards Pico do Arieiro for approximately 3 km. The start of the transect is dominated by housing with the latter part of the transect being mainly planted pine forest	31/07	1100	3	1	35	1.18	0.14
9. Curral das Freiras: Starting at the bus stop outside the Eira do Serrado hotel, a few kilometres north of Câmara de Lobos, this transect follows a steep descent into the town of Curral das Freiros. This transect spans a range of altitudes and consists mainly of laurel and tree heather vegetation, but with some eucalyptus, pine and agriculture	27/07	900–1100	3.5	1	60	5.42	1
10. Palheiro Gardens: Located on the hills to the east of Funchal, these gardens are located in the grounds of the Quinta do Palheiro. A short transect around the gardens passes through many exotic, non-native plants, as well as pine and eucalyptus trees	24/07	500	2	1	35	0	1.43