The story begins in 1886, when the first Monarch was sighted in Gibraltar, and in the same decade when specimens of the butterfly’s hostplant Gomphocarpus fruticosus were already being collected near Viana do Castelo in northern Portugal. Sightings of the butterfly in mainland Portugal between 1932 and 1968 were attributed to vagrancy from the Atlantic islands, and confirmed breeding did not emerge until 2003 and 2004 in the Algarve. Yet the genetics tell a different story. Zhan and colleagues showed that Monarchs established in Portugal earlier than in Spain — before the 1960s — meaning the species likely settled along the Portuguese coast long before entomologists noticed. In the Azores, first sightings date to 1864 on Terceira and Flores, with breeding confirmed on Faial in 1994. In Madeira, the butterfly was observed periodically from 1889 but only became established around 1980, having been recorded in large numbers on neighbouring Porto Santo as early as 1955.

None of this colonisation would have been possible without the hostplants. Monarch larvae feed almost exclusively on asclepiads, and outside the Americas the butterfly depends mainly on alien Apocynaceae of the African genus Gomphocarpus, supplemented by the Central American Asclepias curassavica. Gomphocarpus fruticosus, distributed naturally from southern Arabia to eastern and southern Africa, probably reached Iberia and the Portuguese islands in the 17th and 18th centuries, carried by Portuguese traders from their former African colonies and favoured by its ancient exploitation as a textile plant. Asclepias curassavica arrived even earlier, brought to the Iberian Peninsula in the 16th century by Spanish travellers. The invasiveness of these plants — driven by self-compatible pollination, uniparental fruiting and the potential for hybridisation between the two Gomphocarpus species — allowed them to naturalise along Mediterranean-type streams, fallow land, road verges and gardens throughout the region’s lowlands.

To establish where the butterfly and its hostplants actually occur today, the researchers compiled 136 locations for Gomphocarpus species across mainland Portugal from herbaria, GBIF, citizen-science platforms such as Biodiversity4all, Flora-on and Observation.org, and oral reports from naturalists. They then conducted the first countrywide field surveys, assessing the persistence of hostplant patches in 53 of 69 historical locations between 2016 and 2018, and revisiting 56 confirmed patches in 2020 and 2021. Along 992 kilometres of secondary roads, driven at under 40 km/h across roughly 1,350 square kilometres of the southwest littoral belt and western Algarve, they counted plants, recorded patch geometry and habitat type, and watched for flying adults. The islands were surveyed in 2018: Terceira, São Jorge, Pico, Faial, Flores and Corvo in August, São Miguel in October, and Madeira in September.

The results reveal a sharply uneven geography. Gomphocarpus fruticosus was found from the Minho region in the northwest to the Algarve in the south, but the great majority of patches cluster in the western Algarve around Faro district and the sublittoral farmland of Odemira county in Beja district. In the western Algarve the plant occupies two habitat types: the banks of south-flowing rivers and streams descending from the Monchique foothills, and the fruit-growing lowlands, primarily within or at the edges of orange orchards. East of Messines the plant becomes localized and very rare in central-eastern Algarve. Between the Mira and Tejo rivers, none of the recent locations checked still held the plant, suggesting it may have disappeared from most of the region. Only two large stands survive further north: several hundred plants in an abandoned periurban estate near Viana do Castelo, where the species was first recorded in 1886, and over 1,000 plants on the outskirts of Carnaxide in the Lisbon metropolitan area.

Gomphocarpus physocarpus, first reported from Portugal only in 2000, is far scarcer and differently distributed. In the western Algarve it occurs in just six stands, typically of one to 30 plants, either pure or mixed with G. fruticosus and morphologically apparent hybrids. Near Odemira, hundreds of individuals known in 2017 among greenhouses and fallow ground of an intensive farm were reduced to four plants by 2020 after the land was cleared for shifts in production. In the north, the fate of one relatively large patch in an abandoned yard in Aveiro could be traced through photographs posted on iNaturalist in 2016: Google Earth imagery confirmed that a storehouse was built in its place and the plants were eradicated between 2016 and 2018. Monarch observations from that same neighbourhood, repeatedly posted from October 2013 to January 2017, ceased entirely — a small but vivid case study of how hostplant removal eliminates local butterfly populations.

The persistence data are sobering. The second survey revealed that between 48 and 73 percent of hostplant patches had disappeared since the first survey just a few years earlier — 32 of 63 revisited patches in the Algarve and 8 of 11 in the southwest coastal area. Clearing for agriculture and pasture, removal of roadside vegetation and competition from native species such as Rubus ulmifolius and Scirpus holoschoenus drove most losses. Yet the dynamics are not purely negative: five patches expanded markedly, especially in abandoned orange orchards, and resprouting was often observed in cleared stands, indicating strong metapopulation dynamics in both plant species. The plants’ ruderal nature ties them to the grazing cycle — field preparation eliminates native competitors, fallow allows rapid recolonisation, and conversion to pasture once again mechanically removes the plants.

Against this unstable backdrop, the Monarch’s current distribution in mainland Portugal has contracted to the western Algarve and, more locally, the southwest coast along the Mira valley. Monarchs were confirmed in the majority of patches surveyed in the western Algarve, particularly in abandoned orchards and along road verges of the Silves-Messines area, where the profusion of hostplants and their closeness together sustain the population. In the Mira valley, monarchs were very abundant in 2017 when plants numbered in the thousands, but far fewer were seen in 2020 after roughly two-thirds of the patches were cleared for renewed cattle pasture — even so, 30 and 71 individual sightings were recorded at two remaining clusters of about 50 plants each. The formerly large populations of the Seixe valley, where breeding was first confirmed in Portugal, have almost totally vanished due to land conversion, leaving the Mira population very isolated. In central-eastern Algarve, repeated surveys of the largest remaining patches since 2018 detected no butterflies at all; monarchs there are now accidental at most, probably vagrants from the west.

The islands present a paradoxical picture. In Madeira, hostplants were found in only two locations — Asclepias curassavica in Funchal and Gomphocarpus physocarpus at Jardim do Mar — yet Monarchs were common and widespread, recorded with all life stages at multiple sites across the south, east and north of the island, associated with urban green areas and streamside vegetation. In the Azores the situation is reversed: small hostplant patches were observed on four of seven surveyed islands, including abundant Gomphocarpus on Pico, but the butterflies were rare and restricted to São Miguel and Terceira, with an apparently irregular yearly presence and no detections on Pico and Faial where they were previously reported. The authors note that the species’ establishment in Madeira most likely followed several natural colonisation events rather than the speculated deliberate introduction by British naturalists, since Monarchs had been repeatedly observed there since the late 19th century.

The Portuguese Monarch populations are probably genetically unique, the product of a natural colonisation of Macaronesia and the Mediterranean, and therefore irreplaceable — deserving conservation concern despite the allochthonous and potentially invasive nature of their hostplants. The researchers propose practical solutions: a network of sizeable hostplant micro-reserves across the butterfly’s core range in partnership with collaborative landowners, promoted as touristic destinations in the style of Andalusia’s Monarch butterfly route, and the promotion of Asclepias curassavica as an ornamental in suitably watered private gardens. Because wind dispersal of seeds allows the plants to colonise remote favourable areas, and the butterfly’s strong flight ability drives metapopulation dynamics, the observed decline should be viewed cautiously — but the accumulating cues for a rapid disappearance of the hostplants, and with them the butterfly, from large parts of its range should be a matter of concern. For now, Portugal’s Monarchs hang on in the orange groves, their future resting on the fate of a handful of African plants in a changing agricultural landscape.

Subject of Research: History of colonisation and updated distribution of the Monarch butterfly Danaus plexippus (Linnaeus, 1758) and its hostplants in mainland Portugal, Azores and Madeira

Article Title: History of colonisation and updated distribution of the Monarch butterfly Danaus plexippus (Linnaeus, 1758) and its hostplants in mainland Portugal, Azores and Madeira

Article References: Palma, L., Vasconcelos, S., Palmeirim, A. F., & Cancela, J. P. (2023). History of colonisation and updated distribution of the Monarch butterfly Danaus plexippus (Linnaeus, 1758) and its hostplants in mainland Portugal, Azores and Madeira. Nota Lepidopterologica, 46, 83-101. https://doi.org/10.3897/nl.46.89665

Image Credits: AI Generated

DOI: 10.3897/nl.46.89665

Keywords: History, colonisation, updated, distribution, Monarch, butterfly, Danaus, plexippus, Linnaeus, hostplants, mainland, Portugal