When Doctor of Philosophy student Vaishali Bhaumik cut open specimens of emigrant butterflies as part of her research, she noticed that the abdomen of the female dispersing butterflies were chock full of eggs. Something did not add up – they did not travel light.

“We collected and dissected specimens of lemon emigrant (Catopsilia pomona) and mottled emigrant (Catopsilia pyranthe) butterflies in Bengaluru during their dispersal,” Bhaumik, a graduate student at the National Centre for Biological Sciences told Mongabay-India. “We found the females to be chock full of eggs.”

“It did not make sense in the context of migration…the eggs added to their energy load during their movement,” Bhaumik pointed out. Lemon yellow and light green, fast-flying emigrants have an affinity for plants of the Cassia spp and Senna spp that are common in bustling Bengaluru.

Butterflies show several different types of movement. The two Catopsilia emigrants that piqued Bhaumik’s curiosity are dispersers: moving over relatively short distances for feeding and breeding over several hours or days. Some can seasonally migrate long distances over hundreds of kilometres, like the well-known monarch butterflies in North America.

So Bhaumik and her advisor Krushnamegh Kunte decided to deepen their understanding of dispersal and migration effects on the morphology (form and structure) and reproduction of butterflies. Apart from the two Catopsilia butterflies, the researchers studied five close relatives who do not disperse. They also studied two milkweed butterflies species, the double branded crow (Euploea sylvester) and dark blue tiger (Tirumala septentrionis), that migrate long distances and reproduce only after migration.

Their findings published in the Royal Society’s journal Biology Letters indicate that dispersals between splintered habitats may put extra stress on egg-carrying females by increasing their flight burdens. This has implications for populations’ persistence in a changing environment, especially in an ever-fragmenting natural world, the researchers said in a press release.

So corridors in urban habitats – with plenty of plants that can help the dispersers boost energy levels when they stopover in transit for a swig of nectar and help them flit through without getting hit by vehicles – can sustain populations.

“We do not fully know how the more insidious effects of rapid urbanisation will affect these species,” Bhaumik said while talking to Mongabay-India. “Dust pollution can reduce host plant quality. Higher temperatures may change their movement patterns, etc.”

“At the very least, well-planned corridors that include an abundance of native nectar plants or mud puddling sites can help dispersers maintain energy levels and not get hit by vehicles too much,” Bhaumik said. “The host plants are also common along roadsides, so tree transplantation during road expansions could help sustain urban populations.”

Shrinking habitats

Catopsilia emigrants are cosmopolitan, and their host plants are pretty common in urban landscapes. However, due to urban development and other reasons, the number of host plants available to butterflies in a given area keeps decreasing, explained Bhaumik.

“So as habitats become smaller and isolated due to human-caused disturbances, these dispersers may be in danger because they have to fly longer and farther away to find host plants, and the females carry the eggs during their movement,” explained Bhaumik.

The city of Bengaluru, which was once regarded as the “Garden City” has lost most of its gardens and green spaces, during its transformation into India’s information technology hub. A recent study has argued that neighbourhood parks, despite their small size, can serve as “stepping stones” that facilitate the movement of birds, butterflies, and insects between larger green areas.

Kalesh Sadasivan, who manages a group of butterfly enthusiasts to track migrations in southern India and helps consolidate records, also batted for having urban preserves to conserve local fauna in expanding cities. He did not participate in the study.

“It is true that fragmented habitats can put extra stress on the population – the adults might not last during their journey,” Sadasivan, founder of the Travancore Natural History Society told Mongabay-India. “What we should do is when we plan gardens and parks, we should go for native nectar plants and host plants instead of going for exotics.”

“We can have urban conservation preserves where you maintain the natural habitat and native plants,” Sadasivan told Mongabay-India.

Stepping stone habitats or networks of urban preserves within reach of mobile species can be an alternative to more extensive and continuous habitats, stresses ecologist Gautam Aditya at the University of Calcutta, who works extensively on butterflies in the urban context in Kolkata.

Commending the National Centre for Biological Sciences research as “convincing,” Aditya, who was not associated with the study, remarked that butterfly diversity is “definitely at stake” if they have to go the extra distance searching for their favourite plants.

“But when you are talking about specific butterfly species and its specific needs, the case might not be exactly similar for different fragmented habitats,” Aditya told Mongabay-India.

Echoing the need to plant relevant plants that are liked by the butterflies instead of exotics, Aditya explained that in the United Kingdom, United States of America, Italy, Japan, studies have shown that the stretch of green habitats will determine the extent of species diversity that can be hosted, but it doesn’t mean the woody trees. “We are talking about shrubs, etc,” he said.

Aditya’s research has shown that the highest butterfly diversity exists in suburban Kolkata (96 species), followed by rural (81 species) and urban (53 species) areas.

“It is not in a forest, neither in an agricultural field nor even in a completely rural context,” Aditya added. “The plants that attract butterflies are comparatively higher in suburban areas than in rural areas.”

A growing body of work across the world, such as France and Mexico, has reinforced the idea of “urban protected areas” or “stepping stone habitats”. A September study based on an analysis of tagged monarch butterflies migrating from the United States to Mexico points back to the importance of creating new habitat (with plenty of the monarch’s hostplants, the milkweeds) to ensure the future of the species’ iconic migratory pattern, said a media release.

Energy required for migration

Butterflies are quick to react to environmental changes. They signal ecosystems’ well-being and can be used to judge conservation effectiveness.

India is an important region for butterfly diversity and conservation and hosts 1,400 species (and many endemics). The number of Indian butterflies amounts to one-fifth of the world’s butterfly species. The Western Ghats and the eastern Himalayas harbour one of the richest assemblages of butterflies in the country.

Appearing in a range of hues and patterns, the winged insects are popular among nature enthusiasts and have caught policymakers’ attention in recent years. States such as Maharashtra and Tamil Nadu have designated specific species as their state butterflies.

Expanding on the variety of butterfly movements, Bhaumik said the milkweed butterflies, ie, the double branded crow and dark blue tiger, migrate annually across southern India in response to the Indian monsoon.

Lemon emigrants disperse before the summer monsoon (March-June), while mottled emigrant dispersals occur post-monsoon (October–November). The five close relatives of emigrants do not migrate or disperse long distances at all in India, but mostly breed where they are born.

During the transit, Bhaumik noticed that these emigrants were moving in large swarms. They kept moving continuously, and “so on the surface, the movement looked similar to the milkweed butterflies”. However, there is a difference in the load that they pack in.

Female milkweed butterflies that migrate annually in a predictable manner do so in a state of reproductive diapause. They completely halt reproductive activity. Therefore they are not burdened with eggs during the long migratory flights, investing all their energies in movement instead.

On the other hand, female emigrants – that move over shorter distances – fly in a state of full reproductive maturity, ie, they carry a heavy load of eggs on their flights.

“They invest a lot of energy in reproductive tissue, which makes the ova [eggs],” explained Bhaumik. “This puts them at a disadvantage while flying by making their abdomen relatively heavier, thus requiring higher energy expenditure during flight.”

Krushnamegh Kunte said that by contrast, males of the species always travel light, whether they are migrating, dispersing, or not.

“Thus, dispersals selectively burden the flight of egg-carrying females,” added Kunte.

A lemon emigrant butterfly. Butterfly dispersals between splintered habitats may put extra stress on egg-carrying females by increasing their flight burdens. Photo credit: Krushnamegh Kunte.

But what could be the reason for the stark contrast in reproductive strategies of migrating versus dispersing butterflies? Researchers point to habitat patch and larval host plant availability.

“The emigrant butterfly dispersals coincide with the seasons when their chief host plants (Cassia spp and Senna spp) experience leaf flushing – when they sprout new leaves,” explained Bhaumik. They experience a major flush just before the monsoon, and after the monsoon, there are continuous minor leaf flushes.”

The butterflies’ population numbers explode during these flushes. “Due to the higher availability of resources, more numbers of butterflies can lay more eggs, of which more survive the larval stages and so on,” she told Mongabay-India. “A sort of upstream cascading system.”

So emigrant caterpillars must exploit this flush while it lasts. These plants occur in randomly available patches, depending on whether other butterflies (and their caterpillars) have already been there, the researchers said.

This uncertainty and intense competition for short-lived larval host plant resources mean that female emigrants have to be ready to lay eggs as soon as they find a suitable host plant patch. So a diapause does not precisely make practical sense in this case because it takes time to break the phase, and this subsequently delays courting, mating, and egg-laying, explained Bhaumik.

On the other hand, the annual migrations of milkweed butterflies occur between known habitat patches, so there is some certainty about finding the right habitat patch at a predictable time concerning the monsoon. So, a reproductive diapause seems to be a feasible option for migrating milkweed butterflies, the authors add.

This article first appeared on Mongabay.