Course:CONS200/Butterfly and ant interactions in lowland grasslands in the United Kingdom

The grasslands of the United Kingdom are host to a number of symbiotic relationships between the butterflies of the Lycaenidae and Nymphalidae families and various ant species. Through the early 1900s the extinction rate of invertebrates in the United Kingdom, namely butterflies, has seen a dramatic increase over the extinction rate of vertebrates. A poor understanding of the symbiotic relationship between butterflies and ants has been cited as the reason for their improper conservation.

The complex symbiotic interactions between the two groups varies species by species, from facultative to obligate, mutualistic to parasitic and are an integral part of understanding how to better conserve the butterflies of the area. Although some butterfly species such as the Maculinea arion have experienced severe decline in numbers over the past centuries, successful research and management has allowed the species to change from “vulnerable” to “near-threatened” on the International Union for Conservation of Nature’s Red List of Threatened Species.

Types of interactions

Mutualism

Lycaenid caterpillars use many different strategies to minimize their investment in their interaction with ants. After the initial phase of an encounter, secretion rates of nectar which serves as a reward for ants rapidly decrease as soon as a stable caterpillar-ant association has been established. This process, termed 'enticement and binding', shows that manipulative elements are involved in these facultative mutualisms: caterpillars substitute odour for food rewards. Furthermore, caterpillars adjust their secretion rates according to the number of ants present, the stability of their ant-association, their developmental status and the resources available from their food plants. Caterpillars of the same lycaenid species display a remarkable variability in their profitability as food resource for ants, and this is often reflected in the ant's reaction. Feeding caterpillars of Glaucopsyche lygdamus, P. icarus or Jalmenus evagoras nutrient poor diets, attracted fewer ants than those on a more nutrient rich diet. From the ant’s point of view tending to a caterpillar instead of consuming it is profitable only if the ant colony can derive a larger reward from harvesting caterpillar secretions than from consuming all caterpillars that the worker ants encounter. The nutritive benefits ants acquire from tending lycaenid larvae apparently do not prove to be significantly more than those they could gain from consuming the larvae. However, more significant benefits could be obtained on the long term. Through tending, ants can decrease caterpillar mortality, increasing population size, in comparison to nontended prey species.

Commensalism

The wood ant, Formica rufa engages in a form commensalism with a butterfly of the Nymphalidae family, Mellicta athalia, by spreading the seeds of Melampyrum pratense. M. pratense is an essential plant for the M. athalia as it is the sole location for breeding of the species in a freshly cut woodland habitat. Along with being an essential propagator of M. pratense seeds, F. rufa does not predate on the larvae of M. athalia despite being an abundant presence in all M. athalia habitats. As a result, M. athalia receives the benefit of habitat development by the wood ants but the wood ants receive no positives or negatives in this interaction.

Parasitism

These butterflies lay eggs on low lying foliage, once hatched the larvae feed on leaves until similar size to ant grub. Larvae will then drop to the ground and, mimicking not only the appearance but also the smell of a Myrmica grub through chemical secretions, are picked up by worker ants. Once inside the nest, they continue to imitate ant larvae with chemical signals to ensure that they are fed rather than expelled. However, just looking and smelling right cannot account for the observation that some butterfly larvae are not only treated like ordinary ant larvae but are fed first and fed most, even when the food is scarce. Most recent research shows that larvae also practice sound mimicry, imitating the distinctive sounds made by a queen rather than worker ant larvae this ensures that Maculinea larvae receive royal treatment from their Myrmica hosts. If the mimicry isin't performed properly and an ant doesn’t recognize a caterpillar as one of its own it will eat it. Successfully adopted caterpillars negatively impact for the ant colonies, as ants may neglect their own young in favour of the intruders. But the ants are fighting back. The ant grub are evolving as a result of being parasitized, It’s an ongoing evolutionary arms race. The butterflies are in turn adapting to the ever evolving ants. The more ants there are with a specific chemical profile, the more heavily they were infected, showing that the butterflies had adapted to take advantage of the prevalent ants. This is a perfect example of a co-evolution between two species.

Evidence of the problem

Overall butterfly population decline

The long-term UK butterfly population trends, which run from 1976-2014 for most species, show that 57% of the 56 species decreased in abundance and 43% of species increased. Of the 32 species that declined in numbers over the period, 20 showed statistically significant trends, while of the 24 butterflies that increased, 13 showed significant trends. Thus, of 33 species with statistically significant UKBMS trends, 61% decreased and 39% increased. The occupancy model analysis of the BNM distribution data presents an even more clear-cut picture.

Overall, 70% of species (40 in total) had decreased and 30% (17 species) increased in occurrence over the period 1976-2014. All but two of these trends are statistically robust. Taken together, these two separate lines of evidence demonstrate that 76% (44 out of the 58 species for which at least one type of trend was calculated) of the UK’s resident and regular migrant butterfly species declined in either population or occurrence (or both) over the past four decades.

Maculinea arion decline

Throughout the early 20th century, the population of M. arion in the U.K. was in steady decline due to a decline in rabbit population. The rabbits were a key player in maintaining the correct grass length for the primary host ant, M. sabuleti. Without the constant grazing of rabbits, all but one (farmed) M. arion site became too overgrown for the primary host ant and M. arion populations declined as a result. Early conservationist’s attributed population decline of species of Butterfly in the U.K. to over collecting. Fences were erected as part of a conservation effort to keep out collectors, however this only served to further exclude rabbits, causing further loss of suitable habitat for M. sabuleti. Rapid extinction of M. arion followed and by the time the cause of this rapid population decline was evident, the last U.K. population had gone extinct.

Mellicta athalia decline

Mellicta athalia is another species of butterfly undergoing heavy conservation efforts. M. athalia lives in a few distinct habitats which undergo rapid change into unsuitable environments without appropriate management. The habitat of M. athalia involves very little ground cover, typical of a recently deforested area. It is estimated that these environments, such as recently cut woodlands are only suitable for M. athalia for 5-10 years before forest ground cover exceeds the 20% that is suitable for the species. Along with this, changes in forestry practices towards the growing of tall conifers in plantation forests in England have resulted in severe habitat loss for M. athalia. Wood land ants such as the Formia rufa were once thought to be a major player in the decline of M. athalia, however it is now understood that improper management of M. athalia habitat and habitat loss has lead to a severe population decline. A survey of M. athalia populations and habitats in 1980 identified the species as the most endangered butterfly in Britain at the time.

Repopulation

Succesful reintroduction of M. arion from Öland, Sweden occurred in three U.K. sites from 1983 to 1992. The M. arion individuals that were reintroduced were similar enough to their extinct counterparts to successfully establish colonies in the UK. A fourth reintroduction site failed, presumably due to the difference in climate. By 2008, the butterfly population had successfully re-established 25 colonies containing 1000 to 5000 adults per hectare. The results of M. arion reintroduction in the U.K. have been reflected in similar smaller-scale management across Europe and because of it M. arion’s listing has been changed from “vulnerable” to “near-threatened”. M. arion was among three U.K. butterfly species to meet the Convention of Biological Diversity’s 2010 target to reverse decline of insect species'.

Since 1991, appropriate management of the sensitive habitat of Mellicta athalia has resulted in maintaining and increasing populations in certain colonies. Proper understanding of the ecological requirements of M. athalia has been cited as the reason for their recovery in certain areas. The management regimes that are necessary in maintaining M. athalia populations involves continuous grazing and cutting efforts that provide an open woodland habitat that is required for the species. As ground cover is the essential ingredient in the suitability of the environment, prescribed burning during the winter months has been used as a successful management tool for ensuring M. athalia habitats.As these habitats are quick to shade over, it is necessary for continual management to take place in 4 to 5 year intervals. Large scale nature reserves are now thought to be a viable option in preserving the species given large land area and extensive management. Attempts to reintroduce M. athalia into areas outside their current colonies has not been successful.

Concerns for the Future

Following its significant decline and extinction in the U.K. in 1972, Maculinea Arion has received more scientific interest and conservation effort in the previous 20 years than any other insect species in Europe . As a result of the knowledge gained from studies and conservation efforts, reintroduction was successful in the UK. However, M. arion are still declining and endangered in the rest of Europe. Butterflies, like all species have limits of tolerance to abiotic factors which determine their geographical range. Populations in different parts of the geographical range such as outskirt and centre populations often vary in several aspects, including their ecological requirements and preferences. M. arion populates a variety of habitats using various food plants and host ants throughout its European range, which results in complications in its conservation. Projections of climate-change scenarios predict that many M. arion populations will disappear from Europe over the next half century and southern limit populations of the species are the most threatened.

Carl Bols

A coppiced woodland habitat, ideal for Mellicta athalia

Although there is now a good understanding of the required ecology for Mellicta athalia, there are a few concerns towards the resources required for the future conservation of the species. M. athalia currently inhabits exclusively man-made habitats that undergo rapid change into unsuitability through ecological succession. As a result of this, the concerns towards the future protection of the species involves the commitment to continual cutting, grazing, and burning of the habitats to ensure the species can thrive. This may be a costly effort as it requires continuous habitat management over a relatively short time period. Furthermore, there were efforts in the early 19th century to reintroduce M. athalia, all of which were unsuccessful. These efforts revealed that although there may be suitable habitats for M. athalia outside of its current range the species is too slow at migrating to colonize these areas naturally. Future efforts to reintroduce the species may be possible, but would still require the same costly management of the habitat to avoid overgrowth of grasses and canopy. Another concern is the change in forestry practices in the southern England.There is a movement from coppicing, which creates the proper open habitat for M. athalia, to high forest plantations that shade out the forest floor and create an inhospitable environment for the butterfly species. Should this change in forest practices continue, M. athalia populations will see further declines due to loss of habitat.

Recommendations

Many efforts are already in place towards taking action in the conservation of butterfly species in the United Kingdom, however much larger measures are still needed to be taken in order to maintain and increase biodiversity. A key in ensuring the safety of these butterfly species is to protect the landscapes in which they live. Protected areas already in place in the UK are demonstrating how advantageous it is for butterfly population regeneration to create these undisturbed natural landscapes. Within these intact heterogeneous ecosystems, species co-evolutions, for example the butterfly and ant relationship, remain undisturbed, allowing further interactions. Protected area climate's are also more stable than unprotected and transform at an adaptable rate for butterflies.

Bringing up the topic of climate change, a rising question is how and can butterflies adapt to the rapidly transforming landscapes and shifting climates outside of protected areas? Global butterfly populations are currently shifting in their sizes as well as historic geographical ranges. Monitoring population trends in Great Britain has show that many southern butterfly species are extending their ranges further north, providing evidence that change in climate is altering butterfly habits. While warming temperatures may seem beneficial for butterfly habitats, research put together by 'Butterfly Conservation,' a conservation organization devoted to the protection of butterfly preservation in the United Kingdom, suggests that if the butterfly populations don't increase in size the favourable climate shift won't have much of an impact. As climate change proves to be a key player in the alteration of butterfly population distribution and trends, it is exceptionally important that conservationist and other scientists study how butterfly species react to climate change.

Monitoring the trends among butterfly populations as well as documenting distribution of populations at local scales will aid greatly in recovering future butterfly populations. By accumulating efficient data on current species and populations, it is establishing baseline data to aid in future conservation efforts. Collecting such data is extremely important as it gives light to trends and new habits and dependence of species. If every State began monitoring populations and distribution of populations, this data could be collaboratively shared globally between nations creating a complex worldwide data analysis of butterfly species populations. Sharing data is recommended because it provides every participating party with increased data analyses and gives greater insight into trends.

A highly recommended tactic in conservation is raising awareness at local landscape levels. Landscape-scope conservation is beneficial because it keeps local people educated about issues and ensures long-lasting protection of species if the local people are active and concerned about issues. Conservation at a landscape magnitude is also advantageous because by protecting entire landscapes in which a species inhabits, one is ensuring that the entire habitat of the species remains intact and viable for life. These local-scale conservation efforts should be promoted all around the globe.

Conclusion

For the passed four decades the United Kingdom has experienced a severe decline to their butterfly populations. This is thought to be due to destruction of it's natural landscapes, additionally affecting the species' relationships with various ant species. The imminent pressures of changing climates is also affecting the distribution and range of multiple local butterfly species. The decline of Maculinea arion as well as Mellicta athalia species occurred on account of the lack of understanding that conservationist and scientists had around the butterfly's dependencies on their relationship with various ant species. The uncertainty around butterfly ecology and species interactions needs to be addressed. The decline in butterfly species is a major issue to conservation as butterflies play a significant roles in pollinating flowering plants as they collect their nectar. Without pollination done by butterflies many flowering plants could be extirpated. Butterflies also serve as a central source of nutrients for many bird and smaller animal species, remaining a key part of the food chain in the United Kingdom's grasslands. As the decline of butterflies persists, so does the threat of trophic collapse. What is needed now is more people at local and global scales to be educated and raise awareness on the importance of protecting butterfly species. By preserving and aiding repopulation, we are not only helping the butterfly species, but keeping entire ecosystem functioning and intact. The implementation of more protected areas, investing more time in studying and understand butterfly ecology and their interactions with the landscapes, and getting local people all over the world involved in contributing to these conversation efforts are all necessary actions in ensuring the protection and resurrection of butterfly species not only in the United Kingdom, but around the globe.

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