Jeremy A. Thomas

Rapid responses of British butterflies to opposing forces of climate and habitat change

Habitat degradation and climate change are thought to be altering the distributions and abundances of animals and plants throughout the world, but their combined impacts have not been assessed for any species assemblage. Here we evaluated changes in the distribution sizes and abundances of 46 species of butterflies that approach their northern climatic range margins in Britain—where changes in climate and habitat are opposing forces. These insects might be expected to have responded positively to climate warming over the past 30 years, yet three-quarters of them declined: negative responses to habitat loss have outweighed positive responses to climate warming. Half of the species that were mobile and habitat generalists increased their distribution sites over this period (consistent with a climate explanation), whereas the other generalists and 89% of the habitat specialists declined in distribution size (consistent with habitat limitation). Changes in population abundances closely matched changes in distributions. The dual forces of habitat modification and climate change are likely to cause specialists to decline, leaving biological communities with reduced numbers of species and dominated by mobile and widespread habitat generalists.

Monitoring change in the abundance and distribution of insects using butterflies and other indicator groups

Conservative estimates suggest that 50–90% of the existing insect species on Earth have still to be discovered, yet the named insects alone comprise more than half of all known species of organism. With such poor baseline knowledge, monitoring change in insect diversity poses a formidable challenge to scientists and most attempts to generalize involve large extrapolations from a few well-studied taxa. Butterflies are often the only group for which accurate measures of change can be obtained. Four schemes, used successfully to assess change in British butterflies, that are increasingly being applied across the world are described: Red Data Books (RDB) list the best judgements of experts of the conservation status of species in their field of expertise; mapping schemes plot the changing distributions of species at scales of 1–100 km2; transect monitoring schemes generate time series of changes in abundance in sample populations of species on fixed sites across the UK; and occasional surveys measure the number, boundaries and size of all populations of a (usually RDB) species at intervals of 10–30 years. All schemes describe consistent patterns of change, but if they are to be more generally useful, it is important to understand how well butterflies are representative of other taxa. Comparisons with similarly measured changes in native bird and plant species suggest that butterflies have declined more rapidly that these other groups in Britain; it should soon be possible to test whether this pattern exists elsewhere. It is also demonstrated that extinction rates in British butterflies are similar to those in a range of other insect groups over 100 years once recording bias is accounted for, although probably lower than in aquatic or parasitic taxa. It is concluded that butterflies represent adequate indicators of change for many terrestrial insect groups, but recommended that similar schemes be extended to other popular groups, especially dragonflies, bumblebees, hoverflies and ants. Given institutional backing, similar projects could be employed internationally and standardized. Finally, a range of schemes designed to monitor change in communities of aquatic macro-invertebrates is described. Although designed to use invertebrates as a bio-indicator of water quality for human use, these programmes could be extended to monitor the 2010 biodiversity targets of the World Summit on Sustainable Development.

Distributions of occupied and vacant butterfly habitats in fragmented landscapes

SummaryWe found several rare UK butterflies to be restricted to relatively large and non-isolated habitat patches, while small patches and those that are isolated from population sources remain vacant. These patterns of occurrence are generated by the dynamic processes of local extinction and colonization. Habitat patches act as terrestrial archipelagos in which long-term population persistence, and hence effective long-term conservation, rely on networks of suitable habitats, sufficiently close to allow natural dispersal.

Host specificity among Maculinea butterflies in Myrmica ant nests

SummaryEcological studies have been made of all 5 European species of Maculinea. These confirm that M. nausithous and M. rebeli live underground in Myrmica ant nests for 10 months of the year, as has long been known for the other 3 species. The main discovery was that each Maculinea species depends on a single, and different, host species of Myrmica. This specificity contradicts previous papers and scientific reviews of the relationship between Maculinea and ants. Therefore, early records are re-examined and 3 reasons are given to explain why most are misleading when applied to wild populations. Dependence on a single, rather than any, species of Myrmica explains why Maculinea populations exist in only a small minority of biotopes where their foodplants and Myrmica ants abound. It also explains the puzzling disappearance of Maculinea populations from apparently suitable sites. The discovery that M. alcon and M. rebeli depend on separate species of Myrmica that are not even closely related strengthens the argument that these butterflies are good species.

Successful Conservation of a Threatened Maculinea Butterfly

Bringing Back the Large Blue Flagship endangered species, such as the Large Blue butterfly have driven conservation programs worldwide. However, the Large Blue butterfly (Maculinea arion) became extinct in the United Kingdom. The apparent driver of this extinction was a complex set of events documented by Thomas et al. (p. 80, published online 18 June; see the Perspective by Settele and Kühn). Life-tables and modeling demonstrate how ecological changes, affecting multiple species, cascaded to negatively impact Large Blue populations. When the changes that cause these extinctions were addressed, reintroduction efforts proved successful. Other insects have experienced similar declines and, hopefully, on sites where their known resources remain abundant, a similar approach may be applied. Prediction of population dynamics in relation to habitat requirements has led to a conservation success in the UK. Globally threatened butterflies have prompted research-based approaches to insect conservation. Here, we describe the reversal of the decline of Maculinea arion (Large Blue), a charismatic specialist whose larvae parasitize Myrmica ant societies. M. arion larvae were more specialized than had previously been recognized, being adapted to a single host-ant species that inhabits a narrow niche in grassland. Inconspicuous changes in grazing and vegetation structure caused host ants to be replaced by similar but unsuitable congeners, explaining the extinction of European Maculinea populations. Once this problem was identified, UK ecosystems were perturbed appropriately, validating models predicting the recovery and subsequent dynamics of the butterfly and ants at 78 sites. The successful identification and reversal of the problem provides a paradigm for other insect conservation projects.

The ecology of Myrmica ants in relation to the conservation of Maculinea butterflies

Maculinea butterflies in Europe, and probably most of Asia, are host specific social parasites of various species of Myrmica ants. The latest summary of field data showing the pattern of host specificity by Maculinea is presented. Myrmica ants have been well studied in the laboratory but much less is known about the ecology of their natural populations. While the former is important in understanding the adaptive evolution of Maculinea larval behaviours, the latter is of more practical importance to conservationists charged with the protection of specific populations of Maculinea. The current knowledge of habitat partition, colony growth and colony reproduction within communities of Myrmica ants is summarized in relation to the ecology of Maculinea species. Concepts used in current population simulation models are explained. A key concept is the idea that community structure (both number of species and size and abundance of nests) is controlled by the quantity and quality of suitable nest sites. Some advice is given to conservationists who might need to manipulate Myrmica ant populations in order to maintain a robust population of a Maculinea species.

Protected areas facilitate species’ range expansions

The benefits of protected areas (PAs) for biodiversity have been questioned in the context of climate change because PAs are static, whereas the distributions of species are dynamic. Current PAs may, however, continue to be important if they provide suitable locations for species to colonize at their leading-edge range boundaries, thereby enabling spread into new regions. Here, we present an empirical assessment of the role of PAs as targets for colonization during recent range expansions. Records from intensive surveys revealed that seven bird and butterfly species have colonized PAs 4.2 (median) times more frequently than expected from the availability of PAs in the landscapes colonized. Records of an additional 256 invertebrate species with less-intensive surveys supported these findings and showed that 98% of species are disproportionately associated with PAs in newly colonized parts of their ranges. Although colonizing species favor PAs in general, species vary greatly in their reliance on PAs, reflecting differences in the dependence of individual species on particular habitats and other conditions that are available only in PAs. These findings highlight the importance of current PAs for facilitating range expansions and show that a small subset of the landscape receives a high proportion of colonizations by range-expanding species.

Evolutionary biology: Butterfly mimics of ants

Large blue butterflies are notable for their rarity and ability to dupe ants, and they are endangered. A genetic reconstruction of how social parasitism evolved among them will overturn conservation priorities.

The challenge of conserving grassland insects at the margins of their range in Europe

Abstract Insects have experienced higher rates of decline than other popular taxa in the calcareous grasslands of Europe. This paper reviews reasons why insects are so vulnerable to environmental change, particularly at the edges of their range. Case studies of the ant Myrmica sabuleti and the butterflies Polyommatus bellargus and Thymelicus acteon are used to illustrate the mechanisms responsible for this vulnerability and the need for different management regimes to maintain species’ niches in different parts of their range.

Queen ants make distinctive sounds that are mimicked by a butterfly social parasite.

Ants dominate terrestrial ecosystems through living in complex societies whose organization is maintained via sophisticated communication systems. The role of acoustics in information exchange may be underestimated. We show that Myrmica schencki queens generate distinctive sounds that elicit increased benevolent responses from workers, reinforcing their supreme social status. Although fiercely defended by workers, ant societies are infiltrated by specialist insects that exploit their resources. Sounds produced by pupae and larvae of the parasitic butterfly Maculinea rebeli mimic those of queen ants more closely than those of workers, enabling them to achieve high status within ant societies. We conclude that acoustical mimicry provides another route for infiltration for ∼10,000 species of social parasites that cheat ant societies.