Ecological Correlates of Distribution Change and Range Shift in Butterflies

Increasing evidence suggests that the worldwide biodiversity loss should be attributed to anthropogenic disturbance, particularly habitat loss and climate change. To conserve biodiversity, scientists must identify the factors driving population decline. The ecological traits of a focal species and the traits of species they interact with have previously been correlated with species’ extinction risks and distribution changes. Mattila et al. (2011) analyzed the distribution declines (area of occupancy) and range shifts (extent and direction) of 95 threatened and non-threatened butterfly species in Finland to identify ecological traits that influence species’ distribution changes and range shifts. These traits included larval specificity, resource distribution, dispersal ability, adult habitat breadth, flight period length, body size, and overwintering stage. The results show that the distribution of Finnish butterflies has declined substantially, with the distribution of threatened species’ declining more so than non-threatened species. Additionally, the authors found that the ranges of butterfly species have shifted in both direction and degree, with non-threatened species shifting more so than threatened species. Ecological specialization at the larval or adult stage, as well as poor dispersal ability and large body size, affect both distribution declines and range shifts. These results suggest that highly dispersive generalists will eventually dominate biological communities as result of climate change and habitat fragmentation. However, both non-threatened and threatened species are prone to extinction since both groups possess traits that make them vulnerable to range shifts and distribution declines.—Megan Smith

Mattila, N., Kaitala, V., Komonen, A., Paivinen, J. Kotiaho, J.S., 2011. Ecological Correlates of Distribution Change and Range Shift in Butterflies. Insect Conservation and Diversity. DOI: 10.1111/j.1752-4598.2011.00141.x

Mattila et al. collected Finnish butterfly species data from several scientific papers to assess if threatened and non-threatened species differ in their distribution and range shifts. The authors first categorized the 95 Finland butterfly species as threatened or non-threatened using The Finnish Red List of Species. Butterfly species that were classified as near-threatened, vulnerable, endangered, or critically endangered in the Finnish Red List of Species were classified as by the authors as threatened. The other species were classified as non-threatened.

The authors then determined the distributions, distribution changes, and range shifts of each butterfly species. The distributions were based on the Atlas of Finnish Macrolepidoptera. The distributions are given as the number of occupied 10 km X 10 km grid cells found in the Finnish national coordinate system. The distribution data in the Atlas is categorized into old (before 1988) and new (1988--1997) observations. The authors calculated the distribution changes per butterfly species by finding the difference between the old and new occupied cells, and dividing by the number of old cells. These values were reported as a negative or positive percent, depending on the direction of the distribution change. Range shifts (the movement of the center of the distribution for each species) were measured by taking the difference between the centers of the distributions between the two timescales (old and new). The range shifts were reported in distance (km) and direction (degrees). A figure displaying the direction of range shifts for non-threatened and threatened species and a table reporting the direction of range shifts for all species were constructed. 

Mattila et al. then extracted data from previous scientific papers to determine if the ecological characteristics of Finnish butterflies affect distribution changes and range shifts. First, the authors categorized larval host-plant specificity in Finland into three classes: monophages (feed on a single plant species), oligophages (restricted to one genus of food plants), and polyphages (feed on more than one genus). Monophage data were exclusively used to analyze the effects of resource distribution since their food supply is limited. Plant distribution data were collected from the Atlas of the Distribution of Vascular Plants in Finland and was reported as the number of occupied 10-km grid squares in the Finnish national coordinate system.

Butterfly dispersal information was obtained using a previous paper’s data. Experienced lepidopterists in Finland received questionnaires and were asked to report the dispersal ability index (on a scale from 0 – 10) for each butterfly species. The 0 value represented an extremely immobile species, while the 10 value represented an extremely mobile species. The questionnaires were averaged to obtain the average dispersal ability for each butterfly species.

Additionally, the authors categorized Finnish butterfly habitats into types: uncultivable lands (edge zones next to industrial areas, harbor and storage areas, loading places, un-cropped fields, and other areas that have been impacted by humans), meadows (non-cultivated grasslands), forest edges (roadsides), and bogs. Using these habitat types, Mattila et al. formed an index of adult habitat breadth. This index reports the number of habitat types in which adult butterflies were found. An index value of 1 represents specialist species. Specialist species were confined to one habitat type. An index value of 2 represents intermediate species (those that can inhabit two habitat types), and an index value of 3 represents generalist species. Generalists could occupy three or four habitat types.

The average length of the flight period (days) for each butterfly species was extracted from a previous scientific paper. Wingspan (mm) acted as a measure of butterfly body size because wingspan correlates with body size. Finally, the authors did not include phylogenetic corrections because the information was unavailable, and earlier analyses using preliminary phylogeny showed no change in the results. Two graphs displaying the percent distribution change of species exhibiting larval resource specificity and variation in adult habitat breadth were constructed. Two other graphs demonstrating the effect of body size and dispersal ability on distribution change were also constructed.

Mattila et al. analyzed butterfly distribution changes using standard statistical analyses. They conducted two separate analyses for testing the effects of ecological characteristics (larval specificity and habitat breadth) and life history traits (dispersal ability, body size, length of flight period) on distribution changes since data concerning larval specificity and habitat breadth for 14 northern butterfly species could not be found. The authors analyzed range shifts using circular statistics.

The authors found that the distribution of Finnish butterflies declined on average by 35%. Threatened butterfly species’ distributions declined by 63%, while non-threatened butterfly species’ distributions declined by 26%. The ecological traits driving the distribution declines were larval specificity and adult habitat breadth. In particular, Monophagous butterfly species’ distributions declined more than the distributions of Oligophages and Polyphages. Additionally, the habitat specialists’ and intermediate species’ distributions declined more than the distributions of habitat generalist species, with the largest decline seen in the habitat specialists. Within the habitat specialists, the distributions of species inhabiting semi-natural meadows and bogs declined more than edge specialists. Life history traits that contributed to distribution declines were dispersal ability and body size.

Mattila et al. also found that all butterfly species shifted an average of 22.6 km to the northeast (74.2°). Non-threatened species shifted an average of 30.3 km to the northeast (73.7°), while threatened species only shifted an average of 7.9 km in no consistent direction. The authors asserted that these shifts were caused by changes in climate because Finland’s climatic isotherms move to the northeast, near to where the butterfly species seem to be moving. The directions of the range shifts were not influenced by larval specificity or adult habitat breadth. However, they were influenced by dispersal ability, body size, and flight period length. Species that had better dispersal ability, a smaller body size, and a longer flight period experienced larger range shifts in the direction of the overall, average range shift for the butterfly species.

These results indicate that ecological specialization, whether at the larval or adult stage, contributed to Finnish butterfly species’ distribution declines and range shifts. Specialist species may be incapable of following changes in the environment (i.e. changes in climate), because these species were isolated and confined to small habitat patches. For example, half of the habitat specialist species lived in semi-natural grasslands or natural bogs. These habitats had consistently declined in area since the 1950s-1960s. Therefore, habitat specialization, combined with poor dispersal ability, contributed to the inability of specialists to shift their ranges. Additionally, most specialist species were categorized as threatened species, which may explain why the threatened species did not shift their ranges to the same degree as non-threatened species. Overall, the results suggest that future biological communities will be dominated by generalist species that are efficient dispersers.

Mattila et al.’s findings demonstrate that the ecological traits of Finnish butterfly species influence the distribution changes and range shifts of these species. However, it is imperative to recognize that both threatened and non-threatened species share traits that make them vulnerable to extinction. Therefore, scientists should focus on protecting current, threatened species, as well as species that may be at risk to extinction in the future.