Population dynamics – Distribution of a species of butterfly predicted using geometric variables

The research, published in the latest edition of the magazine Oecologia (Ecology), has made it possible to determine the relative importance of the area which the butterfly Iolana iolas inhabits, as well as the connectivity between the different areas of the habitat (at a maximum distance of two kilometres from each other) in a network of 75 patches (population centres) situated in the south of the province of Madrid (Chinchón).

Three researchers from the King Juan Carlos University in Madrid have therefore presented a line of study on the spatial structure and dynamics of the butterfly different to other quality characteristics in the habitat.

The results of the study, prepared between 2003 and 2006, show that in the Iolana iolas and other species of monophagous butterflies (with clearly delimited habitat requirements), it is possible to predict the dynamics of their populations from the geometric variables of patches, “since most of the characteristics of the habitat are related to the patch area”, explained Sonia García Rabasa, the main author of the article, to SINC.

Researchers have concentrated on factors that determine the distribution, extinction and density of Iolana iolas populations in relation to the habitat patches formed by an endemic plant of the Iberian Peninsular, and host to butterflies, the Colutea hispanica (leguminous plant).

“The study may be of major importance for designing conservation plans for the species, and shows the relevance of geometric characteristics compared to other habitat quality properties which are frequently more difficult to achieve in field conditions”, commented the researcher.

To predict the distribution patterns it has also been important to study the synchrony between populations: “The spatial study and population dynamics of butterflies may also affect the incidence and probability of the extinction of the species”, García Rabasa pointed out.

Study of population dynamics

The fragmentation or division of the species’ habitat areas is one of the main causes of the decline of the fauna and flora, as it produces an increase in local extinctions and a reduction in recolonisation rates. Knowing the situation of the Iolana iolas, which depends on plant extension for survival, scientists have shown the effects of the characteristics of the habitat (topographic factors, microclimate, amount of resources) and the standard geometric measurements (area and connectivity) of the patches.

During the four years of the study, “the extinction, density and occupation rate of butterflies in different areas was purely determined by their size, without affecting the remoteness between the areas or the quality measurements of the habitat”, added the researcher.

Consequently, the scientists only recorded nine extinctions in nine different patches and 15 colonisations in 13 patches over four years. According to the study, all the extinctions were linked to patches or areas with a low or poor production of fruit which would “inevitably” lead to a failure in the local recruitment of butterflies.

As a result, the smallest habitat patches had lower butterfly population sizes and higher rates of extinction, irrespective of their quality of habitat. The population sizes were small, with only tens of individuals in the “best” years, and were even smaller in smaller patches, with fewer than ten individuals.

Variation in the amount of resources, in this case the production of fruit from the host plant (Colutea hispanica), from which the larvae feed, and changes in population density during these years have made it possible to detect a high level of synchrony between different habitat fragments. This high level of synchrony is a risk to populations which might experience mass extinction in all areas experiencing adverse conditions, such as the effects of climate change.