I have been observing butterflies as long as I can remember. Little did I know that I would go on to do my Ph.D. using butterflies as my model system. For my research I focused on how distance between habitat patches and size of the habitat fragments affects butterflies in the lesser-studied grassland-forest complexes in the northern Western Ghats on Goa-Maharashtra-Karnataka borders.
This beautiful landscape comprises of different habitat patches – moist deciduous forests, ridge grasslands and laterite plateau grasslands. Each habitat supports a unique type of vegetation. As these patches support a rich diversity of plants, they in turn support a rich diversity of birds, mammals and around 250 species of butterflies. The blooming laterite plateau grasslands are a nectar bonanza for butterflies while the ridge grasslands and moist deciduous forests have butterfly host-plants, on which the butterfly larvae feed on.
The idea behind my research was to collect data on individual species of these butterflies and classify them into a generalist-specialist continuum based on their abundance and their presence-absence in the different habitat patches. My hypothesis was very simple – if a butterfly is a generalist, it will be widely distributed and present in most of the habitat patches, whereas, if it is a specialist, it will be found only in a particular kind of a habitat and might be restricted only to a few habitat patches. Even though it sounds very trivial you would be surprised to know that there isn’t enough natural history data or quantitative information available at a species-specific level for Indian butterflies. And it was a huge task to get information on the two-hundred and fifty plus butterflies that one finds in the Western Ghats. I managed to get enough data on almost fifty per cent of species.
Once we know if a species is generalist or specialist, we can move on to ask more interesting questions like how does the distance between habitat patches affect the movement of these butterflies in the landscape? The non-habitat between the habitat patches is popularly referred to as ‘matrix’ in scientific literature. How does this matrix influence the generalist-specialist species? Do specialist species find it difficult to overcome these barriers as compared to generalist species? What is the effect of habitat area on the butterfly species? Do generalist species require larger habitat patches as compared to the specialists? Also, how do species physiological traits like wingspan, body size, and life history traits like diet breadth influence their distribution in the landscape? I exactly tried answering these questions once I had information on species ecological traits i.e. the generalist-specialist level of the species that I observed during my study in the Western Ghats.
The results that finally emerged from walking in all those forest patches, getting sun strokes and being bitten by leeches and ticks were very meaningful and probably needed for the conservation of the butterfly species found in the Western Ghats. As one would expect, there were more generalist species as compared to specialist species in the landscape. Species such as Common Emigrant (Catopsilia pomona), Common Crow (Euploea core), Glassy Tiger (Parantica aglea) were present throughout the landscape in almost 70% of the habitat patches. Species such as Common Grass Yellow (Eurema hecabe) and Common Fourring (Ypthima huebneri), specialists at the habitat level, were abundant only in grassland habitat patches and were almost absent in moist deciduous forest patches.
This is an important observation as it shows us that forests may be important for some species, whereas grassland habitats support a completely different set of species. Hence, all the habitats in a landscape are important, not just forests or grasslands alone, but both of them together. Generalist and specialist species had very interesting diet breadths too; generalist species were mostly polyphagous i.e. the larvae of these butterflies fed on multiple species of plants belonging to different families. Specialist species were mostly monophagous, the larvae of these species fed only on a few plant species belonging to the same plant family. Detailed information on the complete list of nectar plants that the adult butterflies use to feed on is still lacking.
Another interesting idea that I tested was the influence of matrix on the movement of these butterflies in the habitat. Are all butterflies able to move equally well across all the different types of habitats? One would expect that butterflies are able to fly; why would they have any trouble moving from one habitat patch to another, right? What I found was that some species, and these are mostly generalist species, like Glassy Tiger or Common Crow can move across habitats more easily as compared to specialist species. The Common Fourring, a specialist species, is highly affected by the surrounding non-habitat. Their abundance drops starkly as one moves from habitat to non-habitat, indicating that specialist species are limited by dispersal capabilities. Here, I would like to highlight the fact that the abilities of a species to move from one habitat to another and how a species perceives the non-habitat are as important as the features of the landscape such as how fragmented the habitat is or the area of the habitat. Species that can move well across habitats will do well in fragmented habitats but species that show high resistance to the surrounding matrix will be the species of conservation concern.
I also found that specialist butterflies found in the Western Ghats are affected by the distance between habitat patches. Specialist species thrived in habitat patches that were located closer to each other. This also relates to their ability to move across habitat matrix. Also, the size of the habitat patch is very important to specialist butterflies; their populations did better in larger habitat patches as compared to smaller habitat patches. This information is very important while designing conservation studies. How a species copes with habitat fragmentation or the size of a habitat patch sometimes has its roots in the evolutionary history of the species. For example, we observed that since specialist species were mostly monophagous, the distribution of these plants therefore might have affected the distribution of these species. For example, the Common Fourring lays its eggs on a particular variety of grass, therefore the Common Fourring was mostly seen in the grassland habitat. This could be one of the explanations why this particular butterfly species was highly abundant in the grassland habitat. Physiological traits such as body-size also affect dispersal of animals in the habitat. Generally, larger butterflies have stronger wings and can cover longer distances. In my study I saw that generalist butterflies were usually bigger in size, had good flight and occupied more number of habitat patches. Specialist species on the other hand were restricted to only a few habitat patches. Thus, the distribution of these butterflies in the landscape can be attributed to their physical limitations, for example the body-size or flying capability, or because of the choice of food and nectar plants, or a combination of both of these traits.
Tropical forests and especially hotspot areas like the Western Ghats lack detailed natural history information about most of the plant and animal species they support. Only when we understand what processes affect the populations of species, can we aim to conserve the species, and this forms the basis for conservation action plans. There is so much variation in how species respond to the effects of habitat fragmentation, the matrix surrounding the habitat and the size of the habitat, that one blanket rule to conserve all species seems difficult. My research highlights the fact that species properties like their flight abilities, diet breadth etc. along with landscape properties such as the level of fragmentation, the size of habitat fragments, vegetation type should be considered while making management plans for species conservation.