Planning Butterfly Habitat Restorations
The following information is an excerpt from Mason 1997, concerning butterfly habitat restoration work in Europe and North America. Read Full Article
Ecologists and entomologists in Britain and the United States have participated in numerous projects aimed at restoring, expanding, and mitigating losses of habitat for native butterflies. Some of these butterflies have been listed as threatened or endangered; others have been lost or are in decline in certain portions of their former ranges. The experiences and observations of these scientists illustrate potential needs and concerns when planning to restore or improve existing butterfly habitat. This paper relates several cases in which complex biotic and abiotic interactions played important roles in the success or failure of butterfly habitat restoration and the reestablishment of butterfly populations.
Although each butterfly species has its own unique combination of habitat requirements and life history, some fundamental principles concerning butterfly biology (New 1991) are significant in planning habitat restorations. Most caterpillars are herbivores, and many are specialists which feed on only one kind or a few related kinds of plants. Therefore, the presence of appropriate larval host plants is the primary requirement of habitat restoration. In addition, many butterfly species require that the larval food plant be in a particular growth stage, of a certain height, exposed to the proper amount of sunlight, or in close proximity to another resource.
Adults typically utilize a wider range of plants or other resources as food, and flight gives them expanded mobility. However, adult dispersal ability varies from species to species. For some, physical features such as a few meters of open space, a stream, a hedge, or a change in gradient create intrinsic barriers to dispersal; other species routinely migrate long distances. Species with greater levels of mobility form "loose" or "open" populations with indistinct boundaries. Such a species may have a wide, relatively continuous geographic dispersion and may be considered rare if found in low density within its range.
In contrast, as many as 85 percent of butterflies in temperate regions may form "tight" or "closed" populations, discrete colonies with distinct geographic boundaries (New 1991). Closed populations may be restricted to particular geological formations, soil types which support characteristic vegetation, or an early successional stage of vegetation. These colonies are often part of "metapopulations" groups of local populations which occupy distinct habitat patches and interact through small-scale movement between colonies. Within a cluster of these habitat patches, a metapopulation can survive for many years, as long as the habitat remains suitable. Extinction of one colony will be offset by recolonization from another. However, when closed populations become isolated due to habitat degradation or fragmentation, they become more vulnerable to locally variable conditions and environmental changes. Also, the adaptive ability of an isolated population may be decreased as the gene pool is no longer refreshed by immigration. Low dispersal ability may then lead to species extinction.
Butterflies, like many other insects, are uniquely adapted to precise, fine-scaled environmental conditions. Meeting these subtle environmental requirements, often indiscernible to the human observer, is essential to restoring habitat for butterflies in human-dominated landscapes.
The essential features of butterfly habitat are not always apparent. Many species are restricted in their resource and habitat needs, and even very closely related species may differ substantially in their biology and behavior. As the example of the Large Blue butterfly shows, intensive investigation of insect habitats and life histories can yield complex and surprising results. Few butterflies have been studied so thoroughly.
The term "plagioclimax" can be applied to a vegetation community in which natural succession is halted by continuous management such as cutting, grazing, or fire (Thomas 1995a). Most plagioclimax vegetation consists of a mosaic of microhabitats and microclimates which is essential to butterflies and other insects which are adapted to the early successional environment. Management for butterfly species which inhabit the ephemeral plagioclimax will require ongoing intervention and a continuity of habitat types in close enough proximity to allow colonization. Some butterflies may require different host plants, habitats, or microhabitats in different years or different seasons. The best breeding habitat in a wet year may be different from the best breeding habitat in a dry year, so long-term conservation of these species requires larger areas which incorporate both habitat types (Thomas 1995a).
In Britain, butterfly reserves protect portions of metapopulations and provide habitat refuges from temporarily unsuitable environmental conditions. Habitat patches can then be re-colonized from these refuges, and the creation of habitat "stepping stones" can prevent colonies from becoming isolated. The probability of recolonization of a restored area, within a given time period, is a function of the distance of the restored area from existing populations and the size of each potential source population (Thomas 1995a). It is possible to model this probability, but the accuracy of such a model depends upon knowing vital population parameters, and they are often unknown.
There are numerous instances in which butterflies have become extinct in reserves, some designed primarily for their conservation, because little or no suitable breeding habitat was created or maintained. These examples should serve as an ongoing reminder that even our best-intentioned efforts may be counterproductive if undertaken without thorough scientific study. Specialist butterfly communities can serve as sensitive indicators of vegetation structure and change, and restorations which successfully create habitat for these butterflies will result in unique sites which contribute to the preservation of regional diversity.
New, T. R. 1991. Butterfly conservation. Oxford University Press, South Melbourne.
Thomas, C. D. 1995a. Ecology and conservation of butterfly metapopulations in the fragmented British landscape. Pages 46-63 in A. S. Pullin, editor. Ecology and conservation of butterflies. Chapman and Hall, London.