Global Warming Causes

Butterfly Ranges and Climate

Research at many scales clearly shows that climate is important for butterfly abundance and distribution. The basic association between climate and insect distribution and abundance has been studied for at least 70 years (e.g., Uvarov 1931, Andrewartha and Birch 1954, Birch 1957, Dennis 1993).The challenge now is to focus on experiments that will integrate our mechanistic understanding of individual and population processes with distribution limits and change.

Large-scale patterns in butterfly distribution are generally con sistent with the hypothesis that environmental conditions may constrain the northern boundaries of many species' ranges, although alternative hypotheses have been proposed (see discussion in Dennis 1993). As with many taxa, butterfly species richness characteristically declines along latitudinal and elevational gradients (Scriber 1973, Gieger 1987, Dennis et al. 1991, Sanchez-Rodriguez 1995). Dennis (1993) conducted a multifactorial analysis of species richness within Great Britain based on a systematic 10-km grid database on butterfly abundance (from the Butterfly Monitoring Program, Pollard 1977). Dennis (1993) recorded that butterfly species richness correlates with July temperature (correlation coefficient, r = 0.85), but also with January temperature (r = 0.59), the number of frost-free days (r = 0.58), and precipitation (r = -0.49). This analysis demonstrates a strong correspondence with environmental factors, particularly summer temperature.

If most species' ranges are constrained by temperature, then during a century of warming the prevailing direction of range shifts should be toward higher latitude and elevation. The 0.7°C warming trend in Europe since the 1890s should favor butterfly range expansion (Dennis 1993), based on physiological and ecological principles of butterfly biology. Parmesan et al. (1999) analyzed butterfly range shifts throughout Europe, and found that out of 35 species with data on their whole range, 63% have shifted northward, whereas only 3% have shifted southward. However, this analysis excluded species for which nonclimatic factors seemed to affect the range. In a separate analysis of the ranges of all British butterflies at a finer resolution, Pollard and Eversham (1995) discovered that many more species have contracted their ranges overall than have shifted. Of 59 resident British butterflies, 30 have experienced major range contractions and only 3 have expanded overall. Pollard and Eversham (1995) and Warren (1992) attribute this discrepancy to widespread habitat loss and degradation in Britain. It is not clear to what extent habitat degradation is sufficiently widespread to affect these species' entire ranges.

Importantly, all expanding British species but one (Ladoga camilla) are common and widespread within their ranges (Pollard and Eversham 1995), thus there may be a connection between local abundance and distribution change. Pollard and Eversham (1995) define as common and widespread those species found at > 80% of survey sites within their range. Fifty-five percent of all common species have expanded their range over the past three decades, while only 3% (a single species) of all localized species have expanded their range in Britain. Autecological studies have revealed a large number of associations between a species' population abundance over time in a given location, and climatic variation (Pollard 1979, Ehrlich 1980, Pollard and Lakhani 1985, Pollard 1988, Pollard 1991, Pollard and Yates 1993,Thomas et al. 1998). Multiple regression analyses show a significant correlation between population numbers and summer temperature for 6 out of 11 species that have expanded range since the 1940s (Pollard and Eversham 1995). Thus common butterfly species seem to track climatic fluctuations in both population size and range size better than other butterflies.

One possible mechanism of range shift resulting from climate change is that populations in an increasingly less favorable location will be more likely to go extinct and less likely to colonize new sites than populations in an increasingly favorable part of the range. This will lead to a shift in abundance, and over time, may lead to a range shift. Parmesan (1996) documented evidence of this type of shift in a North American species, Edith's checkerspot (Euphydryas editha). Population surveys since the 1930s documented the locations of E. editha populations from Canada to Mexico. Parmesan resurveyed populations, recording the number of populations that no longer survived. Significantly more population extinctions had occurred in the southern part of the range and at lower elevations than in the north or at higher elevations, resulting in a 92 km northward and 105 m upward shift in mean population location. This magnitude and direction of shift closely matched mean annual temperature isotherm shift (105 km northward, 105 m upward).

While these results support the hypothesis that climate trends influence the abundance and distribution of many species, there are very few cases where the mechanistic nature of the relationship between climate and distribution change is known. The white admiral butterfly, Ladoga camilla, is one of the best examples in which a detailed historical record is complemented by study of an edge population to attribute a causal mechanism to the association between distribution and climate in Britain. Using key factor analysis, Pollard (1979) determined that the limiting factor for a Ladoga population near the range edge is late larval and pupal development time in June. A cool June yields slow development, prolonged vulnerability to predation, and lower adult population size. Furthermore, a cool July reduces the available flight time for oviposition. The net result is that in cool years mortality is higher and fecundity is lower than in warm years. Pollard compared population sizes and appearance at new sites with May, June, and July temperature since 1900. He found a strong correlation between anomalously warm years and an increase in both new site records and population abundance in existing sites. It is not currently known whether this is a common mechanism linking distributions to climate.