Delayed population explosion of an introduced butterfly

Summary 1 The causes of lagged population and geographical range expansions after species introductions are poorly understood, and there are relatively few detailed case studies. 2 We document the 29-year history of population dynamics and structure for a population of Euphydryas gillettii Barnes that was introduced to the Colorado Rocky Mountains, USA in 1977. 3 The population size remained low (< 200 individuals) and confined to a single habitat patch (∼2·25 ha) to 1998. These values are similar to those of many other populations within the natural geographical range of the species. 4 However, by 2002 the population increased dramatically to > 3000 individuals and covered ∼70 ha, nearly all to the south of the original site. The direction of population expansion was the same as that of predominant winds. 5 By 2004, the butterfly's local distribution had retracted mainly to three habitat patches. It thus exhibited a ‘surge/contraction’ form of population growth. Searches within 15 km of the original site yielded no other new populations. 6 In 2005, butterfly numbers crashed, but all three habitat patches remained occupied. The populations within each patch did not decrease in the same proportions, suggesting independent dynamics that are characteristic of metapopulations. 7 We postulate that this behaviour results, in this species, in establishment of satellite populations and, given appropriate habitat structure, may result in lagged or punctuated expansions of introduced populations.