Kai Ruohomäki, Miia Tanhuanpää, Matthew P. Ayres, Pekka Kaitaniemi, Toomas Tammaru, and Erkki Haukioja. 2000. Causes of cyclicity of
Epirrita autumnata (Lepidoptera, Geometridae) - grandiose theory and tedious practice. Population Ecology 42:211-223. Full text in pdf. |
Creating multiyear cycles in population density demands, in traditional models, causal factors that operate on local populations in a
density-dependent way with time lags. However, cycles of the geometrid Epirrita autumnata in northern Europe may be regional, not local, i.e.
successive outbreaks occur in different localities. We review possible causes of cycles of E. autumnata, under both local and regional scenario,
including large-scale synchrony. Assuming cyclicity is a local phenomenon, individual populations of E. autumnata display peaks but
populations all over the outbreak range fluctuate in synchrony. This assumes that the peaks at most localities are so low that they do not lead to
visible defoliations and easily remain unnoticed. In this scenario, populations are able to start recovery a few years after the crash, i.e. at the
time of the mitigation of detrimental delayed density-dependent factors, such as delayed inducible resistance of the host plant, or parasitism. In
that case, the same factors that lead to crashes also explain the periodicity of cyclic fluctuations. According to the regional cyclicity scenario,
different factors can be important in different phases of the cycle. The key is to identify the factors that tend to produce outbreaks with a
periodicity of about 10 years. Initiation of the increase phase seems to coincide with maxima in sunspot activity but causal connections remain
unclear. Climatic factor(s) associated with the solar cycle could contribute to the large-scale geographic synchrony. |