Tuesday, July 13, 2010

Differentiation in the Common Field Grasshopper

Common Field Grasshopper, Chorthippus brunneus

The seminal work of Clausen, Keck and Hiesey at Stanford University in the 1930s, 40s and 50s on the relationship between plants and climate (1941, The American Naturalist 75, No. pp. 231-250) showed beautifully the influence of phenotypic plasticity on plants native to California. By studying clinal variation in these plants (especially Achillea spp.) over a transect that ran from the coast up to the mountains of the Serra Nevada, they showed that plant size decreased with increasing altitude. Their extensive research also involved taking clones of plants from their individual habitats and replanting them at three different elevations: sea level, 4600 feet and 10,000 feet. They found that races grew well at altitudes similar from where they originated, but often died off at other altitudes.

With plants, such geographic variations in form are often quite obvious. It is less so for animals. Because of this, and the difficulty associated with transplanting experiments, studies on phenotypic plasticity in animal populations are quite few. However a wonderful example was produced in 1999 by Telfer and Hassall (Oecologia 121 pp 245-254) on ecotypic differentation in the common field grasshopper, Chorthippus brunneus.

C. brunneus
is common throughout most of Europe, and can its short chirping song (resembling time signal pips) can be heard in dry grassy areas throughout Ireland from mid-June to August, although it is rarer in the north. 18 to 24 mm in length it flies readily when disturbed, having noticeably long wings and can be distinguished from similar species by its hairy underbelly (Sterry 2004, Collins Complete Guide to Irish Wildlife p. 124).

In studying 27 populations of C. brunneus throughout Britain and Ireland, Telfer and Hassall found that grasshoppers from cooler sites were heavier at hatching, those from nothern sites grew faster but were smaller at adulthood and adults in warmer, sunnier or more southernly locations were larger. They concluded that the observed ecotypic variation in C. brunneus is an evolutionary response to climatic variation.

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