Stable isotopes reveal dietary divergence between dispersal phenotypes in Tetrix subulata pygmy grasshoppers (Orthoptera: Tetrigidae)

Einat Karpestam,Anders Forsman

doi:10.14411/eje.2013.008

Abstract

In some species of insects, individuals with fully developed wings and capable of flying coexist with flightless individuals that lack functional wings. Their diets may differ if long-winged individuals are more mobile and therefore likely to be better at finding and utilizing high quality food resources, or if they have different food preferences or physiological requirements. Despite its potential importance, differences in the diet of dispersal phenotypes have not been unequivocally demonstrated under natural conditions. To test for dietary divergence, we compared natural abundances of carbon and nitrogen stable isotope ratios (d13C and d15N) in long- and short-winged free ranging Tetrix subulata pygmy grasshoppers collected as adults from two natural populations. Overall, this comparison of stable isotopes indicated long-term differences in the diet of the two wing morphs in both populations, but not between males and females of the same morph. We conclude that it is likely that the dietary niches of the long winged and flightless individuals differ under natural conditions. This may reduce intra-specific competition, offset the expected trade-off between flight capacity and reproduction and promote ecological speciation.

Highlights

Differences in the diets of individuals of different sexes, age classes, life-history variants or morphology is well established for vertebrates (Smith & Skulason, 1996; Bolnick et al, 2003)
In the Påryd population, short-winged individuals had lower 15N values than long-winged individuals, this difference between wing morphs was evident in both sexes, and there was no difference between males and females of a particular morph (Tables 1 and 2)
In this study we aimed to test for an effect of dispersal capacity on long-term dietary divergence by analyzing and comparing stable carbon and nitrogen isotopes Tetrix subulata pygmy grasshopper individuals that belonged to either a long-winged flight capable or a short-winged flightless phenotype

Summary

Introduction

Differences in the diets of individuals of different sexes, age classes, life-history variants or morphology (e.g., individuals that differ in the structure of their mouth parts) is well established for vertebrates (Smith & Skulason, 1996; Bolnick et al, 2003). Individuals may vary in diet because they differ in dispersal capacity, ability to exploit patchy resources and in the rate at which they encounter different types of food. In some species of insects, a flight capable morph with fully developed functional wings (macropterous or alate) coexists with individuals that lack wings (apterous) or have poorly developed wings (brachypterous) (Rehn & Grant, 1955; Harrison, 1980; Zera & Denno, 1997). Flight enables individuals to disperse and colonize new habitats it facilitates smaller scale movements between habitat patches within heterogeneous environments (Roff & Fairbairn, 1991; Müller et al, 2001). Early work by Denno et al (1980) on the plant hopper, Porkelisia marginata, indicate that macropterous individuals are better at colonizing and utilizing high quality patches of vegetation. Pushkar (2009) reports that the macropterous form of the pygmy grasshopper Tetrix tuerki is found along the shorelines and on small isolated islands of alluvium in a river, while the brachypterous and mesopterous forms are only found along the shorelines of the river

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