Dietary Overlap of Sympatric Polyphagous Alpine Grasshoppers Includes Invasive Plant Species

Understanding what herbivores eat can provide important information about competitive interaction among sympatric species and the potential for selective feeding to shape plant communities. The flightless alpine grasshoppers Sigaus australis, S. nitidus and S. nivalis are sympatric and abundant in the mountains of South Island, Aotearoa New Zealand. We investigated whether their diets differ among locations, species and sexes, and whether the plants that they ate had changed over 50 years. Mandible morphology of the grasshopper species was compared to infer diet adaptation. Their diet was analysed using gut samples collected from 1969 to 1971 and 2021 to 2023. Despite differences in mandibles suggesting potential adaptation to food plants of different sizes and toughness, we found greater diet differences among locations than among grasshopper species or between sexes. All three Sigaus grasshoppers consume a wide range of plants. Considered as a guild, we found the diet of these grasshoppers was influenced by local environment and season, but that they did not feed at random. Shrubs and herbs including Gaultheria and Lobelia were favoured, but the abundant Chionochloa grasses were avoided. Despite narrow seasonality, fruits and flowers were also important dietary components. Invasive plants including Hieracium, Pilosella and Taraxacum that were rare or absent in the diet 50 years earlier had been eaten by 92% of the grasshoppers sampled in 2021-2023. By selectively feeding on soft groundcover and invasive plants, Sigaus grasshoppers may act as ecosystem architects in the alpine plant communities that continue to be dominated by snow tussock.