Range restricted grasshoppers better conserved in a terrestrial zone than in a riparian zone

Abstract

Riparian zones (RZs) functionally connect aquatic and terrestrial ecosystems, and have azonal and geographically widespread plant communities that differ from those of the neighboring terrestrial zone (TZ). Although well studied botanically, RZs are not well understood in terms of their terrestrial insect diversity, including grasshoppers. The Cape Floristic Region (CFR) is a global biodiversity hotspot with small rocky rivers running through highly diverse sclerophyllous vegetation. It has high levels of endemism among many taxa, including grasshoppers, making it ideal for testing the effect of azonal vegetation on grasshopper assemblages of the RZ, and determining whether conservation efforts should be focused on the RZ as well as the TZ. We determine grasshopper dispersion patterns along the RZ of an important CFR river, and compare these patterns with those of the TZ to understand the habitat occupancy relative to 27 environmental variables of the zones and geographical distribution of the grasshoppers. Forty percent of individuals we collected were CFR endemics. We found only weak differences in the grasshopper assemblages between the RZ and TZ, apparently driven by deep history, complex geomorphology, stressful environmental conditions, a diverse vegetation and land mosaic, and probable high predator pressure. There were two groups: large-sized, well-flighted, geographically widespread generalists that were overall more abundant in the RZ than TZ, and small, flightless or poorly-flighted, vegetation-specialists which are narrow-range endemics adapted to both RZ and TZ, but still more abundant in the TZ. We conclude that although the vegetation of this riparian zone may require some special conservation attention, this is not so for the grasshoppers which overall are best conserved in the TZ.