Terrestrial Invertebrates Strike Back: Aerial and Ground-Based Colonisation of a Dry Riverbed

Abstract

1. Dispersal, either active or passive, plays a key role in community assembly during the colonisation of newly available habitats such as the dry riverbeds of intermittent rivers. These unstable environments are exposed to an ongoing colonisation process, often from adjacent riparian habitats that are local biodiversity hot spots. We aimed to identify the diversity, origin and colonisation pathways of terrestrial invertebrates in the dry bed of an intermittent Mediterranean river.
2. We sampled aerial and ground-dwelling invertebrates during the colonisation of dry riverbeds using cross-vane window traps and pitfall traps. Invertebrate communities were sampled in three habitat types of the intermittent karst Krčić River in Croatia: dry riverbed, riparian and upland habitat. For aerial invertebrates, we differentiated between diurnal and nocturnal colonists and between lateral and longitudinal colonists. All invertebrates were identified to order level, Carabidae, Araneae and Orthoptera to species. Environmental parameters were measured in each habitat.
3. Colonisation of the dry riverbed was primarily diurnal. Aerial invertebrates colonised both laterally, from the adjacent riparian habitat, and longitudinally, along the riverbed. Invertebrate catch was positively correlated with wind speed and direction, indicating the importance of passive dispersal by aerial plankton. The ground-dwelling invertebrate community exhibited rapid dry riverbed colonisation, with riparian habitats supporting the highest diversity and unique taxa. Taxonomic metrics showed different patterns for ground-dwelling invertebrates and each studied taxonomic group.
4. The observed patterns suggest that ground-dwelling Araneae and Carabidae colonised dry riverbed mostly through spillover from the riparian habitat. In contrast, Orthoptera probably colonised the dry riverbed from the upland habitat. Concurrent sampling of aerial and ground-dwelling colonists characterised responses that could not have been detected using a single method.
5. Our study demonstrates that multiple dispersal modes and source habitats are involved in the colonisation of the dry riverbed. Our results underline the importance to conserve riparian and upland areas along intermittent rivers as source habitats during the dry phase. Given the increase in stream intermittency due to climate change, the conservation and restoration of appropriate river landscapes could become more widely relevant in the future.