Aquatic Insect Availability and Use by Ground‐Active Terrestrial Predators Varies Between Habitats of Different Successional Age in a Braided River

ABSTRACT Braided rivers are dynamic ecosystems characterised by frequent flows that rework the braidplain, forming a shifting mosaic of habitats of varying successional age. This dynamic habitat mosaic facilitates movement of resources between aquatic and terrestrial environments, fostering biodiversity and ecological resilience. Anthropogenic impacts such as impoundments, water and gravel extraction and weed encroachment can reduce the ability for the river to rework the riverbed, stabilising and homogenising the braidplain, impacting ecosystem functioning as well as geomorphology. We examined how the availability and consumption of adult aquatic insects by terrestrial invertebrate consumers varied among different‐aged habitat patches within a New Zealand braided river reach with minimal anthropogenic alteration.. Terrestrial and adult aquatic insect availability was quantified at ground level and aerially using pitfall and sticky traps, respectively, in five sites of three different‐aged braidplain habitat types (recently formed bare gravel, intermediate and older vegetated habitats). Stable (δ 13 C and δ 15 N) isotope analyses were used to quantify the use of aquatic insect prey by predatory terrestrial invertebrates (ground‐hunting spiders and predatory beetles) in each habitat type. Aquatic prey availability and consumption by terrestrial predators varied between habitat types, and different‐aged habitats better supported ground‐ and aerial‐active terrestrial predators. At ground‐level, more adult aquatic insects were available and consumed by predatory beetles in bare gravel habitat types. In contrast, older vegetated habitats had higher aerial aquatic insect availability, potentially better supporting aerial‐active consumers. Our results show that a relatively natural braided river reach with minimal geomorphological alteration had spatial heterogeneity in aquatic prey availability, the locations of which differed for aerial‐ and ground‐active consumers during the austral summer. These height‐dependent locations of variable aquatic prey abundance will move at varying timescales within the naturally dynamic habitat mosaic. The observed spatial heterogeneity of resource availability is likely to enhance braidplain‐scale biodiversity and resilience by supporting diverse consumer groups and providing stability during high‐flow events. Understanding the structure of relatively unimpacted braided river systems can support their geomorphological conservation and restoration, thus enhancing their ecological resilience in the face of environmental change. Our results provide evidence that habitat patches of varying successional age, which require frequent bed‐mobilising floods to maintain, can support diverse braided river ecosystems.