Substantial flows of matter and organisms occur between rivers and adjacent terrestrial habitats. Aquatic insect emergence can transport significant proportions of aquatic secondary productivity to adjacent terrestrial habitats where it can provide an important resource subsidy for a host of riparian consumers including birds, bats, and spiders. Recent short-term experimental manipulations of aquatic insect emergence have demonstrated that recipient consumers show a functional or behavioural response to aquatic subsidy alterations. Here, we aim to test whether such findings from short-term experiments can be scaled up in space and time: subsidies occur, but how important are they for riparian consumer populations and communities? Long-term and large scale subsidy manipulations are, however, logistically very challenging and time consuming. To investigate long-term subsidy alterations, we applied a novel approach by exploiting the changes in aquatic communities produced by point discharges of a pollutant into streams and rivers. Abandoned coal mines are a common source of such discharges, which can cause profound changes in the abundance of aquatic insects over entire stream reaches.  Importantly, the ‘treatment’ (mine discharge) alters the aquatic insect production, primarily through changes in physical habitat conditions, but has no direct affect on the riparian habitat thus providing a paired design with aquatic subsidy being altered within a continuous stretch of riparian habitat. Many mine discharges have been flowing continuously for more than 10 years, and so provide a sustained suppression of aquatic insects, and hence subsidy to the riparian zone. We quantitatively sampled aquatic insect larvae, emerged aquatic insects, and entire riparian arthropod assemblage in 15 streams and their adjacent terrestrial habitats, upstream and downstream of the discharges.  

Results/Conclusions

As expected the density of aquatic insect larvae and emerged aquatic insects was dramatically reduced downstream of the coal mine drainage. Importantly, this long-term reduction in aquatic-terrestrial subsidies has resulted in a significant reduction of web spider populations (i.e. numerical response) in adjacent terrestrial habitats. We found no significant difference in the abundance of alternative terrestrial prey.  Our results demonstrate strong direct population effects for recipient consumers but no obvious indirect effect for their alternative terrestrial prey. This suggests that that long-term alteration of spatial subsidies can have wider consequences for recipient consumer populations in connected habitats. Our findings further suggest that the protection of terrestrial species may depend as much on the management of aquatic habitats as it does on the management of the riparian habitats themselves.