The long-term effects of altered aquatic-terrestrial subsidies

Streams and rivers are linked to the terrestrial habitats around them by the exchange of nutrients, detritus and organisms. Most investigations of these exchanges have focused on how aquatic systems are influenced by inputs from the terrestrial environment, but more recent work has identified the potential importance of flows of material from the aquatic habitat to the adjacent terrestrial (riparian) habitat. A substantial part of aquatic animal production is insects, which as they emerge, move into the riparian habitat where they become prey for terrestrial predators (spiders and beetles). The long-term effects of this additional food source (subsidy) on the ecology of riparian predators and their terrestrial prey may be considerable, but have not previously been investigated. This study will address this gap in our understanding and the findings could have important management and conservation implications. If aquatic subsidy is important, 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. The aim of this research is to understand the effects that aquatic subsidy has on riparian food web function. Specifically, we will test whether: 1. terrestrial predators are more abundant where there is a greater aquatic subsidy; 2. input of energy to terrestrial predators changes the impact they have on other terrestrial species (herbivores and detritivores), and the processes (herbivory, decomposition) these species drive; 3. predators respond to increased subsidy by changing their diet, moving into riparian habitats, or having increased population growth; 4. changes in the types of aquatic insects emerging alter the effect the subsidy has on the food web. Previous studies of aquatic subsidies have manipulated aquatic insect emergence over short periods (a few weeks). This is effective at detecting short-term effects (predator movement, diet change), but to test for population changes, and the effects of those changes on other species, we need systems subject to longer-term alterations of subsidy. One novel way in which this can be done is to exploit the changes in aquatic communities produced by discharge of a pollutant into streams and rivers at a fixed point. Abandoned coal mines are a common source of such discharges, which can cause profound changes in the abundance and types of aquatic insects present. Importantly, the 'treatment' (mine discharge) alters the aquatic community, but has no direct affect on the riparian habitat thus providing a paired design with aquatic subsidy being altered within the same stretch of riparian habitat. Many mine discharges have been flowing continuously for more than 10 years, and so provide a sustained suppression of the aquatic insect community, and hence subsidy to the riparian zone. By comparing the terrestrial communities, and food web functioning above and below such discharges we can assess the effect of removing the aquatic subsidy for ecologically relevant periods of time. We will investigate the effects of reduction in the aquatic subsidy below mine discharges at a large number of sites to see how riparian communities respond. We will then carry out detailed examinations of a subset of sites to establish how much energy is entering the terrestrial food web from the aquatic system and whether terrestrial predators respond to the seasonal changes in aquatic subsidy by moving into riparian habitats or changing diet. We will also test whether the response of predators to increased subsidy is limited by the effects of their own predators, such as birds. Finally, we will compare systems in which the proportion of aquatic insects that emerge by crawling on the ground, and the proportion that emerge by flight, differ, to test the idea that these insects are available to different predators (spiders versus beetles), and so affect the terrestrial food web in different ways.