TERRESTRIAL–AQUATIC LINKAGES: RIPARIAN ARTHROPOD INPUTS ALTER TROPHIC CASCADES IN A STREAM FOOD WEB

TLDR

Headwater stream ecosystems are shaped by the interface of aquatic and terrestrial systems, yet the community-level effects of terrestrial arthropod inputs on stream food webs remain poorly understood. The study manipulated terrestrial arthropod inputs and predatory fish presence using greenhouse covers and exclosures in a forest stream in northern Japan to test their effects on the food web. Reducing terrestrial arthropod inputs shifted fish predation from terrestrial to aquatic arthropods, depleted aquatic arthropods, increased periphyton biomass, and demonstrated that terrestrial inputs are a primary driver of trophic cascades among fish, herbivorous arthropods, and periphyton, supporting the importance of energy transfer across ecosystems.

Abstract

Dynamics of headwater stream ecosystems are generally regarded as occurring at the interface of aquatic and terrestrial ecosystems. Terrestrial arthropod inputs can provide an energy subsidy and increase the abundance of predatory fish, and the ensuing effects potentially can cascade through the food web and ultimately affect primary producers. Nevertheless, the community-based effects of such inputs on stream food web dynamics are still poorly understood. We present experimental evidence that terrestrial arthropod inputs have an indirect but prominent effect on a stream benthic community by altering the intensity of fish predation in the food web. Two key elements of the stream food web, terrestrial arthropod inputs and the presence of predatory fish, were experimentally manipulated by using greenhouse-type covers and enclosures (or exclosures) in a forest stream located in northern Japan. When terrestrial arthropod inputs to the stream were experimentally reduced, fish predation pressure shifted dramatically from terrestrial to aquatic arthropods. The ensuing depletion of aquatic arthropods resulted in a subsequent increase in periphyton biomass. This field experiment revealed that terrestrial arthropod inputs were a primary factor controlling cascading trophic interactions among predatory fish, herbivorous aquatic arthropods, and benthic periphyton. These results provide empirical support for the perspective that transfers of energy and biomass from donor systems are frequently significant for the maintenance of biotic communities in recipient systems.