By Nicholas Wright

This blog is the third and final of a three part series on ecological subsidies that appeared throughout summer ’23.

In California’s north coast, the Eel River winds its way through hills with shady slopes carpeted in lush ferns and towering redwoods and sunny ridges covered in brushy chaparral. The South Fork Eel River has been the site of extensive research by UC Berkeley professor Dr. Mary Power that has upended the traditional paradigm in ecology that trophic subsidies from forested watersheds shape river food webs, but subsidies from rivers are unimportant to forests. 

During spring, floating mats of bright green algae grow on top of the water in the river. Aquatic insects like caddisflies and mayflies lay their eggs inside these mats, which provide nutritious food and protection from predators to their young when they hatch. Populations of many different species of these insects will emerge into their terrestrial adult forms in synchrony and then fly away from their algal homes, creating trophic subsidies that flow from the river into the forest.

Dr. Power’s lab has documented just how abundant and diverse the insectivorous consumers of this cross-system subsidy really are. Filmy dome spiders, which weave complex webs to catch aerial insects, grow larger but build smaller webs closer to the river due to the high density of insects. Isotopic analyses show that even spiders whose webs are hundreds of meters from the river consume substantial amounts of river insects (Power et al 2004). Fast-moving wolf spiders, which chase their prey on foot, tracked down algal mats to consume the insects emerging directly from them. Western fence lizards and sagebrush lizards, both thought to be grassland species, were found to be 7x more abundant alongside the river than in meadows during the spring and summer (Power et al 2004). To test whether these densities were because lizards were seeking out insects emerging from the river, or simply trying to warm themselves on the rocks, the Power lab built “subsidy shields”- enclosures that reduced the fluxes of aquatic insect biomass by 70%. Lizards trapped in the enclosures grew 7x slower than lizards trapped in enclosures open to the insects, and lizards placed in open enclosures left those with a subsidy shield to move into enclosures without one (Sabo and Power 2002).

Insects are not the only animals with a terrestrial adult stage whose juveniles consume the algal mats. Pacific tree frog tadpoles from the Eel River were found to grow faster and metamorphose sooner when fed filamentous and epiphytic green algae (which make up the floating mats) than other food sources (Kupferberg 1994). These tadpoles metamorphose into frogs that hop into the damp forest and provide an aquatic trophic subsidy for garter snakes, raccoons, and herons that consume them. 

Aerial insectivores are far more mobile than spiders, lizards, or frogs, and therefore energetically connect rivers to their watersheds across larger spatial scales. Swallows, black phoebes, and six different species of bats consume insects emerging from the Eel River, snatching them out of the air or plucking them off the water’s surface. Using ultrasound detectors, researchers in the Power lab found bat density was the highest directly above the river, and dropped steadily within 50 meters of the banks, and then plateaued at a low density (Hagen and Sabo 2011). Across some transects, there were 30x more bats above the river than there were just 50 meters away! After bats are done with a night of hunting, they head to their day roosts – cavities in old trees and cracks in large boulders, typically many km from the river. In their roosts, bats excrete the insects they consumed, depositing river nutrients in the form of guano. This nitrogen guano is also a trophic subsidy, feeding insect detritivores and providing a potentially important source of nutrients for the nitrogen-limited old-growth conifer forest.

Other species generate far more massive fluxes of nutrients from the water to the forest. After spending years feeding in the prey-rich ocean waters off California’s coast, fall-run Chinook Salmon and Coho Salmon return to the Eel River to spawn. After laying and fertilizing their eggs, the salmon die and their carcasses are washed ashore or dragged on land by predators like black bears, river otters, and eagles. As the nitrogen and phosphorus-rich tissues of the salmon decompose, nutrients enter the soil and are taken up by plants. An experiment conducted in Alaska found that riparian trees receiving nutrients from decomposing salmon grew significantly faster than trees growing along the opposite river bank where salmon carcasses were removed (Quinn et al 2018). In California’s coastal streams, returning adult salmon import ~10x more phosphorous into freshwater than exported into the ocean, creating a vital ecological conveyor belt of nutrients from the ocean to the rivers to the forest (Moore et al. 2011).

Conclusion

Trophic subsidies are complex, cross-system biotic interactions that we are still working to understand. Studying Chinook Salmon reveals the web of trophic subsidies connecting California’s freshwater ecosystems and their watersheds as salmon move through different habitats and stages of their life cycle. As juveniles they consume terrestrial nutrients and energy, preying on insects as parr and floodplain zooplankton as smolt. Then as they die, they return nutrients from water to land in their carcasses, a trophic subsidy all the way from the Pacific Ocean.

Conserving salmon requires conserving entire watersheds–not just the physical riverscapes but also the aquatic and terrestrial biodiversity on which they depend. California’s streams, lakes, forests, farms, and floodplains are ecologically intertwined in a network of trophic subsidies that transcend habitat boundaries. We must protect both aquatic and terrestrial ecosystems, and the trophic subsidies connecting them, if we are going to successfully conserve either.

Nicholas Wright is junior specialist in the Johnson-Jeffres research group. 

Further Reading:

Bastow, J.L., Sabo, J.L., Finlay, J.C., and Power, M.E. (2002) A basal aquatic-terrestrial trophic link in rivers: algal subsidies via shore-dwelling grasshoppers. Oecologia, 131, 261–268.

Hagen, E.M. and Sabo, J.L. (2011) A landscape perspective on bat foraging ecology along rivers: does channel confinement and insect availability influence the response of bats to aquatic resources in riverine landscapes? Oecologia, 166, 751–760.

Kupferberg, S.J., Marks, J.C., and Power, M.E. (1994) Effects of variation in natural algal and detrital diets on larval Anuran (Hyla regilla) life-history traits. Copeia, 2, 446-457.

Moore, J.W., Hayes, S.A., Duffy, W., Gallagher, S., Michel, C.J., and Wright, D. (2011) Nutrient fluxes and the recent collapse of coastal California salmon populations. Canadian Journal of Fisheries and Aquatic Science, 68(7).

Power, M.E., Rainey, W.E., Parker, M.S., Sabo, J.L., Smyth, A., Khandwala, S., Finlay, J.C., McNeely, F.C, Marsee, K., Anderson, C. (2004) River-to-watershed subsidies in an old-growth conifer forest. Polis, G.A., Power, M.E., and Huxel, G.R. (Ed). Food webs at the landscape level. 

Quinn, T.P, Helfield, J.M., Austin, C.S., Hovel, R.A., and Bunn, A.G. (2018) A multidecade experiment shows that fertilization by salmon carcasses enhanced tree growth in the riparian zone. Ecology, 99(11), 2433-2441.

Sabo, J. L., and M. E., Power. River-watershed exchange: Effects of riverine subsidies on riparian lizards and their terrestrial prey. Ecology, 83(7), 2002, 1860–69.