Food web reverberations

Great blue herons are excellent hunters and fishers; at four feet tall, with a wingspan of six feet, an adult heron can be a formidable sight as it wades through marshy shallows, waiting for the perfect moment to strike its prey. Great blue herons are also regulators of the complex interactions that connect them to their fellow eelgrass meadow-dwellers, according to a study recently published in the journal Oikos.

Events that impact one corner of a food web have a way of reverberating throughout the rest of its threads – the organisms that share an ecosystem are linked through many direct and indirect connections. When effects ripple from one end of a food web to the other, scientists call the ensuing changes a ‘trophic cascade.’ The new study suggests that changes to great blue heron populations might set off just such a cascade of changes.

The scientists monitored patches of eelgrass meadow (a major foraging area for great blue herons during the summer months) where herons could freely feed, as well as patches from which the herons were excluded. They monitored organisms at multiple levels of the food web: fish (which are eaten by herons), insects (which are eaten by fish), and the algae that cling to eelgrass blades (which is eaten by insects).

By the end of their nine-week experiment, they found that excluding herons from the meadow influenced all levels of the food web. Fish were more abundant where herons were absent (perhaps, the authors speculate, because fish were “seeking refuge” there), and in those areas where fish were more plentiful, one type of insect commonly eaten by fish was less abundant. The researchers were expecting algae to proliferate in those same areas (where, because herons were absent and fish more abundant, there were fewer algae-eating insects), but instead they found that algae were more abundant in the areas with herons, highlighting the complexity of food webs and the unanticipated effects that can occur in response to changes.

This study “highlights the ecological importance of predatory wading birds,” the authors note. A change in the number of great blue herons in a local ecosystem will affect the birds themselves, of course, but it will also affect all the other organisms connected to them through the food web.

Fish are a staple food for great blue herons.

(Image by Pen Waggener via Flickr/Creative Commons license)