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)

Lake boundaries

“Historically, lake ecosystems were viewed as isolated ‘habitat islands in a sea of lands’ (Forbes 1887).”

So begins a paper recently published in the journal Oikos by a team of scientists working in northeast Germany. The paper they reference in their opening was written by Stephen Alfred Forbes, a prominent biologist from the 1870s until his death in 1930, and “the founder of the science of ecology in the United States,” according to L. O. Howard, writing for the National Academy of Sciences.

Though Forbes was one of the first scientists to investigate how species interact and influence one another, and emphasized the importance of studying a community rather than a single species in isolation, he did not recognize the ways in which the seemingly distinct habitats of lakes and land are interconnected. In his 1887 paper, he wrote:

“The animals of [a lake] are, as a whole, remarkably isolated – closely related among themselves in all their interests, but so far independent of the land about them that if every terrestrial animal were suddenly annihilated it would doubtless be long before the general multitude of the inhabitants of the lake would feel the effects of this event in any important way. … It forms a little world within itself.”

Recent research suggests that the boundaries of the ‘little world’ of a lake are not impermeable – energy and resources are exchanged between lakes and the terrestrial environments in which they are nestled. Sometimes, as in the study recently described in Oikos, the vector of that exchange is an insect with both aquatic and terrestrial life stages.

The researchers added corn leaves to part of two lakes (each lake was divided in half by a plastic curtain). Corn leaves have a different chemical signature than the other vegetation present in or near the lakes, allowing the researchers to trace the fate of the energy in the leaves as it made its way around the food web, from prey to predator.

In the sides of the lakes to which corn leaves had been added, the scientists found values consistent with the leaves in aquatic insect larvae that ate them, the adult forms of those insects, and the terrestrial spiders that ate the insects. In what they describe as a “boomerang flux,” they demonstrated the transfer of terrestrial carbon (in the form of corn leaves) first to aquatic organisms, and then back to a terrestrial setting.

Referring back to Forbes’ idea of lakes as distinct entities, separate from their surroundings, the scientists write: “a viewpoint of such isolation does not reflect the continued back-and-forth ‘boomerang’ cycling of organic matter (and probably nutrients) across the borders of terrestrial and aquatic systems.”

Forbes’ prescient insights into the importance of the relationships among species introduced the world to the concept of ecology and changed the way future scientists would think about and conduct their studies; with time, those future scientists would realize that the interconnectedness among species extends beyond the bounds of Forbes’ original idea of a lake, and beyond the shores of lakes themselves.

Many insects that spend their immature life stages underwater emerge to live in the terrestrial environment as adults. 

(Original image by Bj.schoenmakers via Wikimedia Commons)