Animals depend on consumable energy to live, and that energy can come from a variety of places. If the energy that animals get from their food varies in quality depending on where the animals get their food, what does this mean for birds like the Eastern Phoebe (Sayornis phoebe) that consumes both terrestrial and aquatic food? (Image Credit: Andrew Cannizzaro, CC BY 2.0, Image Cropped).

Aquatic and terrestrial resources are not nutritionally reciprocal for consumers (2019) Twining et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13401

The Crux

In the natural world, ecological subsidies, or the influx of sustenance from one habitat type to another, connect a variety of environments. While research has been conducted on this topic in the past, most of it has dealt with the quantity of energy moving between habitats, but not the quality of the resource itself.

When one habitat (such as an aquatic habitat) is rich in a specific resource that is hard to find in other habitats, subsidies of these resources play a unique role by providing animals and plants with food or energy that they could otherwise not get. The authors of today’s paper wanted to investigate if subsidies from aquatic habitats and terrestrial habitats contain the same amount of that hard to find, valuable resource: highly unsaturated omega-3 fatty acids (HUFAs).

Did You Know: Ecological Subsidies

As stated above, ecological subsidies are simply the movement of energy or resources between two different kind of habitats. Examples of this include aquatic insects like those used in this study (dragonflies, midges, and other flies) which live the majority of their lives in one environment (the water), but then move into terrestrial environments when they mature into their adult forms.

Salmon are another great example of ecological subsidies, as they live much of their lives in the ocean, but then return to freshwater streams to lay their eggs. When they come back they are providing a massive amount of resources to the organisms that live and feed in and around the freshwater streams.

What They Did

The authors used Eastern Phoebe chicks (Sayornis phoebe) as their model organism, as this bird consumes insects from both terrestrial and aquatic habitats. They collected blood samples from the chicks at three different streams in order to analyze the stable isotopes (markers that allow scientists to track where the food that animals eat comes from). In addition to the blood, the authors also collected aquatic and terrestrial insects to compare with the stable isotopes analysis of the blood. The analysis of the insects allowed the authors to determine which groups of insects contained more HUFAs.

The chief question of this study is whether or not the ecological subsidies that terrestrial organisms get from aquatic habitats differ in the amount of HUFAs that they get from their terrestrial food. In order to determine if the HUFAs that animals in these habitats need are coming from aquatic food, the authors collected blood samples from fish in the streams that they sampled. Because the fish are mostly eating food from the stream itself, but also get a limited amount of terrestrial food (such as bugs that fall into the water), this allows for comparisons between two very different animals living in the same habitat.

What They Found

The diets of the Eastern Phoebe chicks differed depending on the stream sampled, with one stream’s chicks eating more aquatic insects than terrestrial, one stream’s chicks eating more terrestrial insects than aquatic, and the third stream’s chicks eating about the same amount from each habitat. Interestingly, the birds acquired the vast majority of their HUFAs from aquatic insects. Like the birds, the fish that the authors sampled ate both terrestrial and aquatic food. Also like the birds, the fish acquired the majority of their HUFAs from aquatic insects.

Problems

Large-scale observational studies like this one take a lot of time and effort from multiple people to succeed, and as such certain limitations are placed on studies in order to make them work logistically. This study analyzed one type of terrestrial animal (the Eastern Phoebe) and only three streams within a relatively small geographic area. These “limitations” are not truly a problem and don’t take away from the findings of this study, but they highlight the need to perform similar studies with other terrestrial animals in other places in order to determine if these results are outliers or are in fact part of a widespread trend.

So What?

This may all seem very removed and abstract when compared with the human experience, but what this study has shown is that animals from one habitat type depend on somewhat limited, but incredibly valuable resources from other habitat types. We harp a lot on habitat connectivity here on Ecology for the Masses, and connections between different habitats like this in nature are not only common, but they appear to be quite important.

A separate experiment that was part of this overall study found that Eastern Phoebe chicks raised with HUFAs grew more and were in better physical condition than those that were raised without HUFAs. What this means is that if adult birds are not able to get these HUFAs for their chicks from aquatic insects, they may end up losing their chicks, which over time could result in population collapse and local extinction. With all of the recent press surrounding the insect apocalypse, studies like this one provide evidence that when one group of animals goes extinct (insects), other groups (birds) may be soon to follow.