Functional Biofilm on Intertidal Flats: A New Conservation Priority for Shorebirds

Intertidal flats are essential foraging areas for shorebirds but are severely impacted by climate and anthropogenic change. In their latest review, Kuwae and colleagues explore the effects of interventions that focus on intertidal biofilm for shorebird recovery.

A version of this post is available in Japanese.

Why shorebird species have been plummeting in numbers worldwide remains a subject of intense speculation and mounting conservation concern. However, the newly recognized importance of intertidal biofilm as a food source on which small-bodied shorebirds depend may provide clues to both a major reason for their decline and a path to recovery.

Photo_2 — Biofilm feeding by a small-bodied shorebird, the Western Sandpiper (Calidris mauri) at the Roberts Bank stopover in the Fraser River estuary, British Columbia, Canada © Jason Puddifoot

All coastal mudflats are coated by biofilm, a thin layer of sticky mucus inhabited by complex microscopic communities made up especially of diatoms (single-celled plants with a silicon skeleton), bacteria, invertebrates and organic matter (detritus). Up until recently, such muddy habitats were considered “wastelands” of little conservation value and the science around biofilm was largely ignored. However, the story has been rapidly changing since the discovery of fine bristles on the tongues of small-bodied shorebird species that enable them to rapidly scoop up globs of biofilm accounting for over 50% of their total diet.

Photo 3+4 — Left: Sampling (© Tomohiro Kuwae) and scanning electron micrograph (© Kazuhiko Ichimi) of biofilm at Komuke Lagoon, Hokkaido, Japan. Right: Morphology of fine bristles on the live tongue tip and scanning electron micrograph of the Red-necked Stint (Calidris ruficollis) © Tomohiro Kuwae

Moreover, many major stopover sites used by shorebirds to “refuel” during long-distance migrations between their breeding and overwintering grounds are biofilm-rich estuarine mudflats that have been subjected to severe degradation, underlining a probable link between biofilm and shorebird populations.

Seven ecological and technical attributes

In this the first publication of its kind, our team used lessons learned from decades of experience from human interventions to recover intertidal flats in Japan, plus recent research on sandpipers feeding on biofilm in the Fraser River estuary on the Pacific coast of Canada, to work out how to improve intertidal habitat for small-bodied shorebirds.

The three improvement goals for shorebirds were: to make a broad spectrum of food resources available, to maximize the energy intake rate for individuals, and to maximize their foraging activities. To achieve these goals, we reasoned that a combination of seven ecologically-based technical features would be necessary:

1) a sheltered coastal environment where fine particles could be deposited to promote biofilm;

2) complex topographical features such as tide pools and hummocks spread over the intertidal flat to enhance the foraging area and reduce competition for shorebirds;

3) a wide, gently sloping intertidal zone to increase foraging chances for shorebirds grazing biofilm as well as probing for invertebrate prey;

4) a range of sediment grain sizes spread across the intertidal area to secure biofilm and the biodiversity of alternative foods;

5) shallow water remaining at the lowest tide (<5 cm) to allow small-bodied shorebirds to forage in the subtidal zone;

6) freshwater/seawater exchange that enhances biofilm production, and

7) an open landscape so shorebirds on the intertidal flats could spot and avoid incoming predators such as falcons.

Fig_1 — Schematic diagram of a tidal flat that meets the proposed seven ecological and technical attributes: during high tide (top) and low tide (bottom). Water colour depicts water depth (deeper shades = deeper water), and sediment colour depicts the elevation for intertidal zones. The dark colour at the upper part of the intertidal zone indicates biofilm on muddy substrates

Application in practice

What comes next in actually putting this new knowledge to work? Shorebird use of an improved mudflat and the biofilm community would need to be closely monitored for numerous years before, during and after improvement, and compared against unaltered control mudflats to properly assess the project success. However, as yet, there have been no concerted attempts to explicitly recover or improve a fully functioning intertidal biofilm habitat for shorebirds, although there is urgent need to start.

Meanwhile, big research challenges such as what makes some types of biofilm so attractive to migrating shorebirds and how the conditions could be replicated in situ remain unanswered. The ecology of biofilm and the world of diatoms remain very much an enigma. In particular, while the seven technical features mesh with goals for providing the “quantity” of biofilm needed by shorebirds for energy, whether the “quality” of the biofilm would be sufficient to generate the essential fatty acids necessary for their migration success remains an important question.

Photo_1 — Flock of small-bodied shorebirds at Boundary Bay on the Fraser River estuary and delta, British Columbia, Canada © Tomohiro Kuwae

Indeed, one of the keys to unlocking the entire story on why small-bodied migratory shorebirds worldwide are declining and the methods necessary to recover them may well lie in understanding how these nutrients from diatoms in biofilm are produced naturally and how to replicate the ecological “machinery” in situ at the right time during migration.

Read the full review: “Seven ecological and technical attributes for biofilm-based recovery of shorebird populations in intertidal flat ecosystems” in Issue 2:4 of Ecological Solutions and Evidence.