Article: Heerhartz, Sarah M., et al. “Shoreline Armoring in an Estuary Constrains Wrack-Associated Invertebrate Communities.” Estuaries and Coasts 39.1 (2016): 171-188.
Hopefully you caught yesterday’s article on protecting shores with a living shoreline, we’ll be talking today about the negative impacts of the “dead” shoreline protections!
It’s hard to imagine a beach as anything but relaxing and peaceful. But imagine for a second you are a small invertebrate, about 1/500th the height of a human. If that were the case, being on a large sandy beach would be the same as standing in the middle of Death Valley. Not so relaxed anymore, huh?
To small organisms, beaches are essentially deserts. Beaches are not productive systems, they aren’t inhabited by a large community of primary producers, but they are surrounded by productivity from land and sea. Therefore, beaches rely on subsidies of organic matter from their surrounding ecosystems. We typically call these subsidies “beach wrack” (Fig. 1). If you’ve been on a beach, you’ve seen beach wrack, but probably didn’t stop to think twice about it. Beach wrack is composed of seaweeds, seagrass, leaves, and wood; a real mix of land and sea influence. To a small beach-dwelling organism, beach wrack is like an oasis in the desert, creating a zone for feeding and shelter. It should be no surprise then, that there are thriving communities of invertebrates inhabiting beach wrack (Fig. 2).
As humans, we love our beaches and have made great efforts to preserve them. One of the efforts made in the past few decades is called shoreline armoring where we build or add hard structures to beaches in order to protect them from erosion (Fig. 3). But armoring might have some unintended consequences as they can disrupt the transition from sea to land and alter beach wrack ecosystems. Here, researchers from Puget Sound, WA compared invertebrate beach wrack communities between armored and unarmored beaches.
Researchers picked 29 paired armored and unarmored beaches within the Puget Sound where pairs had similar physical characteristics. With their sites chosen, they went out and sampled beach wrack at each site for the presence, abundance, and diversity of organisms present. Invertebrates were sampled by placing 1 meter X 1 meter quadrats (a simple square device that provides a frame for counting things within) along a wrack line. Mobile invertebrates were captured and sampled by creating pitfall traps. In addition to quantifying the communities, researchers also employed PVC tubes filled with wrack material and capped with different mesh sizes to either include or exclude certain invertebrates. PVC tubes were left at sites for 30 days, after which decomposition of the wrack was calculated.
Overall, researchers found higher numbers of wrack-associated invertebrates in unarmored beaches as indicated by quadrat sampling (Fig. 4) and pitfall sampling (Fig. 5). When researchers broke down the communities into groups, the trend of greater numbers at unarmored beaches held up, with the exception of aquatic invertebrates in which there were higher numbers at armored beaches. The most striking result was the difference in talitrid amphipods (see Fig. 2) where they were 8.5x more abundant at unarmored beaches! Researchers also found a large number of invertebrate taxa representative of nematodes, beetles, winged insects, molluscs, gastropods, and polychaetes; just to name a few (Fig. 6-8).
Decomposition data was only analyzed from unarmored beaches, but an interesting trend was found that showed decomposition rates were significantly higher when fine mesh was used, excluding many invertebrates (Fig. 9).
We armor beaches with the best intent, but there are unintended consequences that go along with these actions. This research has shown that armoring beaches has a significant and negative impact on beach wrack communities. And while most of us didn’t know these communities even existed, they represent something very important. These researchers have highlighted the amazing amount of connectivity between terrestrial and aquatic ecosystems. In these beach wrack communities lie a diverse collection of organisms that help connect two vastly different systems and help the flow of energy between them. This research has shown that excluding or limiting wrack communities can be detrimental to nutrient cycling. Slower decomposition is not only bad for the communities that depend on a faster cycle but that also means our beaches are going to be covered by wrack for longer periods of time.
My hope is that a study like this opens our eyes to the complexity and connectivity of ecosystems that often goes unnoticed. While there may be casualties in our fight against climate change or in our preservation attempts, we cannot discount the importance of these transitional zones and their inhabitants.