With about a third of the world’s population living within one hundred kilometers of a coastline, coastal areas have been heavily modified to meet the needs of a large population, often at the expense of natural ecosystems. “Shoreline hardening” is the use of engineered structures such as seawalls or breakwaters to stabilize sediments, which provides protection to coastal communities from erosion and flooding.
In order to work toward sustainable coastal management, it is important to consider the effects these structures will have on biodiversity and the ecosystem services we depend on, such as fisheries production and water filtration. There are different types of shoreline protection that may not necessarily perform equally in this respect, and to assess these methods, a team of researchers including MSC postdoctoral researcher Rachel Gittman, Assistant Professor Steven Scyphers, and Associate Professor Jonathan Grabowski examined the impacts of these engineered structures on biodiversity and abundance of species in these areas.
In their recent paper, published in the journal BioScience, Gittman and colleagues conducted a meta-analysis of 32 studies that measured the biodiversity and abundance of species in areas with engineered shorelines versus natural, unmodified shorelines. Overall the authors found that seawalls supported 23% lower biodiversity and 45% fewer organisms than natural shorelines, while alternate methods of shoreline armoring such as riprap revetments and breakwaters appeared to have similar levels of biodiversity and species abundance compared with natural shorelines.
Based on the conclusions of the studies analyzed, the authors explored why there may be a difference between the impact of seawalls and that of other shore structures. One explanation is that seawalls lack the structural complexity needed to provide marine animals with refuge from waves and predators, while the rocks of a riprap revetment or breakwater may provide sufficient structure for them to hide. An alternative explanation is that the placement of seawalls disrupts the intertidal ecosystem by altering the way waves and sediment move, and abruptly cutting the system off from the shore. Since seawalls are hard vertical structures usually placed in the high intertidal zone, the gradual elevation change of a natural shoreline is partially lost, reducing both the quantity and quality of the habitat.
As climate change continues to threaten coastal ecosystems with storms and sea level rise, it is important to make long term coastal management decisions that are sustainable. While ripraps and breakwaters may cause less harm than seawalls, the authors encourage coastal managers to consider all options for shoreline protection, including natural alternatives to shoreline hardening. So-called “living shorelines” may consist of native vegetation plantings, oyster reefs, and other natural additions that reduce erosion while maintaining or even enhancing biodiversity in coastal habitats.
