By Rachel Chan
Coastal regions are densely populated, with much of recent human population growth occurring within these areas (Martínez et al., 2007). As a result, the phenomenon of “ocean sprawl” is becoming increasingly prevalent. Ocean sprawl refers to the replacement of natural marine environments with artificial structures (Firth et al., 2016). With growing coastal populations, land reclamation and artificial coastline defenses seem more and more inevitable. However, these artificial structures have harmful consequences for biodiversity on a local and regional scale. Ecological engineering (eco-engineering), in particular integrated green-grey infrastructure (IGGI), aims to compensate for the detrimental effects of coastal development. While eco-engineering approaches are emerging as possible solutions, there are several key issues that remain to be resolved before IGGI can become a mainstream solution.
To protect growing coastal populations, artificial coastal defenses are being constructed. Habitats such as mangrove forests and seagrass beds are being replaced by seawalls and rock armour to protect coastlines from the impact of waves and tsunamis (Chee et al., 2017). One of the few solutions to create space and protect against erosion is land reclamation. On a grander scale, land reclamation can even be used to build entirely new islands, as has been done in Asia and the Middle East with projects such as the Palm Islands in Dubai (Chee et al., 2017).
However, ocean sprawl contributes to habitat loss and fragmentation, as well as impacts ecosystem functioning. Engineered structures lack topographic complexity compared to natural substrates, which is essential for marine life. Smooth concrete reduces the number of habitat niches and provides less protection from predators and environmental stressors (Loke & Todd, 2016). Overall, the conversion of natural habitats to artificial structures causes functional homogenisation at a community level (Clavel, Julliard & Devictor, 2011). This is because specialists tend to be replaced by generalist species as the habitat becomes more disturbed, moving toward functional homogenisation (Airoldi & Bulleri, 2011). Furthermore, artificial structures facilitate the migration and establishment of invasive species. Diverse communities are more resistant to invasion: since artificial structures are less diverse than natural habitats, they are more vulnerable to invasive species (Firth et al., 2016).
IGGI involves ‘greening’ of grey infrastructure that has already been built, allowing urban areas to support ecosystem services (Firth et al., 2016). Most eco-engineering interventions aim to increase the surface area and habitat complexity of an artificial structure (Chapman & Underwood, 2011). Examples include adding crevices, pits, holes and introducing habitat-forming taxa (Strain et al., 2018). This is simply done through processes like adding structures or drilling. IGGI is a young science, and many of these interventions have only been carried out on small, experimental scales. Nevertheless, this eco-engineering approach shows much potential.
While the research is ongoing, almost all tested IGGI interventions have enhanced species diversity. It is important to note that the degree of improvement depends on the habitat setting and type of organism (Chapman & Underwood, 2011). For instance, fish benefited most from water-retaining interventions in intertidal environments, but benefited more from elevated structures in subtidal environments (Strain et al., 2018). Therefore, there is no one-size-fits-all approach with eco-engineering: interventions should be site-specific and address their main stressors. Since different interventions benefit different taxa, multiple interventions should be used to maximise niche diversity.
On a much larger scale, there have also been several examples of IGGI. These are mostly hybrid approaches that again attempt to enhance artificial structures to restore biodiversity. For example, enhanced shoreline sites were tested in Seattle, USA. A habitat bench was placed in front of an existing seawall, and a constructed pocket beach replaced existing riprap (Toft et al., 2013). These large scale shoreline enhancements provided habitat for invertebrate communities that were different to the conventional shorelines, enhancing taxa richness. The enhanced shorelines also increased the densities of juvenile salmon and larval fishes (Toft et al., 2013).
However, there are several caveats in eco-engineering approaches. As most IGGI approaches have been implemented for research on experimental scales, the benefits observed are largely on a small scale. There has also been a lack of experiments that assess the scaled-up versions of these same eco-engineering interventions (Evans et al., 2019). Furthermore, many of the experiment results have only been observed in the short term. Benefits may not be apparent straight away, or conversely, benefits of an intervention may plateau over time (Strain et al., 2018). These depend on the successional stage of a community, which is determined by the recruitment and growth of organisms. Most of these experiments have been carried out in temperate ecosystems (Strain et al., 2019), so there is no guarantee whether the results would be similar in different locations.
More worrying for marine life, however, is the potential misuse of eco-engineering research to greenwash coastal development projects. While IGGI compensates for environmental damage by altering already existing infrastructure, coastal development still has a large impact on wildlife. New development projects may be viewed as more eco-friendly by the public, and may therefore be more likely to gain consent to proceed (Firth et al., 2020). However, being able to compensate for some environmental damage should not be taken as a green light for new coastal development projects to go ahead without scrutiny. For large projects like land reclamation and constructing artificial islands, there is no amount of ecological design that can offset their environmental harm.
IGGI is incredibly promising and could play a huge role in making current artificial infrastructure more accommodating to biodiversity. However, most projects have been carried out on short time scales and in only a few locations. While outcomes have been positive, results from previous IGGI projects should be expanded to gain a better understanding of the different eco-engineering approaches. This will help in implementing the best approaches based on climates, habitat and stressors, and ultimately aid in future policy development.
References:
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Imperial Bioscience Review 