Habitat destruction and fragmentation

Fig. 1. Marine habitats in Europe, such as kelp forests, support a high diversity of marine life and are threatened by human activity. Photo © BIOMAR

Marine ecosystems are experiencing high rates of habitat loss and degradation as a result of coastal development, exploitation of marine resources, pollution and other human pressures. Coastal wetlands, seagrass meadows, kelp forests, coral reefs, shellfish reefs and many other habitats have been strongly reduced in many parts of the world. These processes are considered among the most critical threats to marine biodiversity.

It has been estimated that, on average, a kilometer of coastline was developed every day between 1960 and 1995. Losses have continued unevenly since then. Global coral reef cover declined by an estimated 14% between 2009 and 2018 alone. Mass coral bleaching driven by ocean warming is now a dominant and accelerating driver of reef loss, compounding the roughly 20% of the world's coral reefs already lost and a further 20% degraded by the end of the twentieth century. Global seagrass meadows have lost an estimated 19% of their surveyed area since 1880, although the rate of decline has slowed or reversed in some regions in recent decades. Mangrove loss has also slowed globally: a net loss of about 284,000 hectares occurred between 2000 and 2020, as gains from natural expansion and restoration increasingly offset ongoing losses.

Habitat loss and biodiversity

A habitat is the environment in which organisms live. Habitats can be characterized by physical conditions such as substrate, water depth, salinity and exposure, but also by habitat-forming organisms. Seagrasses, kelps, corals, mussels and tube-building worms, for example, create biogenic habitats that provide physical structure and living space for many other organisms.

Habitat destruction and fragmentation are major causes of change in marine and coastal ecosystems..

• Habitat destruction or loss refers to the disappearance or degradation of habitat so that it can no longer support the organisms and ecological functions formerly associated with it.
• Habitat fragmentation refers to the breaking up of a continuous habitat into smaller patches separated by other habitat types or artificial barriers.

Habitat loss and fragmentation often occur together, but their ecological effects should be distinguished.

Habitat loss generally reduces the abundance and diversity of organisms that depend on the habitat. Three important consequences are:

1) The loss of resident species.

There is a unique plant and animal diversity living in close association with specific habitats or habitat forming species. For example, invertebrate assemblages associated with mussel beds, epiphytic and epibenthic assemblages communities living in kelp forests or fish communities associated with coral reefs. Destruction of the habitat can therefore cause the decline or disappearance of many associated species. The consequences can extend beyond the direct loss of resident species because species interact through predation, competition, facilitation and other ecological relationships. Loss of an important habitat-forming species can therefore cause changes throughout the associated community.

Fig. 2. The loss of habitats can lead to the formation of barren areas where only few species are able to survive. Photo © NOAA

2) The loss of food resources.

Most biogenic habitats are highly productive compared to simpler habitats. They produce large amounts of organic matter that can be directly or indirectly used by other organisms as food. Most biogenic habitats are highly productive compared to simpler habitats. They produce large amounts of nutrients and organic matter that can be directly used by other organisms as food resources. Habitat-forming species can also trap suspended material or provide feeding grounds for mobile organisms. The habitat loss also implies the loss of these food resources having a negative effect on the survival of other species and the productivity of individual species or communities, with other more profound effects that are likely to propagate along food chains .

3) The loss of ecosystem functions provided by the habitat.

Structurally complex habitats can provide refuge and nursery grounds, stabilize or trap sediment, modify currents and wave action, influence light conditions and nutrient cycling, and contribute to shoreline protection and other ecosystem functions. These functions can be strongly reduced when the habitat-forming organisms disappear. For example, the replacement of macroalgal canopies by turfs affects sediment dynamics on rocky coasts, where fronds prevents accumulation of sediments while turfs tend to trap sediments even on exposed coasts

Effects of habitat loss on biodiversity

Fig. 3. Coral reefs are an important structurally complex habitat that provide food resources and refuge to many fish species. Photo © David Balata

Habitat loss has been generally associated to drastic declines in overall abundance and diversity of marine organisms. For example, in the Wadden Sea, the destruction of biogenic habitats contributed to the regional extinction of at least 26 species during the past 2000 years . Similarly, the loss of seagrass meadows results in a reduction in the number of species and abundance of fishes.

Generally, the environmental changes associated with the destruction of natural habitats promote the arrival and colonization of opportunistic species that can benefit from conditions in disturbed condition. Other species that can favor from disturbed habitats condition are alien species. Once alien species are established they can contribute further to reduce local diversity by interacting with native species.

The ecological consequences of habitat loss depend not only on the amount of habitat remaining but also on its quality and spatial arrangement. Habitat patches that remain physically or ecologically connected can exchange organisms, larvae, propagules, nutrients and organic matter. Loss of this connectivity can impair population persistence and ecosystem functioning even when some habitat remains.

Fragmentation of habitats

Habitat destruction and habitat fragmentation are often discussed together, but they are ecologically distinct. Fragmentation subdivides a habitat into a mosaic of smaller, more isolated patches separated by a different, often less suitable, matrix habitat; this can occur with or without an overall reduction in habitat area. Fragmentation changes the spatial structure of the seascape in ways that affect organisms independently of total habitat loss, through processes such as:

• Edge effects: conditions differ near the boundary of a patch compared to its interior (e.g. exposure, predation risk, light, water flow), and smaller or more elongated patches have a proportionally larger edge relative to their interior.
• Patch size effects: smaller patches typically support fewer species and lower abundances of habitat specialists, though the relationship depends strongly on the mobility and life-history strategy of the species involved.
• Isolation and connectivity: increased distance between patches reduces the exchange of larvae, propagules and mobile juveniles and adults between them, which can limit recolonization after disturbance and reduce genetic exchange between populations.

These effects have been documented across most biogenic marine habitats, including seagrass meadows, salt marshes, coral reefs, mangrove forests and oyster reefs. Fragmentation is a central consideration in the SLOSS (Single Large Or Several Small) debate among conservation biologists over whether it is preferable to protect several already fragmented patches of habitat or a single large area. In practice, the optimal choice depends on the species and processes of interest, and on the degree of connectivity between patches. Because ecological connectivity underpins the exchange of organisms, nutrients and energy across a seascape, restoring or maintaining that connectivity is increasingly recognized as important for the design of effective marine protected area networks and habitat restoration projects, not just the protection of individual patches.

Habitat loss generally has negative effects on biodiversity, whereas studies of marine habitats show that fragmentation per se does not have a consistently negative effect. Effects depend on the organisms involved, habitat configuration, spatial scale and the properties of the surrounding environment. Fragmentation is especially damaging when it strongly reduces ecological or hydrological connectivity.

Human activities as driver of habitat loss and fragmentation

Important causes of coastal habitat loss include coastal construction, land reclamation, dredging, aggregate extraction, ports and harbors, industry and tourism development. In offshore waters, exploration and development of oil and gas activities threaten marine habitats, mainly through discharges of oil and other pollutants, see Coastal pollution and impacts. Recreation can damage sensitive habitats through trampling, anchoring, vehicle use or harvesting. Coastal protection structures can replace natural shore habitats and can impede the landward migration of intertidal habitats. See Ecological enhancement of coastal protection structures for measures that can reduce the impact of coastal defence structures on habitats, and Threats to the coastal zone for a broader overview of pressures on coastal ecosystems.

Fishing can cause extensive physical disturbance of seabed habitats. Bottom trawling can damage structurally complex benthic habitats, including cold-water coral reefs, sponge grounds and biogenic reefs, and repeated disturbance can prevent their recovery.

Dredging, channel construction, roads, causeways, dams and other infrastructure can also fragment habitats or interrupt the movement of water and organisms between them. In estuaries and coastal wetlands, loss of hydrological connectivity can have particularly severe ecological consequences.

Beyond these direct, local pressures, climate change is an increasingly important driver of marine habitat loss. Rising sea temperatures are causing more frequent and more severe mass coral bleaching events worldwide — on average, reefs were affected by severe bleaching roughly once every 25 to 30 years in the 1980s, compared to about once every six years by the mid-2010s. Ocean warming, marine heatwaves and ocean acidification also affect kelp forests, seagrass meadows and other temperate habitats, compounding the effects of direct human activities: Habitats already weakened by local pressures are made more vulnerable to climate-driven disturbance and their capacity to recover is reduced.

Rising sea levels combined with fixed coastal defenses can also lead to a long-term coastal squeeze of intertidal habitats between the defense and the encroaching sea.

Implications for conservation and management

The most effective way to conserve habitat-dependent biodiversity is generally to prevent avoidable habitat destruction and degradation. Management should therefore address the pressures responsible for habitat loss rather than focus only on species that have already declined.

Conservation should consider not only the total area of habitat but also habitat quality, spatial configuration and ecological connectivity. Connectivity can be maintained through networks of habitat patches that permit the movement of organisms or through hydrological connections that allow transport of larvae, propagules, organic matter and nutrients. Different species operate at different spatial scales, so there is no single optimal degree of habitat fragmentation or spacing of habitat patches.

Habitat conservation is important for maintaining the ecosystem services provided by coastal and marine habitats, including:

• regulating services such as attenuation of waves and shoreline protection;
• provisioning services such as fish and shellfish production;
• cultural services such as recreation and tourism;
• supporting ecological processes such as primary production, nutrient cycling and provision of nursery and refuge habitat.

Fig. 4. No take marine reserves are one of the most effective management tools to protect endangered habitats. Photo © David Balata

Long-term and large-scale monitoring is needed to detect changes in habitat extent, quality and distribution and to assess the effects of management measures.

The Kunming-Montreal Global Biodiversity Framework, adopted in 2022, sets targets to have at least 30% of degraded marine and coastal ecosystems under effective restoration and to effectively conserve and manage at least 30% of marine and coastal areas by 2030., building on the earlier assessment that the most effective way to conserve biodiversity, by almost any reckoning, is to prevent the conversion or degradation of habitat. Marine Protected Areas (MPAs) and marine reserves can be effective where damaging activities such as fishing, harvesting, anchoring or seabed disturbance can be controlled within their boundaries. They do not by themselves protect habitats against pressures such as climate warming, sea-level rise or pollution originating outside the protected area. The management measure must therefore be matched to the cause of habitat degradation.

Habitat restoration is another important component of conservation. Restoration programmes now target many marine and coastal habitats, including salt marshes, mangroves, seagrass meadows, oyster reefs, coral reefs and kelp forests. Successful restoration requires not only suitable local environmental conditions but often also restoration of connections with neighbouring habitats. Coastal habitats form interacting seascapes in which organisms, water, sediments, nutrients and organic matter move between different habitat types.

Restoration cannot always compensate for habitat destruction. Protecting intact habitats is generally preferable to destroying them and relying on subsequent restoration.