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Habitat loss and degradation, and their interaction with other threats, are driving declines in animal populations worldwide. One potential approach for mitigating these threats is to create artificial habitat structures as substitutes for lost or degraded natural structures. Here, we provide — to the best of our knowledge — the first general definition of artificial habitat structures and synthesize important considerations for their effective use. We show that such structures represent a versatile conservation tool that has been trialed in a variety of contexts globally, albeit with varying degrees of success. The design of these structures must be well informed by the drivers of natural habitat selection, and their use should be part of an experimental framework to enable evaluation and refinement. We highlight possible ecological risks associated with the use of artificial habitat structures and urge that they not be exploited as inappropriate biodiversity offsets or for greenwashing. Looking forward, cross-disciplinary collaborations will facilitate the development of sophisticated and effective structures to assist animal conservation in this era of rapid global change.

1. Coastal landscapes are being transformed as a consequence of the increasing demand for urban infrastructure to sustain commercial, residential and tourist activities. A variety of man-made structures, such as breakwaters, jetties and seawalls have thus become ubiquitous features of intertidal and shallow subtidal habitats. This transformation will accelerate in response to the exponential growth of human populations and to global changes, such as sea-level rise and increased frequency of extreme meteorological events (e.g. storms). Here, we provide a critical overview of the major ecological effects of increasing infrastructure to marine habitats, we identify future research directions for advancing our understanding of marine urban ecosystems and we highlight how alternative management options might mitigate their impacts. 2. Urban infrastructure supports different epibiota and associated assemblages and does not function as surrogate of natural rocky habitats. Its introduction in the intertidal zone or in near-shore waters can cause fragmentation and loss of natural habitats. Furthermore, the provision of novel habitat (hard substrata) along sedimentary shores can alter local and regional biodiversity by modifying natural patterns of dispersal of species, or by facilitating the establishment and spread of exotic species. 3. Attempts to use ecological criteria to solve problems of urban infrastructure are promising. Incorporating natural elements of habitat (e.g. wetland vegetation; seagrass) into shoreline stabilization can reduce ecological impacts, without impinging on its efficacy in halting erosion. Likewise, improving the ecological value of artificial structures by adding features of habitat that are generally missing from such structures (e.g. rock-pools) can contribute to mitigation of the detrimental effects of urbanization on biodiversity. Management of anthropogenic disturbances (e.g. maintenance works; harvesting) to artificial habitat is, however, necessary if such attempts are to be successful. 4. Synthesis and applications. Increasing our understanding of the ecological functioning of marine habitats created by urban infrastructure and incorporating ecological criteria into coastal engineering are crucial for preserving biodiversity in the face of the growth of human populations in coastal areas and of forecasted global changes. Achieving this goal will need strong collaboration between engineers, managers and ecologists.

The biodiversity benefits of kelp aquaculture and afforestation are increasingly acclaimed as the industry continues to grow and develop globally, however, whether farmed kelp can provide this ecosystem service remains unclear. Using peer-reviewed literature, we evaluated whether kelp farms provide biodiversity benefits, and identified only 23 studies that discussed the effects of kelp aquaculture on biodiversity, half of which were broad reviews that only assessed the concept of ‘biodiversity’ peripherally (e.g. did not focus on specific responses or taxa). There is also a general lack of experimental research on the topic. Based on the evidence, it seems that kelp farms can create habitat via changes to the local environment, particularly through the provision of structure and changed nutrient cycling. While this can lead to increased abundance and diversity among certain taxa (e.g. fouling organisms), it seems that kelp farms typically create novel habitats that support distinct communities not equivalent to natural kelp forests. Moreover, the potential for kelp farms to support biodiversity depends on a range of operational factors, many of which may be at odds with farming objectives that require the harvest and removal of the habitat that farms provide. While more work needs to be done to address the complexity of comparisons between kelp farms and forests, especially at appropriate experimental scales, it currently seems unlikely that kelp farms will act as kelp forests and deliver meaningful biodiversity outcomes. We should instead recognise farms for providing their own valuable services and support restoration and conservation practices of kelp forests to pursue biodiversity outcomes.

Offshore renewable energy provides an increasing component of our electricity supply. We have limited understanding of the potential environmental impacts of these developments, particularly in the move to larger scales. Surfaces provided by devices offer novel habitat to marine organisms, which may allow species to spread to new areas. We used coupled biological and hydrodynamic models to investigate the spread of intertidal marine organisms with pelagic larvae (such as barnacles or gastropods) in the region around south‐western Scotland. We assessed the impact of novel habitat on dispersal and its role in allowing transgression of physical barriers. Model renewable energy device sites provided habitat for pelagic larval particles that would otherwise have been lost offshore. They also provided a source of larvae for existing coastal sites. Many offshore devices fulfilled source and destination (or intermediate connection) roles, creating new dispersal pathways, and allowing previously impossible northward dispersal from the Northern Irish coast to Scotland. Synthesis and applications. New habitat close to biogeographical barriers has implications for existing species’ distributions and genetic population structure. It also affects the spread of non‐native species and ‘climate migrants’. Monitoring these sites for the presence of such species will be important in determining the future ecology of coastal habitat and in maintaining economic aquaculture and marina operations. Future model studies should focus on particular species of importance, taking account of their biology and current distribution.

Habitat creation is a common element of efforts to recover populations threatened by altered habitat. Unfortunately, human-created habitats do not necessarily support re-establishment of target species. We investigated the value of constructed vernal pools as breeding habitat for the endangered Sonoma County population of California tiger salamanders (Ambystoma californiense) by conducting 19 years of larval surveys in 118 pools that were natural (n = 40), constructed (n = 70), or natural with substantial human alterations (human-altered, n = 8). California tiger salamander larval densities in human-altered and natural pools declined over the study period, while larval densities in constructed pools showed non-negative trends. Larval declines in natural pools may be attributed to long-term degradation of the surrounding terrestrial habitat. Mean larval densities over the course of the study were equivalent between constructed and natural pools, when corrected for pool morphometry. In contrast, human-altered pools had lower larval densities. To assure consistent California tiger salamander breeding, pools needed to be ≥30 cm deep and 240 m² in area, with deeper (50–70 cm) and larger (1,600–1,800m²) pools providing increased larval productivity. Over the study, there was no evidence of decrease in the rate of colonization of new pools. We conclude that constructed pools can be important tools in the recovery of imperiled pool-breeding amphibians if proper design elements for the target species are considered.

Landscape and site classifications are increasingly being used in conservation planning and biodiversity management. We examined the utility of a simple typology system for predicting the conservation value of running-water sites in Hungary using aquatic invertebrates. Aquatic invertebrates (444 species) were collected by kick and sweep sampling technique, in a few cases also with a net, at 317 running-water sites covering the entire area of Hungary. On the basis of three criteria (naturalness, altitude and size of catchment area) we obtained a typology scheme distinguishing five running-water types: artificial lowland stream, natural highland river, natural highland stream, natural lowland river and natural lowland stream. We expressed the conservation value of each site using the numbers of native species, unique native species, red-list species, protected species and alien species. Furthermore, the conservation value of each river type was expressed by a measure of beta diversity. Our results show that any interpretation of the effect of a single criterion might be misleading. Consequently, the use of the whole typology system is recommended. The study revealed that all stream types are valuable to a certain extent because they maintain distinct biological communities. We found that the conservation value of artificial watercourses is comparable to that of natural running-water sites. We identified that natural lowland rivers and artificial lowland streams are the ones mostly exposed to species invasions. These findings are essential in maintaining and protecting conservation values of any freshwater ecosystem, and may contribute to management decisions on running waters in Hungary.

Humans use the coastal ocean and its resources as a source of food and energy, as well as for a variety of other purposes, including transportation and recreation. Over the past several decades, uses of the coastal ocean have been increasingly accompanied by the installation of artificial structures. These artificial structures come in different shapes and sizes, ranging from energy and aquaculture infrastructure that incidentally form habitat for marine organisms to artificial reefs that are often deployed intentionally to become habitat. Marine spatial planning has offered a robust framework for siting artificial structures to minimize conflicts with other uses and maximize societal and economic benefits with other intended uses of the seascape, but ecological criteria are seldom considered in the planning process. In contrast, artificial reefs are intentionally sunk to form structured habitat and provide a variety of ecological functions, yet ecological principles are not often incorporated into the siting and planning process. Instead, artificial reefs are sited largely to advance societal and economic benefits and minimize conflicts with other uses, such as shipping traffic, military use, or impacts to sensitive areas. We outline a framework to further incorporate ecological principles into artificial reef siting, design and construction, and evaluation that features place‐based and adaptive management coupled with tenets from experimental field ecology. This framework accounts for complexities of and interactions among ecological, societal, and economic criteria associated with artificial reefs to ensure they meet defined goals.

Habitat complexity of freshwater ecosystems has been decreasing due to human impacts. Therefore, conservation and environmental management actions have intensified in the recent years. Artificial floating islands (AFIs) are one environmental management action intended to promote the populations of aquatic organisms. In this study, we installed eight AFIs in the littoral area of Lipno Reservoir, Czech Republic and covered them with local wetland vegetation to study the impact of this mitigation action on the fish community. The AFIs were sampled by Point Abundance Sampling Electrofishing (PASE). The AFIs were mainly inhabited by juvenile roach (Rutilus rutilus) and perch (Perca fluviatilis), with densities one to two orders of magnitude higher than in the surrounding control sites. Juvenile catfish (Silurus glanis) and adult northern pike (Esox lucius) were apex predators that were recorded exclusively at AFIs. More fish were captured in AFIs than in control areas (up to 29.2 times more). Even AFIs of simple construction harboured significantly more age 0 + and juvenile fish than control areas, confirming their potential to serve as nursery ground for age 0 + fish in lentic systems. Artificial floating islands can be used to support juvenile fish in the conditions of impoverished littoral structured habitat.

Offshore wind farms (OWFs) act as artificial reefs, attracting high abundances of fish, which could potentially increase their local production. This study investigates the feeding ecology of fish species that abundantly occur at artificial habitats, such as OWFs, by examining the short- and the long-term dietary composition of five species: the benthopelagic Gadus morhua and Trisopterus luscus, the pelagic Scomber scombrus and Trachurus trachurus, and the benthic Myoxocephalus scorpioides. We conducted combined stomach content and stable isotope analyses to examine the short- and the time-integrated dietary composition, respectively. Our results indicated that benthopelagic and benthic species utilize artificial reefs, such as OWFs, as feeding grounds for a prolonged period, since both analyses indicated that they exploit fouling organisms occurring exclusively on artificial hard substrates. Trachurus trachurus only occasionally uses artificial reefs as oases of highly abundant resources. Scomber scombrus does not feed on fouling fauna and therefore its augmented presence in OWFs is probably related to reasons other than the enhanced food availability. The long-termed feeding preferences of benthic and benthopelagic species contribute to the hypothesis that the artificial reefs of OWFs could potentially increase the fish production in the area. However, this was not supported for the pelagic species.

Urbanisation results in the loss and alteration of natural wetlands and ponds. However, garden ponds in cities and towns can potentially act as rich reservoirs of aquatic biodiversity and stepping stones for dispersal. Homeowners with a range of different motivations, including biodiversity values, install garden ponds. Here, our main aim was to study whether the design and management choices of garden pond owners depended on the location of ponds (capital city vs. countryside), when ponds were installed (pond age), or whether fish were introduced. We surveyed 834 garden pond owners across Hungary using a citizen science questionnaire, asking questions on pond size, location, construction date and materials, vegetation structure, introduction of fish and management practices. From 753 validated responses, we found that the introduction of fish into ponds and high urbanisation were strongly associated with local features and management practices, especially large ponds with a water circulation feature, irrespective of pond age. A typical garden pond in Hungary is ~ 20 m2, < 10 years old, made of rubber lining, contains fish, aquatic vegetation and circulating water, and is actively managed. There was a spatial separation of ponds based on local features between ponds in the capital city (Budapest) and elsewhere. These findings suggest that garden pond owners in the city were more likely to make different choices in pond design and management compared to owners in regional areas. Our results also suggest that pond owners may primarily select management practices to improve habitat quality for ornamental fish. Our findings have important implications for maintaining aquatic biodiversity in urban areas, where garden ponds may be the only aquatic habitat available.