Need for global conservation assessments and frameworks to include airspace habitat

Article impact statement: The pervasive human-driven decline of life on Earth points to the need for transformative change in the airspace. This article is protected by copyright. All rights reserved.


Introduction
The Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) recently presented its final report on the first global biodiversity assessment since 2005 (IPBES 2019). This exhaustive analysis of the effects of global change drivers on land, freshwater, and marine habitats constitutes an essential guide for the urgent implementation of evidence-based conservation strategies and to inform policy. The engagement of a wide range of stakeholders, from governments and scientists to indigenous peoples, assures the legitimacy of the assessment and has resulted in an informed guide for policy development (Díaz et al. 2019). Despite the intended exhaustive nature of the report, the terminology used when referring to changes in land and water use could hinder the transformative change necessary for effective conservation of a distinct habitat: airspace (3-dimensional region of the troposphere where aerobiological activity occurs [Diehl et al. 2017]).
Because the processes and impacts that occur in water are different from those occurring on land, the driver land-and water-use change is analyzed separately for these environments (IPBES 2019). Similarly, airspace has unique characteristics that make it a distinct habitat with distinct threats (Diehl 2013;Lambertucci et al. 2015;Davy et al. 2017). The airspace habitat extends from the land or water surface to the upper limit of the troposphere and is formed by the basoaerial (up to 1 km in height), mesoaerial (1−8 km), and epiaerial layers ( (Diehl et al. 2017). The airspace as a habitat generally goes unnoticed and is not considered in conservation strategies or legislation, despite increasing disruption, mainly in the basoaerial layer (e.g., buildings, windfarms, aerial artifacts, and pollution) (Lambertucci et al. 2015;Wallace & Holman 2019). Given that the documents generated by IPBES will guide conservation strategies and policies in the coming years, inclusion of the airspace is urgently required to promote the comprehensive protection of biodiversity.

Processes and Threats in the Airspace
New technology has increased the ways humans use airspace, which affects important processes that occur in the air and creates new human-wildlife conflicts (Lambertucci et al. 2015). Species coexist in the airspace, and many use it for breeding, feeding, or dispersing, and some even live in the airspace most, or all, of their lives. For instance, aeroplankton constitute a highly diverse taxon that lives in the air, where they actively metabolize, grow, and reproduce at different elevations (Polymenakou 2012). Some vertebrates, such as swifts (Apus apus), which are airborne for around 10 months/year, spend a great part of their lives in the air (Hedenström et al. 2016). Aerial migratory species use airspace to move between breeding and wintering areas each year. Migratory species represent a large proportion of birds: 70% of the 630 North American bird species is considered migratory (Horton et al. 2019). Moreover, aerial species (birds, bats, insects, etc.) use airspace every day to reach their feeding grounds and return to their roosts.
For large aerial vertebrates, airspace provides a fundamental source of energy that allows them to travel many kilometers over many hours with low energy expenditure (e.g., condors can fly >5 h and >170 km without landing or flapping [Williams et al. 2020]). Airspace is also the main place where a key process for plants, as pollination, occurs, particularly for many of the plant species that provide human food (Klein et al. 2007). Impacts on airspace may, therefore, disrupt key ecological processes and nature's contributions to people (e.g., pollination of crops). This habitat deserves protection for the same reasons as other habitats.

Airspace and Drivers of Global Change
Conservation strategies designed for terrestrial and aquatic environments could be partially effective for aerial species. However, the land-use changes resulting from habitat loss and fragmentation, pollution, and climate change that threaten air-dwelling species require different, specially designed conservation strategies (Lambertucci et al. 2015;Diehl et al. 2017). For instance, changes in wind intensity and direction will affect terrestrial and aquatic biodiversity, but will have a stronger impact on ecological processes dependent on air, such as pollination, seed and fungus dispersion, aerial vertebrate and invertebrate species migration, and food availability for insectivores (Davy et al. 2017;Diehl et al. 2017). If the distinctness of the species, processes, and threats occurring in the airspace is not considered, unique problems may be underestimated and conservation opportunities missed (Diehl 2013;Lambertucci et al. 2015;Davy et al. 2017). Despite big cities and agricultural areas being barriers to many wild terrestrial animals, several flying species still use airspace over these areas or pass over them during migration (Medan et al. 2011;Horton et al. 2019). These anthropic areas are generally not considered conservation priorities. However, specially designed conservation strategies could reduce their far-reaching effects on flying species that depend on airspace, even in human-modified areas (Loss et al. 2015;Davy et al. 2017). The main threats are stationary or mobile structures that lead to collisions and wildlife casualties. The exponential increase in airplanes, drones, buildings, power lines, and wind farms is fragmenting the airspace and increasing human-wildlife conflict (Lambertucci et al. 2015;Loss et al. 2015;Davy et al. 2017). For instance, some species of bats are experiencing serious population declines due to wind farms (Kunz et al. 2007). The fragmentation created by human-made structures may also disrupt the movement patterns of aerial species, forcing them to use more energetically costly or dangerous migration routes (e.g., wind farms on land or water) (Desholm & Kahlert 2005;Kunz et al. 2007). These disruptions may affect species that use airspace and require specific conservation strategies (Davy et al. 2017), although more research is needed given that diverse species may respond differently.
Pollution and climate change are also among the 5 main drivers of global change on land and water, but they affect processes occurring in the airspace as well. Air pollution (chemicals, heat, light, or noise) affects human health (Losacco & Perillo 2018) and wildlife physiology, dispersal, and communication and increases collision risk (Davy et al. 2017;Horton et al. 2019). Light pollution attracts nocturnal migrants to urban areas (La Sorte et al. 2017). Birds have the greatest overall mortality due to light pollution (Loss et al. 2015). Climate change is threatening the survival of pest-controlling bats (Pruvot et al. 2019) and altering the migration patterns and pollination services of aerial species (Magrach et al. 2020).

Airspace Biodiversity in the Current Agenda
The distinct impact of global-change drivers on airspace must be assessed globally so that effective conservation strategies that include aerial species can be developed and the comprehensive changes sought pursued. The conservation approaches needed include development of 3-dimensional aerial reserves that could be temporal and mobile (Diehl 2013;Lambertucci et al. 2015), depending on the focal species, community, or ecological process to be protected. Some of these conservation strategies are included in aeroconservation, which integrates key research areas related to aerial habitat and species conservation (Davy et al. 2017). Worryingly, aerial species and habitat are strongly underrepresented in current biodiversity assessments, which focus mainly on land and water biodiversity threats (e.g., IUCN;Davy et al. 2017). Although the global driver landand water-use change may partially include some aspects of the airspace, IPBES and other assessments do not include the evaluation of impact on biodiversity occurring in this newly defined habitat. For example, migratory flying routes, where millions of animals concentrate, may extend above protected or unprotected land or water, but the airspace along these extensive routes is generally not protected or protection is not specifically designed for many aerial species (Davy et al. 2017). Migratory flying routes that pass over large human constructions (e.g., cities) cause the death of millions of insects and birds (Loss et al. 2015;Horton et al. 2019).
The Convention on Biological Diversity (CBD) is working on the post-2020 global biodiversity framework, and their report will guide future conservation strategies. The CBD (2020), like the IPBES report, does not propose the protection of airspaces among its targets designed to Timing is crucial in considering the concept of aerial habitat and aeroconservation. Including these concepts, although rather new and little known, could make these reports pioneering and lead to successful mitigation and preventive strategies. In the case of anthropogenic development of the airspace, conservation planning could preempt several threats. However, some airspace features (e.g., skyscrapers and windfarms) already cause high mortalities in key pollinator and seed-disperser species (Kunz et al. 2007;Loss et al. 2015;Magrach et al. 2020). We propose that the coming IPBES and CBD assessments change the name of the main global-change drivers to land-, water-, and air-use change and include the impacts of all drivers on the ecological processes occurring in the airspace and associated conservation measures. These suggestions are not minor, given their implications for human health and wildlife conservation. We hope that this and other global biodiversity assessments adopt the airspace habitat in their evaluations. This will aid the design of effective conservation measures and transformative change in all the habitats that support wildlife, given that without a functional airspace, many biological processes that are important for nature and contribute to human life and well-being may become functionally extinct.