High focus on threatened species and habitats may undermine biodiversity conservation: Evidence from the northern Baltic Sea

Conservation policies and environmental impact assessments commonly target threatened species and habitats. Nevertheless, macroecological research provides reasons why also common species should be considered. We investigate the consequences of focussing solely on legally protected species and habitats in a spatial conservation planning context using a comprehensive, benthic marine data set from the northern Baltic Sea. Using spatial prioritization and surrogacy analysis, we show that the common approach in conservation planning, where legally listed threatened species and habitats are the focus of conservation efforts, could lead to poor outcomes for common species (and therefore biodiversity as a whole), allowing them to decline in the future. If conservation efforts were aimed solely at threatened species, common species would experience a loss of 62% coverage. In contrast, if conservation plans were based only on common species, threatened species would suffer a loss of 1%. Threatened species are rare and their ecological niches distinct, making them poor surrogates for biodiversity. The best results are achieved by unified planning for all species and habitats. The minimal step towards acknowledging common species in conservation planning would be the inclusion of the richness of common species, complemented by information on indicator species or species of high importance for ecosystem functioning. The trade‐off between planning for rare and common species should be evaluated, to minimize losses to biodiversity.


| INTRODUC TI ON
Mainstream conservation policies, legislation and environmental impact assessments tend to focus on threatened habitats and species that have elevated risk of extinction. Such species and habitats are typically only a small subset of biodiversity at large. Rarer species are at most imminent risk, so prioritizing conservation efforts for those species is understandable. However, the conservation of the large number of widespread and common species with no special legal status rarely seems to be a concern, although there are ecological reasons why it should be (Baker et al., 2019;Gaston & Fuller, 2008;Jansen et al., 2020). For example, common species may play important roles as foundation species, ecosystem engineers or habitat formers, and they support the coexistence of (rare) species (Ellison, 2019).
Various rare and threatened species depend on common species, either directly or indirectly (Thomsen et al., 2018). Common species are responsible for the functioning of the ecosystem, and they sustain ecological networks, that is interactions between species within communities (Ellison, 2019;Gaston, 2011;van der Zee et al., 2016).
Biogeographic ranges or ecoregions are mostly characterized by common species, due to their frequency of occurrence across geographic and environmental gradients (Gaston, 2011). Common species also provide more information about species richness patterns than the rare ones (Lennon et al., 2004).
The abundance or commonness of a species is rarely considered a value for conservation, as the focus is on preventing harm to rare and threatened species (e.g. the IUCN Red List) (Redford et al., 2013).
Major threats to biodiversity are foremost described via impacts on rare or threatened species, and environmental impact assessments focus on species with legal protection (Simmonds et al., 2020).
Nevertheless, common species are also at the centre of the biodiversity crisis. Habitat loss, environmental degradation, overexploitation and introduction of non-native species first impact common species, with cascading effects on others (Gaston, 2010;Jansen et al., 2020).
With the disappearance of common species, ecosystems lose the ecological roles of common species (Gaston, 2011), and the interactions maintained by common species become simplified (Losapio & Schöb, 2017). The loss of a common, but important species, may also reduce overall ecosystem functionality (Ellison et al., 2005). The decline of a common species may not be easily observed, as monitoring efforts are focussed on threatened species and habitats with legal obligations for protection.
Countries are now expected to plan towards the 30% protected area coverage, outlined in the Post-2020 Global Biodiversity Framework, set out by the Convention on Biological Diversity (CBD, 2021). Here, we link the macroecological debate of common vs rare species to the operational reality of spatial conservation planning, which relies on distribution data for species and habitats (Kujala et al., 2018). We assess how conservation outcomes vary depending on what we protect and how threatened species and habitats act as surrogates for broader biodiversity. Using a comprehensive data set on benthic marine biodiversity from 170,000 field sites in the northern Baltic Sea, on which statistical species distribution models have been developed (Virtanen et al., 2018(Virtanen et al., , 2023, we demonstrate how protecting only threatened species or habitats may undermine efforts to conserve overall biodiversity. Our findings show that the common approach in conservation planning, where legally listed threatened species and habitats are the focus of conservation efforts, leads to poor outcomes for common species (and therefore biodiversity as a whole), allowing them to decline in future ( Figure 1). The reverse does not hold, and in our example by far the most effective results are achieved by conserving both threatened and common species.

| ME THODS
The study area covers 81,500 km 2 of the northern Baltic Sea within the Finnish territorial sea and exclusive economic zone. The southern marine areas are eutrophicated, hypoxic and impacted by various human activities, whereas in the north, species communities of marine origin are limited by a longer ice season and lower salinity (Bonsdorff, 2006;Korpinen et al., 2021;Virtanen et al., 2019).
Since 2004, the Finnish Inventory Programme for Underwater Marine Diversity has collected information on the occurrence and abundance of species from over 170,000 sites. Data cover benthic invertebrates, vascular plants, algae and mosses, based on which statistical species distribution models have been produced at a resolution of 20 m (Virtanen et al., 2018(Virtanen et al., , 2023. These models describe distributions of common and widely occurring species, habitat formers and key species, as well as rare and threatened species. Over the years, targeted sampling for rare and threatened species has also been carried out, which allows the estimation of their likely distribution patterns. The data allow us to estimate how representative the legally Habitats are those listed in the Annex I of the Habitats Directive, with legal national obligations for their protection and with designated Natura 2000 areas. As such, Finland has the responsibility F I G U R E 1 Schematic of the hypothesis of the study. (a) Abundant conservation action is funnelled towards a known threatened/rare species that has its distribution nested inside the distribution of a more widespread, common species. Common species also hosts other, unknown population of the threatened species. (b) Through time, lack of conservation effort allows the populations of the common species to decline significantly, bringing it closer to threatened status. Meanwhile, the rare species continues a slow decline despite the high level of attention received, and the unknown distribution area of the threatened species is lost (a). Decline of populations and the diminishing ranges of common species can have indirect negative effects on threatened species via ecological networks.

(a) (b)
to ensure adequate conservation of these species and habitats, and to maintain their ecological status, for instance in environmental impact assessments. All the rest are species and habitats, with no legal incentives for their protection. Typically, these receive no special attention in conservation or land use planning.
The data used in our case study include 10 threatened species, eight Annex I habitats, and 215 common species, of which 9 are near threatened. The data are geographically and species-wise a comprehensive subset of the known benthic marine biodiversity of the Finnish marine environment. Figure 1 summarizes our hypothesis about how planning with threatened species alone can lead to poor outcomes for biodiversity as a whole.
We used balanced spatial priority ranking as implemented by the Zonation approach to spatial prioritization (Moilanen et al., 2022).
The approach utilizes data, for example on the local occurrence or abundance level of species (Kujala et al., 2018) to generate performance curves that describe the fraction of feature distribution covered at each step of the ranking (Moilanen et al., 2022). The concave form of a curve tells that a species is narrowly distributed, and can be covered with small area, whereas a linearly decreasing curve indicates a widely and evenly distributed species that cannot have its full distribution protected. Complementarity between features is maintained when developing the priority ranking, by iteratively accounting for conservation levels of features (Moilanen et al., 2022).
as low-ranked areas tend to have less biodiversity, compared with high-ranked areas, where loss of each grid cell leads to greater aggregate losses. For a detailed description of using Zonation in marine environments, see Virtanen et al. (2018).
To evaluate the effect of different species or habitats on conservation planning outcomes, we used surrogacy analysis, which is used to measure how the quality of the conservation outcome changes as the quality of data improves (Di Minin & Moilanen, 2014;Rodrigues & Brooks, 2007). The data were divided into two sets: one that drives the prioritization (the surrogate), and one that has no effect on the final results. Zero weights were assigned for features with no effect, meaning that they were tracked but had no influence on the priority ranking, and equal weights were given to surrogates (Appendix S1).
This allowed us to evaluate the performance of common species when prioritization is driven by threatened species, and vice versa.
Three scenarios commonly encountered in conservation planning were analysed: (i) threatened species vs common species, including near threatened (NT) species; (ii) threatened species including NT species vs common species; and (iii) threatened species including NT species and habitats vs common species.
The NT species were tested on both sides of the threatenedcommon division, as they may sometimes be considered in the group of features of concern.
Then for each solution, we calculated the potential impact of conservation planning decisions on threatened and common species using replacement cost analysis (Cabeza & Moilanen, 2006  When the protected area network is optimized for common species alone (including NT species), the threatened species suffer a 1.1% loss compared with the optimal scenario. This means that only 0.1 species distributions lose coverage across the 10 threatened species (10 species *0.011 = 0.1).

| RE SULTS
Accounting for both threatened and common species simultaneously produces a result that has minor marginal losses for both groups, especially at the top 20% and 30% levels of the landscape.
This outcome should be logically preferable to partial optimization and is not changed with the inclusion of habitats or grouping of NT species together with the threatened species (Table 1).

| DISCUSS ION
Our study has demonstrated that focusing solely on the protection of legally listed, threatened and rare species and habitats can result in inadequate conservation outcomes for common species and overall biodiversity. In our Baltic Sea case study, we found major differences in conservation performance when planning was based only on threatened species versus when both threatened and common species were protected. The reason is likely to be that the geographical ranges and ecological niches of threatened species are typically more limited than those of common species. When the focus is solely on threatened species, the low occurrence areas for rare species take precedence over the protection of core occurrence areas of common species.
Some of the threatened species are also dependent on habitats and resources provided by common species, which suggests a high level of habitat nestedness and biotic interaction.
Our results are based on a case study from the northern Baltic Sea, covering a benthic ecosystem. Nevertheless, similar results may occur elsewhere, if-using the language of spatial conservation planningsurrogacy fails. The idea of surrogacy is that information for a subset of biodiversity features will produce a result that is effectively the same F I G U R E 2 Outcome of planning for threatened vs common species. The curves show balanced accumulation curves as function of fraction of landscape protected for the species and habitats. Orange and green curves are for threatened and common species/habitats, respectively. The columns are for (left) planning for threatened species/habitats alone. Here, common species are just tracked, but they do not influence prioritization. (3) The number of data layers in the surrogate is either small in absolute terms or very small relative to the number of features in broader biodiversity (Kujala et al., 2018). Niches of species differ. Random variation will be large in a small sample of species niches, represented by spatial distribution models. (4) There are some relatively widespread and highly species-rich habitats, which are effectively ignored due to their commonness. Failure may occur, if the opportunity is missed for gaining conservation coverage area-efficiently for many species simultaneously. In the present case, the set of threatened species was the surrogate and at least failure modes (1), (3) and (4) (Neeson et al., 2018). We propose that conservation should move from protecting individual rare species and habitats towards protecting also ecological networks, functions and interactions (Valiente-Banuet et al., 2015;van der Zee et al., 2016). More attention should be paid to quantifying species roles in the ecosystem (sustenance, biotic interactions and habitat nestedness). Species could be protected while they are still common rather than waiting for them to first become rare (Gaston, 2010;Gaston & Fuller, 2008;Lindenmayer et al., 2011;Redford et al., 2013). High occurrence levels of many common species can be a surrogate for rarer biodiversity that simply has not been identified or observed (Rodrigues & Brooks, 2007).
Conservation planning should at least be replicated in a unified planning framework, to evaluate the opportunity cost for common species if a protected area network expansion is based on additional coverage for threatened species and habitats alone.
The minimal step would be via considering species richness or indicator species that are not yet threatened.
We urge the countries now planning for the expansion of protected area networks to consider alternative metrics for conservation design, going beyond legal agreements to conserve threatened species and habitats. Conserving only threatened species is the treatment of symptoms, not the cure for the biodiversity crisis.

K E Y WO R DS
commonness, protected area network design, rarity, red list, spatial conservation prioritization, systematic conservation planning TA B L E 1 Results of replacement cost analysis for threatened and common species in prioritization, which shows the loss of mean coverage of species groups compared to maximum attainable under the alternative planning scenarios.

S U PP O RTI N G I N FO R M ATI O N
Additional supporting information can be found online in the Supporting Information section at the end of this article.