**4. Discussion**

Within 30 years, species distribution and habitat modeling for forest management and conservation have much developed technically, but such models have not become mainstream in actual planning. There are probably several reasons for that, as outlined below. Our broad conclusion, however, is that the biodiversity criterion of SFM (and other land use) cannot be met without simplifying and visualizing the living environment in *both* ecologically and socially relevant terms. Despite fair criticism against the misuse of the focal species concept [56,77,78,203,204], there are no clear alternatives for

making non-human perspectives of the environment and its impacts meaningful for stakeholders and the wider public. Carefully prepared maps integrate many technical and cultural tools for such perspectives, including spatial modeling as a major technique.

A detailed treatment of the problems with the surrogacy value of focal species is beyond the scope of this paper (see [56]), but we note that the concept seems to have suffered from two frequent misconceptions. First, that it is primarily about species [77,78,204] and, second, that the concept prescribes its prevailing practice to highlight vertebrates [205,206]. Although Lambeck [54] provided vertebrate examples, his main idea was to operationalize *threats* for conservation *action systematically*. Our case study demonstrated how the diversity of forest taxa can be considered for threat mapping, following from our long-term study of multi-taxon solutions for SFM [207,208]. In analyses on lichenized and wood-inhabiting fungi, we have concluded that focal-species criteria can be met in ca. 2% of the Estonian forest species pool [92,93]. This points at an order of magnitude of a few hundred focal species to be considered for forestry in this country. There is indeed scarce information on most of these species when compared with birds and other vertebrates (and it is likely to remain so), but the question for SFM and conservation is whether the existing information can be organized for the decision-making processes. Specifically, it may not be feasible to monitor all these species in the field, but spatial analysis of their habitats and perspectives using deductive models may be realistic. That, again, does not mean that focal species should be the sole tools to address threats or that they can be used everywhere. For example, useful tools for biologically mega-diverse tropical regions or for indigenous people are probably distinct [209]. However, as a principle, we maintain that non-human species have some integrated and cultural qualities that are not present in the alternatives of measuring physicochemical environments, functional traits in the assemblages, composition, or structure of ecosystems, or their 'services' to humans [38,63,210–212]. Such differences may deepen when it comes to society acting for a change.

Coming to the question of how can spatial analysis help to elaborate and represent meaningful information on selected species in SFM, we identified four key issues for future research and development. Perhaps the crucial one is the *research on social value of spatial mapping tools* for different decision-making processes. We doubt that focal-species models could contribute effectively to the highly formalized field of spatial economic optimization for multi-purpose forestry planning [23], due to general problems with the 'currency' of biodiversity and measurement error [213,214]. Focal-species models are probably more effective as heuristic tools for political processes, when 'windows of opportunity' shift attention on environmental threats [215]. Our review indicated several enlightening models developed for such policy processes (Table 1), but, in general, there seems to be much unused potential. Noting pronounced problems with uncertainties in biodiversity models [213,216], we emphasize that these analyses make sense in the context of particular decisions. For example, our models initially aimed to locate stands for active restoration for biodiversity, but the results changed the perspective to whether such restoration is feasible overall, and for what purpose. Thus, spatial precision became less an issue compared with field-checking of the qualitative predictions of general habitat availability for different species.

A related issue is a *lack of focal-species models on some important ecological dimensions*, notably the biodiversity in forest soils and water bodies [217]. We included these dimensions in our model set, although we faced difficulties with obtaining both species' natural-history information and relevant GIS data. A reason for the former is that the environmental impact research in freshwater and soil domains has traditionally focused on assemblages and taxon groups (including functional groups), not individual species [218–220]. Ecological analyses might reconsider whether group-level treatment is always justified for management [218,221,222], given that red-listing of species—an important part of the focal-species scheme (Figure 1)—has become an accepted formal tool. Inconspicuous species are gaining official protection in those jurisdictions that prioritize conservation status over public awareness. Similarly, protecting rare and threatened species is required by forest management certification systems, such as by the Forest Stewardship Council (FSC) [223]. Even from an awareness perspective, it is not

self-evident that invertebrate species would be ignored by stakeholders. For example, researchers working through media have succeeded in making a long list of 'primeval forest relict beetle' species and their locations attractive for the wider public in Germany [224]. Regarding the GIS data, remote sensing of soil conditions and small water-bodies in biodiversity-relevant terms can probably be better addressed using technological advances already available.

The third issue concerns *mapping units and spatial hierarchies*, with the basic distinction and different practices of gradient- and patch-based models [225]. We found these approaches complementary and used these in different phases of the analysis. However, it is indeed a caveat of most simple habitat models that they neglect the issues of population structure and viability, although these are primary issues when considering environmental threats to a species. An unanswered question is how much population-specific detail is appropriate to still retain the heuristic value of the model for broad questions.

Finally, we highlight that deductive models based on theoretical understanding are most useful for predicting into the (largely unknown) future. Regarding the present, inductive (empirical) models probably outperform deductive models in spatial prediction, and actual measurements of forest conditions may be even more reliable. This means that ecologists developing decision support for SFM and other land use in human-influenced ecosystems should combine approaches [216]. For modeling, a better understanding of the *drivers of future change* is necessary. Our review of the literature indicated that studies tend to predict the long-term future by simply extending the current social and economic context and legislature for many decades. Collaboration with social scientists of futures studies might help forest ecologists to understand better how to compile useful spatial long-term scenarios of land use [226,227].

**Author Contributions:** Conceptualization, A.L.; methodology, A.L., L.R., M.V., and K.R.; formal analysis, R.K. and A.L.; investigation, A.L., R.K., L.R., M.V., and K.R.; data curation, R.K.; writing—original draft preparation, A.L.; writing—review and editing, L.R. and K.R.; visualization, A.L.; project administration, A.L. and M.V.; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Estonian Research Council (grant IUT 34-7) and the Estonian Center for Environmental Investments (grant 16288).

**Acknowledgments:** We are grateful to the species experts who assisted in defining focal species for the case study, notably Piret Lõhmus, Anneli Palo, and Kai Vellak. The Estonian Environmental Agency kindly provided access to the archives of the national forest registry for the modeling input. We thank the Editors of the Special Issue for inviting this review, and two anonymous reviewers for constructive comments on the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
