1. Introduction
Biodiversity is a good indicator of ecosystem health; hence, its conservation is essential for attaining sustainable development. However, anthropogenic climate change presents pervasive and growing threats to biodiversity and has implications for ecosystem services provision [1,2]. These threats make conservation programs even more complicated and unpredictable as species and ecosystems may respond to climate change differently. There is also an increasing number of terrestrial flora and fauna species that are under threat amid limiting conservation resources [3]. One of the challenges facing biodiversity conservation is how to efficiently allocate limited resources to many focal species and/or ecosystems needing immediate conservation attention [4,5]. Thus, the identification of conservation priorities, especially those that are based on species distribution, habitat loss, endemism, and vulnerability as influenced by climate change, is necessary to effectively inform future research and conservation efforts. Information on conservation priorities can subsequently be useful for the development of more efficient local and/or national conservation action plans.
When faced with tough conservation decisions on which species to conserve, one practical approach is to rank different species according to their priorities for conservation. Many conservation prioritization studies have already been carried out worldwide, and the prioritization methods and criteria are very diverse in terms of targets and goals [6]. Conservation practitioners have long debated which criteria should be considered crucial when allocating limited conservation resources. This is because the selection of priorities is influenced by many factors, including legal/policy, administrative, and mandate of the institution or agency involved [7,8]. Governance and conservation policies, which are based on utilitarian considerations, affect what is implemented in a conservation strategy [9,10]. There are also other factors that some authors consider when prioritizing species for conservation, for example, access or availability of the samples, local requirements, and estimated costs of conservation actions [11,12]. Hence, different sets of criteria depending on the objectives and/or interests can be adopted because, to date, there is still no consensus methodology for choosing taxa that should be given priority for conservation. For example, semi-structured interviews were used to gather information about traditional knowledge on target plant species in West Africa, and priority species were identified using a combination of ecological, socio-economic, ethnobotanical, and threats criteria [13]. In an attempt to control the unsustainable collection of native medicinal plants in Brazil, a survey was conducted to gather information about species versatility and establish conservation priorities using only species relative importance and sensitivity index [14]. In terms of fauna, priority setting for Philippine bats was carried out using a practical approach considering information on threats, conservation status, and endemism [15]. A problem-based approach for prioritizing conservation actions for both flora and fauna species was developed in Australia using threats, actions, and costs [16]. With advanced development in technology, conservation priorities of vertebrates and plants in Southeast Asia were mapped through species distribution models using Maxent software [17]. Some issues, however, have also been emerging about the use of different methods and/or criteria, including collinearity effects of related variables (e.g., ethnobotanical and ecological variables), result subjectivity due to ordinal ranking, and quantitative to ordinal data conversion [18,19]. Common mistakes (e.g., hidden value judgments and arbitrariness) have already been reported for the application of quantitative approaches to setting priorities in conservation [20]. Moreover, limited data on some major measures of biodiversity status have also generally restricted conservation prioritization [21].
Another challenge in the optimization of biodiversity conservation efforts is the need to identify the scope of objectives, i.e., whether single species or multiple species-based approaches. The choice of approach is central to the decision-making process in the ranking of conservation priorities, although each approach has its inherent inadequacy. One of the questions from using single species is whether the chosen focal species would represent the conservation needs of the other species. There is also growing uncertainty about the ability of single focal species to adequately protect all species due to myriad differences in life-history strategies, niches, and habitat requirements of multiple species growing in a particular habitat [22,23]. Consequently, conservation efforts have started to concentrate on the multiple-species approach, which is based on conserving several focal species. However, the problem commonly associated with this approach is the availability of data (e.g., habitat requirement, distribution, and population trend) since multiple species are already involved. While no existing guidelines are available yet in literature as to which scope should be used, scope identification may effectively determine conservation priorities by preventing duplication of effort and biases.
The present systematic review determined the knowledge gaps, research trends, and most frequently used criteria and methods in setting conservation priorities for flora and fauna from peer-reviewed articles published from 1990 to 2022. In choosing the review period, we considered the necessary degree of comprehension that is appropriate for the literature search questions that we would like to look into. We also organized the present review into the following questions: (1) what is the proportion of conservation prioritization studies for each country?; (2) which conservation prioritization criteria are frequently used in flora and fauna?; and (3) which methodological approaches are commonly used in prioritizing choices in conservation? The present work will provide us with an integrative understanding of the research trends and methodological approaches to setting conservation priorities. This will also provide a better understanding of the appropriate prioritization methods depending on available resources, well-defined goals, and data availability. Further, the synthesis will help conservation practitioners navigate difficult decisions about the efficient allocation of limited resources for biodiversity conservation.
2. Materials and Methods
2.1. Data Collection
A systematic literature review (SLR) was carried out to collect relevant evidence on the current understanding of a select topic and answer the formulated research questions from peer-reviewed articles published from 1990 to 2022. The SLR was conducted from March to June 2022 and yielded an initial total of 5980 articles. In this SLR, we followed the method used for environmental science, natural resource management, and biological science research [24]. The search databases used were ScienceDirect, PubMed, and Google Scholar, which are the leading search engines for peer-reviewed scientific studies [25,26]. These databases were commonly used in published SLR articles across disciplines, e.g., [27,28]. A study on optimal combinations of database for literature review included Google Scholar as one of the databases to be used as a minimum requirement for adequate and efficient coverage in systematic reviews [29]. As per the guidelines on conducting SLR, multiple databases, ideally one leading database and two subject-focused ones, should be used in systematic search [30]. Papers that used quantitative, qualitative, and combination of the two methods were included in the SLR to ensure an extensive representation of the literature. Further, all articles that employed either experimental or observational designs were also included.
A preliminary search was first conducted to refine the search terms. Thereafter, the final search terms were formulated, considering the most important keywords in each set of search terms, namely, conservation, prioritization, and biodiversity (Table 1). Boolean search strings (i.e., “AND” and “OR”) were inserted in all uppercase letters in the search engine to exclude, broaden, and define the search results, following the required number of strings based on the requirements of the database used. In the present SLR, the AND Boolean operator was used to include both the terms (e.g., “conservation” AND “prioritization”). We used OR Boolean operator to search for the articles that used either of the two search terms (e.g., “flora” OR “fauna”), resulting in more focused results. The advanced search feature in each database was utilized by specifying the keywords or search terms, publication year range, and article type. We did not include more specific terms (e.g., medicinal plants, species name, or specific name of the method) to avoid bias in the search terms used.
2.2. Article Screening
The present SLR followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines in reviewing, screening, and selecting articles, following the four basic steps in Figure 1. The first step is a preliminary identification of relevant articles using title-abstract-keyword domains. In this step, keywords in the search terms that did not occur in any of the title, abstract, and keyword sections of the paper were excluded. Grey literature and articles that are not peer-reviewed original or review articles and not published between 1990 to 2022, were also excluded in the SLR. The papers were screened further by excluding articles written not in the English language, irrelevant, and duplicated. The articles with the same publication year, title, and author were excluded at this stage through Mendeley Reference Manager (version 2.72.0) and pivot table in Microsoft Excel Spreadsheet for some cases. All the articles that passed the first set of inclusion criteria were selected for further investigation and content appraisal using the abstract skim reading. Here, the articles that are not open access or without available free full texts were not included in the SLR. The databases provide links to the PDF copies of the paper, and in case not found, we searched some research websites (e.g., ResearchGate and Google search engine). All abstracts were skim-read and those with unclear results about the topic or research questions were excluded. Thereafter, we performed the suitability assessment or validation by skim reading the main text and focusing only on the results and methodology sections of the paper. All papers with ambiguous results and no detailed explanation of the prioritization methods used were excluded. The quality of the SLR was assessed based on four quality assessment questions:
Did the included papers undergone a peer review process?
Are the databases likely to have provided all relevant studies on the topic?
Are the included papers described the methods and results adequately?
Are the research designs for prioritizing species appropriate?
2.3. Data Extraction, Management, and Analysis
Data were extracted manually from each article and encoded in Google Sheets so that all authors can easily monitor the progress and accuracy of the data extraction. The criteria used for the extraction of information from the selected articles are summarized in Table 2. Publication date (year) was obtained from the top or bottom of the article’s page. The study site country was identified from the “Study site description” of each article to determine the number of studies by country. Studies without any information about the study site were excluded. For articles that mentioned only cities, island, province, and names of permanent plots and communities, the country was searched through Google search engine. The types of conservation approach, criteria used for prioritization, methodological approaches used, and scope of the study were all determined in the materials and methods section of the paper.
After reading through all articles and recording all information for all extraction criteria, we categorized the type of organism (i.e., flora and fauna), types of conservation approach (i.e., single species, multiple species, and ecosystem-based approaches), and study scope (i.e., local, national, and global). The pivot table function in a Microsoft Excel spreadsheet was used for data sorting and categorization. The relative counts of studies by county, criteria used for prioritizing conservation choices, methods for ranking conservation choices, and prioritization approaches in conservation for flora and fauna species were computed using the pivot table. We made sure the spelling of each entry was correct and consistent throughout the spreadsheet to avoid double counting. Terms with the same meaning (e.g., endemism rate and no. of endemic species, population trend and species population changes, etc.) were encoded in the spreadsheet using the same words. Visualization was performed in SigmaPlot (version 10.0) and the ArcGIS software (version 10.7) for mapping the frequency distribution of studies by country.
2.4. Study Limitations
The present systematic review was limited to peer-reviewed articles which are written in English and published between 1990 and 2022. All the included articles are at least indexed in Scopus, and the documents published as brochures or technical manuals were not considered in this study. However, the number of databases used, selection of the review period/duration (i.e., 32 years), selection/quality criteria for article appraisal, and search strategies were based on systematic review protocol for environmental science, natural resource management, and biological science research. Hence, the data we used can already allow us determine the knowledge gaps and research trends about setting conservation priorities.
3. Results
3.1. Frequency of Studies by Country
Across the globe, India has the highest number of studies (i.e., 12) about the reviewed topic for both flora and fauna (Figure 2). Brazil ranked second with 11 studies, followed by the United States of America (USA) and South Africa with seven and six studies, respectively. Canada and Mexico ranked fifth with five total studies. Most of the megadiverse and biodiversity hotspot countries have only 1–3 studies on the topic.
The number of studies started to increase from year 2005 until 2010, gradually declined from 2011 to 2016, and then increased again from 2017 to 2021 (Figure S1). Of the two types of organisms, flora has a higher number of studies compared with fauna.
3.2. Criteria Used for Prioritizing Choices in Conservation
Results of the systematic review revealed bio-ecological criteria as the most frequently used criteria for prioritizing choices in the conservation of both flora (i.e., 41.08%) and fauna species (i.e., 55.42%) (Tables S2 and S3). This is followed by the socioeconomic and management criteria for flora and fauna, respectively. The climatic/edaphic and the taxonomic/genetic variables in flora had the lowest relative count (i.e., <5%), whereas the lowest value (i.e., <10%) in fauna were observed in socioeconomic and climatic/edaphic criteria.
In flora, the species distribution was the most frequently used bio-ecological criterion for prioritization. The adaptability, habitat connectivity, habitat vulnerability, invasive species richness, life history traits, mortality, the potential for recovery, species diversity, species origin, and species vulnerability had very low relative counts, i.e., <1% of the total number of reviewed studies. The economic demand/value of the species had the highest relative count (i.e.,5.73%) within the socioeconomic variables, and this is followed by resource use diversity, ethnobotanical value, and local knowledge/perception. The other socioeconomic variables criteria (e.g., stakeholder/public interest) had values lower than three. In the case of management criteria, conservation status accounted for 11.15% of the total number of reviewed studies, while the other management criteria had less than 3% relative count. All the climatic/edaphic variables (air temperature, precipitation, humidity, topography, latitude, elevation) accounted for only 1.59% of the total number of studies. Lastly, the relative count of the taxonomic/genetic variables ranged from only 0.64% to 2.23% (i.e., evolutionary distinctiveness < phylogenetic distinctness/diversity < genetic diversity/distinctiveness < taxonomic distinctiveness/uniqueness).
A similar pattern was found in fauna, i.e., the species distribution was the most frequently used bio-ecological criteria for prioritization. It accounted for 12.05% of the total number of reviewed studies (Table S3). The habitat fragmentation, ecological distinctiveness, flagship/focal/indicator species, habitat quality/diversity, life history traits, population trend/decline, species abundance, and species richness had relative counts ranging from 3.01% to 4.82%. The other bio-ecological criteria accounted for only less than 2% (e.g., species vulnerability and habitat connectivity). In terms of management variables, the conservation status also had the highest percentage (7.23%), followed by threats (5.42%), conservation actions (3.01%), and management potential, effectiveness, and need (3.01%). Among the socioeconomic variables, the extinction risk accounted for only 3.01%. The other socioeconomic variables (e.g., local knowledge/perception and stakeholder/public interest) were rarely used in the reviewed studies, with ≤1.81%. Taxonomic/genetic variables constitute 9.64% of the total number of reviewed studies in fauna, of which 4.22% and 2.41% used genetic diversity/distinctiveness and evolutionary distinctiveness, respectively.
3.3. Prioritization Methods Used
Here, we found eight commonly used prioritization methods in the conservation of flora species (Figure 3). The point scoring method (PSM), was the most frequently used method, followed by conservation priority index (CPI), correlation analysis, principal component analysis (PCA), species distribution model, and rule-based method. Although lower in relative count compared with the first eight methods, the use of zonation software and regression analysis rank third among the identified methods for prioritizing conservation choices.
In fauna, seven frequently used methods for prioritizing choices in conservation were found (Figure 4). The highest relative count was found for PSM, followed by CPI, species distribution model, regression, principal component analysis (PCA), correlation analysis, and zonation software.
3.4. Level of Prioritization Approaches in Conservation
Results of the SLR showed multiple-species as the most frequently used approach in prioritizing conservation choices in both flora and fauna species (Figure 5). This was followed by single-species approach (21.12–30.53%). The ecosystem and habitat-based approach was used by only 16.41–18.58% of the total number of reviewed studies across species groups.
In terms of commonly used focal species, most of the studies considered several species in setting conservation priorities in most countries (Figure 6). This is followed by using only either rare, medicinal, threatened, or endemic species.
3.5. Scope of Conservation Prioritization Studies
Figure S2 shows the relative count of scope of studies about prioritizing conservation choices for both flora and fauna conducted in the last three decades. In flora, studies with global/regional scope were generally more dominant than those with local and national scopes (Figure S2). Local scope tended to increase from 2020 as the national level of studies decreased. In fauna, the global/regional scope of studies was also more dominant than the other scopes. The local and national scope of studies started to increase from 2000, although the latter scope decreased as the former scope increased from 2015.
4. Discussion
4.1. Conservation Prioritization Studies across the World
The present systematic review revealed a limited number of studies about the topic conducted in the past three decades, particularly from 1990–2003 in megadiverse developing countries such as the Philippines, Madagascar, Papua New Guinea, Peru, and the Democratic Republic of Congo. Biodiversity loss in these countries remains a major environmental problem. Knowledge of how to effectively prioritize biodiversity conservation choices in these countries is crucial for economic growth and poverty alleviation and for attaining global forest goals and targets. In Madagascar, for instance, increasing human pressures result in severe habitat degradation of endemic trees yet conservation decisions are based on very limited data [31]. While threats to biodiversity in the world’s oldest forests (e.g., the Democratic Republic of the Congo and Papua New Guinea) persist due to tension between economic needs and forest conservation, data on prioritizing choices in conservation remains limited based on the result of the present review. A low number of studies about the topic in megadiverse developing countries can also be explained by a lack of resources for science [32]. World bank reported that low-income countries have fewer scientists per population than high-income countries [33]. In Congo, for instance, studies have shown fewer scientific publications due to remoteness, colonialism in science, and lack of research capacity building and support for scientists [34,35,36,37]. Research-related obstacles (e.g., facilities, funding, and expertise) also limit local biodiversity research in developing countries in Southeast Asia [38]. The authors further noted that despite the great appreciation of Southeast Asian countries for the value of biodiversity, national funding for research remains limited, particularly in the Philippines and Indonesia.
Here, the number of studies started to increase from year 2005 until 2010 and from 2017 to 2021, which can be attributed to the significant events in the history of climate change science. In 2005, the Kyoto treaty goes into effect as agreed by major industrial nations, and this further explains the high relative count of studies about the topic in many developed countries. From 2017 to the present, significant increases in mean global temperature and CO2 concentrations have been reported [39,40]. These events may have encouraged more funding for biodiversity research as there is a growing recognition of the importance of biodiversity’s contribution to climate change adaptation [41]. There is also an increasing awareness that climate change and biodiversity crises are fundamentally related [42].
Moreover, we revealed that conservation prioritization-related studies are more documented in flora than fauna in the last thirty years. This can be ascribed to the fact that plants are less mobile than animals and information on plant diversity is generally much better known than animal diversity. Traditional field-based methodologies are also time-consuming due to repeated censuses that need to be done to determine population trends over time and in large areas [43]. Compared with non-mobile plants, conducting a census to determine, for example, the population size, spatial distribution, and geographical extent of highly mobile animals (e.g., migratory birds) can be more labor and capital intensive amid the impacts of the ever-changing climate. A meta-analysis estimated that the distribution of many terrestrial organisms is now shifting to higher altitudes in response to warming temperature [44]. Of the two types of fauna, it was also reported that micro-distributions of non-mobile intertidal invertebrates are more temperature-sensitive than those of mobile invertebrates [45]. Moreover, the use of animals in scientific research, especially the threatened ones, has also long been the subject of long debates, making faunal research limited across borders regardless of its purpose or benefit. Technological advancements, such as the use of consumer-grade drones and convolutional neural networks in collecting and analyzing population-level data [46], could greatly improve the number of conservation prioritization studies for fauna. These technologies can reduce the effort and cost and provide real-time detections for animal surveys and animal tracking for prioritizing choices for fauna conservation [46,47].
4.2. Frequently Used Prioritization Criteria for Conserving Flora and Fauna
Determination of the commonly used criteria can help conservation practitioners choose the optimal set of attributes for setting conservation priorities depending on available resources and objectives. Here, we found that bio-ecological attributes were the most frequently used criteria for prioritizing conservation choices for both flora and fauna species. The Environmental Non-Government Organizations (ENGOs) use 13 scientific and conservation criteria to prioritize targets for conservation use, and six of these criteria are related to biological values, e.g., species richness, rarity, and endemism [48]. The result of the present systematic review can be ascribed to convenience in observing, collecting, and analyzing ground-based and secondary bio-ecological data. Based on the reviewed study, for example, information on species endemism was obtained from secondary sources, including local and regional databases and expert opinion, e.g., [49,50,51]. In Africa, one study used expert opinion at a workshop for acquiring data on species endemism [52]. Similarly, data on the level of species richness conducted in Finland was obtained through an expert elicitation method, which refers to the synthesis of opinions of authorities of a subject [53].
Here, we found that the species distribution was the most frequently used bio-ecological attribute for prioritizing conservation choices for both flora and fauna species. This can be due to a response to the knowledge gap in species distributions, which is a major problem in conservation planning [54]. Prioritization of either single or multiple species can be relatively easier when their distributions are already known. Quantification of the species’ geographic distribution is also a straightforward way to assess the degree of species rarity status by determining the extent of occurrence and the area of occupancy [55]. Distribution patterns of ecologically rare and/or threatened species are of high importance consideration in using species distribution as one of the bio-ecological criteria, e.g., [56,57,58]. Under this approach, the habitat requirements of the target species should encompass the habitat needs of the other species for which the data on species distribution and abundance are limited [59,60]. Moreover, combining species distribution data with environmental gradients and community richness and composition using community-level modelling techniques may be effective for a community-level strategy for conservation prioritization [54]. Also, the increasing threats from climate change may have engendered the need for understanding species distribution patterns to effectively assess the conservation needs of species. As a commonly cited rationale in the reviewed studies, the result can also be attributed to the impacts of climate change (e.g., range shifts and reduction in the climatic suitability of species) on species distributions. This is because it has long been known that the geographic distribution of species is influenced by their interactions with climate [61].
The socioeconomic variables (e.g., local knowledge/perception and resource use diversity) were the second most frequently used criteria for prioritizing conservation choices for flora species but ranked lowest for fauna species. Results can be attributed to limited information on socioeconomic status, e.g., [12], the unwillingness of the respondents to supply information due to personal and/or administrative reasons, and conflict between biodiversity conservation and basic humanitarian needs. Target communities also differ in knowledge of resource use among landscapes depending on their socioeconomic characteristics, such as age, gender, and profession [62], making conservation prioritization of the exploited resource even more complicated. This is because the level of knowledge or perception of resource use could influence the identification of species that can be considered priorities for conservation. There is also a lack of economic valuation studies on socio-economically important species despite funding inputs and research efforts being set in place [63].
The present review also found the inclusion of some new criteria in setting conservation priorities. These are the taxonomic/genetic variables (e.g., evolutionary distinctiveness and genetic diversity/distinctiveness) and management variables (e.g., regional representativeness and cost of management). This can be explained by the emerging need to use several criteria in prioritizing conservation choices and not only the criteria that can be provided by IUCN and other biodiversity-related organizations to avoid inevitable biases [64] using the conservation status rank methods (CSRM). The CSRM ranks species using quantitative or qualitative criteria (Figure 7), such as those in the IUCN system which places species in one of the Red List Categories based on known information (e.g., abundance, life history, habitat requirements, distribution, vulnerability threats, etc.).
4.3. Frequently Used Methodological Approaches in Prioritizing Conservation Choices
Several approaches have already been developed to prioritize species for conservation, but these approaches have been improving in response to contemporary environmental challenges. In this review, the point scoring method (PSM) ranked first among the 39 mentioned methods for both flora and fauna. The PSM uses a series of scores for each flora or fauna species based on easily measured multiple criteria, which can either be summed or multiplied to determine their relative priority scores or obtain ranked species lists [65]. PSM is easier to analyze because of its quantitative, repeatable and objective characteristics compared with the other categorical and qualitative systems (e.g., rule-based method) [65,66]. Although PSM is one of the commonly used methods in prioritizing conservation choices, serious drawbacks are usually associated with this method. It can be misleading due to the complexity of the criteria used, unstandardized weightings of each criterion, limited data, and the lack of objectivity in transforming multiple criteria to a numeric score [67]. A combination of local and global data on species distribution, resource use diversity, species diversity, and ethnobotanical values could mislead the summarization of the scored criteria into a unique Priority Index due to methodological complexities. Multiple criteria may also vary significantly by region, landscape and climate type, species life-history traits, and disturbance frequency and intensity. For example, specialist species with high habitat specialization may be more prone to population declines compared with generalist species. The absence of sophisticated methodologies for collecting field-based data in the least developed countries may also not provide more advanced and real-time databases on species distribution, population trends, vulnerability, and conservation status. Data from developing countries may be more limited compared with those in developed countries due to methodological-related constraints, and to our knowledge, the implications of such constraints remain unknown in conservation science. Although the International Union for Conservation of Nature (IUCN) Red List of Threatened Species has become the most reliable basis of conservation policies developed at the national level, it can be biased toward species that have attracted research interest [68,69].
Moreover, scores for harvesting risks/pressures may also vary significantly depending on culture and ethnobotanical knowledge [70]. The Priority Index can also be subject to methodological uncertainties if the data was obtained from various sources with different ethnobotanical knowledge. Development of a localized and area-based conservation priority setting of socio-economically important species of flora and fauna can be a promising approach to increasing the reliability of the PSM. Because global changes in climate have a direct impact on local biodiversity systems, localized conservation studies may contribute to global conservation action plans through the prioritization of habitats and conservation strategies. In this systematic review, we observed an increase in the relative count of localized conservation prioritization studies in the last 30 years for both flora and fauna. For example, local conservation and the harvesting sustainability of the most popular medicinal plants were evaluated in a local community in Brazil using both biological and cultural criteria [70]. Prioritization of local plant genetic resources for ex situ conservation was also initiated in Israel based on plants’ contributions to local people, industrial and biotechnological applications, and horticultural and forestry potential [71]. These socioeconomic criteria were further combined with other criteria, i.e., local distribution range, abundance, endemism, conservation status, and availability of samples in Israeli collections. In Congo, most of the conservation efforts are generally failing to protect biodiversity because the crucial role of local people is generally excluded in conservation implementation [72]. Specifically, local histories, ecological knowledge, livelihoods, and land rights are normally disregarded in conservation efforts, failing both local people and biodiversity in Congo. This further justifies the need to give more emphasis to prioritizing conservation choices at the local scale.
We revealed that the multiple-species was the most frequently used approach to prioritizing socioeconomically important flora and fauna species across regions. This approach involves a set of species with contrasting characteristics and ecological requirements in species prioritization [73,74]. Without a systematic prioritization strategy, however, conservation of multiple-species can be challenging due to a high number of species to be conserved simultaneously, with each species having contrasting ecological needs. In the present review, the high relative count of multiple-species approach is probably in response to the widespread drawbacks of single-species approach, i.e., using only one charismatic or important species cannot fully encompass the conservation requirements of the other important and probably more threatened species living in the same habitat [75,76]. In the present review, more than 50% of the conservation prioritization studies in some countries, e.g., [Benin, Brazil, China, and India] dealt only with important medical plants rather than rare and/or threatened species. This implies that focusing on a single group of important species can result in inefficient and inadequate conservation and can lead to serious biases towards single species at the expense of other species [77]. Several studies found that such an approach cannot always result in adequate benefits across taxa or ecological processes [78,79]. Single-species approach, however, can encourage more funding and international commitments due to the charismatic appeal of the select focal species [80,81].
Based on our review, there are also no reported standardized quantitative metrics to date that can be used for selecting single or multiple focal species. This could mislead conservation efforts as not all chosen focal species have a significant impact on the structure and function of their natural habitat and can represent the majority of the other ecologically important species living in the same habitat. For example, our heatmap revealed that although India has the highest number of studies about the topic more than 50% of which dealt mostly with medicinal plants, possibly neglecting the rare and most vulnerable endemic species. This can put India’s biodiversity hotspots, which have been insufficiently studied to date, at a much higher risk amid increasing human interference, fragmentation, deforestation, and land use change [82,83].
The dominance of the multispecies approaches in the reviewed studies can also be attributed to noteworthy recent advances in multispecies conservation planning, such as the use of remote sensing, drones, LiDAR, geographic information system (GIS), and convolutional neural networks [77,84,85]. These advances make the conservation needs of multiple species more attainable. This can be supported by a high relative count of the use of the species distribution model (SDM) in prioritizing conservation targets in most of the reviewed studies. SDM is increasingly used in conservation science and management as a powerful tool to understand spatial distribution patterns, identify areas suitable for concerned focal species, and extrapolate relationships between a known distribution and environmental covariates [86,87,88,89]. In India, SDM was combined with spatial hierarchical systematic conservation planning techniques to prioritize habitats suitable for the conservation of endemic species with the aid of Zonation software [90]. The Zonation software, which produces a balanced ranking of conservation priority, was frequently used in most of the studies that used SDM, e.g., [91,92,93]. It can analyze large-scale high-resolution data from different sources, e.g., remotely sensed habitat and species distribution databases [54,94], and this further explains the dominance of the multispecies approaches in the reviewed studies. We can also attribute the highest frequency of multispecies approach to advances in bivariate and multivariate data analysis, which can process information from multiple measurements and demonstrate relationships among variables. The present review showed some of the frequently used bivariate and multivariate data analysis techniques, including correlation and principal component analysis (PCA). For example, data on ethnobotany, economic importance, conservation status, adaptability to climate variations, and ecological criteria were analyzed using PCA for conservation prioritization of useful medicinal tree species in the Wari-Maro Forest Reserve in Benin [18].
5. Conclusions
In this systematic review, we showed the research trends and important knowledge gaps about prioritizing conservation choices for both flora and fauna across regions. A low relative count of studies about the topic was generally observed in biodiversity hotspot countries, and the majority of the studies in countries where the topic is well-documented dealt mostly with a certain group of species (e.g., medicinal plants). While advances have been made over the past three decades, the research progress on the topic in biodiversity hotspot countries is generally slow due to lack of information, support for research, and expertise, the complexity of the problem, and methodological constraints. The present review also showed the span of frequently used criteria and methodological approaches available to prioritize biodiversity conservation choices. There is still no standardized or scientifically defensible set of criteria and methodological approaches for determining conservation priorities. However, the choice should be adapted to the objectives, data, and resources available, depending on the expected results. The choice should also consider the complexity of the method and the time required during implementation. Moreover, the initial choice on whether a single-species, multiple-species, or ecosystem-based approach determines also the expected outcomes. It is also interesting to note that prioritizing conservation choices can be done according to the local context using locally acquired data.
There is a need to develop methods for prioritizing biodiversity conservation investments in megadiverse, biodiversity hotspot countries to assist the government institutions in governance decision-making amid limited resources and increasing poverty rates. We highlight the need to increase not only the conservation prioritization studies but also the scientific efforts on improving biodiversity-related information in hotspot regions for an improved prioritization methodology, particularly in faunal aspect. Moreover, consensus methodology on setting biodiversity conservation priorities must be developed at the local level, but with global significance, so that limited resources can be spent efficiently and effectively for species needing immediate conservation actions, particularly in resource-poor countries.
Conceptualization, J.O.H. and I.E.B.J.; methodology, B.B.P.; software, J.O.H. and B.B.P.; validation, J.O.H. and I.E.B.J.; formal analysis, J.O.H.; investigation, J.O.H.; resources, J.O.H. and B.B.P.; data curation, J.O.H. and I.E.B.J.; writing—original draft preparation, J.O.H.; writing—review and editing, J.O.H., I.E.B.J. and B.B.P.; visualization, J.O.H.; supervision, I.E.B.J.; project administration, I.E.B.J.; funding acquisition, I.E.B.J. and B.B.P. All authors have read and agreed to the published version of the manuscript.
All the data used are already reflected in the article. Other relevant data may be available upon request from the authors.
The authors wish to acknowledge the Philippine Council for Agriculture, Aquatic, and Natural Resources Research and Development of the Department of Science and Technology (PCAARRD-DOST), for funding the CONserve-KAIGANGAN project and finally, to the Department of Environment and Natural Resources-Protected Area Management Board of Samar Island Natural Park (DENR-PAMB-SINP) for issuing a Gratuitous Permit.
The authors declare no conflict of interest.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Enlarge this image.Figure 1. The flow diagram for database search of peer-reviewed original and review articles used for the present systematic literature review.
Enlarge this image.Figure 2. Distribution and number of studies about prioritizing conservation choices for flora and fauna worldwide.
Enlarge this image.Figure 3. Relative count of the identified methods for prioritizing conservation choices for flora species.
Enlarge this image.Figure 4. Relative count of the identified methods for prioritizing conservation choices for fauna species.
Enlarge this image.Figure 5. Proportion of the levels of prioritization approaches in conservation of flora and fauna resources in the reviewed studies.
Enlarge this image.Figure 6. Heatmap diagram showing the relative count of commonly used focal species across countries.
Enlarge this image.Figure 7. Criteria and categories used for prioritizing conservation action using the conservation status rank methods.
The search terms entered in ScienceDirect, PubMed, and Google Scholar databases for the data collection of the present systematic literature review.
| Search Terms | No. of Articles | |||
|---|---|---|---|---|
| Science Direct | PubMed | Google Scholar | Total | |
| “biodiversity conservation” AND “prioritization” | 3241 | 85 | 1630 | 4956 |
| “biodiversity” AND “prioritization methods” OR “biodiversity” AND “prioritization approaches” | 264 | 10 | 453 | 727 |
| “fauna conservation” AND “prioritization” OR “flora conservation” AND “prioritization” | 21 | 10 | 266 | 297 |
The criteria used for the extraction of information from the selected peer-reviewed original and review articles.
| Extraction Criteria | Information Considered and Justification |
|---|---|
| 1. Publication year | Between 1990–2022; to consider the necessary degree of comprehension for the literature search questions |
| 2. Country of study site | Across the globe; to describe the distribution of studies and the trends of publications |
| 3. Type of organism | Flora and fauna; to determine which one is well-studied and which one is not |
| 4. Types of conservation approach | Single species, multiple species, and ecosystem-based approaches; to determine which approach is frequently used for either flora or fauna across the globe |
| 5. Criteria used for prioritization | All mentioned criteria (e.g., species distribution, conservation status, etc.) in the materials and methods section of the paper; to evaluate which criteria are frequently used for either flora or fauna across the globe |
| 6. Methodological approached used for prioritization | All mentioned methods employed (e.g., rule based, point scoring method, etc.) in the materials and methods section of the paper; to evaluate which methods are frequently used for either flora or fauna across the globe |
| 7. Scope of the study | Local, national, and global scopes; to determine the distribution of studies done at various scales |
| 8. Identified research gap | To summarize the research gap between flora and fauna worldwide |
Supplementary Materials
The following supporting information can be downloaded at:
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; BuotJr, Inocencio E 2 ; Byung Bae Park 3
1 Department of Forest Biological Sciences, College of Forestry and Natural Resources, University of the Philippines Los Baños, Laguna 4031, Philippines
2 Institute of Biological Sciences, College of Arts and Sciences, University of the Philippines Los Baños, Laguna 4031, Philippines
3 Department of Environment and Forest Resources, College of Agriculture and Life Science, Chungnam National University, Daejeon 34134, Korea