Introduction
The accelerated rate at which biodiversity is being eroded (Díaz
2019) is moving scientists to call for the conservation of large portions of the Earth (e.g., Locke
2013; Wilson
2016; Baillie and Zhang
2018; Dinerstein et al.
2020). While Protected Areas (PA) are regarded as the cornerstone of nature conservation, concerns are being raised about their capacity to halt effectively the current biodiversity crisis (Laurance et al.
2012; Watson et al.
2014). Interest has thus been increasing in complementary conservation measures, which may integrate and possibly overcome some of the limitations of PA. One such type of measures has been defined and legally ratified by the International Union for Conservation of Nature and the Convention on Biological Diversity as Other Effective Area-Based Conservation Measures (OECMs). These include a broad range of cases, from private estates to ancestral lands, in which conservation is not the primary purpose of land management, as in the case of PA, but consistently delivered as a by-product of other management goals (IUCN WCPA
2019).
SNS are another example of nature protection independent of PA, despite being often spatially overlapping with PA, which has received considerable recognition from conservationists over the last two decades. SNS have been defined as "areas of land or water having special spiritual significance for peoples and communities" (Wild & McLeod
2008). SNS are associated to a wide range of natural features, such as single trees or rock outcrops, rivers, mountains, islands and even entire landscapes (Dudley et al.
2005), although it seems that the majority are sacred groves and forest patches (Dudley et al.
2010). Sometimes, SNS can also consist of built features, such as temples, shrines or monasteries, surrounded by natural or semi-natural areas (Dudley et al.
2009; Frascaroli et al.
2016a,
b).
From a conservation perspective, SNS are often regarded as the oldest form of conservation of habitats and other types of natural resources in human history (Dudley et al.
2005), in some cases having existed for many centuries or even millennia. For instance, the oldest modern PA, the Yosemite National Park, has been considered sacred and protected as such by local people for many centuries, the Bogd Khan Mountain in Mongolia has been protected since the twelfth century and a Buddhist temple at Nikko (Japan) has been constructed 1100 years ago (see Dudley et al.
2005 for details and other examples). Like OECMs at large, it has been suggested that SNS may constitute a large network of “informal” PA (Dudley et al.
2009), potentially reinforcing the official PA networks (Frascaroli et al.
2019) and contributing to conservation at global scale. Indeed, it has been reported that SNS have the same, or even higher, levels of species richness as comparable areas and even official reserves (see e.g. Bhagwat et al.
2005a,
b,
c; Brown et al.
2006a,
b; Boadi et al.
2017). Moreover, SNS sometimes act as refugia for endemic, threatened, rare or specialist species (see e.g. Gunaga et al.
2013; Kühnert et al.
2019) and preserve old-growth trees and forest patches (see e.g. Salick et al.
2007; Tiwari et al.
2010a,
b; Frascaroli et al.
2016a,
b; Stara et al.
2015). In some cases, SNS can also represent the only natural or semi-natural patches of habitat within highly modified landscapes (Dudley et al.
2010). SNS are found in association with both indigenous and mainstream faiths and in all continents, except for Antarctic (Dudley et al.
2005). However, most of the research concerning SNS has so far focused on Africa and Asia (Dudley et al.
2010).
Despite the growing awareness of the importance of SNS for both conservation and local livelihoods, no systematic review based on quantitative or semi-quantitative methods has been published on the topic to date. Existing syntheses have introduced SNS to the scientific and conservation communities and demonstrated their significance in a wide range of contexts (Bhagwat and Rutte
2006; Dudley et al.
2005,
2010). While classic literature reviews have provided major insights, enabling syntheses and highlighting various gaps in our knowledge, the growing number of primary studies offers in addition an opportunity to deploy techniques of systematic review and meta-analysis. Systematic reviews can increase transparency, reproducibility and objectivity by applying strict guidelines throughout the whole review process (Pullin and Stewart
2006; Haddaway et al.
2015). Meta-analyses further develop such results by investigating statistically the size, direction and significance of various treatments across multiple studies (Vetter et al.
2013).
In the present work, we carry out the first review that applies a systematic approach and semi-quantitative methods to address the following question: do SNS worldwide have significant benefits for biodiversity conservation, compared to surrounding areas? We take the comparison of SNS with nearby non-sacred areas as fundamental to objectively evaluate their effectiveness in delivering area-based conservation of nature (see Coetzee et al.
2014 for a similar case). In particular, we asked if the reported effects of SNS on biodiversity varies across: (1) continents; (2) taxa; (3) biodiversity measures. We investigated these three facets through systematic review, while collecting evidence on the state-of-art literature on SNS and paving the way to future evidence-based synthesis.
Discussion
Interest in SNS from a conservation perspective has been growing over the years. However, there have been only scant efforts at systematising available knowledge and drawing key messages regarding their conservation effectiveness at a global scale. Despite the limited number of studies found with suitable criteria, our results somehow support the view, already advanced in a number of local studies (see e.g. Bossart et al.
2006a,
b; Ambinakudige and Sathish
2009; Brandt et al.
2013; Frascaroli et al.
2016a,
b; Nopper et al.
2017; Avtzis et al.
2018a,
b; Shepheard-Walwyn and Bhagwat
2018a,
b) and earlier overviews (Bhagwat and Rutte
2006; Dudley et al.
2005,
2010), that SNS have positive effects on biodiversity across continents and geographical settings. At the same time, our review also underscores that the quantitative research to date has been limited to relatively few areas and taxa, resulting in evident knowledge gaps.
In the first place, the final pool of papers showed strongly uneven distributions, both spatially and taxonomically. As Dudley et al. (
2010), we also found that research has largely focused on two main regions, namely the West-Central portions of India and Africa. These two areas approximate the Western African Forests and the Western Ghats of India, two global hotspots of biodiversity (Myers et al.
2000). At the same time, no papers from Americas, Oceania and large portions of Asia were retrieved. This limited geographical range is likely to hide part of the story, as there are indications that important SNS also occur in those macro-areas, although they have not been so far studied from a quantitative-ecological perspective.
Similarly, we detected a strong bias towards plants, partly reflecting a common taxonomical bias found in ecological and conservation research. Notably, plants are easier—and probably cheaper (Geijzendorffer et al.
2016)—to study than many other taxa, as they can be surveyed with relatively little time and resources and also as they already are much better known than other taxa. Moreover, many SNS are quite small and hence likely more successful in conserving plant species rather than animal species, especially medium to large sized animals (Nilsson and Ericson
1997; Parker
2012). This in turns could have potentially influenced scientists planning to study SNS, catalysing their focus towards plants. Several high-rank taxa which are widely studied in ecology and conservation were totally absent (e.g. fishes) or present in only one study (e.g. lichens), while the microbiota were not studied at all. These spatial and taxonomical biases that we recorded are likely due to the fact that the literature on the topic is small and largely contributed by a restricted group of scientists, specialized in the geographical areas and taxa in question. At the same time, we cannot rule out the possibility that the review protocol we adopted may have contributed to partly exacerbating those biases. For example, some papers focusing on additional geographical areas and taxa were excluded from our final pool because they did not satisfy some inclusion criteria (e.g. Castro and Aldunate
2003; Dafni
2006; García-Frapolli et al.
2007) or they were not present in Scopus at all (e.g. Deb et al.
1997). Indeed, including the grey literature in future synthesis, while further enlarging the inclusion criteria, will definitely help gaining a broader and more detailed comprehension of the phenomenon. We also think that a public call for scientists and practitioners could help catalyse the attention and offer the opportunity to collect larger and more structured datasets.
In the second place, we noted some methodological limitations in the literature, which could be overcome in relatively easy ways. For example, some contributions have surveyed elements of biodiversity at SNS without comparing them with appropriate RS. While studies of this kind can have very high value from the point of view of community ecology, they do not offer a possibility of testing specific hypotheses to benchmark the conservation effect of SNS compared with neighboring areas. In other cases, the possibility to quantitatively synthesize available knowledge can be inhibited by missing information about the statistics employed, such as means and standard errors, or, more generally, by the unavailability of primary data.
In the third place, details about belief systems, management and governance would be key to understand the mechanisms whereby SNS deliver successful conservation, and evaluate their resilience or possible threats in the future (Dudley et al.
2005). Previous research has highlighted how deeply belief systems can affect the relationship between local communities and their environment. Also, different religions have different potential to influence in situ conservation (Mikusiński et al.
2013). Systematic data would enable the testing of possible relations between different faiths (e.g. monotheistic religions, animistic systems) and specific conservation outcomes. However, even basic details about belief systems are frequently omitted in the ecological literature on SNS. Similarly, governance regimes regulate the way communities manage their environmental resources. It has been suggested that one of the key characteristics of SNS is to include a set of mechanisms that contribute to enforcing effective and environmentally beneficial governance (Rutte
2011). However, no studies to date have systematically looked into the governance and management created under SNS and, even worse, often this information is completely omitted from research papers on the topic. This is a gap that should be urgently addressed in future interdisciplinary research.
Besides these limitations, our results provided overall confirmation of the biological importance of SNS worldwide. Indeed, all multinomial tests were statistically significant as well as a part of the post-hoc tests, indicating that SNS positively affect local biodiversity. These effects may be the outcomes of both unique environmental characteristics of SNS, as they often overlap with outstanding natural features (see e.g. Anderson et al.
2005; Frascaroli et al.
2016a,
b), and particular cultural factors and governance regimes, which regulate SNS use and prevent over-exploitation (e.g., Rutte
2011; Marini Govigli et al.
2020). Moreover, while our analysis yielded no definitive proof that SNS are always beneficial to biodiversity, it produced very clear evidence that only seldom they are detrimental (for example, acting as sources of invasive species; see e.g., Liu et al.
2013), despite being areas primarily dedicated to cultural uses. Indeed, in the overall pool of comparisons, positive and neutral effects were significantly more frequent and negative effects significantly less than expected. Similarly, when post-hoc tests resulted in significant outcomes, which occurred in less than half of all cases, positive and neutral effects were always observed more frequently than expected under the null hypothesis, while negative effects were always observed less frequently. The fact that the majority of the post hoc tests was not significant, in contrast, may indicate the lack of a clear trend in the results or suggest that the small sample size was enough to catch some signal at first, but insufficient to confirm the results when checking for the single effects.
Comparisons grouped by continent showed that Africa had the largest proportion of negative effects, although also in this case they remained significantly less frequent than expected, while Asia and Europe showed higher proportions of neutral effects and positive effects, respectively. This is an interesting finding, given that SNS are commonly thought of in connection with indigenous or non-Western context. The evidence we reviewed, in contrast, indicates that they have a significant positive effect on biodiversity also in relatively modernized areas of Europe. A possible explanation may be that the studies on SNS in Europe are often located in relatively less populated and accessible areas, e.g. Central Apennines (Frascaroli et al.
2016a,
b) and Epirus (Avtzis et al.
2018a,
b), while papers from Asia are mostly from Kodagu District in India, which has a moderately high human density. Therefore, we could hypothesise that geographical isolation has favored a continuity of traditional management and governance of those European SNS, although in a context of broader secularization, with positive repercussions on their biodiversity. Nonetheless, we underline that only three studies from Europe were retrieved, hence further evidence is needed to confirm our results and draw general conclusions.
Results by taxa showed that SNS have a significantly positive effect on plants. While the reason for this pattern is not clear, the discrepancies among significant results for different taxa is likely due to the different sample sizes. Moreover, no positive effects were found for invertebrates, although this result was not statistically significant due to the small number of observations. However, there could also be ecological reasons for the lack of positive effects on invertebrates, as groups like butterflies or nematodes, for instance, are known to produce very different assemblages as a response to the type of management and disturbance (see e.g. Hamer et al.
1997; Fu et al.
2000).
Results by type of metrics demonstrated that SNS affect mostly the number of taxa, vegetation structure and cultural uses. Available evidence suggests that all these patterns are likely to be dependent on the activities typically allowed or forbidden at SNS, often codified in the forms of taboos (Colding and Folke
2001). Stara et al. (
2016), for example, reported that grazing and hunting as well as deadwood and non-timber products collection are often permitted at SNS in Epirus (Greece), whereas bans and restrictions are mostly connected to trees. A similar situation has been described with regards to the SNS of Central Italy (Frascaroli et al.
2016a,
b). In both cases, the traditional and extensive use of these sites is likely to have produced a moderate degree of disturbance, which is known to increase the local number of taxa, that is, the so called “intermediate disturbance hypothesis” (Connell
1979). Similarly, taboos related to trees are likely to have promoted the conservation of mature vegetation stands, explaining the significant effect found for vegetation structure (Marini Govigli et al.
2020). Finally, the possibility to collect forest and understory products in a culturally prominent area can explain the persistence of TEK and cultural uses of biodiversity at SNS. While these examples are both from SNS in Europe, the importance of resource-related taboos and collection of useful plants is far from limited to European SNS, having been similarly recorded in other different contexts such as, for example, India (Ormsby
2013) and the Himalaya (Salick et al.
2007).
Overall, our results newly underline the importance of recognizing and possibly ratifying the effectiveness of SNS for area-based conservation, while at the same time considering that they are culturally sensitive areas whose primary purpose is not conservation. This would foster the application of appropriate policies and measures (IUCN-WCPA
2019), while addressing such aims as increasing the coverage of rare or important ecosystems, supporting the conservation of threatened species and enhancing the connectivity of PA networks, among the others. Beliefs and practices associated with SNS should be part of any form of official recognition or institutionalization, as research shows that they are often key to driving the biodiversity patterns found in these areas. In this perspective, the broader framework currently developed with regards to OECMs can serve as a valuable example on which to build, to ratify the role of SNS without assimilating them to PA in the classical sense. At the same time, our review also shows that our knowledge of SNS needs to be systematized, and a number of gaps filled, before the conservation potential of SNS can be fully understood and recognized.
Firstly, it is pivotal to undertake large scale surveys of SNS networks at a national or regional scale, as done for instance by Reynolds et al. (
2016) or Frascaroli et al. (
2019), to acquire a better sense for the extent of those networks and their distribution across biomes and altitudinal gradients. Moreover, such surveys would lay the foundation for an integrated conservation planning, which simultaneously accounts for PA, OECMs and SNS.
Secondly, it would be desirable to elaborate standardized protocols for future studies of SNS (see Borges et al.
2018 for an analogous case). Such protocols should concern the sampling of biological communities, the characterization and classification of SNS (e.g., based on their geographical extent, ecological characteristics, belief systems), as well as the analysis of governance and management mechanisms currently in place. This information would help us disentangle the effect of SNS on different types of organisms across faiths and, likely more importantly, management types, while systematically collecting evidence for future syntheses.
Finally, a number of local studies should address the taxonomic and geographical gaps evidenced by our reviews, to understand whether SNS contribute to conservation of other taxa than just plants and on a truly global scale. In particular, studies from Americas and Oceania are lacking and from high latitudes in general, as well as studies on many animal groups, fungi and other microorganisms. These efforts would advance our understandings of SNS and help maximize their contribution to area-based conservation schemes.