Large terrestrial mammals resurging in a depopulating country

doi:10.21203/rs.3.rs-4937471/v1

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Large terrestrial mammals resurging in a depopulating country

https://doi.org/10.21203/rs.3.rs-4937471/v1

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Large terrestrial mammals have generally declined due to human activity, but the recovery of some populations poses new issues for coexistence. Few studies to date have investigated drivers of this recovery and its impacts on human societies at the national scale. We assessed the process of range expansion of six species of large terrestrial mammals over approximately 40 years and its impacts on human society in Japan, one of the most rapidly depopulating countries. We found evidence that increased agricultural abandonment and decreased snowfall drove the range expansion of large terrestrial mammals. The range of all six species expanded from mountainous landscapes to those closer to human settlements, leading to an increase in conflicts that threaten people’s property and safety. We predict that accelerating depopulation and climate warming could lead to further range expansion of these species, and call for measures to mitigate conflicts and achieve coexistence with them.

Earth and environmental sciences/Environmental social sciences

Earth and environmental sciences/Environmental sciences

Earth and environmental sciences/Ecology

human–wildlife coexistence

population recovery

rewildling

recolonization

climate change

One of the world’s most pressing issue is how to ensure that people and wildlife can coexist and share the environment sustainably¹. Large terrestrial mammals play an irreplaceable role in ecosystems, such as cycling nutrients, dispersing seeds, and controlling populations and behaviours of other species^2–4. Human activities, such as habitat alteration, persecution, and hunting, have caused a drastic reduction in geographical range and population size of large terrestrial mammals⁵. Today, approximately 60% of these species are classified as threatened globally⁵.

Despite this global decline, the recovery of large terrestrial mammals has also been observed in some regions (e.g., Europe⁶ and North America⁷), which is a positive change from an ecological and conservation perspective. However, recovery of large terrestrial mammals can also intensify conflicts with humans, such as crop and livestock damage, vehicle collisions, and even human injury and fatality, posing societal challenges in terms of coexistence^1,8. To tackle these challenges, it is important to understand the dynamics and mechanisms of their recovery as well as its multifaceted impacts on human societies. Given the substantial impact of human activities on the reduction of large terrestrial mammals, changes in human activities and their consequences can also contribute to their recovery⁹. Moreover, range expansion of large terrestrial mammals is expected to be driven by a combination of environmental factors that influence habitat suitability and dispersal from colonized areas (Supplementary Table 1). However, few long-term and large-scale datasets exist to understand the recovery of large terrestrial mammals. As a result, the effects of societal changes on this recovery as well as its impact on human societies are rarely evaluated or often remain speculative.

We address this knowledge gap by analysing long-term nationwide datasets of large terrestrial mammals in Japan, one of the most rapidly depopulating countries in the world. There are currently six species of large terrestrial mammals in Japan (sika deer Cervus nippon; wild boar Sus scrofa; Japanese serow Capricornis crispus; Japanese macaque Macaca fuscata; Asiatic black bear Ursus thibetanus; and brown bear U. arctos). These six species historically experienced severe population declines and range contractions caused by human activities including hunting and habitat loss and degradation associated with agricultural practices¹⁰. However, all six species have expanded their geographical range across the country since the 1970s¹⁰. These expansions are deemed to be driven mainly by recent nationwide societal and environmental changes¹⁰. For example, the human population of Japan has been rapidly decreasing and ageing, particularly in rural areas, leading to nationwide agricultural abandonment and a rapid decline in the number of hunters¹⁰. Furthermore, both snowfall and snow cover—another factor limiting their range expansion—have been decreasing nationwide since the late 1980s due to climate change¹¹.

Here, we assess the nationwide range expansion process of six large mammal species in Japan over the past 40 years, evaluate factors explaining the range expansion, and demonstrate its implications for human society. Modelling changes in the distribution of the six species allowed us to test the following hypotheses: large terrestrial mammals are more likely to expand into areas with more abandoned agricultural land, more rugged terrain, less active agricultural land, less human activity, and less snow cover, due to their preference for higher quality habitats, and those surrounded by more originally occupied areas related to propagule pressure. We also hypothesise that, as a result of the range expansion dynamics described above, the range of the six large mammal species has expanded from mountainous landscapes to human-dominated landscapes, leading to increased levels of conflict with humans.

Four decades of nationwide range expansion

We used distribution data, which were compiled as either the presence or absence of each species in approximately 5 × 5 km grid cells in 1978, 2003, and 2010s^12–15 to calculate the area of occupancy for each species as a measure of their geographical range size (see Methods for more details). Over the past four decades, the six large terrestrial mammal species have markedly increased the area of occupancy (Figs. 1 and 2). Sika deer have expanded their range at the fastest rate among the six species of large terrestrial mammals, with the mean expansion rate of 174.9 km² per year (Fig. 2). This expansion is particularly noticeable in Hokkaido, where few areas now remain unoccupied by sika deer (Fig. 1a). Wild boars, originally found largely in the western half of the country, have expanded their range northeastward (Fig. 1b). Asiatic black bears have also exhibited a prominent range expansion in the western part of the country (Fig. 1c). Brown bears, despite expanding their range at the slowest mean rate of 35.2 km² per year among the six species (Fig. 2), now occupy approximately 80% of Hokkaido where they inhabit (Fig. 1f). The expansion rate of sika deer, wild boar, Asiatic black bear, and brown bear has accelerated after 2003 compared to the previous period, whereas the expansion rate of Japanese serow and Japanese macaque has decelerated (Fig. 2).

Drivers of range expansion

We then investigated the effects of land cover, topography, snow cover, human activity, agricultural abandonment, and propagule pressure on the range expansion of each species by comparing the six variables in newly occupied and unoccupied cells (see Methods for more details). We found that the range expansion of all the six large terrestrial mammal species, except brown bears, was explained by a similar combination of factors, which largely supports our hypotheses (Fig. 3 and Supplementary Table 2). Sika deer, wild boars, Asiatic black bears, Japanese serows, and Japanese macaques were more likely to expand into areas with less agricultural land and less artificial light at night among adjacent areas, while brown bears were more likely to expand into areas with more agricultural land during the first period (1978–2003). Wild boars, Asiatic black bears, Japanese serows, and Japanese macaques were also more likely to expand into areas with more abandoned agricultural land, whereas brown bears showed the opposite pattern again for the period 1978–2003. Terrain ruggedness was significantly associated with the expansion of two species, but the direction of its association varied among species. All six species were more likely to expand into areas with less snow and areas surrounded by more previously occupied areas.

Our results indicate that increasing agricultural abandonment after the 1980s, caused by the rapid decline and ageing of the human population¹⁶, has facilitated the range expansion of large terrestrial mammals, supporting our hypothesis. Abandoned agricultural lands offer habitat for large terrestrial mammals due to its transition towards semi-natural landscapes along with the reduction in human activity¹⁷. Several studies have suggested that agricultural abandonment may have contributed to the range expansion of large terrestrial mammals^6,9, and our results provide robust evidence to support this. Our findings also suggest that recent declines in snowfall and snow cover due to climate warming have also contributed to the expansion of large terrestrial mammals. Large terrestrial mammals often experience reduced food availability and mobility in snow-covered landscapes, which consequently limits their range expansion as well as habitat suitability^18–20.

The range expansion toward areas with less agricultural land and less artificial light at night suggests that these species prefer areas with less human activity. Given the negative correlation between the proportion of agricultural land and the proportion of forest in a grid cell, this result also indicates that the availability of natural habitats (i.e., forests) is more important for the range expansion of those species than attractive food sources (i.e., crops) available on more disturbed agricultural land. In contrast, the range expansion of brown bears in Hokkaido showed different patterns only for the period 1978–2003, and was more likely to occur in areas with more active and less abandoned agricultural land. This can be explained by the generally low level of human activity in agricultural land in Hokkaido, where large-scale agriculture is common, unlike in other regions of Japan²¹. Since 2003, brown bears have continued to expand into lower elevations with even more agricultural land (Fig. 4), which seems to have caused their preference for agricultural land to disappear (Fig. 3), as they experience increasing levels of human activity compared to the previous period.

Impact of range expansion on human societies

Lastly, we assessed the impact of the observed range expansion on human societies. To test whether the six species have expanded into more human-dominated landscapes, we first calculated the mean proportion of three major land cover types (forest, agricultural land, and developed areas) and mean elevation in grid cells newly occupied in 2003 and the 2010s, and compared them with the proportion and elevation in grid cells originally occupied in 1978. We found that the four-decade range expansion of all these six species was characterized by expansion from mountainous landscapes (i.e., high-elevation areas with limited agricultural land and human development) to human-dominated landscapes (i.e., low-elevation areas with more agricultural land and developed areas) (Fig. 4). Combining these results with the identified drivers of range expansions, our findings indicate that large terrestrial mammals have locally selected adjacent unoccupied areas with less human activity for new colonization, but at a larger scale, continuous range expansion from mountainous landscapes has led them to occur closer to human-dominated landscapes.

With their range expansion into human-dominated landscapes, the level of conflicts between the large terrestrial mammals and humans has increased dramatically across the country (Fig. 5). For example, the number of human injuries caused by Asiatic black bears has risen from almost zero in the early 1980s to over 100 per year since the early 2000s^22,23 (Fig. 5). Train collisions with sika deer have also increased in Hokkaido, where their range expansion has been most pronounced, again from almost zero in the 1980s to over 2,000 per year in the 2010s²⁴, and similarly, vehicle collisions with them have also increased, even though the record is available only from 2004²⁵ (Fig. 5). Similarly, crop damage caused by sika deer and wild boar has also increased since 2003 in prefectures where the two species have spread into agricultural areas²⁶ (Fig. 5). The range expansion and population increase of these species has also been reported to have other types of impacts on human societies. For example, the high density of sika deer in Japan is currently leading to forest degradation, which in turn is causing soil erosion and landslides, ultimately threatening human safety²⁷. Sika deer also currently pose public health issues, as they serve as hosts for ticks that transmit zoonotic diseases, such as thrombocytopenia syndrome²⁸.

Our study demonstrates the rapid and substantial range expansion of large terrestrial mammals into human-dominated landscapes and its increasing impact on the society in a depopulating country. Our findings suggest that the reduction of human activities, especially in rural areas, and climate warming have facilitated the range expansion of these mammals. In Japan, depopulation is predicted to intensify at least until 2070²⁹, and in particular the population in mountainous regions is predicted to decrease to approximately 40% of its current level by 2050³⁰. According to climate change scenarios, it is predicted that the nationwide average snowfall in Japan will decrease by at least 30% by the end of the 21st century¹¹. These predicted societal and environmental changes could lead to the continued range expansion of large terrestrial mammals in Japan. However, patterns of range expansion may vary between species and may eventually be limited by the availability of suitable and unoccupied habitats. For example, the geographical range of brown bears is limited to Hokkaido, and since they have already occupied most of the island, further expansion is likely to be limited in the near future. In contrast, since 2003 wild boars have shown a marked expansion into the northeastern part of Honshu, areas characterized by high mountainous landscapes (Figs. 1b, 4 and Supplementary Fig. 1). This expansion may be due to the reduction of limiting factors, such as snow, allowing them to occupy habitats that were previously inaccessible or unsuitable. This suggests that their range is likely to continue to expand.

The general public may develop negative attitudes towards large terrestrial mammals, due to the increase in the frequency and extent of conflict as a result of their rapid range recovery, potentially jeopardizing the achievement of coexistence¹. To ensure that this recovery leads to the coexistence with humans rather than being a temporary phenomenon, Japanese societies must mitigate conflicts with large terrestrial mammals. The lack of experience of sharing an environment with large terrestrial mammals, especially carnivores, means that people lack the knowledge to mitigate conflicts with these species, and often have strong negative perceptions about them³¹. Therefore, it will be necessary to implement proactive education and conflict mitigation measures for people, especially in areas where the new colonisation of these species may lead to conflicts. In Japan, costly and labor-intensive measures (e.g., retaliatory killing and culling) have often been employed to mitigate conflicts with large terrestrial mammals. Our finding that similar environmental factors explain the range expansion of large terrestrial mammals, despite their different ecological characteristics, suggests that landscape-based approaches, such as reducing abandoned agricultural land, may effectively contribute to mitigating conflicts with multiple species on a large scale.

Human populations are expected to decline in many other parts of the world, particularly in East Asia and Europe, due to declining fertility rates, and are likely to concentrate increasingly in urban areas in the future, leading to an acceleration of land abandonment, especially in shrinking societies^32–35. These societal changes are likely to provide opportunities for the recovery of large terrestrial mammals that have been reduced by human activities, but at the same time, may also cause their expansion closer to human settlements, potentially leading to a wide range of conflicts around the world, as we have seen in Japan over the past 40 years. For example, large terrestrial carnivores in Europe are predicted to recolonize human-dominated landscapes in the near future³⁶. Moose (Alces alces), pumas (Puma concolor), and American black bears (U. americanus) are also reported to appear in suburban areas in the United States³⁷. These cases indicate that the range expansion of large terrestrial mammals is a challenge not only for rural communities with declining populations but also for urban communities. Our findings provide unique and important insights into the dynamics, mechanisms, and societal impacts of range expansion of large terrestrial mammals, which can inform practices and policies to conserve those species while mitigating human conflicts around the world.

The recovery of large terrestrial mammals is anticipated to bring back their distinct ecological functions, which is expected to improve ecosystem stability^3,9. However, the contributions of large terrestrial mammals to ecological stability and to humans through ecosystem services are often delivered through indirect processes^38,39, making them harder to perceive compared to conflicts. Therefore, elucidating changes in ecosystem functions and services resulting from the recovery of these species, and their benefits to human societies, can also contribute to the success of coexistence between humans and large terrestrial mammals.

Study area and animals

Our study focused on the four main islands of Japan (Hokkaido, Honshu, Shikoku and Kyushu), excluding smaller islands. The primary vegetation zones in Honshu, Shikoku, and Kyushu are cool temperate and warm temperate vegetation, while Hokkaido, the northernmost island, mostly has boreal and cold temperate vegetation⁴⁰. Approximately 80% of Japan's land area is rugged and mountainous terrain, covered with forests that account for about 67%⁴⁰ (Fig. 4 and Supplementary Fig. 1). The human population in Japan peaked at 128 million in 2008 and has since begun to decline²⁹. This downward trend is expected to continue due to the aging population (over 25%) and lower total fertility rates (1.26–1.54 in the past two decades)²⁹.

Sika deer inhabits all four main islands in Japan. The population of sika deer in Hokkaido was distinguished from those of the other three islands from the statistical analysis due to differences in vegetation zones. Wild boar, Japanese serow, and Japanese macaque inhabit Honshu, Shikoku, and Kyushu. Asiatic black bear inhabits Honshu and Shikoku, but their populations on Shikoku are endangered; hence, Shikoku populations were excluded from the statistical analysis. Brown bear inhabits only Hokkaido.

Distribution range

We used distribution data for six species compiled by the Ministry of Environment^12–15,41. These data were recorded as either presence or absence of each species in each grid cell measuring 3′45′′× 2′30′′ (approximately 5×5 km at Japan’s latitude; hereafter referred to as 5 × 5 km grid cell). We defined a grid cell where species presence was recorded as an 'occupied cell' and a cell where species absence was recorded as an 'unoccupied cell'. In 1978 and 2003, the distribution of each of the six species was surveyed, respectively, through field surveys, interviews, and questionnaires^12,13. In 2014, the distribution of sika deer and wild boar was surveyed in each area where the presence of species had not been identified until 2003, respectively⁴¹. In 2017, the distribution of Asiatic black bears, brown bears, Japanese serows, and Japanese macaques across the country was surveyed again using the same method as in 1978 and 2003^14,15.

Predictor variables

To test our hypotheses, we examined the effects of six predictor variables that are expected to affect the range expansion of large terrestrial mammals in different periods: 1978–2003 for all six species; 2003–2014 for sika deer and wild boar; 2003–2017 for Asiatic black bear, brown bear, Japanese serow and Japanese macaque, respectively. Because these species are terrestrial, areas of oceans or lakes within each cell were excluded from the calculation of predictor variables.

To evaluate the impact of habitat quality, we used five environmental variables (forest cover, cultivation cover, terrain ruggedness index, artificial light at night, maximum snow depth, and area of abandoned agricultural lands). For forest and agricultural land cover, we used the land use data in 2006 and 2014 for expansions in 1978–2003 and after 2003 (i.e., 2003–2014 for sika deer and wild boar; 2003–2017 for Asiatic black bear, brown bear, Japanese serow and Japanese macaque), respectively, obtained from the Digital National Land Information provided by the Ministry of Land, Infrastructure, Transport, and Tourism (https://nlftp.mlit.go.jp/ksj/index.html; accessed July 2022). The mean proportion of cultivation cover as well as forest cover were calculated for each 5 × 5 km grid cell, respectively. We used terrain ruggedness index as topographical variable. We developed a terrain ruggedness index using a 30-m resolution digital elevation model derived from the USGS (https://earthexplorer.usgs.gov; accessed July 2022), and calculated the mean terrain ruggedness index for each 5 × 5 km grid cell. We also used the level of artificial light at night in the final year of each expansion period as a measure of human activities. For example, to analyze the expansion of sika deer in 1978–2003, we used data on artificial light at night in 2003. For artificial light at night in 2003, we extracted data from the global nighttime light composite images (Nighttime Lights Time Series Version 4) of satellite number F15 from the Defense Meteorological Satellite Program’s Operational Linescan System (DMSP-OLS) (https://ngdc.noaa.gov/eog/dmsp/downloadV4composites.html; accessed July 2022). We used the stable_lights data, which contains the lights from cities, towns, and other sites with persistent lighting, including gas flares, while discarding ephemeral events such as fires. For artificial light at night in 2014 and 2017, we extracted data from the NASA/NOAA Visible Infrared Imaging Radiometer Suite (VIIRS) (https://eogdata.mines.edu/products/vnl; accessed July 2022). We used the average-masked data of V.2, which had background, biomass burning, and aurora values zeroed out. We calculated the mean artificial light at night of each year for each 5 × 5 km grid cell. We used the maximum snow depth during the previous year of the final year of each expansion period to minimize the inclusion of snow depth data from the winter following the compilation of species distribution data. For example, to analyze the expansion of wild boar from 2003 to 2014, we used data on the maximum snow depth in 2013. We obtained maximum snow depth data, which recorded in 1 km resolution, from Agro-Meteorological Grid Square Data System operated by the National Agriculture and Food Research Organization⁴². We, subsequently, calculated the mean maximum snow depth for each 5 × 5 km grid cell. All the data were processed using QGIS (version 3.30.2, https://www.qgis.org). For the area of abandoned agricultural lands, we used area of abandoned cultivation, including farmlands, paddy fields, and orchards, of 2005 and 2015 to use for the expansions in 1978–2003 and after 2003, respectively. We acquired the area of abandoned cultivation data collected in the 2005 and 2015 from the Census of Agriculture and Forestry, a questionnaire-based nationwide census conducted every five years by Ministry of Agriculture, Forestry and Fisheries, under a permission, and then calculated the total area of abandoned cultivation for each 5 × 5 km grid cell. To assess the effect of propagule pressure, we used the number of adjacent source cells (See statistical analysis section; Supplementary Fig. 2).

Statistical analysis

A statistical comparison of samples of used and available space, referred to as the "used-available" or "case-control" framework, is an effective approach in animal ecology to quantify the disproportionate space use by animals⁴³. To assess the factors driving range expansion, we compared the newly occupied cells to unoccupied cells, representing used and available areas, respectively, for the range expansion of each large terrestrial mammal species in two different periods i.e., 1978–2003 and after 2003 (2003–2014 for sika deer and wild boar; 2003–2017 for Asiatic black bear, brown bear, Japanese serow, and Japanese macaque). Because of the broad geographical range of our dataset across the country, to minimize potential regional variation in the effect of variables, we first estimated the sources of range expansion and defined available cells as those adjacent to the source cells based on the following assumptions (Supplementary Fig. 2): The expansion of large terrestrial mammals occur sequentially, spreading from cells occupied in the previous period (i.e., originally occupied cells) to cells that are both adjacent and connected by land, based on the reports that the range expansion of large terrestrial mammals typically extends from the areas originally occupied to the surrounding areas at a broad scale^44,45. The originally occupied cell adjacent to newly occupied cells were classified as the source of expansion (hereafter referred to as source cell). Newly occupied cells adjacent to these source cells were then classified as first-stage expansion cells. Next, newly occupied cells adjacent to the first-stage expansion cells but not yet categorized were classified as second-stage expansion cells. The first-stage expansion cells adjacent to the second-stage expansion cells were subsequently classified as the source cells for the second stage of expansion. This process was repeated, with each new stage of expansion identifying source cells from the previous stage, until no further cells could be classified as newly occupied. Among the cells adjacent to each stage’s source cell, those that were newly occupied were defined as “used”, while those that remained unoccupied were defined as “available”.

Due to the uncertainty in estimating source cells, newly occupied cells that were not adjacent to any occupied cells from the previous period were excluded from further analyses. These cells accounted for an average of 4.6% (0.2%-10.0%) of the newly occupied cells in each period. We labelled the used cells as 1 and the available cells as 0, and subsequently contrasted them using conditional logistic regression (via the function clogit in the R package survival⁴⁶) by considering the ID of the source cell as s strata. We excluded grid cells where the area of the ocean and/or lake within the cell exceeded half the total area, to avoid potential overestimation or underestimation for each variable. To improve the convergence of the fitting models and make comparisons of effect sizes easier, all predictor variables were standardised⁴⁷. Next, we tested for multicollinearity among the variables using variance inflation factor (VIF) via the function vif in the R package car⁴⁸. We found that the VIF values above 4, indicating a multicollinearity issue⁴⁹, in all models for each species and period, and that the mean proportion of forest cover had the highest VIF value among all the variables. Therefore, we excluded the mean proportion of forest cover from all models. After excluding the mean proportion of forest cover, all VIF values for all models were less than 4. All models generally had acceptable concordance, ranging from 0.683 to 0.813 (Supplementary Table 2). All analyses were performed with R (version 4.3.3, https://www.r-project.org).

To identify the landscape changes associated with the range expansion of large terrestrial mammals across the country, we calculated the average area of forests, agricultural lands, and human developments, as well as the average elevation within the grid cells occupied by each species in 1978, and those newly occupied by 2003 and beyond.

Acknowledgement

The information used in the analysis was provided by Ministry of the Environment: Biodiversity Center of Japan, Japan Wildlife Research Center, and National Agriculture and Food Research Organization. This work was partly supported by a Grant-in-Aid for JSPS Fellows (Nos. 23K26942, 24H01429) and the Institute of Global Innovation Research at Tokyo University of Agriculture and Technology.

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Large terrestrial mammals resurging in a depopulating country

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Abstract

Figures

Main

Results and Discussions

Four decades of nationwide range expansion

Drivers of range expansion

Impact of range expansion on human societies

Conclusion

Methods

Distribution range

Predictor variables

Statistical analysis

Declarations

Acknowledgement

References

Additional Declarations

Supplementary Files

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