Species Distribution Modeling of Ficus fistulosa Reinw. ex Blume in Java, Indonesia Using a MaxEnt-Based Approach in the BCCVL–EcoCommons Platform

Indonesia possesses one of the largest areas of tropical forests in the world, harboring exceptionally high biodiversity and serving as a critical hotspot for global conservation. Although they occupy a relatively small fraction of Earth’s surface (< 10%), tropical forests support an estimated two-thirds of global species richness [1]. The tropical forests of Indonesia provide habitat for numerous plant and animal species, many of which are endemic and essential for sustaining ecosystem equilibrium [2]. However, the tropical forest in Indonesia declined from 113,500 hectares in 2020-2021 to 104,000 hectares in 2021-2022. This represents an 8.4% decrease and marks a historical low for deforestation in Indonesia since tracking began in 1990 [3].

On the other hand, the decrease in the tropical forests in Indonesia was probably due to climate change. Climate change would alter temperature, precipitation, and habitat availability, which in turn impacts the survival, reproduction, and migration of species. Many species are forced to shift their ranges, while others face heightened risks of extinction due to habitat degradation and environmental inadequacy. Anticipating these changes is crucial for biodiversity conservation, ecosystem management, and the development of effective adaptation strategies. Climate change may pose a significant threat to global species biodiversity within the next century [4]. Tree species vary in their reactions to anticipated climate change. Species are adapting their phenology, behavior, morphology, and geographic distribution in response to climate change [4, 5]. The majority of the species would suffer a lot of suitable habitat area [6]. Climate change has caused species distributions to ascend to greater elevations [7]. Many of these changes are associated with increased temperatures and reduced precipitation [8]. Thus, understanding and predicting the future distribution of species has become increasingly important in the context of global climate change.

The genus Ficus plays a crucial role in many tropical rainforest ecosystems and has been proven to be a keystone resource for both animals and populations [9-11]. Ficus fruit was identified as a vital source of food for frugivores [12-14]. Ficus trees may grow a lot of fruit all year round [15, 16]. The majority of plant species depend on frugivorous mammals for seed dispersion [17]. The frugivorous vertebrates that frequent fruiting Ficus trees include bats, birds, and monkeys [15, 18]. In addition to their advantages for animal life, Ficus trees could potentially function as more effective facilitators for forest restoration compared to other existing tree species in disturbed landscapes. Ficus trees facilitate the regeneration of plant communities that reflect the overall environment [19]. Ficus species considerably contribute to the preservation of the hydrological cycle, influencing the soil water content and soil capacity to retain water [20]. Ficus species are recommended for land rehabilitation in Kuningan Regency, Indonesia, due to their adaptability and contribution to soil restoration [21, 22].

Within the varied members of the genus, Ficus fistulosa, often known as wilodo, stands out as a species of significant ecological relevance in tropical montane forests. F. fistulosa serves as a keystone species in tropical montane forests due to its significant influence on the ecosystem. In Mount Merbabu, Indonesia, F. fistulosa grows at an altitude of 1,500-2,500 meters above sea level and is recommended as a restoration species [23, 24]. As a member of the Moraceae family, F. fistulosa is known for its vital role in maintaining biodiversity, especially as a food source for many frugivores, including birds and primates [13]. Additionally, F. fistulosa is recognized for its ability to store and regulate water, making it a crucial component in water retention areas. This characteristic is particularly beneficial in watershed areas and highland regions, where the species can play a key role in ensuring water availability to downstream ecosystems. The capacity of this species to retain water is not only important for maintaining soil moisture and reducing erosion but also for regulating hydrological cycles, contributing to flood control and groundwater recharge in surrounding lowland areas. The ability of F. fistulosa to endure in ecologically unfavorable conditions enables this species to serve as a restoration plant in the shrub area of Gunung Ciremai National Park, Indonesia [25]. F. fistulosa is also a promising candidate for the development of antivirals against HIV, Hepatitis C, diarrhea, diabetes, malaria, antioxidants, and antimicrobials in the context of herbal therapy [26-30].

Despite its ecological significance, F. fistulosa has become increasingly rare in the wild, especially on the densely populated island of Java. Habitat loss due to land-use changes, deforestation, and agricultural expansion has led to a rapid decline in its population, particularly in critical watershed areas. This decrease is troubling considering the species' function in sustaining environmental stability in these areas. The decreasing presence of F. fistulosa has prompted an urgent need for conservation efforts aimed at identifying suitable habitats that can support its recovery. Furthermore, understanding and predicting the future distribution of species has become increasingly important in the context of global climate change. It is also very important to anticipate these changes, which is crucial for biodiversity conservation, ecosystem management, and the development of effective adaptation strategies. One approach utilized for projecting the future distribution of species is species distribution modeling (SDM). SDM utilizes species occurrence data and environmental variables to assess the probable distribution of species under present and future climatic conditions [31].

In terms of F. fistulosa, SDM can be used to predict the map of potential habitats for F. fistulosa on Java Island, relating to the effects of climate change. Numerous research has examined the effects of climate change on the geographical distribution of native tree species through the application of species distribution models (SDM). For example, Islam et al. [32] created SDM using MaxEnt to predict the effect of climate change on the spatial redistribution of F. benghalensis and F. hispida in Bangladesh. Fungjanthuek et al. [33] constructed an SDM utilizing MaxEnt to evaluate habitat suitability and predict potential habitats of F. squamosa and F. heterostyla in China.

Given the importance of preserving this species for its ecological and hydrological functions, SDM offers a valuable approach to guide conservation efforts and habitat restoration initiatives [34]. By predicting areas where F. fistulosa could thrive, targeted actions can be taken to protect and restore suitable habitats, particularly in upland areas where the species can contribute significantly to the sustainability of water catchment areas. SDMs are the most common type of model used in ecology, evolution, and conservation. They can be used to guide conservation efforts and management strategies, prioritize conservation actions, assess the effects of global change, and figure out how environmental factors affect species responses [35, 36].

This study applies the SDM framework to explore the potential geographic distribution of F. fistulosa in Java. By combining ecological knowledge with predictive modeling, this research's objectives are (1) to simulate the current potential geographic distribution of F. fistulosa in Java, (2) to predict its future distribution under the SSP585 scenario in 2100, (3) to identify the key environmental factors influencing its distribution, and (4) to provide scientific recommendations for conservation planning.

In the current scenario, some bioclimatic variables show a clear threshold effect, where habitat suitability sharply increases or decreases beyond specific climatic values. This suggests that F. fistulosa has optimal environmental conditions within a certain range of temperature and precipitation. However, in the future projections, several response curves exhibit shifts or fluctuations, indicating potential changes in species' climatic preferences due to environmental changes.

Notably, some variables maintain a relatively stable response pattern, suggesting that certain climatic factors may continue to be critical factors of habitat suitability. Meanwhile, other curves show more pronounced variations, which could be attributed to climate change effects, like increasing temperatures or modified precipitation patterns.

These results emphasize the importance of understanding how bioclimatic factors influence F. fistulosa distribution over time. The changes in response curves show that we need to use adaptive conservation measures to make sure this species can survive in the long run, especially if climate change and habitat loss occur.

Figure 6 shows the relative contribution of each bioclimatic variable to the SDM of F. fistulosa in Java, considering both current and future climatic scenarios. These contributions indicate the importance of different environmental factors in determining the species' suitable habitat.

Current

Future

Figure 6. Contribution of each bioclimatic variable in the SDM of F. fistulosa in Java

In the current scenario, bioclim_09 exhibits the highest contribution, suggesting that this variable plays a crucial role in influencing the distribution of F. fistulosa. Habitat suitability for F. fistulosa rises sharply at moderate mean temperatures of around 22-26℃ and declines outside this range. This trend indicates that the species performs best in warm but not excessively hot dry-season conditions typical of monsoonal and sub-humid tropical forests. Temperature significantly influenced the rate of stomatal conductance and photosynthesis. Fig cultivars exhibited adaptive behavior to manage harsh conditions, including elevated temperatures [45]. The preference for stable warmth suggests that F. fistulosa has adapted to microhabitats with moderate thermal stress during the driest months. These results are comparable to those of other F. altissima in Guangxi, China habitats that thrive ideally at a temperature of 20-25℃ [46]. Other variables, such as bioclim_06, bioclim_16, and bioclim_10, also contribute significantly, indicating their strong influence on habitat suitability. Conversely, variables like bioclim_14 and bioclim_18 have minimal impact, implying they are less critical in determining the species’ presence.

Under future climatic conditions, there is a shift in the relative importance of bioclimatic variables. While bioclim_06 remains a dominant predictor, bioclim_12 and bioclim_01 gain increased importance compared to the current scenario. This suggests that future climate change may alter the ecological drivers of F. fistulosa distribution, potentially leading to habitat shifts. The decrease in importance of certain variables, such as bioclim_09, highlights the dynamic nature of species-environment relationships under changing climatic conditions. Future climate forecasts indicate a minor shift in the response curve towards elevated temperature ranges, while the peak suitability narrows. This contraction indicates a decrease in thermal tolerance as heat stress escalates. Under projected climate-change scenarios, increasing temperatures during the dry season could substantially reduce habitat suitability, especially in lowland and mid-elevation areas. As drought and heat stress intensify, F. fistulosa populations are likely to migrate toward higher elevations or persist in wetter microsites. This result is in line with the findings of prior research on the elevational redistribution of species in other places, such as Mount Gongga, Tibet Plateau [47], Natma Taung National Park (NTNP), Myanmar [48]. This pattern highlights the species' reliance on thermally stable environments, such as riparian corridors, shaded forest edges, and moist slopes, where microclimatic fluctuations are reduced.

These findings highlight how important it is to include predictions about climate change in plans to protect F. fistulosa. By identifying key environmental factors affecting its distribution, targeted conservation efforts can be developed to mitigate potential habitat loss and ensure the species' long-term survival in Java. Habitat loss and fragmentation can severely disrupt the mutualistic relationships between F. fistulosa and its animal dispersers by reducing both fruit availability and spatial connectivity among feeding sites. When these networks are disturbed, frugivorous birds and bats—key agents in seed dispersal—tend to show reduced movement, declining population densities, and weakened ecological functions. To mitigate the ecological consequences of fragmentation, conservation strategies should focus on restoring habitat connectivity through well-planned ecological corridors. Corridors linking riparian zones and forest fragments where F. fistulosa naturally occurs can facilitate animal movement, enhance seed dispersal, and stabilize pollination dynamics.

Overall, this model provides valuable insights into the spatial distribution of F. fistulosa, which can inform effective conservation planning and habitat management strategies on Java Island.