Mammals diversity in Indonesian oil palm plantation: identifying its functional role in the agroecosystem

The conventional profile of oil palm plantations that are not environmentally friendly, high-cost, and non-ecologically oriented, are not in line with sustainable development goals. There is a broad understanding of the role of mammalian species in the oil palm plantation landscape to create healthy oil palm plantations. To understand the functional role of these mammals, we identified various research over the last 10 years in Indonesia on mammals found in oil palm plantation landscapes through a systematic literature review approach. We use the Google Scholar search engine by entering search keywords and screening and analyzing the publications obtained. In the final stage, we analyze the possible use of mammals for production and ecological purposes. We found an increase in oil palm research in Indonesia over the last decade. Most of the research was conducted in large oil palm plantations, and the types of mammals found included large and small mammals, with the research focus on looking at the diversity and abundance of mammalian species, biodiversity loss and gain, as well as feeding guilds or tropic levels of mammalian species. The abundance of potential populations as pests is also a concern in several studies. Mammals in the context of contributing to production and ecology have a broad spectrum of roles, including various guild and tropic levels in the food chain, balancing ecosystems, pollinators, pest control, and other essential functions. We also focus on the gaps in research and management activities to achieve a healthy agroecosystem. The result can be served as a guide for future research priorities, conservation strategy, and management planning toward healthy oil plantations.


Introduction
The conventional profile of oil palm plantations that are not environmentally friendly, high-cost, and non-ecologically oriented, are not in line with sustainable development goals.Several studies that have been conducted indicate that oil palm plantations are often blamed as the cause of the decline in biodiversity, especially bird and mammal taxa [1,2,3], due to monoculture planting systems and the absence of the main component of forest vegetation [4].
Otherwise, the characteristic role of mammals in mediating ecosystem process has emerged from recent research [5].The development of ecological studies in oil palm plantations also shows that oil palm plantations can be a habitat for several groups of mammals and play an important role in artificial ecosystems (agroecosystem) [6].Recently, there is a broad understanding of the role of mammalian species in the oil palm plantation landscape to create healthy oil palm plantations.This awareness 1243 (2023) 012005 IOP Publishing doi:10.1088/1755-1315/1243/1/012005 2 encourages the development of a green company that makes biodiversity the main capital for a natural resource-based business.The green company concept optimizes the function of biodiversity to minimize environmental impacts and reduce production costs, thereby increasing economic and ecological benefits in the long term.
With Indonesia's oil palm plantations reaching 16.38 million hectares, it is important to answer the extent of research carried out in this ecological aspect in Indonesia.Many studies on the impact of oil palm plantations on biodiversity have been carried out, but mostly discuss exploitation and cover change, while mammalian biodiversity is still small (only 30% of the total journals found from Google Scholar).
The ecological role of mammals is the main key to explaining the sustainability of the oil palm plantation ecosystem.For this reason, a state-of-the-art study of mammal diversity and its function in the agroecosystem is very important as a scientific basis for the plan for managing biodiversity in oil palm plantations.In addition, this study is important to obtain a theoretical basis that can support problem-solving, especially allegations that oil palm plantations are the cause of biodiversity loss.
A review can be used to find research gaps in research development trends.By using a systematic literature review approach, it is expected to be able to answer the following research questions: 1) what are the research topics related to mammals in Indonesian oil palm plantations?2) where the research locus is mostly used? 3) what types of mammals are found in oil palm plantations?4) What is the role of mammals in the oil palm agroecosystem?This paper discusses research development trends related to the topic of mammal species diversity in Indonesian oil palm plantations by tracing research bibliography over the last ten years including abundance, management, distribution to genetics, and behavioral ecology.In addition, we discussed the research gap in this field of study, the results of which can be used as a scientific basis for realizing a healthy agroecosystem in oil palm plantations.

Materials and method
This study uses a systematic literature review method.The systematic literature review method is carried out by identifying journals related to the topic of study.The next stage is the screening of journals on Google Scholar with the keywords "mammals", "oil palm plantations", "Indonesia" and "richness".
Next, quickly read the abstract and the entire journal to find journals that are eligible for analysis (eligibility) to obtain journals that are eligible for analysis (inclusion).Sources of research information were obtained from journals, articles, and books that were accessed online.Limitation of the research conducted on the object of mammals; in Indonesian oil palm plantations; in English or Indonesian taken from journals, proceedings, and academic manuscripts published in the last 10 years.The search results that appear are then reviewed with the inclusion criteria in Table 1.This systematic literature review data was then synthesized using qualitative and quantitative methods by grouping similar extraction data according to the results measured to answer the objectives.The data that has been collected from various libraries is then analyzed and then looked for similarities and differences for further discussion and conclusions drawn.The total number of articles was then excluded or eliminated because they did not meet the required inclusion criteria.A total of 802 articles that met the criteria for the year and research topic were then re-excluded because they could not be accessed in full text and were not original research.The results of this process produced as many as 447 articles which were then entered into the feasibility testing stage.Feasibility testing is done by reading quickly (screening) the abstract.If there is information in the abstract that is not covered, further reading can be made to the methods section.Based on the results of this feasibility test, 26 journals were obtained to be analyzed and discussed.The following is a flowchart of the inclusion and exclusion stages of the collected articles presented in Figure 1.

Result
We found that the highest average for publications published on mammal diversity in Indonesian oil palm plantations is in 2021, which is 12 journals, while the average publication published at least is in 2014 as many as 4 journals.The three topics that were discussed the most were species richness, abundance and management, while the topics that were discussed the least were genetics.
The summary results of previous studies show that mammalian taxa are susceptible to decreased diversity and only a few species of mammals from primary and secondary forests can live in oil palm plantations.This raises concerns about the sustainability of mammal diversity, especially in oil palm plantations.However, several publications have answered that the conversion of land cover such as open land and other use areas (APL) into oil palm plantations can provide a good ecosystem and biodiversity gain for certain types of mammals [3,7,8].Overall, most of the research was conducted in Riau (34%), followed by South Sumatra (15.45%), and West Kalimantan (11.5%) (Figure 3).Riau is the province with the most mammal research targets in oil palm plantations, with the abundance and diversity of mammals still relatively high.This proves that changes in land use for oil palm plantations, especially in the Riau area, have created new ecosystems for several mammalian groups.Judging from various aspects of the ecological area of the mammalian habitat of oil palm plantations, Riau province has the largest oil palm plantation area among other provinces, namely 2,860.8 thousand hectares or 19.5% of the total area of oil palm plantations in Indonesia (BPS 2021 data).
In addition, we found that behavioral ecology has become a topic that is often discussed in scientific articles on mammals of oil palm plantations in the last 10 years, namely as much as 33%, while species richness is 25% and management is 23%.Furthermore, the abundance of mammals on oil palm plantations is 12%, genetics is 3% and the lowest is about occupancy and distribution which is only discussed in 2% of the number of articles analyzed.Table 2. shows the roles of mammals in the food web according to their respective trophic levels.From the results of the analysis, it was found that there were five groups of mammalian ecological functions based on their diet, namely 6 taxa herbivores namely Sciuridae, Cercopithecidae, Pongidae, Lorisidae, Elephantidae, and Cervidae; a carnivorous group of 2 taxa namely Mephitidae and Felidae; an omnivorous group of 9 taxa namely Mustelidae, Muridae, Manidae, Tupaiidae, Sciuridae, Suidae, Viverridae, Cercopithecidae, and Ursidae; and the frugivorous group of 2 taxa namely Cercopithecidae and Pteropodidae.

Figure 4. Percentage of mammalian role on each tropic level
Based on the role of mammals in the tropic level, it is known that the role of mammals as omnivores occupies the highest position, which is 47.4%.The role of mammals as herbivores is 31.6%,while mammals as carnivores and frugivores have the same percentage of 10.5% (Figure 4).The high omnivore group on oil palm plantations shows the flexibility of diet that does not only depend on one type of food (animal or plant) but can be used for both.This is also supported by the high percentage of Sus scrofa and Paradoxurus hermaphrodites in all analyzed research, which is 30.8%.This illustrates that the new habitat formed in the oil palm plantation environment in Indonesia is favored by several mammalian taxa, especially the taxa Sus scrofa and the taxa Paradoxurus hermaphrodites.In addition, this overall illustrates that the availability of food sources for the omnivore group is still available.The diversity of ecological functions in terms of dietary aspects based on the trophic level of the food web of oil palm plantations proves that oil palm plantations have become a new home for the ecosystems of several mammalian communities.

Figure 5. Percentage of mammalian role as ecosystem services
Figure 5 shows the ecosystem services of mammals in the habitat area of oil palm plantations in Indonesia consisting of pollinators (6.7%), seed dispersers (66.7%), predatory insects (6.7%), prey The results of the representation of Figure 5 show that mammals as seed dispersers have the highest number of taxa, namely 8 family groups with 25 species, while Pteropodidae family as soil fertilizers found only 3 species consisting of Rhinolopus sedulus, Cynopterus minutus, Pipistrelus stenopterus.Judging from the percentage of research articles, Macaca fascicularis and Paradoxurus hermaphroditus species belonging to the seed-dispersing mammal group have 23.1%.Meanwhile, Cynopterus minutus and Cynopterus sphink from the pollinators group, and Callosciurus prevostii, Trachypithecus cristatus, Trachypithecus geei, Macaca nemestrina, Arctogali trivirgata, Hemigalus derbyanus from the seed dispersed mammal group, and Kerivoula pellucida and Rhinolopus pest control from the mammalian group is 3.8%.This confirms that the oil palm plantation habitat provides a suitable place for several groups of Mammals.

Discussions
Evidence-based knowledge is important for research planning and conservation action on oil palm plantations in Indonesia.In this review we found 19 families of mammals found in oil palm plantations, of which the most were from the Suidae family (Sus scrofa) and from the Viverridae family (Paradoxurus hermaphroditus).The species that can be found in almost all types of oil palm plantations is the civet (Paradoxurus hermaphroditus).There are mammal species that are protected under regulations for the protection of plant and animal species in Indonesia, namely orangutans (Pongo pygmaeus), proboscis monkeys (Nasalis larvatus), slow lorises (Nycticebus coucang), Sumatran elephants (Elephas maximus suamtranus), sambar deer (Rusa unicolor), skunks (Mydaus javanensis), pangolin (Manis javanica) and sun bear (Helarctos malayanus).The Mammal group is a biodiversity asset for oil palm plantations that needs to be maintained for its sustainability.
High conservation value (HCV) areas have the highest number of mammal species, especially arboreal mammals such as primates, due to their land cover in the form of forests [7].The more diverse the strata of the canopy, the more types of mammals that can live in it by occupying their respective ecological niches.This is supported by the research results of [9] which shows that the dependence of large mammals on plants is quite high.Areas of high conservation value (HCV) function as a temporary shelter (stepping-stone) or even habitat for several protected and endangered mammal species [9,15].In addition, several articles have stated that river flows in HCV areas function as riparian areas that are important for the existence of wildlife.This area serves to provide water as a habitat component and a link between habitats favored by wildlife (2,7,9).
Long-tailed macaques (Macaca fascicularis) can be found in various types of oil palm plantations even though their location is close to settlements and there are many human activities.This is because long-tailed monkeys have high adaptability and can survive in disturbed habitats [10].Based on the literature review, canopy cover in open oil palm plantation areas causes a lot of light to enter so that mammals, especially large mammals and arboreal mammals do not like it (because the broken canopy cover makes it difficult for this species to move).Oil palm plantation areas close to secondary forests or rubber plantations provide a variety of habitats for mammals [3], this is also in line with [11] old theory which states that an increase in the number of different habitats can lead to an increase in diversity.
The results of the literature review show mammals from the Muridae, Tupaiidae and Sciuridae are the families commonly found in oil palm plantations, while the Felidae, Hystricidae, Ursidae and Tragulidae are families that are rarely found in oil palm plantation areas.Mammal communities in oil palm plantations are involved in a food chain system that maintains the balance of the ecosystem.We found that the abundance of mammal species in the oil palm plantation area is relatively well preserved.Here we divide the trophic level of the food chain system into four levels based on the diet, namely herbivores, frugivores, carnivores and omnivores.The percentage ratio of herbivores: frugivores: carnivores: omnivores shows a data ratio of 3:1:1:4 which means that the value of the frugivorous and carnivorous mammal groups is much smaller than that of herbivores and omnivores.According to [12] the loss of carnivores and frugivores groups is caused by changes in land cover that disrupt the balance of the ecosystem.Furthermore, [13] added that habitat fragmentation in oil palm plantation areas is not beneficial for carnivorous mammals, but only beneficial for the Malay civet group, bearded pigs and the Muriidae group.[14] explained that this large-scale expansion of plant monocultures provides an unnatural environment where many animals adapt quickly and can become pests.The pattern of competition for carnivores does not only depend on mammalian groups but also on other wildlife groups that have the to be more aggressive and adaptive to the oil palm plantation environment as predators such as snakes and owls.
Based on the results of a literature review on the role of mammals in the ecosystem, it is known that there are 7 roles.According to [15], species of mammals from Chiroptera and Primates are mammals that play an important role as pollinating agents, seed dispersers, and controlling insect populations of oil palm plant pests.[16], stated that the community composition of most mid-large sized mammals displayed a clear negative threshold between 45 and 75% in response to increasing oil palm cover.This finding suggests that plausibly a sustainable oil palm landscape should contain a maximum of 45% of oil palm to prevent drastic mammalian community changes.Caution and a strict monitoring program should be implemented in landscapes where the percentage of oil palm is situated at the range of the threshold.Further, from the research of [16], stated that when oil palm cover increased to 75% or more, almost the entire terrestrial mammal community declined abruptly.
The ecosystem services provided by mammals in oil palm plantation areas can have both positive and negative impacts.The results of our analysis show that most oil palm plantation mammals play a role in seed dispersal and only a few fertilize the soil and act as pest predators.Oil palm seeds and other seeds from the HCV forest are scattered throughout the plantation area, even to the HCV forest area, which causes saplings to grow and various other seed plants grow above the ground.If the scattering is carried out in the oil palm plantation area, it is detrimental to the farmers because the oil palm saplings and seed plants that grow will become weeds, otherwise if the seeds are scattered in the HCV forest area, this will increase the diversity of forest vegetation.According to [5] the diverse functional roles of mammals are able to form, modify and even destroy habitats significantly by changing ecosystems and increasing heterogeneity and biodiversity in oil palm plantation areas.The ecosystem services provided will also help maintain the balance of the regional ecosystem.
To complete the analysis, we also identified research gaps in this review.We found that Sulawesi and Papua have rarely been chosen as research sites, with most researchers focusing on Sumatra and Kalimantan.This seems to be related to the number and area of the majority of oil palm plantations in Sumatra and Kalimantan.For research topics, there has not been much research related to mammals compared to other taxa.In addition, the diversity in research topics is still low, where more research discusses the abundance and diversity of mammals but not many studies on ethology and genetic aspects.Most of the research carried out is only done in one time period.There are still very few multiyear researches that produce time series data.Long-term research with continuous data is very important to capture a more complete phenomenon regarding the presence and role of mammals in oil palm plantations.In the long term, it is very important to have more research to answer: what is the prospective role of mammals in palm oil agroecosystems to support biodiversity conservation and sustainable production.

Conclusion
A systematic literature review analysis revealed 19 mammal families in oil palm plantations.The role of mammals based on food classification can be divided into 4, namely herbivore (plant-eating organisms that usually occupy the second trophic level in a food chain), carnivore (any animal that gets food and energy requirements derive from animal tissues), omnivore (any animal that include both plant and animal matter in their normal diet) and frugivore (any animal that subsists totally or primarily on fruit).This review indicated that the role of omnivore is most often found in study articles.Of the 7 roles of mammals in the ecosystem, the most frequently mentioned in the citation is as a seed dispersers, while the least role is the role of insect predator.In the future, more research is needed to reveal in more detail the role played by each taxa (especially for endangered species) in maintaining the balance of the ecosystem in the palm oil agroecosystem.

Figure 1 .
Figure 1.Flowchart of research stages systematic literature review

Figure 2 .
Figure 2. Research development data on Indonesian oil palm plantation land mammals in the last 10years (2012-2022) During the period 2012 to 2022, there are 447 credible and worthy literatures for analysis on mammalian research in Indonesia.Of these, 19 journals discussed the topic of behavioral ecology (33%); 14 topics on species richness (25%); 13 journals on management topics (23%); 7 journals on the topic of abundance (12%); 2 academic papers on genetic topics (3%); 1 journal on the topic of distribution (2%) and 1 journal on the topic of occupancy (1%).

Figure 3 .
Figure 3. Number of publication based on research location and research topic

Table 2 .
Roles of mammals based on tropic level