1. Introduction
The predominant crop in Asia is rice, which is grown on a far larger area of agricultural land than other grains [
1,
2]. The importance of rice in the context of Asia can never be over emphasized as it feeds the most populous part of the world (Indian subcontinent, China and the Far East, Indochina, and South East Asia). Rice fortification in Asia is crucial, particularly in the areas with less developed economies, because it is the livelihood for most traditional farmers and an important grain to tackle malnutrition among children in most developing countries [
3,
4]. Rice has also been traditionally defined a food security, whereby having stable prices for rice indicates a positive marker [
5]. Therefore, depredation by pests, notably rodents, is one of the many difficulties that most of Asia’s rice industry is currently facing [
6].
In Southeast Asia, the most prevalent and significant rat pest species is the rice field rat,
Rattus argentiventer [
7,
8]. Other commensal rodent pest species that can be found in Malaysia include the black rat (
Rattus rattus) and the brown rat (
Rattus norvengicus) [
9]. The greater bandicoot rat,
Bandicota indica, on the other hand, is an introduced rodent pest that is only present in the northern states of Peninsular Malaysia, specifically in the state of Kedah, the state of Perlis, and the island of Penang. A recent observation by [
10] in Sabah’s oil palm has revealed new invasion records for
B. indica. They were also introduced to the Java Island, Indonesia, and Taiwan [
11]. Studies on the DNA suggested the greater bandicoot rat here is not native to the Sundaland region [
12] and chromosome studies have concluded that they were identified as
Bandicota indica nevorivaga [
13]. As a pest, bandicoot rat invasions are feared for several reasons: they are physically large [
11,
14]; adaptable in various landscapes [
15,
16]; have high reproductive rates [
17]; are disease carriers [
18,
19]; and are capable of causing devastating damage, whether towards crops, storage products, or infrastructure [
20,
21].
According to [
22], one of the sustainable rodent control techniques involved a coordinated management strategy in line with the fundamentals of a rodent management system with the aim of preserving the ecological features, specifically ecologically based rodent management (EBRM). This manual promotes rodent pest control that is sensible in terms of social, economic, and ecological elements of the area in question. Intriguingly, studies that specifically address the greater bandicoot rats are lacking, despite the recommendation and encouragement to use ecologically based rodent management to control rodent pests [
22,
23]. Consistent patterns between habitat characteristics and rodent populations in tropical agriculture areas are yet to be determined. Despite the fact that there are numerous rodent ecology studies available, chemicals are used as a method of control [
24,
25]. However, in order to implement an effective EBRM in agriculture, a thorough understanding of the relationships between rodent species and variables in the agricultural ecosystem settings is essential.
Habitat plays an important role in rodents’ survival. For example, bunds will provide a safe haven and nesting site for rodents [
26], while the diversity of small mammals can be influenced by vegetation characteristics [
27,
28]. Other important factors include crop stage and seasons [
28,
29]. The occurrence of rodent pests is linked to environmental factors such as planting season and rainfall [
30]. However, owing to rodent behavioral plasticity, life history traits, and high breeding potential, the control of rodents is notoriously difficult [
31]. It is challenging to design evidence-based management methods for the species without having a thorough understanding of the composition of the small animal species in rice fields and their degree of harm. We thus used the number of trapped rats, existing burrows, and bunds collected in the rice field together with landscape and habitat features as our dataset to investigate the relationships between agricultural habitat and the diversity of rodent communities in the rice field.
The goal of this study was to identify the key habitat structure that affects rodent pest populations in agricultural landscapes, as well as to investigate the relationships between rodent pest populations, farming practices, site-level habitat, and landscape characteristics. Owing to high occurrences of rodent pest populations in the Kedah rice field recently [
32] and their previous records of being notorious pests in agricultural lands focusing on cereal production [
33,
34], we hypothesized that the occurrence of the rodent pests increases with the rice field areas and human alterations such as railways, roads, and residential areas.
4. Discussion
The main finding to emerge from the analysis is that multiple rodent species were trapped from the Jitra rice fields, although they seem to be dominated by the greater bandicoot rats, with much lower numbers of the other species. One of the difficulties in rodent management is dealing with multiple rodent species coexisting in one place, as small rodent populations have unpredictable dynamics [
40]. Unexpected epidemics could result from this, affecting the health, conservation, and economic sectors. With multiple rodent species in an area, their competition for resources can be perilous, especially towards small-scale rice farmers. In addition, different rodent species may have unique traits, ecological requirements, and ecosystem roles that may affect how effective pest management is employed. In Cambodia’s rice field, at least three different species, namely, the fawn-colored mouse (
Mus cervicolor), the tanezumi rat (
Rattus tanezumi), and the red spiny rat (
Mus surifer), were dominant and usually abundant during the wet season [
41], while in Assam, India, a study conducted by Phukon and Borah [
20] reported four rodent species associated with rice fields in Assam, out of which
Bandicota bengalensis was the most predominant species with a relative abundance of 59.76%, followed by
B. indica with 19.08%. Despite that the occurrence of multiple rodent species is common, the situation may cause high crop loss, especially in vast agricultural areas.
This study discovered that several stand-level variables such as bund height, bund width, rice growing stages, and the rice planting season had an impact on rodent abundance in rice fields (
Table 5), while only two landscape-level variables—distance from sampling point to residential area and rice planting season—were highlighted as the most parsimonious model (
Table 6). As the measurements of bund heights and width increase, so does the abundance of rodents in the rice field. A larger size of the bunds would allow the rodents to build deeper and more complicated tunnels and encourage their nesting sites. Allowing the development of large bunds would encourage rodents to nest and spread. The bandicoot rat colonies in India were discovered to be able to build a burrow system that spreads up to 300 m
2 with multiple chambers and openings [
20]. The complex burrows are able to store up to 3 kg of hoarded food, which usually consists of nearby agriculture harvests such as rice, wheat, and sugarcane, which later would impose a threat to farmers’ harvesting yield [
42].
Rodent abundance was also found to have a dynamic correlation with rice growing stages [
43]. Rodents were found to be abundant in the early planting stages and reduced as the harvesting stage started. The reproduction of the rice field rat (
R. argentiventer) was found to coincide with the generative stage of the rice crop [
44]. Similarly, in West Java, breeding occurred during early rice plant production; the booting stage [
40]. The greater bandicoot rats, however, may have a slightly different breeding biology and social organizations [
6]. We agree with [
6] and concluded that their main breeding season would coincide with the monsoonal season. According to [
17], it takes almost one month for a new-born pup to reach maturity. The mean age at sexual maturity of male greater bandicoots was 59.9 ± 2.3 days and that for females was 63.4 ± 1.8 days.
Rodent activity depends on weather conditions [
2,
45,
46]; however, in our study, observation of trapped rodents over rice planting seasons indicated that their abundance is negatively correlated with the wet season. Thitipramote [
6] discovers that, even though the greater bandicoot rats have the capability to breed all year long, their breeding seasons peak during the wet planting seasons in Thailand. Although the abundance of most rodent species was higher during the wet season [
28], our results are consistent with those of [
47], where rodent abundance in Jitra, Kedah is higher during the dry season owing to a drier habitat and forces rodents to wander off in search for food, making them prone to being trapped. We also theorized, based on low females captured in this study, that, during the monsoonal seasons, most females would spend their time underground giving birth and taking care of their young, which explained the negative correlation of rodent abundance with the wet season in the stand-level variable model (slope = −0.498) (
Table 6). The absence of food source in the early rice planting season would allow the rodents to be more responsive to bait offered in the rodent traps installed nearby their burrows, when compared with the harvesting stage, where the food supply is sufficient.
The occurrences of
R. rattus and
S. murinus in the rice fields may be due to aggressive human settlement expansion into the agriculture lands.
Rattus rattus and
S. murinus are known to live alongside human settlements [
2]. They are highly adapted to coexisting with human populations and are particularly ubiquitous in the urban environment. Their occurrences in the rice field can be a threat to crops and storage facilities [
19] as well as become a health hazard towards human [
48]. So far, not much is known about the effects of land conversion and small mammal communities in rice fields in Malaysia and how they affect crop yields, storage, health, and food security—this is an area for future work. As a solution, deep ploughing of fields immediately following harvest would destroy the burrow system, followed by reducing surrounding vegetation density [
27], exposing the rodents above ground and increasing their chances of predation by available predators [
12].
Undergrowth vegetation has no apparent association with the rodent abundance (
Table 5). In other studies, Wells [
49] discovers that vegetation density does not play role in nesting patterns of rodents in Borneo; instead, the nesting behaviour largely reflects the species’ space use. For example, the Whitehead’s rat,
Maxomys whiteheadi, and the red spiny rat,
Maxomys surifer, dominated the terrestrial community and nests below the ground, whereas the large pencil-tailed tree mouse,
Chiropodomys major, was found to utilize higher habitat layers, mostly nests in the canopy. In our study,
B. indica and
R. argentiventer foraged on the ground, but nested mostly below ground, regardless of the condition of vegetation coverage and height in the surroundings. GLMM analysis indicated that the vegetation coverage and height factors do not contribute much towards rodent abundance in the rice field.
In this study, rodent abundance in the rice field increased in the vicinity of residential areas (
Table 6) and rodent burrows were discovered to be built under permanent infrastructure such as roadsides, railways, riverbanks, and other man-made structures that may provide refuge to more rodent pests. Therefore, rice fields nearby these areas require constant monitoring, and our analyses suggest that trapping exercises with covered traps should be conducted around active burrows located nearby residential areas, with high and wide bunds during the early rice planting stage in the dry rice planting season.
4.1. Management Implications
This finding has important implications for helping rice farmers to understand and make informed decisions regarding rodent pest management. GLMM models indicated that bund height and width play a crucial role in rodent abundance (
Table 5), especially for the subterranean species; that is, the rice field rat and the greater bandicoot rats in the rice field. High and wide bunds (such as in main roads and railroads) may allow rodent harborage and increase rodent pest in abundance; in contrast, usage of too low bund sizes may allow overflow of the water in the rice field, thus exposing the rice crops to weeds and other invertebrate pests. It also will hinder any farmer’s activity to manage their lands. Although rice bunds are recommended to be built less than 50 cm in width [
50] and should be within 50 cm (wide) × 30 cm (height) [
51], there is still potential for rodents to nest. Future studies may explore improvements in developing more compacted bunds and the usage of better bund materials in rice fields, which not only provide a stable structure for human assessments and improved water management in rice fields, but at the same time limit the accessibility for any rodent pests for nesting.
Each year, farmers must decide whether or not to allocate funds for rodent management. While predicting rodent outbreaks requires complex calculation of various variables [
52], and eruptive rodent population dynamics can be triggered by interannual variation in environmental factors [
53], the decision to invest in rodent control can be a huge gamble on the crop yield and profit. As many minor or sporadic pests today probably are kept in check by the action of natural enemies [
54], maintaining natural enemies in the rice field can be an asset, as it is not only ecologically based but also environmentally friendly. An established barn owl program in an agroecosystem can provide a constant presence of barn owls (
Tyto alba) to suppress rodent pest population under a tolerable level in the rice field agro-system [
55,
56]. Future studies should also explore other potential candidates of rodent natural predators such as black shouldered kite,
Elanus caeruleus [
57,
58], and how to manipulate the habitat landscape in their favor [
59].
4.2. Limitations of the Present Study
Owing to limited data, this paper could not provide a comprehensive correlation of landscape- and stand-level variables with specific rodent species or a sex-based rodent study, which may require a longer period of observation for such data. We are also unable to provide information on food habits and the type and amount of damage occurring to crops. We also have not discovered a consistent bait to be used in both planting seasons as the changes in rodent abundances could be significantly affected by the change in bait, especially for the different species of rodents. Although bandicoot rats were found to dominate the chart, it is still unclear whether the bandicoot rats are overtaking the rice field rat as the dominant species, as protein-based bait in this study favors the bandicoot rat species. For a better species composition study, we would suggest a bait favored by all species or the use of supporting observations using track plate or camera traps. It is also beyond the scope of this study to examine the relationship of natural predators with rodent abundance in the study area, albeit several natural predators were sighted at least once throughout the study, including barn owls (T. alba), black-shouldered kites (E. caeruleus), brahminy kite (Haliastur Indus), mongoose (Hepestes sp.), Asian water monitor (Varanus salvator), brown rat snake (Ptyas fusca), and equatorial spitting cobra (Naja sumatrana). Exploring other potential biological control agents and developing program to conserve them will allow multiple existence of natural predators in the areas and assist farmers in lowering rodent pest attack incidences.