1.1. Ecological importance of tropical rainforest

Tropical rainforests are the most diverse in the vegetation structure and composition (Figure 1) that supported a diversity of fauna species such as birds, reptiles, mammals, amphibians, and invertebrates, which directly or indirectly depend on them for their survival and existence. They are rich in habitat diversity and provide a variety of resources for the avian species, such as food, habitat, and shelter [6]. Tropical rainforest is vital ecosystem, i.e., it provide crucial ecosystem services such as raw materials, reservoirs of biodiversity, soil protection, sources of timber, medicinal plants, carbon sequestration, and watershed protection [7–9].

### 1.2. Threats to tropical rainforest

Tropical rainforest covers less than 10% of the land area of the Earth, representing the largest biological diversity reservoir, i.e., >50% of known plant species grow in tropical rainforest. Despite being rich in fauna diversity, every year, huge areas of tropical rainforests are being lost and degraded due to human interference [10–15]. It has been stated that 25–50% of the world's tropical rainforest has been lost and degraded due to the land-use change such as

Figure 1. Aesthetic view of tropical rainforest.

deforestation for palm oil plantations, agriculture expansion, cattle ranches, mining, and development of housing societies [16–19], while the rest of the rainforest areas is under a major shift in the dynamic structure and productivity.

It has been reported that Southeast Asia had the highest rate of land-use change (such as deforestation of tropical rainforest for conversion into oil palm plantation, commercial logging for timber and development of human settlement) as compared to other regions [12, 20–22]. Deforestation and fragmentation due to agriculture expansion, human settlement, logging, and fire had altered the plant species composition, richness, and diversity [23–26]. Deforestation and fragmentation, over-exploitation, invasive species, and climate change are the major factors due to which the biodiversity of tropical forest had declined at an alarming rate. For example, some of the fauna species became extinct, while others became threatened and vulnerable due to habitat loss, fragmentation, and degradation.

Changes in the vegetation structure and composition due to deforestation and fragmentation may alter the habitat suitability and food productivity. Habitat suitability, i.e., vegetation structure, species composition, species richness, canopy layers, and food productivity are key drivers, which predominantly influence fauna community parameters such as species composition, relative abundance, species richness, species diversity, and the density of tropical rainforest. Furthermore, it has been stated that the deforestation in humid tropic may be in the range of 4.9–5.7 million ha/year. Likewise, each year, 2.3 million ha of humid forests had been degraded due to logging and fire activities. Similarly, around 2.2 million ha/year tropical moist deciduous and 0.7 million ha/year tropical dry forest has been deforested due to anthropogenic activities [27].

Deforestation may cause habitat loss and fragmentation that adversely affect the population and the community parameters such as species composition, relative abundance, species richness, species diversity, and density of different wildlife species [28–30]. However, the effect of habitat loss and fragmentation on the wildlife species may vary depending on remaining vegetation and the surrounded landscape [31, 32].

Land-use change such as deforestation, i.e., depletion of tree crown cover due to conversion of forested areas in agricultural fields, human settlements, excessive logging, and road constructions are major factors of habitat loss and degradation [10, 33, 34]. The habitat loss and degradation are responsible for biodiversity loss [35], low production of food, and habitat fragmentation [27, 36, 37] that ultimately affects different fauna species. Due to deforestation, large areas become isolated, i.e., temporal refuge, which serves as corridors for different wildlife species, especially bird species [38–40].
