The correct option is Insects.
Explanation
Global biodiversity estimates indicate that the animal kingdom constitutes the vast majority of species on Earth. Within the animal kingdom, arthropods-specifically insects-are the most species-rich group. Tropical rainforests, which host the highest density of biodiversity, contain millions of species that have not yet been taxonomically described.
Detailed Analysis
- Insects: Insects comprise more than 70% of all recorded animal species. Due to their small size, high adaptability, and occupation of diverse micro-habitats (such as high forest canopies and soil layers), a significant proportion of insect species in tropical forests remain undiscovered. Estimates for total insect species range from 5 to 30 million, with only a fraction currently described.
- Mammals: Mammals are macro-fauna and are among the most well-studied taxonomic groups. While new species are occasionally discovered, the probability of finding a large number of undiscovered mammal species is extremely low compared to invertebrates.
- Gymnosperms: This is a relatively small plant group with fewer than 1,000 known species worldwide (e.g., pines, cycads). They are well-documented, leaving little scope for a high number of undiscovered species.
- Bryophytes: While mosses and liverworts are diverse, their total species count (approx. 20,000-25,000) is significantly lower than that of insects. Consequently, the absolute number of undiscovered species is lower.
Key Takeaway: Insects represent the most diverse taxonomic group on Earth, accounting for the majority of global biodiversity and the highest volume of undiscovered species, particularly in tropical ecosystems.