Diverse Species: Insects Rule The Animal Kingdom

Insects are the most diverse animal group, with over 1 million species documented and studied by scientists. This accounts for about 40% of all known living species. Beetles, a subgroup of insects, are the most species-rich creatures known on Earth, with about 400,000 species, including 5,000 species of ladybugs. The success of beetles and insects as a whole can be attributed to their old age as a species, short life cycles, and high levels of speciation, allowing them to adapt and thrive in various ecological niches.

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Insects, with beetles being the most species-rich

Insects represent the largest percentage of the world's known species, with estimates ranging from 1 million to 30 million species of insects inhabiting our planet. This accounts for about 40% of all known living species, and some even suggest that insects make up to 90% of all species of animals on Earth. Beetles, constituting about one-third of all known insect species, are the most species-rich among insects, with approximately 400,000 species, including 5,000 species of ladybugs.

The success of insects, and beetles in particular, can be attributed to several factors. Insects have been around for almost 500 million years, allowing ample time for speciation and the accumulation of species. Their longevity, combined with their short life cycles, has resulted in quicker reproduction rates and more offspring, creating numerous opportunities for mutations that lead to new species. Insects' small size also enables them to survive in a diverse range of habitats, and they can be found everywhere, from snowy mountain ranges to scorching deserts.

Beetles, specifically, exhibit remarkable adaptability. Their outer wings not only facilitate flight but also serve as protective coverings that enable them to burrow underground. This versatility has likely contributed to their successful proliferation. Additionally, beetles' ability to store sperm for delayed fertilization may be another factor in their high species diversity.

The vast numbers of insect species have significant implications for our understanding of biodiversity. The Planetary Biodiversity Inventory project, led by entomologist Toby Schuh, aims to study the entomofauna in tropical forest canopies and shed light on the evolutionary relationships among bug species. These efforts will contribute to a more comprehensive grasp of the insect world and the ecological niches they occupy.

The sheer abundance of insect species underscores their importance in the natural world. Insects, with beetles leading the way, showcase an extraordinary capacity for adaptation and survival, ensuring their dominance in terms of species diversity on our planet.

Mammals, with rodents being the most numerous

Mammals are one of the most diverse groups of animals, with around 5,500 species on Earth. Within this group, rodents are the most numerous, constituting more than a quarter of all mammal species. This equates to approximately 1,500 species of rodents, including rats, squirrels, beavers, capybaras, and porcupines. Rodents are native to all major land masses except Antarctica and can be found in almost every terrestrial habitat, including human-made environments. They are extremely diverse in their ecology and lifestyles, with some species being arboreal, fossorial (burrowing), saltatorial/ricochetal (leaping on their hind legs), or semiaquatic.

One characteristic that defines rodents is their dental structure. They possess a single pair of continuously growing incisors in both the upper and lower jaws, with the absence of other incisors and canine teeth creating a gap known as a diastema. Their incisors do not meet when chewing, and their powerful masseter muscles provide the force for gnawing. This gnawing ability is a defining trait of rodents, derived from the Latin word 'rodere', meaning 'to gnaw'.

Rodents vary significantly in size, with the mouse weighing only 18 grams and the marmot weighing 3,000 grams. Some species exhibit sexual dimorphism, with male-bias observed in ground squirrels, kangaroo rats, solitary mole rats, and pocket gophers, while female-bias occurs among chipmunks and jumping mice. Rodents can be diurnal, nocturnal, or active during both day and night. Their diets vary, with some being herbivorous, while others include both vegetable and animal matter.

While some rodents are considered pests or vectors of diseases such as the plague, others are beneficial to humans. They serve as a source of food, provide fur for apparel, act as test subjects for research, and are kept as household pets. Additionally, rodents exhibit unique behaviours, such as seismic communication observed in the Middle East blind mole rat and footdrumming by the banner-tailed kangaroo rat for long-distance communication and predator warning, respectively.

Although mammals, including rodents, represent impressive numbers in terms of species diversity, insects constitute the largest group of animals. Insects are estimated to have between 1 million to 3 million species, with beetles alone accounting for about one-third of all insect species. Insects have been around for almost 500 million years, allowing for speciation and adaptation to various ecological niches. Their short lifespans and rapid reproduction rates have contributed to the vast diversity of insect species.

Amphibians, with frogs being the most abundant

Insects are the most diverse group of animals, with beetles being the most species-rich creatures on the planet. However, when it comes to amphibians, frogs constitute the largest number of species.

Amphibians are ectothermic, anamniotic, four-limbed vertebrate animals that constitute the class Amphibia. They are superficially similar to reptiles but require access to water bodies to breed. They are highly susceptible to environmental disturbances, making frog populations a good indicator of the health of an environment.

There are approximately 8,000 known amphibian species, of which nearly 90% are frogs. Frogs and toads are among the most diverse animal groups, with a wide variety of unique traits and behaviours. They have complex reproductive needs and permeable skin, and their life cycle typically starts out as aquatic larvae with gills known as tadpoles. At a certain size, the young develop limbs and lungs, and some even lose their tails. They then hop or climb out of the water as adults and spend the rest of their lives on land. This process is known as metamorphosis.

Frogs have many mating strategies, and scientists are still learning about their sex lives. For most species, mature males initiate the breeding process by calling out to females, who then approach and choose a mate. The fertilized eggs, or frog spawn, can incubate for anywhere between 48 hours and 23 days before hatching, depending on the species.

Despite their diversity, amphibians are the most threatened group of vertebrates on Earth. 40% of amphibian species assessed by the IUCN are at risk of extinction, and human-related threats are causing the decline of many frog species.

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Birds, with red-billed quelea being the most common

Birds are a diverse group of animals, with around 10,000 known species alive on Earth today. Insects, however, constitute the largest number of species among animal groups, with an estimated range of 1 to 3 million species. Beetles alone make up about one-third of all known insect species, with approximately 400,000 species, including about 5,000 species of ladybugs.

Among birds, the red-billed quelea is the most common species, with a total post-breeding population of approximately 1.5 billion individuals. This small sparrow-like bird, native to Sub-Saharan Africa, was first described by Linnaeus in 1758, who incorrectly identified its origin as India. It primarily feeds on the seeds of annual grasses but also damages cereal crops, earning it the nickname "Africa's feathered locust." Queleas are highly sociable, travelling in enormous flocks that can number in the millions, creating a spectacle akin to a rolling cloud.

The red-billed quelea, or Quelea quelea, belongs to the weaver family and has three recognised subspecies. It is approximately 12 cm long and weighs between 15 and 26 grams. The breeding males are distinguished by a black or, rarely, white facial mask, surrounded by purple, pink, yellow, or rusty colouring. The bill of the breeding male is bright red, giving the bird its name.

The red-billed quelea's nomadic nature and adaptability have contributed to its success. While subsistence farmers employ low-tech methods like banging pots and pans to scare away the birds, these measures are often ineffective on large-scale farms. The species' high reproductive rate and ability to exploit food sources efficiently have led to an increasing population, despite lethal control methods claiming millions of queleas annually.

The abundance of red-billed queleas has generated challenges, particularly for farmers, as a single flock can decimate a year's worth of crops. However, their population remains resilient, and some have proposed utilising the birds as a food source rather than solely focusing on population control.

Fish, with bristlemouths being the most plentiful

Insects constitute the largest number of species among animal groups, with an estimated range of 3 to 30 million species. Beetles, in particular, are the most species-rich creatures, making up about one-third of all known insect species.

However, when it comes to fish, bristlemouths are believed to be the most plentiful. Belonging to the family Gonostomatidae, bristlemouths are small, deep-sea fishes with elongated bodies, small eyes, short snouts, large mouths, and large jaws. They are commonly known as bristlemouths due to their sharp, bristle-like teeth.

The genus Cyclothone, with 13 recognized extant species, is believed to be the most abundant fish genus and vertebrate genus on Earth. Estimates suggest there could be up to a quadrillion individuals. They are mostly found in the mesopelagic zone of the ocean, at depths of over 300 meters (1,000 feet). Cyclothone species have specialized adaptations to survive in the extreme conditions of the deep sea, including bioluminescence, which helps them communicate and confuse predators.

Tan bristlemouths, a species within the Cyclothone genus, are also considered one of the most abundant species on Earth. They have a relatively large mouth, suggesting predatory behavior, and use light-producing cells for communication and predator avoidance.

While bristlemouths are numerous, they are small and easy prey for larger deep-sea creatures like dragonfish and fangtooths. They feed on zooplankton, small crustaceans, and occasionally smaller fish. Bristlemouths are protandrous, with individuals starting life as males and some transitioning to females at reproductive age. Their reproductive behaviors remain largely unknown due to the challenges of observing them in their deep-sea environment.

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