For diversity:
* Insects: Hands down, insects are the most diverse animal group. They make up over 75% of all known animal species, with estimates ranging from 5 to 30 million different species.
* Nematodes (roundworms): These tiny, often microscopic worms are incredibly abundant and diverse, found in nearly every environment on Earth. Estimates suggest there may be more than a million nematode species.
* Bacteria: While not technically animals, bacteria are incredibly diverse and abundant, with estimates suggesting there are trillions of species. Their diversity is key to life on Earth, as they play crucial roles in many ecosystems.
For abundance:
* Zooplankton: This group of microscopic animals is incredibly abundant in the ocean, forming the base of the marine food web.
* Nematodes: Again, these tiny worms are incredibly numerous, found in almost all habitats.
* Ants: While not as numerous as some others on this list, ants are incredibly abundant in many ecosystems, forming complex social colonies and impacting their environments in significant ways.
Important Notes:
* Difficult to Measure: It's incredibly difficult to accurately quantify the diversity and abundance of most animal groups. Many species remain undiscovered, and even those we know about can be challenging to count.
* Importance of Each Group: Each animal group plays a vital role in its respective ecosystem, contributing to the overall health and stability of the planet. From pollinating plants to decomposing organic matter, the diversity and abundance of all animals are essential for life as we know it.
Ultimately, the answer to your question depends on how you weigh diversity and abundance. But whichever group you choose, it's clear that the Earth is home to a vast and incredible array of animal life.