Why are small land vertebrates less common than small invertebrates?

Question

There are some very small vertebrate species. The dwarf chameleon is a very cute example. But why are vertebrates on that scale so uncommon compared to invertebrates such as arthropods? Warm blooded creatures have issues with their metabolism as they get tiny, but even cold blooded vertebrates seem to be lacking at this scale.

At first I assumed that maybe it had something to do with the amount of time that they have had to evolve. Land vertebrates have been around for a bit less time than land arthropods such as insects, so maybe insects just has more time to take advantage of the niches that come with being that tiny. Looking in the ocean, we can find more small species of fish, and some large invertebrates such as crabs and their relatives, large clams, etc which might support that idea.

I have no idea if this is true though. Are there any biological reasons for why tiny vertebrates, on the scale of a few cm in size, are so uncommon?

Answer 1

I can think of a couple of answers.

1. Vertebrates are generally soft since they possess an inner skeleton and animal cells do not have cell walls. In order to survive falls from a larger height or other mechanical stress, a hard carapace would help tremendously, arthropods already do it and fill the niche by having an external skeleton of chitin (lancelets can exist since water offers little mechanical hazards when compared to the land environment). Hard reptile skin is a contender, however, it may not be sufficient.
2. Animals, such as chameleons which you've described, are staggeringly different when it comes to their innards as compared to arthropods. The differences lie in the respiratory system, circulatory system, the way they excrete, reproduce and so on. As an example, an arthropod cannot have lungs, instead it uses a network of canals in it's body in order to exchange air. Reptiles have lungs, which is why their size is limited as the respiratory system cannot be downsized forever, and it's deevolution can be considered impossible.
3. Smaller vertebrates cannot exist since arthopods and other animals prove to be superior in speed and resilience in their size niche, eliminating any small vertebrates which downsize.