Some species are welcome, some are not. Here is a brief list of what a composter can attract.
Rodents may be attracted. It’s hot, there’s plenty to eat, so it’s ideal to live in!
In addition to putting an anti-rodent wire at the bottom of the composter, you can check if there are entrances (slightly wide slots, these little animals sneak into small holes) and block them. When doing a rotation or distributing the compost, the frightened rodents will leave. It is necessary before the manipulations to be careful to “block” the potential entries, so that they do not return inside as quickly as they left.
Flies and midges
Too much humidity attracts flies and midges.
Add sawdust and mix it with the organic matter, it is effective very quickly.
These small terrestrial crustaceans digest dead organic matter and thus accelerate decomposition. These good compost activators are welcome!
Julida is an order of centipedes, they are detritivorous, consuming dead leaves and wood, decomposed fruits, etc. Sometimes they also feed on a few mushrooms. 
The lithobie is just like the iule, of the family of the centipedes.
Its diet consists of woodbugs, spiders, and other centipedes. 
They can eat up to their weight in waste each day and thus accelerate decomposition. These compost activators are welcome!
The word gastropod comes from gastros, “belly, stomach” and podos, “foot”, literally, the one whose foot is its belly. Thus, we find slugs and snails in this class of mollusk, which can come to decompose organic matter.
Cetonia larvae / cockchafer larvae / European rhinoceros beetle
Cetonia larvae decompose wood.
The larvae of cockchafer, cetonia and those of European rhinoceros beetle (rarer) may look a little alike.
If the larvae of cetonia and those of European rhinoceros are welcome, cockchafers one can be devastating in gardens.
Cetonia larvae and cockchafer larvae are the most common, here is how to tell them apart.
The cockchafer larva has a large head and a small behind (top), the cetonia larva (bottom) is the opposite.
The cetonia larvae are white/grey and the cockchafer larvae are yellow.
Many other species have an essential role, although hardly visible.
Bacteria are the most numerous organisms. They proliferate thanks to the presence of nitrogen contained in bio-waste.
Mushrooms can be very various. They often appear as mold.
Molds play an essential role in the decomposition of plants and the fermentation of food.
There are several types of mites such as:
- oribatid mites which are essentially mycophagous. (They eat mushrooms);
- the gamasina which are predators...
Nematodes are worms that represent a very important part of biological diversity on earth. Over 3,000 species are currently described, they are bacterivorous, detritivorous or micro-predators.
In the compost, they are usually less than a millimeter.
Springtails are divided into over 3000 species. They are generally 2 to 3 millimeters.
Long considered primitive insects; they are wingless and ametabolic ( whose larva strongly resembles the adult).
The larvae usually molt four to five times before becoming sexually mature. They tend to be compared today to crustaceans: many species look like small shrimps and some crustaceans, such as talitres, are also “jumpers”. They were already present in the Devonian, about 400 million years ago, so before the insects.
They contribute to the dissemination and regulation of soil microflora (bacteria, fungi) and play a major role in the circulation of nutrients (nitrogen, phosphorus, potassium, etc.), thus ensuring the availability of essential nutrients to plants.
In the absence of these animals, a large number of elements would remain immobilized within the microbial biomass, their activity of consuming the microflora stimulating the microbial populations and consequently the mineralization of the organic matter of the soil.