It is impossible to count all the plants in a habitat, so a sample is taken. A quadrat is often used to sample plants. It marks off an exact area so that the plants in that area can be identified and counted.
There are some limitations of using a quadrat. Human judgement can be an issue when using a quadrat. For example, some plants may be partially inside/outside a quadrat so there are basic rules that scientists follow that reduce the chance of human judgement affecting results.
It is impossible to find and count all the animals in an area. You can get an idea of the variety and number by taking a sample.
Pitfall traps are often used to sample the small invertebrates living on the ground. You are likely to trap beetles and other insects, as well as spiders and slugs.
How to set up pitfall traps properly: