Questions & Answers about Esta especie es pequeña.
Because especie is a feminine singular noun, and the demonstrative has to agree with it.
- este = this, for masculine singular nouns
- esta = this, for feminine singular nouns
So:
- este libro = this book
- esta especie = this species
Agreement is a very common feature in Spanish.
In Spanish, noun gender is not determined only by the last letter. While many feminine nouns end in -a, plenty of feminine nouns end in other letters too.
Especie is simply a feminine noun, so it takes feminine words with it:
This is something you usually have to learn together with the noun: la especie, not just especie.