To remove an individual species from an entry, first navigate to the entry in question and tap the edit button. On iOS this is called "Edit" at the bottom of the screen. On Android this is a pencil icon in the top right. Select No to live time and position.
Find the species you want to delete, and select it by tapping on the row (not the plus, as that adds a new observation). Scroll to the bottom of the screen and tap Delete Observation.