1. Theme And Rheme Theme Rheme
The main topic of the sentence / the additional information in the sentence.
Must be at least two clauses (has a process) in the sentences for this to work.

2. Snakes
Australia is home to a large number of different species of snakes. Many different species are similar in appearance and are extremely difficult to differentiate from one another while some snakes display high variations in appearance within a particular species. For these reasons experts rely on scale patterns to make a positive identification of a snake.
Source

3. Snakes – Theme & Rheme Theme
Australia is home to a large number of different species of snakes.
Many different species are similar in appearance //
Rheme
and are extremely difficult to differentiate from one another // Rheme
while some snakes display high variations in appearance within a particular species.
Theme Rheme
For these reasons experts rely on scale patterns // to make a positive identification of a snake.

4. Snakes – Rheme to Theme Theme
Australia is home to a large number of different species of snakes.
Many different species are similar in appearance //
Rheme
and are extremely difficult to differentiate from one another // Rheme
while some snakes display high variations in appearance within a particular species.
Theme Rheme
For these reasons experts rely on scale patterns // to make a positive identification of a snake.