Basically the title. They’re going to encounter animals they’ve never seen before, or notice a complete lack of familiar animals. They’re going to notice the none of the plants are the same, and that there aren’t any domesticated plants other than what they may be carrying. They might try to go back the way they came and find it impossible.
In Arizona you can go from Sonoran Desert to forest tundra in as little as 45 miles. Anyone who grows up in Phoenix and vacations in Flagstaff knows the sensation of feeling like you’ve entered a different world several times along the way.
This is a fun read: https://www.desertmuseum.org/desert/habitats_sonora.php?print=y