What protects cells from the particles outside

The deserts of the sahara and australia are associated with

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

Ascientist observes certain changes in the physical features of a particular species over time. she concludes that the species has undergone evolutionary change. she wants to study the animals by analyzing their population. she’ll use . the population of this species is decreasing considerably. if this trend doesn’t change, this species will fall under the category of animals. this is a plato question,i need to make up his biology credit.