[C1] To address this problem, biologists are using a new method by counting species over the course of several years and under varied climatic conditions.
[C2] Amphibians have lived in our planet's ponds, streams, lakes and rivers.
[C3] These declines have occurred both in populated and undisturbed areas.
[C4] Moreover, it is not entirely clear whether the declines are actually part of a natural fluctuation in populations arising from droughts or a scarcity of food.
[C5] For over 20 years now, biologists have been alarmed that certain populations of amphibians have been declining.
[C6] However, offering clear proof of the declining numbers of amphibians has been difficult due to the lack of reliable data on past population sizes.
Which sentence comes first?