challenges that biologists encounter in estimating population characteristics for wild populations

In a group, brainstorm and discuss challenges that biologists encounter in estimating population characteristics for wild populations of (a) whales that migrate along the western coasts of North and South America; (b) algae that live in water bodies receiving excess fertilizers in runoff from cropland; c) caribou that inhabit an Arctic tundra environment; and (d) amphibians that live in marshes.