Freshwater: Why do fresh water salmon swim upstream?
They swim upstream for different reasons. It could be that there is no light at night so they look for the light of the moon. The most common reason is that they lay their eggs upstream because it is safer from predators there and there is more food.

Marine: Are there type of omnivore plants in the coral reef?
Yes, there is the Sea Fan that eats tiny plants and animals.

Desert: How else do animals survive in the dry desert?
I found a website with an organized list of them here.

Forest: Why do tropical forests have poor soil?
"In the rainforest, most of the carbon and essential nutrients are locked up in living vegetation, dead wood, and decaying leaves. As organic material decays, it is recycled so quickly that few nutrients ever reach the soil, leaving it nearly sterile."
I found more information here.

Grassland: What sort of animals are poached and why?
Two of the most common animals poached are elephants and rhinos. The rhinos have a horn that fetches high prices on the black market. Elephants have ivory tusks that also sells wells. Not only that, the elephant has around 1000 pounds of edible meat.

Tundra: What other dangers do the animals in the Arctic Tundra have aside from cold?
Polar Bears are dying due to global warming because the ice under them melts and they drown. Global warming also changes habitats for seals and leaves no nesting grounds for turtles. Also, food and shelter is sometimes scarce for the carnivores living in this region. For animals with fur, poaching is also taken to account because of how much fur costs.