Answer:
Density is nearly 3 cabs per hectare
Explanation:
Given -
There are three three intertidal regions.
Area of each intertidal region is
hectares.
Number of crab in the intertidal regions are as follows -
Intertidal region I - 7
Intertidal region I - 2
Intertidal region I - 6
Sum of crabs in all three regions is equal to

Total area of all three intertidal regions is equal to
hectares
Now density per hectare is equal to total population of crab divided by total area of all the three intertidal regions
Density 
Density is nearly 3 cabs per hectare
Carbohydrates are broken down and they release energy.
Well, to answer your first question, Because there are places on Earth that are both biologically rich — and deeply threatened. For our own sake, we must work to protect them. and for your 2nd question, 3 characteristics of magnetic reversal are: field, bar magnet, and molten material. Hope this helps