Which of the following best illustrates ecological succession?
a. overgrazing causes a nutrient loss from soil
b. grass grows on a sand dune, is replaced by shrubs, and then by trees
c. introduced pheasants increase, and native quail populations disappear
d. decomposition in soil releases nitrogen that plants can use
e. a mouse eats seeds, and an owl eats the mouse

The best illustration of ecological succession is b. the grass growing in the sand dunes is replaced with shrubs which in turn replaced by trees.

Over a period of time, sand builds up into sand dunes above the height of sea level. Once the dunes are fixed, saplings of them will grow towards the sea direction. The plants growing under such harsh conditions like bare ground are called pioneer species.

Such plants are stress tolerant with tough outer layer protecting rest of the plant. These plants can grow into shrubs and in turn tress promoting the ecology to maintain and sustain. These plants after death forms an organic matter for the existing ones and develops water retaining capacity.  

answer; gravity pulls glaciers down hill and as glaciers flow it pick up rocks and soil;