Tortoises with long necks were found to be abundant in regions that had vegetation on a higher level. a few years later, drought hit the region and the vegetation dried up. over time, grasses were observed in the region. shortly thereafter, an increase in short-necked tortoises was observed in the region. what does this change in the species of tortoises suggest?

I believe this is survival of the fittest. so because of the drought there was no more food supply for the long necked tortoises thus causing them to die off and not be able to reproduce and pass of their gene for long neck. where as for the short neck tortoises there was enough food for them to survive and thus being able to reproduce and pass on their gene for short necks.  

The answer would be sand and gravel