Because migration and and other seasonal movements can affect the possibility of finding a mate, stresses, that could affect offspring survival(can  vary with season), and also availability of thing that are available for the offspring.
        
             
        
        
        
Evolution would be the answer you may be looking for.
        
             
        
        
        
I think the best answer is plant growth. My evidence for this is that to hot or to cold would kill them but if warm andwet environment is perfect to grow 
        
                    
             
        
        
        
Plastic bag.cane. cover is very useful for human. plastic mainly environment pollution . it's not used soil digestive . plastic mostly problem environment