Learn English – Is ‘stationary’ singular or plural

Whilst a dictionary search shows that the word is singular but plural in form, it is most commonly used (in my experience) with a verb in singular form.

Math(ematic)s is my least favourite subject.

Math(ematic)s is really hard.

This Ngram viewer helps:

Link to Ngram

EDIT: As @Araucaria points out, the "mathematics are" examples in the Ngram above are not really relevant. See his comment below.

"Dynamics" can be either singular or plural, depending on usage. http://i.word.com/idictionary/dynamics

• The science of Dynamics is singular. "Dynamics is important for Physics majors to study."
  "Group Dynamics is a useful managerial tool."

• a specific instance of "a pattern or process of change, growth or activity" can be called a dynamic. So in contrasting two or more of these you would use "dymamics" as plural. "Your family dynamic was different from mine." "Our family dynamics were different."

So I would say that your first example refers to the study of population dynamics, whereas the second refers to the varying dynamics of two or more populations.