Each creature has a Magic Power associated with it. Higher level creatures have more Magic Power. Your total Magic Power is the total of all the Magic Power from all your creatures added up.

That said, there are two reasons that your Magic Power might go down:
  1. If you just merged Level 4 creatures, those creatures (and their Magic Power) have been replaced by a Nest. Look around for that Nest! Collect the eggs from that nest and hatch them into a Tier 2 creature, and your Magic Power should be better than before!
  2. If you played a level where a creature was summoned into that level with a Creature Portal, then the creature is temporarily not in garden anymore, and thus your Magic Power in garden has gone down. If you find that level, marked by an "In Progress", then go into it and either finish the level or reset it, and the creature will return to Garden, bringing its Magic Power with it.