When the Buddha was talking about compassion and giving in the scriptures, he said that even if we take the rinsing from our bowl, pour it onto the ground or into the water hoping that other beings may feed on it, then it is a source of merit for us, let alone a gift for a human being. Certainly it is considered better Kamma to be able to help human beings, however, as the Buddha has explained here, opening our heart to all beings and seeing that they wish to live and do not want to die is quite important for us for developing our heart and unselfish emotions on a higher level.
However some people tend to focus too much on helping animals and don't understand the importance of helping humans, because humans have a great influence on the animal world. So if we help humans to open their heart more, because they have the capacity to do this, then it is going to benefit animals. So the highest level of giving is usually considered giving the Dhamma to humans, because animals don't have the capacity to develop in this way. But then humans have a responsibility to see that animals have a lot of suffering in their existences and that humans need to be sensitive to the well being of animals, insects etc. that share the world with us.
The Kammic effects if we don't open to animals, well, it shuts down our capacity to be able to put ourselves in the place of other living beings and understand that all living beings wish to live and not die, and wish to be free from suffering. Not being able to understand this interrupts our development of compassion to a higher level. It is important to be able to see the interrelationship between ourselves and other living beings in this world. But if a person is compassionate and generous to humans, then we try not to judge them if they can't have the capacity to have the same generosity towards animals, because we may be putting an ideal onto them, not taking joy in their capacity to be generous to humans.
Some human beings are not even compassionate to humans and treat them like animals. So it is important for us to try not to judge our family members in their capacities that we feel that they should have, as this may have negative Kammic effects for us. Certainly, when we look at the Culakammavibhanga Sutta, if we don't dwell compassionate towards the well being of creatures, it is written that the Buddha said we may be born with, if I remember rightly, ill health, because we are not sensitive to the wish for other beings to live. So the Kammic effect of harming living beings intentionally, which includes animals, is quite deep.