Strictly speaking, no creatures are entirely "necessary."
While there are few (if any) issues to which all of Christendom offers a single response with unanimous support, one belief that is professed widely across traditions as well as Protestant denominations is that the God of the Bible is omnipotent. As such, he does not rely on any particular ecological balance in order to maintain the biological systems that he has created. The point here is not to address any of the concerns raised by environmentalists, and the concept of human stewardship of the creation is vital; the point is simply that, from a Christian perspective, creatures rely on God -- not the other way around.
Your question also calls to mind the effects of "the fall," as described in Genesis, chapter 3:
(17) And to Adam he said, “Because you have listened to the voice of your wife and have eaten of the tree of which I commanded you, ‘You shall not eat of it,’ cursed is the ground because of you; in pain you shall eat of it all the days of your life;
(18) thorns and thistles it shall bring forth for you; and you shall eat the plants of the field.
(19) By the sweat of your face you shall eat bread, till you return to the ground, for out of it you were taken; for you are dust, and to dust you shall return.”
While those verses, when considered in isolation, certainly leave ample room for interpretation as to precisely what the earth was like prior to the fall, there is certainly place for the notion that, prior to man's sin and the introduction of God's curse (v17), the earth was an entirely beneficent place, where problems of scarcity could not have arisen, and there was no biological antagonism toward life generally. There is some support for the belief, then, that those species of flora and fauna that many of us find to be 'aggravating' are uniquely a result of the curse, and did not exist in the Garden before the fall.
Another angle to take in responding to the question would be to reference the notion of God's sovereignty, and the belief, emphasized in Reformed theology, that the essential motivation for all of God's actions is his intent to bring glory to his name. While there are those who would disagree with this belief, it informs the perspective that all of the creatures that have been created exist to bring glory to their ultimate creator -- in fact, that is their only true purpose, with all other effects being secondary -- and each glorifies God in a particular way, whether willingly or otherwise.