Objection 1. It would seem that God preserves every creature immediately. For God creates and preserves things by the same action, as above stated (1, ad 4). But God created all things immediately. Therefore He preserves all things immediately.
Objection 2. Further, a thing is nearer to itself than to another. But it cannot be given to a creature to preserve itself; much less therefore can it be given to a creature to preserve another. Therefore God preserves all things without any intermediate cause preserving them.
Objection 3. Further, an effect is kept in being by the cause, not only of its "becoming," but also of its being. But all created causes do not seem to cause their effects except in their "becoming," for they cause only by moving, as above stated (45, 3). Therefore they do not cause so as to keep their effects in being.
On the contrary, A thing is kept in being by that which gives it being. But God gives being by means of certain intermediate causes. Therefore He also keeps things in being by means of certain causes.
I answer that, As stated above (Article 1), a thing keeps another in being in two ways; first, indirectly and accidentally, by removing or hindering the action of a corrupting cause; secondly, directly and "per se," by the fact that that on it depends the other's being, as the being of the effect depends on the cause. And in both ways a created thing keeps another in being. For it is clear that even in corporeal things there are many causes which hinder the action of corrupting agents, and for that reason are called preservatives; just as salt preserves meat from putrefaction; and in like manner with many other things. It happens also that an effect depends on a creature as to its being. For when we have a series of causes depending on one another, it necessarily follows that, while the effect depends first and principally on the first cause, it also depends in a secondary way on all the middle causes. Therefore the first cause is the principal cause of the preservation of the effect which is to be referred to the middle causes in a secondary way; and all the more so, as the middle cause is higher and nearer to the first cause.
For this reason, even in things corporeal, the preservation and continuation of things is ascribed to the higher causes: thus the Philosopher says (Metaph. xii, Did. xi, 6), that the first, namely the diurnal movement is the cause of the continuation of things generated; whereas the second movement, which is from the zodiac, is the cause of diversity owing to generation and corruption. In like manner astrologers ascribe to Saturn, the highest of the planets, those things which are permanent and fixed. So we conclude that God keeps certain things in being, by means of certain causes.
Reply to Objection 1. God created all things immediately, but in the creation itself He established an order among things, so that some depend on others, by which they are preserved in being, though He remains the principal cause of their preservation.
Reply to Objection 2. Since an effect is preserved by its proper cause on which it depends; just as no effect can be its own cause, but can only produce another effect, so no effect can be endowed with the power of self-preservation, but only with the power of preserving another.
Reply to Objection 3. No created nature can be the cause of another, as regards the latter acquiring a new form, or disposition, except by virtue of some change; for the created nature acts always on something presupposed. But after causing the form or disposition in the effect, without any fresh change in the effect, the cause preserves that form or disposition; as in the air, when it is lit up anew, we must allow some change to have taken place, while the preservation of the light is without any further change in the air due to the presence of the source of light.
Source: Summa Theologica (New Advent)