Copepods have almost become a staple of the marine aquarium systems, serving as (1) algivorous and detritivorous members of the clean-up crew and (2) providing a self-supplying, nutritious live food source for the most finicky corals and small reef fishes. Their presence absolutely makes a captive system functions much more like a natural one.
Given the desirability of copepods, it is understandable why some keepers might be concerned about them getting preyed on by other aquarium animals. Still, this is one of the reasons that copepods are so desirable! They are about as natural and wholesome a food as you could add. So if any aquarium eats them, coral, mandarin fish, shrimp or whatever, it’s a good thing!
Now, in the case of shrimp consuming copepods, the former will be able to catch the latter so seldom that one, two, or even a bunch of shrimp could never significantly impact a healthy pod population. Shrimp simply are not equipped to catch or eat something so small. Most shrimp species could barely even pick up a pod with their relatively large pinchers.
Don’t believe us? Just try to capture a copepod with a small pipette. Whether the pod is on the glass, swimming in the water column, etc., they are fast, reactive, flighty and incredibly elusive. Sure, so are their predators. But there is a really big difference between a mandarin (which has a mouth that is specially adapted to sucking up pods) and a shrimp, whose pinchers are designed for grabbing/holding/shredding much larger objects. Catching a little pod with those things is like catching a fly with chopsticks: Possible, though exceedingly difficult.
Something that might be a larger "threat" to a copepod population than a shrimp is a wrasse. A six-line wrasse for example, can greatly out compete a mandarin dragonet for copepods. There are many other fish that can do the same thing. Why does this matter? Generally, if a hobbyist is concerned about a dwindling copepod population, it is because there is a finicky fish that relies on the pods for food. A six-line wrasse, however, does not typically rely on copepods for food. Having multiply fish eating copepods can be a bad idea if one relies on the copepods, but it is certainly possible if enough copepods are provided. Of course, it depends how large the aquarium is. Larger aquariums can have much more stable populations of copepods. If you aren't concerned about feeding a certain fish, you should not worry about any of your fish or inverts eating copepods.
But this is all beside the real point. Our biggest imperative as keepers of marine animals is to provide the best nutrition for them as possible. So when our fish and inverts eat pods, which are incredibly nutritious, we should be glad! And if it seems they’re not getting enough pods (i.e. the pod population dwindles down), we should take measures to boost the existing numbers. So, if you ever observe your shrimp catching a copepod, cheer it on! And then maybe add another jar of pods, install a refugium and dose some phyto to make many more!
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