Cat and Mouse by Ralph Williams, Science Fiction, Fantasy (Hardcover)

The Harn first came to the Warden's attention through its effect on the game population of an area in World 7 of the Warden's sector. A natural ecology was being maintained on World 7 as a control for experimental seedings of intelligent life-forms in other similar worlds.

Free of natural enemies and competition, it had expanded enormously. So far, the effect in the control world was localized, but this would not be the case when the Harn seeded. Prompt action was indicated.

The Warden's inclination and training was in the direction of avoiding direct intervention in the ecology of the worlds under his jurisdiction, even in the field of predator control. He considered introduction of natural enemies of the Harn from its own world, and decided against it. That cure was as bad, if not worse, than the disease itself.

There was, however, in one adjacent world, a life-form not normally associated with the Harn; but which analysis indicated would be inimical to it, and reasonably amenable to control.

The Warden needed to have a certain very obnoxious pest eliminated . . . and he knew just the pest-eradicator he needed. . . .