"The Easy Way Out", a short story by G. Harry Stine, originally published under the name Lee Correy; also the answer to the old question Short story about alien invader scouts encountering a grizzly bear.
This is a short story in which aliens land on Earth and track, follow and observe animals.
"There is no obvious reason why we can't take over this planet for the Great Overlord."
"There may be several reasons why we can't. It all depends on the native life forms, particularly with regard to their Intelligence Index, Adaptability Index, and, most important, Ferocity Index."
For reasons I forget they follow a chain of increasingly belligerent animals.
Not a very long chain: grizzly bears, wolverines, children. First the grizzly:
The bear stopped fishing and let out a bellowing roar. This panicked the warrior tech who fired a projectile toward the bear's head. Another of the warrior techs got into position But the bear moved . . . fast.
The grizzly brought its huge forepaw down on the closest warrior alien. Armor and all, the warrior splattered.
The next swipe of the huge paw demolished Grahhgh, who had the misfortune to get within range. While trying to get to the second warrior, the bear stepped on Rastharrh, putting part of him out of commission. The bear rose on its hind legs to its full height of eight feet and started to swing again, aiming toward Harmarrght, but the second warrior fired an explosive bolt that caught the grizzly in the roof of the mouth and congealed its brain.
Near the end a wolverine faces down something large (a bear or wolf?)
The bear looked up from its meal and recognized the small bearlike form with its broad ribbon of light brown fur down each side. But the bear was still young and still hungry; it decided to put up a defense of its meal. It had yet to learn that there are few animals of any size willing to tangle with Gulo luscus, the wolverine.
The wolverine simply attacked the bear as though it did not know the meaning of fear. Its flashing teeth and slashing claws were smaller and less strong than the bear's, but sheer meanness was on its side. It ripped in to kill, giving no quarter. After the first encounter in which the bear's huge paw missed in a roundhouse swing, the battle was very short and very one-sided.
The grizzly took the easy way out. It retreated, ambling off into the pine forest as rapidly as it could move.
and when they follow the wolverine it goes to a cabin with a man outside and children playing.
There was a movement behind one of the windows. hen as Norvallk came up on the alert, two human children dashed out of the cabin with yells of delight.
The boy cries "It's wolfey (or something like that)" and starts to play with wolverine
With great consternation, Norvallk watched these two new life forms run fearlessly up to the wolverine.
"Glutton! You're back!" one of them cried.
They dropped to the ground in front of the little animal and began to stroke its coat. The wolverine responded playfully, for it had known these children all its life. They had found it as a cub, half-frozen and starved, somehow separated from its mother. Although these children had raised it as a pet, it often reverted to feral state and disappeared into the hills for days. But it always came back. Hunting was difficult and dangerous; it was easier to be fed on schedule by the children. And the humans were capable of giving it something very pleasurable and desirable: love.
but mother calls them to meal. The wolverine tries to get the boy to continue playing and the boy cuffs it.+
A larger biped appeared in the cabin door. "Boys! Lunch time! Come in Now!"
They started to go, but the wolverine wanted more play and love. It growled and tried to nip at one boy's leg.
The human child turned around and cuffed the wolverine smartly, scolding it as he did so.
The wolverine shook its fur and followed the boys into the cabin.
Aliens look at each other and flee.
"It's the most dangerous planet I know of."
"Well . . . yes. Even our most difficult conquests involved life forms with Ferocity Indexes that we could at least measure. But the drastic measures the Master spoke of might certainly . . ."
"Forgive me for anticipating you," Norvallk broke in, "but those drastic recommendations are likely to involve re-routing of ship lanes away from this vicinity and perhaps even abandonment of nearby outposts."
But we could certainly overcome . . ."
"Again, my apologies. Do you think we could fight the several life forms we saw on that planet without expending millions of warriors and a great deal of equipment? Remember the universal law of living organisms: the Law of Least Effort. This is a big galaxy, and there are more comfortable and less expensive parts of it in which to operate."
"I guess you're right," Harmarrght admitted. "There are easier things to do."