A wildlife predator doesn't have a Dr. Jekyll & Mr. Hyde split personality. Like Nature, it just "is". Without predators to maintain that delicate balance in Nature, the animals they prey upon would multiply & create unhealthy eco-systems.
One good example is the re-introduction of native wolves back into Yellowstone National Park in 1995. Before their re-introduction, elk, etc. because they had no large predators, multiplied and remained relatively non-nomadic in the park. This resulted in decimation of grasslands and plants, etc. through their over grazing which impacted other wildlife, including insects. When wolves were added back into the landscape, elk and other similar species, became more nomadic, including moving to higher elevations, thus alleviating the stress on the environment.
Wolves generally pick the easier targets, such as the aged, the injured & sometimes the very young. But as with all predators, every hunt does not result in success. They may go for days without making a kill.
Like the wolf, the weasel expends a great amount of energy in its pursuit of prey and not every expedition brings success. So here are a few photos of a "softer" side of a weasel, that adorable but "pound for pound", a fierce adversary.
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