Why is more ecologically friendly to eat a salad than a steak?

It's a matter of perspective. There's no doubt that it takes a lot less time, land, water, and caloric energy to grow some lettuce plants and other vegetables to get a salad versus having to raise a steer to get just one steak. However, both come with an ecological cost.

Ground needs to be broken so that the seeds of the vegetable plants can be sown and grow without direct competition from the plants that used to grow there. Soil is exposed and open to being eroded away by wind and rain, and taking even the most nutritious parts of a vegetable plant means less nutrient is being returned to the soil, even if the remnants of multiple breakfasts, lunches, dinners and suppers are composted and returned to that garden soil. Exposed soil also dries out and tends to heat up more so than if it were covered up with grass. Weeds constantly need to be removed because Nature is just trying to cover up that exposed soil to protect it. Weeds still come in even if the soil is covered in mulch.

With the steak, ecological cost is greater if it came from a grain-finished steer than a grass-finished steer. The grain-finished steer obviously needs grain in addition to grass and hay for finishing. Grain must also come from ground that is broken to expose soil so that the crop can be sown and grown then harvested. How much of that ground needs to be broken to feed that single steer? Let's use a typical finisher ration to find out.

A steer at 1000 pound body weight is expected to gain 3 pounds per day. Typically they are finished for 120 days to a target weight of around 1350 pounds finish to slaughter. So, since I prefer to use barley over corn, we will us a ration that is 20.5 pounds of barley grain and 10 pounds of barley silage per day, which translates to 2460 pounds of barley grain and 1200 pounds of barley silage for 120 days of finishing. Expected yields per acre for barley grain and barley silage are 85 bushels (85 bu x 48 bu/lb = 4080 pounds) per acre and 8.5 tons (17,000 pounds) of barley silage, respectively. That means that 3/5ths of an acre (0.6 acre) for barley grain and 0.07 acres for barley silage are needed to finish one steer. Or, in total, that's 2/3rds of an acre or 26,136 square feet.

That's quite a bit of land. That's not counting the amount of land needed for grazing and/or hay production, if you're not counting grass or forage regrowth.

Even if we were considering a grass-finished steer, the ecological cost is lower because grain isn't being used in the diet; grass is, and grass is actually more ecologically-friendly than either crops or gardens because it regrows and keeps the soil covered. Manure from the single steer fertilizes the soil so that the plants can grow. Grass can grow in places where it's not practical to grow either a garden or cropland to grow grain or silage!

We also have to remember that from one steer we can get around 500 pounds or more of ready-to-eat beef which will last a family of four for a whole year.

So the answer to the question is that both are ecologically-friendly in their own ways, provided either are raised or grown responsibly and well with the environment in mind.