They are more subtle, but they are there. And it does have an atmosphere. It's substantial enough that communication to the surface can be lost for months due to planet-spanning dust storms. Yes, it's only 1% the pressure of Earth's at the surface, but that's enough, especially when you allow forces to act over geological time scales.

And yes, they can be as varied as those on Earth. Life doesn't actually increase the biome variety as much as you think it does. The kind of life you get in any given biome on Earth is a direct function of the geology and climate in the area. Input a given altitude, rainfall, temperature, and soil conditions, and you'll get a similar biome anywhere on Earth. Yes, there are different individual species in the rain forests of South America vs the rain forests of Africa, but they're both rain forests. They work as biomes in similar ways. Wherever the local climate and geology support rain forests, rain forests sprout up. The only exception is isolated islands that can't be reached by certain species.

This is why Mars can have the same biome diversity as Earth. The living components of Earth's biomes are a direct mapping to the nonliving components. Earth's living biomes are no more diverse than the underlying geology and climate.

And this is before we even consider Martian life forms, which almost certainly exist. We know of bacteria that exist deep in the Earth's crust that, if you transported them to deep under the Martian surface, would be able to survive and thrive just fine with zero modification. We know Mars used to have vast oceans and all the ingredients necessary to get life started. And we've seen numerous bits of circumstantial evidence of bacterial life present in some capacity on Mars today. While scientists are loathe to affirmatively proclaim life on Mars. The extant existence of bacterial life on Mars today really isn't that an unusual claim. If life could get started on Earth, there's no reason to believe it couldn't have started on Mars. And that's before you consider pansperia. If nothing else, we know life can comfortably exist deep in the planet's crust. And who knows how such life might affect conditions on the surface.