How are animals that eat only one kind of food not horribly malnourished? Do they need a narrower set of nutrients than humans?
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Equine/bovine animals (like cows and horses) can survive pretty much exclusively on grass – which I assume is one of the good examples for your question.
All living things consume energy. Energy is what allows the replication of cells to preserve life. Many plants survive by absorbing sunlight and converting the energy in that warm sunlight into “food” through photosynthesis. They also, however, absorb basic nutrients, like nitrogen, from solid particles in the soil, which supplements the energy they get from photosynthesis. They absorb these nutrients through their roots with the help of water which helps to absorb the minerals. You might think of plants as taking sunlight to heat their oven and the minerals as the ingredients in the meal they “cook” to “eat.”
Animals which primarily depend on grasses and grains as food eat those plants, which have already converted minerals and sunlight into a new, more complex form of energy. The animals “steal” this energy from the plants, having already done a lot of legwork.
These animals have specialized digestive systems and organs which produce the more complex proteins they need to develop muscle and fat, which aren’t obviously available in plants (but the building blocks are there).
Carnivores take this a step further and “steal” the nutrients from animals which already converted the energy from plants into valuable and readily available proteins (among other nutrients). This same concept occurs throughout the chain of life, with fruits and vegetables and grains all creating different concentratioms of sugars, fiber, carbohydrates (a sugar) and proteins and other basic vitamins and minerals which animals eat to quickly and easily obtain the energy they specifically need to live and reproduce.
Humans generally need a wide variety of foods because we have some of the most energy-intensive processes. Our basic living functions could be sustained with some basic sugars and carbs and a little bit of protein, but we have evolved to be able to grab more of that protein through carnivorous diets, and we have successfully been able to reach a wider variety of fruits and vegetables to obtain these nutrients for such a long time. Our brains are also extremely energy-intensive and require fats, oils, and specialized proteins to fuel the specialized higher-thinking regions of the brain. Dolphins are another highly intelligent species and depend on meat for their sustenance, and because advanced predators can obtain these easily through eating meat, our bodies developed to harness that advantage and has allowed our brains to develop further than pretty much all other organisms.
When an animal produces waste, there are insects and bacteria which take *that* specific form of energy and break it down even further into the more basic nutrients found in fertile soil, allowing plants to then reabsorb it in a new cycle. The abundance of specific organisms, and the concentration of certain animals producing that waste, affects the stability of that ecosystem. Grass and plants can only absorb so much of certain nutrients in a given timeframe. Same with the bacteria and insects which break down the waste of animals.
All of this is limited by the availability of basic minerals and nutrients in the earth’s topsoil (where small organisms can break them down and plants can absorb them theough their root systems) and how quickly plants can absorb sunlight for useful energy.
So again, a recap: animals with simple diets obtain simple nutrients, and are very limited in their evolutionary advancement with respect to brain development and advanced processes. However, all nutrients are basically formed by basic building blocks, like nitrogen, hydrogen, oxygen, sodium and even metals, combined with a fuel for combining and using those minerals for “life” processes, which is harnessed from the sun via plants and photosynthesis in relatively large quantities.
Edit: Equine is horses and bovine is cows
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