Plants eat sunlight. Herbivores eat plants. Carnivores eat herbivores. And saprotrophs eat everything. They are the decomposers, the living things that consume dead organic matter. They are earthworms, bacteria, fungi, and protozoa. They are what complete the ignoble Circle of Life.
Please excuse my simple illustration of the delicate complexity of nature, but hey -- at least it's not as simple as yin and yang. Anyway, the moral discourse begins and ends with the ecosystem, so we have to speak in terms that everyone understands.
The apex predator (lion, bear, human) plays an integral role in the balance of an ecosystem. In particular, they control prey species' population dynamics, the impact of which ripples down to the level of plant ecology.
Ecological systems are naturally self-balancing, but they can be dynamic. Even without human interference, plant and animal species can still disappear due to changing climate, changing food sources, competing species, etc. Oddly, the very chaos and dynamism of nature is the 'balance' that sounds so warm and fuzzy off the tongue.
But just because nature is dynamic does not mean that we should sit at the sidelines. There are ecological principles that will bolster entire ecosystems in a way that increases net life -- that is, more lifeforms at all levels, all giving and taking within a robust circle of life.
Organic farming is one great example. But to understand organic farming, we need to first understand industrial farming (where you get you're grains).
Industrial farming first wipes out all life from the given plot of land. Including bacteria. Then, they plant a crop that is genetically altered to tolerate a specific herbicide/pesticide that will continue killing all life on that plot of land, until... Boom. You got grains. So you cut 'em down utilizing cheap labor (migrant illegal labor), ship it across the country on a big truck with a huge engine, and sell them to unsuspecting customers. This is great for grain prices, but bad for human health and the environment.
On the other hand, organic farming uses manure and compost to naturally create nutrient-rich soil that grows healthy vegetables. It uses biological pest control to control pests. Lastly, it's sold locally so it doesn't spoil or make a big carbon footprint. The end result is more lifeforms, no toxins, and better health.
So what about animals? Well, harmony in nature cannot be achieved without predation, and human beings are ancient apex predators. We rely on animals for a complete array of fats and proteins, and it is our duty to participate in this vast, interconnected ecosystem. It only makes sense to hunt overpopulated species and protect endangered ones, and to farm in a way that promotes life at all levels, where each animal lives healthily and happily, and in turn provides nourishment to the next member of the food chain. Pain on an individual level pales in comparison to life on a comprehensive level.