Why are there Animals? A religious answer might be that animals exist to supply humans with protein, to provide companionship for humans, and to round up sheep and pull wagons. The problem with that is that non-human animals existed millions of years before humans. We need a thermodynamic answer.

Animals bring Advantages to Ecosystems

The existence of trophic levels in ecosystems is a well-established concept. A question that seems to never have been asked is, why are there trophic levels in the first place? Why was there a need to expand the original two-component model of producer and decomposer? What was the irresistible force that created the need for animals?

Teleologically speaking, the reason animals came into existence was to facilitate nutrient movement to the decomposers that would increase recycling and increase ecosystem productivity. Their advantage to the ecosystem was greater power output.

A Non-teleological Explanation 

 The first ecosystem consisted of a single-celled eukaryotic producer and a bacterial decomposer. In these first, two-component ecosystems, an autotrophic eukaryotic population produced biomass, and when the older individuals died, they were eaten by decomposers, the prokaryotic bacteria. The first ecosystem feedback loop emerged from the circulation of nutrients released by prokaryotic bacteria and their uptake by eukaryotic cells necessarily in a restricted environment where feedback was possible.  Autocatalytic interactions in the feedback system allowed ecosystems and the organisms they contained to grow in size and power output. Some eukaryotes experienced beneficial mutations that allowed them eat autotrophic cells as a source of energy. That proved easier than to capture solar energy themselves. These became the progenitors of animals.

Once the first consumer (animal) evolved and reproduced, there was competition among the offspring for resources. The offspring that succeeded were the ones with the greatest power output, because they could win the competition for energy. By chance, one group of single-celled consumers banded together in a tube through which plankton-like autotrophic cells would flow. This proved to be a much more efficient way to capture autotrophs, and so this type of animal enjoyed a selective advantage. Throughout evolutionary time, animals increased in complexity around the tube that gave rise to the gastro-intestinal tract.  This increased even further their power output.

Ecosystems

The addition of animals to food chains resulted in a longer food chain and increased biomass of ecosystems. The benefit to ecosystems was increased power output and greater stability.  The combination of autotroph, consumer, and decomposer in this enhanced energy flow enjoyed a selective advantage over cells that were unattached and free-floating or systems with only two nodes.

Migration to Land

In terrestrial systems, nutrients are often the limiting factor for plant growth, so it is important for plants to conserve nutrients. They do this by synthesizing compounds in their leaves that protects them from microbial decomposition. That is good for the plant, but bad for the ecosystem because it slows down nutrient recycling in ecosystems. When plants shed leaves with defensive compounds, nutrients are released very slowly into the soil, and as a result, productivity of the plants declines. But when the first animals appeared, their digestive systems were able to break down the compounds, and nutrients were released back into the soil in a soluble form available to plants. Nutrient cycling was increased and this benefitted the ecosystem.

Ruminants

Ruminants are one of the most diverse and successful groups of mammals. Their ability to acquire nutrition from plants that contain chemical or mechanical defenses is due to fermenting of the leaves through microbial actions in a rumen that facilitates digestion. This may underlie their success in competition with other mammalian species. Breakdown of recalcitrant plant material by microbes in the rumen results in excrement in which nutrients are soluble or almost so. Ruminants are one of the most diverse and successful groups of mammals. Their ability to acquire nutrition from plants that contain chemical or mechanical defenses is due to fermenting of the leaves through microbial actions in a rumen that facilitates digestion. This may underlie their success in competition with other mammalian species. Breakdown of recalcitrant plant material by microbes in the rumen results in excrement in which nutrients are soluble or almost so.

Insects

Insects, of course, are also animals. Like mammals, they also speed-up recycling of nutrients Shredders wander the stream bottom looking for vegetation that has fallen into the water. Using their tearing mouthparts, they rip and shred the leaves as they feed.

Grazers and scrapers are animals that specialize on feed on the biofilm layers. They use rasping mouthparts to scrape the biofilm and algae off of the rocks and vegetation.

Dung beetles feed on feces of higher animals and accelerate the release of nutrients from the feces.

Conclusion

 Life in ecosystems could very well exist without animals. It is just that animals increase the energy-storing capacity and nutrient recycling ability of ecosystems, thereby increasing the probabilities for survival of ecosystems and their embedded species.