Relationships between plants and animals are not just a simple food chain relation. Most animals eat plants or their products, sure. Some plants take nutrients off from dead animals which have been decomposed by fungus. Animals all need oxygen. Plants produce the oxygen that animals need for survival. Looking at this relationship from a superficial perspective, however, it appears that plants don’t really need animals. All they need is some sunlight, water, and nutrients. Animals take advantage of the oxygen plants produced. This suggests that animals are rather dispensable in the relationship. Plants, which controls the primary productivity, get to decide what kinds of animals live and how.
As human beings, a significant portion of our diet comes from plants and their products, including seeds (rice, fruits, some vegetables such as cucumbers that also contain seeds). Seeds are linked to flowering plants, which only came later. Before flowering plants, there were sets of entirely different groups of animals such as some dinosaurs. We take it for granted that we eat seeds. Flowering plants reward their animal or insect partners by providing products in exchange for pollenization and dispersal of seeds. We eat the seeds, and plants hope that we take the seed elsewhere. Plants are fundamentally limited by the fact that they are not mobile, while animals are. Thus plants need animals to maximize its dispersal. Paired with the right animal partner, plants can travel far and quickly, produce more genetic diversity, and possibly much more.
This all makes sense. However, the relationship between plants and animals would go beyond this. Considering plants the primary productivity agent, and animals the secondary productivity agent that feeds on the primary productivity agent. Then the question becomes why primary productivity needs secondary productivity? Is this always the case?
The endosymbiosis theory suggests that chloroplast and mitochondria, once free-living microorganisms, merged into another organism and became its organelle. Over time, the relationship became so tightly bonded that they grow into a single organism, the Eukyrote. The free-living chloroplast may be similar to some free-living unicellular cyanobacteria, while mitochondria, largely mysterious still, could be distantly related to alphaproteobacteria. However, if cyanobacteria were the primary producer, it poses a fundamentally different scenario than that with plants and animals. Cyanobacteria can be mobile themselves, as well as diatoms and other algae. Therefore the fundamental limitation of motility on land plants does not exist in the ocean. So then why aquatic photosynthetic organisms need animals? Perhaps really they don’t!