Thomas Parr, Krista Capps, Shreeram Inamda and Kari A. Metcalf
When animals feed on microbes, plant material, and other animals, the process of metabolizing this food produces waste. As consumed food is digested, it changes size and molecular structure as elements and molecules are plucked from it. The material removed from the food enters metabolic pathways. Important elements are frequently released from these pathways in inorganic forms: carbon as carbon dioxide, nitrogen as ammonium, and phosphorous as phosphate. Some of these are readily reused by plants, algae, and microbes to produce new growth – a process known as nutrient recycling. This nutrient recycling concept, however, seldom considers the fate of the food that is left behind as organic waste (feces). Where does it go? Who uses it and how important is it to them? What are we losing when we lose an animal from an ecosystem?
Our study provides insight into these questions by measuring the amount and chemical composition of wastes produced by stream insects and whether these wastes were readily used by microbes. Insects are a diverse and widespread group of animals who provide many natural benefits from pollinating flowers to supporting fisheries. However, human influences like urbanization and agriculture have led to a sharp decline in their abundances. We found that the organic ‘wastes’ of animals were rich in carbon and nitrogen and were rapidly consumed by microbes. We also found that insects, on average, were releasing more waste in organic forms than inorganic forms.
These two results suggest that estimates of animal contributions to elemental cycles based on the inorganic nutrients released may only capture a third of their total contributions – much more may be released in organic forms. They also suggest that the transformation and release of such microbially desirable organic wastes have important implications for our understanding of how plants, algae, and microbes coexist as they compete for nutrients.