A large proportion of dead organic matter in soils and aquatic sediments may consist of invertebrate feces, which generalist detritivores often include in their diets. Some of the feces derive from grazing insects. In the laboratory, the feces of caterpillars of Operophthera fagata that had grazed leaves of beech (Fagus sylvatica) under the influence of leaching and microbial degradation decomposed faster than leaf litter itself; however, the decomposition rate was much enhanced when detritivorous isopods (Porcellio scabar and Oniscus asellus) fed on the feces (Figure 11.14). Thus, rates of decomposition and nutrient release into the soil from grazer feces can be increased through the feeding activity of coprophagous detritivores.
Feces of detritivores are common in many environments. It some cases, reingestion of feces may be critically important, by providing essential micro-nutrients or highly assimilable resources. In most cases, however, there are probably not marked nutritive benefits of feeding on feces compared with the detritus from which the feces were derived. Thus, the isopod Porcellio scabar gained no more from feeding on its feces, even when these were experimentally inoculated with microbes, than from feeding directly on the leaf litter of alder (Alnus glutinosa) (Kautz et al., 2002). On the other hand, in the case of the less nutritionally preferred leaves of oak (Quercus robur), inoculated feces provided a small but significant increase in growth rate compared to the parent oak leaf material. Coprophagy may be more valuable when detrital quality is particularly low.
A remarkable story of coprophagy was unraveled in some small bog lakes in northeast England (MacLachlan et al., 1979). These murky water bodies have restricted light penetration because of dissolved humic substances derived from the surrounding sphagnum peat, and they are characteristically poor in plant nutrients. Primary production is insignificant. The main organic input consists of poor-quality peat particles resulting from wave erosion of the banks. By the time the peat has settled from suspension it has been colonized, mainly by bacteria, and its caloric and protein contents have increased by 23 and 200%, respectively. These small particles are consumed by Chironomus lugubis larvae, the detritivorous young of a nonbiting chironomid midge. The feces the larvae produce become quite richly colonized by fungi, microbial activity is enhanced, and they would seem to constitute a high-quality food resource. But they are not reingested by Chironomus larvae, mainly because they are too large and too tough for its mouth-parts to deal with. However, another common inhabitant of the lake, the small crustacean Chydorus sphaericus, finds chironomid feces very attractive. It seems always to be associated with them and probably depends on them for food. Chydorus clasps the chironomid fecal pellet just inside the valve of its carapace and rotates it while grazing the surface, causing gradual disintegration. In the laboratory, the presence of chydorids has been shown to speed up dramatically the breakdown of large Chironomus pellets to smaller particles. The final and most intriguing twist to the story is that the fragmented chironomid feces (mixed probably with chydorid feces) are now small enough to be used again by Chironomus. It is probable that Chironomus lugubris larvae grow faster when in the presence of Chydorus sphaericus because of isopods do best when they can eat their own feces
'coprophagy' may be more valuable when detrital quality is low a midge and a cladoceran eat each other's feces the availability of suitable fecal material to eat. The interaction benefits both participants.
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