The Effects of a Top Vertebrate and Invertebrate Predator on the Colonization of the Benthic Community of a Third Order Stream
Linear food chain models often do not fit stream systems and many studies examining trophic structure in streams give conflicting results. It has been stated that the greatest effects are seen when the trophic levels are examined from the top-down. The aim of this experiment was to study the specific effects of two top predators, the rainbow darter (Etheosloma caeruleuml and the crayfish (Orconecles propinquusl on the colonization of a benthic invertebrate community in a stream system. Specifically to study the effect the predators had on invertebrate abundance both singly and together. Also studied was any interaction between the two predators that might aid or hinder in prey capture and consumption. The experiment was performed in Mill Creek in St. Joseph Co. Michigan. Enclosure cages were used to contain predators and also to allow for adequate flow of colonizing invertebrates in and out of the cages. Four treatments were used: crayfish alone, darters alone, darters and crayfish together, and no predators. Results showed that over the course of the experiment colonization did occur in all four treatment groups. This was seen in both invertebrate counts and in chlorophyll a analysis. Among treatment groups the only one that was significantly different from the others in terms of invertebrate abundance was the crayfish treatment. Darters alone had little effect but when paired with crayfish raised the invertebrate counts to a point not statistically significant from the others. It was therefore concluded that the crayfish were able to directly affect invertebrate abundance while the other treatments did not differ and did not directly affect invertebrate abundance. However the darters did apparently indirectly affect the ability of the crayfish to negatively affect invertebrate abundance. It was concluded that the darters affected the behavior of the benthic invertebrates to hinder their capture by crayfish. The overall experiment shows that indirect and direct effects do exist in a stream system and can be observed in the wild in a system that does not exhibit a clear linear food chain model.