Discussion Questions on Nakano, et al. (1999)
BSCI 467
Due November 24, 2014
Nakano, S, H. Miyasaka, and N. Kuhara. 1999. Terrestrial-aquatic linkages: Riparian arthropod inputs alter trophic cascades in a stream food web.
1. In general, give examples of how across-habitat interactions can alter the food web dynamics within one habitat? As a specific example, how does the input of vegetational debris into headwater streams alter the food web dynamics of streams?
Food web dynamics occurring in a focal habitat, in particular, are often influenced strongly by trophic linkages, the movements of nutrient, detritus, and prey organisms across contiguous habitats. Allochthonous inputs of prey species can allow consumer populations in recipient habitats to increase, and these subsidized consumers in turn can depress in situ prey items. In particular inputs of particulate organic matter from terrestrial ecosystems represent an important energy source of production in most headwater streams of temperate deciduous forests. Terrestrial arthropod inputs have an indirect but prominent effect on a stream benthic community by altering the intensity of fish predation in the food web. The ensuing effects on consumers can cascade through the food web and ultimately affect resident primary producers.
2. What was the hypothesis that Nakano et al. was testing in this article?
Terrestrial arthropods inputs have an indirect but prominent effect on a stream benthic community by altering the intensity of fish predation in the stream’s food web.
3. Briefly describe the methods that they used, including where it was done, the way the inputs of arthropods were manipulated, and the way fish were manipulated.
It was conducted in the Horonai stream in the Tomakomai experimental forest. During 4 weeks of summer (June-July) of 1995 the terrestrial arthropods and the presence of predatory fish were experimentally manipulated at the reach scale in the stream. The four experimental treatments (reduced arthropods w/ fish pressnt, natural arthropods inputs with fish present, reduced arthropod inputs with fish absent, natural arthrpods inputs with fish absent ) with two replicating of each, were randomly assigned to 8 twenty five meter long reaches over a 1.3-km stretches of the stream . Each experimental reach was separated from those immediately adjacent by at least 100 m long unmanipulated reaches. There were greenhouse-type covers that covered the entire experimental reach plus 25 meter upstream. Predatory fish were controlled with enclosures or exclosures of 10-mm mesh walls. All fish were first removed from all of the experimental reaches by nine-pass electrofishing and four reaches were stocked with 40 Dolly Varden after 2week. The daily input fall of terrestrial arthropods was estimated from samples collected by five pan traps.
4. What did they measure to satisfy you that their treatments were successful?
They measured different components of dry mass that were decreased by reduced input and compared them to respective natural input, so they can make sure that their treatments were successful.
5. List the measurements they took that were critical for the hypothesis they were testing.
Fish fork length (mm) , Depth (cm), current velocity (cm/s), discharge ( m^3/s), photon flux (%)
6. What are the key points demonstrated from the data illustrated in each of the following:
A. Fig. 2? I see that the drifting input and falling input of natural arthropods are much greater than the reduced. Also I see that there seems to be that we have a 50:50 ration between falling and drifting.
B. Fig. 3? We see that the dietary composition of Dolly Var den by
