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Experimental and theoretical studies often yield conflicting evidence regarding the direction – positive or negative – or magnitude of biotic interactions.

To demonstrate that the direction of interactions can change depending on the timescale of observation, researchers studied stream fishes and terrestrial and aquatic invertebrates in a 1.2 kilometer stretch of forested stream in Hokkaido, Japan. During summer, fish ate mostly terrestrial invertebrate prey entering the stream from the adjacent forest, which diverted fish consumption away from aquatic invertebrates. The seasonal input of terrestrial prey increased fish production, resulting in more fish consuming nearly all the aquatic invertebrates during winter. The resulting indirect effect on aquatic invertebrates was therefore reversed from positive at the seasonal timescale to negative across the entire year. Analyses support the hypothesis that ecological interactions can be dynamic, so that indirect interactions revealed by short-term experiments can change in direction over longer time periods.

Marcarelli, A.M., Baxter, C.V., Benjamin, J.R., Miyake, Y., Murakami, M., Fausch, K.D., Nakano, S., 2020, Magnitude and direction of stream-forest community interactions change with time scale: Ecology, p. online,

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