Survival during adverse seasons reveals size‐dependent competitive ability in a deposit‐feeding amphipod, Monoporeia affinis

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Previous food competition experiments indicated that small Monoporeia affinis (Lindström) amphipods are competitively superior to large conspecifics. If this is the case, large individuals such as adults should be more sensitive than their smaller conspecifics to food shortage during winter. Thus, our hypothesis was that the competitive relationships should be reflected in the winter survival between the different age‐ (size‐) classes. We investigated the effects of starvation on the survival and loss of mass of three age‐ (size‐) classes: juveniles (0+, young‐of‐the‐year) and (1+, one‐year‐old), and adults (2+, two‐year‐old) amphipods in a 10‐week experiment during the winter season. During the starvation period, the survival of the 0+ and 1+ age‐classes decreased gradually with no marked variation between the age‐classes over time, while that of the 2+ age‐class decreased dramatically after 4 weeks. The survival and swimming activity of adult (2+) amphipods were significantly lower than in the 0+ and 1+ age‐classes. The individual dry mass (W) of amphipods decreased with a rate equal to −0.0062W0.95 per day throughout the experiment, with no differences between age‐classes in the allometric scaling of the energetic costs. We found no dramatic decrease in the individual dry mass of 2+ age‐class similar to that in their survival. Using the survival results from our study to rank the age‐classes in terms of competitive ability gave the same rank order as from an earlier competition experiment. These results suggest that a starvation survival experiment prior to adverse seasons could be used to assess the competitive relationships within and between species exploiting the same resource.