Interactions between year classes in the benthic amphipod Monoporeia affinis: effects on juvenile survival and growth

Abstract

The effect of 1-year-old Monoporeia (=Pontoporeia) affinis on the survival and growth of young of the year was studied in laboratory experiments. Amphipods were kept in jars with sediment and a continuous supply of cooled water for 3 months. Adults or juveniles were added to a control density of juveniles. Juvenile mortality increased at higher total densities, and was greater when adults were present. The mean length and dry weight of juveniles decreased at higher densities, but adults did not have a greater effect than juveniles. These density-dependent effects are probably caused by competition for food within and between year classes. Thus, a high abundance of juveniles or adults could reduce the recruitment of juveniles into the adult year-class the following year, and the suppression of juvenile growth could lower fecundity. The results of this experiment support the hypothesis, inferred from field data from the Baltic Sea and Swedish lakes, that intraspecific interactions contribute to fluctuations in the abundance of populations of M. affinis.

Intraspecific competition for food within and between year classes in the deposit-feeding amphipod Monoporeia affinis-the cause of population fluctuations?

“…Results from field surveys conducted by Sarvala (1986) and Lehtonen & Andersin (1998) indicate that both growth and mortality in juvenile Monoporeia affinis are dependent on the density of adults. In addition, Hill (1992) showed similar results in her laboratory study, and found evidence for food competition in adults (1+) when using baker's yeast as source. Along with our results, that growth of juveniles was densitydependent of adults, but only in the presence of added food; there exists a body of evidence that give strong support to the hypothesis that intraspecific competition for food between older age classes and juveniles acts as an important process causing abundance changes in Baltic Sea populations of M. affinis.…”

“…However, in juveniles, a density-dependent trend for mortality was detected at the lowest food level both with and in the absence of adults, which may indicate a small contribution of directly damaging crowding effects. The data therefore cannot exclude the possibility that adults may directly harm juveniles, as suggested by Hill (1992), who found greater mortality in juveniles at high adult densities. Our data showed that juveniles under normal food regimes and high density of adults were only marginally larger than juveniles without food.…”

“…It is therefore plausible that these juveniles would suffer much higher mortality than those that did not compete for food, given longer experimental time (see Fig. 3), suggesting that higher mortality in the presence of adults in Hill (1992) was more likely an effect of competition for food than of direct damage or predation. In our experiment, the mortality in treatments with only juveniles was density-dependent at the medium food level.…”

“…adult mortality and/or adult fecundity). In species that have distinct year classes, 3 experiments indicate that adults depress the survival and/or growth of juveniles: in polychaetes (Kent & Day 1983) and in amphipods (Wilson 1989, Hill 1992.…”

“…Sarvala 1986, Lechtonen & Andersin 1998 and from the Swedish lake Mälaren (Johnson & Wiederholm 1989), where growth and/or mortality of juvenile M. affinis was shown to be density dependent of older age classes. Further, laboratory experiments conducted by Hill (1992) and Goedkoop & Johnson (1994) indicate that growth in juvenile M. affinis depends on the density of adults (1+) and juveniles. However, in neither of these studies was the food level manipulated or estimated, making it difficult to interpret the results and to distinguish between competition for food and other possible crowding effects.…”