Intraspecific trait variation and colonization ability in natural assemblages of zooplankton

Background/Question/Methods

Intraspecific variation has recently received increased interest from community ecologists (Bolnick et al. 2011). Empirical evidence has shown that intraspecific variation can play a positive role in coexistence (Stoll and Prati 2001), community assemblage (Post et al. 2008), and juvenile recruitment (Gamfeldt 2005). I set out to test what traits (e.g. body size, plasticity) of the keystone aquatic herbivore, Daphnia, promote colonization success and to find evidence that niche theory is governing community-level properties (e.g. colonization, productivity, diversity). Niche theory predicts that increased intraspecific diversification should increase the colonization ability of that species, as it has more ecological opportunities. In addition, colonization success of a colonizer should be a function of species richness rather than abundance in the targeted community. Simple mesocosm communities were established within a three trophic level system of primary producers (algae of various size ranges), herbivorous zooplankton, and aquatic insect predators from a source pond. At the end of four weeks, mesocosms were measured for initial numerical abundances, N_o, and initial species richness, S_o, which varied naturally per mesocosm. Different species of Daphnia (D. magna, D. pulex, D. obtusa), with known life-history data, were then added to the mesocosms, and colonization success was monitored by sampling the mesocosms every 2 weeks for an additional 6 weeks.  

Results/Conclusions

Daphnia species exhibited differential success between species and along mesocosm environmental axes. The natural variation of N_o, ranged considerably (312 +/- 158 individuals/L), but S_o was less variable (6.6 +/- 2.5 species). Abundance of Daphnia magna was correlated significantly with total mesocosm abundance (r=0.653, p=0.041) and dissolved oxygen concentration (r=0.712, p=0.032), but not species richness. While these results are preliminary, they show that mesocosm productivity may be linked more strongly to colonization success than niche specification.