Variable effects of competition on two species of duckweed grown in microcosms as part of an introductory biology course

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Brian D. Tomczyk, Todd D. Levine and Susan Roskopf, Department of Life Sciences, Carroll University, Waukesha, WI

Background/Question/Methods
Organisms must obtain and allocate their resources efficiently to survive. This process is further complicated by interactions between species within the community. Stress caused by these interactions necessitates an organism to adapt to its environment. In plants, responses to competition can manifest through altered physical growth in leaves and roots and by changes in numbers of individuals within a population. We explored how interspecies interactions affected the growth of two species of duckweed (e.g., Lemna minor and Lemna trisulca). We used data collected over three years in an introductory biology course to analyze potential differences between the two species when grown in isolation and when grown together. This experiment was carried out in microcosms observed over a two-week period using a standardized format and a community/citizen science approach. After identifying key variables that could be affected by competition for resources and reliably collected by students (e.g., frond count, frond area, and root length), we compiled and analyzed data from across years. Analysis was conducted in R. Through our preliminary analysis of these metrics, we identify distinct effects of competition.
Results/Conclusions
In preliminary analyses, we observed a significantly higher (p<0.05) growth rate of frond area in L. minor populations than L. trisulca, when grown in isolation. Neither species, L. trisulca nor L. minor, changed its growth rate, as measured by total frond area, when grown together. This implies that carrying capacity has not yet been reached in our microcosms or that proximity to the competing species is not a large stressor on these two duckweeds. We did not observe a significant difference in the lengths of the longest root produced by L. trisulca when grown by itself and with L. minor. However, L. minor produced significantly longer maximum root lengths when grown with L. trisulca than alone. The findings from this analysis imply that either L. trisulca is failing to respond to interspecies population stress or L. minor is experiencing greater stress than L. trisulca and is responding to that pressure by growing longer roots. This preliminary analysis highlights the importance of understanding flexible responses of organisms to stress induced by competition. These results also demonstrate that the phenotypic plasticity of duckweed might allow them to avoid density dependent limits to population growth.