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Effects of trophic complexity on invasion success of the exotic cladoceran Daphnia lumholtzi.
Dzialowski, Andrew *,1, Smith, Val 2, 1 University of Kansas, Lawrence, Kansas2 University of Kansas, Lawrence, Kansas
ABSTRACT- We manipulated trophic structure in a series of mesocosms in order to determine whether differences in food web composition could provide biotic resistance to invasion attempts by the exotic cladoceran Daphnia lumholtzi. We created a gradient of trophic complexity consisting of 1-3 trophic levels: algae-only; algae + zooplankton; and algae + zooplankton + invertebrate predators (Notonecta). This gradient was also crossed with two levels of nutrient supply levels to determine whether invasion success was further affected by productivity. We hypothesized that invasion success would be highest in the algae-only treatment due to high food resource availability, and lowest in the algae + zooplankton treatment due to strong food resource competition between the herbivores. Furthermore, we hypothesized that the addition of an invertebrate predator would influence invasion success due to D. lumholtzi's anti-predator adaptations. Daphnia lumholtzi possess extravagant predator-deterrent head and tail spines that are longer than those found on native competitors. Predators therefore could selectively feed upon native zooplankton, potentially leading to lower zooplankton biomass, greater food resource availability, and reduced biotic resistance to invasion. Our initial results show that: (1) Invasion success was high in algae-only mesocosms, and D. lumholtzi's total biomass was positively correlated with water column nutrient availability; (2) D. lumholtzi was unable to invade when other zooplankton competitors were present, regardless of the nutrient supply level; and (3) the presence of an invertebrate predator did not influence invasion success. Our results suggest that the native zooplankton communities of U.S. lakes and reservoirs may exhibit natural biotic resistance to invasion attempts by D. lumholtzi, and that further research is needed to identify the mechanism(s) that are responsible for the successful invasion and range expansion of D. lumholtzi.
Key words: Invasions, Daphnia lumholtzi, Resistance