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Summary 1. The exotic cladoceran Daphnia lumholtzi has recently invaded freshwater systems throughout the United States. Daphnia lumholtzi possesses extravagant head spines that are longer than those found on any other North American Daphnia. These spines are effective at reducing predation from many of the predators that are native to newly invaded habitats; however, they are plastic both in nature and in laboratory cultures. The purpose of this experiment was to better understand what environmental cues induce and maintain these effective predator‐deterrent spines. We conducted life‐table experiments on individual D. lumholtzi grown in water conditioned with an invertebrate insect predator, Chaoborus punctipennis, and water conditioned with a vertebrate fish predator, Lepomis macrochirus. 2. Daphnia lumholtzi exhibited morphological plasticity in response to kairomones released by both predators. However, direct exposure to predator kairomones during postembryonic development did not induce long spines in D. lumholtzi. In contrast, neonates produced from individuals exposed to Lepomis kairomones had significantly longer head and tail spines than neonates produced from control and Chaoborus individuals. These results suggest that there may be a maternal, or pre‐embryonic, effect of kairomone exposure on spine development in D. lumholtzi. 3. Independent of these morphological shifts, D. lumholtzi also exhibited plasticity in life history characteristics in response to predator kairomones. For example, D. lumholtzi exhibited delayed reproduction in response to Chaoborus kairomones, and significantly more individuals produced resting eggs, or ephippia, in the presence of Lepomis kairomones.

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Recently the exotic cladoceran Daphnia lumholtzi has invaded freshwater systems throughout the southern and midwestern United States. We conducted regional surveys of eastern Kansas reservoirs to document the range expansion of D.lumholtzi. Daphnia lumholtzi was found in five of 35 reservoirs sampled in 1994, and 11 of the 35 reservoirs when re-sampled in 1997. In addition, we sampled 40 small ponds inaccessible to recreational boats, within the watershed of an invaded reservoir. We did not find D.lumholtzi in any of these ponds, suggesting that non-human dispersal mechanisms play an insignificant role in the range expansion of D.lumholtzi throughout the United States. Further experimentation, however, is needed to determine if the absence of D.lumholtzi from these small ponds is due to insufficient dispersal mechanisms or the inability of this invader to successfully colonize following arrival. Daphnia lumholtzi has broad limnological tolerances. However, invaded reservoirs tend to be larger in area, have higher Secchi disk depths, and lower total phosphorus, total nitrogen and chlorophyll a levels relative to non-invaded reservoirs. Analyses of preinvasion zooplankton communities indicate that D.lumholtzi may be invading reservoirs in which native Daphnia species are rare. While the long-term effects of the invasion of D.lumholtzi are unknown, it has the potential to dominate late summer zooplankton communities in eastern Kansas reservoirs. Therefore, we need to continue to survey and monitor invaded reservoirs to document the range expansion of D.lumholtzi and determine the long-term implications of the introduction of this invader.

2003. Invasibility of plankton food webs along a trophic state gradient. -Oikos 103: 191-203.Biological invasions are becoming more common, yet the majority of introduced exotic species fail to establish viable populations in new environments. Current ecological research suggests that invasion success may be determined by properties of the native ecosystem, such as the supply rate of limiting nutrients (i.e. trophic state). We examined how trophic state influences invasion success by introducing an exotic zooplankter, Daphnia lumholtzi into native plankton communities in a series of experimental mesocosms exposed to a strong nutrient gradient. We predicted that the attributes of nutrient-enriched communities would increase the likelihood of a successful invasion attempt by D. lumholtzi. Contrary to our original predictions, we found that D. lumholtzi was often absent from mesocosms that developed under high nutrient supply rates. Instead, the presence of D. lumholtzi was associated with systems that had low nutrients, low zooplankton biomass, and high zooplankton species diversity. Using generalized estimating equations (GEE) and multivariate species data, we found that the presence-absence of D. lumholtzi could be explained by variations in zooplankton community structure, which was itself strongly influenced by nutrient supply rate. We argue that the apparent invasion success of D. lumholtzi was inhibited by the dominance of another cladoceran species, Chydorus sphaericus. These results suggest that the interaction between trophic state and species identity influenced the invasion success of introduced D. lumholtzi.