Effect of zebra mussels (Dreissena polymorpha) on trophic state in northern Michigan lakes

Undergraduate Research Exper.Invasive species cause massive damage to the environment, human health, and the economy. Zebra mussels (Dreissena polymorpha) first invaded North America in 1988 and have had dramatic effects on many lake ecosystems. They are prolific filter feeders, effectively decreasing plankton densities and increasing water clarity, thereby altering the trophic state of lakes. I used Carlson’s Trophic State Index to quantify changes in lake trophic state associated with zebra mussels. I examined the effect of zebra mussels on trophic state by comparing lake TSI values pre- and post-zebra-mussel invasion and the influence lake morphometry has on the zebra mussels’ effect on trophic state. Secchi depth, chlorophyll-a, and total phosphorous data were collected at six lakes in northern lower Michigan. Sites were paired in each lake according to depth (15 m). No significant differences were found between shallow versus deep sites among any of the variables. Comparisons of trophic state variables showed no significant changes from data collected in 1973-1974. Zebra mussel density differed among lakes (0 – 3,600 m-2) but showed no correlation with trophic state variables. TSI for Secchi was significantly greater than TSI for TP or chlorophyll. These results suggest that zebra mussel influences on trophic state variables were minor at this early stage in the invasion of these inland lakes.http://deepblue.lib.umich.edu/bitstream/2027.42/57587/1/Stoermer_Danielle_2007.pd

A Test of Baker’s Law: Breeding Systems and the Radiation of Tolpis (Asteraceae) in the Canary Islands

Baker’s law posits that self‐compatible (SC) plants will be more successful than self‐incompatible (SI) plants in long‐distance colonization because a single propagule can establish a viable population. Oceanic islands represent ideal systems to test Baker’s law because insular lineages have, without question, originated from long‐distance dispersal. The dilemma of Baker’s law is that one propagule of an SC plant would establish a population with low genetic diversity, which could limit subsequent evolution. By contrast, a single propagule from an SI ancestor, having originated from an outcrossing source population, would provide more diversity but could not undergo sexual reproduction. We examined this issue by studying the breeding system of members of the flowering plant genus Tolpis (Asteraceae), a small (nine to 13 species), monophyletic lineage in the Canary Islands archipelago. A combination of floral morphology, pollen‐ovule ratio, autogamous seed set, and genetic data indicates that only one endemic species (T. coronopifolia) is effectively SC. The remainder of the endemics are pseudo‐self‐compatible, i.e., are largely SI but capable of low levels of seed set from self‐fertilization. Pseudo‐self‐compatibility remedies the dilemma of Baker’s law: a single propagule can establish a sexual population and yet have sufficient variation to facilitate diversification