Friday, August 6, 2010

PS 103-111: Has a weedy lifestyle evolved? Comparing variation of life history traits between invasive populations compared to their native counterparts

Matthew A. Kaproth and Jane Molofsky. University of Vermont


Studies of species within a genus indicate that certain life history traits are prevalent within invasive species.  Traits associated with invasiveness include small seed mass, short juvenile period, high relative growth rate, and short periods between masting events (Grotkopp et al. 2002, Richardson and Rejmánek 2004).  Using a relatively new invasive (spreading in Japan since 1990), our objectives were to determine how life history traits vary within species between ranges and whether a weedy lifestyle has evolved.  We collected populations of a self-fertilizing, short-lived winter annual, hairy bittercress (Cardamine hirsuta) from two countries in its native European range (France and Malta) and two countries within its introduced range (Japan and the United States).  Populations were grown under uniform greenhouse conditions to increase seed totals and measured for differences in life history traits such as time between each stage (e.g. seed to seedling, seedling to flowering, flowering to seed) and fecundity.  Collected seeds were sown in two experiments testing total lifetime fitness and life history traits under varying propagule pressure (field experiment) and intraspecific competition (greenhouse experiment).


From the primary seed production, we found there is variation in fecundity between ranges (F=3.2394, p=0.0409) and populations (nested within ranges) (F=10.8164, p<0.0001).  Plants from Japan and the U.S. produced more seeds than European plants (France and Malta).  In the U.S. range, Vermont populations were similar in production to Japanese ones, while Massachusetts populations were similar to native French ones.  We found variation in phenology between ranges, where time between stages was shortest in the Connecticut and Vermont populations (total life-cycle <3 months), but was almost 3 times longer in European, Massachusetts, and Japanese populations (total life-cycle 5-7 months).  In addition, germination rates were much lower in European plants than in the invasive Japanese and North American plants (F=25.7159, p<0.0001).  These results indicate that even within a range, populations exhibit variability in life history traits (future studies will determine how introduction history influences the evolution of these life history traits).