Although a great deal of insight into population biology has been gained from close examination of rare or endangered species, invasive exotics also provide instructive models for exploring issues of demographic variability and environmental influences, in addition to helping to develop demographically-appropriate control strategies. Alliaria petiolata, or Garlic mustard, is a biennial exotic invader that is characterized by populations that differ widely in their density, demographic structure and population growth rates. Demographic data have been collected on 35 independent A. petiolata populations in central New Jersey for eight years. Analysis of soil moisture, nutrient content and light availability, in conjunction with population growth rate projections, Life Table Response Analyses and evaluations of demographic parameters reveal the relative importance of a variety of processes in A. petiolata invasions.
As might be expected, all populations combined grew significantly, with an overall lambda of 1.42. However, individual populations revealed highly variable behavior; some disappeared entirely while others disappeared and reappeared one to two years later, presumably rescued by seed bank emergence or long-distance dispersal. Other populations shrank or grew significantly (per capita population growth rates ranging from 0 to 5.2). Some population patterns emerged. Because A. petiolata is a biennial, some populations clearly alternate between being dominated by seedlings and adults in successive years. However, in some populations, both generations are found together, while in others, seedlings are abundant every year, but none of them survive to set seed in year two. LTRE analyses revealed that populations projected to increase in size were characterized by higher-than-average seedling survival from midsummer to fall, the population parameter with the greatest coefficient of variance. Meanwhile, populations projected to decline did so due to lower-than-average overwinter survival, the second most variable parameter, although this differed among years. Both soil moisture and nitrogen availability over time were significant drivers of transition probabilities and population growth patterns, with early-season soil moisture and mid-season nitrogen availability having the strongest associations with per capita growth rate. Thus this study revealed a positive relationship between parameter variability and influence over population growth. It also shows that successful strategies for biological and physical or chemical control of invaders are likely to be dependent on both population structure and environmental quality.