The defining characteristic of invasive populations is that they grow and spread. Population biology gives us the tools to understand how much the growth rates of invasive species vary spatially and temporally, to assess which demographic and ecological factors are most important in driving growth rates, and to compare the potential effectiveness of control strategies that target different aspects of the life cycle. I use population models to study how species interactions influence rates of spread, which involves both interesting mathematics as well as interesting biology (Parker and Reichard 1998, Parker 2000, Neubert and Parker 2004, Swope and Parker 2017).
ECOLOGY, POPULATION DYNAMICS AND CONTROL OF SCOTCH BROOM, Cytisus scoparius
Scotch broom is one of the most aggressive invasive species in the Pacific Northwest. My early work on this plant explored variation in invasion rates across habitats (Parker 2000), and the effects of soil disturbance and seed limitation (Parker 2002) and pollen limitation (Parker 1997) on invasion rates. I later compared the reproductive ecology of Scotch broom and French broom (Parker and Haubensak 2002), and of Scotch broom between Washington and California, where it is pollinated by a wider diversity of native and introduced insect species (Parker et al. 2002). More recently, Karen Haubensak and I have been collaborating with the Forestry Department at Joint Base Lewis-McChord to take a science-based approach to restoring Scotch broom-invaded clearcuts. We are investigating control strategies in a large-scale experiment replicated across five forest plantations. We study how the dynamics of the seed bank and variation in resprout rates shape variability in the need for and success of control.
IMPACTS AND LEGACY EFFECTS OF SCOTCH BROOM, Cytisus scoparius
As a large, nitrogen-fixing shrub, Scotch broom changes N availability and C and N pools when it invades glacial outwash prairies in the Pacific Northwest (Haubensak and Parker 2004). In addition, this species produces high concentrations of alkaloids, which may contribute to reduced plant growth in broom-invaded soils (Haubensak and Parker 2004). There is potential for broom invasion to leave a soil “legacy effect” that persists long beyond removal of the invader. At Joint Base Lewis-McChord, we have seen massive failure of reforestation efforts in invaded clearcuts. We are now studying how soil chemical changes accompanying broom invasion may influence the growth and establishment of Douglas fir seedlings. We believe that broom causes the disruption of ectomycorrhizal fungal (EMF) mutualisms. We are using a series of greenhouse and field studies to disentangle the direct and indirect effects of broom invasion on forest regeneration.
TESTING THE NATURAL ENEMIES HYPOTHESIS OF INVASION: NATIVE, INTRODUCED, AND INVASIVE CLOVERS
A common assertion is that invasions occur because species “leave their natural enemies behind” and are therefore released from pest pressure to become aggressive competitors in their introduced range. Greg Gilbert and I tested the Natural Enemies Hypothesis by comparing herbivory as well as pathogen diversity, infection rates, symptoms, and fitness effects on a suite of 18 native and introduced clover species, adding a novel comparison of introduced species that invade vs. those that don’t. We found little evidence for the idea that The Natural Enemies Hypothesis can explain invasiveness in this system, or indeed, that any sort of escape from pathogens has occurred (Parker and Gilbert 2007).
One possible explanation for a lack of ecological “escape from pathogens” is that rapid evolution in the pathogens quickly increases infection of and virulence on the new hosts, equalizing pest pressure (Parker and Gilbert 2004, Gilbert and Parker 2006). In order to explore the evolutionary processes behind novel host-pathogen dynamics, we created “de novo introductions”, with serial passage experiments on California pathogens with clover hosts collected from Europe and parallel experiments with native and introduced clover hosts collected from California. We found intriguing evidence for rapid evolution in infectivity over five generations in the greenhouse, and in virulence over historical time (Gilbert and Parker 2010).
EVOLUTION AND GENETICS OF INVASION
One of the paradoxes of biological invasions is that introduced species necessarily go through founding events, which can lead to extreme genetic bottlenecks. Yet invasive species can be remarkably successful, despite predicted reductions in genetic diversity and the constraints these might place on evolutionary potential. One possible resolution to this paradox is that multiple introductions and subsequent gene flow may be essential for allowing invasive species to adapt to environmental challenges.
In an invited review paper for Molecular Ecology, Katrina Dlugosch and I reviewed genetic bottlenecks in introduced species (80 plants, animals, and fungi). We found that losses of molecular variation are significant and substantial (15-20%) on average, though they are not ubiquitous (Dlugosch and Parker 2008a). As predicted, allelic richness showed greater losses than heterozygosity, and multiple introductions did cause a significant but small reduction in the degree of molecular diversity that is lost. However, in a companion review of studies of quantitative traits, we found almost no evidence of a loss of diversity in the morphological or life history traits that would be the target of most adaptive evolution.
In her dissertation work, Katrina discovered that the new invader Hypericum canariense shows substantial loss of neutral genetic variation in its new range, yet still demonstrates adaptive evolution of life history traits and phenology (Dlugosch & Parker 2008b).
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