When Who Gets There First Matters—Priority Effects in the Prairie

The tallgrass prairie ecosystem is one of the most threatened in North America. Once covering 70-80% of Iowa, it now covers less than 1/10 of a percent. The remaining fragments are scattered throughout Iowa, often highly degraded and on marginal land. European agriculture and its affiliated machinery have caused much of this transformation. However, individuals, local and state organizations are pushing to restore the prairie through new plantings. The success of these efforts depends on overcoming several obstacles, namely invasion from Eurasian agricultural and ornamental species.

How can invasive plants hurt the establishment of natives? Priority effects holds the answer to this question. A priority effect is the impact of an early arriving species on later community development by constraining which later arriving species can coexist. For example, if species A had a stronger priority effect than species B, species A would more greatly influence what other species could or could not colonize that same space. Research summarized by Wilsey et al 2014 has pointed to a stronger priority effects in exotic, Eurasian species compared to native, tallgrass prairie species. This predicts that Eurasian species will influence later community development than tallgrass prairie species.

A 2014 paper published in New Phytologist by Wilsey et al addressed why priority effects may be stronger in exotic rather than native species. They predicted that the difference in priority effects was due to differences in regenerative traits, or traits related to early development.

Wilsey et al hypothesized that priority effect differences were due to either (1) native and exotic species differing in these regenerative traits or (2) native species differing from cultivated exotic genotypes, but not from wild exotic genotypes (Figure 1). To test these hypotheses, the researchers created 14 matched sets of species, each set containing a native, a wild exotic and a cultivated exotic species (total 42 species). These species shared the same taxonomic family if not tribe and had similar ecological functions (life history, photosynthesis mechanism), spanning the 4 main functional groups of the tallgrass prairie. Additionally, all the exotic species are listed on either national or local invasive species list.  

Figure 1: Illustrated hypothesis of Wilsey et al 2014 (created by Wilsey et al 2014). The x-axis is a combination of regeneration measures with the curves showing the distributions of the native North American individuals with the exotic Eurasian individuals. They had found no evidence of (a). Therefore, that hypothesis was not investigated in their paper.

 

Their design was an additive competition greenhouse experiment. In separate pots, each of the ‘early arriving’ species (the matched 42) were seeded. These seedlings were allowed to grow for 21 days and then a mix of 39 different native species was added as the ‘late arrivals’. 11 pots were started as controls, seeded only with the late arrivals so the researchers could detect how much the early arriving species influenced the growth of the late arriving species. They harvested the plants at the end of the growing season (May 10-September 13) and analyzed diversity, species richness and late arrival biomass (Figure 2).

 

 Figure 2: Illustrated additive comparative design. Each row corresponds to a different early arrival. Each column represents a different time point. Notable findings captioned below.

 

Since the researchers were interested in regenerative traits, they measured timing of emergence, emergence rate, seedling biomass, canopy light capture, and seedling height of the early arrivals. At harvest, they measured the total aboveground biomass of late arrivals, species diversity, and species richness. These measurements described how the community assembled given which species arrived early and were used to estimate the priority effects of each early arrival group (native, wild exotic, cultivated exotic).

The regenerative traits differed significantly between exotic genotypes and native species, but not between wild and cultivated exotic species. Exotic seedlings were larger, emerged earlier and captured more sunlight. Cultivated exotic genotypes had larger biomass than wild exotic than native, but this difference was found to be a small factor in the overall priority effects. Furthermore, these different measures were highly correlated, meaning that if a species was likely to emerge early, they often captured more sunlight and vice versa.

The researchers found much lower diversity at the end of the season in the wild and cultivated exotic pots than in the native pots, often creating monocultures. Because diversity and richness were higher in native treatments compared to exotic treatments, natives were classified as having smaller priority effects. However the most interesting finding was the strong correlation between regeneration traits and community measures. Using principle component analysis, regeneration traits were strong predictors of later establishment diversity and richness. In fact, these traits fully accounted for the variability in diversity, but not total late arriving biomass. In sum, the researchers found evidence supporting hypothesis 1 but none to support hypothesis 2.

This has serious implications since previous research has shown that exotics reduce native abundance not through competition with adult plants but in reducing the establishment of native seedlings. In terms of management practices, these findings suggest focus on establishing natives early before exotics can be fully established. This will prove challenging since approximately 25% of all the species within the Iowa flora are classified as non-native. Additionally other studies mentioned by Wilsey et al (2014) have shown that the impact of priority effects can last for several years, meaning longer time scales for restoration efforts.

But all is not lost. Work by Carter and Blair 2012 has demonstrated that if a representative mix of tallgrass prairie species are used to seed restorations, given time, they can approach similar diversity as remnant prairies. Furthermore, these results are of a greenhouse experiment, meaning that resources are plentiful and weather benign, which rarely happens in the field. The next step for this research is to take the experiment out into the field to see if differences in weather cues and predator activity may alter these outcomes.

Carter, Daniel L. Blair, John M. 2012. Recovery of Native Plant Community Characteristics on a Chronosequence of Restored Prairies Seeded into Pastures in West-Central Iowa. Restoration Ecology 1-10

Wilsey, Brian J. Barber, Kaitlin. Martin, Leanne M. 2014. Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes. New Phytologist 1-10