Lineage and latitudinal variation in Phragmites australis tolerance to herbivory: implications for invasion success

Abstract

Herbivores play a critical role in plant invasions either by facilitating or inhibiting species establishment and spread. However, relatively few studies with invasive plant species have focused on the role of plant tolerance and how it varies geographically to influence invasion success. We conducted a common garden study using two lineages (native and invasive) of the grass Phragmites australis that are prevalent in North American wetlands. Using 31 populations collected across a broad geographic range, we tested five predictions: 1) the invasive lineage is more tolerant to simulated folivory than the native lineage, 2) tolerance to herbivory decreases with increasing latitude of origin of the populations, 3) estimates of tolerance are correlated with putative tolerance traits and plasticity in those traits, 4) a tradeoff exists between tolerance and resistance to herbivory and 5) tolerance has a fitness cost. Response to folivory varied substantially among populations of P. australis, ranging from intolerance to overcompensation. Our model selection procedure deemed lineage to be an important predictor of tolerance but, contrary to our prediction, the native lineage was 19% more tolerant to folivory than the invasive lineage. Tolerance for both lineages exhibited a u‐shaped relationship with latitude. A tolerance–resistance tradeoff was evident within the invasive but not the native lineage. Also, tolerance was positively correlated with belowground biomass allocation, leaf silica concentrations, specific leaf area and plasticity in stem density, and negatively correlated with the relative growth rate of the population and plasticity in putative resistance traits. Lastly, although we did not detect costs of tolerance, our results highlight that fast growth rates can maintain high fitness in the presence of herbivory. Herbivory and plant defense strategies for P. australis lineages in North America exhibit complex biogeographic patterns that cause substantial heterogeneity in enemy release and biotic resistance and, consequently, invasion success.

Plant invasion alters latitudinal pattern of plant‐defense syndromes

“…The net effect is that A. philoxeroides in China still experiences enemy release at high and low latitudes, but not at intermediate latitudes. This finding supports previous work showing latitudinal heterogeneity in the strength of enemy release (Cronin et al 2015, Allen et al 2020. As an evolutionary extension of the enemy release hypothesis, the evolution of increased competitive hypothesis (EICA), which predicts that introduced plants should evolve increased growth and reproduction, and reduced defense (Blossey and Notzold 1995), has stimulated studies on the role of rapid adaptive evolution in the establishment and spread of invasive species (Huey et al 2000).…”

“…Therefore, our findings demonstrate that the difference between native and introduced populations is dependent on environmental factors that vary with latitude. This emphasizes that such latitudinal dependence of population differentiation should be explicitly considered in the experimental design and statistical analysis of studies of the evolution of defense in invasive plants (Bhattarai et al 2017, Allen et al 2020, Croy et al 2020.…”

Opposite effects of nutrient enrichment and herbivory by an alien snail on growth of an invasive macrophyte and native macrophytes

“…For instance, herbivory by two folivores ( Lema daturaphila and Epitrix parvula ) selected for plant Datura stramonium genotypes with intermediate resistance and high tolerance (Carmona and Fornoni 2013). Separately, the plant Senecio jacobaea exhibited both greater resistance to and tolerance of herbivory in the introduced range than in the native range (Stastny et al 2005), while native populations of the invasive grass Phragmites australis did not display tolerance-resistance tradeoff (Croy et al 2020). A positive association between tolerance and induced chemical defence was demonstrated in plant Arabidopsis thaliana (Mesa et al 2017).…”

Response of Five Miscanthus sinensis Cultivars to Grasshopper Herbivory: Implications for Monitoring of Invasive Grasses in Protected Areas

“…Regarding plant responses, previous studies proposed that plant resistance and tolerance to insect herbivory could be expressed in a combination [ 22 ], and it is possible that plant regrowth after damage (any damage in addition to herbivory) represents “a generalized plant response” [ 22 , 23 ]. Croy et al [ 24 ] in their experiments similarly demonstrated that highly invasive Phragmites australis in North America can perform fast growth rates while sustaining herbivory. According to the Evolution of Increased Competitive Ability hypothesis an increased plant growth after damage can be explained by resource allocation from defense to growth [ 25 ].…”