Integral projection model of insect herbivore effects on Cirsium altissimum populations along productivity gradients

Abstract

Prediction of the role of native insect herbivores in the populationgrowth and spatial distribution of native plants within the environmentremains limited. We developed an integral projection model (IPM)to explore the effect of native insect herbivores on the populationdynamics and distribution of the native plant Cirsium altissimum(tall thistle), in two different productivity zones in tallgrassprairie in Nebraska USA. Model parameters were extracted from twofield experiments: a seed addition experiment that demonstrated seedlimitation by insect herbivores on adult recruitment and an insectherbivory exclusion experiment that demonstrated significant herbivoreimpacts on both rosette performance and adult fecundity. Zones differedin elevation, soil moisture and biomass. Using our model, we asked:(1) does insect herbivory reduce plant population growth rate (lambda),(2) does the effect on (lambda) differ between productivity zones,(3) does it primarily operate by limiting growth or fecundity and(4) is there evidence for density dependence in mediating impacton (lambda)? We found that insect herbivory suppressed tall thistlepopulation growth rate, but the magnitude of this effect did notvary with ecosystem productivity. Insect herbivores' effect on lambdaarose primarily through suppression of fecundity. We found no evidenceof negative density dependence countering seed limitation effectsof insect herbivory on tall thistle population growth. The similarmagnitudes of these effects in different productivity zones eliminatevariation in ecosystem productivity as a majority condition in determininginsect herbivores' impact in shaping local distribution of this nativeplant. Overall, the results show that native insect herbivores suppresspopulation growth of this native plant, mediated through seed limitationthat functionally determines adult density, without differences relatedto ecosystem productivity.