Date of this Version
Rose, K. E., F. L. Russell, and S. M. Louda. 2011. Integral projection model of insect herbivore effects on Cirsium altissimum populations along productivity gradients. Ecosphere 2(8):art97.
Prediction of the role of native insect herbivores in the population growth and spatial distribution of native plants within the environment remains limited. We developed an integral projection model (IPM) to explore the effect of native insect herbivores on the population dynamics and distribution of the native plant Cirsium altissimum (tall thistle), in two different productivity zones in tallgrass prairie in Nebraska USA. Model parameters were extracted from two field experiments: a seed addition experiment that demonstrated seed limitation by insect herbivores on adult recruitment and an insect herbivory exclusion experiment that demonstrated significant herbivore impacts on both rosette performance and adult fecundity. Zones differed in elevation, soil moisture and biomass. Using our model, we asked: (1) does insect herbivory reduce plant population growth rate (λ), (2) does the effect on (λ) differ between productivity zones, (3) does it primarily operate by limiting growth or fecundity and (4) is there evidence for density dependence in mediating impact on (λ)? We found that insect herbivory suppressed tall thistle population growth rate, but the magnitude of this effect did not vary with ecosystem productivity. Insect herbivores’ effect on k arose primarily through suppression of fecundity. We found no evidence of negative density dependence countering seed limitation effects of insect herbivory on tall thistle population growth. The similar magnitudes of these effects in different productivity zones eliminate variation in ecosystem productivity as a majority condition in determining insect herbivores’ impact in shaping local distribution of this native plant. Overall, the results show that native insect herbivores suppress population growth of this native plant, mediated through seed limitation that functionally determines adult density, without differences related to ecosystem productivity.