Integral projection model of insect herbivore effects onCirsium altissimumpopulations along productivity gradients

Abstract

Abstract. 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 (k), (2) does the effect on (k) 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 (k)? 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.

Native insect herbivory limits population growth rate of a non-native thistle

“…These findings are consistent with experimental studies showing that insect herbivory often severely reduce the fecundity of thistles (Louda and Potvin 1995;Rose et al 2005Rose et al , 2011Suwa et al 2010). Similar to the native C. altissimum, the late flowering time of C. vulgare combined with the occurrence of late-season herbivores, such as Homoeosoma eremophasma (Pyralidae) (Takahashi 2006), likely prevent successful compensatory seed production (Rose et al 2011).…”

“…However, the results here demonstrate that the magnitude of the effect was more severe for the nonnative C. vulgare. We found that the Δλ (λ herbivores absent − λ herbivores present) was 1.42 for C. vulgare (above), compared to 0.80 for C. altissimum (Rose et al 2011). Further, the effects of native insect herbivory on C. vulgare population growth quantified here were more severe than those observed on average in other native plant populations (average Δλ = 0.04) (Maron and Crone 2006;Miller et al 2009) and in populations of another related native thistle species, Cirsium canescens (Δλ = 0.33) (Rose et al 2005).…”

“…These insects also reduce population growth of the native C. altissimum (Rose et al 2011). However, the results here demonstrate that the magnitude of the effect was more severe for the nonnative C. vulgare.…”

“…3). Native herbivores also limit native plant density and distribution (Louda 1982(Louda , 1983Louda and Potvin 1995;Louda and Rodman 1996;Miller et al 2009;Rose et al 2011). For example, Louda and Rodman (1996) experimentally demonstrated that native insects, not physiological constraints, virtually excluded the native crucifer Cardamine cordifolia from dry, open-sun environments in their native range.…”

Ecosystem carbon exchange in response to locust outbreaks in a temperate steppe

“…Numerous previous studies have demonstrated that, at low density, grasshopper herbivory usually has neutral (Gibson et al 1990;Whiles and Charlton 2006;Rose et al 2011) or limited effects on grassland plant growth (biomass loss <10 %) in North America and China (Hewitt et al 1976;Fry et al 1978;Hewitt and Onsager 1982;Landsberg and Ohmart 1989;Tilman 1992, 1993;Redak and Capinera 1994;Carson and Root 1999;Coupe and Cahill 2003;Kula et al 2005;Fielding 2005, 2008;Orians et al 2011;Zhang et al 2011;Suwa and Louda 2012). However, locusts at a similar density could cause more severe damage (30 % (Loranger et al 2014), or at high elevation (Blumer and Diemer 1996).…”