Biological control and precipitation effects on spotted knapweed (Centaurea stoebe): empirical and modeling results

Anastasia Maines,David Knochel,Timothy Seastedt

doi:10.1890/es13-00094.1

Abstract

The literature is inconsistent regarding the ability of herbivory to control or reduce densities of a major invasive plant species of North America, spotted knapweed (Centaurea stoebe). Here, findings from experimental manipulations of spotted knapweed and long‐term monitoring of seed production and insect abundance were used to parameterize a population matrix model for this species. Model predictions were compared against spotted knapweed densities observed in permanent transects, which were established after the release of biological controls. The model incorporated population‐level compensation for adult mortality (increased growth from juveniles to adults). The model also incorporated newly reported results showing an interaction between precipitation and biological control impacts. We compared predictions from four alternate models against the observed population densities; models were: conspecific density‐dependence, the effects of biological controls, precipitation, and biological control‐precipitation interaction. The best model to explain population declines included the effects of biological control agents. Declines in population growth rates (λ < 1) were only predicted when reduced seed production and increased plant mortality due to biological controls were included. Results suggest that biological controls contributed to declines observed in field studies, and support the contention that biological control attenuates the ability of spotted knapweed to exploit favorable climatic conditions. The results also demonstrate that spotted knapweed control (i.e., conditions where λ < 1) depends upon a relatively large impact of biological control agents through high densities or large per capita impact; both of which are known to vary at our site and elsewhere. At our site spotted knapweed in certain habitats (e.g., disturbed riparian areas) is unlikely to be eradicated by insect herbivory, but spotted knapweed densities found in most other habitats can be reduced by biological control agents. Regardless, the management implications from these findings are that biological controls can intensify the efficacy of other control methods.

Highlights

Understanding the mechanisms for a small portion of nonnative, introduced plant species to become invasive, dominant species in introduced habitats has been a challenging subject for decades (e.g., Davis 2009, Moles et al 2012, Heger et al 2013)
While declines in growth rates (k, 1) were only present under a low-precipitation scenario, the population was ‘controlled’ (k 1) when biological controls were present under all precipitation levels except above-average rainfall
This study is the first to compare the measured effects of biological controls and precipitation on spotted knapweed stages against observed population dynamics in permanent transects. These results support the contention that biological controls have contributed to the observed decline in spotted knapweed populations in Colorado, and support similar findings from Montana and British Columbia

Summary

Introduction

Understanding the mechanisms for a small portion of nonnative, introduced plant species to become invasive, dominant species in introduced habitats has been a challenging subject for decades (e.g., Davis 2009, Moles et al 2012, Heger et al 2013). A common approach is to add host-specific natural enemies from the native habitats and attempt to reduce the fecundity and survivorship of the target species to levels where ecological and economic concerns regarding the presence of the species are diminished. Recent summaries of these attempts note that these approaches range from negligible to remarkably successful (Clewley et al 2012, Stephens et al 2013, Suckling 2013). Demonstration of density reductions and control may require an extended period of time

Methods

Results

Conclusion