Since the end of the 19th century, overall per-capita mobility of humans has increased significantly, leading to increased rates in human-mediated transportation of animal and plant species. The rapid spread of alien organisms, however, may lead to quick and unpredictable changes in ecosystems.
Senecio inaequidens DC. (Asteraceae) is an invasive alien plant from South Africa that was first introduced to Europe 100 years ago and is characterized by an exceptionally fast rate of spread; it contains pyrrolizidine alkaloids that are toxic to invertebrates, livestock and humans.
In the study presented here, laboratory, greenhouse and field experiments on the biology of Senecio inaequidens were conducted, in order to find out if and how herbivory and plant competition influence growth, survival and reproduction of this plant. Specifically, the presence of vertebrate herbivores, molluscs, insects and plant competitors was experimentally manipulated using full factorial and split-plot designs. All experiments were performed at Imperial College, Silwood Park, about 30 km west of London (UK).
Growth and fecundity of Senecio inaequidens were significantly affected by interspecific competition. Vertebrate herbivory only had a significant effect when a closed vegetation cover was present. On artificially disturbed plots, Senecio inaequidens showed high capability to overcompensate for herbivory. Mollusc herbivory significantly reduced fecundity of S. inaequidens. Different ecotypes of Senecio inaequidens showed different amounts of herbivore damage.
One of the most remarkable results of this study was that Longitarsus jacobaeae, a native specialist leaf beetle, freely colonized Senecio inaequidens, indicating that native herbivores might be suitable biocontrol agents of invasive alien plants in the future. Host switching from indigenous to invasive alien plant species in oligophagous insect herbivores may be more common than generally thought.
The results of this study indicate that Senecio inaequidens is likely to invade and persist in heavily grazed or disturbed grassland ecosystems in the near future; because of the toxic compounds this plant contains, it would be advisable to design a preventive management program and to inform the public about the consequences that might be associated with the invasion of this plant.
Table of Contents
1 INTRODUCTION
1.1 Preliminary remarks
1.2 Why study biological invasions?
1.2.1 The borderless world
1.2.2 Biological invasions: concepts and definitions
1.2.3 Invasive plants
1.2.3.1 General overview
1.2.3.2 Important invasive alien plants in Europe
1.3 Herbivory and plant competition
1.3.1 Herbivory
1.3.2 Plant competition
1.4 The special case of Senecio inaequidens DC.
1.4.1 The main reasons for focussing on this species
1.4.2 Phylogenetic position
1.4.3 Country of origin and natural habitats
1.4.4 Invasion history
1.4.4.1 World (excluding Eurasia)
1.4.4.2 Eurasia
1.4.4.3 Germany
1.4.4.4 United Kingdom
1.4.5 Biological characteristics
1.4.5.1 Morphology and life form
1.4.5.2 Reproductive biology
1.4.5.3 Herbivore spectrum and pathogens
1.4.5.4 Physiology, biochemistry and secondary metabolites
1.4.5.5 Plant sociology
1.4.6 Possible ecological impact
1.4.7 Possible economical impact
1.5 Current research and the objectives of this study
1.5.1 Overview
1.5.2 Hypotheses
2 MATERIAL AND METHODS
2.1 Living organisms and how they were obtained
2.1.1 Plants
2.1.1.1 Senecio inaequidens DC. (Asteraceae)
2.1.1.2 Festuca rubra L. ssp.rubra (Poaceae)
2.1.2 Herbivores
2.1.2.1 Tyria jacobaeae L. (Lepidoptera: Arctiidae)
2.1.2.1.1 Biology of the species
2.1.2.1.2 Experiments
2.1.2.2 Longitarsus jacobaeae Waterhouse (Coleoptera: Chrysomelidae)
2.1.2.2.1 Biology
2.1.2.2.2 Experiments
2.1.2.3 Oryctolagus cuniculus L. (Mammalia: Lagomorpha) and molluscs
2.2 Experimental design
2.2.1 Overview
2.2.2 Germination tests
2.2.3 Feeding trials
2.2.3.1 Feeding trials with Longitarsus jacobaeae
2.2.3.1.1 No-choice feeding preference experiment
2.2.3.1.2 Multiple-choice feeding preference experiments
2.2.3.2 Feeding trials with Tyria jacobaeae
2.2.3.2.1 No-choice feeding preference experiments
2.2.3.2.2 Multiple choice feeding preference experiments
2.2.4 Greenhouse experiments
2.2.5 Field experiments
2.2.5.1 Overview
2.2.5.2 Description of the field plots
2.2.5.3 Details on the split-plot design
2.2.5.4 Notes on the response variables used
2.2.6 Additional observations
2.2.6.1 Studies on natural populations
2.2.6.2 Pollination experiments
2.2.6.3 Vegetative reproduction
2.2.6.4 Test for vesicular-arbuscular mycorrhiza
2.3 Statistical Analysis
3 RESULTS
3.1 Germination tests
3.2 Feeding trials
3.2.1 Longitarsus jacobaeae
3.2.1.1 No-choice feeding preference experiment
3.2.1.2 Multiple choice feeding preference experiments
3.2.1.2.1 Statistical analysis
3.2.1.2.2 Results
3.2.2 Tyria jacobaeae
3.2.2.1 Oviposition
3.2.2.2 Development and survival of larvae in no-choice tests
3.2.2.3 Damage to Senecio inaequidens in no-choice tests
3.2.2.4 Multiple choice feeding preference experiments
3.3 Greenhouse experiments
3.3.1 Plant competition
3.3.1.1 Statistical Analysis
3.3.1.2 Results
3.3.2 Herbivory
3.4 Field experiments
3.4.1 Statistical analysis
3.4.1.1 General description
3.4.1.2 Split-plot ANOVA involving rabbit grazing, plant competition, and ecotype effects
3.4.2 Frequency distributions of plant height in Senecio inaequidens
3.4.3 Differences between the two experimental plots
3.4.4 Differences between ecotypes of Senecio inaequidens
3.4.4.1 Ecotype differences in morphological parameters
3.4.4.2 Ecotype differences in invertebrate herbivore damage
3.4.4.3 Treatment effects
3.4.5 Direct effects of plant competition
3.4.6 Direct effects of herbivory
3.4.6.1 Vertebrate herbivores
3.4.6.2 Effects of invertebrate herbivores
3.4.7 Interaction effects
3.4.7.1 Interactions involving invertebrate herbivores
3.4.7.2 Interactions involving treatment and ecotype
3.4.7.3 Interactions between rabbit grazing and interspecific plant competition
3.4.8 Regrowth after clipping
3.4.9 Colonization by Longitarsus jacobaeae
3.4.10 Colonization by other insect herbivores
3.5 Additional observations
3.5.1 Studies on natural populations
3.5.2 Pollination experiments
3.5.3 Vegetative reproduction
3.5.4 Test for vesicular-arbuscular mycorrhiza
4 DISCUSSION
4.1 Introductory remarks
4.2 Feeding trials
4.2.1 Field and laboratory trials with Longitarsus jacobaeae
4.2.2 Feeding trials with Tyria jacobaeae
4.3 Greenhouse experiments
4.4 Field experiments
4.4.1 Differences in morphological parameters, and between experimental plots
4.4.2 Differences between ecotypes of Senecio inaequidens
4.4.3 Herbivory and competition effects
4.5 Host-switching in oligophagous insect herbivores
4.6 Susceptibility of grassland ecosystems towards invasion by Senecio inaequidens
5 FINAL CONCLUSIONS AND NEED FOR FURTHER RESEARCH
6 REFERENCES
Research Objectives and Themes
This thesis investigates the impact of herbivory and plant competition on the invasive alien plant Senecio inaequidens, exploring its potential to colonize grassland ecosystems through a series of field, laboratory, and greenhouse experiments.
- The impact of native European herbivores on Senecio inaequidens performance.
- The influence of interspecific plant competition on the growth, survival, and fecundity of the invader.
- The potential for host-switching by native specialist insect herbivores.
- Ecotype differences in morphological parameters and herbivore damage response.
- Predicting future invasion success and potential management strategies.
Excerpt from the Book
1.2.1 The borderless world
There can be no doubt that the human population has grown to a dangerous size (e.g. UN 2001, Bernstein 2001) and there is still no evidence for a decline in the near future (but see Lutz et al. 2001 and Smil 1999 for long-term predictions). And with this growth in population size, there is more need for resources, be it in terms of spatial, biotic or abiotic resources. Consequently, as man is altering the resource distribution and composition on earth, there is growing evidence for large environmental changes to take place, namely changes in climate, biotic productivity, water resources, atmospheric chemistry, and ecological systems - commonly referred to as ‚global change‘ (see Grime 1997 and Grime 1997; Vitousek, Mooney et al. 1997 for a general overview).
Since the end of the 19th century, major inventions like the invention of the automobile or the aircraft have led to an exponential increase in per-capita mobility of humans. Schafer & Victor 1999 estimate the worldwide traffic volume to have risen from 6 · 1012 person kilometres in 1960, to 28 · 1012 pkm in 1990. Generally speaking, it can be stated that, for all regions of the world, there is a linear correlation between per-capita gross domestic product and per-capita traffic volume. This means that the more industrialized a nation becomes, the more demand for high-speed transportations there will be.
This ever increasing far-distance transport of people and their goods, however, has lead to a breakdown of biogeographic barriers – different types of organisms are brought into regions where they would normally never occur naturally.
Summary of Chapters
1 INTRODUCTION: Outlines the global context of biological invasions and defines the research scope regarding Senecio inaequidens.
2 MATERIAL AND METHODS: Describes the experimental setups, including plant acquisition, herbivore species, and the specific greenhouse and field experimental designs.
3 RESULTS: Presents data from germination, feeding trials, greenhouse competition, and field experiments, including statistical analyses of plant growth and herbivore damage.
4 DISCUSSION: Interprets the experimental results in the context of host-switching, ecological impact, and the susceptibility of grassland ecosystems to the invasion.
5 FINAL CONCLUSIONS AND NEED FOR FURTHER RESEARCH: Summarizes the key findings and suggests future directions for long-term observation and management.
6 REFERENCES: Lists the academic literature and sources cited throughout the study.
Keywords
Senecio inaequidens, invasive species, herbivory, plant competition, host-switching, Longitarsus jacobaeae, Tyria jacobaeae, Festuca rubra, biological invasion, grassland ecosystems, pyrrolizidine alkaloids, plant performance, ecotypes, ecological impact.
Frequently Asked Questions
What is the fundamental focus of this research?
The work examines the ecological impacts of herbivory and interspecific competition on the invasive alien plant species Senecio inaequidens within European grassland ecosystems.
What are the central thematic fields covered?
The study bridges plant ecology, invasion biology, and herbivore-plant interactions, specifically looking at how native European herbivores and competition with native grasses like Festuca rubra affect the invader.
What is the primary objective of this thesis?
The core objective is to determine whether herbivory and plant competition limit the growth, survival, and reproductive output of Senecio inaequidens and if native herbivores are capable of host-switching to this invasive species.
Which scientific methods were employed?
The study utilizes a multi-approach design: greenhouse experiments with controlled conditions, large-scale field experiments using a split-plot design, and laboratory feeding trials to test herbivore preferences.
What is addressed in the main body of the work?
The main body details the morphological and physiological characteristics of Senecio inaequidens, reports the statistical outcomes of various competition and herbivory treatments, and analyzes the role of different plant ecotypes.
Which keywords best characterize this research?
The study is defined by terms such as Senecio inaequidens, invasion biology, host-switching, Longitarsus jacobaeae, plant competition, and herbivory.
Does Longitarsus jacobaeae successfully colonize Senecio inaequidens?
Yes, one of the study's most notable findings is that the flea beetle Longitarsus jacobaeae spontaneously colonized Senecio inaequidens in the field, indicating a successful host-switch from its original host, Senecio jacobaeae.
Are the findings relevant for future management?
Yes, the results suggest that Senecio inaequidens will likely colonize disturbed grassland ecosystems, and the study supports the potential development of preventive management programs based on these ecological insights.
- Quote paper
- Dr. rer.nat. Christoph Scherber (Author), 2002, The effects of herbivory and competition on 'Senecio inaequidens' DC. (Asteraceae), an invasive alien plant, Munich, GRIN Verlag, https://www.grin.com/document/8982
