Evolutionary Ecology of Weeds:  A textbook for bringing evolution into the agronomic classroom

Dekker, J. 2011. Evolutionary ecology of weeds 

 As the famous geneticist T. Dobzhansky has said so rightly, “Nothing in biology makes sense, except in the light of evolution.”  Indeed, there is no other natural explanation than evolution for [biological phenomena].” (Ernst Mayr, 2001, What evolution is)

A weed science textbook.   A new weed science textbook is now available.  This book provides a unique, alternate perspective to weed biology and is the content for Agronomy 417/517, Evolutionary Ecology of Weeds.  The content and structure of this book arose in Agronomy 517, taught since 1991.  It is therefore the fruit that was cultivated and harvested by the many students who discussed and contributed to its development as they learned.  

A biological explanation.  This book is an explanation of the ecology and evolutionary biology of weeds and other colonizing and invasive plants. Weed biology is the ecology and evolution of plants in localities influenced by human activity, notably agriculture.  The focus is on these big WHY, HOW & WHAT questions of weed biology:

    What are weeds?

    Why do we have weeds?

    Why do we have the weed species that we do? (And not others)

    Why do these weeds look and behave as they do?

    How did the weeds we have get to be the way they are? 

    What is the basis of future changes in weeds?

The goal of this book is to provide comprehensive explanations of factual information about weed biology in an evolutionary context as the basis for understanding and management of local weed communities of the future.  The goal is also to provide the reader with a dynamic framework to guide understanding of new observations in the future: a mental 'toolkit' to focus observations of new weed phenomena, a way to understand the fundamental forces in nature that cause weediness.  

Nothing in biology makes sense unless seen in the light of evolution (Dobzhansky, 1973).  Weed and crop management is the management of selection and elimination leading inexorably to the weed adaptations that plague our fields and interfere with our crops.  To understand what we observe in agriculture and want to manage more wisely and efficiently, we need to understand how the evolutionary process works in weed communities.

Seizing and exploiting opportunity.  The thesis of this book is that human disturbance (e.g. tillage, herbicides, atmospheric pollution, frozen winter) creates opportunity spacetime by leaving unused resources (nitrogen, water, light) in a local field with few or no plant neighbors.  Opportunity spacetime is seized and exploited by heterogeneous plant phenotypes with preadapted life history traits expressed at favorable times as the growing season unfolds.  Successful weed populations assemble and interact with crop and other weedy neighbors in their particular locality.  The consequences of successful interactions lead to local adaptation maximizing survival and fitness in that plant community.

Table of contents and course syllabus

1          The nature of weeds                                                                                     
Topics:   What is a weed?  Weeds and human nature.  Weedy traits.  The origins of weeds and crop domestication.  Wild-crop-weed complexes.  World crop-weed species groups.                                            

UNIT 2:  THE EVOLUTION OF WEED POPULATIONS                                                       
2          Evolution, natural selection and weedy adaptation                                                           
Topics:  Evolution.  Natural selection and the process of natural selection.  Adaptation.                                                                                         
3          Formation of the local weed population:  Precondition to natural selection                    
Topics:  Weedy opportunity, weedy habitats, and niches in the local community.  Habitat heterogeneity and dynamics.  Limiting resources and pervasive conditions in local opportunity.  The nature of plant invasions of local opportunity.                                       
4          Generation of genotypic and phenotypic variation:  First process of natural selection 
Topics:   Genotypes and phenotypes.  Generate genetic and phenotypic variation.  Phenotypic plasticity and somatic polymorphism                                                             
5          Survival, reproduction and inheritance:  Second process of natural selection
Topics:   Survive, avoid mortality.  Reproduce the fittest, eliminate the others.  Inheritance:  transmit parental traits to offspring.  Mating systems and inheritance.                                                               

UNIT 3:  ADAPTATION IN WEED LIFE HISTORY                                                   
6          Weed life history                                                                                           
Topics:   Phenotypic life history traits.  Plant life history classification systems.  Ecological demography of plant population life history dynamics.  Evolutionary, trait-based, weed life history population dynamics.          
7          Reproductive adaptation                                                                              
Topics:   Flowering, anthesis, fertilization and birth.  Embryo adaptation:  embryogenesis and dormancy.   Evolutionary ecology of seed heteroblasty.  Observable seed dormancy-germinability regulation life forms.  Propagule adaptation: post-abscission fecundity.
8          Propagule dispersal in space and time                                                        
Topics:   The evolutionary ecology of dispersal structures.  Dispersal in space.  Dispersal in time:   formation of seed pools in the soil.  Propagule germination and recruitment.                

UNIT 4:  ADAPTATION IN LOCAL PLANT COMMUNITIES                                             
Weed-crop communities as complex adaptive systems
9          Neighbor interactions in local plant communities                                      
Topics:   Adaptation to neighbors in the community.  The nature of neighbor interactions in the community.  Strategic roles and traits of interference and facilitation with neighbors.  Effects of neighborhood interactions on plant density, growth and form.
10        Weed community structure, dynamics and biodiversity                
Topics:   Weed communities.  Weed community structure: genotype structuring.  Exploiting opportunity:  weed community dynamics.  Ecological roles-guilds-trades in weed-crop plant communities.  Changes in plant community structure.  Weed community biodiversity.


This new weed science book provides a unique, alternate perspective to weed biology.  A small sample of some of the new concepts in this book include:

Chapter 1:       What a weed is:  sustainable membership in a weed-crop species-group: 
                        The top 79 weed-crop species-groups (e.g. Amaranthus, Brassica, Chenopodium, Panicum, Setaria, Trifolium, Solanum)

Chapter 2:       Evolution never rests: 
                        Cropping systems cause local weed success

Chapter 3:       The process and steps of plant invasion: 
                        What weeds want: the nature and structure of cropping system opportunity

Chapter 4:       The extended phenotype: Weeds-R-Us

Chapter 5:       Weed mating systems: 
                        Heterogeneous, promiscuous sexual mating systems anticipate every cropping system change

Chapter 6:       Timing life history trait expression to seize and exploit cropping system opportunity

Chapter 7:       Hidden blueprint after layby: 
                        When seed dormancy is induced to time future weed infestations

Chapter 8:       Waiting for the right moment to strike: 
                        Formation of agricultural soil seed pools
                        The right moment to attack:  weed seed recruitment patterns ruthlessly track cropping tactics

Chapter 9:       Weed-crop communities as complex adaptive systems: 
                        Emergent behaviors are unpredictable
                        Weed-crop neighbor interactions: 
                                    Is facilitation more important than competition? 
                                    Strategic roles of adaptive traits in interference and facilitation

Chapter 10:     Ecological roles in weed-crop communities
                        Ready for anything:  weed population shifts
                        Weed biodiversity and the myth of the ‘biotic resistance hypothesis’


The Book
Evolutionary Ecology of Weeds is a textbook available free on the internet as a downloadable .pdf file until publication (2013).  Textbook URL:


The Course
Agronomy 417 and 517, Evolutionary ecology of weeds, is offered every Spring semester at ISU to advanced undergraduate and graduate students as both an on-campus and distance education course in a unified classroom.  Course URL: