Invasive Weeds; Are They Really

Special & Unusual?

Weed Definitions

• Plant out of place– Weed Science Society of America

• A plant that interferes with the management objectives of a parcel of land at a given time– Weeds of the West

• A plant whose virtues have yet to be discovered– Emerson, ~ 1870.

Anthropocentric Natureof Weeds

• Definitions centered around humans– not entirely bad

• Humans evolved on earth• Humans use natural resources• Weeds inhibit efficient use of

natural resources

H.G. Baker – Ideal Weed

• 1974. The evolution of weeds. Ann. Rev. Ecol. and Sys. 5:1-24.1. Germination requirements fulfilled many

environments2. Discontinuous germination (internally controlled) &

great longevity of seed3. Rapid growth thru vegetative stage to flowering4. Continuous seed production as long as conditions

permit5. Self-compatible but not completely autogamous or

apomictic6. When cross-pollinated, unspecified visitors or wind7. Very high seed output favorable environments

H.G. Baker – Ideal Weed

• 1974. The evolution of weeds. Ann. Rev. Ecol. and Sys. 5:1-24.8. Produces some seed under wide range

environmental conditions; tolerant and plastic9. Adaptations for short- and long-distance dispersal10. If perennial, has vigorous vegetative reproduction

or regeneration from fragments11. If perennial, brittle, so not easily pulled from soil12. Has ability to compete interspecifically by special

means (rosettes, choking growth, allelochemicals)

Weeds as Strategists

• 2 basic external factors limit amount of plant material in any environment:– Stress– Disturbance

Selection Pressure - Extremes

of Stress & Disturbance Stress High LowDisturbance High: Death Ruderals

Low: Stress Competitors

tolerators

Weeds as Strategists - Extremes of Stress & Disturbance

• 3 possible strategies– Stress tolerators– Competitors– Ruderals

• Weeds as competitive ruderals– disturbance precludes strict competitors– no severe disturbance precludes ruderals– meadows, seasonal grasslands, rangeland

subject to seasonal disturbance, floodplains, lake & ditch margins, arable lands

Why Are Alien Plants Troublesome?

• Noxious weeds primarily of foreign origin– Required by law to manage

• Biotic pressures left at weed origin– plant/plant & animal/plant

interactions• Abiotic pressure ‘new home’

insufficient;– populations increase exponentially

So What’s the Big Deal?

• National Invasive Species Council:– “Society pays a great price for

invasive species – costs measured not just in dollars, but also in unemployment, damaged goods and equipment, power failures, food and water shortages, environmental degradation, increased rates and severity of natural disasters, disease epidemics and even lost lives.”

Invasive Species Impacts

• Anthropocentric:– interfere with land mgmt objective

• interfere with ag production• harmful to human health• impede movement of water• ………

Invasive Species Impacts

• Natural:– cause population declines native spp.– cause native spp. extinctions– cause shifts predator-prey dynamics– alter ecosystem function– decrease ecosystem complexity– ………

Invasive Spp. Impacts

• Economic:– can involve anthropocentric and/or

natural effects

What Plants Should Society Manage?

• No debate list:– leafy spurge– Canada thistle– knapweeds– downy brome– purple loosestrife– salt cedar– toadflaxes– Kudzu

• Debated plants:– damesrocket– blue flax– smooth brome– crested wheatgrass– Kentucky bluegrass– ………

How Can Society End Debate Fairly and

Objectively?• Stop all importations• Assess each spp. thru rigorous

science• Use invasiveness elsewhere as

indicator• Adopt view of neighbors• Objective screens and flow charts

Perception to Cause Harm

• Richardson et al. 2000. Naturalization and invasion of alien plants: concepts and definitions. Diversity and Distributions 6:93-107– definitions & communication may be

a heart of problem• use of naturalization and invasive

interchangeably

Naturalization/Invasion Process

• Plants must overcome various barriers:– Introduction

• overcome geographical barrier

– Naturalization • abiotic & biotic barriers to survival surmounted

and various barriers to reproduction overcome (self-sustaining)

– Invasion• overcome dispersal barriers to produce

reproductive offspring distant from intro site

Definitions

• Alien plant– result of intentional or unintentional

intro due to human activity• Casual alien plant

– alien plant that may flourish or reproduce occasionally• do not form self-replacing populations• rely on repeated intros for persistence

Definitions

• Naturalized plant– alien plant that reproduces

consistently and sustains populations over many life cycles without direct intervention by humans (or in spite of humans)• often recruit offspring freely, close to

parents• do not necessarily invade natural, semi-

natural, or human-made ecosystems

Definitions

• Invasive plant:– naturalized plant that produces

reproductive offspring, often in large numbers, at considerable distances from parent• > 100 m < 5yr by seed• > 6m/3 yr by vegetative reproduction

Definitions

• Weeds:– plants, not necessarily aliens, that

grow in sites where not wanted and usually have detectable economic or environmental effects

– Environmental weeds:• alien plants that invade natural areas• usually adversely affect native

biodiversity and ecosystem functioning

Definitions

• Transformers:– subset of invasive plants that change

character, condition, form, or nature of ecosystems over substantial area relative to extent of that ecosystem• “bad boys” of the plant world• approximately 10% of invasive plants fall

into this category

Excessive use of Resources: - Salt cedar and water

Donors of limited resources: - nitrogen fixers;

- yellow sweetclover

Fire promoters/suppressors: - downy brome increase fire frequency & intensity

Suppressors:Russian knapweed allelopathy

Litter accumulators:Perennial pepperweed

Salt accumulators/redistributors:Tamarisk salt uptake & exudation

Transformation Examples

• Sand stabilizers• Erosion promoters• Sediment stabilizers• …

The Perfect Ranking System?

• Does not exist– too many differing perspectives

• We need objective approach however!– Reichard screen plants not yet here

• Dr. Sarah Reichard, University of Washington– IFAS screen (adapted) for plants already present

• Drs. Randall Stocker, Allison Fox, Univ. Florida– Australian screen

• Dr. John Virtue– NatureServe

• Invasive plants assessment program– Colorado has adopted & adapted CALIPC

screen

What Plants Should Society Manage?

• Weed mgmt is expensive!• Choose battles carefully!• Manage only those weeds

that cause economic and environmental harm– environmental

weeds/transformers

National Invasive Species Council

• Invasive Species Advisory Committee:– Beck et al. 2008. Invasive species

defined in a policy context: Recommendations from the federal Invasive Species Advisory Committee Invasive Plant Science and Management 1(4):414-421.• Clarified definition Executive Order

13112– What do we mean and what do we not mean

Colorado State University Weed Science