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Unformatted text preview: Weed Population Ecology and
Weed Management
Introductory concepts • Ecology: • interactions between organisms & environment • Processes, relations
• Populations, communities,
• Ecosystem: functional relationship of communities and their environment • Ecological Succession: changes in composition of an ecological community
1. Primary 2. Secondary Early stages after disturbance: ruderals, fast growth, produce lots of seed,
r strategists Late stage: Climax, stable, longerlived competitors, vegetative reproduction,
k strategists
•
Weed management keeps succession at early stages • Ecological niche: place, role, function • Competitive exclusion
• Niche differentiation; provides community diversity Weed Population Ecology
…to predict population size over time
under different weed management
practices: A. Simple quantitative approach
1. Exponential growth
N dN
dt • r = rN t Nt = e rt N o intrinsic rate per individual per unit time
• no resource limitation
• low density ruderals after
disturbance 2. Logistic model
dN
dt
N K = rN (or Nt/No = e rt ) dN
dt = KN
rN
K (K= Carrying capacity) t Logistic model N dN
dt K = KN
rN
K Intraspecific
competition Density – dependence:
• survival & fecundity remember K – strategists?
t remember r – strategists? Limitations of Exponential and Logistic models
But r is not always constant & dependent of intrinsic
factors • Survival and reproduction vary with:
• seasons
• age, size of individuals
• relative time of emergence • Also affected by external factors
• competitors, predators, etc.
• environmental constraints …to predict population size over time
under different weed management
practices: More elaborate approaches • Life Tables
• Matrices
Fecundity and survival Life Table
(diagrammatic) Idealized
Weed
Population:
Annual F
E
C
U
N
D
I
T
Y Nt+1 = (Nt x F x g x e)
Adapted from Radosevich, Holt, Ghersa (1997) Overlapping
generations
• biennials: mature
& immatures
coexist
• perennials have
clonal shoots
• annuals with
multiple flowering
episodes F
E
C
U
N
D
I
T
Y Nt+1 = (Nt x p) + (Nt x F x g x e)
Adapted from Radosevich, Holt, Ghersa (1997) B. More elaborate approaches a0
a0 a1 a2 0 5 15 10 0 0 a1 0.1
a2 0 0.6 0 a3 0 0 0.3 M a3
750 0x750 + 5x100 +15x100 +10x50 2500 75
0 x 100
.1x750 + 0x100 + 0x100 + 0x50
=
=
60
100
0x750 + .6x100 + 0x100 + 0x50
0
0 50 0x750 + 0x100 + .3x100 + 0x50 Nt Fecundity (mx)
Survival (lx) 30 Nt+1 M is a transition
matrix Nt = Mt N0 N Fecundity & survival in
matrix M vary as density
increases …and due to weed
management practices t Using the Simple quantitative approach
• When population growth is considered as a
continuous process involving overlapping
generations: Exponential growth Nt/ N o = e rt • If one measures empirically the rate of change of a
population for discrete generations: Geometric growth
• Where λ is the rate of population increase: N t +1 / N 1 = λ • Weed Management aims at λ reduction Predicted Blackgrass (Alopecurus myosuroides)
Population Sizes under Different Tillage
Practices K
K Minimum Cultivation
Rigid Tine Tillage λ Cousens & Moss 1990 Mouldboard Ploughing Time (years) Rate of population Increase, λ (no herbicide)
and min. % control by herbicide to make λ = 1 λ %min Blackgrass
Plough 1.5 50 Direct drill 6.3 88 Cousens et al. 1987 Mathematical models in Weed Science • Tools to integrate complexity
• Predict outcome of complex interactions
• Help detect areas for further research
• Economy of weed management scenarios ...
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 Winter '10
 Fischer

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