Lecture 09 Weed Ecology 2010

Lecture 09 Weed Ecology 2010 - Weed Population Ecology and...

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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, longer-lived 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 = K-N rN K (K= Carrying capacity) t Logistic model N dN dt K = K-N 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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