Are weed population dynamics chaotic?

Abstract

Summary 1.There have been suggestions that the population dynamics of weeds may show chaotic dynamics, and that therefore it will not be possible to predict the impact of changing management regimes on weed abundance. The instability of weed populations is presumed to result either from overcompensating yield-density responses or from threshold management. 2. Using theoretical arguments and empirical evidence we argue that this contention is likely to be incorrect. 3. Overcompensating yield-density responses are unlikely in plant populations and this point has been extensively discussed. Such responses have only been observed in high-density artificially sown stands of weed populations. The form of chaos that results from threshold management is a consequence of high population growth resulting from the cessation of management when weed densities are lower than a threshold level. Consequently the dynamics of such populations may be argued to be extrinsically rather than intrinsically driven. 4. There are many studies that have shown weed populations to be dynamically stable, both spatially and temporally. Here we present an analysis of data from the Broadbalk experiment that demonstrates long-term stability of 12 species of common weeds over a 12-year period. Using parameter estimates derived from the literature we show that the stability of these populations is similar to other annual species, both weedy and non-weedy. 5. We argue that weed population dynamics are more generally better viewed as resulting from the impacts of broad-scale types of management, as well as temporal variability in population numbers. The significance of chaotic dynamics is likely to be minimal.

Characterising spatial and temporal variation in the finite rate of population increase across the northern range boundary of the annual grass Vulpia fasciculata

“…It has recently been argued that this may largely be a reflection of the fact that population growth rates are usually estimated in populations close to equilibrium densities rather than at low densities . Estimates of population growth rates at low densities in a range of annual plants indicate that k may be extremely variable ranging from approximately one to considerably in excess of 100 in some weeds (Freckleton and Watkinson 2002). While V. fasciculata may on occasion reach growth rates of the order of 33, the geometric mean is much closer to two indicating that populations will on average approximately double each year at low densities.…”

Weed Problems, Ecology, and Management Options in Conservation Agriculture: Issues and Perspectives

“…This instability might be either from overcompensating yield-density responses or from threshold management (Freckleton and Watkinson, 2002). Tillage had the largest effect on weed diversity and density in continuous corn, corn-soybean, corn-soybean-winter wheat rotations at Ontario.…”

“…Weed shift has been clearly differentiated by Swanton et al (1993) as weed succession, fluctuation and weed shift deduced from 123 references. The impact of changing management regimes on weed abundance cannot be predicted and the weed population dynamics may show chaotic dynamics (Freckleton and Watkinson, 2002). Undoubtedly, a change in soil tillage led to shift in weed flora composition (Conn, 2006;Montanya et al, 2006).…”

The Evolution of Oscillatory Behavior in Age‐Structured Species

“…For example, despite an understandable and considerable interest in the phenomenon of chaos, very few population dynamical time series are convincingly chaotic (as opposed to noisily periodic; Grover et al 2000;Turchin and Ellner 2000;Bjørnstad and Grenfell 2001). Rather, most field populations exhibit apparently stable or cyclic population dynamics (e.g., Clutton-Brock et al 1997;Saether 1997;Wickens and York 1997;Kendall et al 1998;Siriwardena et al 1998;Bjørnstad et al 1999;Fewster et al 2000;Freckleton and Watkinson 2002). In fact, approximately 30% of ecological systems for which reliable and sufficiently long time series are available are periodic or near periodic (Kendall et al 1998).…”