Interactions between weeds and biological control agents are examined by simple pre- dator/prey biomass analysis. The possibility of discontinuously stable ...
Agriculture, Ecosystems and Environment, 13 (1985) 1--8
1
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
BIOLOGICAL WEED C O N T R O L -- EQUILIBRIA MODELS
B.A. AULD
Agricultural Research and Veterinary Centre, Forest Road, Orange, N.S. W. 2800 (Australia) C.A. TISDELL
Department of Economics, University of Newcastle, Newcastle, N.S.W. 2308 (Australia) (Accepted for publication 7 November 1984) ABSTRACT Auld, B.A. and Tisdell, C.A., 1985. Biological weed control -- equilibria models. Agric. Ecosystems Environ., 13: 1--8. Interactions between weeds and biological control agents are examined by simple predator/prey biomass analysis. The possibility of discontinuously stable systems is demonstrated. The shape of consumption functions for individual predators and populations is shown to be important in their success. The potential for augmentative biological control, where discontinuous stability exists, is shown and the relevance of an ungrazable fraction in weed population persistence is also described.
INTRODUCTION
Analyses of success or failure in biological weed control are comparatively rare. Occasionally parasitism or environmental constraints are suggested as factors responsible for failure or limited success. Moreover there have been few thorough studies in the field or laboratory or speculative analyses (Caughley, 1976) on the dynamics of interacting species. Little progress has been made in providing a rational basis for selecting the most effective biological control agents (Winder and van Emden, 1980). Populations of weeds (prey) and biological control agents (predators) can be modelled by predator--prey systems, although numbers of weeds and agents will be affected by factors other than their respective densities. For the agents these include: aggregative response to host patches, dispersal powers, search efficiency (Beddington et al., 1978) and size of plants being attacked. For the weeds they include: climatic and edaphic factors and interference from other plants. Yet no matter what mechanisms are involved it is clear that there must be adequate numbers and biomass of biological control agents in relation to weeds (Huffaker et al., 1976) for successful biological control programmes. Moreover, the fact that there can be direct interdependence between weed biomass and biological control agent's populations and c o n s u m p t i o n functions often appears to be overlooked. However, Meyers
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(1980) has recognised this in the cinnabar moth--ragwort system in which spatial distribution is also important and White (1980) has observed this interdependence in Cactoblastis c a c t o r u m Berg. and O p u n t i a spp. It is characteristic of effective arthropod biological control projects that predator--prey equilibria appear to be stable (Hassell, 1978). The same appears to be ultimately true in the biological control of weeds (e.g., Nakamura and Ohgushi, 1981). In this paper, possible discontinuity of stability is investigated using simple graphical models based on the predator--prey theory. The interaction between grazing intensity and ability of plants to respond in regrowth, given that they cannot influence the rate of renewal of principal resources (water and sunlight), is explored in this paper. Noy--Meir (1975) adopted the classical predator--prey analysis of Rosenzweig and MacArthur (1963) to study grazing systems. Noy-Meir's (1975) approach is followed to investigate a particular case of grazing systems, the biological control of a weed (prey) by a host-specific predator (grazer). ASSUlVIPTIONS We shall assume that the growth in biomass of the weed species concerned (i.e., combined growth in biomass per plant and in population) is of a logistic form dW
d---t- = rW (1 - W / K )
(1)
where W = weed biomass, r = net proportional growth rate of weed biomass and K = environmental carrying capacity or saturation level. The general form of plant/sward growth is well established (Brougham, 1955; Donald, 1961) and there is evidence that plant population growth is also of that form, although a constant maximum rate may be maintained for long periods in perennials (Harper, 1977). Thus the relationship of weed growth to weed biomass-density has a single maximum (unbroken line in Fig. 1) which m a y form a plateau. Its amplitude and maximum value would be affected by seasonal conditions as well as, for instance, the addition of fertilizer. Various forms of the relationship between predators and prey intensity have been postulated (Huffaker et al., 1976) and found experimentally for predators/herbivores and plant biomass (Noy-Meir, 1975). These are reducible to three main forms: linear, Michaelis and sigmoid. Zimmerman and Malan (1980) have shown a Michaelis type response for C. c a c t o r u m feeding on O p u n t i a ficus-indica L. We shall, initially, consider the Michaelis form C =
W k W+Ks
(2)
where C = level of consumption of the weed biomass, k = maximum rate of the consumption process and K s = half saturation constant. At higher predator population increases, Michaelis curves with higher maxima and steeper
Ph ff'~
/
/
/
f
0..
0 z
8
---P,
a 6U
Wo
w;
wa
w4
WEED BIOMASS per unit area
Fig. 1. Interaction between weed growth rate (solid line) and three population densities
of predators (broken lines) with Michaelis consumption functions. High predator populations (Ph) lead to weed eradication (W0); low predator populations (P1) lead to stable equilibrium (E,) at high weed biomass (W4); intermediate populations (Pi) may lead to stable equilibrium (E3) at W3 or unstable equilibrium (E2) at W2 which could revert to W0 or E 3. slopes near the origin are p r o d u c e d (Fig. 1), i.e., the p a r a m e t e r s k and K s increase. T h e higher c o n s u m p t i o n curves (Ph in Fig. 1) c o r r e s p o n d to higher levels o f p r e d a t o r p o p u l a t i o n , b u t t h e r e m a y be a limit to the e x t e n t to which cons u m p t i o n curves can be p u s h e d up b y increasing p r e d a t o r release. Each curve r e p r e s e n t s a particular level o f p r e d a t o r release and in the time c o n s i d e r e d m o r t a l i t y o f the p r e d a t o r can occur. H o w e v e r , n o significant change in its p o p u l a t i o n size is envisaged, even t h o u g h the weed biomass m a y be altered. Thus, t h e m o d e l is especially relevant t o s h o r t - t e r m ecological situations (e.g., one-seasonal) such as m a y be e n c o u n t e r e d in initial, inundative or augm e n t a t i v e releases (Tisdell et al., 1984). F o r simplicity, changes in p r o d u c t i o n o f w e e d biomass and in c o n s u m p t i o n as a f u n c t i o n o f weed biomass are assumed t o be i n s t a n t a n e o u s . ANALYSIS
F o u r equilibria situations are possible (Fig. 1) (tangential cases are o m i t t e d f o r simplicity). A t high p r e d a t o r p o p u l a t i o n s (Ph) the weed is eradicated (Wo); f o r low p r e d a t o r p o p u l a t i o n s (P1), and equilibrium p o s i t i o n E4 is reached at a high weed biomass W4. (Similar equilibria situations c o u l d o c c u r for
linear consumption functions.) For an intermediate population (with a Michaelis consumption function) (Pi), a stable equilibrium E3 is possible at a relatively high weed biomass W3, but at W2 there is an unstable equilibrium E2 which may lead to eradication of the weed; as soon as W < W2 the population is too low to maintain itself above the consumption rate, and extinction to W0 follows. A sigmoid consumption function can produce two steady states separated by an unstable equilibrium point (Fig. 2). Thus a given population of preda-
1 E2
I "
o-
Oz a ku
z
Wo
W 1
W2
W3 WEED BIOMASS per unit area
Fig. 2. Interactions b e t w e e n w e e d growth (solid line) and a predator p o p u l a t i o n (broken line) with a sigmoid c o n s u m p t i o n function. T w o possible stable equilibria biomasses W~ and W3 are separated by an unstable equilibrium point E 2 at weed biomass W 2.
tors could maintain two quite different weed biomass, one high (W3) and one low (W1); from the unstable equilibrium position, E2, consumption could move in either direction, depending on whether a slight shift was given to either function. A similar multiple equilibria possibility has been observed in relation to fisheries management (Peterman, 1977; Holling, 1978). The above graphical examples can be presented in a more general form, where N represents the weed biomass after consumption, = d___W- C ( t )
dt
(3)
dt
and in equilibrium dN • = 0 dt
(4)
It is assumed that, dW d--/- = F (W)
(5)
and C =
H (a,W)
(6)
where W is the biomass of the weed at time, t, and a is a parameter or set of parameters varying with the release level of the predator. For a particular release level, it might be that a = & and C =
H (~,W) = V (W)
(7)
Equilibria points are f o u n d by solving (5) and (6) simultaneously for different values of a. Given instantaneous adjustments, an equilibrium value is stable for a small displacement in a particular direction if forces return the system to its original equilibrium. An upward displacement in weed biomass from an equilibrium is unstable if the production of weed biomass exceeds predator consumption of the biomass for the displacement, and stable if the opposite is so. A downward displacement of weed biomass is unstable if the displacem e n t consumption by the predator exceeds biomass production of the weed and is stable if the opposite is the case (Figs. 1 and 2). The introduction of lagged adjustments complicates the analysis of stability of the equilibrium an equilibrium that is stable for an instantaneous adjustment may be unstable for a lagged one (Tisdell, 1972; May, 1973, 1975). A further modification of these main possibilities is possible when ungrazable portions of the weed (Wp) remain (Fig. 3), (e.g., rhizome, seed or individuals inaccessible to predators in space or time).
I
/ "
/
/
/
7r'~
0 0
~9
I--
! Wp
/ WEED BIOMASS per unit area
Fig. 3. I n t e r a c t i o n b e t w e e n predators ( b r o k e n line) and weeds (solid line) with an ungrazable fraction ( Wp ).
DISCUSSION Equilibria at high weed biomass (W3, W4 in Figs. 1 and 2) in biological control of weeds is c o m m o n (e.g., Rhinocyllus conicus Froelich on Italian thistle; Goeden and Ricker, 1978). Harris (1980) argues that these low levels of damage still put stress on the target and are therefore useful, especially when combined with additional stress. Other forms of stress may be environmental, reducing weed biomass production, thus ultimately producing an equilibrium at a much lower biomass. Such a situation could apply at the edges of a spreading population. Auld (1969) has shown how relatively low infestations of predators play a part in reducing spread of Ageratina (= Eupatorium) adenophora King and Robinson, although higher infestations of predators in areas favourable for the weed do not reduce local population growth rates. Hosking and Deighton (1980} have demonstrated that the effect of Dactylopius austrinus De Lotto on Opuntia aurantiaca Lindley is enhanced by decreasing moisture availability to the plant, which would reduce biomass production rate. The steeper the initial part of the consumption function (i.e., the greater the searching efficiency of predators; Fig. 1) the greater the likelihood of reaching an equilibrium at low weed biomass or eradicating the weed. This indicates that given the assumptions of the model, r-type biological control agents should be preferred to K-type agents, particularly for scattered weed populations, but spatial aspects of the weed population are not taken into account in this model. It also suggests that selecting for higher per capita consumption m a y be worthwhile. For all forms of consumption function, increasing predator density will reduce weed biomass. However, if the consumption function has a small, constant, initial gradient, the predator population may have to be increased substantially to reach low equilibrium weed biomass. When an unstable equilibrium is reached small increases in predator population will cause a dramatic reduction in weed biomass (cf. Peterman, 1977; Holling, 1978). Berryman (1982) has recognised the possibility of more than one equilibrium and unstable points in analysing pine beetle dynamics. This phenomenon has also been demonstrated in experimental grazing studies with domestic animals, where small differences in plant productivity have caused large differences in animal production (Morley, 1966). The possible existence of intersection points at E 2 (Figs. 1 and 2) means that supplementation of predator populations could have dramatic effects. Using an augmentative approach in the biological control of weeds, supplementing predator populations has recently proved useful in controlling Opuntia aurantiaca in areas where classical biocontrol has been limited in Australia (Hosking and Deighton, 1979) and in Cyperus rotundus L. control in the U.S.A. (Frick and Chandler, 1978). Additionally, some reduction in weed biomass at these points by other
c o n t r o l m e t h o d s c o u l d h a v e a p r o f o u n d synergistic e f f e c t . In this latter case, h o w e v e r , c l u m p e d areas are m o r e likely t o be t r e a t e d a n d this m a y a f f e c t the p r e d a t o r p o p u l a t i o n ( Z i m m e r m a n , 1979). C o n v e r s e l y , if w e e d g r o w t h were i n c r e a s e d (i.e., u p w a r d shift in b i o m a s s f u n c t i o n ) at t h e s e u n s t a b l e p o i n t s , f o r i n s t a n c e b y f a v o u r a b l e c l i m a t i c c o n d i t i o n s , a stable e q u i l i b r i u m at a high b i o m a s s w o u l d be r e a c h e d ( Z i m m e r m a n a n d Malan, 1980). U n g r a z a b l e f r a c t i o n s (Fig. 3) m a y exist in t h e f o r m o f b u r i e d w e e d seeds o r s o m e inaccessible p e r e n n i a t i n g tissue as o c c u r s in C. r o t u n d u s c o n t r o l {Frick a n d C h a n d l e r , 1 9 7 8 ) . T h e ability o f p r e d a t o r s to survive p e r i o d s w h e n n o grazable f r a c t i o n is p r e s e n t is crucial in successful classical biological c o n t r o l . In s o m e cases it c o u l d be w o r t h w h i l e m a i n t a i n i n g s o m e o f t h e w e e d p o p u l a tion in a grazable state t o ensure p r e d a t o r survival, r a t h e r t h a n incur the costs o f r e i n t r o d u c t i o n s o f t h e p r e d a t o r . ACKNOWLEDGEMENT T h e a u t h o r s t h a n k Dr. K.M. M e n z f o r his c o m m e n t s on t h e d r a f t m a n u script.
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