16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/11931
On the inherent instability of the monoculture
Griffon, D.1, Torres-Alruiz, M. D.1
Key words: Stability, Food webs, Network analysis, Agroecosystem architecture.
Abstract
In the last decades has been recognized that monoculture has harmful consequences:
genetic erosion, soil loss, pollution, land concentration, increased poverty and so on.
But, there is another aspect that has been underestimated, the instability that results
of the oversimplification of monoculture’s trophic structure. Here, using network
analysis, we show why the trophic structure of monoculture is inherently instable.
Considering an agroecosystem as a complex network, we propose that for the design
of stable agroecosystems we must generate architectures with redundancy of
relations and homogeneous connectivity, because this compensates and modulates
perturbations.
Introduction
An ecosystem is a complex network, made of many interacting species (nodes). As
result of these interaction emergent phenomena arises, being one of these, the
system stability or homeostasis. With the goal of simplify production, conventional
agriculture break up redundancy and trophic cycles which are present in natural
ecosystems, transforming them in monocultures. Since stability is a phenomenon
observed in natural ecosystems and not in monoculture, we need to study it in the first
ones.
The network analysis of trophic webs can be used to study the interactions between
the components of an ecosystem (Montoya et al., 2006). One advantage of this
analysis is its systemic approach, which allows to explore the whole system and find
patterns. Recently has been suggested (Montoya & Solé, 2003) that the natural
ecosystems trophic architectures can be divided in two classes according to their
richness (S). Those that present low S, are called Poisson, whereas those with high S
are called Scale Free (SF) or Power Law. Poisson nets have a similar number of
trophic relations per species (connections or degrees) and Scale Free have a small
number of species with many connections and most of the species with few
connections. If the richness is related to ecosystem’s architecture, and, if this
influences the potential responses to perturbations (i.e., its stability), this may offer
clues on how to design agroecosystems.
Materials and methods
We simulated Poisson and Powers Law networks, between S=20 to S=150. These
values were obtained from real ecosystem networks information (Montoya & Solé,
2003), and represent the interval on which the architectural change occurs. The
simulations were settled to generate low average degree (i.e., 13) networks according
to empirical data (Montoya and Sole, 2002).
1
PFG en Agroecología, Universidad Bolivariana de Venezuela. Calle Leonardo Da Vinci, Los
Chaguaramos. zip code 1040. Caracas, Venezuela, e-mail: d.griffon@lycos.com.
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/11931
In order to compare the architecture of the networks, we measured the diameter of the
simulated networks. The diameter is a measurement of the cohesion of the food webs
and represents the shortest distance (links number) between the two more distant
species. The diameter was normalized according to the total network size. We also
evaluated how the characteristics of the networks were affected with the removal of
one of their nodes (i.e., species). We removed the most connected node (directed
attack), and the consequences of this perturbation was measured counting the
number of isolated nodes (secondary extinctions). This perturbation simulates the
effect of completely remove an element of the agroecosystem (e.g., a plague, a weed,
etc.).
Results
Network size
a
90
10
0
11
0
12
0
13
0
14
0
15
0
80
90
10
0
11
0
12
0
13
0
14
0
15
0
80
70
60
50
40
30
20
0
70
5
60
10
50
15
12
10
8
6
4
2
0
40
20
30
Diameter
25
20
Percentage of
Isolates nodes
We found that the diameter diminishes as S increases in exponential fashion,
independently of networks architecture. But, for values lower than eighty nodes the
diameter of Poisson networks is considerably smaller than the SF diameter, therefore
the Poisson net is more advantageous because the community is more integrated
(Figure 1a). When perturbed, the number of isolated nodes was always zero for
Poisson networks, whereas for Scale Free networks this number was variable, with an
average of approximately five isolated species (Figure 1b).
Network size
b
Figure 1. a) Changes in Networks Diameter as S increases. In small ecosystems
the Poisson architecture is more advantageous than SF, because it has lower
diameter (higher community cohesion). Continuous line: SF Networks, dashed
line: Poisson Networks. b) Secondary extinction vulnerability for SF architecture
as S increases. Continuous line: changes in the percentage of isolates nodes,
dashed line: overall average of isolates nodes.
Discussion
Our results agree with and complement those of Albert et al., (2000) and Solé &
Montoya (2001). These authors show that Poisson networks are moderately sensible
to random or directed removal, whereas SF networks are very robust to random
removal, but highly sensitive to directed or specific removal. Which is the probability of
randomly remove the most connected node (hub) in both types of nets?. If we
consider the probability (Figure 2) of extracting the most connected node as richness
increases (i.e., p=1/S), we found the following:
90
10
0
11
0
12
0
13
0
14
0
15
0
80
70
60
50
40
30
0,06
0,05
0,04
0,03
0,02
0,01
0
20
Probability
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/11931
Network size
Figure 2. Probability of randomly remove the most connected node
(1) The probability of remove the hub is higher for smaller nets.
(2) The probability of remove the hub is lower for larger nets.
The smaller nets seen on nature have Poisson architecture, whereas in the largest
nets, the architecture is SF (Montoya & Solé, 2003). Consequently, if we remove the
hub from them we find:
a.- Small Poisson networks: the nets do not disarticulate, and therefore, secondary
extinction doesn’t take place.
b.- Large Poisson networks: the nets do not disarticulate, and therefore, secondary
extinction doesn’t take place.
c.- Small SF networks: because (1) happens and the SF network is vulnerable to this
perturbation, this architecture results instable.
d.- Large SF networks: Because large SF nets have similar integrity levels as Poisson
(Figure 1a), and because (2) happens, this architecture is as stable as Poisson.
Nevertheless, extensive evidence (Albert & Barabási, 2002; Newman, 2003) shows
that SF networks have in general more advantageous properties than Poisson ones.
Therefore, if the larger SF nets are as stable as Poisson and more advantageous in
terms of other properties, it is reasonable that natural ecosystem possess this
architecture when the nets are large.
Let us extrapolate these results to the agricultural area. If we recognize that a
monoculture is an ecosystem in which all the species are related by means of some
type of ecological interaction (parasitism, predation, mutualism, competition, etc) to
one species (i.e., the monoculture), we realize its trophic structure is a Star networks
(a net in which all nodes are directly connected to a common hub). The Star network
is unstable when directed attack occurs, because the whole ecosystem cohesion
relies on a single species (in this case, the monoculture). In this architecture, all the
weaknesses of the SF networks in low richness ecosystems are taken to the
maximum, because its centralization. In this context the Star net is the most inefficient
architecture. In it any perturbation will reach the whole system in two ecological
interactions in average. In this architecture any type of self-regulation is impossible.
We think this is the fundamental reason of all imbalances inherent to the monoculture.
If our goal is to design stable agroecosystems, we must copy the topologies found in
nature. Our results indicate that the way to do so, in low diversity ecosystems (like
agroecosystems) is generating architectures in which all the bets are not in a single
species. Redundancy of relations and similar connectivity is the key of the stability,
because these compensates and modulates perturbations in the network.
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/11931
Conclusions
Monocultures are ecosystem with an inherently imbalance architecture, in which all
the weaknesses seen on low richness SF networks ecosystems are taken to the
extreme. In this architecture any type of self-regulation is impossible and ecosystems
functionality is loss.
If our goal is to design stable agroecosystems we must enforce redundancy of
relations and similar connectivity for all system species, because this compensates
and modulates disturbances in the trophic network.
Acknowledgments
We wish to express our gratitude to Iliana Chollet, Roberto Ruggiero, Dayaleth
Alfonzo and Glenda Briceño for their helpful comments and encouragement.
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