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Comparison of Time Series from Ecosystems and an Artificial Multi-Agent Network Based on Complexity Measures

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Comparison of Time Series from Ecosystems and
an Artificial Multi-Agent Network Based on
Complexity Measures
Michael Hauhs
1
, Jennifer Koch
1
, and Holger Lange
2
1
Bayreuth Center for Ecology and Environmental Research (BayCEER),
University of Bayreuth, D-95440 Bayreuth, Germany
{michael.hauhs, jennifer.koch}@bitoek.uni-bayreuth.de,
http://www.bayceer.uni-bayreuth.de
2
Norwegian Forest Research Institute (Skogforsk), N-1432
˚
As, Norway
holger.lange@skogforsk.no,
http://www.skogforsk.no
Abstract. We investigate ecosystem dynamics by analyzing time se-
ries of measured variables. The information content and the complex-
ity of these data are quantified by methods from information theory.
When applied to runoff (stream discharge) from catchments, the infor-
mation/complexity relation reveals a simple non-trivial property for a
large ensemble (more than 1800) of time series. This behaviour is so far
not understood in hydrology. Using a multi-agent network receiving in-
put resembling rainfall and producing output, we are able to reproduce
the observed behaviour for the first time. The reconstruction is based
on the identification and subsequent replacement of general patterns in
the input. We thus consider runoff dynamics as the expression of an
interactive learning problem of agents in an ecosystem.
1 Introduction
Ever since the invention of Tierra [1], Artificial Life at the level of whole (virtual)
ecosystems has attracted ALife researchers. A minimal notion of an ecosystem
requires that it is open to abiotic fluxes of mass and energy and contains life.
In (real) terrestrial ecosystem research, hydrological headwater catchments are
considered as protoypical examples for whole ecosystems. They are defined as
the basins of attraction for rainfall fields, i.e. as the units of water transfer
from rainfall to runoff in a stream. They have the advantage that only abiotic
criteria and observations are needed to identify and characterise its boundaries.
A disadvantage lies in the fact that they are often large units of study which are
difficult to manipulate experimentally.
The input-output relationship between rainfall and runoff in catchments is
often assumed to be describable by simple black box models [2]. However, these
models capture only some linear short-term aspects of the dynamics. A closer
look using nonlinear methods of time series analysis reveals a stunningly intri-
cate dynamics in particular for runoff on all time scales [3–5]. In this paper, we
use methods from information theory to quantify information content and com-
plexity as nonlinear measures of the runoff data. An important class of linear
models (autoregressive models and their extensions) is fundamentally unable
to reproduce these quantities from the observations, although these measures
are short-term only and the fitted models do reproduce simple linear charac-
teristics (autocorrelation structure) by design. Another observation is that the
relationship between information and complexity for runoff follows a simple one-
parametric curve, a nontrivial property which lacks explanation from hydrolog-
ical models or a process understanding thus far. It is currently unclear whether
models based purely on hydrology and abiotic transport are able to reproduce
this property. However, taking biological freedoms into account opens up for new
model classes which focus on behaviour directly.
We thus consider these ecosystems here as an ensemble of interacting or-
ganisms (agents), which receives input (rainfall) across the abiotic boundaries,
manipulates it, and releases output (runoff). We use the term interaction in a
technical sense as defined in [6]. We employ an artificial ecosystem consisting
of a multi-agent network to study the influence of interaction upon time series
generated by the network. Agents are able to make autonomous decisions de-
pending on their internal strategy parameters. The requirements for the agents
are here that they are capable of adapting to their local environment and mi-
grate between different localities [7], [3]. Following learning strategies that seek to
identify repetitive patterns in the input, the agents may maximize their nutrient
access or efficiency, e.g. with respect to reproduction, in an evolutionary setting.
Identified patterns are ”used” (extracted), and thus the input transformed by
substitutions. We will show in the following that pattern substitution is a key
ingredient to reproduce the simple property observed in runoff time series.
2 Information and Complexity Measures
2.1 Quantifying information content and complexity in time series
To calculate the values for information content and complexity, time series have
to be transformed into a symbol sequence (with λ indicating the size of the
alphabet, here always λ = 2). Using the same transformation method renders
the values for different time series comparable. The estimation of values for
information content and complexity is based on part-intervals of a certain length
L, called words. Thereby not only the details of the value distribution of these
words, but also transition probabilities are of interest for some measures [4].
An especially suitable information content quantifier for many environmental
data sets is the Mean Information Gain (MIG) [4], [8]. This measure quanti-
fies the information gained on average, if L-word i is followed by L +1-word
j, which differs from i only in the last symbol. With transition probability
p
L,ij
=
n
L+1,j
n
L,i
, and event frequency p
L,ij
=
n
L+1,j
NL+1
, used to estimate the
weighted average, Mean Information Gain is [8]:
H
G
=
λ
L
i,j=1
p
L,ij
log
2
p
L,ij
(1)
Fluctuation Complexity (FC) [9] accounts for information loss at transition
from L-word i to L-word j. It is the statistical fluctuation of the net information
gain:
σ
2
FC
=
λ
L
i,j=1
p
L,ij
log
2
p
L,i
p
L,j
2
(2)
Reny´ı Complexity defined by [3], [5], is based on differences of Reny´ıentropies
from conjugated orders:
C
R
(α)=
2
L ln 2(1 α)
H
R
(α) H
R
1
α

, (3)
with Reny´ıentropy
H
R
(α)=
1
1 α
log
2
n
i=1
p
α
i
for α =1. (4)
The short term dynamics of natural and artificial time series is assessed by
using the above presented concepts of randomness, information and complex-
ity. The methods were developed in information theory and statistical physics
(Symbolic Dynamics, [10]). The information content of the time series is a mono-
tonically increasing but nonlinear function of randomness; thus it is quantified
by a first order measure. A second order measure is expected to show low val-
ues at a coarse sampling rate (data close to noise), low values as well at very
high sampling rate (redundant measurements) and a maximum somewhere in
between [5]. This is in accordance with an intuitive notion for complexity. Here
we choose the Fluctuation Complexity or FC as such a quantifier, it is based on
transition probabilities [9].
These two quantities have the desired features, as can be demonstrated e.g.
for binary Bernoulli sequences (Fig. 1). MIG is nonlinearly proportional to ran-
domness, being more sensitive to structural changes in the region of low random-
ness, FC exhibits a maximum and vanishes for constant as well as completely
random sequences. The combined result of the randomness and complexity anal-
ysis characterises a time series in MIG/FC plots, whereby the two measures form
the axes of these plots (see figures below). The black curve shown in these figures
represents the theoretical maximum that can be attained by a random process.
These figures will be used to assess the similarity between observed catchment
behaviour and the behaviour attained in our simulations.
Any constraints in the dynamics or behavioural patterns will lower the MIG-
/FC value of a time series. It turns out that if this lowering occurs in a consistent
manner across the range of randomness its distance from the limiting curve is
0 10 20 30 40 50 60 70 80 90 100
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Randomness [%]
Information/Complexity
Complexity − FC
Information − MIG
.
Fig. 1. Mean Information Gain (MIG - ”Information”) is chosen as first order mea-
sure (information) and Fluctuation Complexity (FC - ”Complexity”) as second order
measure (complexity). These are here shown as functions of a randomness parameter
(range p [0, 1/2]: interpreted as scale of randomness from 0% to 100%) in a binary
Bernoulli-Process [3].
assessed by fitting the exponent α of the Reny´ı Complexity. In the limit of α =1
the Reny´ı Complexity becomes identical with the FC [5].
In the subsequent results all time series have been (statically) partitioned at
the median into a binary alphabet. The statistics of words with different length
from this alphabet is then used to calculate first and second order complexity
measures and the distances from the limit curve. In all examples below, four-
letter words were used.
2.2 Application to runoff data
A window technique was used to apply the measures locally in long-term time
series. The window length for runoff data from catchments was typically 4 years
at daily resolution. The length of artificial data sets generated without use of the
agent network was 250000. Window length for results from the agent network
was 3400. We inspected different window lengths from 25 to 20000 (not shown).
When applied to long-term runoff data sets at daily resolution, a unique
parametrisation of the Reny´ı Complexity results (Fig. 2, α =1.28). So far it had
not been possible to reconstruct artificial time series with such a property by
stochastic or deterministic generators. One of the conjectures to explain these
difficulties suggested this behaviour as a signature of (indirect) interaction among
the organisms within the catchment [11].
.
Fig. 2. Data sets collected from real world ecosystems (hydrological catchments). The
black curve gives the limiting case for α = 1. Each blue dot represent a long-term
runoff data set (>30 years), green triangles are tropical catchments. The red line gives
afitoftheReny´ı Complexity for α =1.28 [3].
2.3 Generating the signatures of universal Reny´ı Complexity with
artificial systems
Here we tested this conjecture without and with a multi-agent network. The net-
work is able to simulate a parallel decision process by the action of agents which
affect a realised stochastic process, here the supply with external nutrients that
limit growth and proliferation of the agent populations. Interaction among the
agents in this network is indirect only [12]. Before employing the agent network
we tested the effect of a two step realisation process in which we firstly realised
a time series with specified randomness (by choosing the Bernoulli parameter)
and secondly identified and replaced (enhanced) patterns within these series.
The first test without the network was necessary to show that a selective deci-
sion was sufficient for reconstruction of the observations and that this had to be
done after the random process had been realised. This procedure is described
below.
In order to assess the impact of (interactive) decisions on information and
complexity of time series we generated examples of the Bernoulli-Process that
forms the limiting curve in figure 2 (for α =1.0) with 250000 points drawn
from a binary alphabet. Then we searched these realisations for one or two
general patterns (e.g. where the four letter words interpreted as integer where
monotonously increasing in 3 (4) subsequent overlapping words). Such local gen-
eral patterns where replaced by either random sequences or by a unique pattern
(a fixed permutation of the original sequence used for all realisations in the range
of randomness) while conserving the occurrences of letters. Figure 3 shows the
results for local randomisation (α =1.06) and pattern replacement (α =1.32).
Depending upon the pre-selected pattern much higher α where possible.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Mean Information Gain (MIG)
Fluctuation Complexity (FC)
Bernoulli−Curve
.
Fig. 3. Artificial time series with varying Reny´ı Complexity. The dash-dotted line
represents the fit of the Reny´ıComplexityα =1.06 on Bernoulli-Process realisations
with different range of randomness, where general patterns were replaced by random
sequences. The dotted line (fit of the Reny´ıComplexityα =1.32) is the same for
replacement by a fixed permutation of the original sequence. Dots in the lower right
corner are not in line with tted Reny´ı-Curve. Different symbols stand for different
Bernoulli parameters.
The resulting artificial time series thus included the complete set of short-
range patterns that are produced by the stochastic process, except for one or
two that were manually removed afterwards. The deviation in the random (lower
right) corner of the diagram is subject of further investigations. Now we can look
for a multi-agent simulation where a similar signature is produced from indirect
interaction among agents competing for nutrients. We use the agent-network
Pool-World for this purpose.
3 Multi-Agent Network: Pool-World
Pool-World [13] is a system in which agents interact indirectly by uptake of
limited resources provided by the environment. Agents evolve under selective
pressure expressed through the temporal and spatial variability of these resources
in input time series. Here the input has been parameterised to represent ranges
of simple or random behaviour.
The question is what is the minimal interactive behaviour within Pool-World
in order to generate time series with similar characteristics as in real ecosystems?
The criterion for comparing input to output is restricted to the position of these
streams in the MIG/FC diagram (see above). [13] showed that even with only
random input of resources the dynamics of the evolving (interacting) agents
reveal a complex long-range dynamics. Here we study the short term measures
only.
Fig. 4. The network consists of places (dark-grey) between which agents (light-grey)
may move. Resources (geometric symbols) have prespecified input functions for each
of the places, including zero inputs.
The multi-agent network consists of a number of connected 0-dimensional
places (Fig. 4). The agents may move along the connections, but they have to
pay for it by using internal resources. Resources are represented by geometric
symbols. The external supply with resources is provided by specified functions as
simple, random or complex. Input streams may differ among places and among
nutrient types (here indicated by shape). Agents compete for them by taking
them up through input interfaces with an individually specified resource affinity.
Affinities may evolve during reproductive events. Agents reproduce non-sexually
when a resource threshold is reached. They die when failing to encounter a
minimum resource load or when the reach a maximum life span. Resources not
taken up are ”drained” from places by an exponential export function. Agents
are only able to interact indirectly through uptake of resources. They are not
equipped with memory in theses runs. The only persistent adaptive state is the
uptake preference parameter.
In former runs it had been shown that with these parameter settings special-
ists and generalists may evolve among agents. The former tend to stay in one
place specialising on one resource type, while the latter tend to move more freely
around. The program is run as a JAVA application (on one single machine). For
details see [7], [13].
Here we used simulations with up to four places. Output of the resources
”drained” was monitored (from each place of the network, but separately for each
resource type) and population levels at each place were observed. We conducted
fifteen scenarios with one place, varying lifetime, reproduction rate, nutrient
input fluxes and decay rate of the nutrients (which has direct effects on nutrient
residence time in the places). Reproduction rates in all scenarios were chosen
near to 1 for holding population levels low. Scenarios with two, three and four
places, showing different topologies and so called ”desert”-places (almost no
nutrient input), served for investigating migration. Nutrient based migration was
tested as well as population based migration. Scenarios with four places included
variations of migration threshold. Altogether we analysed 195 output time series
and 44 population time series with information and complexity measures.
Only some variations showed an effect at all. Changing migration from nutri-
ent to population based showed no effects, as well as migration threshold. High
decay rate values affected that the nutrients left the places fast. Especially life-
time and reproduction rate variations had strong effects. Populations adaptation
to available nutrients happend much faster at higher lifetime and reproduction
values.
4Results
We tested between 1 and 4 places with varying connections and up to three
nutrient input fluxes at each place. We used only parts of the time series after
initial transient increases in population levels (Fig. 5). The outflow levels are
in these later phases low due to strong competition among agents, migration
and adaptation effects. In the typical example shown in figure 5 long-term levels
appear stationary though bursts occured (in population and in outflow time
series).
0
5
10
0
2
4
6
1 2 3 4 5 6 7 8
0
10
20
2 3 4 5 6 7 8
10
20
30
40
Data point [× 10
−4
]
Number
Outflow "B"
Outflow "G"
Outflow "R"
Population
Fig. 5. Typical results for outflow and population time series (one place). Initial tran-
sient increases were cut, the remaining parts show low nutrient outflow levels, two
nutrient outflows (”B” and ”R”) show bursts.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Mean Information Gain (MIG)
Fluctuation Complexity (FC)
p
Input
= 2
p
Input
= 1
p
Input
= 5
p
Input
= 0.5
p
Input
= 0.2
p
Input
= 0.05
p
Input
= 0.1
.
Fig. 6. The black curve gives the limiting case for α = 1. With increasing nutrient
input, nutrient outflow positions in information-complexity diagram shift along the
limiting Bernoulli-Curve from simple to random. Each dot stands for one window of a
time series with corresponding nutrient input.
We show only one typical result here. Many similar series where produced
with varying parameters for agents, nutrient inputs and network topologies.
The outflow of resources from all runs did not deviate from the maximum
curve (α =1.0). By variation of the input parameter (from 0.05 to 5; the higher
the input, the higher is the number of different values observed in output time
series) we were able to shift the position of the time series for nutrient export
along the limiting curve from simple to random (Fig. 6). In this case we did
consider only result without any burst phenomenona (see for example the burst
of the ”R” or ”B” nutrients in figure 5 above). The bursts induced deviations
from the limiting curve. These effects were transient. That is why we studied the
information and complexity measures resulting when two curves from different
positions along the limiting curve were concatenated (Fig. 7).
These intermediate positions in the information-complexity diagram display
transient behaviour when the window moves between the two concatenated time
series and cannot be fitted by a consistent α-value. This effect is very different
from the one observed in the catchment data set.
The last figure shows the population time series at various locations (Fig. 8).
These time series have a similar characteristic deviation from the limiting curve
across the various degrees of randomness (α =1.22).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Mean Information Gain (MIG)
Fluctuation Complexity (FC)
.
Fig. 7. Two different time series from the limit curve (solid dots) concatenated. When
the measures are analysed by the window technique intermediate positions below the
limiting curve result. Dashed lines indicate the trajectories taken by transient alpha
values, when the windows moves from on to the other part of the two time series.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Mean Information Gain (MIG)
Fluctuation Complexity (FC)
Bernoulli−Curve
Renyí−Curve, α = 1.22
Population
Fig. 8. Population time series (about 40 locations). The dashed line represents the fit
of the Reny´ıComplexity(α =1.22).
5 Discussion and conclusion
We were able for the first time to artificially generate a similar signature in a
information-complexity diagram as in real world data sets. However, when using
a multi-agent network for this purpose, this signature appeared in the population
dynamics of the agents and not in the nutrient export from the network. Clearly
more topologies, and agent features could be tried out with this AL simulation.
One drawback with the current setup has been that competition among agents
was fierce and largely successful. Hardly any of the nutrients was exported at all
and export rates were rather low (Fig. 5).
The two step procedure by which a random process has been realised first and
then a selection/decision is applied to it was so far the only procedure capable
of producing one-parametric reductions from the limiting curve. This indicates
that an interpretation of the universal behaviour of runoff data is consistent with
an interactive (online) influence of biota: they decide depending on the innate
strategies based on the realised precipitation available. Rainfall is an example of
the random process. When realised as local rainfall pattern the vegetation decides
and thus imprints this decision as a pattern upon the time series. The result is a
set of behaviours that can be described as active removal or substitution of some
of the realised patterns in the input, i.e. as an active filter. This is in accordance
with the recent observation [14] that biological and physical time series from
the same environmental system might behave qualitatively different: whereas
biological data exhibit nonlinear deterministic dynamics, the physical variables
are best described in linear stochastic framework. We believe that this can be
modelled much easier interactively.
So far no closed form (analytical) model of transport processes in catchments
has been successful at rigorously describing the physical process. The combina-
tion of a stochastic process with an active decision by agents may be a way to
show why it has been so difficult to conceptualise water transport in ecosystems
as a purely physical process. AL simulation have been criticised for being difficult
to compare with observations from real Life. The above example demonstrates
how an AL simulation can be linked to Life at the ecosystem scale and that also
hydrological data sets can be used in this context.
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Open systems are part of a paradigm shift from algorithmic to interactive computation. Multiagent systems in nature that exhibit emergent behavior and stigmergy offer inspiration for research in open systems and enabling technologies for collaboration. This contribution distinguishes two types of interaction, directly via messages, and indirectly via persistent observable state changes. Models of collaboration are incomplete if they fail to explicitly represent indirect interaction; a richer set of system behaviors is possible when computational entities interact indirectly, including via analog media, such as the real world, than when interaction is exclusively direct. Indirect interaction is therefore a precondition for certain emergent behaviors.
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