A major part of the support for fundamental research on aquatic ecosystems continues to be provided by the Natural Environment Research Council (NERC). Funds are released for ”thematic” studies in a selected special topic or programme. ”Testable Models of Aquatic Ecosystems” was a Special Topic of the NERC, initiated in 1995, the aim of which was to promote ecological modelling by making new links between experimental aquatic biologists and state-of-the-art modellers. The Topic covered both marine and freshwater systems. This paper summarises projects on aspects of the responses of individual organisms to the effects of environmental variability, on the assembly, permanence and resilience of communities, and on aspects of spatial models. The authors conclude that the NERC Special Topic has been highly successful in promoting the development and application of models, most particularly through the interplay between experimental ecologists and formal modellers

Maberly, Stephen

Reynolds, Colin

English

Aquatic Commons

44 STEPHEN MABERLY AND COLIN REYNOLDS 
TESTABLE MODELS OF AQUATIC SYSTEMS: 
A NERC SPECIAL TOPIC FOR INTEGRATING 
EXPERIMENTS AND MODELLING 
STEPHEN MABERLY AND COLIN REYNOLDS 
(Dr S. C. Maberly and Professor C. S. Reynolds, 
Institute of Freshwater Ecology, Windermere Laboratory, 
Far Sawrey, Ambleside, Cumbria LA22 OLP, England.) 
Introduction 
A major part of the support for fundamental research on aquatic ecosystems 
continues to be provided by the Natural Environment Research Council 
(NERC). In recent years, the Council has implemented a more proactive 
approach to supplement the system of standard research grant applications by 
individual scientists. Here, funds are released for "thematic" studies in a 
selected special topic or programme. This allows a co-ordinated research 
effort to be made on subjects of general interest and topical importance. 
"Testable Models of Aquatic Ecosystems" was a Special Topic of the NERC, 
initiated in 1995, the aim of which was to promote ecological modelling by 
making new links between experimental aquatic biologists and state-of-the-art 
modellers. We believe that the exciting progress achieved by the programme 
will be of general interest to readers of Freshwater Forum. 
The foundation of this Special Topic was the recognition that sound 
management policies for any ecological system depend upon an ability to 
predict the likely range of outcomes resulting from both natural and 
anthropogenic environmental change. Although all such predictions are 
essentially extrapolations from past experience, their accuracy can be 
improved dramatically when they are made on the basis of a detailed 
understanding of the internal dynamics of the system. Mathematical models 
are a particularly powerful way of developing hypotheses about the causal 
mechanisms that underlie the dynamics of natural systems. The programme 
was devised to catalyse the complex tasks of formulating and testing such 
models by enabling continuous close collaboration between experimental and 
field biologists and modellers. In this way, models could be produced, 
checked, refined and, importantly, falsified by new experimental results. 
The Topic covered both marine and freshwater systems. Although 
dynamically distinct and often treated as separate disciplines, marine and 
freshwater ecosystems are characterised by similar physical environments and 
biological processes at the individual, population and community levels. 
Knowledge and understanding that is encapsulated into models for one system 
may prove to be directly relevant to the other. 
TESTABLE MODELS OF AQUATIC SYSTEMS 45 
Awardees and topics 
NERC allocated £890k to the support of the "Testable Models" Special Topic. 
This was used to fund eleven awards, including three studentships, and 
supported three annual workshops. Most of the projects started in January 
1995 and continued for three years, although one project was a 1-year 
"scoping study" and one PhD did not start until October 1997 (see Table 1). 
The programme may be subdivided into three general areas of focus: 
"organisms to populations", with four projects; "populations to communities", 
also with four projects; "spatial models", with three mutually interlinked 
projects. 
Organisms to populations 
The four projects in this section each sought to study aspects of the responses 
of individual organisms to the effects of environmental variability, and to 
build models of populations incorporating the aggregate of individual 
behaviours. 
(1) Nutrient interactions between a diatom and two harmful dinoflagellates. 
This approach is exemplified in experiments on nutrient interactions between 
a diatom and two harmful marine flagellates (Project 1, Table 1), which were 
then used to develop models describing the rates of population increase for 
each of the species represented. However, the first models failed to simulate 
the results from multi-species cultures. Re-analysis of the results has led, for 
the first time, to the development of a usable and reliable (i.e. tested) model 
describing the effects of simultaneous nitrogen and silicon deficiencies on the 
marine diatom used in the experiments. Using this model, it is possible to 
simulate the outcome of the multi-species experiments. Experiments on 
predation by copepods on the three algae (which included toxic species) failed 
to demonstrate a clear advantage of being toxic. Subsequent consideration of 
the growth dynamics of toxic algae using mechanistic models suggests that 
there is little or no significant cost to being toxic, while any advantage in 
being toxic would have to operate at the level of discouraging predators rather 
than killing them. 
(2) Predator-prey interactions. This pilot study on testable mathematical 
models of predator-prey interactions (Project 2, Table 1) applied formal 
mathematical descriptions, using fuzzy logic (which introduces elements of 
unpredictability into logical processes), to behavioural responses of the 
marine brown shrimp (prey) during attacks by juvenile cod (predator). Simple 
rules were established which defined the escape and exclusion envelopes of a 
shrimp in response to an attack. Unpredictability is an important component 
of the escape response of brown shrimp that reduces the ability of predators to 
learn escape-patterns. Fuzzy modelling techniques offer scope for more 
46 STEPHEN MABERLY AND COLIN REYNOLDS 
TESTABLE MODELS OF AQUATIC SYSTEMS 47 
representative models of natural predator-prey interactions, as this 
unpredictability can be taken into account. 
(3) Population consequences of spawning decisions in fish. Traditionally, 
the fields of animal behaviour and population dynamics have been separate 
despite the fact that animal behaviour clearly can influence individual, and 
hence population, success. A simulation model (Project 3, Table 1) linking 
behaviour to population dynamics of fishes, which can be tailored readily to 
other systems, was developed and field-tested for the European bitterling 
Rhodeus sericeus. This fish lays its eggs in the siphons of freshwater mussels 
(Unionidae) and so the spawning site can be clearly defined and characterised. 
The study showed that production of juvenile bitterling was correlated with 
abundance of mussles although predation by perch Perca fluviatilis also had 
an influence. Experiments showed that bitterling tended not to lay eggs in 
mussels that were already carrying bitterling emryos, so reducing embryo 
mortality. The work produced the most comprehensive test, at the population 
level, of a model including behaviour and population dynamics. The model 
can be used to predict the consequences, for the fish population, of 
environmental change or habitat loss due to hydrological changes or changes 
in the mussel population, and could also be adapted for other fish species or 
animals of conservation interest. 
Populations to communities 
The four projects in this section each developed models on the assembly, 
permanence and resilience of communities, with special reference to the 
effects of seasonal changes on the patterns of community development, and to 
the ways in which communities are affected by stochastic events and spatial 
heterogeneity. 
(1) Investigating permanence and community assembly using protozoa. For 
the first time, the long-term persistence properties of all combinations of 
species from a species pool was systematically examined to estimate the 
number of sets capable of long-term coexistence, using microcosms 
containing protozoa (Project 5, Table 1). The susceptibility of each coexisting 
set to invasion, and long-term end-states following invasion, were determined 
and used to construct a transition diagram of the assembly pathways for the 
experimental species pool. This provided a unique opportunity to identify 
characteristics of the assembly map that have been speculated upon in the 
ecological literature (e.g. assembly cycles). These data were used to test, for 
the first time, two predictions from permanence theory (a new, qualitative 
approach to measuring stability in dynamical systems), and the results 
provided qualified support for the theory. The predictions from a qualitative 
Markov model based on the assembly map, which predicts the frequency of 
occurrence of each species in a multiple-habitat system, were derived and 
48 STEPHEN MABERLY AND COLIN REYNOLDS 
examined. Species distributions were shown to depend on the assembly 
process, rather than on the more commonly examined species extinction and 
colonisation rates. 
(2) Investigating the assembly of phytoplankton communities with 
PROTECH. The late-starting Project 6 (Table 1), on the assembly of 
phytoplankton communities, began with several existing models that have 
been developed in recent years to simulate the dynamics of nuisance blooms. 
Since some of these, at least, are capable of delivering authentic outputs, it 
was felt that they could be directed to elucidate the ecological principles 
underpinning selection of adaptive traits, succession, competitive exclusion, 
the effect of inoculum bias, and the extent to which diversity is maintained by 
disturbance and stress. The essential groundwork of validating the model 
assumptions, verifying the model provisions and testing the sensitivity of the 
models, has been completed, and the performances of species, individually 
and collectively, in defined environments are currently being tested. The 
studentship continues until September 2000. 
(3) Annual development of communities in sandy bays. Diverse 
communities of fish and epibenthic crustaceans develop in sandy bays through 
a succession of ordered recruitment events. Individual-based models were 
developed, taking into account the processes of settlement, mortality 
(including predation), and size-dependent depth preference and density-
dependent growth rate. The models take into account spatial and temporal 
variation resulting from migration of mobile species and spatial distribution of 
predators (Project 7, Table 1). The final model successfully synthesised 
known and novel ecological phenomena controlling growth and survival of 
plaice Pleuronectes platessa in a way that would have been impossible with a 
single-process model. Since the plaice is a commercially important species, 
the model should be a valuable aid to the prediction of variation in production 
of juvenile plaice among different sites and years. 
(4) Effect of deterministic processes, stochastic events and spatial 
heterogeneity on rocky shore communities. Rocky shores show a great 
complexity of pattern and process but very few paradigms about the 
underlying causes. Models were developed on algal zonation, algal-limpet 
patch dynamics and dogwhelk foraging. In Project 8 (Table 1), models based 
on foraging by individual limpets and its influence on algal patch dynamics 
were developed using a cellular automaton approach. These showed the 
importance of limpet homing frequency on patch dynamics. A complex spatial 
model based on the location of individual limpets was able to generate the 
algal patchiness observed on the shore. Spatial patterns of predation by 
dogwhelks on barnacles were modelled. Tests of different hypotheses showed 
that field patterns could only be matched by taking mortality risks for the 
dogwhelk into account. 
TESTABLE MODELS OF AQUATIC SYSTEMS 49 
Spatial models 
Spatial variability is an important factor that affects the outcome of predation 
and competition, and this was addressed by three linked projects (9, 10 and 
11 in Table 1) in which the modelling work was carried out under Project 
10. 
(1) Foraging and predation risk: exploiting patchy environments. Tidal 
movements of young fish and crustaceans onto and away from the shore 
generate an intense, pulsed trophic signal in coastal food webs. Four 
hypotheses were modelled to explain the cause of dense bands of the shrimp 
Neomysis integer which move behind advancing tides (Project 9, Table 1). 
Foraging and resource gradient models were rejected as unlikely because 
there was little evidence that Neomysis was able to discriminate between areas 
with different resource levels and because in situ gradients of food disagreed 
with the model requirements. A random diffusion model was unable to predict 
the distributions found in the field. However, a hydrodynamic refuge model 
reproduced the clumped distribution of Neomysis in the field and was 
supported by experimental evidence. This model implies that Neomysis 
actively selects water depths where water-flow is low and this prevents 
removal of the animals from the estuary by water currents. 
(2) Mobility patchiness and species interactions in a well-characterised 
stream community. Studies on communities of insect larvae in Broadstone 
Stream have continued for over twenty years. There are two main predators, a 
caddisfly Plectocnemia conspersa and an alderfly Sialis fuliginosa. An inter-
generation model was devised for the two predators and two species of their 
prey (stoneflies, Leuctra and Nemurella), the first time this has been achieved 
for a stream system (Project 11, Table 1). The results showed that there was 
little apparent interaction between the prey: the two stoneflies did not avoid 
one another to minimise "attack" by predators in high densities of prey. The 
model predicts that prey density was close to the environmental carrying 
capacity and that the predators have little impact on prey density. This 
prediction, which runs counter to the expectations of stream ecologists, is 
currently being tested directly by a large-scale field experiment, in which 
Sialis is removed from a section of the stream. 
Benefits of the programme on "Testable Models" Special Topic 
Although the findings are presented above in resume form, it is apparent that 
each of the projects has achieved a considerable measure of success in 
attaining its objectives and contributing an advance in knowledge and 
understanding that will lead to publication in the scientific literature; indeed, 
several papers have been published already. Our task here is simply to 
comment upon the execution of the Special Topic in its entirety and to 
50 STEPHEN MABERLY AND COLIN REYNOLDS 
consider the benefits that the "Testable Models" programme may have 
brought. 
One evident achievement has been to establish the value of models to the 
development of ecological thought. It is clear, for instance, that non-modellers 
or reluctant modellers among the awardees have gained confidence in their 
use and versatility, and now seek their further development. It is interesting to 
note also that several of the projects (notably 7 and 9) used their models to 
falsify and reject interpretative hypotheses. This is an important application of 
model philosophy to scientific method. 
In several cases (Projects 1, 3, 8, 11), models have been developed which 
simulate, with a high probability and an acceptable precision, the processes 
under consideration. These models did not always correspond to biological 
preconceptions. Hildrew (Project 11) showed that adult insects dispersed 
rather shorter distances from their home streams than was thought to be true, 
which carries implications for the dynamics of population recruitment and 
survivorship. There were also surprises in the extent to which predator 
performance, and thus predator exclusion, impacts on the behaviour of the 
prey populations. In the development of sandy bay communities (Project 7), 
the importance of ontogeny in governing the seasonally changing species 
interactions was powerfully demonstrated. This is an important factor that 
cannot be excluded from the understanding of food-web effects. 
A final benefit of modelling which became apparent was that gaps in 
information or understanding are highlighted by the rigorous approach of 
modelling. Thus, even in a well-studied stream, much information needed by 
the modellers had to be specially collected (Project 11). 
Conclusions 
Researchers habitually proclaim the benefits of their work to others in the 
same field. Measured against its own ambitious objectives, the Testable 
Models Special Topic has been highly successful in promoting the 
development and application of models, most particularly through the 
interplay between experimental ecologists and formal modellers. The scheme 
has crossed other interdisciplinary boundaries and facilitated active discussion 
and collaboration among freshwater and marine scientists. The progress it has 
achieved will be of interest to freshwater and marine biologists everywhere. 
However, perhaps most importantly, it has successfully proselytised the 
modelling cause within the participating population of scientists within the 
Special Topic and, hopefully, it eventually will do likewise throughout the 
wider community of aquatic biologists. 


Testable models of aquatic systems: A NERC special topic for integrating experiments and modelling

http://aquaticcommons.org/4623/1/SMaberly.pdf

Testable models of aquatic systems: A NERC special topic for integrating experiments and modelling

Abstract

Similar works

Full text

Available Versions

Aquatic Commons