Copyright  © January, 2003, D, GRIFFITHS, ALL RIGHTS RESERVED


Duncan Griffiths

Competitive exclusion (refered to here in as“CE”) is nothing new by any means, it exists all around us, it occurs in bacteria, fungi, plants and both lower and higher animals, even in commerce. CE exists everywhere every second of the day. Without it evolution would not be as advanced as it is today nor would we have so many different species inhabiting the planet.

The historic extinction of the Dinosaur's and the fact that many evolved and took to the oceans and the sky's and formed populations of birds and fishes, was as direct result of CE

Darwin spoke of CE as long ago as 1859 when he studied finches on the Galapagos Islands, in his theory “natural selection”, as one of the basic fundamentals behind the survival and evolution of the species…

Competitive Exclusion is a new tool for the koi hobbyist in helping to eliminate pathogenic bacteria from the pond eco system, and help keep our koi from harmful ulcers and infections, by the very use of such a tool we will also be excluding useful bacteria and organism's from the water column, this may be brewing a storm for potential disaster, remember, as smart as man likes to think himself in out manoeuvring Pathogens, to date humans as the *ultimate* Competitor, are at best holding our own and at worst losing the direct competitive fight with bacteria and virus.

We use antibacterial chemicals to fight bacteria and they become immune and stronger, we use antibiotics and bacteria learn real fast how to defeat this threat and again become stronger, its not unreasonable to also assume that CE will become useless as bacteria learn to deal with this threat.

Below is theory based on scientific fact on just how bacteria organism's can out manoeuvre CE when CE threatens an organism’s way of life, in more ways than you would imagine possible, but then again although CE is new to the koi hobby it is in fact as old as time itself, and far from being a new concept has been responsible for so many species living on the planet today.

It’s a fact that CE works for some hobbyists, but it’s also a fact that for some it is the start of a disaster. I realize that the article contained here in will be viewed at best controversial, but I would ask you to just keep an open mind when reading it, look at the bigger picture, and try to imagine what happens when you exclude an organism, because sure as eggs are eggs CE is not long term target specific and you will not only exclude some pathogens but also essential friendly organisms as well, its a fine Balance that holds any eco system together.

We (Humans) as a species are competing with nature by felling huge rain forests, it all seems harmless enough, but in the process complete organisms both micro and otherwise are becoming extinct and eco systems are falling apart, and the knock on effects occurs not only in the immediate vicinity of the tree losses but the failures in the eco system due to such losses can be felt thousands of miles away, Net results are a loss of vegetation and therefore animal habitats , increased desertification, global warming, and the loss of eco-organism’s and natural predation, and a complete break down of the eco systems.

These are all forms COMPETITIVE EXCLUSION.




For those who are not aware of the CE theory, CE works like this:

Imagine on an African plain roams a strong herd of grazing animals, these animals exist on this plain by natural events I.E. ample resource to support an unknown stable population. A plague of locusts descends on the plain and in no time at all eat all the grazing pastures, in this event there is no resource left for the herd and they simply die out, or migrate to another un-utilised resource.

When this principle is applied to the pond eco system, its easy to see that we could substitute the grazing herd and

call these the pathogenic bacteria in the pond, and our bottled CE application is represented by the locust, the only difference is our bottled bugs/locusts once they have eliminated the resources cannot take flight to a new area and start over, so in the instance of the pond eco system those bugs die as well.

So for the koi hobbyist its easy to see that if you have harmful pathogenic bacteria in residence in the pond eco system, and you want to discount their numbers you can, by the use of CE, saturate the system with so called harmless bacteria that compete with the pathogenic bacteria  for the limited food resource that’s available, in so doing both species scavenge the system for food till the food is exhausted then both organisms die out( in theory), you may be thinking well what happens now that your harmless bacteria in the CE application have died out?, no problem because we will be placing a weekly maintenance dose of CE in the system to mop up any newly formed food resource that has been made available from the waste by-products from the fish, thus we never allow this food source to build up to support any significant populations of pathogenic bacteria.

but remember CE is not target specific, in so much as ce will not target filter bacteria because they utilise i different foos source CE will target almost any other bacteria in th epond be they good or bad.

And here in may lie the problem. In essence this is a over simplified explanation of CE.


Competitive exclusion Terms




The negative effects which one organism has upon another by consuming or controlling access to a resource that has limited availability.  {Keddy 1989}

The flow of energy through Trophic levels follows the second law of thermodynamics, there is less and less energy available to each successive Trophic level, resulting in the classic Trophic level pyramid. 

Important! This decline is NOT evidence that energy is lacking or not available.

Competition puts a premium on efficiency this assumes a division or labour amongst a population and is the ultimate reason for
the amount of species we have at the present day.






If competitors are very similar in their resource needs they may not be able to co-exist together, one or both may become
extinct as a result of the competitive reaction = CE principle = complete competitors cannot co-exist

Ecologically similar species *can* co-exist if there are 1 or more major differences in their ecological niches
CE says that two or more species cannot co-exist on a single limited resource in the same community " if there is no difference between the realized niches of the two competing species, or if such differentiation is precluded by the limitations of the habitat then one species will eliminate and exclude the other" {Begon and Mortimer 1986}

One species will use resource faster and more efficient than the other, thus reproduction is more rapid, helping to eliminate the inferior competitor, this model was derived by  “Lotka a Volterra” who modified the logistic CE to incorporate population
growth, as a result of inter-specific competition (G.F Gause) confirmed CE in the following experiment.    

Two species of Paramecium were grown together although they both proliferated when they occupied their own space and therefore food source, when grown together P, Caudium was out competed and eliminated by P. Aurelia





Inter-specific competition occurs when two or more species rely on the same or similar limited resources, in this case they can have a negative impact on one another, this may be in the form of interference competition {fighting} or exploitative competition {consumption or use} as the population density of one species increases it may limit the other species population.

Further with inter-specific competition the fitness and virility of an organism is negatively effected Therefore natural selection will favour the organism that can escape these conditions and resource elsewhere.


Typical example, A pack of wild dogs hunt and kill their prey for food for the pack; if present at the time of the kill stronger predators will move in on the kill and the dogs are driven off and are forced to watch and wait to see what's left, when the stronger predator has eaten its fill it will move away and allow the original hunters back to the feed, provided there are no more hunters around higher up the Trophic level pyramid the dogs will be left the scraps of their own kill, if there is another predator which is on a par with their Trophic level they will fight it out for the scraps left.

So nature has a predetermined Trophic level pyramids, an organisms position in the pyramid is dependant on its competitive abilities therefore, dependant on what animals are present at the dogs initial kill and at what level they sit in the pyramid directly influences at what point in time the dogs will be allowed back into the food chain {if ever} even if they did make the original kill.



A case study that the koi hobbyist can relate to is; it has been known for some time that competitive exclusion exists in fish parasites in particular, Dactylogyrus. 

D. anchoratus and D. extensus are prime examples, when both subspecies try to inhabit the same host it was found that when low aggregates of both species were found co-existence was just possible but when high aggregates were present,  D. extensus  was the superior competitor and would exclude D. anchoratus. Making co-existence between the two species impossible in
high numbers.


A. Ozer

Ondokuzmayis Üniversitesi, Su Ürünleri Fakültesi, 57000 Sinop, Turkey.


Co-existence between potentially competing species, Dactylogyrus anchoratus and D. extensus parasitising common carp (Cyprinus carpio L) from the two localities in northern Turkey has been studied. The distribution of these two dactylogyrids on mirror and scaled varieties of common carp according to season and the sex of the host at two environmentally different sampling stations is presented. Dactylogyrus anchoratus was recorded with an infestation prevalence of 21.79% at Çobanlar (farm) and 34.04% at Bektaºaga (lake), whilst D. extensus,
the dominant species, was recorded with infestation prevalences of 98.71% at Çobanlar (farm) and 85.1% at Bektaºaga (lake). A possible competitive exclusion by Dactylogyrus extensus over D. anchoratus was determined.





Intra-specific competition leads to stabile populations in that dependence of birth and death rates result in the population approaching its carrying capacity because eventual birth rates equals death rates. Limiting factors include water, space, light and food

Growth rate is slow at first due to a limited number of individuals, this is called the Lag time.

As individuals become sexually mature the population increases as the environment resource are exploited called the Exponential phase.

Carrying capacity is reached due to limiting factors such as food , space, light and water which results in death rates equalling
birth rates called the Stable phase

A shortage of, food, water, Light or space causes the carrying capacity to decrease the is called the Death phase where death rates exceed birth rates

The Carrying Capacity is the maximum population size an environment can support an environment that reduces birth rate is called Environmental Resistance factors such as competition disease and predation contribute to this.



An example of intra-specific competition is in a pride of Lions, the pride will only support one male, if two males are present they will fight sometimes to the death or until one withdraws, this is based on pure genetics and food resource for the pride I.E. the stronger lion survives to pass on his superior genes to the offspring/cubs, ensuring his genetics survive and ultimately a genetically superior Pride, and there is not enough food to support more than the carrying capacity of the pride for the environment. 

This particular form of competition has nothing emiediatly to do with food availability although it does have a direct baring on it,
as the female will provide all food for both the cubs and the male, this competition is primarily  to do with the male's need to
sow his seed and ensure the survival of his genes, so much so that if a male outsider challenges an older male in a pride and
either drives him off or kills him, the new more dominant male will kill all the cubs of the previous male, and destroy all genetic evidence of him and his offspring. The female after realising her cubs are gone will come into season and be ready to mate
again in about three days, and she is happy to mate with the murderer of her young because she realises the new male has
superior genetics.

Another example of this directly linked to the koi hobby would be in the Biological pond filtration. A pond filter in an ideal world would be at its biological carrying capacity I.E. the filter is colonised with bacteria to the amount of organic waste a population
of fish produce, (Intra-Specific competition stable phase). If the fish population living in the eco system increases and therefore the amount of organic waste in the water the filter population of bacteria will grow in response, (no competition Exponential phase) however, if the population of fish decreases and therefore the amount of organic waste decreases the population of filter bacteria will decrease in direct response. (Direct competition Environmental resistance).




Increased evidence of morphological differences of an organism between species has taken place when in direct competition in
the same habitat. 




Resource partitioning is the division, either artificially or naturally, of environmental resources by co-existing species so that their particular niche differs by one or more significant factors.

E.g., Both species access the same food source but one species favours a portion of the source that differs significantly, either chemically or environmentally, to that favoured by the other species.



Over evolutionary time, competition among different species may even lead to a shift by one (or both) species into a slightly different niche space.

Differing subspecies of Dactylogyrus  will co-exist inter-specifically in the gill of the host, in other words differing species of Dactylogyrus can only exist by each sub species partitioning  the gill and favouring a slightly different location of the gill


SIMKOVA A, GELNAR M, SASAL P. Aggregation of congeneric parasites (Monogenea: Dactylogyrus) among gill microhabitats within one host species (Rutilus rutilus L.). Parasitology 2001 123 : 599-607.

Parasite aggregation within the gill arches of 1 host species (Rutilus rutilus L.) was studied in the case of 9 congeneric monogeneans belonging to the genus Dactylogyrus. Both intra- and inter-specific aggregation, were followed by considering gill arches as independent patches. Parasite species were found to be aggregated both intra- and inter-specifically. We showed that the intensity of parasite infection was positively related to intra-specific aggregation only
for the more abundant species. No relationship was found between intensity of parasite infection and niche size when correcting for total parasite intensity. This may suggest that when parasite intensities are low, intra-specific
competition may not have a strong effect on parasites. Conversely, when looking at the evolution of niche size and
intra-specific aggregation for each species separately, intra-specific aggregation decreased for the most abundant
species (D. crucifer) suggesting that this species is more competitive than others when total parasite intensity increases. When considering inter-specific aggregation, Dactylogyrus species were found to be positively and negatively aggregated. Following the prediction of morphological and ecological similarity for Congeneric species, Dactylogyrus species similar in morphometry of attachment parts tend to be more positively aggregated among their gill
microhabitats than Dactylogyrus species with morphometric differences in attachment parts




When a resource that two organisms compete for becomes depleted, one organism will tend to emigrate to find another
unutilised resource in response to the difficult competition.




CE principle presumes that one of the above situations is present and true, hence one species will always win.  However, a number of extenuating circumstances make CE laws useless!  Because examples of excluded organisms are lost {ghosts of competition past} these are the organism’s that were not accounted for and probably, in the case of the pond eco-system,
many bacterial species are unknown but hold the eco-system together by their very presence and occupation. The demands of these largely unknown and unmapped bacteria on an eco-system result in the completion of the eco-balance.

 In a pond’s eco system, CE is part of everyday life, as pond organisms compete with one another, this maintains the ecological pond balance.



This is what Dr David Moriarty of the university of Queensland wrote in an article on shrimp farming:


“Many pathogens appear to have mutated to more virulent forms than were present a decade ago, thus aquatic
farming comes under attack even if poor water conditions are present or not”

No surprises here.


“In Thailand this year a shrimp farmer who was using colloidal silver in all feed stuffs experienced massive mortalities from vibrio species this was managed by the mass introduction of Norfloxacin in all feed, this managed the problem
very well however when all microbials stopped, all shrimp died from vibriosis in two days, it appeared in this short
length of time a highly virulent strain of luminous vibrio sp had developed in response to the excessive use of silver and antibiotics”.



“In Thailand its common practise to cleanse the ponds which are sometimes one hectare in area, with chlorine, prior to re-emptying and refilling ready for the introduction of shrimp stock

It was found that just prior to reintroduction of shrimp stock there is a massive resurgence in numbers of vibrio harveyi just after the chlorine was removed.

It’s thought that the chlorine reduces the numbers of direct and indirect competition to such an extent that they are able to increase in numbers rapidly also these were found to be more pathogenic in nature”.


This demonstrates when an organism’s competition is excluded how fast the surviving bacteria and possibly the pathogenic bacteria, gain ground in the eco system



“If antibiotics or antimicrobial compounds are used to reduce counts of bacteria some bacteria will survive, these will usually be found to increase in virility and numbers because they have the right genetics for survival, some of these may have escaped exclusion because they were present in a bio film in a pipe, in a filter, or on a fish or even in the gut of the fish, gram negative bacteria can and will exchanges genetics across the species and evolve rapidly”


There are several interesting observations here, not least is the ease with which bacteria can escape the exclusion process and the most virulent can exchange genetics across to other non related species. And no surprise the
bacteria with the best genetics will ultimately be the best survivors.


“Resistance plasmids encoding for many antibiotic resistant genes were transferred   between pathogenic and non pathogenic bacteria in several environments including sea water, the rate of gene transfer between vibrio cholerae and Aeromonas salmonicida increased 100 times

This work raises questions not only about the use of antibiotics in aquaculture but the use of closely related pathogenic species as probiotics. Not only antimicrobial resistant genes, but also genes for virulence can be transferred by R plasmids and transposons. As the R plasmids can transfer genes between widely different bacteria in the gram negative spectrum, it was though to be extremely unwise to use Vibrio or pseudomonas for example as probiotics”.


Interestingly we know certain CE products have pseudomonas present!


And finally


“Species composition of a microbial community, such as in a pond will be determined partly by stochastic phenomena, that is (pure chance) and partly by deterministic and predictable factors that allow one species to grow and divide more rapidly than others, and thus dominate numerically. Chance favours those organisms that are in the right place at the right time to respond to a sudden increase in nutrients e.g. from the lysis of algal cells or the decaying of waste or unwanted food.We are still a long way away from knowing all the factors that influence the process of competitive exclusion and the controlling factors in the process of numeric growth rates of bacteria undergoing competitive
exclusion because the complete species composition that make up a natural environment in a pond eco-system are still
to a greater extent Unknown”.

 The last statement reinforces the fact that when competitive exclusion in use we have absolutely no idea exactly
how many different organisms we are excluding from a pond eco-system. From a hobby stand point we only care as long we can influence Aeromonas and pseudomonas count. This fact is not in dispute, as we know we can influence these two species, but even significant reductions in these two species can give a false sense of security.


 Effects of Competition on Communities:

On an ecological time scale (i.e. within one generation of organisms), competition will often decrease biodiversity. This is
because if two organisms are both competing for the same resource, the stronger competitor will often win out (and drive the other population to very low numbers - or even to zero). This phenomenon is sometimes called the CE principle. It can be illustrated with an example:

Two species of paramecium (paramecium are single celled organisms) are grown together in a well-stirred flask that is regularly restocked with nutrient media. If we follow the growth of these two populations over time, we will generally see something like the following pattern:

The population of species one starts off growing faster, but eventually species 2 (the better competitor) takes over, and out competes species 1 - driving its population to zero.

A mathematical model that describes the interaction shown in the graph above is given below.
This is called the Latka-Volterra model (named after two ecologists). It starts with the familiar equation for logistic growth:

And then adds to that by including terms that account for the competitive effect of species one on species two (and vice versa). This is in the form of two equations, given below.

Most of the symbols here are ones we are familiar with (N, r, t, and K). The alpha symbol represents the effect of competition.
In the first equation, this is the effect of species two on species one. In the second equation, alpha represents the effect of species one on species two.

So, if the CE principle says that when two species compete that one will perish - why do we see so many species in nature that seem to co-exist? This is because the natural world has more than one niche (a niche is set of resources that an organism requires and utilizes to survive). Remember the stirred flask mentioned above? That was an artificial environment that only contained one functional niche - there was only one food source, and it was well mixed all the time. Natural environments are much more heterogeneous, thus providing more potential niches for different species to exploit. In fact, the resource availability in an environment (i.e. its "niche space") is sometimes called an "n-dimensional hyper-volume" - that is a "space"
of not 2 or 3 dimensions, but of some indeterminate number ("n") - there are many axes of this "hyper-volume" and each of these axes may represent a different resource.

Fact, Conjecture or Theories? Take your pick! 



We know for some koi keepers CE is successful in driving pathogens way down (again no argument) but we know for some it has failed to exclude pathogens sufficiently to eliminate disease or aid healing and, for some, it has apparently had disastrous consequences (showing an actual increase in disease).  So for those that may of experienced poor or detrimental results when using CE, Below is some theory based on this same science that may help in some way to explain why it sometimes does not work.


If we think back to the locust example, in the pond eco system we now have to factor the following extenuating circumstances into the equation.


The introduction of CE could result in natural and essential Bacteria in the fishes gut being out competed, which results in increased food passage as the bacterial competition increases the need for nutrients. Net result not only would this have a negative impact on essential bacteria, the total population of bacteria would compete with the host (fish) for nutrients from the food it eats( as the food source we are trying to eliminate are in the main carbon based) , further the reduced number of natural essential bacteria would result in reduced production of bacterial by-products essential to optimum intestinal function; is reduced and caan lead to a reduced immune system.


It’s common knowledge that bacteria can be out competed in the water column but there is a possibility they cannot be out competed in the mucus layer, because of the biological make up of the mucus layer results in a food source in the mucus and this is constantly being replenished I.E. sloughing off of dead host cells, whilst its generally agreed that the mucus layer in an aquatic Teleosts incorporates natural immune antibodies these are a general antibody of antigens found commonly occurring in nature and are not as a result of previously encountered pathogenic antigens, they will inhibit but not eliminate all bacteria


The most virulent strains, are amongst the strongest competitors and are most likely to be the main survivors of CE only now we have upset the eco- balance, we have eliminated the competition!  {The ghosts of CE}


When using CE for the first time, some hobbyists have taken base line readings of Aeromonas and Pseudomonas, e.g., a reading for a typical pre treated pond might be something like 150,000 CFUs (colony forming units) per cc of water. This would be pathogenic Aeromonas and Pseudomonas combined.  But, we usually don’t check for the presence of other organisms or bacteria in the eco-system, or even what variety and strains of anaerobes (Yes Anaerobes) that is present in any typical CE application that we are about to use. In truth in a typical pond eco-system, we do not know what all the beneficial organisms that are present, because they are simply not all mapped.

So as previously mentioned after a typical treatment, we have absolutely no idea what the GHOSTS OF CE were!  Why? Because we never check! We send our after sample off to the lab and only check for Aeromonas and Pseudomonas.  And, its no secret or surprise even to sceptics that we can significantly reduced these counts to say sub 25000 CFUs per cc. But we never know by how much we have reduced the numeric count of other species of bacterial (or their types) that are essential in keeping the eco system balanced and therefore go some way towards holding Aeromonas and Pseudomonas in check.  Remember this is all part of natural competition. Further, we don’t know how much of the so-called useful bacteria in the CE application is left. We assume none.

Neither do we know how much, if at all, we have diminished the essential bacteria of the gut.  The maker’s of the CE products only claim is that nitrifying bacteria are unaffected. I would say that some important questions are unanswered?


If the numeric counts of these later bacteria of the non-pathogenic kind is a figure near zero, we could have created an ecological problem in the pond.  A proposition not so unreasonable given the opinions above by scientists that pathogenic bacteria are given to such virulence that they can out compete out grow and out number most non-pathogenic bacteria given even half a chance  Therefore, this would seem to be an acceptable thesis based on current available science. 
We do know that most pathogenic bacteria favour differing sites and forms of attack, so it follows that they cannot all occupy the same niche in the eco-system, this is of particular interest as the principles of CE Says two organisms cannot co-exist in the same niche! And presumes that CE competes with pathogens in one Niche or another not both? Which I would presume would tip a so-called level playing field in favour of harmless bacteria or pathogenic bacteria as far as CE is concerned.  

Assuming we have by the use of CE significantly reduced a typical numeric count of Aeromonas and pseudomonas to a figure of sub 25000 CFUs.  Adding to this we know we probably cannot influence the numeric count in mucus coat of the fish or the filters or the walls of the pond or the fish.. The final hypothetical tested numeric figure of 25000 cfu’s per cc of water, would be for Aeromonas and Pseudomonas and would be obtained purely from the water column because that’s what we ask of the laboratory. This figure would not be obtained from the mucus layer of the fish the filter media or pipes or pond walls, which could possibly multiply that figure by factor of who knows? Only now we have now eliminated the competition for these pathogenic bacteria?



The above does not figure into the equation such occurrences as quality control or pathogenic bacteria being contained in the bottle and therefore adding to the pathogenic load in the eco- system and/or storage problems! Or even dare I say, some form of CE taking place right there in the bottle, leaving the contents minus some or most of the species it should have contained, after all if you buy into the fact that CE is a capable application you also have to buy into the fact CE *CAN* take place within the stored species of bacteria in the bottle if left for some time and not in the right environmental conditions I.E refrigerated or with the top loose? 


I will leave it here as this was one of those projects that once I got into the research it found it was a huge-massive subject, it was like looking into a hole and the more you looked the bigger the hole got,

But I hope it has stimulated your interest and interested you enough that you would like to know more, to say this subject is big is an understatement. But I will leave it with these final few words,


Nature abhors a vacuum.  If a resource is un-utilized or under utilized, natural events usually take place to bring the situation back into "balance." We sometimes may not like nature’s way of balancing out the equation.