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Positive and negative interactions. I nterspecific competition. Competition  is an interaction between individuals of the same or of different species membership, in which the fitness of one is lowered by the presence of the other. Predation. Herbivory is a form of parasitism.

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Positive and negative interactions

Interspecific competition

Competition is an interaction between individuals of the same or of different species membership, in which the fitness of one is lowered by the presence of the other.


Herbivory is a form of parasitism

Symbiosis is any type of relationship where two individuals live together

Amensalism is a relationship between individuals where some individuals are inhibited and others are unaffected. 

Parasitoidism is a relationship between two individuals in which one member benefits while the other is not allowed to reproduce or to develop further

Parasitism is any  relationship between two individuals in which one member benefits while the other is harmed but not killed or not allowed to reproduce.

Commensalism is a relationship between two individuals where one benefits and the other is not significantly affected.

Mutualism is any relationship between two individuals of different species where both individuals benefit.

Mutualism is the way two organisms of different species exist in a relationship in which each individual benefits. Mutualism is the oposite to interspecific competition.

Client– service relationships

In plant succession early arriving plants pave the way for later arrviing by modifying soil condition.


Mutualism is often linked to co-evolutionary processes

Facilitation is a special form of commensalism and describes a temporal relationship between two or more species where one species benefits from the prior (and recent) presence of others.

Facilitation generally increases diversity.

Intraspecific competition

Canis lupus

Mytilus edulis

Scramble (exploitation, diffuse) is a type of competition in which limited resources within an habitat result in decreased survival rates for all competitors.

Contest (interference) competition is a form of competition where there is a winner and a loser 

Mate competition


The variance in distanceis much less than the meandistance

Territoriesimply a moreor less evendistribution of individuals in space


Territoriality is a form of avoidance of intraspecificcompetition


Home range


Home range

Home ranges might overlap

Density dependent regulation and diffuse competition

The stem self thinning rule

Leaf area L increases with plant density N

L=lNwhere L is the average leaf area per plant. This area and mean plant weight w increase with stem diameter by

l=aD2 and


Trees is a forst have certain distances to each others


The -3/2 self thinning rule

Modified from Osawa and Allen (1993)

Density dependent regulation of population size results from intraspecific competition

Density independence

Density dependence

Tribolium confusum

Data from Bellows 1981. J. Anim. Ecol. 50

Density dependence

Density independence

Vulpia fasciculata

Data from Ebert et al. 2000. Oecologia 122

Data from Allen 1972, R. Int. Whaling Comm. 22.

Salmo trutta

Density dependence

Peak reproduction at intermediate densityy

Density independence



First order order recursive function of density dependent population growth

Nicholson and Baily model




Georgii Frantsevich Gause (1910-1986)

Competitive exclusion principle

In homogeneous stable environments competitive dominant species attain monodominancy.

Paramecium caudatum

Paramecium aurelia

Joint occurrence

Data from Gause 1943, The Struggle for Existence

Applying this principle to bacterial growth Gause found a number of antibiotics

Interspecific competition

Tribolium castaneum

Tribolium confusum

Data from Park 1954. Phys. Zool. 27.

Two species of the rice beetle Tribolium grown together compete differently in dependence on microclimatic conditions.

The Lotka – Volterra model of interspecific competition

Alfred James Lotka (1880-1949)

Vito Volterra (1860-1940)

At equilibrium: dN/dt = 0

Certain conditions allow for coestistence

If carrying capacity differs one species vanishes

If competitive strength differs one species vanishes

The Lotka Volterra model predicts competitive exclusion

But the oberserved species richness is much higher than predicted by the model.

The model needs

stable reproductive rates

stable carrying capacities

stable competition coefficients

Grassland are highly diverse of potentially competing plants

It needs also homogeneous environments

Randomy fluctuating values of r, K, a, and b.

a > b

K1 > K2

Unpredictability and changing environmental conditions as well as habitat heterogeneity and aggregation of individuals promote coexistence of many species.

Competition for enemy free space (apparent competition)

Ephestia kuehniella

Plodia interpunctella

Venturia canescens


Data from Bonsall and Hassell 1997, Nature 388

Predator mediated competition might cause extinction of the weaker prey

Character displacement and competitive release

Chalcosoma caucasus

Interspecific competition might cause species to differ more in phenotype at where where they co-occur than at sites where they do not co-occur (characterdisplacement)

Rhinoceros beetles

Chalcosoma atlas

Interspecific competition might cause a lower phenotypic orecologicalvariability of two species at sites where both species compete.

Competitivereleaseis the expansion of speciesniches in the absence of interspecificcompetitors.



Bodey et al. 2009. Biol.Lett 5: 617

Raven + Crows




Canada lynx and snowshoe hare

Specialist predator

Generalist predator




Trade-offs in foraging

Maximum yield

Animals shouldadopt a strategy to maximuzeyield


Stopping point





Predictedenergyintake from travel and handlingtime

Great titsforageatsite of differentquality

How longshould a birdvisiteachsite to haveoptimalyield?



Predictedenergyintake from traveltime






Parus major




Cowie 1977

Specialist predators and the respective prey often show cyclic population variability

12 year cycle

Cycles of the predator follow that of the prey

Cycles might be triggered by the internal dynamics of the predator – prey interactions or by external clocks that is environmental factors of regular appeareance

Canada lynx and snowshoe hare

Hudson’s Bay Company

Data from MacLulick 1937, Univ. Toronto Studies, Biol. Series 43

Bracyonus calyciflorus

Chlorella vulgaris

Most important are regular climatic variations like El Nino, La Nina, NAO.

Data from Yoshida et al. 2003, Nature 424

The Lotka Volterra approach to specialist predators

e: mortality rate of the predator

r: reproductive rate of the prey

faN: reproductive rate of the predator

f: predator efficieny

aP: mortality rate of the prey

a: attack rate

The equilibrium abundances of prey and predator

In nature most predator prey relationships are more or less stable.

  • Any deviation from the assumption of the Lotka Volterra model tends to stabilize population:
  • Prey aggregration
  • Density dependent consumption
  • Functional responses

The Lotka Volterra models predicts unstable delayed density dependent cycling of populations

Environmental heterogeneity and predator prey cycles

Typhlodromus occidentalis

Eotetranychus sexmaculatus

Simple unstructured environment

Heterogeneous environment

Habitat heterogeneity provides prey refuges and stabilizes predator and prey populations

Functional response

Type II Holling response

Type III Holling response

Type I response

Microplitis croceipes

Calliphora vomitoria

Predator attak rates are not constant as in the Lotka Volterra model

Microplitis croceipes

Calliphora vomitoria

Variability, chaos and predator prey fluctuations

Lotka Volterra cycles with fixed parameters a, e, f, r.

Lotka Volterra cycles with randomly fluctuating parameters a, e, f, r.

Stochasticity tends to stabilize populations

Dynamic equilibrium

Any factor that provides not too extreme variability into parameters of the predator prey interaction tends to stabilize populations.

Fixed parameter values cause fast extinction.


Plant defensesagainstherbivors

Many plants produce secondary metabolites, known as allelochemicals, that influence the behavior, growth, or survival of herbivores. These chemical defenses can act as repellents or toxins to herbivores, or reduce plant digestibility.

Alcaloide (aminoacidderivatives):

nicotine, caffeine, morphine, colchicine, ergolines, strychnine, and quinine

Terpenoide, Flavonoids, Tannins

Mechanicaldefenses: thorns, trichomes…


Mutualism: Ant attendance, spiderattendance


Functions of herbivores in coral reefs

Negative feedback loops occur when grazing is too low

Positive feedback loops occur when grazing is high

Herbivorous fish (Diadema)

Reduced structural complexity

Decreasing fish recruitment

Increased structural complexity

Increasing fish recruitment

Low coral cover

Low grazing intensity

High coral cover

High grazing intensity

Overfishing of herbivorous fish might cause a shift to algal dominated low divesity communities

Decreasing coral recruitment

Increasing algal cover

Increasing coral recruitment

Decreasing algal cover

Hay and Rasher (2010)