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

Positive and negative interactions

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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. - PowerPoint PPT Presentation

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Positive and negative interactions


Interspecific competition

Herbivory is a form of parasitism

Competitionis aninteractionbetweenindividuals of the same or of different species membership, in which thefitnessof one is lowered by the presence of the other.

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

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.

Mutualismis any relationship between two individuals of differentspecieswhere both individuals benefit.

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

Symbiosis is any type of relationship where two individuals live together

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

Mutualismis 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


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.

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

Intraspecific competition

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

Mytilus edulis

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

Canis lupus

Mate competition


Territories imply a more or less even distribution of individuals in space

Territoriality is a form of avoidance of intraspecific competition



Home range

Home range


Home ranges might overlap

The variance in distance is much less than the mean distance

The stem self thinning rule

Trees is a forst have certain distances to each others

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



Modified from Osawa and Allen (1993)

Density dependent regulation and diffuse competition

The -3/2 self thinning rule

Density independence

Density dependence

Density dependent regulation of population size results from intraspecific competition

Vulpia fasciculata

Density independence

Density dependence

Tribolium confusum

Data from Ebert et al. 2000. Oecologia 122

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

Density independence

Density dependence






First order order recursive function of density dependent population growth

Nicholson and Baily model

Salmo trutta

Peak reproduction at intermediate densityy

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

Competitive exclusion principle

Georgii Frantsevich Gause (1910-1986)

In homogeneous stable environments competitive dominant species attain monodominancy.

Paramecium aurelia

Paramecium caudatum

Joint occurrence

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

Data from Gause 1943, The Struggle for Existence

Interspecific competition

Tribolium confusum

Tribolium castaneum

TemperatureHumidityPercentage winsTribolium confusumTribolium castaneumHotMoist0100TemperateMoist1486ColdMoist7129HotDry9010TemperateDry8713ColdDry1000

Data from Park 1954. Phys. Zool. 27.

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

Alfred James Lotka (1880-1949)

Vito Volterra (1860-1940)

The Lotka Volterra model of interspecific competition

At equilibrium: dN/dt = 0

If competitive strength differs one species vanishes

If carrying capacity differs one species vanishes

Certain conditions allow for coestistence

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

It needs also homogeneous environments

a > b

K1 > K2

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

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

Grassland are highly diverse of potentially competing plants

Competition for enemy free space (apparent competition)

Data from Bonsall and Hassell 1997, Nature 388

Plodia interpunctella

Venturia canescens

Ephestia kuehniella

Predator mediated competition might cause extinction of the weaker prey


Character displacement and competitive release

Rhinoceros beetles

Chalcosoma caucasus

Chalcosoma atlas

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 (character displacement)

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

Competitive release is the expansion of species niches in the absence of interspecific competitors.


Raven + Crows

Dietary width

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



Erigone atra

Generalist predator


Specialist predator


Canada lynx and snowshoe hare


Trade-offs in foraging

Searching time

Prey quality


Maximum yield

Stopping point

Animals should adopt a strategy to maximuze yield

Optimal foraging theory











Parus major

Great tits forage at site of different quality

How long should a bird visit each site to have optimal yield?

Predicted energy intake from travel time

Predicted energy intake from travel and handling time

Hollings optimal foraging theory

Cowie 1977

Hudsons Bay Company

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

Data from Yoshida et al. 2003, Nature 424

Specialist predators and the respective prey often show cyclic population variability

12 year cycle

Canada lynx and snowshoe hare

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

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

Bracyonus calyciflorus

Chlorella vulgaris

The Lotka Volterra approach to specialist predators

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

The equilibrium abundances of prey and predator

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

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

Environmental heterogeneity and predator prey cycles

Eotetranychus sexmaculatus

Typhlodromus occidentalis

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

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

Calliphora vomitoria

Microplitis croceipes

Type I response

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.

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.

Stochasticity tends to stabilize populations

Dynamic equilibrium


Feeding StrategyDietExampleFrugivoresFruitRuffed lemursFolivoresLeavesKoalasNectarivoresNectarHummingbirdsGranivoresSeedsHawaiian HoneycreepersPalynivoresPollenBeesMucivoresPlant fluids, i.e. sapAphidsXylophagesWoodTermites

Plant defenses against herbivors

Alcaloide (amino acid derivatives):

nicotine,caffeine,morphine,colchicine,ergolines,strychnine, andquinine

Terpenoide, Flavonoids, Tannins

Mechanical defenses: thorns, trichomes


Mutualism: Ant attendance, spider attendance


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

Functions of herbivores in coral reefs

Negative feedback loops occur when grazing is too low

Increasing algal cover

Decreasing coral recruitment

Low coral cover

Low grazing intensity

Decreasing fish recruitment

Reduced structural complexity

Positive feedback loops occur when grazing is high

Decreasing algal cover

Increasing coral recruitment

High coral cover

High grazing intensity

Increasing fish recruitment

Increased structural complexity

Herbivorous fish (Diadema)

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

Hay and Rasher (2010)