Competition Flashcards
Liebig’s Law of the Minimum
- First proposed by Carl Sprengel
- Popularized by Justus von Liebig
- Growth is limited by the scarcest resource
- Limiting factors vary, may include space
- Ex: photosynthesis in an oligotrophic lake: even if all other nutrients are readily available, low levels of CO2 would limit photosynthesis and therefore primary production; photosthynthesis will increase if CO2 added, but only to the extent that it is added, unless something else become limiting
- When one factor is very limiting = competitive exclusion happens FAST
- When there is no one extremely limiting factor, organisms have time and energy to compete
Competition
- interactions between individuals (same OR different species)
- Negative effect on fitness
- Occurs when necessary resources are limited and must be shared (i.e when there is overlap in niche)
Ecological Niche
- Sum of a species use of biotic and abiotic resources in its environment
- role of a species in an ecosystem
- both spatial and temporal
- includes where it lives, what it eats, when it is active, the abiotic conditions it tolerates (temperature, salinity etc)
Niche dimensions
- One-dimension
- Two-dimensional
- reality; ecological niches have many dimensions
- The more dimensions you look at the less overlap there is
Fundamental Niche
- Niche that could theoretically be occupied in the absence of competition and predation
Realized Niche
- Actual occupied nice
Example of Fundamental vs. Realized Niche in orange-crowned Warbler vs. Virginia’s Warbler
- Two insectivorous wood warblers with overlapping home ranges and similar resource use
- Removal experiments: how does one do without the other?
- In absence of Orange-crowned: Virginia chose different nest sites (more maple stems), male Virginia’s fed incubating females more often; Parent Virginia’s fed nestlings more often
- In absence of Virginia: general nest predation rates were lower; more young survived long enough to fledge
- Conclude: Virginia warbler’s realized niche is NOT their fundamental niche; stronger competitor is orange-crowned; orange-crowned not limited by competition but maybe by predation
Types of competition
- Interference: direct interaction between individuals
- Exploitation - indirect: consumption of resources
- Apparent - indirect: for species that are preyed upon by same predator
- Intraspecific: between individuals of the same species
- Interspecific: between individuals of different species
Intraspecific interference competition
- individuals of the same species competing directly for resources such as food, mates, space
- ex: hartebeests competing for mates, barnacles competing for space
Interspecific interference competition
- Individuals of different species competing directly for resources such as food or space
- ex: lions and hyenas compete for food, corals of different species compete for space
Intraspecific exploitation competition
- use of resource by one individual makes it unavailable for use by another
- intraspecific when the individuals involved are of the same species
- not necessarily a fight; might just stake out territory and mark it with a scent
Interspecific exploitation competition
- use of resources by a species makes it unavailable for the others
- ex: hyenas eat carcass before the vultures can
- ex: competition for light and other resources
Intraspecific Apparent indirect (for species that are preyed upon by same predator)
- density-dependent predation or parasitism: the presence of more individuals of the same species results in increased predation or parasitism rates
- ex: lynx and hare; lynx is predator of snowshoe hare - higher rates of predation if there are more individuals; so hare predation increases with density
Interspecific apparent competition
- presence of one species results in increased parasitism or predation on another species
- ex: variegated leaf hopped (invasive): negative effect on native grape leafhopper; provides host for shared egg parasitoid; presence of variegated leafhopper increased parasitoids for BOTH them and native grape
Competition affects fitness
- fitness is the ability to reproduce successfully
- Intraspecific: ex: great tits; individuals removed to lower densities (reduce competition) = territory size increased; lower densities resulted in more eggs (greater fitness) SO competition affects fitness
- Interspecific: ex: warblers; both species of warblers have higher fitness when the other is absent
Competition affects evolution
- Principles of evolution by natural selection; variation, heritability, natural selection, survival of the fittest
- if heritable traits confers a competitive advantage, they confer a fitness advantage which affects evolution
- competition may be one of the main driving forces of evolution
Mechanisms of Competition - Consumptive competition
- one individual inhibits another by consuming a shared resource
- the most common mechanism in terrestrial environments
- exploitation: indirect
- can be behavioural, phenotypic, genetic
- ex: ants and rodents in same desert eat same seeds; exclusion studies = ant colonies up 71% in the absence of rodents; rodents up 18% in number and 24% in biomass in absence of ants
- usually intraspecific OR between similar species
- Behaviour ex: group of dogs sit and beg for treats, best of birds chirp and open mouth for food
Mechanisms of Competition - Pre-emptive
- Occurs when individuals occupy space
- when a physical resource is occupied by one organism and therefore unavailable to others, along with the resources it provides
- exploitation (mostly)
- most common mechanism of competition in marine systems
- Ex: barnacles; two species that live in stratified distribution in intertidal region observations; balanus more concentrated in lower and chthalamus more concentrated in upper; both have free swimming larvae and can settle anywhere on rocky shoreline so why don’t the two types grow together? So he removed from upper area and saw balanus didn’t replace; because could not survive are with so much desiccation; so Balanus’ fundamental and realized niche are the same; then he removed balanus from lower and saw that chthalamus replaced it (therefore more successful competitor) fundamental niche and realized niche for chthalamus is NOT the same
- Behavioural, genetic, phenotypic
- Behaviour: Ex; penguins maintain personal space by stretching out wings
Mechanisms of Competition - Overgrowth
- When one organism grows directly over another; depriving it of light (plants) or water-borne food (sessile aquatic organisms)
- Interference (direct)
- Ex: barnacles competing for space, zebra mussels in the great lakes, Kudzu
Mechanisms of Competition - Chemical
- Production and release of chemical toxins or growth inhibitors (allelopathy)
- Interference competition: direct
- Allelochemicals; influence growth, survival or reproduction of other organisms either in a positive or negative way
- When allelochemicals have negative influence – chemical competition
- Ex: interspecific; black walnut prevents plants from other species from growing near them
- Ex: intraspecific; bryophyllum young plants grow near old plants but they can’t put roots down too close to the parent because they won’t survive – parent produces growth inhibiting allelochemicals
- Allelopathy may be linked to successful invasion by exotic species
- Ex: diffuse knapweed (Eurasian invader) successful in part because native plants are not adapted to its allelochemicals
- Only occasionally recorded in animals; ex: monomorium ants excrete chemical substance that is irritating to competitors
Mechanisms of Competition - Territorial
- Aggressive behavioural exclusion of organisms from spatial territories
- Common among species (intraspecific)
- Also found between species (interspecific)
- Generally considered to be interference but could also be exploitation
- Common among terrestrial like songbirds - ex: bell miner; Aggressive social birds, eat sweet physillid insect secretions; Chase away all other birds; When removed, other birds move in and eat insects
Mechanisms of Competition - Encounter competition
- Non-territorial aggressive encounters between individuals that has negative effects on one or both
- Interference
- Common among animals, especially terrestrial
- Often competition for a mate
ex: ungulates battling
Sexual selection
- important driving force in evolution
- when competition is for mates, may lead to runaway evolution or sexual selection
Coexistence
- Two species may exploit the same resources at different times of the day or year (but still live in same place/territory)
- If competing species are affected differently by fluctuating environmental conditions, they may coexist
- Ex: bromus mollis does better than erodium botrys under normal conditions; but in drought erodium does better; Therefore if live in territory with periodic droughts, they can both coexist
Modes of coexistence -Niche Differentiation/Character Displacement
- Share resource need; have similar niche and competition is therefore common
- Species with similar resource uses may partition resources when together
- Also called niche or resource partitioning
- Ex: insectivorous birds living in coniferous forests forage at different heights; enables different species to coexist even though they are using the same types of resources – when not coexisting they may use more of the tree
- Character displacement is process where differences in genetically controlled traits emerge from these kind of selective forces
When occur together – sympatric; when doesn’t occur together – allopatric - Ex: Darwin’s finches - when they occur in allopatry the two species have similar beak sizes, when they cooccur in sympatry then their beak sizes differ from one another – allows them to exploit different parts of the niche
- Ex: two species of snails differ in size when they occur together; when they occur separately almost the same size
- Criteria: sympatric vs. allopatric character differences, characters must have known effect on resource use, must have genetic basis, competition must be demonstrated and must be correlated with character similarity, differences in character must not be due to differences in resource availability
- May lead to speciation; ex in Galapagos finches
Modes of coexistence - Competitive Networks
o No single species is a better competitor than all other competitor species; not one species that is dominant over all the others
o Simulation experiments show that complex networks could lea to various scenarios of coexistence
o The more limiting factors that there are, the more species that can coexist
o Empirical evidence: corrals overgrow each other when competing for space; if we took out one species one species would be outcompeted
Modes of coexistence - Compensatory mortality
o Greatest mortality falls on the dominant competitor
o Ex: tube worms and anemones competing for space; when neither species stressed tube worms dominant; in mid tide area anemones dominate because tube worms become exposed conditions which damage them (interspecific)
o Intraspecific: a justification for hunting?
- Hunting: top-down predation, or culling the dominant competitors
- Hunting large males, you remove some of the competition for resources and will improve overall health of population; increase reproductive rate and survivorship of remaining – may work for some ungulates
- Doesn’t seem to work for carnivores; ex: Cooley: low kitten survival for population exposed to hunting
- Why does it work for ungulates but not carnivores? Density of the population (ungulates usually more dense, so removing some frees up resources for the others) Density dependence
- Resource acquisition
Modes of coexistence - Spatial/Temporal changes in the environment
o The environment is not stable; different species or individuals dominate under different condition
o Ex: paradox of the plankton; how is it possible to have so many species coexist when they use similar or same resources
o Intermediate disturbance hypothesis: highest diversity occurs at intermediate levels of disturbance
- Frequency of disturbance, time since disturbance, size of disturbance
- Ex: long leaf pine ecosystems – dominated the south east and central US; when settlers came they cut down majority and replanted other types of pine (because long leaf pines are slow growing) – and these long leaf ones dominate; forest fires would disturb the area and long leaf pines are fire tolerant - use prescribed burns to keep them alive/dominant
- Intermediate disturbance hypothesis is supported by many but not all studies o Fugitive species: produce many small offspring capable of widespread dispersal that can colonize open areas
- In an area that is often disturbed, weaker competitors may do fine just because they get there first
- Ex: marine gastropods: Nerita (snail) is the competitive dominant over Celllana (limpet) but Nerita can never colonise all areas at all times – high variability in space and time of the recruitment of the larvae
Intraspecific example of spatial/temporal changes
- Research question: how are meadow vole populations affected by fragmentation?
STUDY: Enclosed experimental landscapes, released 6 pairs of voles onto each landscape (one just patch and one 4 fragmented), trapped within and between patches to measure vole densities throughout the breeding season - Saw more females in fragmented landscapes; why? The one patch makes more competition happen; because they’re all fighting in the same patch, but if they split up from patches they don’t compete as much
- Positive effect of fragmentation; reduction in intraspecific competition
what happens in the absence and presence of competition?
- In the absence of competition, individuals would presumably select the optimal niche - In the presence of competition, the niche exploited has to be broader
Biological invasions and competitive exclusion
o Natural; storms, debris
o Anthropogenic; commerce (ex: salmonids, goats), sport (ex: fishing, hunting), familiarity (ex: cats), biocontrol (ex: ladybird beetles), accidental (ex: zebra mussels)
The invasion process
- introduction
- establishment
- population growth
- dispersal
- Ex: Rates of species invasion in Hawaii
- Before human settlement happened 1 in every 300,000 years, then 1 in 30K ad in the past century 1 in every 3 weeks
