Community Assembly Flashcards
lecture 4/5
What is community assembly?
Species present in a local community is a subset of species that exist in broader species pools.
Global -> continent/regional -> biome/habitat -> local ecological community
What is a regional species pool?
The set of species present in a region from which local communities are assembled.
e.g. - all the plants in South Africa
Where did this theory originate from?
Theory of island biogeography - MacArthur and Wilson 1963.
Ultimately led to the filtering of this process of regional species pools.
Whether species will persist or not on the island - immigration and extinction balance.
close islands have high immigration levels, and far-away islands have low immigration rates.
large islands have more species with low extinction rates; small islands have fewer species with high extinction rates.
Each island is a regional pool with a subset of habitat pools
What is the historical influence on regional biota?
Natural occurring barriers that separate species over long periods of time.
biogeographic barriers decreasing immigration rates.
Wallace line
Floral Kingdoms (Good 1947)
Zoogeographic realms (Holt et al 2013 Science)
How are regional pools determined/origins?
Biogeographic filters (keep the global diversity …-> Immigration / Emigration -not as important
Speciation/extinction.
Examples of regional pool origins
Australian flora (Crisp and Cook 2013)
Galapagos birds (Valente et al 2015)
What are biogeographic filters?
Deep ocean trenches
e.g., The great American Interchange following the closing of the Isthmus of Panama.
South American species pool was homogenized when species from North America migrated down.
What are environmental barriers?
different environments that border on each other.
Species don’t occur everywhere.
e.g. Tree lines - trees drop out at some altitude in the system.
What is an ecological niche? First definition
John Grinnell
The Grinnellian niche = “ …. the ultimate distributional unit, within which each species is held by its structural and instinctive limitations” (1924)
Every species has a physiological, morphological, and behavioral profile that makes it suitable to occupy a particular space offered by nature.
NICHE = part of the environment that a species can occupy
Regional habitat species pool
Different biomes = different climates, different
abiotic drivers, different vegetation contexts
finer habitat difference than biome differences:
Jonkershoek & cederberg
Type 1 and Type 2 curve graph
slide 23 lecture 4/5
What is a species pool?
Set of species available to colonize a local community.
Originates through evolutionary/biogeographical processes and adaptation to macro-habitats.
How does a community get assembled from the species pool?
Observation: not all species present in the species pool are present in each community
Implication: Local processes are important for community assembly:
Biotic interactions:
* COMPETITION
* Predation
* Facilitation
* Mutualism
dispersal
What is competition?
A mutually negative interaction among species at the same trophic level directly or indirectly resulting in reduced fitness.
What is exploitative competition?
depletion of limiting resources.
What is interference competition?
exclusion by aggressive interactions
What is apparent competition?
reduction of a species though influence of a second species of predation.
What is the Lotka-Volterra model?
lecture 4_5 slide 28 + 29
dN1/dt = r1N1 ((K1-N1-aN2) /K1)
dN2/dt = r2N2 ((K2-N2-βN1) /K2)
N1, N2 : population size
r1, r2 : maximum population growth rate
K1, K2 : carrying capacity
α, β : interspecific competition
logistic growth:
dN1/dt = r1N1(1- N1/K1)
When α, β < 1 : interspecific competition is weaker than intraspecific competition. STABLE CO-EXISTENCE of species.
When α, β > 1 : interspecific competition is stronger than intraspecific competition. COMPETITIVE EXCLUSION of weaker
competitor.
What is the competitive exclusion principle? (Gause 1936)
Two species must differ in some aspect of their niche in order to coexist.
What is an ecological niche? Second definition
Second definition – Charles Elton
The Eltonian niche = “ …. the status of an animal in its community, to indicate what it is doing and not merely what it looks like” (1927)
NICHE = functional role of a species in a community
Niche is a property of the species and not the environment.
Defined by biotic interactions and resource-consumer dynamics.
What is an ecological niche? Third definition
The Hutchinsonian niche “… an n-dimensional hypervolume, enclosing the complete range of conditions under which an organism can reproduce itself”
NICHE = physical and biological conditions needed to maintain a stable population size NB
What is the realised niche?
ecological niche a species actually occupies in the presence of other species
(and dispersal limitation).
What is the fundamental niche?
ecological niche a species could occupy in the absence of other species (and
dispersal limitation).
What is competitive exclusion?
Exclusion by negative biotic interactions
Testing the role of competition in structuring natural communities, predictions
1) Interspecific competition should be common (paradox of plankton - Hutchinson 1961)
2) Potential competitors in a community should exhibit differentiation of their realized
niches.
3) Niche differentiation should manifest as morphological (functional trait) differentiation of coexisting species (principle of limiting similarity)
4) Niche / functional trait differences between populations of a species living alone and those coexisting with a competitor (character displacement)
5) Competitors should have negatively associated distributions (ie species that are too similar should not co-occur)
NIche/Resource partitioning
species alter the use of their niche to avid competition, by dividing resources among them.
What is limiting similarity?
How different species need to be to coexist, limit to similarity of competitors for coexistance.
What is character displacement?
Galapagos bird species - bill depth changes when there are competitors in the system.
Functional character is displaced due to competition.
What are checkerboard distributions?
Some species never co-exist by themselves or as part of a larger combination.
Spider example - 4 different forms of spiders have different niches. Multiple species of bark spiders do not co-occur. Arises because of competitive exclusion.
How does a community get assembled from the species pool?
Prominent role of competition…..but not always.
Species need to occupy different realized niches to coexist.
What maintains diversity in the face of competition?
Tropical rainforest example
different kinds of trees competing for the same basic resources on small plot of land, yet they all co-exist.
- Biotic factors - competition
- Mutualism - root symbiosis, pollination.
- Facilitation - modification of some biotic/abiotic component
- exploitative factors - benefiting of now species to another.
What is the mutualistic interactions?
Benefit and benefits - both organisms benefit.
2 types of mutualism:
obligate - species-specific (fig-wasp e.g.)
facultative - generalized (plant-fungus e.g.)
What is facilitative interactions?
One organism’s modification of some component of the abiotic or biotic environment enhances the colonization, recruitment, and establishment of another.
facilitation and competition often operate in balance.
facilitation strongly influences local assembly in stressful
environments.
What is exploitation interactions?
Interactions where one species benefits at a cost to the
other.
It can directly reduce the diversity in systems.
Predators can enhance species coexistence - exploiter mediated coexistance.
Janzen-Connell Hypothesis = If pests are host-specific, then pests will reduce recruitment where hosts are most abundant (for seeds and seedlings = near adults) freeing up space for other plant species.
How does a community get assembled from the species pool?
LOCAL PROCESSES:
Prominent role of competition…..
Species need to occupy different realized niches to coexist. (this ensures that intraspecific competition exerts stronger control on populations than interspecific competition – allowing coexistence).
What is dispersal limitation?
Not all species in the species pool can reach the local community:
-because they have different dispersal traits / modes - determinism
-because of the stochasticity of dispersal - stochasticity
functional traits influence dispersal capacity > same applies at finer sclaes.
HIgh spatial turnover in fynbos diversity as a result of speed dispersal by ants?
How is dispersal limited to communities?
Long-term seed trapping studies can be used to see how frequently seeds arrive at potential recruitment sites.
Neutral theory of biodiversity.
Hubble 2001
many basic ecological patterns can
be explained by dispersal limitation
The dynamics of the local community are driven by:
- speciation/extinction in the species pool
- dispersal from the species pool to the community
- competition between equivalent species
WITH NO NICHE DIFFERENCES
- the debate has stressed the importance of dispersal limitation /
historical processes, in addition to niche difference (competition)
in community assembly
- Neutral theory provides a null hypothesis against which niche
assembly can be tested
Non-equilibrial view - competition-colonization balance
Intermediate Disturbance Hypothesis
Non-equilibrial view of community assembly.
disturbance - relatively discrete event that:
1. removes individuals
2. disrupts availability of space and resources
3. changes in physical environment.
What is ecological succession?
a non-seasonal, directional, and continuous patterns of colonization and extinction on a site by species population.
Non-equilibrial view – storage effects
Storage Effects
What if recruitment depends on environmental conditions, but environmental conditions fluctuate, and species respond differently to these fluctuations?
All species have some conditions under which their populations can grow = ‘good years’.
The benefits of good years are stored through bad years – storage mechanisms would be things like seed dormancy or longevity of adults.
Storage buffers species against competitive exclusion during bad years, thus promoting co-existence.
How does a community get assembled from the species pool?
LOCAL PROCESSES:
COMPETITION
Stabilizing mechanisms: reduce the influence of competition (ie prevent competitive exclusion)
- Other interactions (exploiter mediated coexistence)
- Dispersal limitation
- Non-equilibrial dynamics
