II. Competition and ecological niche theory Flashcards
What were the findings from the ‘plant traits practical’?
Positive relationship beteen Leaf Mass Area and Wood Density -> trees investing in high WD that also invest in higher LMA are more resistant, durable, mechanically strong.
Massive variability in traits even with a small sample e.g. 200 to 100,000 LA:SA ratio.
Different plants have different traits that enable them to thrive in a specific habitat, but can’t have optimum for everything -> they will ‘trade off’ one trait for another.
What is an ecological niche?
‘An abstract concept - an N-dimensional hypervolume where the dimensions are all environmental conditions and resources [biotic and abiotic] which determine the persistence of a population through influencing their birth or death rates’ (Hutchinson, G., 1957).
How do we determine whether a species is within/outside of its niche?
Inside: birth rate => death rate (population will be maintained or increase)
Outside: death rate > birth rate (population will decline)
How can you quantify whether a species is in/out of their niche? What is the difficulty with this?
Experimentally i.e. moving species to see its resource limits (translocation).
Information synthesis i.e. look at performance based on certain traits, function of its environment. (Holt, 2009).
Remains “abstract” since the understanding we have of a plant’s function can only be based on the variables chosen to look at.
What are the competitive resources in a tropical rainforest? Why do variables change? What does this mean?
Light, water and nutrients.
Variables change across space and altitude (changes in radiation, wind, CO2, humidity).
Variables also change as plants grow i.e. hydraulics, bark thickness.
Means niche differentiation is massive!
What are the two types of competition?
Inter-specific: traits differ between species so differentially compete for resources by developing different traits; but niche overlap occurs results in competition
Intra-specific: traits can differ between plants of same species e.g. tree height/size, canopy position -> compete for same resources
What is ‘niche overlap’ and its relation to competition and traits?
The extent of difference in traits between two different species determines niche overlap.
Similar functional traits resultes in HIGH overlap where EXCLUSION of one species if it has a worse competition ability is more likely. Competition is higher.
Highly differential traits results in LOW overlap where co-existence is possible.
What is ‘Negative Density Dependence’?
When greater density is negatively correlated with persistence BUT more complicated than that. In the context of a tree stand, the low density is only beneficial if trees are mixed. Monocultures are not good because they increase competition like crazy (fill same niche), especially if high density.
What is the Janzen-Connel Hypothesis?
The further from the mother tree, the density of seeds declines i.e. dispersal less effective with distance. BUT probability of seedling survival conversely increases because further from similar species, decreases density and competition, niche less likely to be filled.
Not just to do with resource competition, but also predator pressure (e.g. in tropical RFs) -> higher pest pressure with monocultures because can support higher pest population
What is ‘Positive Density Dependence’?
Where the presence of one species facilitates survival of others i.e. when survival improves with larger number of species
What are some drivers of PDD (facilitation)?
Increased shade = reduced radiative/heat stress
Increased leaf litter puts nutrients in soil
Increased potential for hydraulic lift - water drawn up from deep soils by trees, help to redistribute water to shallow soil layers
Positive effect on pollinators (the opposite of the Janzen-Connell hype); especially if pollinators very specific to a species
Greater presence of symbiotes e.g. mycorrhizae fungi
What is Neutral Theory and how does it contrast to theories of competition for survival?
Hubbell (2001): all individual species are exactly equal i.e. probabilities of birth, death, survival etc.
Ecosystem models do not incorporate evolutionary dynamics, competition, functional traits.
Used to predict equilibium number of species in a given environment: assume zero-sum gain per species (add one, lose one), resource space always full.
Eventually all species go extinct, but immigrants and ecol drift maintain the same numbers. So, ecological drift is random, turnover is constant.
Scenario J: species max, spaces full
Scenario D: disturbance event, pop declines
Scenario J: species replaced purely as function of the numbers of individuals of each species prior disturbance, immigrants occasional.
Is NT correct? What has it contributed?
No - theoretically unlikely that species don’t actively compete with one another, unrealistic.
BUT it acts as a null model - assumes all species differences cancel out to produce equal fitness, which fits distribution patterns in forests.
Have found that it fits the species patterns and population behaviour in tropical rainforest and coral reefs!
More applicable to areas that are highly diverse (200-250 species per ha). Ecological drift/null model works as a function of diversity.
What did Contreras et al. (2011) find?
That we need an improved understanding of how competition can be measured and influence individual tree growth to aid projections -> fuel and fire management.
Evaluation of different measures - shows the complexity of tree competition. No single universal index.
What did Holt (2009) find?
Exploring whether an ecological niche can be characterised as the abstract mapping of population dynamics onto an environment, how to quantify. Shows the complexity of the concept, multiple definitions, models, methods.
