GEOG20009 Lecture Notes - Lecture 9: Competitive Exclusion Principle, Ecological Niche, Biological Dispersal
15 May 2018
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Department
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Professor
LECTURE 9: SPECIES COEXISTANCE
• How do individual pairs of species manage to coexist in same environment when they are
competing for resources (food, habitat)?
ECOLOGICAL NICHE
• Ecological concept that allows us to understand how
species live in nature
• Usually presented in an illustration
• Species will have optimal fitness environmental
conditions & using resources
• Helps us understand between various environmental
gradients and the fitness of organisms (capacity to survive & reproduce)
COMPETITION & COEXISTANCE
• Competitive exclusion principle: complete competitors
(species that compete fully across every important
environmental gradient or resource) cannot coexist
• Theoretically:
o Species C cannot coexist with species B unless it
eats other food or differs from species C in
substantive ways
o Species A can coexist with both species B & C
• Has been observed in lab experiments
In the Field
• In the field: many examples of species coexisting without extinction – disregards theory
o Zooplankton: live in coexistence where theory says they shouldn’t, as some as better competitors –
should drive the rest to extinction, yet didn’t happen
• Idea: notion that environment shifts frequently enough that competition never runs its full course
• Take model, then add variability of space and time
o In model, species will theoretically go extinct – from subtle to large environmental changes
MODERN MODELS
Neutral Models of Species Diversity – Hubbell (2001)
• Composition of communities & extinction driven by random events – no mechanisms
that specifically promote coexistence
• Focus on communities at a larger spatial scale and longer temporal scale
• Proposes there is no real difference between species in their dispersal capacity or the way
they use habitat: suggesting animals/plants move in and out of communities that are
driven by chance events
• Species migrate (m) into local communities (with abundances of n) from a regional species
pool based on abundance in the region (values of P)
• There are no strong differences in habitat use or dispersal ability; entry of individuals into
local communities is random & driven by abundances (P) only
• Species thus go extinct more or less at random
• Evidence? Explains some patterns, but species do differ and density dependence does
occur
Mechanisms that promote coexistence
• A mechanism – an outcome – that ensures species have strong positive population
growth when they reach low densities will ensure they do not go extinct
• Graph: propose that instead of looking at extinction, growth from low densities
Fluctuation-Independent Models
• Mechanisms that do not depend on environmental variability
• Niche portioning (coexistence of species with low niche overlap)
• Frequency-dependent predation (compensatory mortality – predators switch to eat
whatever prey species is not abundant)
Questions
• What do we mean by
species coexistence
and why do we need
to examine it? -
Understanding the
niche and Niche
overlap &
competitive exclusion
• Alternative views –
Hubbell’s neutral
theory
• Models of
coexistence –
fluctuation-
dependent and
fluctuation-
independent models
• Evidence
Mechanism: a cause & effect
that promotes species
coexistence, and promotes
growth from low densities
Document Summary
Ecological concept that allows us to understand how species live in nature: usually presented in an illustration. Species will have optimal fitness environmental conditions & using resources: helps us understand between various environmental gradients and the fitness of organisms (capacity to survive & reproduce) Competitive exclusion principle: complete competitors (species that compete fully across every important environmental gradient or resource) cannot coexist. Species c cannot coexist with species b unless it eats other food or differs from species c in substantive ways. Species a can coexist with both species b & c: has been observed in lab experiments. Understanding the niche and niche overlap & competitive exclusion: alternative views . Hu(cid:271)(cid:271)ell"s (cid:374)eutral theory: models of coexistence fluctuation- dependent and fluctuation- independent models. In the field: many examples of species coexisting without extinction disregards theory: zooplankton: li(cid:448)e i(cid:374) (cid:272)oe(cid:454)iste(cid:374)(cid:272)e (cid:449)here theor(cid:455) sa(cid:455)s the(cid:455) should(cid:374)"t, as so(cid:373)e as (cid:271)etter (cid:272)o(cid:373)petitors should dri(cid:448)e the rest to e(cid:454)ti(cid:374)(cid:272)tio(cid:374), (cid:455)et did(cid:374)"t happe(cid:374)
