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NRES 310 lecture
Niche dynamics
5 November 2007
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Forms of interactions between species.
Competition: each species has a negative effect on the other, does not need to be
balanced.
Commensalism (facilitation): one species benefits without affecting the other.
Mutualism: both species benefit
Predation & parasitism: one species benefits to the disadvantage or detriment of the
other
NICHE DYNAMICS
Any discussion of competition must begin with the concept of ecological niche.
Niche: functional role and position of the organism in its community.
Habitat is address, the niche is job.
Niche must include every aspect of the environment that limits the distribution of the
species.
Hutchinson hypervolume  multidimensional # axes (correspond to independent
physical or biological variables that affect the abundance of that species) that make up the
niche of an animal. defined by the set of resources and environmental conditions that
allow a single species to persist in some particular region.
Used niche to describe the range of physiological and biological conditions, including
limiting resources needed for a species or population to maintain a stable or increasing
population size.
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Temperature
NRES 310 lecture
Niche dynamics
5 November 2007
Foraging
height
Seed size
Foraging
height
Seed size
Cannot draw all or dimensions on a graph, but you can envision a sphere that
theoretically completes the niche.
Problem: This approach assumes we know what to measure, we often measure what we
can rather than what is important or we measure so many things that the important
variable is lost in the chaos.
Habitat Use: is uniquely defined by what is available.
Habitat Selection: Used – Available (+ result is selection, - is avoidance, 0 used
proportional to availability.)
Niche is often dealt with in terms of what is available.
** If habitat selection is related to survivorship and fecundity then it is linked to
population dynamics (never been documented).
1. If you just look at use to describe niche dynamics, then you cannot evaluate what
is available.
2. Use and Availability, may miss importance, because something very important
may be used in proportion to what is available and you would not be able to
document or evaluate it.
3. Importance = use x availability (rescaled to 100%).
4. Consider: USE, SELECTION, and IMPORTANCE
5. THEN you may be able to deal with niche.
NICHE
1. Fundamental niche – full range of variables to which it is adapted, an area that an
animal would occupy in the absence of competitiors.
NRES 310 lecture
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Niche dynamics
5 November 2007
a. Set of resources and environmental conditions that allow a single species
to persist in a particular region, often conceived as a multidimensional
space.
b. This is rarely, if ever, seen in nature because the presence of competing
species restricts a given species to a narrower geographic range of
conditions.
2. Realized niche – area that an animal occupies in the presence of competitors.
a. Set of resources and environmental condition constrained by competition
or predation that allow a single species to persist in a particular region.
b. Interspecific competition excludes a species from certain areas of its
fundamental niche.
Fundamental niche
Realized Niche in the
presence of 5 superior
competitors
Competition is dependent upon the number of competitors and how much niche
overlap among them.
Ecologists usually confine themselves to one or two niche dimensions, such as
feeding, and space use.
Niche of an animal can change over time.
Example : insects with complex life-histories can occupy one niche as larvae and
another as an adult.
Fish, niche space changes as the organism matures by size (switches from plankton
when small to eating small fish as grows). Also changes Interspecific relationships
ie. Interspecific competition.
Principle of Competitive Exclusion: postulated by Gause 1934. Complete
competitors cannot cowxist.as a result of competition two species hardly ever occupy
NRES 310 lecture
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Niche dynamics
5 November 2007
similar niches, but displace each other in such a manner that each takes possession of
certain kinds of food and modes of life in which is has an advantage over its
competitors.
Basically 2 species cannot occupy the same niche. Untestable because it cannot
be disproved, because either result exclusion or coexistence can be attributed to the
principle.
frequency
frequency
Intraspecific
competition
diet
Interspecific
competition
diet
At some point intraspecific competiton > interspecific competition because
individuals have the same demands for resources.
Resource Partitioning or “Ghost of Competititon Past”
For example: today we measure the niche of two similar species that are
sympatric and detect no measured differences:
So is it past competition resulting in narrower niche OR physiological differences
let animals fit together in the absence of competition.
Possibly both are true, but it is impossible to measure with correlation studies.
Only demonstrable with removal experiments.
2 possible outcomes of competition
Competitive exclusion  can occur particularly with species without evolutionary
history together.
NRES 310 lecture
Niche dynamics
5 November 2007
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Co-existence  more common with natural communities.
Example: National Bison Range
Mountain
Goats
Forested
benches –
mule deer
Steppe
grasslands
elk
Bighorn sheep
Prairie –
Bison and
pronghorn
Riparian
White-tailed
deer
Grasseaters – bison, elk, sheep
spatial diversity where dietary niche overlaps.
Forbeaters – white-tailed deer, mule deer, pronghorn, goats
Spatial separation  dietary niche overlap is greatest
Spatial overlap  dietary separation
NRES 310 lecture
Niche dynamics
5 November 2007
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This is especially true is spatial overlap can occur. To find out if competition is
occurring you must find a way to separate niche axes.
NICHE EXCLUSIVITY – Opimality Models or Optimal foraging models.
Bison
Body size,
Bite size,
&
Handling
time
Bison &
deer
Deer
Bison,
deer, &
rabbits
Deer &
rabbits
Rabbits
Zones of niche exclusivity
Bison, deer,
rabbits, &
voles
Deer, rabbits,
& voles
Rabbits &
voles
Voles
everybody
Deer, rabbits, voles
& grasshoppers
Rabbits, voles,
grasshoppers
Voles &
grasshoppers
Grasshoppers
Quality and Abundance of Vegetation
*No population dynamics included
*could add sexual segregation (for body size, bite size relationships)
*species with small bite sizes are regulated to high quality food.
*allometric relationships determine zones of competition
Get co-existence based on zones of exclusivity – allows exclusivity and competition.
Optimal  energy maximizing v. time minimizing strategies.
Old models do not include predation.
New Models bring in predation.
What are the units of selection? Energy? Protein?
A: successful reproduction
May be good for looking at niche theory. Niche exclusivity and niche partitioning, but it
makes removal experiments too simple. It does provide ideas for experiments and allows
for ideas to model niche partitioning.
Remember predation is not included here.
Most species need to avoid predators and they reduce predation by utilizing “enemy-free
space”
NRES 310 lecture
Niche dynamics
5 November 2007
Enemy free space does not refer to a particular location, but rather the set of conditions
that minimize the impact of predators, in a similar manner to how niche refers to the set
of conditions that allows species to co-exist.
Need to briefly discuss apparent competition
Defn: a species entering a community is similar enough to another that even if they do
not directly compete the two species together can support greater predator populations
that one alone, which has a greater negative impact on both species by predators than
would if either occupied the area alone.
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