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Species distributions
What determines the
distribution and abundance of a
species in space and time?
Mapping and measuring the Range
3 types of range maps
1. Outline maps
• Display range as an irregular area
• Limitations: highly generalized, ignore gaps
2. Dot maps
• Plot points on a map where species have actually been
found.
• Limitations: very limited inference/spatial coverage
3. Contour maps
• Indicate variation in density and abundance within a
range.
• Limitations: underlying data, interpolation, temporal
The Distribution of Populations
• Population Growth and Demography
• Malthusian Theory
– All organisms have the inherent potential to
increase their numbers exponentially.
– However, because resources limit growth, and
many environments are unsuitable, no organisms
actually increase indefinitely.
Ecological niche
• Describes the “ecological space” occupied by
an organism
– Resource “space”
– Environmental tolerance
– Ecological role
• Helps to understand
1. main factors (environmental, competitive) which
limit a population’s growth
2. mechanisms/factors that influence species’
geographic distribution
Grinnell’s niche (1917)
• Habitat concept; emphasis on where the species lives
Elton’s niche (1927)
• Functional concept; emphasis on the trophic position
Black rhino
White rhino
N-dimensional Hutchinsonian niche
• Multi-dimensional space of
resources that a species needs to
survive and reproduce
• Functionalized the niche concept
Fundamental vs. realized niche
• Fundamental niche : species niche in the absence of
any interspecific competition and predation
• Realized niche: species
niche in the presence
of competing and/or
predator species
• Reflected in geographic
distributions of species
The niche concept
Useful conceptual framework; but… Niche alone
cannot explain all patterns of species distribution and
abundance:
• Source – sink populations
Unsuitable habitat
Sink population
Suitable habitat
Source population
The niche concept
• Also, some favorable habitats may be uninhabited just
by chance (historical factors)
The niche concept
• Additionally, temporal fluctuations in the environment
can determine species presence/absence
What limits a species’ geographic range?
What restricts a species’ fundamental
niche to the realized niche?
Liebig’s law of the minimum
• Originally developed for agriculture
• yield is proportional to the amount of the most
limiting nutrient
• Population is limited by single most
limiting factor
• Oversimplification
• Interaction among multiple factors
What limits a species’ geographic range?
What restricts a species’ fundamental
niche to the realized niche?
• Physical environment
• Disturbance
• Biological interactions
Tolerance curve and gradients
• Gradients can
represent physical,
disturbance, or
biological factors
Physical limiting factors
Temperature
• Saguaro distribution limited by low temperatures
Physical limiting factors
Temperature
• Bark beetles overwintering temperature limits
• Climate change
Physical limiting factors
Temperature
• Eastern phoebe winter range (food limited)
Physical limiting factors
Precipitation (moisture)
• Orographic precipitation and vegetation gradients
Physical limiting factors
Soil Moisture
• Xerophytes
– Full sunlight in dry soils
• Mesophytes
– Wetter and more shaded environments
Physical limiting factors
Solar radiation – light saturation
S
Physical limiting factors
Solar radiation – light limitation
Physical limiting factors
Wind
Timberline and wind
• Determined by temperature at
broad scales
• Wind critical local limiting
factor
Disturbance
• Fire, volcanism, floods, hurricanes, etc.
• Great impact on species distributions by killing
individuals, changing resource availability &
environmental conditions , altering species
interactions
• Also biotic disturbances:
insect outbreaks, pathogens,
herbivore grazing, etc.
Disturbance
Example:
1896
• Fire suppression in the
Patagonian steppe allows
Austrocedrus chilensis
expansion
F.P. Moreno
1986
T.T. Veblen
Biological interactions
Competition (negative-negative)
Some definitions
• Exploitative : using resources and therefore making them
unavailable for other species
• Interference: “aggressively” denying the use of resources
to other species
• Diffuse competition: one species is affected by multiple
species that collectively diminish a shared resource
Biological interactions
Competition
• Kangaroo rats in the
southwest
Biological interactions
Competition
• Plant allelopathy
(interference)
• Most commonly influences
fine scale distributions
Black walnut
Biological interactions
Predation (positive-negative)
• Broad definition also includes herbivory, parasitism
Biological interactions
Predation - herbivory
• Predator distribution influenced
by prey (resource)
Biological interactions
Predation
• Prey distribution influenced by predator/s
• Yellowstone Lake cutthroat trout (native) & lake trout
(invasive)
Cutthroat trout populations
diminish with introduction of
lake trout
Brown trout
Population structure of cutthroat
shifts few reach adult stage
Biological interactions
Predation
Parasitism
• Alcon blue butterfly larva
• Fools ants with chemistry by
mimicking the surface chemicals
that the ants have on their own
brood
• Reduces ant reproductive output
Biological interactions
Mutualism (positive-positive)
• Can be obligate or nonobligate
• Obligate mutualisms have
stronger effects on species
distributions
Acacia-ant
mutualism
Biological interactions
Mutualism: mistletoe and its seed dispersers
Yellow fruits disperser: mockingbird
Green fruits disperser: marsupial
Biological interactions
Facilitation (positive-zero or positive-positive)
Presence of one species aids
another
• Silene acaulis increases plant
species richness & abundance
Molenda et al. 2012
Biological Interactions
Principle of Competitive Exclusion (Gause’s Principle)
• Species cannot coexist in a community if their niches are
identical
• Exclusion takes time
• Only applies if the ecological factors are constant
• Many exceptions observed in natural systems
The paradox of the plankton, Hutchinson 1961:
• Phytoplankton communities reveal an astonishing
biodiversity, whereas classical competition theory
suggests that only a few competing species can
survive on a fixed abundance of resources.
Resolution of the Paradox
• Non-equilibrium conditions
• Temporal variation in environmental
conditions
• Disturbances
• Spatial heterogeneity of resources
• Biological interactions
– Complex “webs” of biological interactions
Interaction among factors
• Interacting factors may have more extreme
consequences on species distributions than any
factor alone
• Most commonly the interaction among multiple
factors determine species distributions (and not a
single factor alone)
Interaction among factors
• Temperature, precipitation, solar radiation influencing
the life zones on a mountain environment
Interaction among factors
• Predator mediated coexistence
• Keystone species (Paine 1969)
Interaction among factors
Relative importance of physical and biological limiting factors
• On harsh physical environments competition is less
important than on resource rich environments as a
factor affecting species distributions.
Relative importance
of facilitation versus
competition is
dependent on
environmental
setting
Michalet et al 2006