Download - 03-Distribution of Species
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 Suitable habitat
Sink population 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
• Gradients can represent physical, disturbance, or biological factors
Tolerance curve and gradients
Physical limiting factors
• Saguaro distribution limited by low temperatures
Temperature
Physical limiting factors
• Bark beetles overwintering temperature limits
• Climate change
Temperature
Physical limiting factors
Temperature
• Eastern phoebe winter range (food limited)
Physical limiting factors
• Orographic precipitation and vegetation gradients
Precipitation (moisture)
Physical limiting factors
• Xerophytes
– Full sunlight in dry soils
• Mesophytes
– Wetter and more shaded environments
Soil Moisture
Physical limiting factors
Solar radiation – light saturation
S
Physical limiting factors
Solar radiation – light limitation
Physical limiting factors
Timberline and wind
• Determined by temperature at broad scales
• Wind critical local limiting factor
Wind
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:
• Fire suppression in the Patagonian steppe allows Austrocedrus chilensis expansion
T.T. Veblen
1986
1896
F.P. Moreno
Biological interactions
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
Competition (negative-negative)
Biological interactions
• Kangaroo rats in the southwest
Competition
Biological interactions
• Plant allelopathy (interference)
• Most commonly influences fine scale distributions
Competition Black walnut
Biological interactions
• Broad definition also includes herbivory, parasitism
Predation (positive-negative)
Biological interactions
• Predator distribution influenced by prey (resource)
Predation - herbivory
Biological interactions
• Prey distribution influenced by predator/s
• Yellowstone Lake cutthroat trout (native) & lake trout (invasive)
Predation
Brown trout
Cutthroat trout populations
diminish with introduction of
lake trout
Population structure of cutthroat
shifts few reach adult stage
Biological interactions
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
Predation
Biological interactions
• Can be obligate or non-obligate
• Obligate mutualisms have stronger effects on species distributions
Mutualism (positive-positive)
Acacia-ant
mutualism
Yellow fruits disperser: mockingbird
Green fruits disperser: marsupial
Biological interactions
Mutualism: mistletoe and its seed dispersers
Biological interactions
Presence of one species aids another
• Silene acaulis increases plant species richness & abundance
Facilitation (positive-zero or positive-positive)
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)
• Temperature, precipitation, solar radiation influencing the life zones on a mountain environment
Interaction among factors
Interaction among factors
• Predator mediated coexistence
• Keystone species (Paine 1969)
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.
Interaction among factors
Relative importance of facilitation versus competition is dependent on environmental setting
Michalet et al 2006