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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 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