Lecture 19

Question 1

Why are organisms where they are?

Answer 1

slide 4

Question 2

Ranges & Distributions

Answer 2

• Range: the spatial extent of a
  species
• Distribution: the spatial extent of a
  population
• Individuals do not occupy the entire
  distribution because not all areas
  are suitable habitat
• Species with extensive geographic
  distribution typically encounter a
  wider range of environmental
  conditions and are generalists

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Question 3

Ranges & Distributions

Answer 3

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Question 4

Endemism

Answer 4

• Endemism: Small restricted
  geographic ranges; found in a
  location and nowhere else
• Constrained by isolation
  and/or narrow habitat
  requirements
• Often of conservation concern
• Often on islands

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Question 5

Range Expansion

Answer 5

• Range size can change over time with shift in climate, resources, change in
  competitors or predators, or introduction

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Question 6

Evolutionary Implications of Distributions

Answer 6

• Isolation & dispersal influence
  migration and gene flow
• Migration reduces population
  differentiation
• Range size impacts genetic drift
• Allele frequency changes as a
  result of random chance more
  common in small populations
• Ecological conditions (biotic
  and abiotic features of the
  ecosystem) influence the
  strength and nature of natural
  selection

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Question 7

The Ecological Niche

Answer 7

• Ecological niche: the range of
  environmental conditions and
  essential resources needed for
  a species to survive and
  reproduce
• Conditions must fall within an
  organism’s environmental
  tolerance for it to persist
• Organismal traits are related to
  their environment: they
  determine where they will
  thrive as well as limit where
  they can exist
• Each combination of
  environmental conditions
  present unique constraints on
  organisms
• Adaptations allow species to
  thrive under a specific set of
  environmental conditions
• These conditions reflect
  requirements and adaptations:
• Physiological
• Morphological
• Behavioral

slide 13-14

Question 8

The Ecological Niche

Answer 8

• Developed by Joseph Grinnell
  and Charles Elton in 1920’s
• Grinnell focused on habitat and
  the limitations of the physical
  environment
• Elton emphasized species
  interactions
• G. Evelyn Hutchinson: niches are
  multidimensional!
  “N-dimensional hypervolume”
• Each axis is a variable relating to
  resource needs or environmental
  constraints

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Question 9

Fundamental Niche

Answer 9

• Fundamental Niche: the range of
  environmental conditions tolerated by
  a species
• Describes the potential habitat space
  of an organism and constraints on
  where it can be found based on
  environmental conditions
• Each species has limits beyond which
  it cannot survive
• As environmental conditions change in
  time and space, potential distribution
  changes

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Question 10

Realized Niche

Answer 10

• Realized Niche: the actual habitat
  occupied by a species as a
  consequence of species interactions
  and barriers to movement
• The realized niche is a subset of the
  fundamental niche
• Describes the actual habitat space of
  an organism based on where it has
  been found
• Reflects the abiotic constraints
  (environmental tolerance) AND biotic
  interactions

slide 19-20

Question 11

Quantifying Niche Space

Answer 11

• Ecological niche
  models (also known
  as bioclimatic
  envelope model,
  species distribution
  model) relate
  climate to known
  distribution
• Results of model can be used to map potential geographic distribution under
  past, present, and future conditions

slide 21-24

Question 12

Realized Niche: Species Interactions?

Answer 12

• Species interactions shape the
  realized niche too!
• Negative interactions
  (competition, predation, disease)
  can limit the realized niche
• Positive interactions
  (commensalism, mutualism) can
  modify the fundamental niche by
  directly or indirectly enhancing
  survival and reproduction of
  other species
• Species interactions also interact
  with the abiotic environment

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Question 13

Realized Niche: Overlap

Answer 13

• Competitive exclusion principle: two species cannot coexist if they occupy
  exactly the same niche (competing for identical resources)
• The resource gradient will be divided among the species to minimize overlap
• Species may evolve to reduce niche overlap resulting in niche partitioning and
  increasing specialization

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Question 13

Niche Partitioning

Answer 13

• Each species exploits a
  portion of resources
  unavailable to others
• Niche space can be
  divided:
• Spatially: where the
  organism physically is
• Temporally: when the
  organism uses the habitat
• Behaviorally: what the
  organism consumes (diet,
  resources, light, nutrients)

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Question 14

Competitive Exclusion

Answer 14

• Species distributions can be restricted by
  physiological tolerance and competition
• Zonation results from exclusion by a
  dominant species

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Question 15

Niche Partitioning – Competitive Exclusion

Answer 15

• Elevation creates different
  ecological niches
  characterized by
  temperature, vegetation
  type, and food resources
• Example: Different species
  of chipmunks occupy
  different altitudinal zones
  because of a combination
  of thermal tolerance and
  competitive exclusion

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Question 16

Character Displacement

Answer 16

• When niche partitioning involves
  phenotypic adaptation
  (morphology, behavior, physiology)
  directional selection leads to
  character displacement

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Question 17

Niche Partitioning – Character Displacement

Answer 17

“Ghost of competition past” when character displacement is presumed to
have occurred in the past to lead to present day niche partitioning

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Question 18

Habitat Heterogeneity

Answer 18

• Resources are not usually
  homogeneous in an environment
• Increasing environmental variation
  (heterogeneity) typically results in
  increasing beta diversity as species
  are restricted to subsets of the
  habitat
• This means more unique species
• Environmental variation supports
  higher phenotypic (and by extension
  genetic) variation for natural
  selection to act on
• More habitat = more species!
• Habitat heterogeneity produces different types of niche space to occupy
  allowing more species with narrow niches to fit
• Most species will be specialists, few will be generalists because of competitive
  exclusion
• Sources of habitat heterogeneity:
• Disturbance (e.g., treefall)
• Vertical structure (forest)
• Elevation
• Land cover
• Patterns of nutrients and microclimate
• Variation in food resources
• Seasons
• Habitat
  heterogeneity
  facilitates niche
  partitioning
• Species niches
  become more
  narrow
• Species become
  more specialized
• More specialized
  species are more
  likely to have
  restricted ranges
  (endemic)

slide 32-34

Question 19

Eco-Evolutionary Relationships

Answer 19

Evolutionary ecology
focuses on identifying
phenotypes (traits) that are
ecologically important and
understanding how and
why those traits vary in
different environments

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Question 20

Environmental Gradients: Clines

Answer 20

• Clines are measurable, gradual changes in a
  phenotype (trait) associated with a continuously
  varying environment
• Environmental variation creates variation in
  natural selection across a species distribution
• Gene flow along the environmental gradient
  maintains continuous phenotypic variation
• Example: Along an elevational gradient
  precipitation and temperature decrease with
  altitude. This produces a continuous variation of
  phenotypes correlated with altitude in
  Arabadopsis thaliana.

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Question 21

Environmental Gradients: Ecotypes

Answer 21

• Clinal variation may not transition smoothly from one habitat to an adjacent
  habitat if the environment abruptly changes
• Ecotypes are populations adapted to
  unique local conditions with differences
  maintained by natural selection

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Question 22

Phenotypic and Environmental Variation

Answer 22

• Phenotypes (traits) are influenced
  by the environment
• Because the environment is
  variable in space and time,
  phenotypes are variable
• Phenotypic plasticity can influence
  trait variation within a generation
  – the same genotype produces
  different phenotypes depending
  on the environment
• Adaptation can influence trait
  variation across generations
  through natural selection

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Question 23

Adaptation: Trait-Environment Relationships

Answer 23

• Adaptations are tightly linked to the ecological
  niche
• Adaptations increase fitness of an individual under
  a given set of environmental conditions
• Environment imposes constraints on processes
  related to fitness: survival, growth, reproduction
• How adaptations enable an organism to thrive in
  one environment and not in another is key to
  understanding the distribution and abundance of
  species.
• Adaptations arise because of the interaction of
  individuals with their environment leading to
  differential survival and reproduction (natural
  selection)
• Environment: selective landscape
• Trait: selective target
• Environmental change: selective agent

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Question 24

Adaptation: Trait-Environment Relationships

Answer 24

• MANY types of adaptations that
  correlate with niche
• Across the tree of life all organisms
  must first be able to tolerate the
  abiotic environment
• Physiology related to abiotic
  conditions (thermal tolerance and
  desiccation tolerance) are strongly
  tied to distribution
• Diet specialization and crypsis are
  other common environmentally
  correlated traits in animals

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Question 25

Plant Adaptations: Temp. & Precipitation

Answer 25

• Leaf area and shape relate to heat dissipation
• In habitats with abundant water, larger leaves are more effective at cooling via evapotranspiration
• In habitats with little water, smaller leaves are more effective at cooling via convection
• Leaf characteristics reduce light absorption and water loss: thick cell walls, hairs, waxy
  coating
• Water availability influences roots vs leaves investment
• More water = more leaves, less water = more roots
• Cold is a problem too! Anti-freeze compounds protect plants (at a cost to the plant)

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Question 26

Animal Adaptations: Temp. & Precipitation

Answer 26

• Behavioral:
• Migration
• Burrowing
• Diapause (arrested development in
  insects)
• Nocturnal activity
• Movement between sun / shade / water
• Physiological:
• Water extraction from food, concentrated
  wastes
• Shivering, panting, sweating
• Dormancy (estivation, hibernation) &
  torpor
• Heat storage / night dissipation
• Supercooling protection from freezing
• Morphological:
• Fur, feathers, fat
• Coloration
• Vascular modifications

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Question 27

Darwin’s Finches

Answer 27

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Question 28

Key Points to understand

Answer 28

• What factors shape where organisms are where they are?
• What is an endemic species?
• What is an ecological niche? How does the realized niche
  differ from the fundamental niche?
• What does modelling niche space tell us (what is it used for)?
• How do disturbance and species interactions modify niche
  space?
• How does distribution and niche space relate to adaptive
  (natural selection) and nonadaptive (genetic drift) evolution?
• What is the relationship between habitat variation and
  species diversity?
• What key adaptations and plants and animals are closely tied
  to niche space?
• How does the ecology and evolution of Darwin’s Finches
  illustrate each of the concepts above?

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