Outline

1
Outline

• Introduction
• Habitat Selection theory
• Source-sink
• Balanced dispersal model
• Real world implications
• Ecological traps
• Jays data

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Habitat Selection

• Developed to relate individual foraging decisions
  to spatial distributions in abundance.
• Assumes habitats are chosen based on their
  relative evolutionary costs and benefits.
• Assumes that foraging profits convert to fitness.
• Predicts that abundances in a patch should be
  determined by patch quality.

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The Ideal Free Distribution

• IF Animals are
• IDEAL Capable of perfectly assessing costs and
  benefits of their current location.
• FREE Capable of uninhibited movement to a new
  location.
• THEN,
• Animals will be spaced so that fitness (or some
  measure of it) is equal everywhere.

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Ideal Free Foraging

• Fish distributed in tank so average energy intake
  is equal among all individuals

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Habitat Selection

• How does spatial variation in habitat quality
  affect populations???

N 100
N 10
N 50
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The Ideal Free Distribution of animals across
habitats
Habitat 1 higher quality than 2
Fitness
(0,50)
K1
K2
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The Ideal Free Distribution of animals across
habitats
Fitness Isocline Abundances in each habitat
where fitness are expected to be equal under IDF.
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Different patterns of habitat heterogeneity
different isodars
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Habitat selection summary

• When conditions are right
• Ideal knowledge and freedom to get there.
• Animals are distributed so that fitness are equal
  across heterogeneous habitats.
• Implies, density is a good predictor of habitat
  quality.

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What limits IDF??

• Not Ideal
• Imperfect information
• Balancing short vs. long-term costs and benefits
  difficult
• Energy intake vs. predation risk
• Short term vs. long term rewards
• Not Free
• Energetic costs of moving and selecting habitat
• Dominance from other individuals
• Territoriality

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Source-sink theory

• What happens to populations when they utilize two
  types of habitat, one where average fitness is
  gt1, and one where average fitness is less lt 1?
• History
• Holt 1985. Predator in habitat with and without
  prey
• Pulliam 1998. BIDE models (reading)
• Rapid growth in field after these
• Primarily theory, but empirical studies are
  catching up

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Source-sink model (Schmida and Ellner 1984, Holt
1985, Pulliam 1988, etc.)
SOURCE HABITAT On average, bgtd, so population
grows - egti, net exporter of animals
SINK HABITAT bltd, so population declines igte,
net importer of animals
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Pulliam 1988

• Simple model with density dependence in source
  habitat
• Limited of breeding sites in source
• Sinkunlimited of poor breeding sites
• Animals perform habitat selection never occupy a
  poorer breeding site when a better one is still
  available

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Pulliam 1988
Figure 2
Source
Sink
Success of any ind. in the sink
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Pulliam 1988

• Implications
• Can have many individuals in sink
• Density may be a POOR indicator of habitat
  quality
• Realized niche might be bigger than fundamental
  niche!
• Communities may include both source and sink
  species
• Immigration plays a role in community structure

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Balanced dispersal (McPeek and Holt 1992, Lemel
et al 1997)
- Individuals have positive fitness in both
habitats - Habitats have different carrying
capacity (K) - Conditional dispersal is
favored K2 / K1 m12 / m21
OR K2 m21 K1 m12 Equal numbers in
both directions
M12
M21
K2
K1
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Predictions/assumptions of the models
Balanced dispersal
Source - sink

• Sinks are present
• Differences in demographic variables across
  habitats.

• No Sinks
• No differences in demographic variables across
  habitats.

• Biases in movements between habitat patches of
  different quality

• No biases in movements

• No prediction regarding density-dependent
  dispersal

• Negative density-dependent dispersal

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Implications of spatial theory

• Implications
• Movement patterns and the spatial patterns of
  abundance will depend on an interplay between
  population dynamics and the relative differences
  in habitat quality (both average fitness and rate
  of decline in fitness with increasing density)
  across space.

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Implications of spatial theory

• Density alone is a unreliable indicator of
  habitat quality.
• IF IDF present, then density quality
• If not, density may be poor predictor of quality
• Long-term demographic data required to assess
  habitat quality
• Factors inhibiting historic habitat selection can
  negatively impact species
• Limiting movement
• Altered cues for selecting high quality habitat
  result in Ecological Traps

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Implications of spatial theory

• Ecological Traps. Habitat selection cues shaped
  by evolution result in species preferring sink
  habitats
• maladaptive habitat selection
• source-sink population dynamics can be generated
  by anthropogenic changes in landscapes that occur
  so quickly that organisms no longer make optimal
  habitat selection decisions. Individuals select
  the same habitats as their ancestors but these
  decisions no longer provide high fitness. Remes
  2000.
• Birds in grazed, burned, and undisturbed
  prairies.
• Management more food AND more predators.
• Birds select habitat based on food, then nests
  get eaten. (Shochat et al. 2005)
• Restoration frogs in natural vs. human-made
  ponds
• Marine fish reserves

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Implications of spatial theory

• Studies in sinks may yield incorrect conclusions
  about factors regulating populations
• Movement likely plays a key role in population
  dynamics
• Different species will have different
  propensities for SS vs IDF type distributions.
• Wind/current dispersed species with passive
  dispersal
• Actively moving species that can sample habitat
  and choose to remain

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Spatial theory and the real world

• Theoretical models provide an initial basis for
  how we perceive and think about natureit is
  vital that they be tested
• Research Goal Relate field data and spatial
  models to create better models

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Summary of study

• 7.7 years of data
• 23,000 captures from biweekly samples
• 3 species analyzed

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16g
40g
145g
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Cotton rat movements
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Prairie vole movements
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Deer mice movements
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RESULTS Looking for evidence of SS or BD

• No Sinks present
• Population growth rates positive on all blocks
• Female reproduction and survivorship (both sexes)
  similar across all blocks
• Habitat quality varies across blocks
• 8 year averages in abundance differ by block

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Jolly- Seber average abundances by block
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Balanced dispersal prediction no biases in
switching

• Tested for biases in switching between pairs of
  blocks (chi-square)
• Of 37 possible tests across all species, none
  show biases in switching so movement was balanced
  between patches

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Balanced Dispersal negative density-dependence
Cotton rats
0.1
0.09
B
0.08
Y 0.078 - 0.007X R2 0.764
0.07
B
0.06
Proportion switching
0.05
0.04
0.03
0.02
B
B
B
0.01
0
0
2
4
6
8
10
12
Average abundance (MNKA)
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Conclusions from movement data

• For all three species there are no sinks
• The balanced dispersal model is strongly
  supported by the data
• Patches exist with different carrying capacities
• Movement between patches is balanced
• Movement is negatively density dependent