Cryptic Density Dependence

1
Competition and Density Dependence in Marine
Fishes
Why worry about density dependence? Issues
detecting density dependence When does density
dependence occur? Interspecific competition
2
Issues in Detecting Density Dependence

• The naive experimentalist
• Presume you set up 10 experiments, with 5 low
  abundance treatments, and 5 high abundance
  treatments
• Run each treatment for multiple generations
• You want to measure density dependence by
  measuring the average recruitment for each
  treatment

3
The Naïve Experimentalist
The naïve experimentalist switches treatments
whenever There is a high recruitment event in
the low abundance (switch to the high
abundance treatment) There is a low recruitment
event in the high abundance treatment (switch
to the low abundance treatment)
Per Capita Recruitment
Low Abundance
High Abundance
4
The Naïve Experimentalist in Practice
The treatment is determined by the response
You tend to oversample high recruitment events at
low spawner abundances and undersample high
recruitment events at high spawner abundances
5
The Observation Error Problem

• Two types of statistical error
• Process error (actual noise in the biological
  response)
• Observation error (errors in measuring biological
  response)
• Ignoring observation error leads to large biases
  in estimates of density dependence!

6
Density Dependent Overwinter Mortality in
Bluefish?
Age-1 CPUE
Wiedenmann and Essington, 2006 CJFAS 631934-1943
7
Is there really density dependence in bluefish?

• Calculated the probability of getting observed
  results, if there was no density dependent
  relationship
• Only if error variance was huge, would you expect
  to get observed results

Wiedenmann and Essington, 2006 CJFAS 631934-1943
8
Cryptic Density Dependence
Local sites vary with respect to habitat
quality Habitat quality dictates intensity of
density dependence Sites will be settled at a
rate proportional to habitat quality High
quality -gt High Settlement (low density
dependence) Low quality -gt Low Settlement (high
density dependent sites) When looking across
sites, there is little apparent density dependence
9
Crypic Density Dependence
Thalossoma hardwicke survivorship across sites
Shima and Osenberg. 2002. Ecology 84 46-52
10
When does density-dependence occur?
Consider a species with several life history
stanzas Each stanza has unique set of
habitat, food, predation risk
Adult
early juvenile
late juvenile
larval
We know that density dependence has to occur in
one of these linkages
11
Identifying Density Dependence
Early Juvenile Abundance
Larval Abundance
Late Juvenile Abundance
Early Juvenile Abundance
Adult Abundance
Late Juvenile Abundance
12
Critical Stages and Density Dependence in Striped
Bass
Kimmerer et al. (2000) CJFAS
13
Kimmerer et al. 2000 Results
Density dependence at a late life history stanza
(YOY- Age 3) dampened variability due to early
stanza density-independent mortality
14
Intra and Interspecific Competition

• What is competition, and how is it measured?
• Problems detecting / inferring competition
• Rules for coexistence for competing species
• Evidence for alternative types of competition

15
What is Competition?

• The use of resources by an organism that thereby
  reduces the availability of the resource to
  others
• The depressing influence of density on
  (population) growth rate
• Negative biological interactions between 2 or
  more individuals that occurs when a necessary
  resource is in limited supply
• An interaction in which one organism consumes a
  resource that would have been available to, and
  might have been consumed by another.
• Mutual demand for a resource.

16
Two categories of competition
Exploitative use of resources that eliminates
or transforms the resource and thereby reducing
its availability to others Interference
making resource unavailable for others, without
affecting the abundance of that resource
17
Competitive Exclusion Principle Two species
cannot coexist on the same limiting resource
but species seem to share quite a few resources!
Feeding Habits of Bigeye and Yellowfin Tuna
King J.E. and Ikehara I.I. 1956. Comparative
study of food of bigeye and yellowfin tuna in the
central pacific. Fish. Bull. 57 61-85
Competitive exclusion principles hold for
closed systems without stage-structure or
environmental forcing
18
Litmus Test for Detecting Competition

• Must demonstrate that species share some resource
  that is in limited supply OR
• That resource use shifts in the presence of a
  competitor AND
• Must demonstrate that limitation in this resource
  ultimately affects population size

19
Look out for Tautologies! If there is a
negative relation between the abundance of two
species, they must compete for some limiting
resource If there is overlap of resource use
between two species, then that resource must not
be limiting If there is no overlap of resource
use, then those species must not be competing
with each other.
20
Competition Between Bluefish and Striped Bass?
21
Interpreting Resource Use
Buckel and McKown, 2002
Compared food habits and local spatial
distributions in nearshore habitats (juvenile
habitat) Striped Bass Diets Between 30 96
Invertebrates Bluefish Diets Between 7- 24
invertebrates Little spatial overlap at the
scale of sampling stations What does this mean?
Does this support the notion of competition?
22
Resource Partitioning 101
Theory tells us that in the presence of a
competitor, a species niche will be compressed
When alone (allopatry)
When together (sympatry)
B
A
23
Selectivity and Resource Use
Competition for rocks
Hornyhead Chub (Nocomis biggatus)
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Why is this important?
Competition is often fueled by selectivity This
is often overlooked in analysis of supply and
demand
Leiostomus xanthurus Spot
Dagge, M.J. and Govoni, J.J. 1996
It is concluded that ichthyoplankton predation
in these regions rarely has any significant
effect on copepod population dynamics, and
therefore larval competition for food is unlikely
to contribute to density-dependent mortality.
25
Lottery Competition in Open Communities
Over Supply of Potential Settlers
Limited Number of Settling Sites
26
Peter Sales Lottery Competition Model
Assumptions and Observations Within particular
ecological guilds, one sees Little
specialization for food types Little
specialization for habitat types
Sale, P.F., 1977. Maintenance of high diversity
in coral reef fish communities. Am. Nat. 111
337-359
27
Pomacentrus wardi
Plectroglyphidodon lacrymatus
Stegastes apicalis
28
Peter Sales Lottery Competition Model
Assumptions and Observations Within particular
ecological guilds, one sees Little
specialization for food types Little
specialization for habitat types Individuals
hold territories throughout their lives (strong
founder effect) When fishes die, space is
rapidly refilled by residents and new
colonists No tendency for space initially held
by one species to be taken by the same species
Sale, P.F., 1977. Maintenance of high diversity
in coral reef fish communities. Am. Nat. 111
337-359
29
Lottery Competition in Open Communities
Over Supply of Potential Settlers
Limited Number of Settling Sites
30
Lottery Competition Continued
Persistence of species does not require resource
partitioning Presumes little or no
density-dependence after settling
31
Readings

• Mumbay testing lottery competition hypothesis
• Shima and Osenberg Cryptic Density Dependence
• Wiedenmann and Essington Evaluating density
  dependent overwinter survivorship of bluefish