2. Complete competitors cannot coexist (Gause’s law)
-predicts that ecologically similar species will
partition resources to reduce competition
The idea is simple, but
demonstrating that interspecific
competition is currently occurring
is difficult in natural populations.
How can we tell whether current interactions are due to
competition? And how can we separate interactions over
food resources from interactions due to breeding
requirements?
3. Solution? Birds during the non-breeding season!
• no breeding requirements (only individual survival)
• compare locations of sympatry
(species coexist) to allopatry (species
found alone) to test whether they have
experienced interspecific competition
(selection for divergence only in
sympatry)
• also include ecological effects due to seasonal changes (may
change the intensity of competition)
I will test hypotheses related to three topics:
• Interference competition
• Exploitative competition
• Physiological effects of coexistence
Allopatry
Sympatry
4. TBVI (resident in the Bahamas)
Thick-billed vireo (Vireo crassirostris)
Weight: 14.1 g
Bill length: 8.83 mm
Wing chord: 61.6 mm
WEVI (breeds in south-east US;
winters in the Bahamas and
Mexico)
White-eyed vireo (Vireo griseus)
Weight: 11.9 g
Bill length: 7.45 mm
Wing chord: 60.2 mm
5. Allopatric
WEVI
Sympatric
TBVI and
WEVI Allopatric
TBVI
I visited three locations to test for evolutionary effects of
species coexistence (comparing allopatry to sympatry): Abaco
Island, the Bahamas (sympatry); San Salvador Island, the
Bahamas (allopatric TBVI); and Los Tuxtlas,
Mexico (allopatric WEVI). At each location, I
tested for the
ecological effects
of declining
resource abundance
by comparing data
collected in the fall
[Sept-Nov] to the
winter [Jan-Mar].
6. Direct interactions between individuals over a shared,
limited resource
• direct interactions can involve signalling (color,
posture, chemicals, auditory cues, etc) and fighting
(biting, attacking, striking, etc)
• types of resources include mates, offspring-rearing
locations (nests, burrows, holes, etc), predator-free
space, environmental refuges (basking sites, moisture-
rich areas), and food
-but mates are only shared if the species hybridize; offspring
locations if it is during the breeding season; predator-free space
if predation is a major selective pressure; and environmental
refuges if individuals are vulnerable to those stresses. Food,
however, is always necessary to both species.
7. Question:
Do TBVI and WEVI engage in interference
competition?
Hypotheses and predictions:
1) If heterospecific aggression is beneficial, it will
be higher in individuals from sympatry
compared to allopatry, while conspecific
aggression will not differ.
2) If the benefits of excluding individuals
outweigh the costs, territories will be mutually
exlusive.
8. Methods:
Territory holders were color-banded for individual
recognition.
I used taxidermy mounts and playback of songs/chatter
to simulate territorial intrusions.
Each bird received a 3 minute trial, 3 minutes of silence,
then a second 3 minute trial. I systematically alternated
the order of conspecific and heterospecific trials among
territory holders to control for presentation order.
I recorded the movement (time spent within 0-2m and 2-
5m of the mount, and time spent attacking the mount)
and vocal responses (number of songs and chatters) of
each territory holder.
10. Results (using GLMMs with identity link):
691 trials; 54 individual TBVI and 53 individual WEVI
responded to at least one trial.
Species differences: TBVI were much more aggressive than
WEVI (attacked and spent time closer to the mount)
(p<0.0001).
Intra vs interspecific aggression: Both species were more
aggressive to conspecifics than heterospecifics (p<0.001).
Evolutionary effects: Both conspecific and heterospecific
aggression higher in sympatry compared to allopatry for
TBVI (p=0.03); no difference for WEVI.
Ecological effects: No clear season effect.
11. Methods: I used conspecific
playback and GPS units to
map the boundary of
territory holders (TBVI:
n=49; WEVI: n=41).
Results: Conspecific
territories barely overlapped
(<10%), while heterospecific
territories had a lot of
overlap (>50%).
Example of TBVI and WEVI territories in
sympatry
12. Do TBVI and WEVI engage in interference
competition?
Yes, but it is asymmetrical and perhaps not an
adaptive response
1) Heterospecific aggression AND conspecific
aggression were higher in sympatry compared to
allopatry. Cannot conclude whether one or both
behaviors were under selection in sympatry.
2) Conspecific territories were mutually exlusive, but
heterospecific territories overlapped.
Surprisingly, the high levels of heterospecific
aggression by TBVI towards WEVI did not
result in exclusive territories.
13. Indirect interactions between individuals over a
shared, limited resource
• indirect means that individuals may never
meet or physically interact, but both consume
the same resource so there is less available for
individuals of the other species
• resource acquition involves detection (seeing
and identification), capture (handling time)
and consumption (eating it)
-Food is the most common resource shared during
exploitative competition.
14. Question:
Do TBVI and WEVI engage in exploitative
competition?
Hypotheses and predictions:
1) If resources are shared between species, diet
will diverge between individuals from
sympatry compared to allopatry.
2) If rainfall affects arthropods and fruits, then the
abundance of food will decline from fall to
winter mirroring decreases in rainfall.
15. Methods:
Clipped 2mm of claw from each
individual.
Each claw was analyzed for nitrogen stable isotopes
(δN):
-all individuals obtain their nitrogen from their food
source, and this accumulates up the food chain
-therefore, nitrogen isotopes reflect trophic level
-this can separate birds eating fruit from those
eating herbivorous insects (caterpillars, leaf
hoppers) and those eating carnivorous arthropods
(predaceous beetles, spiders)
16. Results (ANOVAs; TBVI: n=151, WEVI: n=92):
Species differences: WEVI fed higher on the food
chain than TBVI (p=0.0004).
Evolutionary and ecological effects: WEVI fed higher
on the food chain in the winter than the fall in
symptry but did not change their diet in allopatry
(p=0.002). TBVI did not
change their diet between
sympatry/allopatry or from
the fall to the winter.
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
2.50 3.50 4.50 5.50 6.50 7.50
Nitrogenstableisotopevaluesin
thewinter
Nitrogen stable isotope values in the fall
Individual TBVI were consistent in their
diet from the fall to the winter.
17. Methods:
Every 200m along trail conducted a 50m transect
perpendicular to the trail by collecting arthropods
at two sampling points: 25m and 50m.
I used beat sheets (nylon on a PVC pipe frame) to
collect all arthropods shaken from four trees at
each sampling point.
I also estimated the number of fruits on all Bursera
simaruba trees within 5m of
the 50m transect (both species
have been seen to consume
this fruit).
18. Results (repeated-measure ANOVAs; n=80
transects per season):
Ecological contrast: The number of arthropods
declined from fall to winter (p=0.017). Fruit
abundance did not change from fall to winter.
Evolutionary contrast: The decline in arthropod
abundance was stronger in sympatry than in
allopatry (p=0.042).
19. Do TBVI and WEVI engage in exploitative
competition?
Yes, but it is asymmetrical.
1) Diet diverged between individuals from sympatry
compared to allopatry. WEVI consumed the
higher trophic levels (ie. spiders) in sympatry.
2) Abundance of food declined from fall to winter,
especially in sympatry. Indicates there should be
stronger competition in winter than fall. The
presence of two vireo species in sympatry may
have resulted in decreased food availability.
20. Direct physical interactions are costly in terms of
time away from feeding and energy expended;
receving those interactions are costly for the
same reasons.
Indirect interactions are costly when the more
energetically favorable food is consumed by
another species, leaving less desirable or less
profitable food.
Therefore, both types of competition, whether
interactions are adaptive or not, may result in
costs. But how do you measure ‘cost’?
21. What is corticosterone?
-hormone released in the blood in response to a
stressful event
-stress includes predation attempt, severe
environmental weather, lack of food, etc
-causes behavioral and physiological changes,
including increasing activity (more likely to find food)
and activating gluconeogenesis (releasing
carbohydrates from muscle for energy)
-if present at elevated levels for long periods, can cause
individuals to lose body mass
-the abundance of this hormone is a common way to
assess whether an individual is ‘stressed’ and thus
experiences a ‘cost’ by losing energetic reserves
22. Question:
Do TBVI and WEVI experience physiological
costs from coexistence?
Hypotheses and predictions:
1) If coexistence is stressful, both species will have
higher corticosterone and lower body mass in
sympatry compared to allopatry.
2) If resource decline is stressful, both species will
have higher corticosterone and lower body
mass in the winter than the fall.
23. Methods:
I bled each bird at capture (=baseline corticosterone) and after
30 minutes (=acute corticosterone).
-30 minutes of handling is considered a stressful event, and
elicits an increase in this hormone
I spun the blood in a centrifuge, separated the plasma from the
red blood cells, then froze the plasma at -80°C until
analyzed using an enzyme immunoassay kit.
Results: (ANOVAs; TBVI: n=42; WEVI: n=34)
Baseline corticosterone levels were higher in sympatry than
allopatry (TBVI: p=0.024; WEVI: p=0.05).
-this indicates that the presence of the heterospecific
increased stress
-higher corticosterone can cause increased foraging effort
and the release of stored energy reserves
24. TBVI body mass was
negatively related to
acute corticosterone
(p=0.0008).
0
2
4
6
8
10
12
14
16
18
0 10 20 30 40 50 60 70 80
Bodymass(g)
Acute plasma corticosterone concentration (ng/mL)
-the more corticosterone in your
system, the more you use skeletal
muscle as an energy source,
resulting in lower body mass
Body mass also declined from the fall to the winter
in TBVI (p<0.0001), but not in WEVI. Neither
species showed a difference in body mass in
sympatry compared to allopatry.
25. TBVI are dominant and really aggressive towards
the subordinate WEVI, yet their territories
overlap.
WEVI eat different food when they coexist with
TBVI, especially when resources decline.
TBVI and WEVI are more stressed when they
coexist, and TBVI are stressed as resources
decline.
Both species experience negative effects of
coexistence. This is the first time this has been
demonstrated during the non-breeding season.
26. Why do we care?
-as the climate changes and habitat is lost, ecologically
similar species will be forced to share smaller fragments
of habitat, potentially leading to an increase in the
occurrence and intensity of interspecific interactions.
-previous studies have shown that territory location,
quality of diet, and physiological condition can all affect
survival, and thus fitness and population processes.
-effects on individuals during the non-breeding season
have been shown to carry-over to the breeding season in
both migrants and residents, affecting both survival and
reproduction, and thus fitness.
-microevolutionary processes: subpopulations under strong
selection to adapt to conditions on the wintering ground
(i.e. presence of heterospecifics) may experience a
mismatch to conditions during their breeding season.
27. Funding:
UC Mexus small grant
Gerace Research Center grant
AOU research grant
Ralph Schreiber reseach grant
UCLA Dept of EEB research grants
NSERC PGS fellowship
Committee:
Greg Grether
Catherine Sugar
Tom Smith
Peter Narins
Field/lab work:
E. Rutherford, E. Quiros,
M. Akresh, E. Curd, M. Rensel
Logistic support:
Gerace Research Center
Friends of the Environment
Los Tuxtlas Research Station
BEST Commission, the Bahamas
C. Macias Garcia, UNAM
