COMPETITION AND COEXISTENCE BETWEEN A MIGRANT AND RESIDENT BIRD DURING THE NON-BREEDING SEASON Kathryn Peiman, PhD candidate UCLA Dept of EEB
COMPETITION AND COEXISTENCE BETWEEN A MIGRANT AND RESIDENT
BIRD DURING THE NON-BREEDING SEASON
Kathryn Peiman, PhD candidateUCLA Dept of EEB
BackgroundComplete competitors cannot coexist
(Gause’s law)-predicts that ecologically similar species will
partition resources to reduce competitionThe 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?
BackgroundSolution? 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 haveexperienced interspecific competition (selection for divergence only insympatry)
• also include ecological effects due to seasonal changes (maychange the intensity of competition)
I will test hypotheses related to three topics:• Interference competition• Exploitative competition• Physiological effects of coexistence
Allopatry
Sympatry
Study speciesTBVI (resident in the Bahamas)Thick-billed vireo (Vireo crassirostris)Weight: 14.1 gBill length: 8.83 mmWing chord: 61.6 mm
WEVI (breeds in south-east US; winters in the Bahamas and Mexico)White-eyed vireo (Vireo griseus)Weight: 11.9 gBill length: 7.45 mmWing chord: 60.2 mm
Study locations
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 effectsof declining resource abundance by comparing data collected in the fall[Sept-Nov] to thewinter [Jan-Mar].
Interference competitionDirect 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.
Interference competitionQuestion:
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.
Interference competition - aggression
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.
Interference competition - aggression
TBVI attacking mount of WEVI
Interference competition - aggression
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).
Intra vs interspecific aggression: Both species were more aggressive to conspecifics than heterospecifics.
Evolutionary effects: Both conspecific and heterospecific aggression higher in sympatry compared to allopatry for TBVI; no difference for WEVI.
Ecological effects: No clear season effect.
Interference competition: territories
Methods: I used conspecific playback and GPS units to map the boundary of territory holders.Results: Conspecific territories barely overlapped (<10%), while heterospecific territories had a lot of overlap (>50%).TBVI and WEVI territories in sympatry
Interference competitionDo 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.
Exploitative competitionIndirect 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.
Exploitative competitionQuestion:
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.
Exploitative competition - diet
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)
Exploitative competition - diet
Results (ANOVAs):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 7.002.503.003.504.004.505.005.506.006.507.007.50
Nitrogen stable isotope values in the fall
Nitr
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sta
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isot
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valu
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Individual TBVI were consistent in their diet from the fall to the winter.
Exploitative competition – resource availability
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 specieshave been seen to consume this fruit).
Exploitative competition – resource availability
Results (repeated-measure ANOVAs):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).
Exploitative competitionDo 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.
Physiological effectsDirect 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’?
Physiological effects - corticosterone
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
Physiological effectsQuestion:
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.
Physiological effects - corticosterone
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: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
Physiological effects – corticosterone and body mass
TBVI body mass wasnegatively related to acute corticosterone (p=0.0008).
0 10 20 30 40 50 60 70 8002468
1012141618
Acute plasma corticosterone concentration (ng/mL)
Body
mas
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)
-the more corticosterone in your system, the more you use skeletal muscle as an energy source, resulting in lower body massBody mass also declined from the fall to the winter (p<0.0001) in TBVI, but not in WEVI. Neither species showed a difference in body mass in sympatry compared to allopatry.
Summary
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.
Broader importanceWhy 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.
AcknowledgmentsFunding:UC Mexus small grantGerace Research Center grantAOU research grantRalph Schreiber reseach grantUCLA Dept of EEB research grantsNSERC PGS fellowship
Committee:Greg GretherCatherine SugarTom SmithPeter Narins
Field/lab work:E. Rutherford, E. Quiros, M. Akresh, E. Curd, M. Rensel
Logistic support:Gerace Research CenterFriends of the EnvironmentLos Tuxtlas Research StationBEST Commission, the BahamasC. Macias Garcia, UNAM