Transcript CSULA Symposium 2011_HK - Cal State LA

Life at the edge:
local adaptation and range limits for two estuarine
sea slugs
Hanna Koch
Dr. Patrick Krug
California State University, Los Angeles
Local Adaptation
Selection
for low
salinity
tolerance
Selection
for high
thermal
tolerance
Migration
(gene flow)
Env. 1 - Los Angeles
Env. 2 – San Fran
•Natural selection produces adaptation, but:
• Slxn doesn’t always favor same traits in every habitat
• Animals don’t always stay in one place
•Adaptation results from selection on a heritable trait
• Gene flow opposes adaptation
Range Limits
• Represent species’ limits to adaptability
• Useful for ID’ing & testing limiting factors that
shape distributions/abundances of organisms
•
•
•
•
Geographic barriers
Abiotic (temperature, salinity)
Genetic Constraints (what inhibits adaptation)
Biotic Interactions (competitive exclusion)
Intertidal ecosystem
• Model system:
– Ideal for range limit studies
– Easily tracked along 1-D, narrow shoreline
• North & south endpoints
– Distribution spatially & temporally restricted by tides
• Exposed to extreme temps & salinities
• No subtidal refuge
• Climate change can lead to range shifts
Alderia modesta - Alderia willowi
Vaucheria longicaulis
•Estuarine sea slugs
•Live on exposed mudflats of salt marshes along North
America’s Pacific coast
•Live & feed on alga that grows on the mudflats
willowi, modesta
Dynamic Range Boundary
Bodega Bay
Alderia modesta
Northern sp.
Tomales Bay (HI)
Tomales Bay (OF)
Alderia willowi
Southern sp.
SF Bay
range edge vs. range center
• Tomales Bay (stressful):
– range edge
– least freshwater input compared
to surrounding bays (Bodega & SF)
A. willowi
Tomales Bay
Range edge
• Tectonically formed
• Los Angeles (optimal):
– range center
– More stable environment than TB
• Warmer temps
• 4-fold less precipitation
• Preferred higher salinities
LA
Range center
Hypotheses
1. The range-edge population is more locally adapted
to low salinity stress than range center
2. Local adaptation over the rainy season is more
pronounced at the range edge
3. Low salinity tolerance is a genetic,
heritable trait & is favored at the
range edge
Methods- measuring low salinity tolerance
(time to death)
Vital Staining
2 ‰ SW
Results 1 (summer):
local adaptation across range
ANOVA: F2,57 = 10.85, p < 0.001
Mean Time to Death (min)
A. willowi
A
B
Los Angeles
Bolinas
(Range center)
B
B > A,
p < 0.001
Tomales Bay
(Range edge)
Range edge exhibits greater local adaptation to low salinity
stress, even during summer before the winter rains
Results 2: local adaptation over growing season
Range Center
Mean Time to Death (min)
ANOVA: F1,38 = 6.0, p = 0.019
A. willowi
Range Edge
ANOVA: F1,38 = 244.12,
p < 0.0001
A. willowi
2 days
5.4 hrs
3.7 hrs
3 hrs
September ’10
December ‘10
September ’10
December ‘10
Populations become more adapted over rainy
season & even more so @ range edge
Mean Time to Death (min)
Results 3:
low salinity tolerance is a heritable trait
A. willowi
A. willowi
Range Center
Range Edge
Generation 0
Generation 1
Generation 2
Low salinity tolerance is (A) genetically based
(B) favored at the range edge
Mean Time to Death (min)
ANOVA: F1,38 = 10.4, p < 0.005
~3 days
~2 days
A. modesta
modesta’s ability to
withstand lower
salinities for longer
provides greater local
adaptation &
competitive advantage
over willowi in SF
A. willowi
Results 4:
comparing low salinity
tolerance between sister
species
Conclusions
1. Gradients in salinity drive variations in local adaptation for
A. willowi across its range
– @ range center, LA: nat’l slxn on low S tolerance is relaxed
•
Slight local adaptation, only seen over rainy season
– @ range edge, Tomales Bay: strong slxn on low S tolerance
•
Strong local adaptation, Low S tolerance conferred to offspring
2. A. willowi northern range limit currently fixed @ TB
– Genetic constraints
3. In SF, the most adapted willowi still never reach same level of
tolerance to very low salinity as modesta does
Overall Study Significance
•
If we can better understand how tolerance for the physical
environment limits the geographical range of species, then
we can make better predictions of potential ecosystem
responses to climate change, which is critical to effective
management and conservation
Thank You
• The Krug Lab:
Dr. Patrick Krug, Dr. Jann Vendetti, Betsy Shimer,
Dominique Gordon, Matthew Garchow, Angela
Llaban, Julia Vo, Diane Rico, John Martin
Climate Change
• As global temperatures rise:
– Changes in salinity & rainfall patterns
– Worldwide range limit shifts
• Previous studies suggest poleward (upper) bounds
set by physiological limits; equatorial (lower)
bounds set by interspecific competition
Daily exposure to 2‰ water
Daily exposure to heat stress
…correlated
with
seasonal
shifts in
temperature
& salinity
Seasonal
shifts in
presence /
abundance…
Population
Dynamics in
Mill Valley - SF
25
Mean Total Monlthy Rainfall (cm)
Bodega Harbor
Tomales Bay
20
Mill Valley
15
10
5
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Abiotic Stress
• Salinity gradients can determine distributions
w/in estuaries
• Salinity has not yet been linked to N-S range
endpoints in a intertidal animal, yet is critical
for estuarine taxa dist. Along coastlines w/
strong latitudinal gradient in precipitation like
NE Pacific