Transcript Climate change and range shifts in marine communities, PAT EDIT

Climate Change and Range Shifts
Hopkins Marine Station, site of Barry et al. 1995
Example 1: Barry et al. 1995
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Hopkins Marine Station is located in
Monterey Bay
In 1931, ecologists sampled a
transect (red line) across the intertidal
zone. They recorded the identities
and densities of every species on the
transect.
In 1993, Barry and colleagues
relocated bolts marking the transect
and repeated the original survey.
They then divided the species into
three groups: (1) southern species,
extending far to the south, but not far
north of Monterey (2) northern
species, extending far to the north but
not south of Monterey, and (3)
cosmopolitan species, occurring over
a very broad latitudinal range.
Barry et al. 1995: changes
in species composition
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They found that southern species
increased abundances on the transects
(red dots, frame A), northern species
deceased abundances (blue dots, frame
A, and cosmopolitan species showed no
net change (green triangles, frame A).
Solid symbols indicate statistically
significant changes.
Plot B is a different representation of the
same trend: the means of the ratios of
densities (1993/1932) are plotted with
respect to species group.
The tends are consistent with the
interpretation that southern species are
expanding- and northern species
receding to higher latitudes. Such
latitudinal rage shifts have been
observed for terrestrial species.
Barry et al. 1995: changes
in species composition
• southern species abundances ↑
• northern species abundances ↓
• cosmopolitan species: no change
• means of ratios of densities (1993/1932)
Trends are consistent with hypotheses that:
- southern spp. are expanding
- northern spp. receding to higher latitude
 similar latitudinal rage shifts have been
observed for terrestrial species
solid = P<0.05
Barry et al. 1995:
temperature records
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Over the 60 years of the
comparison, sea surface
temperatures recorded at
Hopkins Marine Station increased
(plot A).
Comparing 11-year time spans
before the original survey and resurvey revealed higher
temperatures preceding the resurvey, with the most extreme
differences occurring in summer
(plot B).
They propose that the
temperature rise favored
southern species and disfavored
northern species. However, the
specific mechanism of effect is
not known.
Example 2: Holbrook et al. 1997
•Sea surface temperature
records indicate a major shift
in temperature regimes
occurring in 1975-1976.
•Such shifts are part of a
normal multi-decadal cycle
in the north Pacific ocean,
but global climate change
appears to be affecting the
cycles, making the upward
part of the cycles greater
than the downward, so that
long term temperatures are
ratcheting up.
Example 2: Holbrook et al. 1997
Sea surface temperature records
indicate major regime shift in
1975-1976
 part of a normal multi-decadal
cycle in north Pacific
Global climate change may make
warm cycles warmer
 long term temperatures are
ratcheting up
Holbrook et al. 1997:
shift in the fish fauna
• They observed that after
the shift in temperature
regime (arrows, plots A
and B) fishes species
became less diverse at
two survey sites (plot A),
southern species
increased and northern
species declined in
relative abundance (Plot
B).
Holbrook et al. 1997
• The authors are careful to point out that the negative
correlation between temperature and either fish diversity
or the abundance of northern species does not reveal
the mechanism of change.
• Higher sea surface temperature could directly affect fish
physiology, lowering survival or reproduction of northern
species. However, increases in surface temperatures
could indirectly affect them by diminishing their food
supplies. Cold surface water often causes high fertility,
high surfaces temperatures often restrict nutrient
availability, ocean productivity, and hence food
availability.
• Or the correlation could be a shear coincidence. More
work is needed.
Correlation vs causation
Negative correlation between °C and (a) fish diversity, or
(b) abundance of northern species, does not indicate any
mechanism or causation for change
Higher sea surface temperature could have:
- direct effects on fish physiology, lowering either survival
or reproduction of northern spp.
 cold surface water often causes high fertility
- indirect effects – e.g., diminishing food supplies
 high surfaces temps can restrict nutrient availability,
ocean productivity, and hence food availability
... or the correlation could be coincidence; more work needed
Example 3: Harley (2011)
Harley presents evidence to support of the idea that the extinction of
mussel beds in some areas of the Salish Sea (Puget Sound) results
from temperature stress with climate change.
Harley 2011: The hypothesis
• Harley bases his explanation on the refuge hypothesis (Connell
1970, Paine 1974)
• Under this hypothesis, the lower limits of the mussel zone are set by
predation from the sea star Pisaster ochraceus. Upper limits are set
by temperature stress.
• Wave exposure (topographic differences in average levels of wave
action, including up-shore wave wash) affects temperature stress.
On shores with high wave action, wave wash alleviates temperature
stress, so that mussels can survive at relatively high shore levels.
On shores with low wave action, wave wash does not reach up the
shore, and temperature stress extends to lower shores levels,
eliminating mussels from higher shore levels.
• The sea stars are unaffected by these differences in wave action, so
that the lower boundary of the mussels (lower limit to the refuge)
remains at the same shore level regardless of wave exposure.
• Therefore, from wave exposed to sheltered shores the vertical range
of the refuge (predator-free space) is compressed, according to the
hypothesis.
Diagram of the refuge Hypothesis (Connell 1970, Paine 1974)
A real mussel bed
in an apparent spatial refuge.
Harley 2011
Wave exposed: broad zones
Sheltered: compressed zones
• Harley predicted that as temperature stress increases with global
warming, invertebrate zones in sheltered areas should compress
further, eventually causing extinction.
Harley 2011: the sample design
Along a west-east transect, he compared temperature and
zonation as it exists today (grey squares) and a change in mussel
and barnacle zonation from 1957 to 2010 (gray circles). Lower
graph shows low tide temperatures measured at sites indicated by
squares.
Harley 2011: experimental design
low tide temperatures
measured at sites
Along east-west transects, compared:
 °C & zonation today
 change in mussel + barnacle zonation
from 1957 to 2010 (gray circles)
Harley 2011: results from 2009
The upper limits of barnacles and mussels measured on 2009 sites
(squares in previous slide) are negatively correlated with temperature (plots
A-D). Sea star heights measured at low tide show no statistically significant
relationship with low tide temperatures (plot E).
Harley 2011: results from
historical comparison
• From 1957 to 2009, air temperatures
measured in Victoria, a city midway
along the west-east transect, increased
steadily (plot A).
• Over the same time span, upper limits of
the two species of mussels declined
down shore, while lower limits showed
no statistically significant trend (plot B).
• Harley also observed that some mussel
beds in the wave sheltered eastern
extremity of the transect went extinct.
• He concludes that increasing
temperature stress with global warming
caused the upper limit of the refuge to
fall, vertically compressing the mussel
zone. The naturally compressed mussel
zones in the east eventually vanished.
Trends from 1957-2009
- air temperatures in Victoria (city
midway along transect) increased
- upper limits of barnacles & mussels
moved down shore
- lower limits showed no significant change
barnacles
- some mussel beds in wave-sheltered
eastern end of transect went extinct
 conclusion: increasing temp. stress
caused upper limit of refuge to drop,
vertically compressing mussel zones
- naturally compressed mussel zones
in the east just disappeared
mussels
Imagining alternative explanations
• In the following slides we learn about…
– Pisaster foraging behavior
– Temperature effects on Pisaster feeding rates
– the coincidence of environmental factors and
mussel bed loss in Southern California
About sea star
foraging behavior….
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Most studies of Pisaster predation report
densities made at low tide, when the sea
stars are resting.
But Pisaster moves with the tides to
forage. On sheltered shores, where the
lower limit of prey may occur at high
shore levels, the sea stars may forage to
high shore levels. On wave exposed
sites, they do not forage as high. In plot
A, the x-axis measures wave exposure
as high tide bottom flow speed; the Yaxis is shore level. Open symbols
indicate mean sea star heights at low
tide, solid symbols indicate sea star
heights at high tide), triangles show
highest reach of seas stars, the extreme
limit of high tides.
Panel B shows the corresponding
abundance (mean per cover) of mussels
over the wave exposure gradient.
Data are from sites in Barkley Sound,
roughly 50 miles from the Salish Sea
Transect.
Inviolable refuges?:
Sea star “Browse Lines”
Sea stars sometimes
forage above
the lower boundary of
mussel beds removing
small, preferred prey,
including barnacles and
small mussels.
Their foraging appears to
be more flexible than
assumed by the refuge
hypothesis.
Feeding rates of Pisaster are
temperature dependent
• Sanford (2000) observed feeding rates of
Pisaster as sea surface temperatures changed
with cycles of coastal upwelling. When water
temperatures dropped, sea stars feeding rates
decreased, when temperatures increased,
feeding rates accelerated.
• Lab studies showed that sea star feeding rates
were sensitive to water temperatures when
submerged, but not to a normal range of air
temperatures during simulated low tides.
Changes in mussel abundance on Bird Rock, Catalina
Island, Southern California.
D. R. Blakeway and C. D. Robles, Department of Biological
Sciences, California State University at Los Angeles, CA 90032.
Study site
on Bird Rock
Bird Rock east end, winter 1984
Bird Rock east end, March 1987
Bird Rock east end, April 1991
Bird Rock east end, March 1993
Bird Rock east end, October 1999
Using a camera on a pole one can take overhead views, which are easier to analyze
Bird Rock east end photo-mosaic, March 2001
Image analysis software can be used to covert panoramas to an overhead view
Mussel abundance Bird Rock East, 1983-2000
What happened in 1983-86 and 1991-93?
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Time
SST Catalina Ridge/Two Harbors, 1982-2001
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25
20
20
15
15
5
Jan-82
5
Jan-84
Jan-86
Jan-88
Jan-90
Jan-92
Jan-94
Jan-96
Major el nino (warm water) events in blue
Jan-98
Jan-00
Zooplankton abundance, SoCal Bight, 1950-2000
Could the recent (since the late 1970s) decline in zooplankton
abundance be the result of declining ocean production? If so, what are
the implications for mussels?
250
53
200
64
65
62
33
56
150
100
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57
51
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0
1950
85
72
66 68
52
54
78
67
69
81
75
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60
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86
80 82
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74
58
83
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1960
8889 90
1970
1980
years
Severe El nino years in orange.
94 95
91
93
92
1990
96
97
98
2000
Assuming that mussel zones are indeed
collapsing in the Salish Sea, can you
formulate an alternative hypothesis to
Harley 2011?