Download Research Paper/Writing Sample Impacts of Climate Change

Abstract
The increase in the Earths’ average annual temperature over that last 100 years has been
well documented. According to the U.S. Environmental Protection Agency, the Earth's average
temperature has risen by 1.4°F over the past century, and is projected to rise another 2 to
11.5°F over the next hundred years (http://www.epa.gov/climatechange/basics/). This increased
average temperature has had a profound effect of terrestrial ecosystems. These effects include
changes in the geographic distribution of species, the altering of species life cycles (e.g. flowers
blooming earlier in the year), and timing of species migrations. The pelagic marine environment
is not immune to impacts from global climate change. Studies have documented changes in the
geographic and depth distribution of several pelagic planktonic taxa (Edwards and Richardson
2004). However, the potential effects of climate change on pelagic biodiversity are not well
understood. Here we focus on the known and potential impacts of global climate change on the
biodiversity of pelagic fish.
Introduction
The distribution of pelagic fish is dependent on many factors including water
temperature, salinity, the availability of prey, and oceanic currents. Observations and theory
suggest that marine species respond to ocean warming by shifting their geographical and depth
ranges (e.g. Perry et al. 2005; Parmesan 2006; Hiddink and Hofstede 2008; Mueter & Litzow
2008; Dulvy et al. 2008). As species shift their ranges they will be invading the range of other
species; thus initiating a complex series of biotic interactions (e.g. interspecific competition) the
outcome of which are difficult to predict. Here we review current literature on climate change
and pelagic fish to determine the known and potential impacts on their biogeography, biotic
interaction, and physiology. In addition, we review future projections of species distribution to
evaluate the potential effects on biodiversity.
Potential Impacts of Climate Change on Species and Populations
Species Range Changes
All species live within a certain range of environmental conditions in which they are able
to tolerate. This is sometimes referred to as a species envelope of tolerance. Some species may
be able to thrive in a wide range of environmental conditions while others may have a very
narrow envelope. Whether broad or narrow, these limits form the large-scale boundaries of the
distribution of species.
One of the most important variables in a species envelope of tolerance is temperature.
Temperature controls the distribution of many species of pelagic fish. For instance, large
endothermic species like Atlantic Bluefin Tuna (Thunnus thynnus L.) are able to thrive over a
large temperature range and are found in the subtropical and temperate waters of the North
Atlantic Ocean, the Mediterranean, and Black Seas. Block et al. (1998) found Atlantic Bluefin
Tuna were able to tolerate water temperatures as low as 6°C. In contrast, their smaller relative,
the Yellowfin Tuna (Thunnus albacares) can survive in the temperature range of 10°C-33°C but
prefer warmer water.
Current theories on the distributional response of pelagic fish to climate change suggest
that as oceanic temperatures increase, species ranges will move poleward (Rose, 2005). Based
on the literature reviewed for this report, there is no clear evidence that this has occurred for
pelagic fish. However, researchers (Rose, 2005; Brander, 2007) have looked at recent
documented warming events to understand what may happen as oceanic temperature rise. Rose
(2005) looked at the distribution of both demersal and pelagic fish in the North Atlantic during
the period from 1920 to 1940. During this time, the area of the North Atlantic from Greenland
to Norway warmed significantly, by as much as 3-4°C (Taning, 1948). During this warming
event, Rose (2005) found: (i) some pelagic species such as capelin (Mallotus villosus) and
Atlantic herring (Clupea harengus) expanded their ranges northward; (ii) some local species (e.g.
herring) increased in density; (iii) spawning grounds for herring and capelin changed; and (iv)
species decreased in density where the water became too warm. This event can be viewed as a
possible analogue for global climate change.
From a fisheries perspective, the warming trend noted above and corresponding change
in species distribution was considered a positive event (Brander, 2007). However, over longer
temporal scales, the effect of oceanic temperatures changes on species distribution is not well
understood. Thus researchers have turned to models to predict the future ranges of marine
species. Bioclimate envelope models, alternatively called environmental niche models, have
been widely used to predict distributions of plants and animals (Cheung et al., 2009). A
bioclimate envelope can be defined as a set of physical and biological conditions that are suitable
to a given species. Thus, shifts in species distributions can be predicted by evaluating changes in
bioclimate envelopes under climate change scenarios. Cheung et al. (2009) used such a model to
predict the distribution range of over a thousand marine fish and invertebrates. They looked at
three climate change scenarios assuming high (720 ppm), medium (550 ppm), and low
(concentration not given) levels of greenhouse gases (Cheung et al. 2009). Consistent with what
was observed during the warming period discussed above, their model predicts a climate changeinduced range shift under all three scenarios. Specifically, range limits were projected to shift
poleward by a median of 600 km for pelagic fish species.
While simple bioclimate envelope model predictions are consistent with historical
observation for range shifts, Lavergne et al. (2010) have criticized the use of these models to
make future predictions on biodiversity. They argue that:
1.
Species niches are often labile.
2.
Many species are capable of rapid adaptive evolution in response to stresses such as climate
change.
3.
Ecological interactions will affect rates of species range shifts.
4.
Each species will respond differently to climate change (species assemblages will not
respond cohesively).
They recommend that species distribution models be developed that integrate evolution and
species interactions. However, models such as these are currently under development and no
studies were identified in the available literature where these models were used to evaluate future
patterns of biodiversity.
Predator/Prey Asynchrony
As indicated above, each species will respond to climate change differently. Speciesspecific differences in the reaction to climate change can be particularly important when
interacting species are considered (Schweiger et al. 2008). Tropically interacting species may
become asynchronous causing a temporal or spatial mismatch. An example of spatial mismatch
is when a large predatory pelagic fish shifts its range northward and down in the water column
due to temperature changes. However, its preys range remains the same due to their dependence
on some local resource. An example of a temporal mismatch could refer to how growth and
survival of fish larvae (predator) depends on this production being synchronous with that of their
main food items, i.e. early stage zooplankton (prey). However, if the timing of the fish spawning
and the plankton bloom becomes asynchronous, survival of the larval fish could be seriously
threatened.
Rapid Acclimation/Adaptation
Significant changes in ocean water temperature are a serious threat to the survival of
many pelagic species of fish. Many species of tropical pelagic fish, such as billfish (family
Istiophoridae), already live near their thermal tolerance limit (Western Pacific Fisheries
Management Council website at http://www.wpcouncil.org/). These species are particularly
vulnerable at the larval and juvenile stages. However, recent research has shown that many
species have the ability to rapidly acclimate/adapt to significant temperature changes. Donelson
et al. looked at the capacity of tropical reef fish to acclimate and adapt to water temperature
increases. The authors reared siblings from eight wild parental lineages of tropical damselfish
for two generations at water temperatures predicted under different climate change scenarios. To
evaluate a fish’s ability to acclimate, the authors looked at their capacity to meet increased
oxygen demand at elevated temperature. The result of this study showed that the fish have fully
regained their aerobic capacity after two generations. It is unclear how universal this
phenomenon is. However, it is likely that some species of tropical pelagic fish will be able
rapidly adapt/acclimate to changes in oceanic temperature due to climate change.
Invasion and Ecological Interaction
As discussed above, both the empirical data and modeling suggest that climate change
has the potential to shift the ranges of pelagic fish. As their ranges shift poleward, species will
invade ecosystems where they were not found previously. Hellman et al. (2007) list a series of
environmental filters that a species must pass through to become a successful invasive. They
state “A species must travel across major geographic barriers to its new location. The ability of a
species to pass through the transport stage depends on the rate at which propagules are moved
from one site to another and their viability upon arrival. Second, a species must survive and
tolerate environmental conditions at the arrival site. Third, a species must acquire critical
resources, survive interactions with natural enemies, and possibly form mutualistic relationships
at the new site. Finally, the species must spread, establishing populations in new sites across the
landscape.”
The end results of invasion are dependent on many variables and many outcomes are
possible including: (i) the new species may fill an empty niche and have little impact on the
ecosystem; (ii) the new species may not be able to compete with the local species and have little
impact of the ecosystem; (iii) the new species may be a better competitor than the local species
and become the dominant species in the ecosystem; (iv) the new species may have no natural
enemies and become the dominant species in the ecosystem; and (v) the new species integrates
itself into the ecosystem and competes for resources with the local species thus causing a decline
in the population of the local species. No empirical or modeling data was found in the literature
pertaining to invasions in pelagic ecosystems. However, many high trophic level pelagic species
(e.g. Bluefin Tuna) are already subject to intense pressures from fisheries around the world.
Thus it is likely that any increased competition for resources will have a negative effect on their
overall population.
In possible invasion outcomes (iii) and (iv) the new species has major competitive
advantages as they are able to outcompete native species or their numbers are not kept in check
by natural predation. In these cases, invasion may lead to the local extinction of one or more
native species. These location extinctions can have the effect of lowering the local (alpha)
diversity of the invaded ecosystem. For species with low thermal tolerance, regional or even
worldwide extinctions are possible under extreme climate change scenarios. Complete
worldwide extinctions of species may have a profound effect on global biodiversity of pelagic
fish.
Potential Impacts of Climate Change on Biodiversity
As discussed above, climate change has the potential to effect species and populations in
many different ways. Range shifts are expected to cause the rearrangement of ecosystems,
including the creation of novel communities. Range shifts will also results in local extinctions as
species vacate portions of their range. In addition, global extinctions are possible for species
living at their thermal tolerance limits. However, some species will be able to adapt/acclimate to
temperature increases relatively quickly. Thus given all these factors, it is difficult to assess
future trends in the biodiversity of pelagic fish. Thus researches have either looked to the past
for antecedents to climate change or used models to predict future trends in biodiversity.
Trends in Biodiversity in the Fossil Record
In their evaluation of the association of biodiversity and temperature, Mayhew et al.
(2012) looked at the fossil record to determine past biodiversity patterns. They tested whether
temperature is statistically linked with variation in global biodiversity and taxonomic rates. In
this study they analyze marine invertebrate biodiversity patterns for the Phanerozoic Eon. Their
findings indicated that “The geographic distribution of life on Earth supports a general pattern
of increase in biodiversity with increasing temperature.” These finding are consistent with
current latitudinal diversity patterns where biodiversity is highest in the warmer lower latitudes
but decrease as you move towards the poles. Thus higher oceanic temperatures do not
necessarily mean a decrease in biodiversity. However, it should be noted that the biodiversity in
Phanerozoic Eon likely developed from evolutionary processes that occur over long time
periods. Based on current projections, significant climatic change is expected over the next 50 to
100 years. Thus while some species may have sufficient time to acclimate/adapt to these
changes it is likely to short a period of time for evolutionary processes to give rise to new
species.
Biodiversity Models
Above we documented some of the problems associated with using bioclimate models to
predict future species distribution. However, recently researchers have recognized that these
models can be improved by incorporating critical processes such as dispersal and population
dynamics. As discussed above, Cheung et al. (2009) used such a model to make predictions of
species range shifts for both demersal and pelagic fish resulting from climate change. They then
evaluated the potential impacts to biodiversity resulting from these range shifts. First they
calculated the current species richness in each model grid cell by overlaying distribution maps of
all sampled species.
Then, using a bioclimate envelope model, they projected the change in
distributions of species of marine fish under high-, medium- and low- climate change scenarios.
They then calculated the number of newly occurring species (i.e. invasions) and the number of
locally extinct species in each cell.
The model results predicted general patterns of species invasion and local extinction
across all three climate change scenarios. As would be expected from historic data, high intensity
of species invasion was predicted to be concentrated in high latitude regions, specifically the
Arctic and the Southern Ocean. Local extinctions were predicted to be most common at low
latitudes, the Southern Ocean, the north Atlantic, the northeast Pacific coast and in semi-enclosed
seas (e.g. Mediterranean Sea). Species turnover was predicted to be highest in the Arctic and the
sub-polar region of the Southern Ocean. These results show that biodiversity in the high latitude
regions are likely to be highly sensitive to climate change. Generally, pelagic fish will extend
poleward as global oceans warm up. This will lead to high invasion intensity and overall
increases in species richness in high latitude regions. Biodiversity in tropical regions is likely to
be impacted by higher rates of local extinction. As discussed above, tropical pelagic fish tend to
live near their thermal tolerance limits. Thus, these animals are projected to move to colder
habitats in higher latitudes leading to local extinction in the tropical regions. Thus the
biodiversity of pelagic fish is likely to decrease at lower attitudes.
The authors did not find a rate of global extinction of marine species that is comparable
to the high rate of extinction (15–37%) predicted for terrestrial organisms. They attributed this to
the high dispersal ability of marine species which allows them to more easily find suitable
habitat.
Summary
The potential effects of global climate change on pelagic fish are complex and
multifaceted. Bioclimate models predict that the warming ocean will causes range shifts for
many species as they move poleward and deeper in the water column toward cooler
temperatures.
However, each species will respond differently to climate change which may
results in predator/prey asynchrony. Some species living near their thermal tolerance limit may
be at risk of local or even global extinction. However, recent studies have shown that rapid
acclimation/adaptation is possible in only a few generations. Range shifts are expected to cause
the rearrangement of ecosystems, including the creation of novel communities. The winners and
losers in these new ecosystems will be the result of a complex series of biotic interactions which
are hard to predict. However, bioclimate model results suggest that the biodiversity of pelagic
fish will increase at higher latitudes and decrease at lower latitudes.
Predicting the effects of climate change on bluefin tuna
(Thunnus thynnus) spawning habitat in the Gulf of Mexico
Block, B.A., Dewar, H., Farwell, C., Prince, E.D. A new satellite technology for
tracking the movements of Atlantic Bluefin tuna. Proc Natl Acad Sci U S A. 1998 August 4;
95(16): 9384–9389.
Cheung, W.L., Lam, V., Sarmiento, J.L., Kearney, K., Watson, R., and Pauly, D.
Projecting global marine biodiversity impacts under climate change scenarios. Fish and
Fisheries , 2009, 10, 235–251