Ecosystems

In one of the most comprehensive syntheses of the impacts of climate change on species, Parmesan synthesizes 866 peer-reviewed papers, written from 1899-2003, that depict climate change impacts on global biota. Her conclusion: human-induced climate change has already affected biodiversity. The review covers phenologic changes, interactions across trophic levels as species' timing mismatches, range shifts, and changes in evolutionary processes. Parmesan notes that springtime has arrived earlier on every continent but one; that many predator-prey relationships have changed as a result of mistiming; that numerous species are migrating poleward; that disease vectors are altering ranges and populations with negative consequences; and that ranges of many species are contracting, leading to extinction of individual species.

Implications: This extraordinarily thorough review indicates that we are already seeing major climate-related consequences to global biodiversity. It implies a similar reduction in the services that people derive from biodiversity, such as medicines, water filtration, air purification, carbon dioxide sequestration, and others.

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Regehr, Amstrup, and Stirling document the decline in polar bears in the Arctic as a result of sea ice retreat in our warming climate. Polar bears depend on ice floes to feed from (the ice gives them a resting place as they swim to find their prey), as well as to breed. The scientists found that the rate of cub survival in the southern Beaufort Sea had significantly decreased from the periods between 1967-1989 and 1990-2006. In addition, in performing skull measurements and weight analyses, they found that the adult male polar bears in the last two and a half decades were far lighter than those captured before 1990.

Implications: The decline in polar bear cub survival, as well as adult size and weight, has paralleled a loss of sea ice, which the species depends on. The results from this study are consistent with other assessments of climate impacts on the Arctic. The status of the polar bear is currently being considered by the U.S. Government, as it may warrant listing as a threatened species. The World Conservation Union (IUCN), the world's leading body on cataloging threatened species, has already added the polar bear as a threatened species in its 2006 "Red List".

Many of those living in the western U.S. have noted an increase in forest fire activity. However, it has previously been challenging to determine causality -- is it a change in management or climatic factors that lead to the change? A new study by Westerling et al. suggests spring and summer warming, compounded by an earlier snow melt in the spring months, which leaves trees more combustible for longer periods of time, is to blame. Using a comprehensive database of wildfire activity in the western U.S. since 1970, as well as management and climatic data, the authors rely on statistical associations to explain the cause of this phenomenon. The authors demonstrate that recent years of earlier snow melt increased fire burn area more than 6.5 times that of previous years. Human-induced climate change, which they assert is a causal mechanism in current burning patterns, has brought drier conditions to the North American boreal regions. These impacts have fostered the growth of the size and length of burns.

Kasischke and Turetsky reach similar conclusions. Tracking the fire regime in the North American boreal region over the period from 1959 to 1999, the authors obtained data on fire size, fire events, and seasonal patterns and found that while the area burned by human-ignited fires has decreased over the four decades studied, the burned area in the region has doubled between the 1960s/70s and 1980s/90s. Moreover, the frequency of fire events on a large scale has increased. The authors attribute the recent fire regime to warming trends.

Implications: Increased forest fire activity has sociological, cultural, and ecological repercussions. Fire regime changes will not only challenge managers in the North American boreal regions but may also impact the ecosystem processes in the region. Perhaps most significantly, drying conditions coupled with larger fire events could lead to the burning of organic soil layers. This will further impact biodiversity composition, growth rates, and nutrient cycles. The studies suggest an additional financial impact: U.S. federal agencies spend over US$1 billion per year on fire fighting, and U.S. forest land plays a significant role in the livelihood of many communities.

Conservation biologists Malcolm et al. explore the impacts of a doubling of CO2 levels on endemic species (species that are confined within the given area) in areas of rich biodiversity, or "hotpots." They identified 25 hotspots, which house 44 percent of the world's plant species and 25 percent of vertebrates, and ran 14 runs with general circulation models and global vegetation models. While some species in hotspots were able to migrate and survive, others were left more vulnerable to climate change, including those located in the hotspots of the Caribbean, Southwest Australia, Tropical Andes, Cape Floristic Region, Indo-Burma, and the Mediterranean Basin. Malcolm et al. found that under doubled CO2 levels, some hotspots lost 39-43 percent of species. This loss translates into a potential extinction of over 55,000 endemic plant species and 3,700 endemic vertebrate species. The authors found that the extinction predictions were not unique to hotspots; other ecosystems could face similar extinction events.

Implications: Absent strong climate policy, it is anticipated that atmospheric CO2 concentrations would double well before the end of the century, with concomitant global warming. This research confirms that climate change will be one of the leading factors of species degradation in decades to come, with the loss of thousands to tens of thousands of endemic species.

Collecting data from 16 atmosphere-ocean general circulation models and 52 model runs, scientists Scholze et al. examined the ecosystem response to temperature rise between 1961-1990 and that projected for 2071-2100. They used three different temperature scenarios: less than 2 degrees C increase, 2-3 degrees C increase, and more than 3 degrees C. Under each warming scenario, risk of ecosystem change increased. For example, fire events are exacerbated, and the risk of forest transformation to non-forest ecosystems are greater (e.g. while historically less than 5 percent of the land was transformed, this rises to 43 percent with a rise of 2 degrees C; 75 percent with a rise of 2-3 degrees C; and 85 percent with an increase of more than 3 degrees C). Moreover, the authors demonstrate that one of their modeled terrestrial land sinks, which ordinarily sequesters carbon, turns into a source of carbon under the greater-than 3 degrees C warming scenario, creating an additional warming feedback.

Implications: Higher temperatures result in higher risks to ecosystem resiliency, and such responses are non-linear. Even small temperature shifts (less than 2 degrees C) are still significant.

  • Jonzen, Niclas; Linden, Andreas; Ergon, Torbjørn; Knudsen, Endre; Vik, Jon Olav; Rubolini, Diego; Piacentini, Dario; Brinch, Christian; Spina, Fernando; Karlsson, Lennart; Stervander, Martin; Andersson, Arne; Waldenstrom, Jonas Lehikoinen, Aleksi; Edvardsen, Erik; Solvang, Rune; and Nils Chr. Stenseth. "Rapid Advance of Spring Arrival Dates in Long-Distance Migratory Birds." Science 312(5782): 1959-1961. 30 June 2006.
. 4 May 2006.

As climate change takes its toll on the world's ecosystems, species will vary in their adaptive response to change, sometimes adversely impacting one another as food chain dynamics shift. One example is the shift in migratory birds, which can be expected to time their movement around breeding and food availability. While it is expected that migratory birds traveling only short distances would be able to understand environmental cues better than long-distance migratory birds who cannot detect changes in their faraway destination, Jonzen et al. find that European long-distance migratory birds are changing their spring arrival dates in Scandinavia more than short-term migratory birds. The scientists also demonstrate that bird arrival dates in the Mediterranean have moved forward, suggesting the advance in arrival is triggered earlier in their journey. The authors propose that altered migration timing will lead to evolutionary changes -- as individuals able to adapt to climatic changes are favored.

Both et al. provide additional documentation for this phenomenon, studying the interaction between climate change and the habits of the pied flycatcher, a long distance migratory species which depends on caterpillars for food. They examined the timing of peak caterpillar populations and arrival dates of the flycatchers in nine populations in the Netherlands. They found that due to warming, the peak availability of prey-caterpillar populations was occurring earlier in the season, and that by the time the birds arrived, they often did not have enough food for their nestlings. According to Both and colleagues, this led to a 40 percent population decline of the flycatcher over the past 20 years.

Implications: Temporal peak food availability is common in many food chains, and the mistiming between predator and prey species is not a novel phenomenon. While these climate-change driven trends are already being manifest in migratory species, it can be anticipated that additional species will be affected as the food chain is further modified due to climatic shifts.

Not only are species' ranges shifting and population dynamics and breeding seasons being altered in our warming climate, but micro-changes to species, as genetic composition shifts, are transpiring as well. Balanyá and colleagues document genetic changes in populations of a fly species Drosophila subobscura in three continents over the course of roughly two and a half decades. They find that of 22 population groups, 21 altered their genotypes to those of low latitude populations which live in warmer climates. They attribute this genetic shift to the species' adaptation to climate change. Sarup and colleagues document a similar change to a different Drosophila species. They show that in 11 populations of Drosophila buzzatii located in southeastern Australia selection for adaptive traits to warming was occurring. Thirteen of the nineteen traits that are relevant to adaptive capacity to climatic geographical constraints were altered, as more resilient genotypes were selected for.

Implications: The studies suggest both that warming is occurring, and also that species are beginning to adapt. However, while the genetic shifts in the studied Drosophila species were relatively rapid due to short generation lives, other species may not have the capacity to adapt as easily. Genetic change of this type is symptomatic of disruption of natural ecosystems and will no doubt have consequences for population dynamics and species resiliency to changing climate conditions.

Grebmeier and colleagues document a dramatic ecosystem change in the northern Bering Sea that has been contemporaneous with higher atmospheric and ocean temperatures, as well as increased sea ice loss. Where previous conditions favored species that relied on ice cover, such as marine mammals like walruses and whales, the ecosystem is now being overtaken by pelagic fish species.

Implications: This study has far-reaching consequences for marine ecosystems and productivity. Ecosystem composition can be expected to be altered under warming temperatures as marine mammals and seabirds suffer from ice loss, and pelagic fish species increase in the absence of predators. Simultaneously, some commercial fish stocks may be negatively impacted as species migrate out of fishing zones or die off.

With the advent of satellite technology, the ocean can be scanned within a few days and comparative measurements of ocean productivity in multiple marine regions easily obtained. Doney uses such satellite measurements to determine the health of phytoplankton, small marine organisms that live in the top layer of the ocean. He finds that phytoplankton productivity trends parallel climatic changes: as sea level temperatures increase, phytoplankton productivity decreases. He attributes this decline to warming-induced inhibition of ocean layer mixing, which limits the upwelling of nutrients (e.g. iron and nitrogen) from lower ocean layers.

Implications: Phytoplankton is a key underpinning of the marine food web, and loss of productivity could have significant ramifications for other marine species and fisheries. It is likely that marine ecosystem productivity will decline if warming increases at its current rate. In addition, phytoplankton are a natural sink for atmospheric CO2, which is absorbed during photosynthesis. Therefore, their loss may also have a feedback effect on global CO2 concentrations.

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