Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

Impact of global climate change on individual ecosystems F 4.2

235

(Fitzharris, 1996). Chapin and Shaver (1996) have

proven by experiment a possible change in the composition

of species in tundra ecosystems on the basis

of increased CO 2

concentration and temperature.

Over nine years the mean temperature of a tundra

area in Alaska was increased by 3.5 ºC and the lighting

strength and nutrient levels modified. The populations

of the most common plant species increased,

by contrast the lichen and herb species that form the

main food source for caribous declined. The international

tundra experiment has not yet been able to

confirm such a clear reaction to climate change. The

different reactions of individual plant species in Sweden,

Canada and Alaska observed over many years

varied to the point that no general conclusion may be

drawn for growth or biotope composition in tundra

regions (Henry, 1997).

F 4.2.3

Coastal ecosystems

Coastal ecosystems are particularly sensitive to climate

change, such as for example mangrove forests,

coastal marshes, estuaries and river deltas, dune systems,

low islands and coral reefs; the latter are discussed

in the next section (IPCC, 1996b; Markham et

al, 1993). The rate of sea level rise is crucial in that

context as is the strength and frequency of storms

(Markham et al, 1993). However, the change in the

transportation of sediment in the rivers by dam construction

and other flood protection measures also

influence coastal ecosystems. Over longer periods of

time coastlines fall and rise as a result of geological

processes. The Mississippi delta accounts for around

41 per cent of total coastal wetlands in the US and in

this century an area of around 40 hectares has been

lost every day. Climate scenarios (business as usual)

predict in this context by the year 2100 an additional

loss of 39 per cent of the current area (Reid and

Trexler, 1991).This jeopardizes numerous animal and

plant species and at the same time an effective CO 2

sink is being lost.

Mangroves cover an area of 20 million hectares or

25 per cent of the tropical coastline. The 34 known

tree species from nine families form a unique habitat

with typical adaptation such as air roots and other

halophyte strategies. Reconstructions of past situations

indicate that mangroves can only tolerate sea

level rises of up to 12cm a century (Ellison, 1994) – at

that rate they would have extreme difficulty adapting

to the predicted sea level rises. Mangroves are also at

risk, however, by increased water temperatures and

changes in salinity and sediment levels.

Climatic changes also have far-reaching ecological

consequences for other flat sedimental coasts, such as

for example the German North Sea coast (Reise,

1993). First of all probably at a warming of 2–4 ºC

there will be an influx via the English Channel of

species that are similar to the Eem period in the last

interglacial and could increase the diversity by 20–40

per cent. With temperature changes however there

would not just be an expansion of the species, but

also a change in their vertical distribution by tidal

zone. The frequency of harsh winters has a particular

impact on the populations in the Wadden Sea (initially

mass extinction and migration of fauna, but

then in the subsequent summer above-average

settlement of young animals with high biomass).

Warming would therefore tend to create high biodiversity,

but lower biomass with crucially reduced

food supply for fish and sea birds. Higher summer

temperatures would probably intensify the impact of

hypertrophy (green algae mats, oxygen deficiency in

sediment) (Reise, 1993). Overall, the North Sea mudflats

are expected to have a reduced capacity to store

and remineralize organic materials.

F 4.2.4

Coral reefs

Coral reefs can exist at water temperatures between

18 and 30°C. They achieve optimum growth between

25 and 29°C, this being just barely below their upper

lethal temperature limit. Increased water temperatures

triggered by climate change could therefore

impair the capacity of reefs to live and function or at

least increase their vulnerability to other stress factors.

The clearest impact of increased temperature is

the bleaching of coral. The coral lose 60–90 per cent

of their zooxanthels (monocellular algae) that live in

symbiosis with them.The zooxanthels also lose 50–80

per cent of their photosynthetic pigments (Hoegh-

Guldberg and Smith, 1989; Kleppel et al, 1989; Porter

et al, 1989). Corals can recover again within a few

weeks or months by regenerating zooxanthels if the

stress factors are removed (Wilkerson et al, 1988).

Immediately after bleaching corals demonstrate

reduced skeletal growth and interruption of their

reproductive cycle. In addition, their resistance, for

instance to coverage by algae, is strongly impaired. If

external stress factors persist, the coral polyps ultimately

die off. In Panama, Colombia and on the

Galapagos Islands some rare coral species have

already disappeared locally (Glynn and de Weerdt,

1991). When reef-forming corals die reefs are settled

by other benthic fauna. Since animals that use coral

as a food source are not impaired to the same degree

by the stress factors, the feeding pressure on the

corals can increase and bring about additional losses

(Glynn, 1996).

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