Island Ecosystems

Since the time of Darwin, it has been recognized that organisms of oceanic islands are notoriously vulnerable to extinction.  Having been isolated for millions of years oceanic island fauna (animals) and flora (plants) appear to be extremely vulnerable to many effects of climate change (e.g., temperature and carbon dioxide increases, precipitation changes, sea-level rise, and changes in the frequency and intensity of extreme events).  It is important to remember that climatic conditions (temperature, precipitation, etc.) strongly determine where particular plants and animals can live, grow, and reproduce.  Some species and ecosystems are so strongly influenced by the climate to which they are adapted, that they are very vulnerable to even modest climate changes.  Some island ecosystems are already constrained by climate as well as geography and are likely to face extreme stress from projected climate changes while some species could disappear entirely.  For example, increased ocean temperatures and possible changes in ocean circulation patterns will affect coastal ecological systems, such as mangrove forests and coral reefs and important marine resources such as fisheries.  Often there is very little that can be done to assist ecosystems in adapting to the projected speed and amount of change.

Environmental and Societal Impacts

Pacific Islands: The Hawaiian Islands are larger, have more variety in topography and climate, and have higher numbers of unique species than most other Pacific Islands.  Hawaii also has the highest proportion of extinct and endangered species of anywhere in the US.  Loss of about two-thirds of the area of Hawaii's natural habitats has been a primary reason for the species losses so far; however, now biological invasions (introduction of non-native species) pose the greatest threats to the native plants and animals.  Climate change has the potential to make the islands more conducive to invasive species. 

Pacific Islands are vulnerable to invasion due to great isolation and due to high numbers of species found only on the islands.  The example of the brown-tree snake (Boiga irregularis) on Guam could serve as a cautionary note for all oceanic islands that non-native species can explode in island habitats with unexpected consequences.  The brown tree snake probably arrived on Guam via ship cargo from the New Guinea area and the first inland sighting was in the early 1950s.  Some forested areas now have populations up to 13,000 snakes per square mile.  The brown tree snake is responsible for essentially wiping out the native forest birds of Guam.  Twelve species of endemic (native) birds have disappeared from the island, and several others survive in such low numbers that they are close to extinction. 

A similar botanical (plant) example of the unintended consequences of non-indigenous (non-native) species invasions and impact is seen in the invasion of the South American tree Miconia calvescens in Tahiti.  There it has spread very quickly, wiping out many native plants, and has come close to dominating the forest canopy.  Miconia is becoming a concern for Hawaii as well.  Probably because of its attractive purple and green foliage, it was introduced to Hawaii as an ornamental plant -- a single tree in the Oahu Wahiawa Botanical Garden in early 1960.  By 1964, it had reached Hawaii and by the late 1960s or early 1970s it had reached Maui.  Based on a 1995 discovery of a fruiting tree over 10 meters tall, it is estimated that it reached Kauai by the early 1980s.  Its presence in Hawaii poses a threat to native plant species in habitats receiving 75-80 inches or more of yearly precipitation.  The vulnerability of oceanic islands to non-native plant and animal invasions cannot be underestimated. 

In Samoa, as well as on other high islands of the Pacific (e.g., the Society and Marquesas, Tonga, the Cook Islands, and the Hawaiian Islands) relatively intact biodiversity is centered in high-elevation cloud forests, which can be exceptionally vulnerable to climate change.  The upper and lower limits of cloud forests are determined by the altitude of a persistent cloud zone, generally set by the elevation of the winds and by the temperature and humidity levels.  Pacific Island cloud forests are particularly vulnerable to climate change, because relatively small climate shifts are likely to trigger major local changes in rainfall, cloud cover, and humidity.  Such climatic disruptions will likely favor the movement of invasive non-native species into otherwise intact ecosystems.  Numerous population crashes of native birds, reptiles, and amphibians have already occurred in ecosystems in the mountain cloud forest of Monteverde, Costa Rica, apparently resulting from warming oceans and warming and drying of the atmosphere in the area.  These examples could be similar to potential problems in Pacific Island cloud forests with climate changes. 

The cloud forests of East Maui, Hawaii (Haleakala National Park and neighboring reserves), for example, are home to numerous native species, with approximately 90% of the native flowering plants and invertebrates endemic (native) to Hawaii.  Nine endemic bird species in the Hawaiian honeycreeper family occupy these cloud forests.  One reason these birds survive in the higher forest is because the mosquitoes that carry avian (bird) malaria are limited at higher elevations by cooler temperatures.  Therefore, any warming that allows the mosquitoes to move farther up the mountain could threaten the birds, five species of which are already listed as endangered by the US Fish and Wildlife Service. 

Caribbean Tropical Forests:  The islands in the Puerto Rican Bank (Puerto Rico and its out-islands e.g., Vieques and Mona) have been largely striped of native vegetation and have extremely dense human populations.  They also face a wide range of environmental problems, including extinction of native plants and animals.  The introduction of non-native species, particularly vertebrate animals, is yet another difficulty facing the native plant and animal communities of this area. 

In the past, tropical forest conservation efforts generally considered the threat of climate change as quite insignificant compared to land-use change and other human impacts.  However, a better understanding of tropical ecology is now leading many scientists to believe that tropical forests can be very sensitive to climate change.  Increasing importance is being given to climate change factors other than warming, such as changes in hydrology (the water cycle), rainfall patterns, the frequency and intensity of storms, and fires.  These factors could have far-reaching impacts.  In comparison to Hawaiian forests, Caribbean forests are well-adapted to disturbance (e.g., storms and fires).  Possible increases in the frequency or intensity of hurricanes, floods, droughts, and fires could lead to new and greater stresses and drastic changes in forest structure and composition.  For example, a computer model run for the Luquillo reserve of Puerto Rico showed that stronger hurricanes could reduce the numbers and health of the trees, and favor the development of fast-growing, short-lived and weedy species, including invasive species. 

Mangrove Forests:  Coastal wetlands and mangroves of the Caribbean and the Pacific are vital areas for sustaining populations of birds, juvenile fishes, and invertebrates.  They also serve as ‘coastal stabilizers' providing important storm and flood buffers between the sea and coastal communities and absorb nutrients.  Mangrove forests have the capacity to adapt to sea-level rise, although this is being limited by human activities along the coast that interfere with mangrove growth.  One study suggests that mangrove communities will do better in high-sediment environments where strong tidal currents redistribute sediment rather than in low-sediment environments, like most small islands. 

In the pine rocklands (e.g., Sugarloaf Key) in the lower Florida Keys is an area where pine trees grow directly on rock on very low-lying islands.  These pines depend on lenses of freshwater that are being gradually eliminated by salt water as sea level rises.  The result has been that mangroves have replaced the pines.  This transformation will likely continue as sea level rises.  On the other hand, mangroves in the Caribbean are more likely to be affected by changes in precipitation than by higher temperatures and rising sea levels because they require large amounts of fresh water to reach full growth potential.  A decrease in rainfall in the Caribbean could thus reduce mangrove productive potential and increase their exposure to full-strength seawater.  

Coral Reefs:  Some scientists suggest that coral reefs are among the most sensitive ecosystems to long-term climate change.  Recent widespread coral bleaching, particularly associated with the 1997-1998 El Niño event, highlight the sensitivity of coral reef ecosystems to temperature increases.  Coral bleaching could be expected to continue and accelerate with increased ocean temperatures associated with climate change.  Bleaching occurs when the coral animal expels all or part of the algae (plant) that lives within it, when the pigments in the algae decline drastically, or when there is some combination of the two.  Bleaching is a stress reaction that can be caused by many individual or combinations of conditions: high or low water temperature, high levels of ultraviolet radiation, long exposure to air, freshwater dilution, high sedimentation, or various pollutants.  While low temperatures have generally been considered a major reason that coral reefs are not found in cooler climates, reefs can also be damaged by increases in temperature because they are already near their maximum survival temperatures in summer. 

While various species and populations could respond differently, in general, coral are likely to bleach but survive if unusual high temperatures occur for less than a month.  Such a short-term exposure usually allows corals to recover.  However, continued high temperatures can result in long-term stress that can cause life-threatening damage.  Even a less than deadly stress can make corals highly susceptible to infection.  In the late 1980s and 1990s, after localized bleaching in 1982-1983, bleaching became a regular and pervasive problem in the Caribbean and began to appear in the Pacific and Indian Oceans as well. 

©P.Grabhorn Coral reef ecosystem.

Corals in the Caribbean are currently affected by enormous numbers of international tourists, high volume of ship traffic, and fishing.  Pacific Ocean coral reefs are some of the world's healthiest overall; about 70% are rated in good-to-excellent condition.  But 30% are rated fair-to-poor and many are dying, while human impacts are growing.  The region's extremely diverse corals, mangroves, and sea-grasses are already pressured by deforestation, agriculture, construction, pollution, and fishing. 

Climate-related changes could affect coral through more than simply increased water temperatures.  In some Pacific Ocean locations, changes are expected in sea levels that would expose corals to the air.  In the Caribbean, erosion/sedimentation can be caused either directly through floods or indirectly through increased erosion following fires associated with drought conditions.  In addition, hurricanes and typhoons can damage coral reefs.  Unhealthy coral reefs are the most susceptible to destruction by hurricanes and typhoons.  Without the reefs, low-lying islands would be susceptible to rapid erosion and destined to eventual disappearance.   

Economic Impacts

The invasive brown tree snake in Guam is responsible for economic impacts as well as the loss of bird species.  For example, snakes crawling on electrical lines frequently cause power outages.  More than 1,200 power outages have been attributed to snakes since 1978.  The power interruptions have resulted in numerous problems ranging from food spoilage to computer failures and all include significant expenses for the population. 

The US Geological Survey Hawaiian Ecosystems at Risk (HEAR) project suggests that “the silent invasion of Hawaii by insects, disease organisms, snakes, weeds, and other pests is the single greatest threat to Hawaii's economy and natural environment and to the health and lifestyle of Hawaii's people. --   According to HEAR, millions of dollars in crop losses, the extinction of native species, the destruction of native forests, and the spread of disease can already be attributed to alien (non-native) pest (plant and animal) invasions, with many more harmful pests now threatening to invade Hawaii and cause further damage. 

The cost to society of the loss of natural ecosystems is difficult to determine.  Coastal wetlands and mangroves are critical areas for maintaining growing and healthy populations of birds, juvenile fishes, and invertebrates.  They also protect the islands from wind driven waves that cause erosion, are important flood buffers between the oceans and coastal communities, and absorb nutrients and pollutants.  

In the United States, the majority (84 percent) of coral reefs are found in Hawaii with the remainder found in Guam, Florida, the Virgin Islands and Puerto Rico.  Coral reefs are valuable -- in Hawaii they contribute almost $1 billion annually to the economy through recreation, coastal protection, and fishing. 

For many of the islands, being showcases of natural ecosystems is part of their image and is crucial to their tourist economies.  Without the beauty of these natural areas, there would be little reason for tourists to come.  The economic impacts of the loss of tourism would affect all parts of the islands economic and social systems.  

Strategies to Address Potential Impacts on Island Ecosystems

There are a limited number of specific options that can assist island ecosystems to cope with a changing climate.  Some of those options include the following:

• Attempting to slow biological invasions

• Reducing other stresses on the ecosystems, e.g., reducing pollution and limiting usage;

• Strengthening and enforcing policies that protect critical habitats;

• Improving understanding of the local effects of climate variability and change; and

• Increasing awareness of tourists and the public concerning the value of species and biodiversity.