Or, why is climate change a global (not local) issue?
For scientists, the challenge of trying to understand the impacts of climate upon the future of life on earth is daunting. Issues such as land use, cloudiness, precipitation, shifts in nutrient distribution, air and water pollution, and our legacy of past disturbance all need to be interwoven into models and predictions.
One significant issue of concern for the future of life on earth concerns the survival of tropical rain forests. This ecosystem houses an estimated half (or more) of the diversity of species inhabiting our planet, so its health is of critical priority. Many tropical species have narrow tolerances for temperature, and also elevation. With climate change, tropical organisms have three options: 1. adapt quickly to rapid changes in their local climate: 2. move locations to find their climate of choice somewhere else; or 3. become extinct. For most species, their ability to locate and then shift to new geographical regions as their local climate changes is doubtful. Some live specifically in certain cloud conditions midway up tropical mountains; some live happily at the tops of equatorial mountains. But, as temperatures continue to rise, the level of cloud cover and moisture shifts upwards. Hence, the mid-elevation species may move up the mountain range for the short-term, but that begs the obvious question — where will the ones at the top go? In theory, they will become extinct.
In 2007, the Intergovernmental Panel on Climate Change (IPCC) found 28,586 cases of biological shifts of organisms from global warming, but only seven were from tropical ecosystems. In Amazonia, tropical deforestation is increasing due to roads and human clearing activities. Forests rapidly become soybean pastures to supply an increasing American glut for this crop; but few research projects have made scientific predictions as to where these species might shift as their forests degrade and their climate changes.
In the wet tropics of Australia, the golden bowerbird that lives on slopes in montane tropical regions is projected to become extinct if warming increases by as little as 3 degrees centigrade in Queensland which could happen as early as 2050. However, there is not yet any scientific study of latitudinal range shifts in the tropics, mainly because there is very little temperature shift at all in these low elevation tropical rain forests. In contrast, the easiest option for organisms to survive rising temperatures is to move upward on slopes until – but of course – they reach the top of the mountain. Species that live at the tops of mountains will either contract their ranges significantly, or else go extinct.
Queensland currently has had 0.8 degrees Centigrade of warming. The Herbert River ringtail possum is predicted to go extinct with a few degrees of warming. But because the Australian tropics are small (despite their biological importance), they are not broadcast as one of the most “at risk” global regions. In north Queensland, biologists studied the populations of insects, birds, and mammals along the slopes to assess the vulnerability of different species, so they could determine where to allocate the most funding for conservation. The questions they asked were: How sensitive are species to either be resilient or else adapt to changes in climate? And what about possible mitigation? To accurately predict the fate of a species, lemuroid ringtail possums were studied for their current climatic range and the predictions for the future. With two degrees of warming (accompanied by increases in precipitation), the models indicated only one or two sites of survival. With four degrees of warming, total extinction is likely. Temperature warming is only one component of predicted climate change. With higher temperatures will likely come changes in rainfall patterns, differences in monsoon patterns, possible fires, and influx of diseases including fungi, bacteria, molds and insect vectors. And even more complex is the fact that western slopes may have slightly different climates than east-facing slopes, that valleys have different sunlight patterns than mid-elevations that tall trees have different microclimates than young forests, and that even something as subtle as vine cover may alter the climate of the forest floor. So making climate change predictions for the world’s tropical biodiversity ain’t easy.
In the case of tropical epiphytes along the Barva Transect of Costa Rica, tropical biologists modeled that – of a total of 650 species in one local forest – 340 species may disappear from the lowlands and 18 species may disappear altogether at the top of the mountain if temperature increases just a few degrees.
Growth rates of tropical trees are slowing down with warming temperatures, another indication that tropical ecosystems are under increasing stress as our planet warms.
In Mt. Kinabalu, Borneo, moths are moving uphill. Using data from a moth species survey conducted in 1970, biologist I-Ching Chen (working at the world-acclaimed climate change lab of University of York) repeated the same survey in 2008, nearly 40 years later. During this time the temperatures have warmed 0.7 degrees Centigrade. Her results show that the upper boundaries of range of moths have shifted, moving uphill approximately 200 meters for most species.
In Manu National Park in the Peruvian Andes, trees are also shifting. This is also called “species migrations” where species are predicted to move uphill or northward, depending on the available habitat. Ken Feeley has conducted the first study of shifts in a tropical plant species. Trees in plots are being censused every few years along an elevational gradient, so that biologists can determine how trees are surviving at different elevations and temperatures ranging from 8.5 to 23.8 degrees centigrade along the gradient. They hypothesize that trees will migrate upslope approx. 9 meters per year, given increasing temperatures of 0.05 degrees centigrade per year which is what IPCC predicts. They used basal area to indicate survival. Approximately 32 of 37 tree genera or 87% of Andean trees are moving upslope at a rate of 3.5 meters elevation per year, which is less than predicted given the warming temperature changes. This means that some tree species may suffer larger losses of habitat. But, like most climate change models created by scientists, this assumes “perfect” conditions. However, in the Andes, human factors such as cattle grazing and burning will interrupt a tree species’ ability to migrate up the hill, providing what conservation biologists affectionately call a “grass ceiling” or impermeable barrier. Weinmannia spp., for example, may lost approximately 61% of its potential habitat if the tree cannot shift upslope faster than its current rate. Tropical forests are responding to warming temperatures through migrations; but “slow migrations” will result in decreased habitat for many species. And worst of all, human factors may have dire consequences to these upward shifts of tropical forests.
In the north temperate ecosystems, many species are shifting northward, including butterflies, mammals, insects, and even plants. For example, the American pika is quickly disappearing as temperatures rise, causing the disappearance of the climate required for its high-elevation communities.
The data is solid – species around the globe are shifting in response to changing climate. In geological history, this has occurred before and many species adjusted, fluctuated, and adapted. But now a new untested hurdle exists for all biodiversity – can species adjust, fluctuate and adapt given all the human impacts to natural systems? With fragmented forests, polluted waters, and extensive highway grids, can species migrate or shift their ranges and/or their habits given the challenges of navigating the extensive human degradation to the landscape? Perhaps only the most resilient will survive, in which case we can look forward to a world of cockroaches, slim molds, and squid. Or perhaps humans will need to invest in enormous amounts of “biodiversity aid” to the survival of species that contribute to human health. Stay tuned, as humans continue to play Russians roulette with our planet.