Birds Are In Trouble!

An interesting paper came out last week in PLoS-Biology: Projected Impacts of Climate and Land-Use Change on the Global Diversity of Birds by Walter Jetz, David S. Wilcove and Andrew P. Dobson. You can view some bloggers' responses on The DC Birding Blog, Field Of View and Living the Scientific Life and media coverage here, here and here.

The authors of the paper collected information about all known ranges of land birds and made a mathematical model for predicting how those ranges will be affected by global warming on one hand and the land-use on the other by years 2050 and 2100. They use four Millennium Ecosystem Assessment scenarios. Two of those scenarios assume a global response to environmental problems and two assume a local (fragmented) response. Also, two of them assume a proactive approach to environmental threats, while the other two predict that most problems will be dealt with reactively, i.e., after they happen.

The results differ between scenarios, but even under the best scenario, a strikingly large number of bird species are predicted to lose substantial proportions of their ranges, likely leading to their eventual extinction. Depending on the scenario, approximately 400 (of the 8750 species studied - they omitted seafaring and coastal species) will lose half or more of their range by 2050. That number may increase to 900-1,800 species by 2100.

The main message of the paper is that global warming will disproportionately affect species in higher latitudes, while the land-use will be more detrimental to the tropical species. Why?

In the tropics, there are many species of birds competing for local resources. How do they partition those resources? In one (or both) of two ways: one is to become specialists, e.g., to nest and feed on particular local plants; the other is to divvy up the territory through competitive exclusion. As a result of both of these mechanisms, the tropical species tend to cover very small ranges and, thus, have rather small population sizes to begin with. Clear-cutting a patch of forest may entirely wipe out the range of a local species which, when displaced elsewhere into the neighboring woods, will not be able to compete against the local tenants and will likely go extinct.

In the higher latitudes, the number of species is smaller, the ranges are larger and the population numbers are greater. Here, effects of global warming on the plants will have a greater effect on the range-sizes than land-use (this is for the most part already developed world not in a frenzy of clear-cutting forests any more). As a result, the ranges will become more fragmented and patchy, which can lead to extinction of some species.

The authors are quite upfront about the limits and underlying assumptions of their model, particularly the assumption that avian ranges will remain static, i.e., that the birds will not move their ranges to higher latitudes and/or altitudes. It has already been well documented that birds (as well as other organisms, e.g., insects, plants, mammals and fish) are in fact responding to global warming by changing their ranges: some are gradually moving to higher latitudes (i.e., in the Northern hemisphere, their ranges are shifting North), some retain their breeding grounds while shifting their migratory routes to different wintering grounds, while others are abandoning migration altogether.

Of course, as authors note, land-use and global warming are interconnected: clearing forests increases the albedo of the area so more of the Sun's energy is absorbed instead of being reflected back out into space. The agricultural use of chemicals and their runoff into the oceans kills dinoflagellates which perform about half of Earth's CO2 absorption and O2 release (the clear-cut rainforests provided the other half). At the same time, global warming affects human populations and activity, introducing droughts into already poor areas, thus motivating further destruction of forest in order to provide agricultural land.

Let's assume that there is a fifth scenario, the most pessimistic one, that assumes there will be no response to environmental troubles at all, or too little too late (i.e., let me play an Alarmist here). Thus, both clear-cutting and global warming continue at the current rate. What will happen? According to the model in this paper, land-use will result in destruction of habitat in the tropics and poor countries, leading to mass extinction of local, small-area/small-population avian species. Global warming will bring in the droughts into the areas at higher latitudes, changing the nature of the plant cover, fragmenting the species ranges and leading to at least local extinctions of many more avian species in currently temperate zones.

But, if we reinstate the fact that avian species will move their ranges, what will be the result? In the tropics, there is not much place to move - the specialists have to stay where their host plants and food are. If they try to move, they will encounter different vegetation, different avian competitors who are better adapted to the local conditions, and different predators they are unfamiliar with. In other words, tropical birds have nowhere to go.

In the temperate zones, birds will shift their ranges to higher latitudes and, if we do nothing about global warming, will in the end, all end up at the ends of their continents. The narrow strips of the northernmost coasts of Europe, Asia and North America will become grounds for ferocious competition for limited resources between all those immigrant species. Many will go extinct. Others will survive at small ranges in small populations. There is nowhere else to go, as by this time, there will be no more
Arctic to fly to - it will be all ocean after the Arctic ice melts.

In the Southern hemisphere, the birds of South America may island-hop to and spread across the new lush tropical forests of Antarctica, but the birds of Africa, Australia and Pacific islands will not be able to make such a big leap and will concentrate on the southernmost edges of their continents until many of them go extinct due to competition.

And even this bleakest scenario makes an assumption that makes the picture look prettier. Species cannot just get up and go and move their ranges with no consequences. Species are parts of their local ecosystems. In those ecosystems, some species will shift their ranges faster, some slower, some not at all (changing the timing of annual events instead, i.e., adapting in time instead of space). The predators and prey will leave each other - the former trying to adapt to new prey, the latter trying to avoid new predators. The flowers and pollinators will split their ways. The mutualists may part ways as well. The remodeling of ecosystems will occur, i.e., we cannot expect entire intact ecosystems to migrate to higher latitudes in synchrony. Disruption of ecosystems by such remodeling will certainly lead to extinctions of numerous organisms, way before they reach the edges of their continents.