Retreating to higher ground

Green ringtail possum

Author

Joe Douglas

As humans we are pretty good at living at a range of temperatures, adapting to life at most latitudes and able to find insulation and shelter or cool ourselves in the shade, as and when we get uncomfortable. Of course, the targeted maximum of a 2°C increase of global average temperature set down in the Paris Climate Agreement isn’t an attempt to limit the risk to the structural integrity of ice-creams on July afternoons. Preventing a global temperature increase of this kind is about avoiding human tragedies on a massive scale, from whole islands and cities being engulfed by rising seas, droughts that destroy vital crops year after year, and extreme weather events that flatten homes and lives. The 2°C mark, despite being an objective and quantifiable change, is therefore largely symbolic. Stepping away from global increase to a more local, less apocalyptic scale – in the city of Oxford for example – I imagine people would probably welcome such a modest increase in temperature when braving the wind and rain on their daily commutes.

Localised warming, however, even to the tune of just a few degrees, can be bad news for other less generalist species than humans. Although many species are capable of adapting their behaviour and physiology to new temperature ranges, many more require very specific conditions to survive. Such species typically live in high biodiversity areas, like rainforests, where each species fulfils a very particular ecological niche – eating specific species, foraging or hunting using a very specific method, living in specific places, and so on. This means that small changes, if they occur too quickly for a population to adapt, can have profound consequences.

In Queensland, Australia, for example, the rainforests are home to a great diversity of possum species. One of these species, the green ringtail possum, P. archeri, (pictured) is found only at elevations above 300m limited by its inability to successfully regulate its body temperature at warmer lower altitudes. This is because it has ‘adaptive heterothermy’, meaning that as temperatures rise above and beyond 30°C it ceases to increase its rate of perspiration, in an attempt to avoid severe dehydration. The narrow ecological niche of the green ringtail possum restricts it to the forest canopy, where it obtains most of its fluid from eating foliage. Poisonous compounds in the leaves it eats – a defence against overgrazing – stop the possum from quickly rehydrating itself, eating less often so it has time to break down the harmful compounds it ingests. These possums evolved over many millennia to take advantage of a highly stable ecosystem and climate, which favoured adaptive heterothermy. But with localised warming in the Queensland region, this fine balance is being disturbed, and the green ringtail possum and other species are being constrained to smaller and smaller territories at higher altitudes, which can’t support the same number of possums.

Hawaii creeper

Proving the influence of climate change on ecosystems is a relentlessly difficult task, especially when there are other major forms of human disturbance at work. The complex web of interactions between living and non-living factors in an area often produces chains of effect that can magnify the effect of localised warming. In Hawaii, the destruction of lowland habitats to make way for agriculture has forced most native bird species uphill to habitats on the mountain slopes. However, the current threat to the birds is no longer agriculture, but avian malaria carried by an introduced species of mosquito, C. quinquefasciatus. As the malarial parasite can only successfully infect new hosts at temperatures above 13°C, the native birds are only contracting malaria at middle elevations, preserving refuges in the highest areas. Incremental localised warming, however, is already robbing them of this sanctuary, and famous birds such as the Hawaii creeper (pictured) are becoming endangered. The retreat of specialist species uphill to cope with rising temperatures is doomed to be a losing game. Although for some of them are still able to expand upwards, at some point their islands in the sky will shrink to nothing and their retreat will be halted once and for all.


​Relevant studies: Krockenberger et al. (2012) ‘The limit to the distribution of a rainforest marsupial folivore is consistent with the thermal intolerance hypthesis’; Benning et al. (2002) ‘Interactions of climate change with biological invasions and land use in the Hawaiian Islands: Modeling the fate of endemic birds using a geographic information system.’