Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 290

species with nowhere to go with further warming - tropical lowland species that do not
have an upward corridor to higher elevations or latitudinally from where they are now
through which they can migrate - there is another ecological problem. This is, what
replaces the world's most tropical species that already live in tropical lowlands and
that have either migrated away or died in situ ? There is no terrestrial biome warmer
than the tropics. So in the event of further warming above today's levels we can
expect increased tropical lowland ecological attrition in addition to those at the
highest of tropical altitudes. This tropical lowland ecological attrition will be more
severe than the temperate lowland attrition in developed nations discussed at the start
of this section.
The seriousness of this tropical ecological attrition may not at first seem obvious;
after all, in theory a lowland tropical species should be able to migrate latitudinally
towards the poles or upwards. In the tropics, and indeed elsewhere, a species respond-
ing to climate change will find it easier to shift its range vertically than horizontally as
the vertical migration required to compensate for, say, half a degree warmer climate
necessitates a far shorter vertical migration than a horizontal one. Indeed, in the trop-
ics this difference between the more easy vertical and the harder horizontal latitudinal
migration to compensate for temperature is greater than in temperate zones: vertical
temperature gradients in most places on Earth are steep, around a 5.2-6.5 ◦ C decrease
per 1 km elevation but this distance is far less than the horizontal distance required,
which is more than 1000 times more: you need to leave the tropics first before there
is cooling with increased latitude. Once beyond the tropics in the temperate zone,
latitudinal cooling is around 6.9 ◦ C per 1000 km at latitudes around 45 ◦ south or
north: these are the latitudes of places such as Bordeaux in France, Minneapolis
in the USA, or Oamaru in New Zealand. In addition, there is the aforementioned
landscape fragmentation problem that is both natural (the Earth's geography being
not one where all lowland elevations are connected by corridors that gradually rise
because many places, especially of mid-elevations, see their route to higher eleva-
tions blocked by intervening warmer lowlands) and anthropogenic, with intervening
human settlements or agricultural lands (i.e. the landscape fragmentation already
mentioned a number of times) inhibiting migration. But there is the lowland tropical
ecological problem: irrespective of whether there are appropriate migration corridors
and species migrate, or they locally go extinct in situ , what replaces them? At the
tops of tropical mountains, species that go extinct are replaced by others tolerant to
slightly warmer conditions migrating from below, but what replaces those current
tropical species that will no longer live in the even warmer tropical lowlands: tropical
lowlands are the warmest biomes on Earth (not including extreme niches such as
volcanic springs). So, we are faced with this prospect of lowland tropical ecological
attrition in addition to the extinction of alpine-allied species at the tops of tropical
mountains. All this leads us to consider the seriousness of this problem.
In 2008 the US ecologist Robert Cowell, the German zoologist Gunnar Brehm and
colleagues attempted to get a handle on this issue. They analysed altitudinal range
data for four large survey data sets of plants and insects: epiphytes (plants that live
on the surface of other plants but do not derive nourishment from them), understorey
Rubiaceae (the family that includes blackberry and raspberry), Geometridae (a large
family of large-winged moths with distinctive larvae) and Formicidae (ants). The