Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 50

established instantaneously. Indeed, taking beetles as an example, a specialist plant-
eating beetle needs for its food plants to be already established before it can migrate
into an area and, if it is in turn preyed upon by a carnivorous beetle, then a popula-
tion of the plant-eating beetle needs to established before the carnivorous predator
can be sustained. All this takes time; meanwhile, the climate has already changed.
Consequently the remains of species are not ideally suited as palaeothermometers
(except in a very broad-brush sense). Instead the remains of species are better viewed
as indicators of the ecological impact of climate change. We shall return to this later
when discussing the impact of climate change (Chapters 4, 5 and 6) and especially
the likely effects of current global warming (Chapters 7 and 8) for which in some
instances there are palaeo-analogues.
Of course, the above techniques only work if we have exact knowledge of species'
climatic preferences: that is to say, that we can study live members of the species
today. When trying to ascertain the climate many millions of years ago (as opposed
to hundreds of thousands of years ago) we have to increasingly rely on fossils of
species that are no longer with us. Consequently the next best thing that can be done
is to look at the nearest living relatives of the fossil species. Even if this does not give
us as exact an idea of the past climate it still gives us a good idea and can provide
corroborative evidence alongside other palaeoclimatic indicators. The fundamental
assumption behind analysis of nearest living relatives is that the climatic tolerances
of living taxa can be extrapolated without modification to ancestral forms. The prob-
lem here is that this oxymoronically implies evolutionary stasis. However, confid-
ence in nearest-living-relative species analysis can be increased if making a number
of (hopefully reasonable) assumptions and adopting operational practices. Namely:
(1) that the systematic and evolutionary relationship between the extinct species and
the living one is close; (2) being able to look at as large a number of fossil and
nearest-living-relative pairs as possible, assuming that this will rule out individual
species' idiosyncrasies; (3) using living relatives that belong to a diverse and wide-
spread higher taxon as this increases the chance of finding a closer match with the
fossil; and (4) using plant groups that have uniformly anatomical or physiological
features that are constrained by climatic factors. Nearest-living-relative analysis can
therefore provide us with a broad-brush picture of climate change many millions of
years ago. (For examples see Allen et al., 1993.) However, as this topic is largely
driven by the present need for science to underpin policy concerns over current
global warming, much of this text's focus is on more recent change in the past
300 000 years or so (the past two glacial-interglacial cycles) and likely future change
due to anthropogenic global warming. Over this recent and short time horizon, many
of the species that were around then are still with us. (Of course, there are always
exceptions and many large animal species, or megafauna, outside of Africa have
become extinct in the last quarter of a million years due to human, and combined
human and climatic, pressures. African large animals were more resilient as they
co-evolved with humans.)
Naturally, individual species are not alone in responding to climate change and it
is often necessary to use more than one species to ascertain the timing and degree
of the same climate change. For example, Post and Forchhammer (2002) published
research on caribou and musk oxen on opposite sides of Greenland, demonstrating