Polar and alpine environments - Fundamentals of the Physical Environment - page 620

Plate 24.15 Cushion krummholz of dwarf spruce (Picea glauca) at arctic treeline on the western side of Hudson Bay, Canada.
Protection to the tree is given at ground-level by snow, in contrast to the bare stems above.
Photo: Bill Barr

unpredictable climate in polar latitudes leads to simpler
biological communities. Many polar species are endemic,
having evolved and adapted to the specific environmental
conditions. Thus they are more at risk than species in
temperate regions to extrinsic stochastic and deterministic
processes which might disrupt the composition of, and
interactions between, species within the ecosystem. Such
disruptions could, for example, take the form of climate
warming, or degradation and loss of habitat by human
activity.
The stability and integrity of ecosystems differ con-
siderably between polar and temperate regions. We have
seen in Chapter 22 how species diversity declines along a
gradient from equator to poles, and how ecosystem func-
tioning depends on species diversity. In temperate regions
the 'redundancy model of ecosystems' is appropriate, as
there is a high probability that 'spare' species within an
ecosystem can substitute for any species lost, and replace
its functional role (see p. 552). The more species present
in an ecosystem the greater the community's 'insurance'

against any disruption caused by stochastic or determin-
istic events.
Arctic marine ecosystems have a more complex
structure than equivalent terrestrial ecosystems. They
reflect the 'keystone species model of ecosystems', where
the loss of a single keystone species leads to the complete
collapse of ecosystem functioning and stability. An
example is shown in Figure 24.20 , which shows energy
flows through the marine food web of Lancaster Sound,
Arctic Canada (75 N) (Welch et al . 1992). This trophic-

dynamic model has been constructed following the
principles discussed in Chapter 21. Phytoplankton, ice
algae and kelp fix 89 per cent, 10 per cent and 1 per cent
of the gross primary production (GPP) respectively.
Primary production peaks sharply from June to August,
when light and temperature are favourable, and when
summer retreat of sea ice allows more primary production
in open water. Average primary production is 60 g carbon
m -2 yr -1 , but there are large variations, depending on
ocean currents and nutrients. A few species of amphids,