15.5 Near-Fatal Impact
257
15.4 Pa Defence with Falling Populations
The defence of a pa must always have been concentrated on the defence of its
perimeter. The most naturally defensible sites, because they are situated on crags
and cliffs or rivers, will have the best naturally protected perimeters and so the least
stretch of unprotected perimeter to defend. Despite their huge size, volcanic cones
are an obvious choice for a pa because the steepness of the cone is an obvious
defence against attack and only the entry to the cone (the area of lava spill) needs
to be defended. But most pa are built on hill-top ridges, long thin sites with a very
long perimeter in comparison to their area. The obvious technique for fortifying
these pa, as over time the numbers of their defenders fell, was by digging transverse
ditches. These could cut a long thin rectangle into a series of squares which could
be defended sequentially. Transverse ditches would have enabled a smaller number
of warriors to defend a ridgetop pa. With all warriors concentrated initially in the
most vulnerable square, they had the best chance of successfully defending the pa.
If overwhelmed, they could retreat successively to the other squares. Digging transverse ditches allowed for the most intelligent organization of the available warriors.
For example, if a ridgetop pa of 1,200 m 2 is 60 m long and 20 m wide, it has a
perimeter of 160 m. If it is cut by two transverse ditches, two of the three squares,
each 20 m by 20 m, have a 60 m perimeter to defend at a time and the last has 80.
With a perimeter of 60 m versus 160 m to handle initially, fewer than half the men
needed to defend the pa without transverse ditches could hold it against an enemy.
Clearly in any pa the addition of a second and/or third ditch and palisade also
reduced the numbers needed to defend the perimeter by a half or two-thirds. Our
argument is that the addition of fortifications in a context of falling populations
makes sense as a means of continuing to defend optimally sited large pa – and
indeed any pa – when over time there were successively fewer warriors available to
defend them.
15.5 Near-Fatal Impact
The human consequences of the high rates of population decline in Little Ice Age
New Zealand, obvious from sequential fortifications for pa and from our demographic analysis, show the risk of extinction increasing with time as over 356 years
populations estimated in millions shrank to numbers of less than 200,000.
The arrival of European ships at a time when populations were biologically
stressed and declining not only increased the risks of extinction but also undoubtedly intensified the physical and cultural impacts of contact. Populations effectively
reduced by 1801 to about 8% of their numbers in AD 1445, less disease-resistant
because of prolonged stress, had to combat a battery of European diseases for which
they had no genetic or acquired immunity.
The highest rate of population loss for the Maori population occurred after
European contact in the period from 1801 to 1858, a rate of loss from 1.72% per
annum to 1.92% per annum. This is 2.4–2.5 times as high as the averaged loss rate
257
15.4 Pa Defence with Falling Populations
The defence of a pa must always have been concentrated on the defence of its
perimeter. The most naturally defensible sites, because they are situated on crags
and cliffs or rivers, will have the best naturally protected perimeters and so the least
stretch of unprotected perimeter to defend. Despite their huge size, volcanic cones
are an obvious choice for a pa because the steepness of the cone is an obvious
defence against attack and only the entry to the cone (the area of lava spill) needs
to be defended. But most pa are built on hill-top ridges, long thin sites with a very
long perimeter in comparison to their area. The obvious technique for fortifying
these pa, as over time the numbers of their defenders fell, was by digging transverse
ditches. These could cut a long thin rectangle into a series of squares which could
be defended sequentially. Transverse ditches would have enabled a smaller number
of warriors to defend a ridgetop pa. With all warriors concentrated initially in the
most vulnerable square, they had the best chance of successfully defending the pa.
If overwhelmed, they could retreat successively to the other squares. Digging transverse ditches allowed for the most intelligent organization of the available warriors.
For example, if a ridgetop pa of 1,200 m 2 is 60 m long and 20 m wide, it has a
perimeter of 160 m. If it is cut by two transverse ditches, two of the three squares,
each 20 m by 20 m, have a 60 m perimeter to defend at a time and the last has 80.
With a perimeter of 60 m versus 160 m to handle initially, fewer than half the men
needed to defend the pa without transverse ditches could hold it against an enemy.
Clearly in any pa the addition of a second and/or third ditch and palisade also
reduced the numbers needed to defend the perimeter by a half or two-thirds. Our
argument is that the addition of fortifications in a context of falling populations
makes sense as a means of continuing to defend optimally sited large pa – and
indeed any pa – when over time there were successively fewer warriors available to
defend them.
15.5 Near-Fatal Impact
The human consequences of the high rates of population decline in Little Ice Age
New Zealand, obvious from sequential fortifications for pa and from our demographic analysis, show the risk of extinction increasing with time as over 356 years
populations estimated in millions shrank to numbers of less than 200,000.
The arrival of European ships at a time when populations were biologically
stressed and declining not only increased the risks of extinction but also undoubtedly intensified the physical and cultural impacts of contact. Populations effectively
reduced by 1801 to about 8% of their numbers in AD 1445, less disease-resistant
because of prolonged stress, had to combat a battery of European diseases for which
they had no genetic or acquired immunity.
The highest rate of population loss for the Maori population occurred after
European contact in the period from 1801 to 1858, a rate of loss from 1.72% per
annum to 1.92% per annum. This is 2.4–2.5 times as high as the averaged loss rate