Harvest management - Ecological Applications: Toward a Sustainable World - page 203

mortality rather than adding to it. Incidentally, this is a problem with fi sheries
models too. These generally accept that there is density-dependence in natural
mortality so that the decline in population density brought about by harvesting
reduces natural mortality and allows the population to rebound. This is the basis
of the hump-shaped yield curves, but it is only a 'reasonable' assumption that has
rarely been demonstrated. Taken overall, the proposals for sustainable beech for-
estry, removing a small number of trees (sometimes by helicopter) that will be
replenished by natural recruitment, conform to best practice and are used in many
forests around the world - including the one described by Elena in Guatemala
(Section 7.1). However, the plan was not acted upon. At a time of a general election
in New Zealand, public opinion that native forests should be left completely
untouched led to government rejection of the proposal.

7. 3 . 3 A forest bird of
cultural importance

Deep in the New Zealand beech forest are found small populations of one of the
largest pigeons in the world, known to the indigenous people as kereru ( Hemiphaga
novaeseelandiae ). The species is legally protected because the formerly large popula-
tions have shrunk due to habitat loss and, in particular, predation by ferrets and
other mustellids introduced to New Zealand in an abortive attempt to control rabbits
(Section 6.1.1). The demise of kereru has been particularly painful for Maori people,
for whom it was important both for sustenance and cultural reasons: dying female
elders (kuia) would be given a kereru. Many Maori hope that with appropriate action
some populations can be brought back to their former status, which would permit
a small harvest of animals for cultural purposes.
A population viability analysis (much like the age-structured forest model) was
performed, using population sizes and carrying capacities, and birth and death rates
estimated from real populations. The birds breed fi rst when 1 year old, produce one
egg per year (although as few as 40% may breed in a bad year) and may live to at
least 14 years. Up to 81% of adults survive each year, but as few as 9-15% of eggs
and juveniles survive, mainly as a result of predation of eggs and nestlings by mus-
tellids, as well as rats and other introduced mammals. Simulated populations all
went extinct within 25 years, whether modeled for mainland or island populations:
only the most optimistic scenario for an island population persisted for 100 years
(w it h 100% of females breeding each year and 15% of young surviving). However,
even this modeled population quickly went extinct if an annual harvest was taken.
Figure 7.12 shows the probability of kereru extinction on the island for four levels
of mammalian predator control. Only with the highest level of control (essentially
a 100% reduction in mammalian predators) could a small annual harvest of four or
fi ve kereru be taken without increasing the risk of the population going extinct.
Against this background, Maori have implemented plans to reduce predation and
protect habitat in mainland reserves (mammals trapped or poisoned; predator-proof
fence erected). They say they will only lift their rahui (prohibition of any harvest)
'when the pigeons are once again a pest in nearby orchards'.

7. 4 Evolution of
harvested
populations - of fi sh
and bighorn rams

Size-selective harvesting, whether for bighorn sheep with the largest horns or for
the biggest fi sh to sell at market, can be expected to result in rapid evolution, by
selecting for animals that mature at a smaller size. The reasoning goes like this. In
an unharvested population a few small individuals may mature earlier than the rest.
If the population is then harvested in a way that takes mainly large individuals, the