Principles of In-Situ Management of Wildlife Communities

PRINCIPLES OF IN-SITU MANAGEMENT OF WILDLIFE COMMUNITIES

SYMPTOMS – DIAGNOSIS – TREATMENT – CURE

A species is THREATENED (with extinction), ENDANGERED or VULNERABLE ((Remember that these may be the result of a past decline.) because:

• It exists in very low numbers
• It’s range is very small
• It is known to be in decline

1. SYMPTOMS

A very few species naturally exist at very low numbers or with very restricted ranges (usually island endemics). These species are always at risk of extinction as a result of demographic and/or environmental stochasticity.

Such examples include: the Black Robin (Petroica traversi) of Chatham Island, which is currently endangered with only 200 individuals known to be left.

• Hood Mockingbird (Mimus macdonaldi) Espanola – Vulnerable – 2,500
• Kakapo (Strigops habroptilus) New Zealand – Critically Endangered – 124

But, REMEMBER, most species “don’t wander towards extinction, they are pushed” and so, to remove the threat of extinction, we must identify and threat the original causes of the decline which has resulted in the low numbers or restricted range.

Extinctions may be divided into two categories, driven extinctions and stochastic extinctions.

1. Driven extinction: whereby a population’s environment changes to its detriment and rate of increase falls below zero. The population declines. Perhaps this lowering of density frees up resources to some extent, or lowers the rate of predation, but this is not sufficient to counteract the force of the driving variable, and the population finally goes extinct.

Included in this category are extinctions caused by environmental fluctuation and extinctions caused by catastrophes. The latter are viewed here as simply large environmental fluctuations.

1. Stochastic extinction: whereby a population fails to solve the “small population problem.” The effect of chance events, which would be trivial when numbers are high, can have important and sometimes terminal consequences when numbers are low.

(a) Extinction by demographic malfunction: whereby a population goes extinct by accident (chance) because it is so small that its dynamics are determined critically by the fortunes of individuals rather than by the law of averages. In those circumstances a population is quite capable, by chance configuration of age distribution or sex ratio, of registering a steep decline to extinction over a couple of years even though its schedules of mortality and fecundity would result in an increase if the age distribution were stable.

(b) Extinction by genetic malfunction: whereby a population at low numbers for several generations loses heterozygosity to the extent that recessive semi-lethals are exposed, average fitness therefore drops, and the population declines even further and ultimately to extinction. The loss of an allele from the genotype is an event resulting from the lottery of random mating. Although each individual loss is unpredictable, the average rate of loss, as a function of population size, can be predicted fairly accurately.

These mechanisms do not exclude each other entirely but they are sufficiently distinct that we treat them separately. Although the relative contribution of these mechanisms is unknown, enough anecdotal information is available to suggest that the driven extinction is by far the most prevalent.

Extinction by demographic malfunction is probably the second most important but it usually requires that the population be driven to low numbers before demographic stochasticity can operate

2. DIAGNOSIS

In the crisis of the moment it is far too easy to confuse observation with explanation (an introduced predator is present therefore it must be the problem – so we take the introduced predator away and the population will be grand)

So the first step in averting extinction is to recognise the problem. Many species have slid unnoticed to the brink of extinction before their virtual absence was noticed. The smaller mammals and birds, and the frogs and reptiles, are more likely to be overlooked than are the large ungulates and carnivores – choosing charismatic animals is a general occurrence.

The second step is to discover how the population got into its present mess.

• Is the cause of decline a single factor or a combination of factors? (Our predator?)
• Are those factors still operating?
• If so can they be nullified?

The cause of a decline is established by application of the researcher’s tools of trade: the listing of possible causes and then the sequential elimination of those individually or in groups according to whether their predicted effects are observed in fact. This is the standard toolkit of hypothesis production and testing.

It is essential that the logic of the exercise be mapped out before the task is begun. The listing of potential causes is followed by a formulation of predictions and then a test of those predictions. The efficiency of the exercise is critically dependent on the order in which the hypotheses are tested. Get that wrong and a 3-month job may become a 3-year project. In the meantime the population may have slid closer to the threshold of extinction, so time is important.

This is where we can introduce the “endangered species recovery analysis” – a five-step process that can be used to determine the best treatment to employ.

ENDANGERED SPECIES RECOVERY ANALYSIS

1. Firstly, we must confirm species is declining or was formerly more abundant/widespread
2. Study the species’ natural history for knowledge of and a feel for its ecology and status
3. When this background knowledge is adequate to avoid silly mistakes, list all conceivable causes of decline

1. For each possible cause look for links with the decline: Has hunting pressure increased in line with decline? Does the range overlap with that of an introduced predator?

• If there is an apparent link, you have a testable hypothesis. Don’t assume that the answer is provided by “folk wisdom” (cf. the case of the Dodo)

1. Test each hypothesis in order of probability (These can be trial treatments)

Detecting trends

First determine if there is a decline, second, determine the rate of the decline. This tells you how urgent the situation is.  Remember that many species in decline may appear relatively common until just before they become very rare (cf. African Elephant). A frequent mistake in conservation is that because a species is common it has no conservation problems (and therefore does not merit monitoring or study).

Contraction of range

Often we have only a ‘guestimate’ of population size from which to determine rates of decline. An alternative sign or trouble is range contraction (and extinction of local populations). Such local extinctions may not be uncommon but may throw up testable clues as to the factors determining overall range and abundance