Subj: Ecocentral 10
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Well, Alan Mcgowen said to All on 03-12-94 16:44
about: Ecocentral 10....
AM> From: Alan McGowen <alanm igc.apc.org>
AM>
AM> /* Written 1:09 pm Apr 12, 1992 by alanm in cdp:sci.environmen */
AM> /* ---------- "ECO CENTRAL 10" ---------- */
AM> ECO CENTRAL 10
AM>
AM> Extinction and the Four Horsemen
AM>
AM> Extinction can be either *deterministic*, when a resource or
AM> environmental condition essential for life is removed, or
AM> *stochastic*. Deterministic extinctions are wrought by
AM> catastrophes large and small: a volcanic eruption, an asteroid
AM> impact, a clearcut. The occurrence of the catastrophes themselves
AM> may be stochastic, but the extinctions they produce are
AM> deterministic. Depriving plants of light for many months will
AM> kill them, acidifying a lake sufficiently will kill fish,
AM> depriving a population of monkeys of their forest with a clearcut
AM> will kill the monkeys. In general, when the environment moves
AM> outside the range of the tolerances that define a species' niche,
AM> deterministic extinction occurs. The characteristic time to a
AM> deterministic extinction by catastrophe is unrelated to
AM> population size, i.e. a population of size 2N does not
AM> typically wait far longer than one of size N.
AM>
AM> *Stochastic* extinctions, on the other hand, are more probable
AM> the smaller a population is, and the characteristic time to
AM> extinction by stochastic processes increases with population
AM> size. The waiting time for extinction by *demographic
AM> stochasticity* increases extremely rapidly with N, and only very
AM> tiny populations are vulnerable to extinction by this means.
AM> However, tiny populations are quite vulnerable, and the _coupe de
AM> gras_ of extinction is frequently dealt to a remnant population
AM> by demographic fluctuations. The time to extinction increases
AM> more slowly with K for *environmental stochasticity*. Although
AM> the details are model-dependent, under fairly plausible
AM> assumptions about the behavior of the environmental fluctuations,
AM> the time to extinction increases roughly logarithmically with K
AM> [Leigh].
AM>
AM> Thus the possibility exists of determining a K for a *Minimum
AM> Viable Population* (MVP), such that if the population is smaller
AM> than MVP, extinction is rapid, while if it is greater, the chance
AM> of extinction is low and the population will be expected to
AM> persist for a long time. The greater the population is than the
AM> MVP, the lower the chances of extinction. The problem of
AM> determining an MVP is called *population vulnerability analysis.*
AM>
AM> The Four Horsemen
AM>
AM> There are four extinction processes that need to be considered
AM> when doing a population vulnerability analysis. These "four
AM> horsemen" are four positive feedback processes that drive
AM> populations towards extinction. [Gilpin]
AM>
AM> The *demographic process* occurs when chance lowerings of N and
AM> increases in var(r), make the population more vulnerable to
AM> extinction. It does not matter how the changes in N or var(r) are
AM> produced, by demographic randomness or environmental fluctuation.
AM> The point is that when N is decreased and var(r) [or var(K)]
AM> increased the population is less stable, and more likely to
AM> experience further fluctuation. The demographic process is
AM> usually the proximate cause of a stochastic extinction.
AM>
AM> The *fragmentation process* operates through the spatial
AM> distribution of a population. As habitat is fragmented (e.g. by
AM> many clearcuts, roads, or scattered developments), larger
AM> populations are carved up into smaller, more isolated ones. The
AM> smaller populations are at greater risk of extinction by the
AM> demographic process. As they go extinct, the mean distance
AM> between the small populations increases, and reestablishment by
AM> dispersal becomes less likely, i.e. effective fragmentation
AM> increases, exacerbating the fragmentation process.
AM>
AM> The fragmentation process is generally slower than the
AM> demographic process.
AM>
AM> The remaining two "horsemen" operate through genetics.
AM>
AM> The *fitness loss process* occurs as a result of *inbreeding
AM> depression*. In small populations, genetic drift is strong and
AM> removes population genetic diversity. One result of this is that
AM> gene loci become *fixed* to single alleles with no other variants
AM> in the population. Because genetic drift is quite random, the
AM> fixed loci are as likely to have a deleterious allele as the only
AM> one remaining as a beneficial one. Also, the entire population is
AM> now homozygous at the fixed locus, i.e. both copies of the gene
AM> are the same allele, rather than two different alleles (a
AM> condition called *heterozygosity*.] Both the fixation of
AM> deleterious alleles and loss of heterozygosity (which protects
AM> against the effects of deleterious recessives) take their toll on
AM> the organism. It becomes less fertile, shorter-lived, less
AM> resistant to diseases, less good at predation if a predator and
AM> less good at escaping from predators if a prey. These effects can
AM> be very large, substantially reducing r, and this reduction of r
AM> is inbreeding depression. The reduced r leads to a reduced N,
AM> and a further increase in inbreeding, and further fitness loses.
AM>
AM> The fitness loss process is generally slower than the
AM> fragmentation process.
AM>
AM> The *adaptation loss process* is another result of the loss of
AM> genetic diversity due to genetic drift in smaller populations.
AM> The rate of adaptation is proportional to genetic diversity, a
AM> result known as the fundamental theorem of natural selection,
AM> first proved by Ronald Fisher. As population genetic diversity
AM> decreases, so does the ability of the species to track changes in
AM> the environment (e.g. evolving diseases, climate shifts, changes
AM> in abundance of different prey species or plants, etc.) This
AM> leads to a gradual loss of adaptedness to the environment,
AM> placing the population at a subtle competitive disadvantage, or
AM> increasing predation pressure on it, and generally reducing its
AM> absolute fitness (its numbers). This of course leads to still
AM> stronger genetic drift, more loss of population genetic
AM> diversity, slower rates of adaptation, and still more loss of
AM> adaptedness to the environment.
AM>
AM> The adaptation loss process is the slowest -- the most insidious
AM> -- of the four. The only way to protect against adaptation loss
AM> is for the population to be large enough that loss of alleles to
AM> genetic drift is balanced by mutation and natural selection. This
AM> requires a quite healthy population size.
AM>
AM> * * * * * * *
AM>
AM> When there is no extinction crisis in progress, the extinction
AM> rate is more or less balanced by the speciation rate, or even
AM> slightly exceeded by the speciation rate, since species diversity
AM> has increased throughout the Phanerozoic. The conditions for
AM> speciation to occur are much like those for adaptation to be
AM> maintained, except more so: i.e. a species which has many locally
AM> adapted populations and much geographical variation is most
AM> likely to produce new species. This generally requires an even
AM> larger total population size than protection against loss of
AM> adaptation does.
AM>
AM> Of course speciation is also a stochastic process, and there is
AM> some possibility that a species with small numbers may
AM> nevertheless speciate: it seems quite clear that new species
AM> almost always arise from tiny *founder populations*. However,
AM> most tiny populations succumb to the four horsemen of extinction.
AM> In order for a few to speciate, many must become extinct. Thus
AM> the probability of speciation is increased when a species
AM> constantly produces many such candidate founder populations. This
AM> is most likely when the species has a very large total K
AM> throughout a large range, and much local variation of selection
AM> pressures acting upon it, i.e. a diverse environment throughout a
AM> large range.
AM>
AM> Thus we can think of a "fifth horseman" operating at an even
AM> slower rate than the adaptation loss process: a *speciation loss
AM> process* in which contraction of ranges and reductions of
AM> environmental diversity lead to decreased opportunities for
AM> speciation. When the speciation rate is exceeded by the
AM> extinction rate, as local populations become extinct the
AM> diversity of local selection pressures decreases and the ranges
AM> of species shrink, leading to still lower speciation rates. To
AM> reverse this process would require significant expansions of
AM> ranges and increases in local environmental diversity (especially
AM> species diversity) to restore local variation of selection
AM> pressures.
AM>
AM> Refs
AM>
AM> Gilpin, Michael E and Michael E. Soule, 1986. Minimum viable
AM> populations: processes of species extinction. In _Conservation
AM> Biology: The Science of Scarcity and Diversity_, Sinauer,
AM> Sunderland, Massachusetts.
AM>
AM> Leigh, E.G. 1981. The average lifetime of a population in a
AM> varying environment. J. Theoretical Biology 90:213-39
AM>
AM>
AM> -!---------
AM> Alan McGowen
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AM> -!-
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