Small and other fragile populations

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Small and other fragile populations
Lonely George
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Conservation of tropical biodiversity

• Biodiversity the main challenges
• A rationale for biodiversity conservation
• Lessons from island biogeography
• The value of biodiversity
• Overexploitation a scrutiny of wildlife trade
• Small and other fragile populations

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Small and other fragile populations

• Island populations
• Population Viability
• Introduced species

New Zealand from space
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12 extinct bats of the world
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Extinct plant taxa of the worldSource IUCN Red
Data List 2000
N79 (73 species 6 subspecies), 63.3 from
islands.
N61 mammal extinctions, 68 from islands.
N128 bird extinctions, 82 from islands.
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Biota on Islands

• Limited colonization (plants, invertebrates,
  birds) and slow recolonization.
• Absence of predators and large herbivores.
• Absence of defense mechanisms against predation,
  herbivory, parasitism, pathogens y competition
• Loss of flight ability
• Ground nesting
• Absence of chemical and morphological defense in
  plants, no fast regeneration upon attack.

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Lakes as islands Nile perch (Lates nilotica)
drove to extinction several hundred cichlid
species.
Lake Victoria
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Lake Victoria
Nutrients from agriculture and organic waste
Haplochrominae feed on algae
Overfishing
Algae proliferation and eutrophication
Decline in Haplochrominae
Predation by Nile perch
Low oxigen at great depths
Eihhornia crassipes invades the lithoral
Decline in Tilapia fisheries
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Small and other fragile populations

• Island populations
• Population Viability
• Introduced species

The dynamics of small populations are peculiar.
Small populations may go extinct, even in absence
of the original cause for their decline.
Intrinsic demographic and genetic phenomena, as
well as environmental stocasticity, affect the
survival chance of small populations stronger
than that of large populations.
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Population Viability see Caughley Sinclair
1994. Wildlife Ecology and Management.

• Why populations go extinct
• Demographic problems
• Genetic problems
• Effective population size
• Environmental changes

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Steps toward extinction

• The most frequent cause of extinction is a
  modification in the environment
• Modification of the habitat
• Increase in hunting and predation
• Competition with another species
• Toxicity introduced into the environment
• Introduced disease
• Other forces start to act upon a population
  decline THE small POPULATION SYNDROME.

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Population Viability

• Why populations go extinct
• Small size resulting from deterministic forces
• s.th. essential is removed or s.th. lethal is
  introduced K declines.
• Small size resulting from stocastic forces
• Environmental
• Catastrophic
• Demographic
• Genetic
• Forces of fragmentation (subpopulations)
• Interaction between these forces is the rule a
  vortex of population reduction.

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Exponential growth for a population growing at
rate r between time 0 and t.

• e the base of natural logarithm (approx. 2.72)
• t units of time
• er the factor by which the population increases
  each unit of time (exponential rate of increase)
• r birth rate mortality rate
• rates are typically age related
• Population Increase Rate
• dN /dt rN

Population size
N(t)
N(t) N(0) ert
N(0)
0
t
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The logistic equation describes the growth of a
regulated population

• e the base of natural logarithm (approx. 2.72)
• t units of time
• r birth rate mortality rate
• rates are typically age related
• K the carrying capacity of the environment
• Population Increase Rate
• dN /dt r N (1-N /K )

Population size N
K
N(t) K/ 1erti
K/2
0
i
Time
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Demographic Factors

• Population Growth Rate r or intrinsic rate of
  natural increase or exponential growth rate
• Predictable if age distribution is stable
• Demographic Stocasticity
• Natural variation in death and birth rates
• If age distribution not stable or small
  population
• r deviates from life table
• (survival fecundity of left over
  individuals)

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Demographic Factors

• LARGE populations - deterministic outcome
  (certain outcome, driven by means)
• Small populations stocastic outcome (uncertain
  outcome, driven by chance).

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Example nature reserve 200 km2, density
0.01/km2, two individuals (one male, one female),
yearly survival probability p0.9, and yearly
probability of producing one offspring m0.95.
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Deterministic Effect

• The variance of the deterministic value of r in a
  constant environment is
• Environmental and demographic stocasticity
  combine to determine Var (r)
• Large populations Var (r) cancels out.
• Small populations Var (r) can mean extinction.

VarD ( r ) Var ( r ) 1 / N Var ( r ) 1 is the
contribution of the demographic behaviour of the
average individual (approx. 0.5, if intrinsic
growth rate rm0.28).
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Deviation from expected growth rate as a result
of stocastic variation VarSt(r)
Conclusion extinction by chance is probable if N
lt 30 !
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Environmental Stocasticity Vara( r )

• Main source climate.
• Annual variation has a greater effect on r than
  demographic stocasticity (it is assumed that for
  N gt 5000 the effect of demographic stocasticity
  is zero)
• It is not reduced by population growth (N)
• Its interaction with Var ( r ) in small
  populations greatly affects the probability of
  extinction.

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Genetic Variance

• Variation in traits between individuals has two
  origins
• Environmental
• Genetic variance
• additive genetic variance (effect of alleles
  within and of genes between loci)
• Dominance effect (dominant vs. recessive)
• interaction between the loci

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Genetic Diversity

• Heterozygosity the
• proportion of
• heterozygotic loci
• in one individual
• (lt 10).
• Genetic Diversity the amount of heterozygosity
  present in the population.
• Genetic Diversity number of alleles present
  in the population. Not relevant for conservation.

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Decline in heterozygosity

• Alleles are lost because matings are not by
  chance and because there is differential
  reproductive success between individuals.
• There is a loss of alleles, even if there are
  neither inmigrations nor mutations. This loss
  reduces heterozygosity.
• The rate of decline of heterozygosity depends on
  the population size N.

Small POPULATIONS
- Genetic drift -
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Genetic Problems

• In small populations
• Tendency for breeders to be related
• Genetic drift becomes important
• Frequency of genes in a population is determined
  by chance rather than evolutionary advantage.

Expression of recessive deleterious mutants
Low levels of heterozygosity High levels of
homozygosity
Reduced mean fitness
Reduced ability to respond evolutionarily to
changed environment
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Genetic Problems

• Homozygosity and heterozygosity
• A chromosome has gene pairs in each locus
• There are two alleles per locus, one by the
  father, the other by the mother
• Two equal alleles in one locus homozygotic
• Two different alleles in one locus heterozygotic
• Genotipic Frequency relative proportion of
  different combinations of alleles in the
  population
• Methods electrophoresis or DNA sequencing

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Inbreeding depression

• Strenght of association is very variable from
  species to species.
• rule of thumb
• Effective population size of 500 individuals
  should avoid these genetic problems.

Fitness
Homozygocity
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Inbreeding Depression

• Recessive alleles, with some deleterious effect,
  are maintained in the population, masked by their
  corresponding, dominant allele.
• The genetic pool of a population contains many
  recessive, (sub)lethal alleles.
• The loss of heterozigosity reduces the fitness of
  the (individuals and the) population.

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Inbreeding depression in small populations

• The depression starts if the population size is
  kept small for a long time
• 1. The frequency of matings between relatives
  increases, the genetic drift increases.
• 2. The heterozygosity of the progeny is reduced.
• 3. Semilethal effects of recessive alleles are
  exposed.
• 4. Fecundity is reduced and mortality increases.
• 5. The population declines even more ...
  Extinction becomes inminent.

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Inbreeding depression facts perspectives

• Mortality is 33 higher among progeny of matings
  between parents and offspring or between siblings
  (Ralls et al. 1988).

• Inbreeding depression is rare in populations of
  more than 25 individuals.
• Genetics problems arise from small population
  sizes over a long time. This is rarely the case
  with releases (reintroductions) of small, but
  fertile, populations.

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Inbreeding depression facts perspectives

• Low heterozigosity does not always lead to
  endogamic depression (selection eliminates
  exposed, deleterious alleles). The cheetah seems
  to illustrate this point.
• A minimum, viable population size cannot yet be
  defined in genetic terms.

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Effective population size Ne(genetic)

• Ne the size of a genetically idealized
  population to which the actual population is
  equivalent in genetic terms.
• Ideal Population family size follows a Poisson
  distribution, sex ratio 5050, non-overlapping
  generations, matings by chance, growth rate is
  zero.
• Ne tends to be less than Ideal Population and
  less than the real population census. In general
  Ne approx. 0.4 N

A real population loses genetic variability at
the same rate as an Ideal Population of half its
size.
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Effective population size Ne(genetic) examples

• If sex ratio of breeders not 11 (e.g. 100 males,
  400 females) N500 but Ne 320.
• If population size varies from generation to
  generation, then Ne is disproportionately
  influenced by the smaller sizes. E.g. The
  sequence 500, 100, 200, 900, 800, mean N500, but
  Ne 258.
• Estimate of several hundreds for a viable
  population size (Ne) implies an actual census (N)
  figure several times Ne

For formulae see Lande Barrowclough (1987)
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Effective population size Ne(demographic)

• Ne size of a population which growth rate is
  the same as the study population and that has a
  stable age distribution and even sex ratio.
• Delta N is a linear function of the proportion of
  females in the population. The capacity to
  recover from a decline increases if females are
  predominant.
• The distribution of ages affects the recovery
  capacity of a population.

In small, wild populations it is convenient to
have a female bias, while in captive populations
an even sex ratio is desirable to minimize
genetic drift.
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Effective population size and mating systems
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Apparent and actual reproduction of male
Stripe-backed wrens. Breeding subordinates
elevate Ibm and decrease average age at
reproduction (after Parker Waite).
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PVA of Trichechus manachus
Population Vulnerability Analysis
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Trichechus manachus
Costa Rica
Go to PVA Vortex
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Lessons What is a small population?

• 500 50 or less of the variance in a trait is
  due to genetic drift, the rest to natural
  selection population genetically healthy.
• 50 a loss of 1 in genetic variance per
  generation is not problematic.
• 10 MVP for a microorganism that reproduces by
  fission.
• The size of the small population will depend on
  the species characteristics and its environmental
  condition.
• The MVP size depends on the population concerned
  and on ones judgement of acceptable risk.

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Introduced Species

• Effects
• Loss of native species
• Displaced by competition
• Driven to extinction by predation
• Modification of the habitat
• Hybridization
• Modifications of the living community
• Physical alterations of the environment
• What makes an invasion likely?
• Not all invasions are desastrous

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Species Introductions

• Islands in the Pacific an extreme case
• Small areas, isolated from recolonization
  sources.
• High endemism.
• Settlers introduced exotic species.

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Case study Hawaii

• High endemism, many
• introduced species.
• 2000-4000 years ago arrival of humans and
  exotics.
• 1/2 of endemics extinct with the arrival of
  polynesians gt1000 years ago, ¼ with the arrival
  of Europeans, 1/8 threatened today.
• Increase of 9,000 to 12,000 species, but loss of
  endemics ... erosion of biodiversity?
• Much prey for one predator the mongoose. No
  competitors (no reptiles).
• Heterolochia acutirostris, nectarivorous bird,
  extint in 1907 by goat herbivory and introduced
  pigs.

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The brown tree snake

• Boiga irregularis originally from Australia and
  New Guinea, introduced accidentally with a
  military shipment exterminates from the 1940s
  until 1986 the 10 forest birds of Guam (2
  endemic).

Guam
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Origins of introductions

• European colonization
• Horticulture, agriculture, hunting and fishing
• Biological control
• Accidental transportation
• Range expansion

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Increase in number of exotic species in the U.S.
(OTA 1993)
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Accidental transportation

• Zebra mussel (Dreissena polymorpha), native of
  the Caspian Sea, accidentally introduced into the
  Great Lakes and associated rivers in 1988,
  outcompetes and chokes out native species.

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Convention on Biological Diversity

• Art. 8 (h) prevent the introduction of, control
  or erradicate those exotic species which threaten
  the ecosystems, habitats or species.
• IUCN Invasive Species Specialist Group

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Alien species invade the
planet The crazy
ant, the brown tree snake, the small
Indian mongoose, the Nile
perch, strawberry
guava, the water hyacinth, the zebra mussel and
the brushtail possum
are all wonderful species
in their own habitats. But like an
unwanted house
guest they can be a pest when they invade
ecosystems to which they
are alien species. In
fact these species have inflicted enormous
environmental and
economic damage
throughout the world.
These eight invasive offenders are among the
100 worst alien species, according
to a report issued by IUCN - the World
Conservation Union.
http//www.iucn.org/ (28.5.2001) http//www.iucn.
org/biodiversityday/index.html
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Successful Invasives

• High fecundity and reproductive rate, pioneer
  species, short generations, long lived.
• High dispersion rate
• Successful reproduction of one parent
• Vegetative or clonal reproduction
• Phenotypic plasticity
• Large native range
• Habitat generalist
• Broad diet (poliphagy)
• Human follower

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Vulnerable Communities

• Same climate as source of invasives
• Early succesion stage
• Low diversity of native species
• Absence of predators/parasites of invasives
• Absence of native species similar to invasives
  (morphology and ecology) empty niches
• Candid prey due to absence of predators
• Absence of fire in evolutionary history
• Trophic web with few connections
• Disturbed by humans

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Community effect of species removals
Food web

• Complexity
• Species position

SECONDARY EXTINCTIONS
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Communities disturbed by humans

• Dominance of exotics depends on the degree of
  alteration of the landscape.
• Human activity fosters
• Fragmentation of the landscape
• Establishment of exotics
• Increased environmental heterogeneity with high
  contrast
• Extinctions
• Increment in total number of species

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Human disturbance facilitates invasions
Degradation of Southeast Asian forests by logging
and farming (Harrison 1968)
Only introduced rats
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Which introductions/invasions are successful?

• Most are not (10-40 success).
• Overall 32 of predator introductions were
  harmful. Cats 64 of 59 introductions.
• Harmful cat introductions on oceanic islands
  71, on mainland or peninsular islands 30.
• Autoecology is a key factor.
• If successful at low densities.
• If low vulnerability to endogamy.
• If no competitors encountered.
• If no new diseases encountered.
• If carrier of diseases that decimate competitors.

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Exotic red fire ants (Solenopsis invicta)
decimate northern bobwhites (Colinas virginianus)
in Texas

• Fire ants attack and disturb bobwhites,
  particularly at the nestling stage, and may
  compete for food items, such as insects (Allen et
  al. 1995).

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Which species might face extinction upon an
invasion?

• Rare species (low abundance)
• Low viability under reduced population size.
• Marked population oscillations
• Environmental variation
• Predator prey interaction
• Trophic specialization
• Life history

Ebenhard, T. 1988. Introduced birds and mammals
and their ecological effects. Swed. Wildl. Res.
131-107
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Qué invasives causarán más extintions?

• A veces depende of the comunity hospedera, no of
  the invasor.
• A veces no. P.ej. peces Gambusia sp. of the este
  y centro of N-América son devastadores of native
  en diversas comunityes.
• 32 of predator introductions were harmful.
  Gatos 64 of 59.
• Gatos en islas 71 perjudiciales, Gatos en
  tierra firme o islas peninsulares 30.
• Otros devastadores ratas, cabras, bivalvos zebra
  (Dreissena polymorpha of the mar
  Caspio,Fig.10.8), hormigas of fuego (Solenopsis
  saevissima, Fig.10.4), y en plantas el kudzu.

Ebenhard, T. 1988. Introduced birds and mammals
and their ecological effects. Swed. Wildl. Res.
131-107
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Qué extintions of native resultan en más pérdidas?

• La extintion of species clave generan cambios
  drásticos en la comunity (extintions
  secundarias).-FIG. 8.12
• - specie of plantas en la base of cadena trófica
  simple
• - predators en cadenas tróficas complejas (efecto
  of cascada si la presa compite con muchas
  species)
• - predators polifágicos
• - herbívoros polifágicos sin predators en
  comunityes simples (p.ej. cabras en islas
  oceánicas)

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No siempre es desastroso (Ariel E. Lugo Inst.
For. Trop. Puerto Rico)

• Éxito of species exóticas
• absence of enemigos naturales
• absence of parásitos
• absence of competidores (nichos vacíos) poco
  impacto sobre el ecosistema.
• Colonizadoras agresivas, rápido crecimiento y
  alta fecundity? v.s. Conditions of the comunity
  invadida

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Not all introductions are harmful (Ariel E. Lugo
Inst. For. Trop. Puerto Rico)

• Eichhornia crassipes in the Amazon
  inconspicuous, in Florida and Lake Victoria a
  pest (slow and eutrophic waters).
• Forest of exotics and natives do not differ
  ecologically (73 variables compared structure,
  composition and functioning) not even in the
  case of Eucalyptus.
• Exotics as a restoration tool for understorey and
  soil enrichment through nitrogen fixation (Myrica
  faya en Hawaii).
• Delonix regia endemic and threatened plant in
  Madagascar found refuge in Puerto Rico.

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Managment strategy in a changing world

• Hundreds of exotic insects are introduced into
  Hawaii for the control of agricultural pests.
• Humana activity initiated thousands of years ago.
  Introduction and managment of exotics are an
  intrinsic and continuous process in a world
  governed by humans.
• Mitigate damage and potentiate benefits.

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Conclusions

• Native, island organisms are fragile and
  vulnerable to species introductions.
• Not all introductions are successful and not all
  successful ones are desastrous.
• But anticipating which introductions will be
  desastrous is highly complex.
• Small populations are particularly vulnerable to
  extinction due an interaction vortex of
  demographic, genetic and environmental effects.
• Population declines call for inmediate attention.

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