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What are the differences between the small and declining population paradigms?

- continuum, relative importance of factors shift over time

small population (SP):
- rich in theory
- promotes local & very intensive management
- firefighting
e.g. vaquita: < 30 individuals

declining population (DP):
- less rich in theory
- general principles concerning diagnosis and management of causes of decline
- less intensive management over larger areas
- proactive


What is an example of the declining population paradigm affecting small populations?

Spoon-billed sandpiper (CE)
- previously 3000 pairs now <200
- low breeding successes: disturbance and predation
- large scale winter habitat loss
- low recruitment: hunting

i.e. not yet experiencing stochasticity, inbreeding, allee effects


How can small and declining population paradigms act together?

- Atitilan grebe

factors driving decline change over time:
- DPP: habitat destruction, exotic species
- SPP: earthquake, hybridisation

- multiple factors interact (can be synergistic)
- diagnosing & thus reversing causes of decline is essential, but difficult


What are difficulties with declining populations discovery?

- difficult due to geographic or taxonomic biases in monitoring
- many species are data deficient
- need to exploit other methods


How are declining populations discovered?

- naturalist recordings

- haphazard naturalists' recordings can reveal past population sizes/population crashes


How are declining populations discovered?

- local people

- interviewing local people e.g. fisherman can give accurate idea of population declines if shifting baselines taken into account


How are declining populations discovered?

- spatial variation in factors driving decline

- look for areas where factors driving decline are intense

e.g. habitat loss, over-exploitation, introduced species


How are declining populations discovered?

- biological traits

- idea that ecological and life history traits predispose species to population declines in response to human activities

e.g. body size
- hunting
- bigger animals mean
more food
- reproduction
- bigger animals have a
slower growth rate


What are the weaknesses of using biological traits to identify declining populations?

- Collen et al., 2011
- species traits weaker predictors than environmental traits

- Fritz et al., 2009
- geographic transferability is limited
- body size only a good predictor in the tropics


What is meant by the cause of a decline?

- demographic causes
e.g. reduced surival/breeding success/recruitment

- environmental causes
e.g. poisoning from DDT/pesticides


How to diagnose cause of a decline?

- simulation model

Population Viability Analysis (PVA)
- requires knowledge of relationship between demographic traits & all external factors
- for most species, knowledge barrier too great to implement with sufficient speed
- need plenty of data


How to diagnose cause of a decline?

- comparative approach

- list plausible factors for decline
- identify populations that differ in these environmental conditions through time/space
- allows environmental factors causing decline to be identified


How to diagnose cause of a decline?

- comparative approach: timing

- compare timing & decline of environmental change

e.g. dramatic corncrake decline when machine cutting of hay (vs hand cutting) reaches 60%


What are problems with comparing timing?

several environmental factors often change simultaneously
- e.g. agricultural intensification
- more machinery/chemicals/hedge
removal/higher stocking densities

monitoring is too infrequent to
identify precise timing of change in environment or population decline,
e.g. many plants
- spatial scale of population &
environmental monitoring differ
e.g. fish stocks & marine wildlife
- time lags can distort relationships


How to diagnose cause of a decline?

- comparative approach: different environments

- use when not possible to compare populations pre and post decline
- can compare declined populations with undeclined populations at other sites


correlation does not always = causation
- positive correlation between rat poison & rat population doesn't mean poison isn't good for rats


How to diagnose cause of a decline?

- comparative approach: different environments - radiated tortoise

Radiated tortoise:
- madagascan endemic
- harvested for food & pet
trade, pop thought to be
- trade bans, but no
evidence that trade was
causing the decline

compared densities in commercial areas (1 per km2) and remote sites (2500 per km2)

supporting evidence:
i) densities increase with distance from urban areas
ii) commercial hunters travel increasing distances c.200 km


How to diagnose cause of a decline?

- check for individual movements: great tits

- say in first year 50 tits in woods, 50 in hedgerows
- second year 50 tits in woods, 10 in hedgerows

- does this mean hedgerows have declined in quality?
- not necessarily true

- buffer effect: if habitat quality varies then highest quality sites will be occupied first & abandoned last
- if tits prefer woods & can move between woods &
hedges then any factor that drives a population
decline (e.g. a cold winter) will result in greater pop
decline in hedges


How to diagnose cause of a decline?

- test for all possible causes of decline

- rarely possible due to lack of data
- common problem is
lack of information from
non-breeding grounds of
- need to acknowledge bias


How to diagnose cause of a decline?

- test for all possible causes of decline: albatross

- many studies only test the
favoured hypothesis
- can lead to incorrect

Rolland et al., 2009
- albatross all threatened by long-line fishing
- in Indian yellow nosed albatross, cholera is more important


How to diagnose cause of a decline?

- test for all possible causes of decline: New Zealand birds

- major declines in NZ native birds: introduced
predators & associated factors

- disease ignored: avian malaria equally likely as
causal agent

- major factor on other islands, present in introduced &
native birds & known to cause mortality


Recovery case studies:

- Linum cratericola

critically endangered
- discovered at 2 sites in 1966
- extinct at 1 sites invaded by an exotic shrub
- also a large number of feral goats

hypothesis that grazing invasion causing decline
- 1997
- 2 groups of plants enclosed in goat-proof fence & invasive shrub cleared
- intensive goat control in general area
- population size increased until goats broke into enclosure

- often need to manage more than 1 threat to a species


Recovery case studies:

- skylark

- decline of 53%
- almost all of decline on
- higher productivity on
spring sown cereals than
winter cereals
- reduced food availability

- skylarks struggle to forage in tall thick winter cereals
- leaving undrilled patches in winter cereal fields
- fields w 2 skylark plots per herctare have significant benefits
- mostly use them for foraging but increases breeding success
- more chicks, heavier chicks