Download From populations to communities

Reminder
 Quiz – Chapters 11 and 8: Friday December 17
 Mustafa: syllabus. See email.
 To all: follow your own deadlines please 
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A few points
 Populations exist in the context of the whole
community
 Each is within a whole web of interactions
 Each responds differently to the prevailing abiotic
conditions
 …how do biotic and abiotic factors combine to
determine dynamics of species populations?
 What is the importance of the concept of
metapopulations?
 What about food webs?
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Multiple determinants of
dynamics of populations
 Why are some species rare and others common?
 Why does a species occur at low population densities in
some places and in high densities at others?
 What factors cause fluctuations in a species’ abundance?
 To understand even a single species in a single location –
need to know:
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Physicochemical conditions
Level of resources available
Organism’s life cycle
Influence of competitors, predators, parasites…
And how all these factors influence abundance through
effects on birth, death, dispersal, and migration
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‘devil in details’
 A record of numbers alone: What do they tell us? What
don’t they tell us?
 Thus – need information on age, sex, and size
 Correlations with external factors.
 High intensities of disease late blight in potato crops occur
15-22 days after a period in which min temp > 10C and
humidity > 75% for 2 consecutive days
 Correlations – suggest (not prove) causal relationships
 Cause requires a mechanism
 Pop too large … what could happen?
 Correlation does not tell us which of the options could
happen.
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What does stability mean?
 Stability does NOT mean “nothing changes”
 Population: may have complex dynamics, flux, and
stability
 Small, sand-dune plant (androsace septentrionalis):
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

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150 - 1,000 new seedlings / m2 each year
Mortality: reduced pop by 30% - 70%
Pop kept to same limits
50 plants always survived to fruit and produce seeds
for next seeds
 Stability?
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Stability or Change?
 Do we look for constancy of populations? Or do we
emphasize the fluctuations?
 If emphasize constancy: look for stabilizing forces
within populations to explain why populations do
not exhibit unfettered (unrestrained) increase or a
decline to extinction
 If emphasize fluctuations: look to external factors
(weather? Disturbance?) to explain changes
 Can the two sides be in agreement?
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Stability or Change?
 How is abundance determined? How is abundance regulated
 Regulation…
 Regulation: tendency of a population to decrease in size when it is
above a particular level to increase in size when below that level
 Can occur only due to one or more density-dependent processes that
act on rates of b , d and/or movement (remember: Chapter 5)
 Detected in 80% of studies of insects that lasted > 10 years
 Determined:
 Precise abundance of individuals will be determined by combined
effects of all the factors and all the processes that affect a population,
whether dependent or independent of density
 Weather typically major determinant
 Apple thrips: weather accounted for 78% of variation in #
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Key factor analysis
 Can distinguish between what regulates and what
determines the abundance of a population
 Then learn how regulation and determination relate to
one another
 By: key factor analysis (better to name it: key phase
analysis)
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Calculate k-values for each phase of the life cycle
k-values measure the amount of mortality
The higher the k-value, the greater the mortality
k = killing power
[read box 9.1]
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Key factor/phase analysis
 How much of the total mortality tends to occur in each of the
phases? Mortality = all losses from population
 What is the relative importance of these phases as determinants of
year-to-year fluctuations in mortality, and thus of year-to-year
fluctuations in abundance? [is the toll roughly the same year to
year?]
 Then phase mortality is low: total mortality is low: pop is large. And
vise versa.
 A phase with a k-value that varies randomly will – by definition have
little influence on changes in mortality
 So what is needed: to measure the relationship between phase
mortality and total mortality: regression coefficient of former on
latter
 Larger regression coefficient: key phase causing pop change
 Regression coefficient at zero: random phase mortality
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What about densitydependency?
 Plot k-values for each phase against # present at
start of the phase
 For density dependency: k-value would be highest
when density is highest. Right?
 For beetle population
 Key phase: summer adults
 Density-dependent: older larvae
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Wood frog (rana
sylvatica)
 Key phase determining
abundance: larval period, due
to year-to-year variations in
rainfall
 Density-dependent: adult
phase (due to competition for
food)
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Dispersal, patches, and
metapopulation dynamics
 Migration can be a vital factor in determining and/or
regulating abundance
 Disease : important role when populations are
fragmented and patchy (as many are)
 Abundance of patchily distributed organisms: determined
by properties of two features: ‘habitable site’ and
‘dispersal distance’
 Metapopulation: if pop comprises a collection of
subpopulations, each one of which has a realistic chance
both of going extinct and of appearing again through
recolonization
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American
pika
 Metapopulations: N, middle, and S network of patches
 Northern network: high occupancy throughout study period (1972-1991)
 Middle network: more variable and much lower occupancy
 Southern network: steady and substantial decline
 When isolated (in simulation): northern network was fine, but middle and
southern networks crashed
 When entire network simulated as a single entity: middle network – stable,
southern network: periodic collapses
 Consistent with data
 So? – northern network acts as a net source of colonizers that prevent
middle network from collapsing -> delaying extinction -> allow
recolonization of southern network; southern
network: transient behavior
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Temporal patterns in
community composition
 Patch dynamics. Disturbances open up gaps
 2 fundamentally different kinds of community organization
 Founder-controlled
 When all species are good colonists and essentially equal competitors;
Species are approximately equivalent in their ability to invade gaps and
can hold gaps against all comers during their lifetime
 Examples: tropical reef fish; vacant living space: limiting factor
 Competitive lottery
 dominance-controlled
 Some species are strongly superior competitively
 Community succession
 Opportunistic, early-succession  species with poorer powers of dispersal
 mid-succession  climax stage; # of species increases (colonization) then
decreases (competition)
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Community succession
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Plants and animals in
succession
 Just because plants dominate succession – does not
mean that animals necessarily follow
 Sometimes: animals determine nature of plant
community
 Heavy grazing. Trampling. [box 9.4]
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Succession in a patchwork:
size and shapes of gaps
 Has climax been reached? A matter of scale
 Many successions take place in a mosaic of patches,
with each patch, having been disturbed
independently, at a different successional stage
 Large gaps
 Centers: colonized by species that travel great
distances (relatively)
 Small gaps
 Colonized by established individuals around periphery
of gap
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Food webs
 Let’s examine systems with at least 3 tropic levels
 Plant – herbivore – predator
 Let’s then consider direct and indirect effects that a
species may have on others on the same or other
trophic levels
 Effects of a predator on individuals of its prey. Clear.
What about effects of the predator on the prey’s
resources? Or on other predators of that prey?
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Food webs: indirect and
direct effects
 What would happen when a species ire removed
from a community?
 Increase in abundance of a competitor
 Increase in abundance of a prey
 Or
 Competitor decrease in abundance
 Decrease in abundance of a prey
 Why? Direct effects < indirect pathways
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Cats, rats, birds
 Feral cats  threaten local birds
 Cats eat rats and birds; Remove cats; Exotic Rats have less
pressure on them; Rats attack the endangered flightless
parrot (Of 21 chicks that hatched between 1981 and 1994, nine
were either killed by rats or died and were subsequently eaten
by rats) ; parrots translocated to an island w/o the exotic
predators
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
Cat control in 1982 arrested a sharp decline in Kakapo numbers, and they have
recently increased under the Kakapo Recovery Plan. Red arrows indicate breeding
years. Numbers become less precise before 1995, with the 1977 figure perhaps out
by 50 birds. The 2009 figure is as of April 11, 2009.
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http://nzgames.org/
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Trophic cascade
 Predator reduces abundance of its prey  impacts
trophic level below  prey’s own resources increase
in abundance
 Or
 Predator reduce intermediate predator -> increase
herbivore  decrease plants (4 trophic levels)
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 2 year experiment: predation by birds – Glaucous-winged
gulls and oystercatchers excluded from large areas in which
limpets were common
 Excluding the birds  increased overall abundance of 1 of the
limpet species (L. digitalis) , but a second limpet species
became rarer (L. strigatella), and a third did not vary in
abundance – the third (L. pelta) is the most commonly eaten
by the birds. Hmm…how?
 L. pelta is the birds’s favorite food and it is did not increase
with the removal of the birds?
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 L. digitalis – light colored; occurs on
light-colored goose barnacles
 Dark L. pelta – on dark California
mussels
 Predation by birds  reduces areas
covered by goose barnacles. So
removing predation?
 Increasing barnacles -> decrease in
area covered by mussels [more
competition from barnacles]
 Third species (L. strigatella) is
inferior competitively to ohers;
increase in L digitalis  decrease in
L. strigatella  released pressure on
L. pelta so remain unchanged
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Birds -> limpets (and
barnacles/mussels) – and algae!
 Birds eat limpets. Limpets eat fleshy algae.
 Birds eat barnacles. More space for algal
colonization.
 Birds excluded. Algae cover…
 Decreased
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Food webs
 Top-down?
 Predators controlling prey
 Bottom-up?
 Plants can limit herbivores
 “Why is the world green?” (Hairston et al, 1960)
 Because top-down control predominates? Green plant
biomass accumulates because predators keep herbivores in
check
 Or “is the world prickly and tastes bad” (Murdoch, 1966;
Pimm, 1991)
 A world controlled from the bottom up may still be green
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To be continued
 Page 313
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