Download Lecture 7 Ecology and species assemblages

Populations & Communities
I.
Population Ecology
A. Density
B. Factors affecting density
II. Community Ecology
A. Trophic Relationships
1) Trophic pyramid
2) Food web
B. Structure & Local Species Assemblages
C. Species Interactions
Population Ecology
A. Population density = # of individuals of a
species per unit area or volume
Intrinsic rate of increase = maximum rate at
which a population of a given species
can increase under ideal conditions
Factors affecting density
1) Density dependent
factors
2) Density independent
factors
Governing forces:
• Species
interactions
(comp/pred)
• Abiotic factors
(tolerances)
• Historical factors ~
dispersal
2-5x increase in densities of Batrachoseps
attenuatus on islands in SF Bay over those on
mainland due to lower predation pressure on
the islands
P. teyahalee
P. metcalfi
Chihuahuan Desert - precipitation ranges from 50 to 400 mm
Lizard species’ densities studied over 5 year period, exhibited fluctuation
correlated with precipitation.
Schall & Pianka 1978
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Compared Aus & N.A.
Turtle & frog richness
pos. correlated to
annual rainfall, neg.
mean annual hrs
sunshine
Lizards had opposite
pattern
Snakes positively
correlated w/ mean
annual temp & rain
Community ecology
Community = All of the organisms that
inhabit a particular area; an assemblage
of populations of different species living
close enough for potential interaction
A. Trophic Relationships
1) Trophic pyramid
2) Food web
B. Structure & Local Species Assemblages
C. Species Interactions
Structuring forces
Gause’s Principle
1960’s & ’70’s: Competition
structures communities
Ecological niche
Current view: physical environment
predicts composition
trophic pyramid
trophic structure
trophic levels
quaternary
tertiary
secondary
consumer
primary consumers
primary producers
Detritovores - decomposers
Guild =
energy flow through
ecosystems:
much is lost at each trophic level
2) food web:
The structure of a community &
assemblages
1. Structure = species diversity, species
richness
– Diversity =
– Richness =
– Species assemblages - Subset of species
being considered in a community
•
General trends
Gradients in Species Richness
Latitude/ Altitude – more species
found lower (taxon. widespread)
Tropical lowland herp diversity
peaks around 150-200

Some exceptions (salamanders,
turtles, lizards(!))
Why the trends?
1) Climactic features vary with increases in
latitude
2) Habitat heterogeneity
HABITAT HETEROGENEITY
• The greater the range of environmental conditions,
the more kinds of topography, soil conditions,
microclimate and habitat – the greater the
heterogeneity, the greater the biodiversity of a
landscape
– Physical or Biotic
– Spatial or Temporal
– Fixed or Dynamic
• Higher heterogeneity =
What drives these gradients?
Many proposed mechanisms:
Productivity
Historical (lag)
Structural diversity
Climate (seasonality)
Pattern, Process, Mechanism
Mechanism- factors affecting
individuals (e.g. competition)
Process- population-level effects of
individual interactions (neg b/w
species)
Pattern- community level (presence/
absence, resource/microhabitat use,
etc.)
PPM, cont’d

Cause and effect relationships
among these 3 levels often assumed

Attribute negatives between 2 spp to
using same prey (interference) or
aggression (exploitative)
However, different mechanisms can
produce indistinguishable patterns
PPM, cont’d

Differences in prey use b/w spp
competition, or
 independent evolution?
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Process may be difficult to detect
Resource-limited years (Dunham 1980)
 ‘Ghost of competition past’

• Differentiation, displacement
Species Interactions
-
1) Competition
2) parasitism
predation
S1
-
S2
+
S1
S2
-
3) mutualism
+
S1
S2
+
1) Interspecific Competition for limited resource
•
Interactions lead to either decrease in
abundance or some component of fitness
•
the 2 species diverge in their use so the
co-existence is possible
Interspecific competition results in
microhabitat differences
• Microhabitat parapatry
results from intense
competition between P.
cinerus & P. shenandoah
• P cinerus, more
aggressive – P.
shenanadoah becoming
restricted to dry slopes
Species Interactions:
Competition

Williams (1983)
examined
microhabitat use
of 9 Anolis
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Sun/shade
Perch diameter
Perch height
Reduces
interactions – but
a result of comp?
Ex: Anolis lizard sp. perching sites in the Dominican Republic
Alternative explanations?
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Environmental
tolerances (T, water
loss)
Preferred prey
Other researchers
have shown
determinism in
Anolis composition
on islands
Determinants of
Community Structure
Often many interacting factors both
abiotic and biotic
Competition in Caribbean Anolis:
2-9 species on islands; few predators,
lizards abundant- resources limiting
Interspecific competition may be
alleviated by resource partitioning

Anolis competition
Removal of an aggressive species
showed habitat expansion by
another (Jenssen 1973)
 Enclosure experiments revealed
greater partitioning b/w a species
pair from a more resource-limited
island (Pacala & Roughgarden 1985)

Predation
• Negatively affects prey (they are
consumed)
• Most predators feed on more than one
prey species (dependent upon
abundance)
Predator and prey populations follow a series of synchronized fluctuations
The prey population grows exponentially, and reproduction in
the predator population is a function of the number of prey
consumed
As a single predator population increases, the single prey
population decreases to a point at which the trend is reversed
The two populations rise and fall, oscillating in a predictable
manner
Lotka-volterra model -simplest model of predator-prey interactions,
predict greater stability w/more “links”
Species Interactions:
Predation
Predation- easier to establish than c
Often, more abundant prey species are taken –
modify community structure
Submergent behavior- prey respond by
reducing activity
Restrictive activity times, places
promote prey-switching in predator
mediate competitive coexistence
Species Interactions:
Parasitism
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~1/2 all animals are parasites
often specialized; differential effects
Shift P-P, competitive interactions
Rarely studied at the community level
Pathogens may cause local extinction:

B. boreas in Colorado by Aeromonas
Abiotic factors: Habitat
complexity

Pianka (1967)
Plant height
diversity best
predicted species
richness of flatland
desert lizards
Abiotic factors:
Physiological tolerances

Each species has different tol/ pref
Trade-offs
 Distinct vs. shared preferences
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Temp, precipitation, ET are often
correlated with a species’ range, and
its abundance therein
Weather-induced
community changes
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Whitford & Creusere (1977) – fecundity
and composition in Chihuahuan Desert
lizard community enhanced in wet years
Pechmann et al. (1989) # species
metamorphosing from ponds related to
hydroperiod
Longer-lived spp’s may endure poor
weather conditions better
Anthropogenic effects
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Dispersal:
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Polynesians– gecko introduction
Anglers– introduced salamander bait
Seri- Sauromalus
Landscape changes: Maya cultivated much
of Yucatan, favoring open-habitat species
Local scale: Single-tree harvest in
Amazonia promoted heliothermic lizards;
forest species retreated (Vitt et al 1998)
Prey of Brazilian Colubridae
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Vitt & Vangilder (1983) explained
resource-use with present-day
ecological factors
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Cadle & Greene (1993) assert that
much of observed prey use was due
to ancestry
No invertebrate specialists
Vitt & Vangilder (1983)
Too much competition
w/ insectivorous
mammals, too many
small snake
predators, suitable
microhabitat lacking
Cadle & Greene (1993)
Insect eating snakes
are rare/ absent in
neotropics
Anuran-eating snakes are
speciose
V&V:
Convergence on this
resource due to
frog abundance
and year-round
availability
C&G:
Large proportion of
the frog-eaters
have common
ancestry
Salamander competition
Hairston (1949)
Distributional
patterns in GSM
suggest interspecific comp
Density
manipulations
confirmed geog
variation in degree
of competition
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Larval amphibian
assemblages
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Extremely dynamic systems
Size-dependent predation/competition
 Composition turnover
 Hydroperiod; nutrient flux
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Spatial partitioning
In Thailand,
different tadpole
species use
distinct aquatic
zones
Heyer (1973)
Trade-offs- puddle vs. pond
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Selective forces differ depending on larval
environment
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Fast-drying ponds- promote quick
development
Permanent ponds- predatory contingent
promotes submergent behavior
Predation & Competition
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6 tadpole species at different newt densities
(Morin 1983)
Low predation, 4 comp dominant species
suppressed 2 inferior tadpoles
High predation, inferiors survived better
3 species developed faster with heavy predation
Priority Effects
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Competitors/ predators arriving first
can influence late-comers
Hyla pseudopuma avoids ovipositing
where predators/competitors are
present
 Ambystoma opacum oviposit in fall
If they survive winter, they feast on
other amphibian larvae
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Tropical anuran assemblages
Up to 80 anuran spp in sympatry
 Diverse reproductive modes
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Aquatic larvae
 Eggs carried on back (terr & aq)
 Oviposit on land, carry eggs to water
 Oviposit on leaves over water
 Direct development
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Why does aquatic egg & larval
development seem to be avoided?
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Magnusson & Hero (1991)
Looked at water quality, desiccation,
predation and competition
 Experiments revealed heavy predation
by other anuran larvae and
invertebrates probably favors more
terrestrial reproductive modes
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Vocalization partitioning
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Tropical choral aggregations may prohibit
effective sexual selection
Responses to distinct environmental cues
among species may affect species at a
breeding site
Temporal partitioning
Call-character displacement:
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Allopatric species pairs have more similar calls
than in sympatry
Selection favors species recognition
Mutualism
• Beneficial symbiosis –
2 species dependent
upon each other and
both benefit from the
association
• Examples:
Red-backed Salamanders help make soil better for plants and animals when they
tunnel through it. Nutrients in the soil get mixed and plants can pull them into their
roots more easily. Small animals such as mites and beetles find it easier to move
around in the soil.