Population Ecology I
Distribution & Abundance of
Populations & Species
Molles & Cahill (2008) – Chapter
10
Topics:
• “Population Ecology” defined
• “Population” defined
• Issues:
– Population distribution
• Clumped
• Random
• Regular
– Population density
– Population abundance
– Rarity and Vulnerability to extinction (practical
applications in conservation biology, pest control)
Population ecology
• Addresses the distribution, abundance, and
density of populations.
• Addresses population structure and dynamics
• Addresses interactions between organisms (e.g.
competitive, exploitative, mutualistic interactions).
• Defines community structure in terms of these
interactions.
• Supported by evolutionary theory &
ecophysiology.
Population defined
• A group of potentially interbreeding
individuals of a single species
• A group of individuals of a single species
inhabiting a specific area.
– Highly localized group of individuals
representing a fraction of the total population
of a species, or
– All of the individuals of a species across its
entire range.
Links to evolutionary theory
(review Chapter 4)
Links to physiological ecology
(much of what we have covered)
Common garden & transplant experiments (Clausen, Keck & Heisey 1940)
This classic study design distinguishes acclimation from adaptation
and helped define the concept of “ecotype” (c.f. “genotype).
Molles (2008)
Chapter 8, p188
Tolerance limits – central theme in population ecology
…but note that population ecology also considers
biotic aspects (e.g. species interactions)
Population distribution
• Distribution limits (range) often defined by physical factors.
Range of three kangaroo species in Australia. These species’ ranges can be
defined by environmental conditions, but biotic factors may also be involved.
Fig. 10.2, Molles & Cahill, 2008
Tiger beetle distribution limited to
cool environments
Suggests clear physical & physiological tolerance limits
Figs. 10.3 & 10.4, Molles & Cahill, 2008
Leaf Pubescence (hairs) – why be hairy?
(many reasons!)
Glandular hairs (trichomes) on Cannabis sativa
http://en.wikipedia.org/wiki/Leaf_hair
For example, leaf pubescence can
greatly reduce a leaf’s radiation load
and reduce it’s evaporative water loss
Pubescent leaves of Salvia apiana
Photos: Dwight Beltz
Cross-section of S. apiana
(note abundant leaf hairs)
Species often distribute themselves along climatic
(temperature or moisture) gradients.
http://en.wikipedia.org/wiki/Encelia
Fig. 10.5
Molles & Cahill 2008
Within a region, species may further subdivide their ranges
by microclimate
Ehleringer et al. have shown that pubescence
in Encelia sp. (wild sunflower) leaves follows
environmental conditions (microclimate).
Figs. 10.6 & 10.7, Molles & Cahill, 2008
Molles 2008
Intertidal region – Zonation the result of both physical
and biotic factors
Molles 2008
Zonation in the intertidal - Barnacle species have different ranges (work of Connell
Demonstrated that Balanus sp. is more vulnerable to dessication than Chthamalus sp.)
Zonation among barnacles in the upper intertidal appear to be the result of both
abiotic factors (tolerance limits) and biotic factors (competition)
Figs. 10.8 & 10.9, Molles & Cahill, 2008
3 types of species distribution:
• Random
• Regular
• Clumped
Again, both biotic and abiotic factors
may determine the pattern.
Fig. 10.10, Molles & Cahill 2008
Behavioral interactions (aggression) may affect distribution
(example from field studies of stingless bees)
Molles 2008
Distribution patterns of Creosote bush…
… change over time as shrubs grow (due to resource limitations)
Molles 2008
Mature creosote bushes “partition” the soil habitat
to avoid competition for soil resources
(Evidence that both biotic and abiotic factors determine plant distribution)
Molles 2008
Distribution varies with scale
• At large scales, organisms’ distributions
tend to be clumped based on resource
availability.
• At smaller scales, species interactions
(e.g. competition) may result in “habitat
partitioning”
Warbler Feeding Zones
Fig. 1.2
Molles & Cahill 2008
• MacArthur found that warblers were able to coexist by feeding
in different zones of trees (habitat or niche partitioning).
• Morse (1980) found that aggression between warbler species
maintained their distinctive feeding zones.
Crow distribution over the continental US – note clumping
(compare to figure 10.15, Molles & Cahill 2008)
Molles 2008
Bird populations tend to concentrate around key
resources (food, habitat).
Fig. 10.16, Molles & Cahill, 2008
Plant species tend to be distributed along
environmental gradients according to physical
factors (Whittaker et al. )
Santa Catalina Mts. Arizona
Great Smoky Mts. Tennessee
Figs. 10.17 & 10.18, Molles & Cahill, 2008
Population Density
• Often linked to organism size –
suggesting resource limitations
Fig. 10.22,
Molles & Cahill, 2008
Animal size vs. population density
Fig. 10.23
Molles & Cahill 2008
Plant size vs. population density
These size-density patterns suggest competition for
resources influences population density
Fig. 10.24,
Molles & Cahill, 2008
Rarity and vulnerability to extinction
• Rabinowitz (1981) defined 3 factors
influencing commonness/rarity:
– Geographic range (extensive vs. restricted)
– Habitat tolerance (broad vs. narrow)
– Local population size (large vs. small)
Species become more vulnerable to extinction if these 3
factors are combined
See fig. 10.26 Æ
See fig. 10.15b Æ
… but extinction can easily occur even if one of these
factors is strong enough (see next example).
Fig. 10.25,
Molles & Cahill,
2008
Example of the passenger pigeon (Ectopistes migratorius) Huge population sizes (billions), and broad range…
Former range of the passenger pigeon
…but had a very specific nesting requirement (old growth forests)
http://en.wikipedia.org/wiki/Passenger_Pigeon
Ectopistes migratorius – the Passenger Pigeon
Ça n’existe plus!
http://en.wikipedia.org/wiki/Passenger_Pigeon
Practical applications of population ecology:
1) Pest control
2) Species conservation
We can apply principles of population ecology to
manage populations more effectively (reduce
populations of unwanted individuals or enhance
population levels of desired species).