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 bedefined by environmental conditions, but biotic factors may also be involved.

Fig. 10.2, Molles & Cahill, 2008

Tiger beetle distribution limited to cool environments

Figs. 10.3 & 10.4, Molles & Cahill, 2008

Suggests clear physical & physiological tolerance limits

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

Cross-section of S. apiana(note abundant leaf hairs)Photos: Dwight Beltz

Species often distribute themselves along climatic (temperature or moisture) gradients.

http://en.wikipedia.org/wiki/Encelia

Fig. 10.5Molles & Cahill 2008

Within a region, species may further subdivide their ranges by microclimate

Ehleringer et al. have shown that pubescencein Encelia sp. (wild sunflower) leaves followsenvironmental conditions (microclimate).

Figs. 10.6 & 10.7, Molles & Cahill, 2008

Molles 2008

Molles 2008

Intertidal region – Zonation the result of both physical and biotic factors

Figs. 10.8 & 10.9, Molles & Cahill, 2008

Zonation in the intertidal - Barnacle species have different ranges (work of ConnellDemonstrated 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)

3 types of species distribution:

• Random• Regular• Clumped

Fig. 10.10, Molles & Cahill 2008

Again, both biotic and abiotic factors may determine the pattern.

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

Molles 2008

(Evidence that both biotic and abiotic factors determine plant distribution)

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

• 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.

Fig. 1.2Molles & Cahill 2008

Crow distribution over the continental US – note clumping

Molles 2008(compare to figure 10.15, Molles & Cahill 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. 10.23Molles & Cahill 2008

Plant size vs. population density

Fig. 10.24,Molles & Cahill, 2008

These size-density patterns suggest competition for resources influences population density

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

Fig. 10.25,Molles & Cahill, 2008

See fig. 10.15b

See fig. 10.26

… but extinction can easily occur even if one of these factors is strong enough (see next example).

Example of the passenger pigeon (Ectopistes migratorius) -Huge population sizes (billions), and broad range…

http://en.wikipedia.org/wiki/Passenger_Pigeon

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!

Practical applications of population ecology:

1) Pest control2) 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).