POPULATION DISTRIBUTION AND ABUNDANCE Chapter 9

Chapter Concepts
Physical environment limits geographic distribution of species
On small scales, individuals within pops. are distributed in random, regular, or clumped patterns; on larger scales, individuals within pop. are clumped
Population density declines with increasing organism size
Rarity influenced by geographic range, habitat tolerance, pop. size; rare species vulnerable to extinction

Populations
Ecologists define a population as group of individuals of single species inhabiting specific area.

Habitat
Physical environmental conditions that allow individuals of species to survive AND reproduce

Habitat quality
Ability of environmental conditions to support repro and survival

Habitat area/volume
Resource concentration
Time
High habitat quality = organisms acquire many resources; high survival + repro = large pop.
Population numbers vary with habitat quality

What defines Habitat Quality?

Habitat Quality = Habitat Space X Resource Concentration X Time to Acquire Resources

Figure: Pop. numbers vary with habitat quality

Distribution Limits
Physical environment limits geographic distribution of species
Organisms can only compensate so much for environmental variation

Geographical range
Geographic area where species is found (based on macroclimate, salinity, nutrients, oxygen, light, etc.)

“Large-scale” patterns of distribution:
Refer to variation in species abundance w/in range
due to variation in habitat quality

Kangaroo Distributions and Climate
Caughley - relationship between climate + distribution of three largest kangaroos in Australia
Macropus giganteus – eastern greyEastern 1/3 of continenttemperate forest, tropical forest
Macropus fuliginosus – western grey southern and western regions temperate woodlands and shrubs
Macropus rufus – redarid / semiarid interior

Distributions largely based on climate: Fig. 9.2

Kangaroo Distributions and Climate
Limited distributions may not be directly determined by climate.

Climate often influences species distributions via:
food production
water supply
habitat
incidence of parasites, pathogens and competitors

Tiger Beetle of Cold Climates
Tiger beetle (Cicindela longilabris) - higher latitudes + elevations than other NA species

Schultz found metabolic rates of C. longilabris are higher and preferred temps. lower than other species
Physical env. limits species distributions

Figure 9.3: metabolic rates of C. longilabris higher; preferred temps lower than other beetle species

Distributions of Plants Along a Moisture-Temperature Gradient
Encelia spp. distributions + variations in temp and precipitation:

Fig. 9.5

Distributions of Barnacles - Intertidal Gradient

Organisms in intertidal zone have evolved different degrees of resistance to drying
Barnacles - distinctive patterns of zonation within intertidal zone

Connell found pattern in barnacles:
Chthamalus stellatus restricted to upper levels; Balanus balanoides limited to middle and lower levels

Distributions of Barnacles Along an Intertidal Gradient
Balanus - more vulnerable to desiccation, excluded from upper intertidal zone
Chthamalus adults excluded from lower areas by competition with Balanus
Competition? How do we know that Balanus outcompetes Chthamalus?

Figures 9.8 and 9.9

Distribution of Individuals on Small Scales
Three basic patterns:
Random: equal chance of being anywhere
Regular: uniformly spaced
Exclusive use of areas
Individuals avoid one another
Clumped: unequal chance of being anywhere

Mutual attraction between individuals
Patchy resource distribution

Figure 9.10

Importance of scale in determining distribution patterns:
At one scale pattern may be random, at another scale, might be uniform:

Distribution of Tropical Bee Colonies
Hubbell and Johnson predicted aggressive bee colonies have regular distributions;
Predicted non-aggressive species have random or clumped distributions

Hubbell and Johnson results:
4 species with regular distributions were highly aggressive
Fifth non-aggressive and randomly distributedWhat causes overall pattern? Figure 9.11
Behavior!
Aggressive bees were uniformly spaced due largely to their interactions.
Non-aggressive species were random - did not interact.

Distributions of Desert Shrubs
Traditional theory suggests desert shrubs are regularly spaced due to competition
Phillips and MacMahon - distribution of desert shrubs changes from clumped to regular patterns as they grow

Hypothesis:
Young shrubs clumped for (3) reasons:
Seeds germinate at safe sites
Seeds not dispersed from parent areas
Asexual reproduction

Distributions of Desert Shrubs
Phillips and MacMahon proposed as plants grow, some individuals in clumps die = reducing clumping
Competition among remaining plants produces higher mortality
Eventually creates regular distributions: Fig 9.13

Brisson and Reynolds
Dug up roots, map distribution of 32 bushes
found competitive interactions with neighboring shrubs influences distribution of creosote roots
So what?
Creosote bush roots do not overlap with nearby plant roots
Only 4% overlap between bushes: Fig. 9.14

Distributions of Individuals on Large Scales
Bird Pops North America
Root - at continental scale, bird pops have clumped distributions (Christmas Bird Counts)
Clumped patterns in species with widespread distributions

Similar distribution pattern for species with small range: few “hot spots” Fish crow
Brown et al. (1995)
Relatively few study sites gave most records for each bird species in Breeding Bird Survey
Density = number individuals per unit area/volume

Sedentary organisms: plot approach
Moving/secretive organisms: mark/recapture
Relative abundance = percent cover, CPUE

Estimating density
Sedentary animals and plants
Plot methods
Area of known size
Randomly located plots
Count individuals in plots
Average / plot
Density = average no. / plot area

Example: maple trees
20 randomly located plots, 10 x 10 m squares (area = 100 m2)
Average sugar maple stems per plot = 4.5
Unit area for trees = hectare (10,000 m2)
Density = 4.5 maples per plot / 0.01 hectare plots = 450 maples / ha

Example: zooplankters
35 lake water samples, 50 ml each
Average copepods per sample = 78
Unit volume for zooplankton = liters
Sample volume = 0.05 l
Density = 78 copepods per sample / 0.05 l samples
= 1560 copepods / l

Organism Size and Population Density
Population density declines with increasing organism size
Why?
Bigger organisms need more space and resources
Bigger organisms have lower repro rates

Damuth (1981)
Pop density of 307 spp. of herbivorous mammals decreased with increased body size: Fig. 9.19

Peters and Wassenberg (1983)
Aquatic invertebrates had higher pop densities than terrestrial invertebrates of similar size;
mammals have higher pop densities than birds of similar size: Fig. 9.20

Plant Size and Population Density
Plant population density decreases with increasing plant size
Underlying details different from animals
White (1985)
Tree seedlings can live at high densities, but as trees grow, density declines until mature trees are at low densities

Rarity and Extinction
Rabinowitz - 7 forms of rarity
commonness classification based on (3) factors:
Geographic Range of Species
Habitat Tolerance
Local Population Size

Rarity
Non-rare populations have large geographic ranges, broad habitat tolerances, some large local populations
All seven other other combinations create some kind of rarity
= risk of extinction
Rarity
Rarity I
Large Range: Broad Habitat Tolerance: Small Local Pops
Peregrine Falcons
Rarity II
Large Range: Narrow Habitat Tolerance: Small Local Pops
Passenger Pigeons
Rarity
Rarity III
Small Range: Narrow Habitat Tolerance: Small Pops
Mountain Gorilla

Example: NA suckers
White sucker - large range
Broad habitat requirements
Large body size

Yacqui sucker - small range
Narrow habitat requirements
Small body size

Summary
Physical environment limits geographic distribution of species
On small scales, individuals w/in pops. are distributed in random, regular, or clumped patterns; on larger scales, individuals w/in pop. are clumped
Population density declines with increasing body size
Rarity influenced by geographic range, habitat tolerance, pop size; rare species vulnerable to extinction