1. Population Distribution and Abundance
Chapter 9
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2. Outline
Distribution Limits
Distribution Patterns
Organism Size and Population Density
Commonness and Rarity

3. Introduction
Ecologists usually define a population as a group of individuals of a single species inhabiting a specific area.
Characterized by the number of individuals and their density.
Additional characteristics of a population include age distributions, growth rates, distribution, and abundance.

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

5. Niches
Niche: Summarizes environmental factors that influence growth, survival, and reproduction of a species.
Grinnell’s definition focused on the effects of the physical environment
Elton’s definition included biotic and abiotic factors

6. Niches
Hutchinson defined niche as:
n-dimensional hyper-volume
n equates the number of environmental factors important to survival and reproduction of a species.
Fundamental niche - hypervolume
Realized niche includes interactions such as competition that may restrict environments where a species may live.

7. Kangaroo Distributions and Climate
Caughley found a close relationship between climate and distribution of the three largest kangaroos in Australia.
Macropus giganteus - Eastern Grey
Eastern 1/3 of continent.
Macropus fuliginosus - Western Grey
Southern and western regions.
Macropus rufus - Red
Arid / semiarid interior.

8. Kangaroo Distributions and Climate

9. 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.

10. Tiger Beetle of Cold Climates
Tiger Beetle (Cicindela longilabris) lives at higher latitudes and elevations than most other species in NA.
Schultz et. al. found metabolic rates of C. longilabris are higher and preferred temperatures lower than most other species.
Supports generalization that the physical environment limits species distributions.

11. Tiger Beetle of Cold Climates

12. Distributions of Plants Along a Moisture-Temperature Gradient
Encelia species distributions correspond to variations in temperature and precipitation.

13. Distributions of Barnacles Along an Intertidal Exposure Gradient
Organisms living in an intertidal zone have evolved to different degrees of resistance to drying.
Barnacles show distinctive patterns of zonation within intertidal zone.
Connell found Chthamalus stellatus restricted to upper levels while Balanus balanoides is limited to middle and lower levels.

14. Distributions of Barnacles Along an Intertidal Gradient
Balanus appears to be more vulnerable to desiccation, excluding it from the upper intertidal zone.
Chthamalus adults appear to be excluded from lower areas by competition with Balanus.

15. Distributions of Barnacles Along an Intertidal Gradient

16. Distribution of Individuals on Small Scales
Random: Equal chance of being anywhere.
Uniform distribution of resources.
Regular: Uniformly spaced.
Exclusive use of areas.
Individuals avoid one another.
Clumped: Unequal chance of being anywhere.
Mutual attraction between individuals.
Patchy resource distribution.

17. Distribution of Individuals on Small Scales

18. Distribution of Tropical Bee Colonies
Hubbell and Johnson predicted aggressive bee colonies would show regular distributions while non-aggressive species would show random or clumped distributions.
As predicted, four species with regular distributions were highly aggressive.
Fifth was non-aggressive and randomly distributed.
Prospective nest sites marked with pheromones.

19. Distributions of Desert Shrubs
Traditional theory suggests desert shrubs are regularly spaced due to competition.
Phillips and MacMahon found distribution of desert shrubs changes from clumped to regular patterns as they grow.
Young shrubs clumped for (3) reasons:
Seeds germinate at safe sites
Seeds not dispersed from parent areas
Asexual reproduction

20. 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.
Brisson and Reynolds found competitive interactions with neighboring shrubs appear to influence distribution of creosote roots, Larrea tridentata.

21. Distributions of Individuals on Large Scales
Bird Populations Across North America
Root found at continental scale, bird populations showed clumped distributions in Christmas Bird Counts.
Clumped patterns occur in species with widespread distributions.
Brown found a relatively small proportion of study sites yielded most of records for each bird species in Breeding Bird Survey.

22. Plant Distributions Along Moisture Gradients
Whittaker examined distributions of woody plants along moisture gradients in several North American mountain ranges.
Documented moisture gradient from moist canyon bottoms up to the dry southwest-facing slopes.
Tree species showed a highly clumped distribution along moisture gradients, with densities decreasing substantially toward the edges of their distribution.

23. Plant Distributions Along Moisture Gradients

24. Organism Size and Population Density
In general, population density declines with increasing organism size.
Damuth found the population density of herbivorous mammals decreased with increased body size.
Peters and Wassenberg found aquatic invertebrates tend to have higher population densities than terrestrial invertebrates of similar size.
Mammals tend to have higher population densities than birds of similar size.

25. Organism Size and Population Density

26. Plant Size and Population Density
Plant population density decreases with increasing plant size.
Underlying details are very different.
Tree seedlings can live at very high densities, but as the trees grow, density declines progressively until mature trees are at low densities.

27. Commonness and Rarity
Rabinowitz devised commonness classification based on (3) factors:
Geographic Range of Species
Habitat Tolerance
Local Population Size
Populations that are least threatened by extinction, have extensive geographic ranges, broad habitat tolerances, and some large local populations.
All seven other combinations create some kind of rarity.

28. Rarity
Rarity I
Extensive Range, Broad Habitat Tolerance, Small Local Populations
Peregrine Falcon
Rarity II
Extensive Rage, Large Populations, Narrow Habitat Tolerance
Passenger Pigeon

29. Rarity
Rarity III
Restricted Range, Narrow Habitat Tolerance, Small Populations
California Condor

30. Review
Distribution Limits
Distribution Patterns
Organism Size and Population Density
Commonness and Rarity