Population Distribution and Abundance

1. Population Distribution and Abundance Chapter 9

2. 1. 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 (number per space = # / area. • Additional characteristics of a population include age distributions, growth rates, distribution (location in space), and abundance (number of individuals).

3. 2. What factors in the physical environment limit Distribution? • Physical environment limits geographic distribution of a species. Organisms can only compensate so much for environmental variation. Climate – temperature, moisture, light, soil fertility (N, K, P, minerals) primary abiotic limits.

4. 3. Kangaroo Distributions and Climate • Caughley - climate and kangaroo distribution Macropus giganteus - Eastern Grey • Eastern 1/3 • Macropus fuliginosus - Western Grey • Southern and western • Macropus rufus - Red • Arid / semiarid interior.

5. Kangaroo Distributions and Climate

6. 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.

7. 4. 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.

8. Tiger Beetle of Cold Climates

9. 5. Plants affected by elevation and climate Plant species vary along an elevation gradient (Merriam’s theorem). Moisture, temperature limits

10. 5. Distributions of Plants along a Moisture-Temperature Gradient • Encelia species distributions correspond to variations in temperature and precipitation.

11. 5. Plant Abundance along Moisture Gradients • Whittaker - woody plants N. American Mts. • moisture gradient from moist canyon bottoms up to the dry southwest-facing slopes. • Tree species showed a highly clumped distribution w/ moisture gradients, density decreasing at edges.

12. 5. Plant Abundance Along Moisture Gradients

13. 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.

14. Distributions of Barnacles along an Intertidal Gradient

15. 7. Types of spacing of individuals (small scale) • Random:uneven distribution of resources. No pattern • Uniform (regular): Exclusive use of areas, Individuals avoid each other. • Clumped: Unequal chance of being anywhere. • Mutual attraction between individuals. • Patchy resource distribution.

16. 8. Factors influencing small scale distribution • Food • Competition • Nesting spaces • Plant distribution • Parasite load • Predation – safety in numbers

17. How are life history and small scale spacing related? • Hubbell and Johnson predicted aggressive bee colonies would show regular distributions while non-aggressive species would show random or clumped distributions. • four species (uniform) were highly aggressive. • Fifth non-aggressive and randomly distributed. • Prospective nest sites marked with pheromones.

18. 10. Why do Desert Shrubs show clumped distribution patterns? • 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

19. Desert shrub Distribution

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. 11. 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. 12. Metapopulations • A metapopulation is made up of a group of subpopulations living on patches of habitat connected by an exchange of individuals. • Alpine Butterfly - Roland et.al. • Lesser Kestrels - Serrano and Tella.

23. 13. Organism Size and Population Density • density declines with increasing organism size. • Damuth - density of herbivorous mammals decreased with increased body size. • Peters and Wassenberg - aquatic invertebrates have higher population densities than terrestrial invertebrates of similar size. • Mammals tend to have higher population densities than birds of similar size.

24. Organism Size and Population Density As body size increases: pop density decreases Aquatic invertebrates have higher density than others of same size Mammals tend to have higher density than birds (food availability).

25. 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.

26. 14. What factors contribute to 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.

27. 15. Types of Rarity • Rarity I • Extensive Range, Broad Habitat Tolerance, Small Local Populations • Peregrine Falcon • Rarity II • Extensive Rage, Large Populations, Narrow Habitat Tolerance • Passenger Pigeon • Rarity III • Restricted Range, Narrow Habitat Tolerance, Small Populations • California Condor