What are the limitations that keep population sizes from continuing to grow exponentially?

1. What are the limitations that keep population sizes from continuing to grow exponentially?

2. Factors that limit population growth: • Some kind of environmental disaster (that kills individuals outright). • Shortage of essential resources (starvation, desiccation). • Elevated levels of poisons or stress-inducing factors in the environment. • Interactions with other species (disease, predators, interspecific competitors)

3. Environmental “disasters”: Mount St Helen before 1980 Mount St Helen after the volcanic eruption in 1980

4. Yellowstone Fire, 1988

6. Two general classes of limitations: Limitations that are independent of population size. Limitations that depend on population size. density-independent limitations density-dependent limitations

7. Density-dependent limitations on growth are often linked to overcrowding: • 1) Shortage of essential resources per individual can lower birth rates or increase death rates (or both): • the amount of available energy source (food, light) • the amount of essential nutrients in the food or environment • water • space (territory size or quality, nesting sites, predator-free space) • 2) Elevated levels of poisons or stress-inducing factors in the environment can lower birth rates or increase death rates (or both): • toxic waste produced by individuals in a population • growth-inhibitors exuded by individuals in a population • hostile interactions between individuals in a population 3) Interactions with other species can can lower birth rates or increase death rates (or both): • essential resources consumed by other species • harm done by other species (being eaten, injured or infected)

8. Poppy Fraction of sown seeds producing a mature plant Density of seeds sown (no per m2) An example of nutrients limiting population growth (self-thinning in plants): (Data from Harper and McNaughton 1962)

9. Bynoe’s Gecko Pied flycatcher Desert clicker Examples of territory ownership limiting reproduction:

10. An example where toxins limit population growth: Ethyl alcohol is the waste product of fermentation. Above 15% alcohol by volume, yeasts cannot ferment.

11. Some plants produce auto-toxins: allelopathy: Allelopathy is the exudation of substances by adult plants that suppress the seed germination or seedling growth. (After Wilson and Rice 1968)

12. An example where stress of crowding limits population growth: Crowding alone, in mice and rats, can cause mortality rates to increase and birth rates to decrease, through stress-related abnormal behavior.

13. An example where many species compete for the same resource: space (barnacles and limpets in the intertidal zone). Owl limpet

14. An example where toxins limit the growth of another species: Bacteria growing on agar Bacteria-free space Penicillum colonies.

15. An example where predators check the population sizes of their prey:

16. Examples of conditions that favor disease propagation:

17. So there is checked and unchecked growth. There are density-dependent and -independent checks on population size. Imagine a population of ducks at a lake. How would you determine if the population is a) checked or unchecked, b) whether the population is checked by density-dependent or -independent mechanisms?

18. Sagebrush Lizard in Zion National Park: (After Tinkle et al. 1993)

19. Sagebrush Lizard in Zion National Park: (After Tinkle et al. 1993)

20. Sagebrush Lizard in Zion National Park:

21. Yearlings were larger at low population density. • Larger lizards laid more eggs. • Yearlings were larger in high rainfall years. • Food limitations probably were limiting hatchling growth, exposing them to greater risk of being eaten, and reducing female fertility. (After Tinkle et al. 1993)

22. Annual growth rate (1986-1987): • - 3% • +20% Wild donkeys in Australia: site 1: 3.3 animals/km2 site 2: 1.5 animals/km2 (After Choquenot 1991)

23. 6-month old donkeys were 9% larger at low density. • Animals had a 60% higher kidney fat index at low density. • Male donkeys reached sexual maturity earlier at low density. Food limitations (grasses) probably were limiting juvenile growth and development. Juveniles probably died from starvation or consequences of malnutrition.

24. Pond snails in Michigan ponds: (After Eisenberg 1966)

25. Eisenberg’s manipulative experiment: • 12 identical snail-proof cages set up along the margins of a • pond in spring. • Addition of adult snails in four cages (5x). • Reduction of snails numbers in four cages (1/5th ). • No change in four cages (natural density, control). (After Eisenberg 1966)

26. Eisenberg’s manipulative experiment: Differences in the numbers of young snails or eggs were non-significant between treatments (After Eisenberg 1966)

27. Food limited egg production: egg production was directly proportional to the amount of spinach fed per adult.

28. Do birth or death rates limit growth at high density? A greater proportion of hatchlings died when lizard density was high. Surviving lizards were also smaller and smaller lizards lay fewer eggs. Sagebrush lizard Mares had the same pregnancy rate at high or low density. Juveniles in the high density population were smaller and had a 3x higher chance to die. Wild donkeys Adults lay fewer eggs when adult snail density was high. The same proportion of adults and juveniles died, independent of adult density. Pond snails

29. Summary: • Many factors can change population birth and death rates, thus population size. • Effects on population growth are classified as either density-dependent or independent. • Density-independent effects = can happen any time, independent of population size. • Density-dependent effects = are correlated with population size. • Next time we see that density-dependent effects are responsible for creating stable population sizes.