Population: Regulation

Transcription

1 Population: Regulation MODULE 11: POPULATION: REGULATION UNIT 2: INDIVIDUALS AND POPULATIONS Objectives At the end of this series of lectures you should be able to: Define terms. Distinguish between different types of density Discuss population fluctuation and cycles and their potential causes. Describe metapopulations and processes that influence their dynamics and persistence. Explain the extinction threats that face small populations. Discuss dispersal of organisms 1

2 Density Crude density is the number of individuals per unit area. Ecological density is the density within the habitat space actually inhabited or utilized by the species. Patterns of Population Growth Exponential Growth Rapid increase in population size Conditions are favorable Introduced to a new region Few competitors Few predators or disease Predators or competitors are removed. 2

3 Patterns of Population Growth Logistic Growth Population size maintains relatively stable Population fluctuations Variation in carrying capacity Abiotic conditions change Effects cascade thourgh the biotic portion of the ecosystem Biotic community changes Relatively abundance New species colonizations or introductions Extinctions Limits to Population Growth Biotic factors Predators Food supply Competitors Parasites Pathogens Mutualists 3

4 Limits to Population Growth Biotic factors can act in a topdown or bottom-up fashion. Refer to the direction of regulation through the food chain. Bottom-up Regulation Act on the population via limitations imposed by lower trophic levels. Example: El Niño-driven cycles of the population abundance of Galápagos penguins During an El Niño, warm water from the western Pacific flows eastward toward South America and the Galápagos. The cold, nutrient-rich water is replaced with warmer, less nutrientrich water. The marine food base collapses and the penguin population declines radically 4

5 Top-down Regulation Cyclic dynamics of the red grouse (Lagopus lagopus) populations in Great Britain. Caused by the density-dependent effects of a parasitic nematode. When the parasite burden was experimentally reduced, the grouse population did not crash. Line colors: No treatment 5% treated 10% treated 20% treated Population Fluctuations Density independent Weather Density dependent Genetics Stress Disease Limited resources (Food, shelter, etc.) 5

6 Density-Dependent Factors Act as negative feedback systems on populations Density-Dependent Factors Can be identified by the correlation between mortality or reproduction and population density. The correlation often involves time lag. 6

7 Density-Independent Factors Their effects do not change with population size. Abiotic factors (e.g., weather, fire, floods). Often unpredictable. Density-Dependent vs. Density-Independent Population regulation dn/dt = rn(k-n)/k Densitydependent Densityindependent Represent disturbance. May prevent the population from reaching the equilibrium (K). 7

8 Density-Dependent and Density- Independent Factors Interact Example: Populations of the desert bighorn sheep (Ovis canadensis) Amount of the rainfall determines forage amount Variability in the rainfall determines the degree of densitydependence Density-dependent factors (competition) dominate in the areas of less variability in the rainfall Population Cycles Some populations show regular fluctuations at regular intervals of time. Hormonal Behavioral Food supplies but why do they cycle? Predator prey cycles Sunspot cycle 8

9 Interactions Among Factors Isle Royale Orange wolves; blue moose. 9

10 Population Extinction Different uses of extinction Deterministic vs. stochastic factors Fluctuations in λ or r increases extinction risk Small population size increases extinction risk 10

11 Small Populations Small populations face a greater threat from extinction than do larger populations. Demographic stochasticity Allee effect Genetic stochasticity Inbreeding Environmental stochasticity Catastrophes 11

12 Metapopulations A group of isolated populations linked by immigration. Characterized by repeated extinctions and colonizations. If a population does become extinct, it could be recolonized by immigrants from another population Stability of the metapopulation is greater than the stability of the populations alone. Stability of metapopulations are threatened if immigration is limited. Fragmentation 12

13 Dispersal Movement of individuals Immigration Emigration Migration Colonization Jump dispersal Diffusion Secular migration Distinguish between dispersal and dispersion 13

14 Mechanisms of Movement Active dispersal Vagility Passive dispersal Pagility What types of adaptations would favor each type of dispersal? A Conceptual Model 14