(Chapter 40 Study Notes)

Population Ecology and the Distribution of Organisms

Introduction to Population Ecology

Population ecology is the study of populations in relation to their environment, including environmental influences on population density and distribution, age structure, and variations in population size. Understanding these dynamics is essential for predicting changes in population size and for managing species and ecosystems.

Scope of Ecological Research

Levels of Ecological Study

Ecology can be studied at multiple levels, each focusing on different aspects of the interactions between organisms and their environment.

• Global Ecology: Examines the influence of energy and materials on organisms across the biosphere.

• Landscape Ecology: Focuses on the exchanges of energy, materials, and organisms across multiple ecosystems.

• Ecosystem Ecology: Studies energy flow and chemical cycling among the various biotic and abiotic components.

• Community Ecology: Investigates interactions between species in a community.

• Population Ecology: Analyzes factors that affect population size and how and why it changes over time.

• Organismal Ecology: Explores how an organism's structure, physiology, and behavior meet environmental challenges.

Earth's Climate and Biomes

Global Climate Patterns

Climate is the long-term prevailing weather conditions in a given area. It is influenced by solar energy and the planet's movement in space.

• Latitudinal Variation: Sunlight intensity varies with latitude, affecting temperature and climate zones.

• Global Air Circulation: Warm, moist air rises near the equator, cools, and releases precipitation, creating wet tropics and dry deserts at 30° latitude.

• Seasonal Variation: The tilt of Earth's axis causes seasonal changes in sunlight intensity and day length.

Ocean Currents and Climate

• Surface water circulation in oceans (e.g., Gulf Stream, California Current) redistributes heat and affects coastal climates.

Influence of Water Bodies and Mountains

• Large bodies of water moderate climate by absorbing and releasing heat.

• Mountains affect climate by causing air to rise and cool, leading to precipitation on windward sides and dry conditions on leeward sides.

• Lake Effect Snow: Occurs when cold air moves over warmer lake water, picking up moisture and depositing it as snow on the downwind shore.

Terrestrial and Aquatic Biomes

• Terrestrial Biomes: Major life zones characterized by vegetation type (e.g., tropical forest, desert, tundra).

• Climograph: A plot of the annual mean temperature and precipitation in a region, used to compare biomes.

• Aquatic Biomes: Characterized by physical and chemical environment, such as lakes, rivers, and oceans.

• Zonation in Lakes: Includes littoral, limnetic, photic, aphotic, benthic, and pelagic zones, each with distinct communities.

Factors Affecting Species Distribution

Limiting Factors

The geographic distribution of species is limited by a combination of biotic and abiotic factors.

• Dispersal: The movement of individuals or gametes away from their area of origin can limit distribution.

• Biotic Factors: Interactions with other species, such as predation, competition, parasitism, and disease.

• Abiotic Factors: Non-living factors such as temperature, water, oxygen, salinity, sunlight, soil nutrients, and pH.

Example: Removal of sea urchins increases seaweed cover, demonstrating the effect of biotic interactions on distribution.

Population Dynamics

Population Size and Density

• Population Size: The number of individuals in a population.

• Population Density: The number of individuals per unit area or volume.

• Population size changes due to births, deaths, immigration, and emigration.

Equation for Population Change:

$\text{Change in population size} = \text{Births} + \text{Immigrants entering population} - \text{Deaths} - \text{Emigrants leaving population}$

Patterns of Dispersion

• Clumped: Individuals aggregate in patches, often where resources are abundant.

• Uniform: Individuals are evenly spaced, often due to territoriality or competition.

• Random: Position of each individual is independent of others, often where resources are consistent.

Life Tables and Survivorship Curves

Life tables summarize the survival and reproductive rates of individuals in specific age groups within a population.

• Population Growth Rate: Determined by birth rates, death rates, immigration, and emigration.

Example: The logistic growth curve demonstrates how populations grow rapidly at first and then level off as they approach carrying capacity.

Age (years)	Number Alive at Start of Year	Proportion Alive at Start of Year	Death Rate	Average Number of Female Offspring per Female
0-1	653	1.000	0.614	0.07
1-2	252	0.386	0.322	0.80
2-3	171	0.262	0.187	1.13
3-4	139	0.213	0.417	2.09
4-5	81	0.124	0.568	1.99
5-6	35	0.054	0.600	1.49
6-7	14	0.021	0.786	1.18
7-8	3	0.005	1.000	0.00

Survivorship Curves: Graphs that show the proportion of individuals surviving at each age. There are three idealized types:

• Type I: Low death rates during early and middle life, then an increase among older age groups (e.g., humans).

• Type II: Constant death rate over the organism's life span (e.g., squirrels).

• Type III: High death rates for the young, but lower death rates for survivors (e.g., oysters).

Population Growth Models

Exponential Growth Model

Describes population growth in an ideal, unlimited environment. The rate of increase is constant and does not slow as the population grows.

$\frac{dN}{dt} = rN$

• r: Intrinsic rate of increase

• N: Population size

Example: African elephant populations in Kruger National Park showed exponential growth when not limited by resources or predation.

Logistic Growth Model

Describes population growth that is slowed by limiting factors as the population size increases and approaches carrying capacity (K).

$\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$

• Population growth rate decreases as N approaches K.

• Populations may overshoot K before stabilizing.

Example: Paramecium and Daphnia populations in laboratory cultures demonstrate logistic growth, with Daphnia sometimes overshooting K.

Life History and Population Regulation

Life History Traits

• Traits that affect an organism's schedule of reproduction and survival, such as age at first reproduction, frequency of reproduction, and number of offspring produced.

• Trade-offs exist between reproduction and survival (e.g., caring for more offspring may reduce parental survival).

Example: In kestrels, increased brood size reduces parental survival to the next year.

Population Regulation

Example: Predator-prey interactions, such as moose and wolf populations on Isle Royale, show fluctuations due to density-dependent regulation.

Summary Table: Key Population Ecology Concepts

Concept	Definition	Example
Population	Group of individuals of the same species in an area	All elephants in Kruger National Park
Carrying Capacity (K)	Maximum population size an environment can sustain	Lab culture of Paramecium
Exponential Growth	Growth without limits	Early elephant population growth
Logistic Growth	Growth with limits, levels off at K	Paramecium population in lab
Density-Dependent Regulation	Regulation affected by population density/Factors whose effects increase as population density increases (e.g., competition, predation, disease).	Predation, competition
Density-Independent Regulation	Regulation not affected by density/Factors that affect populations regardless of density (e.g., weather, natural disasters).	Weather events

Additional info: These notes integrate and expand upon the provided lecture slides, including definitions, equations, and examples for a comprehensive overview of population ecology and the distribution of organisms.