Population Ecology Explained: Insights into Species Interactions and Growth

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

o     Explain the terms, population density, population growth, immigration, emigration, birth rate and death rate.
o     Outline the population sampling techniques
o     Identify the factors which affect population size.
o     Distinguish between density dependent and density independent factors that affect population size.
o     Explain the concept of biological pest control.
o     Outline the advantages and disadvantages of biological pest control.
o     Explain the concept of ecological succession.
o     Explain how organisms in aquatic and terrestrial habitats are adapted to their habitat

POPULATION DYNAMICS

Population dynamics is the study of short-term and long-term changes in the size and age composition of populations, and the factors influencing these changes. It deals with the way populations are affected by birth and death rates, and by immigration and emigration.

Characteristics of Population Ecology

Population ecology is the study of how populations of organisms interact with their environment, focusing on factors that influence population size, structure, growth, and dynamics. Key characteristics of population ecology include population density, distribution, demographics, growth rate, and carrying capacity, each of which contributes to understanding how populations thrive, decline, or remain stable within ecosystems.

1. Population Size

Population size is the total number of individuals in a particular species within a defined area at a given time. It’s a fundamental measure in ecology, as it can indicate the health of a species, potential for reproduction, and vulnerability to extinction.

Examples: Large populations can withstand more environmental fluctuations, while small populations are more prone to extinction due to disease or resource scarcity.

2. Population Density

Population density measures the number of individuals per unit area or volume. It provides insight into the interactions among individuals and their environment.

High Density: Often leads to competition for resources, social interaction, and an increase in disease transmission.
Low Density: May result in reduced mating opportunities but less competition for resources.

3. Population Distribution (or Dispersion)

Distribution refers to the spatial arrangement of individuals within a population. There are three main types:

Clumped Distribution: Individuals are clustered together in patches, often around resources (e.g., water or food).
Uniform Distribution: Individuals are evenly spaced, often due to territorial behavior or competition.
Random Distribution: Individuals are spaced unpredictably, usually when resources are abundant and evenly distributed.

4. Age Structure

Age structure describes the proportion of individuals in different age categories within a population. It’s critical for predicting future population growth trends.

Young Population: Populations with a larger proportion of young individuals often grow quickly.
Older Population: Populations with a higher proportion of older individuals may experience slower growth or decline.

5. Sex Ratio

The sex ratio is the proportion of males to females in a population. This ratio can influence reproductive rates and thus the overall growth rate of the population.

Balanced Ratio: Often seen in stable populations with equal birth and survival rates for both sexes.
Skewed Ratio: Can occur due to environmental pressures or social structure, affecting reproduction.

6. Population Growth Rate

Population growth rate is determined by birth rates, death rates, immigration, and emigration within a population. Growth patterns are typically categorized as either exponential or logistic.

Exponential Growth: Rapid, unrestricted growth under ideal conditions, represented by a J-shaped curve.
Logistic Growth: Growth that slows as the population nears carrying capacity, forming an S-shaped curve.

7. Carrying Capacity (K)

Carrying capacity is the maximum population size that the environment can sustainably support based on available resources like food, water, and shelter. When populations exceed carrying capacity, resources become limited, leading to increased competition and potential decline.

Dynamic Nature: Carrying capacity can change due to environmental factors, such as climate, resource availability, or human impact.

8. Limiting Factors

Limiting factors are environmental constraints that regulate population growth. They can be either density-dependent or density-independent:

Density-Dependent Factors: Increase in intensity as population density rises, such as competition, predation, disease, and parasitism.
Density-Independent Factors: Affect populations regardless of density, including natural disasters (e.g., floods, fires) and climate extremes.

9. Survivorship Curves

Survivorship curves show the rate of survival of individuals over time and are categorized into three types:

Type I: High survival rates in early and middle life with a decline in later years (e.g., humans).
Type II: Consistent survival rate throughout life (e.g., birds).
Type III: Low survival rate early in life, with higher survival for those that reach maturity (e.g., many plants, fish).

10. Life History Strategies

Life history strategies refer to how organisms allocate resources for growth, reproduction, and survival. These strategies are typically classified into r-selected and K-selected species:

r-Selected Species: Characterized by high reproductive rates, minimal parental care, and shorter lifespans (e.g., insects, many fish).
K-Selected Species: Have lower reproductive rates, extensive parental care, and longer lifespans (e.g., mammals like elephants).

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Population Growth

Population Growth is the change in a population over time, or the change in the number of individuals in a population per unit time. Population growth is determined by four factors, births (B), deaths (D), immigrants (I), and emigrants (E).

The population growth of a period can be calculated in two parts, natural growth of population (B-D) and mechanical growth of population (I-E).

Growth can be both positive and negative i.e. growth can be increasing or decreasing. The factors which limit the growth of population are collectively called environmental resistance and can be grouped into abiotic and biotic factors.

Population Growth Rate

Population Growth Rate is the rate at which the number of individuals in a population increases in a given time period.

Factors Affecting Population Size

Several factors influence population size, determining whether a population grows, declines, or remains stable. These factors are often divided into biotic (living) and abiotic (non-living) components and can be either density-dependent or density-independent. Here’s a breakdown of the main factors affecting population size:

1. Birth Rate (Natality)

Birth rate, or natality, is the frequency of births within a population over a specific period. A high birth rate typically increases population size, assuming other factors like death rate and emigration remain stable.

High Birth Rate: Leads to population growth.
Low Birth Rate: May contribute to population decline, especially in long-lived species with slow reproduction rates.

2. Death Rate (Mortality)

The death rate is the frequency of deaths in a population over time. A high death rate can decrease population size, while a low death rate can contribute to growth, particularly when coupled with a high birth rate.

High Mortality Rate: Often reduces population size, especially if deaths occur among breeding individuals.
Low Mortality Rate: Allows population growth, as more individuals survive to reproduce.

3. Immigration

Immigration is the arrival of individuals from other areas into a population’s habitat. This can introduce new members into a population, increasing its size and genetic diversity.

Increased Immigration: Boosts population size, adds genetic variation, and can strengthen the population’s resilience to environmental changes.

4. Emigration

Emigration is the movement of individuals out of a population’s area, often leading to a reduction in population size.

High Emigration Rates: Reduce population size and potentially limit genetic diversity.
Low Emigration Rates: Help maintain population stability or allow growth if birth rates are high.

5. Availability of Resources

The availability of resources, such as food, water, and shelter, directly influences population size. Resources act as limiting factors, as population growth typically requires a sufficient supply to sustain individuals.

Abundant Resources: Promote growth and can support larger population sizes.
Limited Resources: Can limit population growth, leading to increased competition and potentially reducing population size.

6. Predation

Predation occurs when one species preys on another, impacting population size by reducing the prey population. Predation pressure can lead to natural population control and affect birth and death rates within prey and predator populations.

High Predation Pressure: May reduce prey population size significantly, potentially affecting the predator population as well.
Low Predation Pressure: Allows prey populations to grow, which may eventually increase predator numbers as more food becomes available.

7. Disease and Parasitism

Diseases and parasites can significantly impact population size, especially in high-density populations where infections spread quickly.

Disease Outbreaks: Can reduce population size by increasing mortality, especially if the disease is highly contagious and deadly.
Parasitism: Weakens individuals, potentially reducing reproductive success and survival rates, leading to lower population growth.

8. Competition

Competition for limited resources occurs both within a population (intraspecific) and between populations (interspecific). This competition affects population growth, especially when resources are scarce.

Intraspecific Competition: Individuals of the same species compete for food, mates, or territory, influencing birth rates, death rates, and population density.
Interspecific Competition: Different species compete for shared resources, potentially limiting growth for one or both populations.

9. Environmental Conditions

Environmental factors such as climate, weather, and natural disasters are powerful density-independent factors that can impact population size.

Climate and Weather: Extreme temperatures, droughts, and storms can reduce food and water availability, directly affecting survival.
Natural Disasters: Events like floods, wildfires, or earthquakes can lead to sudden, sharp declines in population size by destroying habitats and killing individuals.

10. Carrying Capacity (K)

Carrying capacity is the maximum population size an environment can sustainably support based on available resources and environmental conditions. Populations tend to grow until they reach their carrying capacity, at which point growth levels off or declines.

Exceeding Carrying Capacity: Leads to overpopulation, resource depletion, and eventually a decline in population size as individuals struggle to survive.
Stabilization at Carrying Capacity: Population size fluctuates around a balance where birth and death rates equalize, maintaining a stable population.

11. Human Activity

Human actions, such as habitat destruction, pollution, hunting, and resource exploitation, can significantly influence population sizes, especially for wildlife.

Habitat Destruction: Reduces available living space, forcing populations to decrease or move.
Pollution and Climate Change: Can introduce toxins, alter habitats, and lead to shifting population distributions or declines.
Hunting and Overexploitation: Directly decrease population sizes of targeted species, sometimes to the point of endangerment or extinction.

12. Intrinsic Factors (Life History Traits)

Intrinsic factors such as reproductive rates, lifespan, and survival strategies also impact population size. These traits are built into a species' biology and determine its natural growth potential.

Reproductive Strategies: r-selected species, which reproduce quickly and produce many offspring, tend to have rapid population growth, while K-selected species have slower growth due to longer lifespans and fewer offspring.
Survival Strategies: Species with high parental care or long lifespans tend to maintain stable populations with slower growth rates.

13. Genetic Diversity

Genetic diversity within a population affects its ability to adapt to environmental changes and resist diseases. Low genetic diversity makes a population more susceptible to extinction from disease, environmental changes, or inbreeding issues.

High Genetic Diversity: Increases adaptability and resilience, supporting population growth over time.
Low Genetic Diversity: Leads to inbreeding, increased vulnerability to diseases, and limited ability to adapt to changes.

14. Allee Effect

The Allee effect occurs when a population’s growth rate decreases as the population density decreases, often due to challenges in finding mates or achieving effective group behavior.

Positive Allee Effect: Populations grow better at higher densities, benefiting from social behaviors like cooperative hunting or communal defense.
Negative Allee Effect: Low-density populations may struggle to reproduce or survive, especially if individuals are isolated, leading to potential population declines.

15. Social Structure and Behavior

Social dynamics within a population, such as hierarchy, group cooperation, and competition, can influence reproduction rates, access to resources, and overall survival.

Cooperative Behavior: Groups that work together (e.g., pack animals like wolves) can improve hunting efficiency and defense, boosting survival.
Dominance Hierarchies: May limit reproductive opportunities for certain individuals, thus controlling population growth.

16. Migration Patterns and Seasonal Movements

Seasonal migrations and other movement patterns affect population size and distribution by relocating individuals temporarily or permanently. This is especially true for migratory species like birds, fish, and some mammals.

Seasonal Migration: Helps populations access resources during different times of the year, supporting population stability and growth.
Permanent Migration: If a population permanently leaves an area, it reduces local population size but may boost size in new regions.

19. Biotic Interactions (Mutualism, Commensalism)

Relationships with other species, such as mutualistic or commensalistic interactions, can positively influence population size. These interactions may provide food, shelter, or other benefits that help populations thrive.

Mutualism: Both species benefit, enhancing growth (e.g., bees and flowering plants).
Commensalism: One species benefits without affecting the other (e.g., barnacles on whales), indirectly supporting population survival and growth.

21. Trophic Structure and Food Web Dynamics

The trophic structure within an ecosystem—how organisms feed on each other—determines resource availability and competition levels. Predators, prey, and competitors in the food web can have significant impacts on population size.

Top-Down Control: Predators regulate prey populations, which in turn influences plant and resource availability.
Bottom-Up Control: Availability of primary producers (plants) influences herbivore populations and, subsequently, predator numbers.

22. Physiological Tolerance and Environmental Conditions

A population’s survival often depends on its physiological tolerance to environmental conditions such as temperature, salinity, humidity, and pH.

High Tolerance: Species with broader tolerances (e.g., generalists) can inhabit more environments, allowing for population stability.
Low Tolerance: Species with narrow tolerances (e.g., specialists) are more vulnerable to environmental changes, leading to potential population declines.

24. Climate Change and Habitat Alteration

Climate change alters habitats, impacting food availability, reproduction rates, migration patterns, and survival, often leading to population shifts or declines.

Temperature and Weather Extremes: Affect the availability of water, food, and shelter, altering population dynamics.
Sea-Level Rise: Leads to habitat loss for coastal and marine species, affecting population stability and forcing migration.

25. Human-Induced Selective Pressures (Anthropogenic Effects)

Human activities like hunting, fishing, and selective harvesting can impose selective pressures that impact population size, behavior, and genetic diversity.

Selective Harvesting: Overharvesting specific species (e.g., large fish) can alter population structure and reduce size.
Urbanization and Land Use: Fragment habitats, isolate populations, and decrease available resources, negatively impacting population size.

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Density-dependent and -independent factors

Density Dependent Factors

They are density dependent because these factors relate to the size or density of the population. These include predation, epidemic, disease, parasitism, competition, food availability, accumulation of toxic waste. These factors lower the population and eventually stabilize it at a particular point.

In some populations, the rate of increase in numbers, the population growth rate, decreases as the density increases. For example, in studying the dynamics of bacteria or yeast, the population is seen to increase very quickly at first and then begins to decrease. The initial period of rapid growth may be due to the supply of adequate growth conditions, such as suitable temperature, food materials and oxygen. As growth progresses, poisonous waste products of metabolism from the organisms build up in the growth medium and the supply of food oxygen decrease. This makes the environment less favorable for growth. The result is a decrease in the rate of population growth. This type of population is described as self-limiting: it is density dependent because the growth rate depends on the numbers present in the population. If a graph is plotted to represent how such populations change with time, a sigmoid or S-shape is obtained.

Density Independent Factors

These are factors which can affect dense or sparse population irrespective of the level of population. They include natural disaster such as floods, earthquake, fire, storms, and volcanic eruption.

Other population growth rates are density independent. Their growth curves appear J-shaped. There is an initial period of rapid increase in the population as a result of favorable growing conditions. This allows for a rapid initial growth until a peak value is obtained. Unless this rapid growth is checked by other factors in the environment, the increase continues indefinitely, until the food supply is exhausted. Beyond this point the rate of reproduction drastically decreases and the growth rate decreases. This type of population growth is termed density independent because the regulation of growth rate is not tied to the population density until the final crash.

Growth Curves

Populations have characteristic growth curves. Growth in a density-dependent population is described by a S-shaped or sigmoid curve. In a density-independent population, growth rate is described by a J-shaped or exponential growth curve.

Exponential growth curve

An exponential growth curve represents density- independent population growth as the regulation of growth rate is not tied to the population density until a final crash. Exponential growth cannot continue indefinitely because growth is restricted by environmental resistance such as food supply, accumulated waste products, increased competition and predation.

An exponential growth curve has two phases:

i. The lag phase during which growth is slow because the population is small and is becoming established.

ii. The exponential growth phase during which growth is accelerated as the population increases, until an environmental limit causes a population crash.

Common examples of populations exhibiting this type of growth are those that show 'boom and bust' cycles, such as algal blooms and some species of insect.

Sigmoid Growth Curve

The sigmoid growth curve describes the growth of a density-dependent population. The growth rate depends on the number of individuals present within the population. The population becomes stable when the carrying capacity of the environment is reached.

Carrying capacity is the number of individual that the environment can support.

The sigmoid curve has four phases:
(i)       The lag phase where population density initially shows a slow increase.
(ii)     The exponential growth phase where growth accelerates.
(iii)    The deceleration phase where growth slowest down due to competition between individuals for limited resources.
(IV)    The equilibrium phase where little or no growth takes place because the carrying capacity of the environment has been reached   and the number of births and deaths are approximately equal.

The population will only grow when its size is lower than the carrying capacity of the environment. This has practical implications, for example, in fish production. The fish population can be maintained as a continuous source of food if the population size is maintained below the carrying capacity at the exponential phase of growth, where replacement individuals is highest. If we overfish, the population will be reduced to the lag phase and take longer to recover. To limit the growth of a pest, it is always better to reduce the carrying capacity of the environment rather than reduce the population size, which only allows exponential growth to begin again.

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Methods of Determining Population Size or Density

1. Direct Counting:

a. Quadrat: A quadrat is a square wooden frame which may be divided into small squares by means of string or wire fastened across the frame. The size of the frame depends on the area under study and the size of the organism being sampled. The quadrat is normally carefully thrown at random within the habitat and where it lands, the area of ground enclosed carefully studied. This normally involves counting the number of each species of plant within the frame. The plants partially enclosed are counted as half. The random is repeated several times.

b. Line transect: This is a method of sampling plant distribution and it is mostly used with the quadrat. A string is laid down in a straight line across all or part of the habitat. Sampling of vegetation is then carried out by placing quadrats at regular intervals along the string.

c. Point frame: a point frame may also be used in conjunction with a quadrat to find percentage cover. This is a piece of wood with ten holes drilled in it and large pins put through the holes. The quadrat is marked off into ten equal strips and the point frame is put down in each strip in turn. Where a plant touches a pin, it is identified and recorded. This means that 100 points will be touched in the quadrat, so percentage cover is easily obtained by this method. If, for example, a species of grass is touched 20 times, its percentage cover is 20%. The total number of percentages recorded (including that of bare ground if no plant is touched by a pin) should add up to 100%. The mean (average) percentage cover of a plant species over a certain area can be obtained by throwing the quadrat several times and finding the percentage cover in each quadrat. The mean percentage cover for a particular plant species can then be calculated as the sum total of the percentage cover in each quadrat, divided by the number of quadrats sampled.

d. Butterfly net: Butterfly net may be used to catch flying insects such as butterflies,

e. Sweep net: Insects on the ground and also some land vertebrates can be trapped with a sweep net.

f. Pooter: It used in collecting small animals e.g. ants, found among leaf litter, on vegetation and in cracks and crevices.

Ecological instrument for population sampling

g. Light traps: Light traps use an ultraviolet or black light to attract insects, especially moths, where they are collected in a trap or attracted to a white sheet and selectively identified and removed.

h. Pitfall trap: This is a way of sampling the populations of small animals (usually invertebrates) in the habitat. A simple trap is just a hole in the ground with straight sides, or a large jar with smooth and slippery sides buried in the ground containing food as bait. Traps are placed randomly within the habitat and are examined regularly (at least once a day). Any organisms in the trap are collected, identified, counted and then released. Large trap may be used to sample small animals and lizards.

2. Capture-Mark-Release- Recapture Method

A number of animals of a certain species are captured either using a stout net, an empty bottomed box or any other suitable method. The animals captured are then marked with a non-poisonous dye or nail vanish and released into the same area of the habitat from which they were removed. Enough time is then allowed for the marked animals to mix randomly with the rest of the population in the habitat. A second sample is then taken in exactly the same way as the first sample and the number of marked animals recaptured is recorded.

methods of determine population size and population density

The value obtained is called the Lincoln index. This method works best with animals whose movements are limited by geographical barriers e.g. animals in a pond (aquatic)

N/B:

o Marking the animals does not impede the movements.
o Marking the animals does not increase the chances of their being detected by predators.

o It is also assumed that changes in the population due to death, birth, immigration and emigration are insignificant during the period of sampling.

Reasons Ecologists study populations

Ecologists study populations to understand how organisms interact with each other and their environments, which is essential for predicting changes in ecosystems, conserving biodiversity, managing resources, and addressing environmental challenges. Here are some key reasons ecologists focus on populations:

1. Understanding Ecosystem Dynamics

Population Interactions: Populations do not exist in isolation; they interact with each other and influence ecosystem functions. By studying populations, ecologists can see how species interactions (e.g., predation, competition, mutualism) affect ecosystem health.
Energy Flow and Nutrient Cycling: Populations are a fundamental part of energy transfer through food chains and contribute to nutrient cycling. Tracking these dynamics is essential for understanding how energy flows through ecosystems and how resources are cycled.

2. Predicting Population Trends and Environmental Changes

Climate Change Impact: Studying populations helps ecologists predict how species may respond to changes in temperature, precipitation, and other environmental shifts due to climate change. For example, they can determine which species are likely to thrive, migrate, or decline as habitats transform.
Population Modeling: Ecologists use data from population studies to create models that predict population growth, decline, or stability. These models help forecast future population sizes and are crucial for managing natural resources sustainably.

3. Conservation and Biodiversity Protection

Endangered Species: Understanding the characteristics and needs of a population allows ecologists to devise strategies to protect endangered species, helping them recover and ensuring genetic diversity is maintained.
Ecosystem Stability: Healthy populations contribute to biodiversity, which is critical for ecosystem resilience. By studying populations, ecologists identify which species are "keystone" or foundational to ecosystem health, guiding conservation efforts.

4. Managing Resources and Sustainable Practices

Agriculture and Fisheries: In agriculture, understanding population ecology helps manage pest control, while in fisheries, ecologists use population data to avoid overfishing and ensure sustainable fish stocks.
Forestry and Wildlife Management: For effective management of forests, game, and other natural resources, population studies help determine sustainable harvesting rates, protecting species from depletion.

5. Public Health and Disease Control

Disease Dynamics: Many human diseases originate or spread within wildlife populations. Studying population ecology helps ecologists understand how diseases spread and evolve within populations and identify species that act as reservoirs or vectors for pathogens.
Human Population Growth: Insights from population ecology are also applied to human demographics to address issues like resource allocation, urban planning, and population control measures.

6. Mitigating Human Impacts on Nature

Habitat Destruction and Fragmentation: Human activities impact population structures, causing fragmentation and isolation of populations. Studying how populations respond to habitat destruction helps develop strategies for wildlife corridors and habitat restoration.
Pollution and Invasive Species: Population studies help assess the impact of pollution and invasive species on native populations, aiding in creating policies and practices that minimize ecological damage.

Population studies are fundamental to ecology as they provide insights into the stability, resilience, and sustainability of ecosystems. By understanding population dynamics, ecologists help ensure ecosystems remain balanced, biodiversity is protected, and natural resources are managed responsibly for future generations.

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