Marine Population Dynamics

Marine Population Dynamics Study Notes

Introduction

• Marine Population Dynamics: Focus on how populations of marine species change over time.

• Lecturer: Dr. C. Nicolai Roterman, University of Portsmouth.

Learning Objectives

• By the end of this session, students should be able to:

  • Describe and define what is meant by population dynamics.

  • Appreciate what determines a population's size.

  • Understand the reasons why we care about population size and dynamics.

Coverage Points

• Topics to be covered in the lecture:

  • Birth & death rates, recruitment, mortality, and top-down predation.

  • Trophic cascades, carrying capacity, and extinction.

  • Case studies for practical understanding.

Importance of Population Dynamics

• Key Links: Population dynamics connect with concepts such as:

  • Life Histories

  • Diversity & Abundance

  • Biogeography

• Ecological Interconnections: Every organism is interconnected with others and ecosystems.

  • Quote by John Muir: “When one tugs at a single thing in nature, he finds it attached to the rest of the world.”

Reasons to Study Population Dynamics

• Population Management: Essential for sustainable management of populations.

• Understanding Changes: Enables comprehension of fluctuations in population sizes.

• Conservation Efforts: Particularly important for endangered species and management of pests/invasive species.

• Fisheries Management: Aims to maintain and maximize the sustained yield (MSY).

Population Definitions and Dynamics

• Population: A group of individuals of the same species occupying a specific area and sharing a gene pool over time.

• Population Dynamics: The study of how populations of a species change over time, focusing on questions such as:

  • What explains average population abundance?

  • What causes fluctuations in abundance?

  • How resilient is this population?

• Population Size Variability: Populations can inhabit spaces from cubic centimeters to millions of square kilometers.

Key Concepts and Terminology in Population Dynamics

• Population Size Influencers:

  • Per capita population growth rate: A measure of how quickly the population size changes, determined by:

  • Birth rates

  • Death rates

  • Immigration rates

  • Emigration rates

  • Exponential Growth: Occurs when the per capita growth rate is constant, leading to initial rapid increases in population size followed by decline.

  • Minimum Viable Population (MVP): The smallest population size necessary for a species to survive in the wild.

Density and Carrying Capacity

• Population Density: Generally, population density declines with increasing organism size.

  • John Damuth (1981): Established the relationship between body size and population density; larger animals require more resources, resulting in lower density.

• Resource Availability: Determined by factors such as food, habitat, and water availability.

  • As density increases, competition arises:

  • Intra-Specific Competition: Competition among individuals of the same species.

  • Inter-Specific Competition: Competition between individuals of different species.

• Carrying Capacity (K): Maximum sustainable population size of a species in a given area, influenced by resource availability.

Population Ecology and Size Relationships

• Population Density Across Species: Density varies widely among organisms:

  • Bacterial populations can exceed $10^9$ per cubic centimeter.

  • Phytoplankton densities often exceed $10^6$ per cubic meter.

  • Large mammals and birds typically average less than one individual per square kilometer.

Distribution Patterns in Populations

• Patterns of Distribution: Individuals within a population may:

  • Attract, repel, or ignore each other, affecting distribution:

  • Random Distribution: Individuals occur anywhere with equal probability.

  • Regular Distribution: Individuals are uniformly spaced throughout the environment.

  • Clumped Distribution: Individuals live in areas of high local abundance separated by areas of low abundance.

Mathematical Models of Population Dynamics

• Closed Population Model:

  • No immigration or emigration presented.

  • Births (B) and deaths (M) influencing population size (N):

  • Formula: $N<em>{t+1} = N</em>t + B - M$

  • Where:

    • $N_{t+1}$ = Population size next year

    • $N_t$ = Current population size

    • $B$ = Births per year

    • $M$ = Deaths per year

Marine Microbes Population Dynamics

• Marine Microbial Populations:

  • Populations can reach up to a million in just one milliliter of seawater.

  • Microorganisms make up over 98% of ocean biomass, existing in diverse environments and acquiring energy from various sources.

Population Dynamics in Closed Systems

• Exponential Growth Phases: When resources are limited and conditions are stable:

  • Exponential Phase: Rapid population increase.

  • Stationary Phase: Growth levels off (dying cells = dividing cells).

  • Death Phase: Exponential decrease in living cells.

Case Study: Vibrio Bacteria Population

• Study in 2018:

  • Examined the relationship between bacteria populations and nutrient limitations in an oligotrophic region of the Atlantic.

  • Results indicated that bacteria populations correlated with aerosol particles from Sahara dust storms, which replenished nutrients thereby promoting plankton growth.

  • Virus populations targeting bacteria were also correlated, showing complex interdependencies affecting dynamics in these marine microbes.

Open Systems and Carrying Capacity

• Open Systems: Differ from closed systems as they account for resource replenishment and population dynamics.

• Logistic Growth Curve: Represents populations in open systems considering factors such as resource availability, predation, disease, and growth rates.

Fish Population Dynamics

• Modeling Fish Populations:

  • Essential for understanding responses to exploitation; must appreciate behaviors in unexploited states.

  • Biomass Model:

  • Formula: $B<em>{t+1} = B</em>t + R + G - M$,

    • Where:

    • $B_{t+1}$ = Biomass in one year

    • $B_t$ = Current biomass

    • $R$ = New recruits' biomass

    • $G$ = Growth in current fish biomass

    • $M$ = Mortality of fish.

Recruitment Processes in Fish Populations

• Recruitment: Refers to the addition of new, young organisms to a population.

  • Influenced by various factors such as food availability and predation rates. Recruitment tends to be density-dependent, with survival rates affected by fish population density.

Life History Traits and Recruitment

• Contrasting Species Life Histories:

  • Tunas vs. Sharks:

  • Significant implications for population dynamics and resilience against fishing practices.

  • Typical egg/life cycle numbers for tunas and sharks highlight the differences in reproductive strategies.

Mortality Processes in Fish Populations

• Mortality: The natural death of fish, accounting for many processes such as:

  • Predation and disease.

• Important for understanding fishing impacts and resilience of stocks.

• Expressed as mortality rates relative to age.

Dispersal and Population Connectivity

• Dispersal Significance: Critical for maintaining population ranges; can connect semi-isolated sub-populations into larger metapopulations.

  • Reasons for dispersal include:

  • Response to food supply changes.

  • Adapting to changing environmental conditions.

Metapopulations in Marine Environments

• Metapopulations: Rare in marine contexts; tend to remain connected through ocean currents, as seen in coral reefs and hydrothermal vent examples.

Population Decline and Extinction Risks

• Causes of Population Decline: Linked to resource depletion, increased competition/predation, and environmental changes.

  • Example of Steller’s Sea Cow:

  • Extinct due to overhunting and ecological disruptions, highlighting vulnerability factors to extinction.

Factors Preventing Extinction

• Populations least likely to go extinct typically:

  • Have extensive geographic ranges.

  • Exhibit broad habitat tolerances.

  • Maintain large local populations.

Historical Extinctions and Modern Risks

• Carlton et al. (1999): Reviewed historical extinctions, suggesting a greater frequency of extinctions than recorded due to research and taxonomic expertise biases.

Case Study: Steller’s Sea Cow

• Details:

  • Extinct by 1768 due to excessive hunting and ecosystem collapse caused by loss of the keystone sea otter, which maintained habitat ecosystems through predation on sea urchins.

Trophic Cascades Overview

• Trophic Cascades: Strong indirect interactions affecting entire ecosystems. Occur when a change at one trophic level impacts others. Examples include:

  • Sea otters impacting kelp forests by controlling urchin populations.

Trophic Cascades Case Studies

• Wolves: Impact on river ecosystems through indirect effects on vegetation by controlling herbivore populations.

• Sea Otters: Recent studies depict their role in promoting ecosystem resilience through management of kelp populations and carbon sequestration benefits.

Conclusion and Summary

• Reviewed the following:

  • Definitions and importance of population dynamics.

  • Factors influencing population size and dynamics including birth/death rates, recruitment, mortality, and ecological concepts like trophic cascades and carrying capacity.

  • Emphasized the necessity of understanding population dynamics in marine ecology through detailed case studies.