Study Notes on Intraspecific Population Regulation

Chapter 11: Intraspecific Population Regulation

Introduction

• Intraspecific population regulation examines how populations of the same species interact and affect their size and growth.

• Important questions:

  • Why can no population grow indefinitely?

  • What limitations do members of the same species impose on growth?

  • How do these interactions regulate population size?

11.1 The Environment Functions to Limit Population Growth

• Exponential Population Growth Model:

  • Equation: $\frac{dN}{dt} = (b - d)N$

  • Assumptions of the model:

  • Unlimited essential resources.

  • Constant environment.

  • Reality:

  • Resources are limited in natural populations.

  • The environment is not constant.

• Resource Demand:

  • Increasing population density leads to higher resource demand.

  • When resource consumption exceeds replenishment:

  • The resource base shrinks.

  • Mortality increases; fecundity decreases, or both.

11.2 Birthrate and Death Rate

• Change in Birthrate:

  • Formula: $b = b_0 - aN$ where:

  • $b_0$ is the maximum birthrate under ideal conditions (exponential model).

  • $a$ is the slope.

• Change in Death Rate:

  • Formula: $d = d_0 + cN$ where:

    • $d_0$ is the minimum deathrate under ideal conditions.

    • $c$ is the slope.

• Rewrite the exponential model considering varying birthrate and death rate:

  • $\frac{dN}{dt} = [(b<em>0 - aN) - (d</em>0 + cN)]N$

  • Consequences:

  • Birthrate drops as population size increases while death rate rises.

  • Rate of population growth slows.

• Population Growth Dynamics:

  • If d > b, population size declines.

  • If $d = b$, growth rate is zero: $\frac{dN}{dt} = 0$.

  • Sustainable size:

  • $N = \frac{(b<em>0 - d</em>0)}{(a + c)}$ (where $K$ is the carrying capacity).

11.3 Logistic Growth Model

• Logistic model formulation:

  • $\frac{dN}{dt} = rN(1 - \frac{N}{K})$.

  • Components:

  • Exponential growth: $rN$.

  • Effect of carrying capacity: $(1 - \frac{N}{K})$.

• Growth dynamics:

  • When N << K, the term is close to 1.

  • As $N$ approaches $K$, growth slows to zero.

  • Maximum growth rate occurs when $N = \frac{K}{2}$ (inflection point).

11.4 Density Dependence

• Carrying Capacity (K):

  • Negative feedback exists between population size and resource availability.

  • Higher density --> lower per capita resource availability.

• Regulatory Mechanisms:

  • Density-dependent mortality increases with population density.

  • Density-dependent fecundity decreases with population density.

• Factors Affecting Density-Dependent Regulation:

  • Reduced resource availability,

  • Changes in predation patterns,

  • Spread of disease.

• Also involves density-independent factors like floods, fires, and storms.

11.5 Competition and Resource Limitations

• Competition:

  • Arises when individuals exploit limited resources.

  • Types:

  • Intraspecific competition occurs among individuals of the same species, leading to competition for a common resource.

• Responses to Limited Resources:

  • Scramble Competition:

  • All individuals equally suffer from resource limitation.

  • Contest Competition:

  • Some individuals claim sufficient resources.

• Outcomes:

  • Scramble competition can lead to extinction.

  • Contest competition primarily affects unsuccessful individuals.

11.6 Mortality Influences

• Resource competition leads to mortality suppression, enhancing resource availability for survivors.

• Evidence from studies:

  • Monarch butterfly egg density affects survival probability.

• Self-thinning:

  • Increased mortality due to competition allows remaining individuals to access more resources.

  • Documented in plants and aquatic organisms.

11.7 Reproductive Impacts

• High population densities can reduce fecundity, affecting reproduction rates across species.

• Patterns in varying species:

  • Increased age of sexual maturity in high-density conditions affecting reproductive output.

11.8 High Density Stress

• Elevated density can trigger stress responses:

  • Hormonal changes reduce growth and reproduction.

  • Stress can impair immune function, leading to heightened disease vulnerability.

11.9 Dispersal Dynamics

• Dispersal often results from high density, driven by competition and resource limitations.

• Some individuals may successfully find new habitats, but high competition may restrict effective density regulation through dispersal.

11.10 Social Structures and Territoriality

• Social behavior can limit population size through domination and aggression.

• Home Range vs Territory:

  • Home range: area regularly used.

  • Territory: defended area preventing others' access.

• Territorial behavior includes aggressive displays and vocalizations to manage space and resources.

11.11 Plant Competition

• Plants preempt resources through their physical presence and root competition.

• Strategies include shading and resource depletion, leading to spatial competition in fixed locations.

11.12 Inverse Density Dependence in Small Populations

• The Allee effect emerges at low densities, where individuals experience reduced birth and survival rates due to challenges in finding mates or cooperative behaviors.

11.13 Density-Independent Factors

• Environmental factors (temperature, precipitation) can influence populations but do not biologically regulate growth.

11.14 Conservation Implications

• Understanding minimum viable population size (MVP) is crucial for conservation efforts, determining the number of individuals needed for long-term survival.

• The MVP varies with species characteristics and requires adequate habitat to maintain population health.