IB Biology Study Guide: Populations, Communities, and Ecology

Fundamentals of Biological Organization and Ecology

Population: Defined as a group of organisms belonging to the same species, inhabiting the same geographic area at the same point in time.
Community: Defined as a group of various populations that live together and interact with one another within a specific, given area.
Biodiversity: This term refers to the variety of all living things within a particular ecosystem. This exhaustive category includes plants, animals, fungi, and other microorganisms.

Measuring Biodiversity Using Simpson's Reciprocal Index

The Equation: The Simpson's Reciprocal Index is calculated using the following formula:

$D = \frac{N(N-1)}{\sum n(n-1)}$

Variable Definitions: * $D$ : The Diversity Index. * $N$ : The total number of organisms belonging to all species found in the study area. * $n$ : The number of individual organisms of a particular species.
Usage: This index is utilized to measure biodiversity by quantifying two main factors: how many different species are present in an ecosystem (species richness) and how evenly the individuals are distributed across those species (species evenness).

Practical Application: Comparing Specific Sites

Site 1 Data: Species A = 6; Species B = 1; Species C = 1. Total $N = 8$ . * Calculation: $D = \frac{8(8-1)}{6(5) + 1(0) + 1(0)} = \frac{56}{30} = 1.86$
Site 2 Data: Species A = 4; Species D = 4. Total $N = 8$ . * Calculation: $D = \frac{8(8-1)}{4(3) + 4(3)} = \frac{56}{24} = 2.33$
Conclusion: Site 2 shows a higher diversity index than Site 1.

Population Sampling and Estimation Techniques

Random Quadrat Sampling: This method involves placing quadrats (square frames of a known size) randomly within a defined study area. Researchers count the organisms found within these frames, determine the average density per quadrat, and extrapolate that average to estimate the total population of the entire area.
Capture-Mark-Release-Recapture (Lincoln Index): This technique involves capturing a sample of animals, marking them in a way that does not harm them or affect their survival, and releasing them back into the wild. After a period that allows for dispersal, a second sample is captured. The ratio of marked to unmarked individuals in the second sample is used to estimate total population size.
Transect Line Collection: Data is collected along a physical line placed across an environment. Scientists record the organisms encountered at specific, regular intervals along this line to study changes in distribution relative to environmental gradients.

Estimation Problems and Calculations

Barnacle Estimation: A researcher counts 150 barnacles within 10 quadrats (each size $0.1\,m^2$ ). The total area is $200\,m^2$ . * Total sampled area: $10 \times 0.1\,m^2 = 1\,m^2$ * Estimated population for $200\,m^2$ : $150 \times 200 = 30,000$
Dandelion Estimation: A student uses a $0.5\,m \times 0.5\,m$ quadrat to sample dandelions in a $100\,m^2$ field. They place 10 quadrats and find 20 dandelions. * Area of one quadrat: $0.25\,m^2$ * Total sampled area: $10 \times 0.25\,m^2 = 2.5\,m^2$ * Density: $\frac{20}{2.5} = 8\,\text{dandelions}/m^2$ * Estimated population for $100\,m^2$ : $8 \times 100 = 800$

The Lincoln Index Formula and Assumptions

Formula: $P = \frac{M \times N}{R}$ * $P$ : Estimated total population size. * $M$ : Number of individuals caught and marked in the initial capture. * $N$ : Total number of individuals caught in the second capture (recaptured sample). * $R$ : Number of marked individuals found in the second capture.
Example Calculation (Mountain Gorillas): Captured/Marked ( $M$ ) = 25; Recaptured ( $N$ ) = 24; Marked in recaptured sample ( $R$ ) = 8. * $P = \frac{25 \times 24}{8} = 75\,\text{gorillas}$
Assumptions of the Lincoln Index: 1. The population size remains constant during the study (no significant births, deaths, or migrations). 2. Marked animals mix evenly and randomly back into the general population. 3. Marks do not fall off, fade, or negatively affect the animal's chance of survival. 4. Every individual has an equal probability of being captured in the samples.

Population Growth Dynamics and Modulations

Carrying Capacity ( $K$ ): The maximum population size of a species that a specific environment can sustainably support over time, given the available resources.
Density-Dependent Variables: These are factors whose influence on population size changes based on the density of the population. Examples include disease, competition for resources, and predation.

The Sigmoid (S-shaped) Population Growth Curve

Lag Phase: Growth is slow as the initial population adapts to the new environment and establishes itself.
Exponential Growth Phase: The population increases rapidly at its maximum biotic potential because resources are abundant and limiting factors are minimal.
Transitional Phase: The rate of growth begins to decelerate as resources become harder to find and competition increases.
Plateau / Stationary Phase: The growth rate stabilizes at zero ( $\text{births} + \text{immigration} = \text{deaths} + \text{emigration}$ ). The population levels off around the carrying capacity ( $K$ ).

Modeling with Simple Organisms

Rationale: Organisms like yeast or duckweed are used for growth studies because they reproduce very quickly, are easy to maintain in controlled environments (agar plates or water cups), and allow for the observation of full population cycles in a compressed timeframe.
Yeast Death Phase: Unlike the standard sigmoid curve, yeast studies often include a Death Phase, where the accumulation of toxic waste products or the total depletion of nutrients causes the population of viable cells to plummet after the plateau.

Limiting Factors and Trophic Control

Top-Down Control (Trophic Cascade)

In this scenario, the top predator controls the dynamics of the lower levels of the food chain.

Absence of Foxes: With no foxes, the frog population increases (loss of predation). Increased frogs consume more snails, causing the snail population to decrease. Fewer snails mean less herbivory on plants, leading to an increase in the plant population.
Increased Fox Population: More foxes lead to a decrease in frogs. Fewer frogs allow the snail population to boom. An abundance of snails results in over-consumption of plants, causing the plant population to decrease.

Bottom-Up Control

In this scenario, nutrient supply and primary productivity control the higher levels of the food chain.

Absence of Fertilizer: Lack of nutrients leads to decreased plant growth. Fewer plants support fewer snails. The reduction in snails limits the frog population, which in turn reduces the fox population.
Adequate Fertilizer: Increased nutrients boost plant productivity. More plants provide more food for snails, supporting a larger frog population and, ultimately, a larger fox population.

Interspecific Relationships and Interactions

Interspecific Relationship: Any biological interaction occurring between members of different species.
Herbivory: The consumption of living plant tissue by animals.
Predation: One organism kills and eats another living organism for energy.
Interspecific Competition: Individuals of different species compete for the same limited resources (e.g., food, space).
Mutualism: An interaction where both species benefit. * Obligate: The relationship is necessary for the survival of both organisms. * Facultative: Species benefit from each other but can survive independently.
Parasitism: One organism (parasite) lives on or in another (host), gaining benefits at the host's expense.
Pathogenicity: The capacity of microorganisms (bacteria, viruses, fungi) to cause disease in a host.
Niche: The specific functional role or "occupation" a species plays within its ecosystem.

Statistical Analysis: The Chi-Square Test

Purpose: Used in ecology to determine if there is a statistically significant association between the distributions of two different species.
Methodology: Comparing Observed Frequencies (from field data) against Expected Frequencies (calculated assuming no association).
Null Hypothesis ( $H_0$ ): States there is no significant association between the species; any observed overlap or avoidance is due to chance.
Result Interpretation: If the calculated chi-square value is less than the critical value (usually at $p < 0.05$ ), the null hypothesis is accepted.

Ecological Succession and Stability

Succession: The process of gradual change in an ecosystem's species composition over time.
Primary Succession: Occurs in an entirely new environment where no soil previously existed (e.g., cooled lava flows, retreating glaciers).
Secondary Succession: Happens in areas where an existing community has been disturbed (e.g., after a forest fire or flood) but the soil remains intact.
Cyclical Succession: Repeating patterns of change caused by regular environmental disturbances.
Arrested Succession: When a community is prevented from reaching its final stage due to constant external disturbances (e.g., grazing or human activity).
Climax Community: A stable, mature community that has reached a steady state and undergoes little change over time.

Competition and Predator-Prey Dynamics

Invasive Species: Non-native organisms that spread rapidly and cause ecological or economic harm by outcompeting native species. Example: The Zebra Mussel, which clogs waterways and disrupts local food webs.
Predator-Prey Cycles: These populations fluctuate in related cycles. For example, the Arctic Fox and Snowshoe Hare populations show oscillations where the predator population peaks shortly after the prey population reaches its highest point.
Competitive Exclusion Principle: States that two species competing for the exact same resources cannot coexist indefinitely in the same niche. One will inevitably be more efficient, leading to the extinction or displacement of the other.
Resource Partitioning: An alternative to exclusion where species alter their use of the niche (e.g., feeding at different times or heights) to divide resources and reduce competition.