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Interaction and interdependence - Populations and communities

Grade 12IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A population is a group of organisms of the same species living in the same area at the same time. The size of a population (NN) is influenced by natality, mortality, immigration, and emigration.

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The carrying capacity (KK) is the maximum population size that a particular environment can sustain indefinitely, given the food, habitat, water, and other necessities available.

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Population growth follows two main patterns: Exponential growth (J-shaped curve), represented by dNdt=rN\frac{dN}{dt} = rN under ideal conditions, and Logistic growth (S-shaped curve), which levels off at KK.

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Limiting factors can be density-dependent (e.g., competition, disease, predation) or density-independent (e.g., natural disasters, temperature, weather).

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A community is an assemblage of different populations interacting in a specific area. Interactions include herbivory, predation, and symbiosis (mutualism, commensalism, and parasitism).

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The ecological niche of a species includes its spatial habitat, its feeding activities, and its interactions with other species. There is a distinction between the fundamental niche (potential) and the realized niche (actual).

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Competitive exclusion principle states that two species cannot occupy the same niche in the same environment for long; one will eventually outcompete the other.

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Keystone species have a disproportionately large effect on their environment relative to their abundance, maintaining the structure of the community.

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Succession is the process of change in the species structure of an ecological community over time, categorized into primary succession (starting from bare rock) and secondary succession (following a disturbance in existing soil).

📐Formulae

N=n1×n2nmN = \frac{n_1 \times n_2}{n_m}

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

Population Growth=(B+I)−(D+E)\text{Population Growth} = (B + I) - (D + E)

💡Examples

Problem 1:

An ecologist wants to estimate the population of woodlice in a garden. They capture n1=60n_1 = 60 woodlice, mark them, and release them. A day later, they capture n2=50n_2 = 50 woodlice, of which nm=12n_m = 12 are marked. Calculate the total population size (NN).

Solution:

N=60×5012N = \frac{60 \times 50}{12} N=300012N = \frac{3000}{12} N=250N = 250

Explanation:

Using the Lincoln Index (capture-mark-recapture method), the total population is estimated by multiplying the initial marked sample by the second sample size, then dividing by the number of recaptured marked individuals.

Problem 2:

Calculate the Simpson's Diversity Index (DD) for a community with three species: Species A (n=10n=10), Species B (n=5n=5), and Species C (n=3n=3). Total N=18N = 18.

Solution:

First, calculate n(n−1)n(n-1) for each species: Species A: 10×9=9010 \times 9 = 90 Species B: 5×4=205 \times 4 = 20 Species C: 3×2=63 \times 2 = 6 ∑n(n−1)=90+20+6=116\sum n(n-1) = 90 + 20 + 6 = 116 N(N−1)=18×17=306N(N-1) = 18 \times 17 = 306 D=306116≈2.64D = \frac{306}{116} \approx 2.64

Explanation:

Simpson's Diversity Index (DD) measures the diversity of a community. A higher value of DD usually indicates greater biodiversity, suggesting a more stable and ancient ecosystem.