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Ecology - Species, Communities, and Ecosystems

Grade 11IBBiology

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

🔑Concepts

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A species is a group of organisms that can potentially interbreed to produce fertile offspring; cross-breeding between different species usually results in infertile hybrids.

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Autotrophs (producers) synthesize organic molecules from simple inorganic substances, such as CO2CO_2 and H2OH_2O, using energy from light or oxidation of inorganic chemicals.

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Heterotrophs obtain organic molecules from other organisms. They are classified into: Consumers (ingest living or recently killed organisms), Detritivores (ingest non-living organic matter via internal digestion), and Saprotrophs (secrete digestive enzymes onto organic matter for external digestion).

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A community is formed by populations of different species living together and interacting with each other in a specific area.

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An ecosystem consists of a community and its interactions with the abiotic environment (non-living factors like pHpH, temperature, and humidity).

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Nutrient cycling is essential for ecosystem sustainability; while energy flows through ecosystems and is lost as heat, chemical elements like CC, NN, and OO are recycled indefinitely.

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Mesocosms are small experimental areas used to investigate ecosystem sustainability under controlled conditions, requiring energy availability, nutrient cycling, and waste recycling.

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The χ2\chi^2 (Chi-squared) test is used to determine if there is a statistically significant association between the distribution of two species using quadrat sampling data.

📐Formulae

χ2=∑(fo−fe)2fe\chi^2 = \sum \frac{(f_o - f_e)^2}{f_e}

fe=Row Total×Column TotalGrand Totalf_e = \frac{\text{Row Total} \times \text{Column Total}}{\text{Grand Total}}

df=(r−1)(c−1)df = (r - 1)(c - 1)

💡Examples

Problem 1:

An ecologist uses 100 quadrats to study the association between a moss species (MM) and a fern species (FF). The results show: Both present in 20 quadrats, only Moss in 30, only Fern in 10, and neither in 40. Calculate the expected frequency (fef_e) for 'Both Present' and determine the degrees of freedom (dfdf).

Solution:

  1. Calculate totals: Total Moss = 20+30=5020 + 30 = 50. Total Fern = 20+10=3020 + 10 = 30. Grand Total = 100100.
  2. Expected frequency for 'Both Present': fe=50×30100=15f_e = \frac{50 \times 30}{100} = 15.
  3. Degrees of freedom: Since it is a 2×22 \times 2 contingency table, df=(2−1)×(2−1)=1df = (2-1) \times (2-1) = 1.

Explanation:

The expected frequency represents the number of quadrats where both species would be found if their distributions were completely independent. The degrees of freedom for a standard association test between two species is always 11 because there are 2 categories (present/absent) for each species.

Problem 2:

In a sustainability study of a mesocosm, why must CO2CO_2 be present for the system to survive long-term?

Solution:

Autotrophs within the mesocosm require CO2CO_2 for the process of photosynthesis: 6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2.

Explanation:

Without CO2CO_2, producers cannot synthesize the organic compounds (glucose) needed to support the food chain. The CO2CO_2 is typically replenished by the cellular respiration of both autotrophs and heterotrophs: C6H12O6+6O2→6CO2+6H2O+ATPC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP.

Species, Communities, and Ecosystems - Revision Notes & Key Diagrams | IB Grade 11 Biology