Organisms and Populations

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding. In this chapter you will study the attributes that a population has but an individual does not: birth rate and death rate expressed per capita, sex ratio, and age distribution shown by age pyramids that are expanding, stable or declining. You will learn how population density is measured using total counts, percentage cover, pug marks, faecal pellets or relative abundance, and how density changes through natality, mortality, immigration and emigration, summarised as Nt+1 = Nt + [(B + I) - (D + E)]. The chapter then explains exponential growth, dN/dt = rN, which gives a J-shaped curve, and logistic growth, dN/dt = rN(K - N)/K, which gives an S-shaped curve limited by carrying capacity K, along with life history variation. Finally, you will explore population interactions: predation, competition including Gause's competitive exclusion principle and resource partitioning, parasitism, commensalism and mutualism, with examples such as the fig and wasp and the Mediterranean orchid Ophrys.

Detailed Worksheet: Organisms and Populations Section A - Definitions (10 marks) 1. Define a population. Name any four attributes of a population that are not shown by an individual. (2 marks) 2. What is carrying capacity? Represent it on a logistic growth curve. (2 marks) 3. State Gause's competitive exclusion principle. What is resource partitioning? (2 marks) 4. Distinguish between commensalism and mutualism with one example of each. (2 marks) 5. What is brood parasitism? Give one example. (2 marks) Section B - Calculations and Applications (15 marks) 6. A pond had 20 lotus plants last year, and through reproduction 8 new plants were added this year. In a laboratory population of 40 fruit flies, 4 died in a week. Calculate the birth rate of the lotus population and the death rate of the fruit fly population, stating the units. (3 marks) 7. A population of deer is 500 at the start of a year. During the year there are 120 births, 40 deaths, 30 immigrants and 10 emigrants. Using Nt+1 = Nt + [(B + I) - (D + E)], calculate the population at the end of the year and its percentage growth. (3 marks) 8. A bacterial population grows exponentially with r = 0.693 per hour. Starting with 1,000 cells, calculate the population after 1 hour, 2 hours and 5 hours (take e^0.693 = 2). What shape of curve does this growth produce? (3 marks) 9. A population follows logistic growth with r = 0.2 per year and K = 1,000. Calculate dN/dt when N = 100, N = 500 and N = 900. At which population size is growth fastest? (3 marks) 10. Identify the type of interaction (+, -, 0) for each pair: (i) cuckoo and crow (ii) orchid growing on a mango branch (iii) Balanus and Chthamalus on the rocky coast of Scotland (iv) lichen fungus and alga (v) Cuscuta and its host (vi) sea anemone and clown fish. (3 marks) Section C - Diagrams (10 marks) 11. Draw and label the exponential (J-shaped) and logistic (S-shaped) growth curves on the same axes, marking the carrying capacity K, and write the equation for each. (4 marks) 12. Draw age pyramids for expanding, stable and declining human populations, labelling pre-reproductive, reproductive and post-reproductive age groups. (3 marks) 13. Draw a labelled diagram showing the mutualistic relationship between a fig tree and its pollinator wasp, indicating pollination and egg-laying. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. The prickly pear cactus introduced into Australia in the early 1920s spread over millions of hectares until a cactus-feeding moth from its natural habitat was introduced. Explain this outcome using the role of predators, and analyse why invasive species spread rapidly in new habitats. (5 marks) 15. Explain the defence mechanisms evolved by prey against predators, using the Monarch butterfly, cryptic colouration in frogs and insects, and plant defences like thorns and chemicals such as those in Calotropis. Analyse why most herbivores avoid Calotropis. (5 marks) 16. The Mediterranean orchid Ophrys uses sexual deceit to get pollinated. Explain the mechanism and analyse what would happen if the colour patterns of the female bee change during evolution. Relate your answer to the concept of co-evolution between plants and pollinators. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw graphs and diagrams neatly in pencil with labelled axes. Show working for all calculations.