Nobody can count every plant in a forest or every crab on a beach. Ecologists sample instead, and then estimate. Section 1.7 of the book explains what biodiversity means, the levels at which it is measured, and the field methods used to measure it.
Chapter 1 of the PECTAA Biology 11 book is Biodiversity and Classification. After the classification of viruses (section 1.6), section 1.7 turns to biodiversity and how it is assessed. This note is for FSc Part 1 (1st year, Class 11) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Biodiversity. The word comes from "biological diversity". It means the variety of life forms in different ecosystems, covering the diversity of species, genes and ecosystems. It also includes how living things interact with each other and with their environment.
Two terms from the margin.
- Ecosystem: a dynamic, interactive system of living organisms and their physical environment. It includes all biotic (living) and abiotic (non-living) factors.
- Niche: the role or function of an organism or species within an ecosystem. It covers its habitat, its interactions with other organisms (predation, competition, symbiosis) and its part in energy flow.
Levels of biodiversity assessment.
- Species level: identify and count the different species in an area. Species diversity includes the number of species and also their relative abundance and distribution.
- Genetic level: the variety of genetic information within all the individuals of a species. This helps a species adapt to and survive environmental change.
- Ecosystem level: the range of habitats, such as forests, wetlands, grasslands and deserts, and how each ecosystem works and supports overall ecological health.
Why random sampling matters. The book gives five reasons:
- Minimises bias: every part of the area has an equal chance of being sampled.
- Gives reliable estimates: data can be analysed statistically for species richness, abundance and distribution.
- Allows comparisons: standard collection methods let different areas or habitats be compared.
- Improves representativeness: samples cover different parts of the area.
- Supports conservation: accurate assessments identify areas of high conservation value and track changes over time.
Methods to assess distribution.
- Quadrat sampling: divide the area into a grid and sample inside randomly chosen squares (quadrats). Suited to plants and sessile (fixed) organisms, for example on a forest floor.
- Transect sampling: lay a line or strip across the area and record species at regular intervals along it. Suited to change across environments, for example from the high tide line to the low tide line on a coast.
- Aerial surveys: aircraft or drones record organisms over large areas, such as birds across a wetland or large mammals on a savanna.
Methods to assess abundance.
- Point counts: record the number of individuals seen from a fixed point over a set time. Common for birds and other mobile animals.
- Mark–recapture: capture, mark and release individuals, then recapture later to estimate population size and density. Useful for animals that are hard to count directly.
- Quadrat counts: count individuals in each quadrat and scale up to estimate total abundance.
- Capture–recapture models: extensions that allow for things such as varying capture chances and movement between areas.
- Remote sensing: satellite or drone images assess abundance and distribution over large or hard-to-reach areas.
Key points
- Biodiversity is measured at three levels: species, genetic and ecosystem.
- Species diversity is more than a count; abundance and distribution matter too.
- Distribution methods ask "where?"; abundance methods ask "how many?".
- Random sampling is the base of fair, comparable estimates.
Worked examples
Example 1 (choosing a method). A student wants to know how seaweeds change from the top of a rocky shore to the water's edge.
- The question is about change across a zone, so it is about distribution.
- Use transect sampling: lay a line from the high tide line to the low tide line and record species at regular intervals.
Example 2 (choosing a method). A team wants to estimate how many lizards live in a scrub area.
- Lizards move and hide, so direct counting is hard.
- Use mark–recapture: capture, mark, release, then recapture later and use the proportion of marked animals to estimate the population.
Example 3 (applying a level). Two wheat fields have the same species, but one is sown from many seed varieties.
- The difference lies within one species.
- This is diversity at the genetic level, which helps the crop cope with environmental change.
Common mistakes in the exam
- Treating niche as habitat. Habitat is only part of a niche; the niche is the organism's whole role.
- Counting species only. Species diversity also needs relative abundance and distribution.
- Choosing quadrats for fast-moving animals. Quadrats suit plants and sessile organisms.
- Mixing distribution and abundance methods. Transects show distribution; mark–recapture estimates abundance.
- Leaving out abiotic factors from the definition of an ecosystem.
Exam technique
- For "explain" questions, give the method, what it measures and one example from the book.
- Use the five reasons for random sampling as a numbered list.
- In comparisons, use a two-column table: quadrat against transect.
- Check your board's notice for the sections in your paper.
Practice questions
- Name the three levels of biodiversity assessment. Answer: Species, genetic and ecosystem levels.
- Which method suits sampling grass on a lawn? Answer: Quadrat sampling.
- Give one reason random sampling reduces bias. Answer: Every part of the area has an equal chance of being sampled.
- Which abundance method is common for birds? Answer: Point counts.
- Define abiotic factors with one example. Answer: The non-living parts of an ecosystem, such as temperature or water.
Ready to practise this? Continue in 1st Year Biology.