Populations and sustainability (6.3.2)
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Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:
- Content: A clear outline of the content covered by the delivery guide
- Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.
|Content (from A-level)
The content from the specification that is covered by this delivery guide is:
|6.3.2 Populations and sustainability|
|(a)||the factors that determine size of a population||To include the significance of limiting factors in determining the carrying capacity of a given environment and the impact of these factors on final population size.
M0.1, M0.2, M0.3, M0.4, M0.5, M1.3, M2.5, M3.1, M3.2
|(b)||interactions between populations||To include predator–prey relationships considering the effects on both predator and prey populations
AND interspecific and intraspecific competition.
|(c)||the reasons for, and differences between, conservation and preservation||To include the economic, social and ethical reasons for conservation of biological resources.
HSW7, HSW9, HSW10, HSW12
|(d)||how the management of an ecosystem can provide resources in a sustainable way||Examples to include timber production and fishing.
|(e)||the management of environmental resources and the effects of human activities|
To include how ecosystems can be managed to balance the conflict between conservation/preservation and human needs e.g. the Masai Mara region in Kenya and the Terai region of Nepal, peat bogs
The study of populations and sustainability gives another opportunity to cover and incorporate practical and mathematical skills for example, Practical skills: PAG3 (sampling techniques), PAG10 (investigation using computer skills),
PAG12 Research skills.
Mathematical skills: M1.1, M1.2, M1.3, M1.5, M1.6, M1.7, M1.9, M1.10, M1.11, M2.1, M2.2, M3.2, M3.5, M3.6.
A range of sampling techniques can be used as a basic introduction to the use of and understanding of terminology and concepts encompassing population dynamics, in a variety of ecosystems. The use of quadrats and transects are fundamental in assessing populations of plants, animals, fungi, algae, lichens etc.
As a basic exercise towards developing the best technique to use when sampling populations within a given area, it may be useful to do an activity to estimate the optimum quadrat size, which may vary dramatically depending on the organism and environment being investigated. This can be a useful starter activity also, which can then be built upon for further investigations.
Students can be introduced to various methods of estimating population size such qualitative techniques (% cover, ACFOR), and quantitative techniques (first hit, and all hit point sampling), and these techniques may be practised in different sized quadrats.
To determine the optimum quadrat size, a quadrat of 10 cm by 10 cm is set up, and the populations within recorded. The size of quadrat and recording procedure is then gradually increased until reaching a size of 2 m by 2 m. The point at which the most NEW species are recorded in the shortest time given i.e. the most economical then ascertains the optimum size. To calculate this, a line graph can be plotted with quadrat size against number of species counted, to visually demonstrate that the point at which the graph levels off indicates the most economical quadrat size for that particular area. All hit point sampling is the best technique for this purpose as it allows students to recognise when new species are recorded.
The other aforementioned techniques may then be practised in random 1 m square quadrats to enable comparisons to be drawn in terms of accuracy and efficiency; afterwards the advantages and disadvantages of each technique can be evaluated. A simple key will be required to enable students to identify species, or types of species. These can be obtained from websites such as the Field Studies Council, although it often works better if the site is assessed before hand and a key constructed to suit the terrain/purpose.
Populations may be sampled on heathland/grassland, sheltered and exposed shores, in fresh water habitats, on hillsides, and areas undergoing succession to assess the changing communities and the various biotic and abiotic factors influencing each in addition to considering inter and intra specific competition. Woodlands also provide scope for assessing population changes depending on tree density and variety, and in such environments moisture levels may also be taken into account and measured.
When assessing freshwater invertebrates (which can be used as indicators of pollution in river water) kick samples are required to collect individuals residing in the sediment. Students may identify and count invertebrates at points along a river from source to mouth, whilst also measuring physical and chemical variables including width, depth, bedload size, flow rate, temperature, pH, oxygen concentration, calcium carbonate, nitrates, phosphates, and ammonia. Students also studying Chemistry will benefit from carrying out titrations to calculate dissolved oxygen before converting to saturation levels, as chemical titrations are often poorly understood and so this method provides an opportunity for deeper understanding (by practical analysis), whilst relating the results to a biological context i.e. effect on population size and species diversity.
In order to assess abiotic factors which affect population size during any field investigation, climatic variables may be measured including air temperature, soil pH and temperature, light intensity, wind speed, humidity, and exposure / turbulence.
Type of transect used in an investigation may vary as with quadrat. When investigating the effects of zonation on the seashore for example, an interrupted belt transect allows enough data to be collected on algal and animal populations efficiently, and allows for consideration of limiting factors afterwards. Such limiting factors will influence the community and population density within each zone and will include desiccation, predation, salinity, temperature (especially variable within rock pools), exposure, food supply, and reproduction. Kite diagrams are useful visual representations of changing communities from data collected.
The effect of limiting factors on final population size can also be discussed following investigation in terms of abundance, diversity, and species adaptations of which there are many on the rocky shore. Predator-prey relationships can also be ascertained with such studies and techniques such as mark/release/recapture (MMR), another useful tool in investigating the population size within a CLOSED population of a semi-mobile species. On the rocky shore, dog whelks (Nucella lapillus) are commonly used for this purpose, the marker being Tipp-Ex® or nail varnish (the use of such can also be evaluated). The animals are collected from a specific location, counted, marked and released, and several days later animals are re-collected from the same location (usually a 1 m square quadrat), and the number of individuals marked and unmarked counted, to be used in calculations later. The Lincoln index is the most commonly used calculation for this purpose but has many limitations, which again are useful for discussion.
The FSC website is an excellent resource providing numerous sources of information covering similar activities to those described above. It provides links to freshwater ecology activities and human impacts, moor land and heathland management, rocky shore analysis, as well as salt marsh and sand dune dynamics. Information on Woodland ecology and management can also be found here.
Another source of information on field work techniques may be found at Science & Plants for Schools. This outlines methods used to measure distribution of species across a footpath (and therefore may also be used in conjunction with geography fieldwork on footpath erosion), and includes the definition of ‘species richness’ as well as helpful identification guides.
In terms of environmental management, this issue may be raised when investigating succession on a hydrosere (often done via a line transect with point sampling, along with a quadrat at each end), and discussed with relation to species at risk, and whether or not human intervention is necessary to protect /maintain such an environment. The advantages and disadvantages can be weighed up and debated. Such sites are often SSSI’s so are habitats for rare and threatened species, but at the same time it may be assumed that succession allows new species to colonise the area and increase diversity.
Approaches to teaching the content
A practical approach to the teaching can keep the students engaged and reinforce knowledge from earlier in the specification. Real life examples can help bring the topic to life and aid teaching and learning.
Common misconceptions or difficulties students may have
Students can struggle with understanding that the carrying capacity of any environment or habitat is determined by biotic and abiotic factors, which may be limiting factors, thereby preventing population explosion and keeping the number of individuals in each population within a community relatively steady.
Students often struggle in understanding ecological concepts when fieldwork techniques are not undertaken and are able to develop a much deeper understanding through actual investigation. This part of the course relates directly to the issues and concepts taught in 6.3 – Ecosystems, and the techniques outlined are also relevant in this area i.e. to measure the distribution and abundance of organisms, succession, and investigating different types / size of ecosystem such as woodlands, coastlines, and heathlands where biotic and abiotic factors are also considered. The differences between inter and intra specific competition should be explored to prevent confusion, and related to fieldwork tasks undertaken.
Conceptual links to other areas of the specification
This last section of the Biology A specification links back to teaching Module 4, particularly 4.2.1 Biodiversity, as well as 6.3.1 Ecosystems. These topics can also be used to reinforce knowledge and development of the practical planning, implementing, analysis and evaluation skills of teaching module 1.
Many other parts of the course share common links with this area of the specification, for example: species adaptations link to respiration, chemical production, enzyme activity, osmosis, photosynthesis, and avoidance behaviour. There are also several opportunities to link with geography such as when measuring physical changes along a river and abiotic meteorological variables in addition to pollution levels. Lichen species are also often assessed to determine levels of and types of pollution, and can also be used to link to geographical work.
In addition to places such as the Galapagos Islands, there are many other vulnerable sensitive ecosystems in less well-known and more remote locations, which are about to undergo significant change, and the impacts of such change can be discussed as a case study.
St Helena Island, a British Island in the South Atlantic, is one such place with a variety of endemic organisms both plant and animal, which are currently in decline, but could be put under even greater pressure once tourism levels rise as an airport is due to open by 2017. Until such time, the Island has only ever been reached by ship. The length of the journey and time taken to wait for return travel severely restricts tourism on the island at present, and conservation efforts are underway to try to protect native endemic species such as the wire bird, and the extremely rare She Cabbage tree, amongst other organisms including Britain’s most endangered invertebrate – the spiky yellow woodlouse.
A wealth of information on such species, their habitats, status, and conservation efforts underway to protect them can be found via the St Helena National Trust.
At one time, sugar cane harvesting and fishing (exports also) were important sources of income for St Helena Island but due to the effects of over fishing by international vessels and soil erosion through deforestation, these industries have completely declined with the island now unable to export fish at all. There are measures to attempt to control illegal fishing, but this is difficult in practice for such a small community to enforce.
New Zealand flax was once also harvested in large quantities on the island, leading to many of the problems associated with deforestation. Students could be asked to think about how this could have been managed in a sustainable way.
Approximately 700 miles north of St Helena Island is Ascension Island, another British Island, and a dependency of St Helena Island. Green turtle populations are monitored here as they migrate from South America to lay their eggs on the beaches of Ascension.
Tourism is very limited on Ascension due to its use as a military base but visits to turtle nesting sites are arranged and visitors are educated on the conservation of these animals and sites are restricted where necessary.
Ascension Island’s bird populations such as the Frigate, and Wide Awake Tern have declined dramatically since the introduction of feral cats to the island in an attempt to control wild rat populations, a method of control that was not properly assessed beforehand.
A wealth of information can be found on the importance of St Helena’s vulnerable ecosystems by clicking on the selection of Resource links, and the St Helena National Trust are very willing to respond to queries.
The fieldwork techniques described in the Curriculum Content section may be applied to many different environments to allow students to grasp the concepts and put them into context, as well as reinforcing the ideology and terminology introduced. Building skills from initially establishing an optimum quadrat size, and then applying sampling techniques within quadrats with the additional use of line, belt, and interrupted belt transects will allow students to gain a better understanding of what they are investigating in terms of population dynamics and enable them to develop the confidence to design appropriate investigative techniques based on prior knowledge.
Such fieldwork also enables students to grasp the meaning of statistical tests that are put into context when used in actual investigations whether to compare species diversity, assess relationships, or determine specific differences between populations of the same species. Many of the techniques have advantages and disadvantages that can be discussed to allow improvements to be made and provide excellent practice in evaluation skills.
There are several websites which may be of use to encourage a better understanding of population dynamics including, the one on the right. This website includes a quick reference to factors determining population size and may be used to justify ideas and field work results.
There are many opportunities for the use of statistical analysis, for example, comparing the diversity of two areas using the Simpson’s Diversity Index (useful in areas undergoing succession, and woodlands), or looking for correlations using Spearman’s rank and Student t test, if for example an investigation was carried out comparing limpet shape (height and width) on exposed versus sheltered shores.
The website introduces the use of Simpson’s Diversity Index for measuring differences in population size and type.
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