Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Biodiversity within a community

Biodiversity is obviously biological diversity - the variety of life.

In order to live, organisms need a number of things:

Green plants need water and minerals from soil, carbon dioxide and oxygen from air, light, and a suitable temperature.

Animals need some of the same things, but more than anything they need plants; herbivores use them for food, and by proxy so do carnivores. And they mostly need oxygen, which is a waste product fom plants. Plants also provide shelter for animals. But animals can have negative impact on plants, as well as on other animals.

All living organisms also need a place - a part of the environment - to provide all of these.

Individual organisms live in their own habitat, alongside other individuals of the same type - the same species - forming groups called populations.

Other species have similar requirements, and they share some of the same area. There is competition for resources within and between the different populations, which collectively make up a community.

One animal species has much more impact on other species and the environment than any other - and much of that impact is negative.

Hunting, habitat loss and climate change are the main factors affecting biodiversity these days.

Five mass extinctions are recognised in the distant past. Most are thought to have been caused by rapid climate change.

Biodiversity is a topic that is often in the news these days - but this is mostly due to
reduction in numbers within animal populations, and extinction of species.
Speciation - the formation of new species from existing populations - is rather a rare event.

The slippery slope to extinction

The IUCN Red List uses these Categories and Criteria for classifying species at high risk of global extinction.
Least Concern,
Near Threatened,
Critically Endangered,
Extinct in the Wild
and Extinct.

Friends of Biodiversity

redlist (2K) The International Union for Conservation of Nature's Red List of Threatened Species "has evolved to become the world's most comprehensive information source on the global conservation status of animal, fungi and plant species.
The IUCN Red List is a critical indicator of the health of the world's biodiversity."
The most recent report states that more than 26,000 species are threatened with extinction - more than 27% of all assessed species.
WWF (2K)
The World Wildlife Fund (WWF) has a "mission to conserve nature and reduce the most pressing threats to the diversity of life on Earth".
And the WWF-UK's mission is "to create a world where people and wildlife can thrive together".

CFBlogo (6K) The Center for Biological Diversity is another organisation with a campaigning attitude and some snappy slogans:
"Our Mission: Saving Life on Earth"
"Because diversity has intrinsic value, and because its loss impoverishes society, we work to secure a future for all species, great and small, hovering on the brink of extinction."
I particularly like their free endangered species ringtones.

Enemies of biodiversity


Humans kill animals for a number of reasons. When populations of food species are expected to recover, this is described as sustainable.
However animals get killed on a large scale for other reasons which cannot be justified except greed.
elephants (12K)

The African Elephant Loxodonta africana is currently categorised as Vulnerable.
Africa's total elephant population was around 415,000 in 2016, a decline of around 111,000 over the previous decade. This dramatic drop in numbers was almost entirely due to poaching to provide ivory.


Monoculture is growing one species of plant in an area, to the exclusion of other plants and the animals which would be at home there.

Tropical-Rainforest (389K)
palm-oil-plantation (58K)

Large areas of tropical rain forest which are havens of biodiversity have been cleared to make space to grow oil palms. Palm oil plantations in Borneo and Sumatra have been blamed for destruction of forests in which Orangutans live.
Biologists study living organisms, and need to summarise how populations and communities are interacting.

Species richness

Within an area, it is possible to identify different organisms (species) and count or at least estimate their numbers. In fact the word population is generally taken to mean not just the species but also their number, whether studying a small area or the whole planet Earth.

The term species richness is used as a measure of the number of different species in a community.

A long list of species implies that the community is well balanced and 'healthy' so no species is having an adverse effect on others, even though some species feed on others!

Wildfowl counts

There are a number of organisations that carry out counting of birds and other species.

I am quite impressed by the Montrose Basin Local Nature Reserve (link below) in the North East coast of Scotland which is an interesting point in the migration of birds from the North to the South and back again.

There are monthly counts of wildfowl and waders on the Basin and the results give an interesting picture of migration and population dynamics. Over the years they have built up quite a long species list - 213 species. The numbers are often quite impressive, too. 75,000 Pink Footed Geese were counted on 5th Oct 2018.

The British Trust for Ornithology is "Looking out for birds" but they seem to charge for data about counts of birds in wetlands..

Species abundance

Of course some organisms will not be so easy to see or count; there will be fewer secondary and tertiary consumers than primary consumers - fewer carnivores than herbivores. The observed number of individuals of a species is called their abundance.

Some animals can be counted directly. Birds may be counted at the garden bird-table or perhaps at a lake using a hide. The population of other animals may also be estimated using a variety of techniques.

The top 10 garden birds

Big Garden Birdwatch 2018
United Kingdom results - mouseover for pictures (and scientific names)
This year, you saw:
1 House sparrow
2 Starling
3 ↑Blue tit
4 ↓Blackbird
5 Woodpigeon
6 Goldfinch
7 ↑Great tit
8 ↓Robin
9 ↑Long-tailed tit
10 ↓Chaffinch

Arrows indicate up or down from last year

Index of diversity

The term species diversity takes into account both species richness and the abundance of those species.

An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species, and it can be used to describe or compare communities in different areas.

There are several different versions - different indices of diversity. The most common one is calculated from the expression:


where N = total number of organisms of all the species
and n = total number of organisms of each species
and Σn(n-1) means the sum of each value for n(n-1) for each species.

This mathematical method gives different emphasis to some species than others. Notably species that only show up once are ignored by the subtraction of 1 in the n-1 term.

A population with minimum diversity has a diversity index of 1; a value of greater than 1 shows more diversity, and this can be used in comparisons between different areas, or 'snapshots' of the same area at different times.

Some example calculations

The first three of these all total 80.
Even abundance distribution of 8 species
Diversity_indexall10 (17K) 80x79/720 gives 8.78 - a reasonably high value
8 species but abundance not so even
Diversity_index1to24 (17K) In this example two species are cancelled out by the n-1 term, so the value is lower (4.77).

Only one species
Diversity_indexjustone80 (8K) In this case there is minimum diversity and n=N so the index turns out as a fraction of 1/1
An actual example from an exam
Diversity_index_2015 (19K) Just 2 marks for this one - or 1 for either numerator (24180 or 156 x 155) or denominator (9014)

Astonishingly there are other indices of diversity, but they mostly take the same data and process it differently.
Bart_Simpson_200px (18K)
Simpson's index of diversity D is given by:
SimpsonIndex (1K)

D can have a value between 0 and 1.
0 signifies no diversity, and 1 signifies infinite diversity.

One swallow does not make a summer!

Twitchers twitching twitchers (125K) This bunch of enthusiasts are zooming in on an Eastern Crowned Warbler seen in the UK for the first time
Benny the Beluga Benny the Beluga (61K) Beluga whales belong in Arctic waters.
I don't think they will start a colony in the Thames.

Even though twitchers (bird watchers) will go to great lengths to spot a single bird which is unexpected outside its usual habitat, such sightings underline the insignificance of the event, as it generally takes two such organisms to reproduce and establish a colony.

Same goes for whales, I think.

Neither of these would contribute to a Diversity Index!

Biodiversity is reduced by farming techniques

What we see as farmland today used to be "natural" i.e a mixture of species of plants - notably trees and a variety of plants of all sizes, and native animals. These have been replaced with various types of grassland used as pasture for raising stock, and just a few crops: wheat and other cereals, sweetcorn, oilseed rape (Canola), potatoes and beans are the main types. There are just a few types of animals: mainly cows, sheep, and pigs which are kept as livestock, and their stocking density (number of animals per unit area) is quite high. The biomass of their populations greatly exceeds the corresponding biomass of wild animals which would have originally inhabited the area.

Many agricultural practices have the effect of reducing competition between natural plant species and crops.
Ploughing suppresses the growth of perennial species of plants as the soil is turned over to expose fresh soil ready for the planting of seed of crop plants. Rolling and harrowing fields may have similar effects.
Combining several fields into one large one for more efficient access by farm machinery has led to the grubbing out (removal) of hedges together with the loss of ditches, and small tree-lined ponds in fields sometimes used by cattle have similarly been filled in.

Crop spraying with pesticides:
insecticides to kill insect pests,
fungicides to kill fungi which infect crops and reduce yields, and
herbicides to kill competing weeds
can all have consequences apart from the desired 'clean kill'.

Insecticides can also have the unwanted consequence of killing bees and other insects required for pollination of fruit crops, as well as reducing numbers of butterflies and moths, not all of which are pests.
Insecticides and fungicides can build up on and in crops and these residues can have unwanted effects on consumers.

Many practices which today are seen as undesirable on a global scale - wholescale deforestation of large areas to remove trees for timber and 'slash and burn' practices to allow wild areas to be cultivated by Man - have been carried out in the fairly distant past in Europe and the land thus became used for agricultural purposes.

And of course the rising human population has placed pressure on farmers to provide consistent supplies of food ....

The balance between farming and conservation

Although for economic reasons farmers need to allocate most of the area of their farmland to the production of crops and livestock, there is a possible balance to be achieved between monoculture - the growing of single species - and partial reversion to wild species. Uncultivated strips of land alongside hedges give shelter to and provide a habitat for (small) animal species, and undisturbed plant species provide seeds for birdlife.

Delayed harvesting of crops may have beneficial effects: hay (sun-dried grass) may be cut later allowing wild flowers to spread their seeds. This may also benefit ground-nesting birds, giving them time for their young to develop to independence.

Meadows are low-lying land adjacent to rivers. These may be subject to seasonal flooding, but they support a variety of native species, often well-liked "wild flowers". Cultivating this land, and re-planting with seeds of grass for pasture purposes, will reduce the number of species and hence the biodiversity.

On a global scale, the conversion of large areas of wild land to cultivated land for growing crops causes habitat loss for animal species with the consequent reduction in populations. Some animals migrate over large areas and there have been a number of initiatives to limit the destruction of natural biomes leaving migration corridors for wildlife.
woodland (174K) Mixed woodland
wheatfield (221K) A wheatfield - with a little biodiversity in the distance

Deforestation within Brazil to release land for the cultivation of soybeans is a contentious issue - especially as the soybeans are mainly used to provide cheap, high protein food for animals.

Other related topics on this site

(also accessible from the drop-down menu above)

Similar level

Species and taxonomy - Putting a (scientific) name to to a species, and seeing how it relates to others
Evolution processes - The basic stages of Evolution - which results in new species - increasing Biodiversity
Allopatric speciation - Evolution of new species based on geographical isolation
Sympatric speciation - Evolution of new species based on reproductive isolation, not geographical isolation
Populations in ecosystems - Ecological terms, estimating population size using quadrats and transects, Mark-release-recapture method, and Succession, with examples from Surtsey and Sand dune succession

Web references

What is biodiversity and why does it matter to us?
The Guardian warns us that the number of animals living on the Earth has plunged by half since 1970, and the number of tigers has plunged by 97% in the last century.

The IUCN Red List of Threatened Species.

Species Directory - from the World Wildlife Fund

Big Garden Birdwatch - 2018 results
Over the UK, 420,489 of you counted a staggering 6,764,475 birds. Once again, the noisy and gregarious house sparrow took the top spot, with the starling in second place. See this web page for the top 10 birds in the UK.

Welcome to the Wildlife of Montrose Basin - Check out the bird species lists

A Condition in Indonesian Tropical Rainforest - An oddly worded plea for reduction in deforestation

The Effects of Palm Oil - How does Palm Oil Harm Orangutans and other wildlife?

Daily Mail online - Hundreds of twitchers descend on quarry after rare Asian bird never before seen in the UK flaps into view

The big five mass extinctions from Cosmos - a quarterly science magazine aiming to inspire curiosity in 'The Science of Everything'

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