IB Biology SLProtecting BiodiversityPaper 1 & 2Core idea~10 min read
Biodiversity
“Biodiversity” sounds like it just means “lots of animals”. It does not. It is a measurement, and it works on three separate levels — habitats, species, and the alleles inside those species. A field of wheat is full of life and has almost no biodiversity at all.
📚 What you need to know
Biodiversity is the variety of life in a specified area. That area can be a pond or the whole planet.
It is studied at three levels: ecosystem (habitat), species and genetic diversity.
Species richness = how many species. Species evenness = how evenly the individuals are spread between them. You need both to be high.
High biodiversity makes an ecosystem more stable and resilient to change.
Around 8.7 million species are thought to exist; only about 1.2 million have been described so far.
Global biodiversity rises when speciation outpaces extinction. It is higher now than at any point in the past.
Classification is pattern recognition, so it is partly subjective — “lumpers” group species together, “splitters” separate them.
The three levels
Any question about biodiversity is really asking about one of three things. Get clear on which, and the rest of the topic falls into place.
Read each panel as a question. How many habitat types? How many species? How many versions of each gene? All three matter, and they are measured separately.
Ecosystem (habitat) diversity
This is the range of different habitats within an area. A coral reef scores highly because it is full of microhabitats — crevices, overhangs, sand patches, different depths — each of which supports different organisms with different niches.
A large sandy desert is the opposite. Conditions are much the same for hundreds of kilometres, so there are very few habitat types and biodiversity is low.
Species diversity
This is the one students get wrong most often, because it is made of two separate measurements.
Both are neededRichness = how many species | Evenness = how evenly individuals are shared between them
The top-right plot is the trap. It contains four species, so its richness is high, but almost every individual belongs to one of them. That is low evenness, and it is not a diverse community.
If an exam gives you a table of species counts, do not just count the rows. Look down the numbers column. If one species has 900 individuals and the other four have three each, say “high richness but low evenness” — that phrase is usually the mark.
Genetic diversity
This is the number of different alleles present, either across a whole species or within one local population. Two things build it up:
the proportion of genes that have more than one allele, and
how many different alleles each of those genes has.
Genetic diversity also exists between populations of the same species. Two populations living in different places face slightly different selection pressures, so their allele frequencies drift apart — which is exactly where speciation starts.
Why it matters: genetic diversity is a population’s supply of raw material. If the environment changes, a population with many alleles has a decent chance that some individuals already cope. A population with almost identical members has nothing for selection to work on.
Biodiversity and resilience
The more biodiverse an ecosystem is, the more stable and resistant to change it tends to be. The reason is straightforward: with many species and many alleles, one bad event rarely removes everything at once.
🤔 Why low diversity is fragile
Picture a forest dominated by just one or two tree species. A new pathogen arrives that attacks one of them. Because so much of the forest is that one species, a huge fraction of the trees die at the same time — and everything that depended on them, from insects to birds to fungi, goes with it. The whole ecosystem can collapse from a single event. In a forest with fifty tree species, the same pathogen removes a slice and the rest keeps functioning.
How many species are there?
Nobody knows exactly, and that is a genuinely interesting problem.
Current estimates suggest roughly 8.7 million species on Earth.
Only about 1.2 million have actually been recorded and classified.
If the estimate is right, the large majority are still undiscovered.
The number has never been fixed. The fossil record shows huge numbers of species that no longer exist, lost to extinction. It also shows that there are far more species alive today than at most points in the past, because speciation has been running faster than extinction over the long term.
The balance
Speciation rate > extinction rate → global biodiversity rises
NOS: classification is pattern recognition
Here is why “how many species” is such a slippery number. Counting species requires deciding where one species ends and the next begins — and that decision is a judgement, not a measurement.
If species are classified correctly, the estimate is accurate.
If two genuinely separate species are lumped together, the estimate comes out too small.
If one species is wrongly split in two, the estimate comes out too large.
Taxonomists tend to fall into two camps. Lumpers pay more attention to similarities and group organisms together. Splitters pay more attention to differences and separate them. The same specimens, examined by both, can produce different species counts — which is a nice example of scientists using pattern recognition rather than reading off a fact.
Worked examples
WORKED EXAMPLE
Plot A contains 6 species with 50 individuals of each. Plot B contains 9 species: one with 290 individuals and eight with 5 individuals each. Compare the species diversity of the two plots. [3]
Step 1: compare richnessA = 6 species, B = 9 speciesPlot B has the higher species richness.Step 2: compare evennessA: 300 individuals shared equally — 50 each. B: 330 total, but 290 in one species (88%)Plot A has far higher evenness; Plot B is dominated by one species.Step 3: judge overall diversityPlot A has the higher species diversity, because diversity needs high richness and high evennessDo not stop at “B has more species”. That is only half the answer.
WORKED EXAMPLE
Explain why an ecosystem with high biodiversity is more likely to survive a new plant disease than one with low biodiversity. [3]
Step 1: what high biodiversity means hereMany species are present, and each has many alleles, so individuals are not all the same.Step 2: what the disease doesThe pathogen only attacks certain species or certain genotypes, so it cannot wipe out everything at once.Step 3: the consequenceSurviving species continue their roles, so the ecosystem stays stable and can recover — it is more resilientIn a low-diversity ecosystem one pathogen can remove the dominant species and the whole system collapses.
WORKED EXAMPLE
Suggest why estimates of the total number of species on Earth vary so widely. [3]
Point 1: most species are unrecordedOnly around 1.2 million of an estimated 8.7 million have been described, so the total is extrapolated rather than counted.Point 2: classification is subjectiveDeciding where one species ends and another begins is a judgement, so lumpers and splitters reach different totals.Point 3: sampling is unevenSome habitats, such as deep ocean and soil, are far harder to survey than others
💡 Exam tip
Learn the three levels as a list: ecosystem, species, genetic. Questions often ask you to name them.
For species diversity, always mention both richness and evenness, even if the data only obviously shows one.
Link high biodiversity to stability and resilience, and say why: no single event removes everything.
Quote the figures if you can: about 8.7 million estimated, about 1.2 million described.
For NOS questions, use the words subjective, lumpers and splitters, and say how each affects the estimate.
“Biodiversity of a specified area” — the area always has to be defined. Say which area you mean.
⚠ Common mix-up
Treating richness alone as diversity. 1000 species with 995 of them down to a handful of individuals is not diverse.
Confusing evenness with abundance. Evenness is about how balanced the numbers are, not how many organisms there are in total.
Thinking a green landscape means high biodiversity. A plantation or a lawn is green and almost monoculture.
Mixing up genetic diversity and species diversity. Genetic diversity is alleles within a species.
Saying biodiversity has always been falling. Over geological time it has risen, because speciation outpaced extinction. The problem is the current rate of loss.
Assuming the species count is a hard fact. It depends on how species are classified, which is partly a judgement call.
Up next: Causes of Extinction — what actually pushes a species over the edge, and the two case studies you are expected to know.
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