EPHM7110 Chap.8 The Big Five Threats to Biodiversity
The Big Five Threats to Biodiversity
Five drivers, and the discipline of not naming all five
The course takes its list of major pressures from the international conservation body that also maintains the Red List and names five: habitat that is lost or degraded; alien species that people introduce and that then spread; wild resources taken faster than they rebuild; pollution together with disease; and a climate altered by human activity.
Learning the list takes a minute.
Using it well is the whole chapter, because almost every real decline involves more than one of them and only some are reversible on a management timescale.
Impact and trend are separate questions
The summary the course draws distinguishes how strongly a driver has acted over the last century from whether its impact is currently decreasing, continuing, increasing or rising very rapidly. Those come apart often.
A driver can have caused enormous historical loss while its current trend is flat, which makes it a poor target for new spending, and another can have caused little loss so far while rising steeply, which makes it the one worth preventing now.
An answer that reports only the historical column will consistently recommend yesterday's priorities.
What survives a clearance is fragmented
Direct loss comes from logging, mining and damming; encroachment from urbanisation, road networks and industrial areas; intensification of agriculture into monoculture removes habitat value from land that is still green, and abandoning practices that favoured wildlife does the same from the other direction.
Whatever remains is fragmented, and fragmentation carries three separate consequences. Edge effects arise from the abrupt transition between adjacent habitats and act along a perimeter that grows while the area does not. Small area interacts with everything about small populations.
Isolation turns a population into a network of patches whose recolonisation depends on whether an individual can cross the gap.
Invasive is a narrower word than introduced
An invasive alien species is a non native species introduced by people that causes harm to native wildlife, threatens human health and safety, or causes economic damage.
Most introduced species do none of those and never establish, so calling every non native species invasive inflates the problem and spends effort on the harmless ones. The stated reason the harmful ones are so damaging is specific: no native predators keep them under control, so the density dependent ceiling is missing.
Introduced predators and introduced pathogens are both in this family and behave differently, because a predator can sometimes be trapped out while a pathogen already in the environment usually cannot.
What this chapter covers
- 01
The five named drivers and where each one bites hardest
- 02
Historical impact against current trend, and why they come apart
- 03
Direct loss, encroachment and agricultural intensification
- 04
Edge effects, small area and isolation as three separate consequences
- 05
Why an invasive species is defined by harm rather than by origin
- 06
Introduced predators against introduced pathogens
- 07
Over exploitation and the harvest record that evidences it
- 08
Climate change as a regional driver, and the local contrasts it cannot explain
Allocate one species decline across the five drivers
- 3Name the drivers the evidence supports and what indicates each.
- 2Name the drivers that should not be asserted here, with reasons.
- 3Choose the measure and justify it by operation and reversibility.
Key terms
- Habitat Fragmentation
- The breaking of continuous habitat into separated pieces, raising edge exposure and isolating what remains.
- Edge Effect
- A change in conditions along the abrupt transition between adjacent habitats, acting on the perimeter of a patch.
- Invasive Alien Species
- A non native species introduced by people that harms native wildlife, threatens human health and safety, or causes economic damage.
- Over Exploitation
- taking wild individuals faster than the population can rebuild itself.
- Introduced Pathogen
- A disease organism carried into a new region by human activity, where resident species have no resistance to it.
- Indicator Species
- A species whose condition is used to track an environmental change affecting a wider system.
The Big Five Threats to Biodiversity FAQ
What is wrong with calling every non native species invasive?
The definition turns on damage, not on origin. Most arrivals fail to establish and most of those that do establish harm nothing, so applying the label everywhere overstates the scale of the problem and, worse, spreads limited eradication effort across organisms that were never going to cost anything.
Reserving the word for arrivals that demonstrably injure wildlife, health or the economy keeps the priority list short enough to act on.
Why is a regional driver a weak explanation for a local contrast?
Because something acting across a whole region acts on all the sites in it. If four streams have lost a species and two neighbouring ones have not, whatever separates them has to differ between them, and a pressure that reaches every stream equally cannot be that separator.
It may still be the largest long term pressure on all six, which is a different claim and needs a different kind of evidence, usually a shifted season or a moving range boundary.
Two fragments hold the same area as one block. What has changed?
The perimeter and the separation. Cutting a block into pieces leaves the amount of habitat unaltered while multiplying the boundary along which conditions from outside penetrate, and it puts distance between the pieces so that animals unable to cross the gap are confined to whichever piece they are in. Both consequences are geometric, which is why the loss can be quantified before any ecology is done.
Exam move
Take one declining species you can find a decent account of and write the five drivers down the page, then force yourself to write the specific evidence each one would require beside it. Cross out every driver for which that evidence is missing. What remains is your answer, and its shortness is the point.
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