Threats to biodiversity (i) Overexploitation

Suggested learning outcomes for content delivered in this sub-section Describe the exploitation and recovery of whale or fish populations. Investigate the reductions of populations and if species can recover to previous levels. Explain the effect of small populations on evolutionary responses to environmental changes (the bottleneck effect). Analyse the rates of reproduction within small populations and examine the effects of inbreeding. Investigate the relationship between genetic diversity and viable populations.

Threats to biodiversity (ii) Habitat loss

Measure/describe species richness data from habitat fragments within the Caledonian Forest or for tiger populations. Describe the processes that cause habitat fragmentation. Investigate the impact of degradation of habitat fragments on interior species. Explain the use of habitat corridors for recolonising habitats. Suggested learning outcomes for content delivered in this sub-section

Key terms Overexploitation and habitat loss

Genetic variation o Variation in alleles of genes. o Occurs both within and among populations. o Genetic variation is important because it provides the raw material for natural selection.

Genetic diversity Measure of genetic differences within and between individuals, populations and species. The variety of genetic material within a single species of organism that permits the organism to adapt to changes in the environment.

The bottleneck effect o A population bottleneck (or genetic bottleneck) is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing.

Habitat fragmentation o The separation of an organisms preferred habitat. o Habitat fragmentation can be caused by geological processes that slowly alter the layout of the physical environment or by human activity.

Habitat fragmentation o Pathways of natural habitat occur within larger areas that have been fragmented. o They attract species and act as safe passages for species between neighbouring habitats.

Reduction of population numbers: can species come back from the brink? American bald eagle removed from the endangered species list in 2007.

The bottleneck effect Loss of genetic variation means that the population may not be able to adapt to environmental changes or pressures, such as climate change or a loss of available resources. The genetic variation needed for natural selection will have drifted out of the population, which could result in extinction. If there is enough genetic variation then the species can still recover but will lack genetic diversity, making it susceptible to disease. Recovery of species Extinction of species Bottleneck event Time Population number

Northern elephant seals An example of a population bottleneck

Northern elephant seals have reduced genetic variation, probably due to a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since risen to over 30,000, but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. An example of a population bottleneck

Investigating habitat fragmentation