ka8: threats to biodiversity

• With overexploitation, populations can be reduced to a low level but may still recover. Some species have a naturally low genetic diversity in their population and yet remain viable
• The bottleneck effect — small populations may lose the genetic variation necessary to enable evolutionary responses to environmental change.
• In small populations, this loss of genetic diversity can be critical for many species, as inbreeding can result in poor reproductive rates
• The clearing of habitats has led to habitat fragmentation. Degradation of the edges of habitat fragments results in increased competition between species as the fragment becomes smaller. This may result in a decrease in biodiversity
• More isolated fragments and smaller fragments exhibit a lower species diversity
• To remedy widespread habitat fragmentation, isolated fragments can be linked with habitat corridors.
• The corridors allow movement of animals between fragments, increasing access to food and choice of mate. This may lead to recolonisation of small fragments after local extinctions
• Introduced (non-native) species are those that humans have moved either intentionally or accidentally to new geographic locations
• Those that become established within wild communities are termed naturalised species
• Invasive species are naturalised species that spread rapidly and eliminate native species, therefore reducing species diversity. Invasive species may well be free of the predators, parasites, pathogens and competitors that limit their population in their native habitat. Invasive species may prey on native species, out-compete them for resources or hybridise with them
• In Biology ‘exploiting’ a natural resource means to make the best use of it. Humans have been overexploiting species for hundreds of years by removing and using up individuals at a rate that exceeds the species maximum rate of reproduction
• When overexploited, populations can be reduced to a low level but may still recover.  If overexploitation continues at a sustained rate for a long period of time it can lead to the extinction of the species.
• A common example of exploitation is overfishing. In the past fish stocks have been depleted without thought for the ability of stocks to replenish themselves.  In the UK, strict EU quotas are now enforced to prevent overfishing. Since quotas have been introduced some fish populations have shown signs of recovery.
• This is a measure of genetic differences within and between individuals, populations and species.  The variety of genetic material within a single species permits the organism to adapt to changes in the environment.
• 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.
• if species are overexploited or suffer a drastic decrease in population from a natural disaster e.g. a flood, their population can decrease to a critical level.

  Small populations may lose the genetic variation necessary to enable evolutionary responses to environmental change.
• This loss in genetic variation leads to inbreeding which in turn results in poor reproductive rates. It could result in the extinction of the species.
• If there is enough genetic variation then the species can still recover but will lack genetic diversity, making it susceptible to disease.
• Some species do have naturally low genetic diversity in their population and yet remain viable, e.g. elephant seals.

  
• The clearing of habitats has led to habitat fragmentation. The process of fragmentation of a habitat results in the formation of several habitat fragments whose total surface area is less than that of the original habitat. Therefore habitat is being lost.
• Degradation of the edges of habitat fragments results in increased competition between species as the fragment becomes smaller.
• More isolated fragments and smaller fragments exhibit a lower species diversity. If the numbers of species increase they are forced to invade the interior habitat at the expense of the interior species, this results in a loss of biodiversity. 
• One possible solution of habitat fragmentation is to link isolated fragments with habitat corridors.  A habitat corridor is composed of a narrow strip or series of clumps of quality habitat by which species can move between disconnected fragments of habitat.
• The corridors allow movement of animals between fragments increasing access to food and choice of mates.  This may lead to recolonisation of small fragments after local extinctions.
• Introduced species are organisms that have been moved by humans to a new geographic location.  This can happen accidently, e.g. shipping, tourism, or on purpose e.g. pet trade.
• Over time introduced species become established in the wild community.  They are now called naturalised species.

   
• Some naturalised species become invasive species.  This happens when a species spread rapidly in the wild community and eliminate native species.

  
• Commonly these species lack competitors, pathogens, parasites or predators from their native ecosystem.  This allows them to spread rapidly and become invasive.  They may also prey upon or outcompete native members of the community for resources.
• Only a minority of introduced species have become invasive.  Unfortunately invasive species have a negative impact on native communities and the economy.
• 1. Economy

  Japanese knotweed was introduced 200 years ago and has become an invasive species.

  It is able to grow through hard structures, e.g. foundations of buildings and car parks, and subsequently vast sums of money are spent trying to eradicate it.
• 1. Loss of Biodiversity

  The grey squirrel was introduced from America and has become an invasive species.

  The grey squirrel outcompetes the native red squirrel for resources.  The grey squirrel also carries a virus called squirrel pox that only affects the red squirrel.

   The native red squirrel is rapidly facing extinction.