Over-harvesting & Introducing Non-native Species (Cambridge O Level Biology)

• A result of increased demand for food and other natural resources is the tendency to over-harvest animals and plants
• Animals can be over-harvested in the following ways:
  • Hunting of wild animals for sport, food or fur to the point of near extinction or complete extinction
  • The exploitation of animal populations for their body parts eg. ivory from elephants' tusks
  • Over-fishing
• Crops can be over-harvested which can reduce the mineral content of the soil to critical levels
  • This usually arises from over-intensive farming practices
• All kinds of over-harvesting can increase pollution and reduce biodiversity

• Making food production more intensive means producing food more efficiently with a finite amount of land and other resources
• Modern technology has increased food supply substantially in the following ways:
  • Agricultural machinery has replaced humans and improved efficiency due to the ability to farm much larger areas of land
  • Chemical fertilisers improve yields - fertilisers increase the amount of nutrients in the soil for plants, meaning that they can grow larger and produce more fruit
  • Insecticides and herbicides - these chemicals kill off unwanted insects and weed species, meaning that there is less damage done to plants and fruit lost to insects (insecticides), as well as reducing competition from other plant species (herbicides)
  • Selective breeding - animals and crop plants which produce a large yield are selectively bred to produce breeds that reliably produce high yields

Examples of Intensive Farming Diagram

Modern agricultural processes allow for cultivation of much larger areas of land for crop plants

• Monoculture farming means that on a given area of agricultural land only one type of crop is grown (eg trees for palm oil grown in Indonesian rainforest)
• This large scale growth of a single variety of plant does not happen naturally in ecosystems, where there are usually many different species of plants growing which, in turn, support many species of animals (high biodiversity)
• In monocultures, biodiversity is much lower
• Another issue with monocultures is the increase in pest populations – if a particular pest feeds on a crop, farming it in large areas repeatedly means there is an ample supply of food for the pest, causing the population to increase
• Often farmers will spray insecticides onto crops in order to control the pests. This leads to:
  • harmless insects being killed as well
  • pollution by pesticides (which are often persistent chemicals which accumulate in food chains)
  • in many instances where they are used repeatedly for specific pests, the pests may eventually become resistant to them, reducing their effectiveness

 Palm oil production has increased rapidly over the last 30 years

• A species that has moved into an ecosystem where it was previously unknown is an invasive species
• An invasive species can occur naturally as a result of a species migrating or expanding their habitat but most recorded incidents of invasive species have been caused by humans
• In the past humans have:
  • Knowingly collected and traded species between countries via ships
  • Unknowingly provided transport for non-native species to a new ecosystem
  • Introduced non-native species deliberately as a biological control for pests

• • Several natural population controls exist for Japanese knotweed in its natural habitat in Japan
  • The irregular climate and the deposits of volcanic ash over the ground limit its growth
  • A German botanist brought the plant to the UK in the 19th century because he admired its beauty
  • Because the UK does not possess the same environmental factors the plant was able to grow unchecked
  • Since the 1800s it has spread across the UK and become a major problem
  • It grows at a rapid rate, with its strong roots breaking up tarmac and blocking out all sunlight for the native plant speciesJapanese knotweed (Reynoutria japonica) is the UK’s most invasive non-native plant species

Problems with invasive non-native species

• The biological process of evolution often brings balance to an ecosystem
• Through evolution, the environment a species lives in strongly influences the adaptations that the species evolve to live in that environment
• A non-native invasive species will have evolved adaptations for survival in different environmental conditions so when they are introduced into the new ecosystem this can upset the balance
• In a new ecosystem invasive species will have few or none of the natural population controls that existed in their previous ecosystem:
  • They will have no natural predators or competitors

• As a result, they can increase in number at a rapid rate
• This can affect the processes within an ecosystem
  • Competition may occur between invasive species and native species that occupy a similar niche with the native species getting displaced or pushed to extinction. It could be competition for things such as prey, soil nutrients, light and space
  • Many invasive species can be over-successful predators causing a massive decline in their prey species
  • Invasive species can introduce new diseases, to which the native species have no natural immunity
  • The biodiversity of an ecosystem is impacted which reduces its productivity

• Humans can also feel the knock-on effects of an invasive species taking over an ecosystem
  • The spread of novel diseases and irritants of the skin/breathing system directly affects human health
  • The economy of a country can be severely impacted by the costs of trying to control invasive species and their negative effects
  • In the past travel has been brought to a standstill by invasive species, with some plant species prone to blocking up waterways

The Cane Toad in Australia

• In the early 1900s, there was a major problem with the sugarcane crop in Australia
• An insect pest was destroying the crop and causing major economic losses for many farmers
• It was decided that the non-native cane toad (Bufo marinus, from Hawaii) should be introduced so that it could act as a biological control
• After a short time, the numbers of cane toads increased rapidly due to a lack of natural predators and they spread into other habitats outside of the sugarcane plantations
  • This had a knock-on effect on other species:
  • The cane toad is toxic when eaten
  • The northern quoll, which is an endangered marsupial carnivore, declined steeply in numbers as they preyed on the cane toad
  • Other amphibian species face increased competition for food and resources
  • The eggs of ground-nesting birds are often eaten by cane toads

The cane toad (Bufo marinus)

Charles J. Sharp, CC BY-SA 4.0, via Wikimedia Commons