17 Threats to Biodiversity

Prof. Daizy Batish

Learning Objectives:

• Statistics of biodiversity loss
• What types of species is most vulnerable to extinction?
• Major threats to the biodiversity

Introduction

Variability in the species composition of plants, animals and microorganisms on earth is imperative for the sustenance of healthy, balanced and thriving ecosystems. However, growth of human population and rising consumption levels are disturbing the biodiversity components, ecosystem integrity and sustainable use of ecosystem services. Complex interactions of various demographic, socio-economic, cultural and technological factors have created a chain of unnatural anthropogenic activities ending up with irreversible loss of various plant and animal species. The major drivers of biodiversity loss such as habitat degradation and fragmentation, invasion by alien species, enhanced population, pollution, and exploitation levels and anthropogenic climate change are being discussed in this chapter.

STATISTICS OF BIODIVERSITY LOSS

A WWF report suggests that 52% of the world’s biodiversity (including terrestrial, freshwater and marine diversity) has been lost in a 40 year span between 1970 and 2010, freshwater diversity being affected most. IUCN has stated that 75% of the genetic diversity of agricultural crops, 75% of the fisheries stock and one-third of the reef building corals around the globe are either threatened or lost. 42% of the amphibians, 25% of mammals and 19% of the reptile species among the faunal species whereas 71% of magnolias, 63% of cycads and 34% of conifers among the floral species are categorized as threatened with extinction. Other estimates suggest that current rate of species extinction is estimated to be as rapid as 1,000 and 10,000 times as compared to the average rate during last few billion years. In fact, the sixth major extinction would be the greatest event in past 65 million years (since the disappearance of dinosaurs). Some studies also suggest that within next two to three hundred years, half of the mammal and bird species worldwide may become extinct. However quantifying the rate of extinction is difficult as the total biodiversity on earth is yet to be discovered and the biodiversity monitoring is insufficient.

Figure 1 The direct and indirect drivers resulting in biodiversity loss (Source: Global Biodiversity Outlook 2 [www.cbd.int/gbo2/])

WHAT TYPES OF SPECIES IS MOST VULNERABLE TO EXTINCTION?

• Rare species (as listed by IUCN)
• Species with restricted geographical range (such as the organisms dwelling on the island ecosystems)
• Species occurring at the sites of high human disturbance/ exploitation Species with large/ specialized/ stable habitat requirements
• Species with slow reproductive rate
• Plants with ineffective dispersal ability Species being heavily predated
• Seasonal migrants
• Species with lesser genetic variability
• Species with no prior contact with humans
• Species closely related to the extinct or threatened species

MAJOR THREATS TO THE BIODIVERSITY

a) Habitat loss/ degradation/ fragmentation

Any sort of change incurred by natural or anthropogenic activities in the local environmental conditions may pose a threat to the plant/ animal/ microbial diversity sustaining there. In fact the habitat degradation and fragmentation, caused by human-induced land use changes is the main factor driving biodiversity loss. Human interference in the natural self-sustaining ecosystems have shrunk the habitats of different plant and animal species and limited the supply of resources essential for their survival. Activities such as urbanization, expansion of agricultural lands, industrialization, deforestation, mining, damming and dredging of rivers, and draining of wetlands, estuaries and mangroves are majorly responsible for turning a habitat unsuitable for the local species.

Terrestrial ecosystems

Deforestation is one of the direct causes of species extinction in the terrestrial ecosystems. Forest ecosystems bears a major portion of the world’s terrestrial biodiversity, provides innumerable ecosystem services and play a critical role in the operation of global bio-geochemical cycles. However, forest lands are being cleared up for the expansion of agricultural fields, residential areas, and industries and for obtaining fuel, fodder, furniture, raw material for paper and pulp industry, medicinal plants, and precious herbs, thereby destroying the most dependable ecosystems on earth. Tropical rainforests are known to withhold the highest percentage of the world’s flora and fauna. However, these biomes have been highly disturbed as a result of massive destruction of forests and its conversion into other land-uses. Habitat fragmentation (splitting of large units of habitat into smaller parts) in the Amazon rainforests has resulted in the significant biodiversity loss. If the next thirty years would experience a similar deforestation rate as now, 5 to 10% of the species would be at the verge of extinction due to habitat loss. This indicates the strong need of sustainable forest practices. But in the view of increasing demands that are estimated to be doubled over next fifty years, it is difficult to control the rate of destruction of forests.

Desertification is another reason of habitat loss, driven by the similar reasons as that of deforestation such as extension of agricultural processes, urbanization, industrialization and overexploitation. Poor land management, further provoked by global warming and climatic change has degraded the organic content, fertility, and water retaining abilities of the soil converting them into deserts. The 25-40 % decline in soil organic content has been observed over past 25 years. The desertification not only makes a land unsuitable for human usage but also leave the local plant and animal species homeless.

Marine ecosystems

Occupying the 70% of earth’s total surface, oceans are considered as the largest ecosystems regulating the global climate, bio-geochemical cycles, a high percentage of world’s food production and various other economic aspects. Constant degradation of coastal and marine environment due to rising fishery trade, overexploitation of resources, rising pollution, poor near-shore land use practices, discharge of industrial wastes etc. have reduced the fisheries production and threatened the aquatic biodiversity. On the other hand, coral reefs, another important and diverse ecosystem, are experiencing a high level of exploitation and pollution.

The rising levels of water in the oceans have shrunk island ecosystems, disturbing the flora and fauna sustaining there.

b) Invasive species

When any organism is migrated to a new geographical area, there is a chance that the alien species may become invasive and can affect the native species in several different ways. It can parasitize or predate upon resident species, compete for the resources, bring unfamiliar diseases, modify habitats and ecological conditions, disrupt the existing food web, hinder the environmental interactions and threaten endemic species. The invasive alien species can directly replace the native species through completion, predation or allelopathy or it may transform the structure and composition of ecosystem, thereby indirectly repressing or excluding the native species. Recently, the rate and risk associated with the introduction of alien species have escalated significantly because of the rising human population, increased travel, trade and tourism, climate change, and habitat fragmentation.

The invasive alien species (IAS) are considered as the second leading cause of biodiversity loss after habitat destruction and the major cause of species extinction in most island ecosystems. The invasive species may be introduced intentionally by humans for hybridization/ cultivation/ ornamental purposes or it may get migrated accidently along with the goods being exchanged between the countries or through the people travelling across the world or via long distance dispersal as in case of plants. In marine ecosystems, the introduction of alien species can result from hull fouling, ballast water from ships or through aquaculture and aquarium releases. Such introductions are as old as the history of human migration to different parts of the world, but in the recent past, particularly since the 17th century, introduction of invasive alien species have become more and more frequent and resulted in nearly 40% of all the animal extinctions. Similarly, nearly 300 species of crustaceans, molluscs and fishes have entered the Mediterranean Sea from the Red Sea through the Suez Canal since 1891 that have now dominated over the native species. In the Fynbos biome of South Africa, 80% of the threatened species are endangered because of invading alien species. Many countries have established systems to prevent and control the migration of alien species, to predict the likelihood of alien species becoming invasive and the potential ecological and economic cost they may incur. To effectively combat the challenges posed by invasive alien species, there is a need for global networking and communication at different levels.

c) Pollution

Different pollutants entering our environment via release of excessive amounts of nitrates/ phosphates from agricultural run-off, emission of green house gases through deforestation and burning of fossil fuels, dumping of sewage into the water bodies etc. has severely influenced the natural ecosystems of Earth. Pollution may not be directly responsible for extinction, but it disrupts the habitat conditions of an organism. Persistent organic pollutants, drugs, fertilizers/ herbicides/ pesticides, acid rain, heavy metals, and plastics etc. can cause environmental toxicity and since it is difficult for the organisms to reside in the modified surroundings, they are either forced to migrate or adjust. As a result, the species that could not cope up with the changing environment generally dies out.

For example acid rain can alter the pH of the soil and water bodies thereby, affecting the feeding options, breeding conditions and species composition of the locale. Poisonous gases present in the atmosphere may cause allergies, respiratory problems and fatal diseases in many species. Nitrates and phosphates released from the fertilizers run off from the fields into the water system and can cause non-habitable zones for the freshwater animals and plants known as “dead zones”. Plastics, oil and waste disposed in the rivers, lakes and oceans make it difficult for the fishes and sea birds to survive. Similarly, bioaccumulation is another example in which the top carnivores suffer high toxicity due to the multiplication of heavy metal concentrations through the food chain as seen in Killer whales.

d) Human population and overexploitation

With an exponential increase in human population around the globe in last few decades, the pressures on biodiversity have significantly enhanced. Increasing population has resulted in excess demand of resources generally accomplished by the unhealthy practices such as poaching, targeted hunting, overfishing and over-harvesting. This has led to the killing of numerous species over the past few decades, resulting in great loss of biodiversity. Fisheries have been the most vulnerable group in terms of overexploitation especially after World War II  because of technological developments like refrigeration, sonar, larger nets, and onboard processing. According to IUCN, overfishing has reduced commercial fish stocks by 90% in the last several years. The cod fishery in the Northwestern Atlantic Ocean has witnessed a population collapse due to intense harvesting. On the other hand, overhunting and poaching of the top predators of the food chain resulted in the disruption of the entire food web. The ‘mega-fauna extinction’ caused due to overharvesting is one such major example.

e) Climate change

Climate plays a crucial role in deciding the distribution of species across the globe. Climate change has therefore, influenced the composition of flora and fauna particularly the keystone species thus, disturbing the entire ecosystem. Increase in the amount of green house gases in the atmosphere, particularly the CO2 affects the physiological functioning of plants. Being primary producers, any change in the plant community of an area directly or indirectly influences the secondary or tertiary consumers dependent on them. As climate warms, species thriving in the plain areas will migrate towards higher altitudes whereas the temperate species will be threatened. It may also result in phenological changes among insects, birds, plants and mammals. Change in the life cycle patterns has already been evidenced in certain migratory birds and insects. Disturbed weather situations may result in behavioral changes in pollinators, pests and pathogens, thereby disturbing the delicate balance that exists in the wild. Rising ocean temperatures and diminishing Arctic sea ice can shift vegetation zones, significantly affecting the marine biodiversity. A report from Intergovernmental Panel on Climate Change (IPCC) estimates that 20-30% of plant and animal species are ‘likely to be at increased risk of extinction’ if greenhouse gas emissions continued at the current rate. Climate change may also promote distribution and abundance of invasive species, thus indirectly promoting a threat to biodiversity. Small and fragmented ecosystems such as islands, coral reefs, mangrove swamps and coastal wetlands are more vulnerable to the changes in temperature as compared to the large ecosystems. With further shifts in climatic conditions, biodiversity will face more severe threats.

Some other factors that could be held responsible for wiping out any taxon includes epidemics, warfare, and UV radiation penetrating on earth due to depletion of ozone layer (considered a primary threat to amphibians).

Table 1 Threats in terrestrial ecosystems and their underlying causes.

Table 2 Threats in marine ecosystems and their underlying causes.

CONCLUSION

It can be concluded from Table 1 and 2 that every factor considered responsible for biodiversity loss is interlinked and generally promote each another. Since all these factors have been operating in the environment all together, the synergistic threats to the biodiversity are possibly much more severe than we can quantify or estimate.The pressures built on biodiversity are, mainly driven by economic development and increasing demand for the biological resources. It is important to understand that loss of biodiversity will result in lack of genetic stocks and ecosystem services thereby, jeopardizing the economic development. Decreased biodiversity may also interfere with essential ecological processes such as pollination, maintenance of soil fertility, flood controls, water purification, assimilation of wastes and the cycling of carbon and other nutrients. Mitigation efforts are therefore, needed to be put forward to conserve biodiversity.