The latter phenomenon can be temporary or permanent, depending on whether the environmental degradation that leads to the loss is reversible through ecological restoration / ecological resilience or effectively permanent (e.g. through land loss). Global extinction has so far been proven to be irreversible.
Even though permanent global species loss is a more dramatic phenomenon than regional changes in species composition, even minor changes from a healthy stable state can have dramatic influence on the food web and the food chain insofar as reductions in only one species can adversely affect the entire chain (coextinction), leading to an overall reduction in biodiversity, possible alternative stable states of an ecosystem notwithstanding. Ecological effects of biodiversity are usually counteracted by its loss. Reduced biodiversity in particular leads to reduced ecosystem services and eventually poses an immediate danger for food security, also for humankind.
|“||You know, when we first set up WWF, our objective was to save endangered species from extinction. But we have failed completely; we haven’t managed to save a single one. If only we had put all that money into condoms, we might have done some good.||”|
|— Sir Peter Scott, Founder of the World Wide Fund for Nature, Cosmos Magazine, 2010|
Locally bounded loss rates can be measured using species richness and its variation over time. Raw counts may not be as ecologically relevant as relative or absolute abundances. Taking into account the relative frequencies, a considerable number of biodiversity indexes has been developed. Besides richness, evenness and heterogeneity are considered to be the main dimensions along which diversity can be measured.
As with all diversity measures, it is essential to accurately classify the spatial and temporal scope of the observation. "Definitions tend to become less precise as the complexity of the subject increases and the associated spatial and temporal scales widen. Biodiversity itself is not a single concept but can be split up into various scales (e.g. ecosystem diversity vs. habitat diversity or even biodiversity vs. habitat diversity) or different subcategories (e.g. phylogenetic diversity, species diversity, genetic diversity, nucleotide diversity). The question of net loss in confined regions is often a matter of debate but longer observation times are generally thought to be beneficial to loss estimates.
To compare rates between different geographic regions latitudinal gradients in species diversity should also be considered.
- Habitat loss and degradation
- Climate change through heat stress and drought stress
- Excessive nutrient load and other forms of pollution
- Over-exploitation and unsustainable use (e.g. unsustainable fishing methods) we are currently using 25% more natural resources than the planet
- Armed conflict, which disrupts human livelihoods and institutions, contributes to habitat loss, and intensifies over-exploitation of economically valuable species, leading to population declines and local extinctions.
- Invasive alien species that effectively compete for a niche, replacing indigenous species
- Human activity has left the Earth struggling to sustain life, due to the demands humans have. As well as leaving around 30% of mammal, amphibian, and bird species endangered.
In 2017, various publications describe the dramatic reduction in absolute insect biomass and number of species in Germany and North America over a period of 27 years. As possible reasons for the decline, the authors highlight neonicotinoids and other agrochemicals. Writing in the journal PLOS One, Hallman et al. (2017) conclude that "the widespread insect biomass decline is alarming."
Food and agriculture
In 2019, the UN's Food and Agriculture Organization produced its first report on The State of the World’s Biodiversity for Food and Agriculture. This warned that "Many key components of biodiversity for food and agriculture at genetic, species and ecosystem levels are in decline." The 2019 IPBES Global Assessment Report on Biodiversity and Ecosystem Services asserts that industrial farming is a significant factor in collapsing biodiversity. The health of humans is largely dependent on the product of an ecosystem. With biodiversity loss, a huge impact on human health comes as well. Biodiversity makes it possible for humans to have a sustainable level of soils and the means to have the genetic factors in order to have food. 
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...at the first Earth Summit, the vast majority of the world’s nations declared that human actions were dismantling the Earth’s ecosystems, eliminating genes, species and biological traits at an alarming rate. This observation led to the question of how such loss of biological diversity will alter the functioning of ecosystems and their ability to provide society with the goods and services needed to prosper.
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