Biodiversity refers to the variation of life at the level of genes, species, and ecosystems. Due to selective pressures such as natural selection and genetic drift, plants, animals, bacteria, and other forms of life all evolved particular adaptive functions to better survive and reproduce. The scientific community has identified over a million distinct eukaryotic species and estimates that approximately [8.7 million](https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001127) eukaryotic species are currently living on Earth.

[Why does biodiversity matter?](https://pub.epsilon.slu.se/10240/7/wardle_d_etal_130415.pdf) Natural processes tend to work best when a diverse set of organisms are present and able to perform specialized functional traits or niches. Species that form [mutually beneficial relationships](https://jhr.pensoft.net/article/75855) with other organisms often outcompete those that don’t, resulting in a highly complex ecological system that performs essential environmental services like biomass production, water filtration, carbon/nutrient cycling and more. Robust biodiversity stabilizes ecosystems and is a form of risk mitigation. Genetic variation ensures that some organisms within a population can withstand harsh environmental pressures. (That said, [evidence of these macro claims about biodiversity are limited](https://pub.epsilon.slu.se/10240/7/wardle_d_etal_130415.pdf) due to the sheer complexity of systems.)

The [main drivers of biodiversity loss](https://ipbes.net/global-assessment) are invasive species, habitat loss, pollution, overexploitation (fishing/hunting/harvesting), disease, and climate change. Each of these drivers are downriver consequences of increasing resource consumption, which is historically correlated with human population growth. In an ideal world, more efficient resource use could decouple population growth from resource consumption, easing the aforementioned pressures.

Biodiversity loss most commonly refers to the loss of species within a specific ecosystem. What happens to an ecological system when a species or multiple species drastically decreases in population size, or goes locally/globally extinct? It [depends on the organism’s function and the ecosystem in which it resides](https://pub.epsilon.slu.se/10240/7/wardle_d_etal_130415.pdf), but to simplify, the following may occur: 

* **Another organism takes or fills the vacated ecological niche**. This can have negative effects on an ecosystem if the new organism, typically an invasive species, imperfectly performs a functional niche and has other adverse effects on the environment. Still, there can be neutral or positive effects depending on an ecosystem’s resilience, aided by existing biodiversity.
* **No organism fills the ecological niche and the ecosystem changes significantly.** Some organisms are so specialized that no other organism can replace its function in the short term. This can and typically does have cascading negative effects across trophic levels, especially when a keystone species disappears. The distribution of plants and animals, and even geologic characteristics can be altered.
* **No organism fills the ecological niche and nothing happens. Sometimes nothing appreciable happens, perhaps because the organism’s functional role was relatively insignificant or was filled without disturbance.**

  


Some ecosystems, like tropical rainforests, are inherently more diverse than others. These are called hotspots. Biodiversity is a comparative term. How can we measure it? [Species richness](https://en.wikipedia.org/wiki/Species_richness) tracks how many species are in a given community but it does not take into consideration species [abundance](https://en.wikipedia.org/wiki/Abundance_(ecology)). Some ecologists measure biodiversity across [different scales](https://www.globalforestwatch.org/blog/data-and-research/how-scientists-measure-biodiversity/). [Tracking the number of species extinctions](https://ourworldindata.org/extinctions) is another rudimentary way to assess the state of biodiversity, but the metric is only so useful with an uncertain denominator.

Let’s narrow the scope to look at a few high-impact species. [A keystone species](https://www.nature.com/scitable/knowledge/library/keystone-species-15786127/) is an organism with a disproportionately large (often moderating) impact on ecosystem structure thanks to high leverage functional traits. Keystone species are typically apex predators that make their mark via hunting but certain mutualist organisms such as pollinators and engineering organisms like beavers also qualify. Removing these species from an environment is considered extremely detrimental to an ecosystem. 

Two high profile keystone species in North America are the wolf and the grizzly bear, both of which are macro-organisms that consume a variety of flora and fauna. Historically, grizzlies ranged from Canada down to Mexico and west of Kansas across to the Pacific Ocean. Wolves could be found throughout the [vast majority of America](https://en.wikipedia.org/wiki/Wolf_distribution#/media/File:North_American_gray_wolf_subspecies_distribution_according_to_Goldman_(1944)_&_MSW3_(2005).png). This is no longer the case. In the mid-20th century, wolf and grizzly populations plummeted in the contiguous United States due to excessive hunting and habitat loss. Fortunately, these species have since made remarkable comebacks in recent decades thanks to reintroductions spurred by their addition to the [Endangered Species Act (ESA)](unsafe:ESA). 

The wolf’s controversial reintroduction into the Greater Yellowstone Ecosystem is a [seminal example of trophic cascade](https://www.sciencedirect.com/science/article/abs/pii/S0006320711004046), potentially contributing to a reduction in elk populations, the reemergence in plant life and increased beaver populations (though correlation vs causation is [still debated](https://www.nps.gov/articles/the-big-scientific-debate-trophic-cascades.htm)). Wolves have since recolonized a handful of western and upper midwestern states. The gray wolf was officially delisted from the ESA in late 2020, and the federal government handed over population management to the states. Several states have since reinstated wolf hunting and harvesting, [potentially reducing gray wolf populations once again](https://www.scientificamerican.com/article/wolf-populations-drop-as-more-states-allow-hunting/). Colorado’s [Proposition 114](https://cpw.state.co.us/learn/Pages/CON-Wolf-Management.aspx), to reintroduce the wolf to the state, was passed in 2020.

Grizzly bears have made a slower albeit [encouraging recovery](https://www.fws.gov/mountain-prairie/es/species/mammals/grizzly/20210719_distributions&RZs_website.jpg) and can now be found in Montana, Washington, Idaho and Wyoming. After a five year review, the bear remains [listed as a threatened species](https://www.fws.gov/mountain-prairie/pressrel/2021/03312021-Grizzly-Bears-Lower-48-States-Retain-Threatened-Status-Under-ESA.php) on the ESA and is still illegal to hunt. 

# **Half Earth**

Habitat loss and climate change are [significant drivers of biodiversity loss](https://onlinelibrary.wiley.com/doi/10.1111/gcb.12148). The spatial configuration of remaining habitat (or lack thereof), called habitat fragmentation, further contributes to the magnitude of loss. Early research in the field found that large amounts of connected habitat are required to support viable populations of flora and fauna, especially predators like bears and wolves. There’s still [debate about this hypothesis](https://www.sciencedirect.com/science/article/abs/pii/S0006320718305779), with [some research](https://onlinelibrary.wiley.com/doi/full/10.1111/geb.13059) finding that fragmented areas are more diverse than larger singular areas. ([Progress](https://media.nature.com/original/magazine-assets/d41586-020-02210-x/d41586-020-02210-x.pdf) on improved measurement techniques continues in the meantime). Certainly, it’s essential to prioritize the [protection of biodiversity hotspots](https://www.science.org/doi/10.1126/sciadv.aat2616).

Regardless, as demonstrated by the research of E.O. Wilson — the founder of sociobiology and advocate for environmental conservation — more land begets higher species richness. It was his belief that humans must set aside a full 50% of Earth for preservation in order to have diverse healthy functioning ecosystems at a global scale. Dr. Wilson wrote a book on the topic and spent the last stage of his career [advocating for the idea](https://www.half-earthproject.org). Just how much progress is needed to reach a Half-Earth scenario? IUCN’s [2020 Protected Planet Report](https://livereport.protectedplanet.net) estimates that roughly 17% of terrestrial lands (and 8% of marine ecosystems) on Earth are protected. The quality of that land is at least in part degraded, but let’s take that number at face value. To reach Half-Earth, global society must protect a further 33% of land on Earth. To most, this seems like a moonshot, at least in the short-term. However, in 2021 the [White House committed](https://www.whitehouse.gov/briefing-room/presidential-actions/2021/01/27/executive-order-on-tackling-the-climate-crisis-at-home-and-abroad/) to a 30x30 initiative, which proposes that 30% of both land and sea be protected by 2030. The UN’s Post-2020 Global Biodiversity Framework is seriously contemplating adopting 30x30 in 2022. If governments believe this short term goal is achievable, Half-Earth may be achievable too.

Variables to consider when answering this last question include [public sentiment concerning conservation](https://www.pewresearch.org/science/2019/11/25/u-s-public-views-on-climate-and-energy/) (the status of wolves and grizzly being a good proxy), the severity of climate change, global population growth, and more. Above all, land use and availability is the potential limiting factor. How much land is available to protect? About [50% of the United States’ land](https://www.ers.usda.gov/topics/farm-economy/land-use-land-value-tenure/major-land-uses/) is currently used for agriculture and only 3% for urban areas. Globally, there’s a [similar trend](https://ourworldindata.org/land-use). There are a billion acres of cropland and pasture, some subset of which could potentially be protected. Any conversion of these lands hinges on demand for food and progress on agriculture innovations such as increasing crop yield, [vertical farming](https://www.metaculus.com/questions/8189/global-interest-in-vertical-farming/) and plant-based or [cultivated meat](https://www.metaculus.com/questions/7064/5-cultivated-meat-products-sell-in-the-us/).

I’m confident in a doubling of conserved lands over the next 28 years. I predict that at least 30% of terrestrial land will be conserved by 2050 and believe we could reach 35-40%. Demand for rangeland will decrease with the normalization of cultivated meat, further spurred by the burgeoning animal rights movements. Vertical farming will supplement the need for cropland but traditional farming will remain a necessity. Climate change mitigation efforts will drive more land restoration and protection. I further predict that global conservation will fall short of Half-Earth by 2050, but that it will be closer than most expect, to the benefit of biodiversity everywhere.