“Biodiversity hotspot Karnataka pays a high price for thoughtless govt actions”, “Montreal unveils pollinator plan ahead of UN biodiversity conference”, “European Research: Biodiversity computing in the "hotspot" Crete” – At one point or another, you have probably come across the term in a headline, on the news or in your day-to-day profession. As a result, you probably already have an idea what biodiversity is. Walking down the street you see the trees starting to lose their leaves. Some of the leaves on the ground are pointy, others rounded. Some are big, some are small. And some trees don’t lose their leaves at all. No person you meet has the same face (well, except for identical siblings). Be it the eyes, the facial structure, the skin tone, the hairline – something is different. It might make you wonder why these differences are there. Why all of this diversification? Why have so many different living organisms evolved?
Evolution and Speciation
Before these questions can be answered, we first need to establish a few key concepts and dive into molecular biology [1, 2]. Our genes, sequences of DNA found in the cores of our cells, are our base plan. They are the unmodified instructions on how to make an organism. In humans, we have two gene sets. One copy come from our mother, the other from our father. These genes, however, are susceptible to changes, such as mutations. Each gene codes for specific proteins and changes in the gene sequence lead to changes in the resulting protein.
Usually, these changes are a disadvantage for the organism because some of the functions linked to the protein don’t work. Rarely, these random mutations may actually be beneficial to the individual. However, if a genetic change is beneficial or not depends almost entirely on the environment, where selective pressures dictate which individuals survive to pass on their genes and which do not. A minor mutation not affecting any core functions may lead to a gain in fitness for this particular individual, making it more likely that it will reproduce and spread this mutation throughout the local population. This means that future generations are born better adapted to their specific environment. The better adapted individuals are to their environment, the more “niche” their survival strategy becomes. They specialise into a specific ecological niche, making them very well adapted to their surrounding environment. Eventually, these related individuals in different environments become so different from each other that they can no longer reproduce with descendants of their closest common ancestor that specified into other niches. New species are created.
In other words, they start to branch out, as depicted in the concept of the tree of life. The tree is not to be understood as the highest point being the pinnacle of evolution, but the outermost branches being the surviving descendants of species that came before them. In the long term, all species that are maladapted eventually die out due to selection through the ever-changing environment or they evolve and diversify into new ecological niches that have been created.
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This process doesn’t take place overnight though. Depending on selective pressures and generation time, it can take an extremely long time for new species to evolve naturally. However, human activity is changing the environment so rapidly and so intensely, that the pace of evolution in most complex organisms such as mammals is too slow to keep up.
A lack of biodiversity creates a homogenous pool of genes of organisms using the same natural resources and that are similarly vulnerable to pathogens. Not to mention collapsing ecosystems that are no longer self-sustainable.
To have a real-life example, let’s take agriculture where homogenous fields of crops are planted in one spot. This may be the intuitively most efficient way to use the land, but it makes these crops dependent on human support through fertiliser, pesticides, and water supply – otherwise the ecosystem collapses.
Despite the increases in crop yield through selection and modification of plants, studies such as the BIODEPTH experiments that took place between 1995 and 2001 all over Europe found that on average with each additional grass type that was planted in the same area, the yield of hay increased substantially. Also, the smaller animals living in the area grew larger too, due to having more available food sources. This leads to an overall increase in health of the ecosystem. 
The Jena Experiment which started in 2002 in Germany found similar results. By studying the soil composition, they discovered that the plants also extract more nitrate, leading to cleaner surrounding waters which have further downstream effects such as healthier freshwater inhabitants. As a last example, the BEF experiment in the subtropical forest of China concluded that biodiverse forests are up to twice as productive and fixate much more Carbondioxide than non-diverse forests. The results of each of these experiments have been attributed to the effective sharing of resources and sharing of work between the plants. A more genetically diverse set of plants is better at utilising the surrounding environments and automatically occupy the niches they can best adapt to.  This means there is less competition between the plants and a sharing of resources. Additionally, the amount of yield became more stable, varying less between years, due to increased resistance to pathogen damage, especially those specialised on one plant.
Modern Agriculture and Monocultures
Now if we return to our example of modern agriculture, we must recognise that most of our crops are grown in monocultures. Monocultures offer many benefits, such as simplicity and lower initial costs for farmers, lower competition for light and land space between plants and an effective control option against pathogens in pesticides. However, the fundamental core of this practise lies in our human ability to “out-adapt“ crop pathogens and protect the plants from damage and simultaneously increase crop yield through technological advances and supply of nutrients. As a result, these crops are almost completely dependent on humans and the pathogens are selected on traits that improve their resistance to the pesticides we use. In turn, we come up with new pesticides or methods to combat them. This agricultural strategy is based on active combat against pathogens which costs time, money and energy - three things most people would rather invest in other things, especially when there is a viable alternative with much less long-term hassle.
Instead of actively trying to control the ecosystem using pesticides, monocultures and competing in arms races against an ever-evolving cast of pathogens, we should instead take a more passive role and support existing and cultivate new biodiverse systems that end up producing a larger yield with far less investment and with less strict dependence on technological advancement.
As you see: Adaptation into ecological niches ensures a species’ short-term survival. This answers our questions from the beginning of the article as to why so many different species have evolved. By diving into emerging niches, organisms can specialise. This leads to an optimal usage of natural resources by the organisms in that environment through shared labour and distribution of excess, as well as a higher resilience against pathogens – that’s why we should care about biodiversity.
UWW 172 Einführung in die Nachhaltigkeit, UZH, „Biodiversität“, Prof. Dr. Schmid, Bernhardt
UWW 172 Einführung in die Nachhaltigkeit, UZH, „Anthropogene Selektion“, Prof. Dr. Kokko, Hannah
Hector, A., Bagchi, R. Biodiversity and ecosystem multifunctionality. Nature 448, 188–190 (2007). https://doi.org/10.1038/nature...
Yuanyuan, H. Impacts of a species richness on productivity in a large-scale subtropical experiment. Science 362, 80-83 (2018). Comment on “Impacts of species richness on productivity in a large-scale subtropical forest experiment” | Science
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