Biodiversidad I: _{Definiciones y patrones de diversidad}

What is biodiversity?

The term “biodiversity,” a contraction of “biological diversity,” refers to the variety of life on Earth. The term stems from the Greek word bios (life) and the Latin word diversitas (difference or variety). In combination, the two words describe the enormous range of living things from tiny bacteria to the largest animal, the Antarctic blue whale, or an even larger organism called a honey fungus that grows to several miles in diameter (Casselman, 2007).

The human understanding of biodiversity likely began long ago. Our hunter-gatherer ancestors would have needed to be aware of the diversity of plant and animal life they depended on for survival (Tallavaara et al., 2017). By the 300s BCE, the Greek philosopher Aristotle observed that plants and animals could be sorted into groups based on how they looked and behaved. His work led to the approach we use today to classify and assign scientific names to living things (see our Taxonomy I: What's in a name? module).

Since Aristotle’s time, we’ve come a long way in describing biodiversity. The official definition is “the variability among living organisms from all sources, including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems” (Convention on Biological Diversity, 2006). Biodiversity includes the variety of living organisms, the diversity of genes they carry, and the variety of ecosystems in which they live. This official definition includes three levels of biodiversity: species diversity, genetic diversity, and ecosystem diversity.

Species biodiversity

Species diversity is the most commonly measured level of biodiversity. Current estimates suggest that between 5 million and 10 million living species currently exist on Earth (Costello et al., 2013; Wilson, 2018). Why is there such a huge range in the estimates? To date, about 2 million species have been accounted for, meaning they have been assigned formal scientific names by people who discovered them. Based on the rates of naming of new species, the majority have yet to be discovered. While scientists think they have identified nearly all bird and mammal species, there are millions of species of fungi, bacteria, and other organisms that have yet to be identified. For example, the approximately 100,000 known fungi are thought to be less than ten percent of existing species (Sigwart et al., 2018). So, estimates of 5 to 10 million total species on Earth are based on the rate of discovery of new species and projections of how many more are likely to turn up.

Counting species

Counting all the species on Earth is no simple task. Think about the magnitude of 5 to 10 million species compared to the total from your own local count.

One of the earliest published counts of species diversity was made in 1982 by American biologist Terry Erwin. He wondered how many species of beetles and other arthropods (invertebrates with jointed legs) lived in the tropics. Erwin “fogged” 19 tropical trees with insecticides and counted nearly 1,200 species of beetles that fell out. From his observations and counts, Erwin noted various beetles’ dependence on particular tree species. By estimating about 50,000 species of tropical trees, Erwin came up with a staggering tally of 30,000 species of beetles and other tropical arthropods (Erwin, 1982).

While many of the assumptions behind Erwin’s estimates are debated, such as the degree to which beetles are specialized to certain trees, his work spawned a flurry of interest in tallying up all the species on Earth (Ødegaard et al., 2000). Scientists all over the world are collectively trying to figure out global species diversity. Fogging and other collection techniques are still used today, but judiciously and alongside other methods that are less destructive. For example, insects are sampled by attracting them to lights and netting, after which they can be released (Montgomery et al., 2021).

Today, in calculating species diversity, scientists include not only “richness” (the number of different species counted) but also “abundance” (how many members) of each species counted. Relative abundance gives you information about the species’ influence on the ecosystem. For example, while an individual grass plant may have a small impact on the characteristics of an ecosystem relative to an oak tree, the sheer abundance of grasses in a meadow ecosystem makes it an excellent habitat for animals like grass snakes and voles.

Ways of knowing

Indigenous Peoples’ ways of knowing are particularly valuable in estimating biodiversity. In fact, research has shown that indigenous and other local knowledge about biodiversity is as accurate as data collected via Western science techniques (Danielsen et al., 2014). Indigenous homelands tend to have high biodiversity because of the ways they are managed to sustain natural resources that people depend on directly. For example, in New Zealand, Māori whale expert Ramari Oliphant Stewart was mentored in the natural environment by her elders from the Ngāti Awa, Rongomaiwahine, and Ngāti Mahuta tribes. At age 10, she became a “whale rider”, which signifies someone with special knowledge about and relationship to whales (Morris, 2020).

Discovering new species and adding them to the tally of biodiversity on Earth requires continued global collaboration across different communities and knowledge keepers. A project called The Encyclopedia of Life (EOL) is cataloging all living species into an open-source biodiversity information repository that anyone can add to and access. Similarly, the Map of Life (MOL) project is a similar collaborative effort to map the locations of every species in the world.

However, biodiversity goes even deeper than the species level.

Genetic diversity

Cataloging species by the way they look is a reasonable way to understand Earth’s diverse ecosystems. However, genes “code for” (determine) the very characteristics that set species apart from one another (see our DNA II: The Structure of DNA module). As the raw material for natural selection, genes are the building blocks of species diversity as it changes over time (Hughes et al., 2008). All the variability that makes life capable of adapting to changing environmental conditions has accumulated within the pool of DNA. This is genetic diversity (Convention on Biological Diversity, 2021).

Genetic diversity helps buffer species against environmental change by ensuring that at least some individuals survive disease or other catastrophes. It’s like keeping money in different places to buffer against change (Lynch, 2016). You might keep some at home, some at the bank, and maybe some in a car or other location. If your home is robbed or the bank fails, you still have part of your money elsewhere. Similarly, in a population of organisms with high genetic diversity, some are likely to be resistant to a particular disease or parasite and survive to reproduce and ensure the continuation of the species.

The consequences of losing genetic diversity are apparent in many areas, including agriculture. As industrial farms have worked to identify and use high-performing individual species, they have also reduced the genetic diversity of industrial crops. As a result, industrial crops are at a much higher risk of being wiped out by disease or parasites. For example, large-scale loss of corn to the Southern Corn Leaf Blight epidemic of 1970-71 brought attention to the importance of genetic diversity. One-billion dollars of U.S. corn was wiped out by a fungal infection because the corn genes had become so homogeneous (lacking diversity) that most of the crop lacked resistance to the disease caused by the fungus (Bruns, 2017).

The perils of losing genetic and species diversity highlights the importance of being able to measure and track it. While research on DNA dates to the late 1800s, the first successes in determining an actual DNA sequence of genes came in the 1970s (Jou et al., 1972). Building on these advances in 2003, Canadian molecular biologist Paul D. N. Hebert developed a technique called DNA barcoding that identifies species from a short segment of the genetic code (Hebert at al., 2003). A DNA barcode is a genetic signature of an organism. It’s like the codes you can scan to read the price of a product or look at a restaurant menu, except a DNA barcode provides information about the DNA of organisms (Figure 1).

Hebert heads the International Barcode of Life (iBOL) Consortium, an international group of scientists that aim to collect the genetic signature, or barcode, of every species on Earth. It’s like the Encyclopedia of Life, but catalogs DNA rather than other features of organisms. The iBOL database makes genetic diversity information openly available to anyone who wants to access it.

Ecosystem diversity

Another way to view biodiversity is at the level of ecosystems. An ecosystem is a community of organisms interacting with their physical, or nonliving, environment. Ecosystem diversity refers to the variety of ecosystems that exist in a defined area, something visible to early naturalists.

In the early 1900s, Prussian explorer Alexander von Humboldt laid the foundation for understanding ecosystem diversity, inspired by his expedition to the American tropics. Humboldt’s Tableau Physique (1807) was one of the first formal attempts to delineate biodiversity at an ecosystem level. As shown in Figure 2, he mapped plant species in the Andes Mountains, showing how they changed with altitude.

Humboldt’s mapping was ahead of its time. Yet, scientists today recognize the limitations of his mapping, particularly in finding exact upper and lower limits of vegetation types (Moret, 2019).

Compared to species or genetic diversity, ecosystem diversity is harder to measure. The boundaries of most ecosystems are not a sharp line, but instead a gradual transition from one community of organisms to another (Cofrin Center for Biodiversity). A city ecosystem might have an obvious boundary, say between a park and a road, or a coastal area between land and sea. But typically, ecosystem boundaries are less clear. To test this out, look online for an aerial photo of the region you live in, and try to draw lines delineating the ecosystem boundaries.

Regardless of whether you look at biodiversity through a species, genetic, or ecosystem lens, it invites questions: What creates patterns of biodiversity? Why is one area more diverse and another less diverse? As you will see in the next section, the most visible global pattern in biodiversity is how it differs across latitude.

Biodiversity and latitude

Take a look at the map below (Figure 3). Red areas indicate more species. What do you notice about the distribution of species on Earth?

For at least two centuries, naturalists have noted that biodiversity increases as you go from the poles to the tropics. This is called a Latitudinal Diversity Gradient (LDG). Inspired by the biodiversity he saw in the Andes, Humboldt mapped the first isothermal (temperature) bands onto the globe. In 1817, he published a map which, while based on limited data, showed how temperatures change over the globe (Klein, 2018; Humboldt, 1817). Building on Humboldt’s mapping, in 1876, British naturalist Alfred Russell Wallace reported, “Animal life is, on the whole, far more abundant and more varied within the tropics than in any other part of the globe, and a great number of peculiar groups are found there which never extend into temperate regions.” (Wallace, 1876; Dowle et al., 2013).

The tropics are close to the equator (defined as 23.5 degrees north or south), while temperature zones are further (defined as between the Tropic of Cancer and Arctic Circle, or Tropic of Capricorn and Antarctic Circle). Since Humboldt’s work, the LDG has become an accepted part of the scientific understanding of biodiversity. The LDG established that biodiversity is concentrated near the equator (that is, at lower, tropical latitudes). This is true whether you count species on land or in water, and it is true across all kinds of life - from single-celled organisms to plants and animals. Tropical rainforests house more than half of the world’s known species, despite covering just seven percent of Earth’s land surface (Primack and Morrison, 2013). Judging from fossils, the LDG is a pervasive pattern of life on Earth. In fact, fossil evidence suggests it has existed for 270 million years or more.

But the underlying question remains: Why do the tropics have higher biodiversity? Many hypotheses have been proposed, and scientists are still grappling with this key question. For example, in wondering what drives the LDG pattern, Chinese geobiologist Haijun Song and colleagues mapped latitudes of more than 50,000 marine fossils described in a database. They identified a 5-million-year period with no LDG beginning about 252 million years ago. During this period, levels of biodiversity were similar from the poles to the equator. Song attributes the pattern to intense global warming −a greenhouse interval −that overheated the tropics and forced more animals poleward (Song et al., 2020).

Song’s results support a hypothesis that heat drives the LDG.

Hypothesis 1: Light and heat

As early as the 1960s, scientists recognized that tropical ecosystems cycle nutrients quickly than temperate ecosystems, i.e., nutrients like nitrogen move through the tropical environment faster, demonstrated by various studies (Vitousek and Sanford, 1986). Nutrient cycling requires energy, which comes from sunlight (see modules The Carbon Cycle, The Nitrogen Cycle, and The Phosphorus Cycle). More year-round sunlight near the equator means more energy supply for plants to take up nutrients and grow. Plants are at the base of food webs (the connection of all food chains in a single ecosystem), making their food through photosynthesis. As a result, their productivity is essential to supporting other organisms; this is also known as primary productivity.

Primary productivity is measured in various ways, such as calculating total plant biomass or measuring the carbon that plants incorporate from photosynthesis. Figure 4 shows the biomass of plants in different types of ecosystems. What do you notice about the biomass in tropical ecosystems (starting at the left side of the graphic) compared to other ecosystems?

Measures of primary productivity show that it is about twice as high in the tropics as elsewhere. And productivity is not just about the sunlight available for photosynthesis. With more sunlight comes heat, so climates near the equator are hotter. According to Kinetic Molecular Theory, atoms and molecules are in constant motion and move faster when they are warmer (see our Kinetic-Molecular Theory module). When molecules have more energy, the chemical processes that affect biological processes, like those regulating growth and reproduction, also go faster. This helps explain the high plant productivity of the tropics (University of Southern California, 2008).

This speeding up of tropical ecosystem processes may also cause the quicker evolution of new species. Studies have found that the DNA molecules making up genes evolve faster in the tropics. Changes in DNA may ultimately result in new species (called “speciation”), which adds to biodiversity. Some scientists, therefore, call the tropics a “cradle” for biodiversity (Jablonski et al. 2006).

So, if conditions in the tropics speed up nutrient cycling, productivity, and evolution, the outcome is more species diversity. But, even if we can explain why more species arise in the tropics, why don’t they spread out into other areas?

Hypothesis 2: Out of the tropics

Biologists propose that environmental conditions keep species from spreading out of the tropics. Many species have a long evolutionary history of living in the tropics. If they are adapted to a warm, humid climate, they might not tolerate other conditions (see our Adaptation: The Case of Penguins module). The outcome is a wealth of biodiversity in the tropics that has adapted to tropical conditions and cannot live elsewhere (Brown, 2014).

Studies of how organisms are distributed provide evidence that supports this hypothesis. For example, Iranian biologist Sana Sharifian studies the geographic distribution of mangrove crabs. She wondered whether she could predict where different species live based on environmental factors like sea surface temperature and other ocean conditions. Using more than 8,000 records of where mangrove crabs have been found, Sharifian calculated species richness and plotted it by latitude (Sharifian et al., 2020). In Sharifian’s graphic (Figure 5), blue dots represent the number of mangrove crab species, while colored bands represent temperature. Would you say that sea surface temperature is a good predictor of where species of mangrove crabs live?

Mapping species richness by latitude revealed that the highest mangrove crab diversity is in tropical waters, especially in the Indo West-Pacific, indicating that temperature is the best predictor of where they live.

Species accumulating in the tropics eventually spread into higher latitudes as they evolve adaptations for cooler climates. For example, American geophysicist Dave Jablonski examined fossils of marine bivalves (two-shelled clams, oysters, etc.) from the past 11 million years and plotted where and when each species originated. He found that the tropics have been “an engine of global biodiversity,” producing most of the new bivalve species, which then expanded their ranges towards the poles over thousands of years. But, even as their ranges expanded, nearly all of them continued to live in the tropics. In Jablonski’s view, this makes the tropics both a “cradle” (where species arise) and a “museum” (where species remain) for biodiversity (Jablonski et al., 2006).

Besides the stable, warm conditions of the tropics, their high biodiversity may also relate to their complexity.

Hypothesis 3: Heterogeneity

Because they support high plant diversity, tropical areas have more variety of habitats (heterogeneity). A tropical forest is made up of multiple layers of plant species that differ as you move from the ground to the tree canopy. Within an ecosystem, each organism has a habitat niche, defined by the resources it uses. This layering may support high biodiversity by providing more unique niches for species.

For example, American biologist Jonathan Huie is one of a group of biologists working to understand how animals in the tropics reduce competition by occupying different habitats. As a graduate student, he examined the features of tropical anole lizards and categorized them according to their lifestyle—how they use the habitats (Huie et al. 2021).

The diagram (Figure 6) shows where you find species of anole lizards in a tropical forest ecosystem. What might you conclude about how they share the habitat?

On both islands and mainland South America, scientists like Huie find that anole lizard species sort out into lifestyles of ground, grass-bush, trunk-ground, trunk, trunk-crown, twig, and crown-giant. By using different parts of the habitat, species rely on unique sets of resources. And the more heterogeneous a habitat, the more species can share it, that is, the more biodiversity.

Scientists still debate whether the high biodiversity of the tropics and the Latitudinal Diversity Gradient as you head towards the poles is due to light, temperature, stability, heterogeneity, or other factors. Explaining the LDG is a challenge that involves many scientific fields. Biologists, ecologists, geologists, and other specialists continue to gather evidence.

Find your location on a map and note your latitude. Does latitude explain the biodiversity around you? Think about both the count you did outside where you live and the life you see in your region. Note what else (besides latitude) may help explain the pattern of your local biodiversity.

Biodiversity on islands

“Islands are tumultuous places; raised from the oceans or divided from continents, they undergo change at a pace faster than most other biomes. The species that colonize and persist upon islands react and adapt to this constant change.”

– James C. Russell, 2019

Islands, as fragments of land surrounded by the ocean, are a special case when it comes to biodiversity. R. H. MacArthur and E. O. Wilson (1967) proposed the Theory of Island Biogeography, which states that biodiversity should increase with island area and closeness to other landmasses. Since islands are separated by ocean waters and not all species can fly, float or swim across, islands that are more isolated islands should have fewer species. And smaller islands should have fewer species because they offer a lower diversity of resources. Thus, you would expect the smallest, most isolated islands to have the lowest biodiversity.

The predictions of Island Biogeography Theory have proved correct in most circumstances but fail to explain the whole picture. For example, consider the Hawaiian Islands. Hawai’i is the biggest island in the archipelago (a collection of islands), and all the Hawaiian Islands are really far (more than 9,000 km or 5,600 miles) from mainland North America. Based on Island Biogeography Theory, what would you predict about the biodiversity on Hawai’i compared to its increasingly smaller neighbors of Maui, Oahu, and Kauai? Do data on island size plotted against species richness support your prediction (Figure 7)?

Island Biogeography Theory predicts that the biggest island, Hawai’I, as the highest biodiversity. In fact, Hawai’i has the lowest biodiversity, with species richness increasing as islands get smaller. While Island Biogeography Theory is accurate in many circumstances, scientists are coming to understand other important factors. Estonian ecologist Madli Jõks modeled the expected species richness on groups of islands and found factors besides island size to be important (Jõks and Pärtel, 2018). In the case of Hawai’I, island age comes into play. The smallest islands are older, having formed earlier from volcanoes building up from the ocean floor. Their higher biodiversity can be explained by more time for species to colonize them.

Value of biodiversity

"Biodiversity is an essential heritage for all humankind...Stopping its loss, and guaranteeing the continued functioning of the earth's ecosystems− both marine and terrestrial− should be a high priority for everyone."

– United Nations Secretary General Kofi Annan, 2003

Why do we care about biodiversity?

Losing species means losing the interactions that they have with other species, which can lead to a cascade of species losses (Valiente-Banuet et al., 2014). For example, local extinctions of wolves in Yellowstone National Park resulted in fewer predators for elk populations, causing them to grow. The growing elk populations reduced the streamside willows they graze on. As a result, beavers no longer had the slow-moving water around willows that they rely on and disappeared from Yellowstone. So, the loss of a single species may have far-reaching effects on an ecosystem. Supply of water, formation of soil, cycling of minerals, and maintenance of climate are among other ecosystem services that may be disrupted.

For example, Brazilian ecologist Julia Astegiano finds that as habitats get degraded, the diversity of pollinators like bees goes down. The loss of pollinator diversity leads to shifts in plant diversity. This can result in “community collapse,” where just a fraction of the former species survives (Astegiano et al., 2015). Due to the loss of insects, plants, and other species, agricultural and urban areas tend to have lower biodiversity than wild areas (Rogan and Lacher, 2018).

The significance of biodiversity was first acknowledged broadly when Ghanaian Kofi Annan, then U.N. Secretary-General, called for a Millennium Ecosystem Assessment in 2005. The assessment detailed the effects of ecosystem change on humans and concluded that biodiversity and our human well-being are inextricably linked (Millenium Ecosystem Assessment, 2005).

Since then, many have invested in studying and conserving the complex living system that is biodiversity. As we continue to learn more about what determines biodiversity, we become better equipped to manage it. But sustainably managing the diversity of life does not mean there will be no changes. Rather, it calls for an intentional approach to tracking and managing change.