It sounds rather implausible that there would be a single cure to heal our natural environment, reverse the rate of extinction of animal species, improve human health, and dampen the on-going changes in our climate.
Perhaps there is one. More specifically, one that embraces a deeper acknowledgment of the relevance of life’s diversity in general. Biodiversity has become a buzzword. It refers, in broad strokes, to a wide-ranging biological variety of life. But the more intriguing question is: Why is it considered so important?
During uneasy times of a palpable changing climate and a deep-felt pandemic, it might be helpful to take a step back, get to the roots of these issues, and flesh out the meaningful interactions between biodiversity, climate, and global health.
Seeing the Wood for the Trees
Before we kick-off, a bit of terminology is in order. The richness of species points to the number of different species in a certain area, whereas species abundance designates the number of individuals per species. And evenness provides information about the relative distribution of species abundance in a community.
Petra Tschakert et al. define biodiversity as “a measure of variation and richness of living organisms at a particular scale.” Vicki Medland applies a similar definition but emphasizes making a distinction between genetic variability within a population, the diversity in species, and the degree of variety in ecosystems.
The latter description connects well with the explanation given by the Convention on Biological Diversity, which describes biodiversity as “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.”
Ian Swingland furthermore argues that any geographical area impacts biodiversity in a number of ways, including “through its richness in numbers of species, through the endemism (or geographical uniqueness) of these species […], and on the basis of degree of threat.”
Because of the changing relative importance of these factors on the local, national, regional, or global scale, Swingland points out that it is far from a straightforward endeavor to establish a single definition of biodiversity. Not only that, “[t]his plethora of terms and definitions”, he continues, “is one of the major stumbling blocks to reaching agreement in problem solving and decision making.”
Setting the issues of definition and policy implementation aside, there are nevertheless two popular biodiversity measurement metrics in use: the Shannon-Wiener function and Simpson’s Diversity index.
The Shannon-Wiener function takes into account both species richness and the average of evenness, and the function tends to rise when biodiversity flourishes. Simpson’s index, in contrast, reflects the probability that two randomly sampled individuals belong to the same species: the stronger the dominance of that species, the lower the biodiversity, and thus the lower the index.
A general characteristic of biodiversity seems to be that the more we move towards higher latitudes, the less diverse the biological ecosystem is. One explanation is that life was more abundant in the tropics during the last glacial period, which ended approximately 15,000 years ago. A second one falls back on a higher energy intake along the equator (due to the Sun), as opposed to temperate and polar regions, though the exact relationship between solar energy and increased speciation remains unclear.
What is more, there are certain regions on our planet that deserve our utmost care — they are called biodiversity hotspots. These places are distinguished by a geographically concentrated biological richness, while at the same time being under substantial threat by human activity. Although the current 36 hotspots only take up 2.4% of land globally, over half of all vascular plants and 43% of land vertebrates are unique to these critical areas.
For instance, in Madagascar, about 90% of the total number of plant and animal species are endemic, and the country has lost nearly 90% of its forests. Other examples include the Tropical Andes in South America and Sundaland in Southeast Asia with more than 15,000 endemic plant species. In these regions, 75% and 93% of the natural habitats have vanished, respectively.
In the next two sections, I will examine the significance of biodiversity from two perspectives: its relationship with climate change, and its connection to emerging diseases.
Climate Change Entanglement
First of all, it goes without saying that restored or conserved natural habitats, such as forests and woodlands, can contribute to the removal of carbon dioxide (CO₂) from the atmosphere. As a case in point, Yin Li et al. ascertain, based on their research on subtropical forests in China, that “biodiversity conservation might be an effective way for enhancing tree carbon storage in natural, species-rich forest ecosystems.”
Less CO₂ in the atmosphere leads to easing the pressure of a warming planet — carbon dioxide is a greenhouse gas — as well as indirectly ensuring less air pollution deaths, according to research conducted by Mark Jacobson in the United States.
Moreover, in the context of their work on Australian vegetation, researchers Rachel Standish and Suzanne Prober reveal additional perks to a rich plant diversity other than carbon sequestration from the air, namely reinforced soil carbon storage, stability, and ecological resilience towards disturbances.
On top of all that, reforestation boosts the availability of ecosystem services, e.g. nutrient cycling (which comes with significant consequences for the quality of water and climate), fuel wood for cooking, medicines, construction materials, fruit, and freshwater regulation for farming.
Even so, Shibu Jose and Sougata Bardhan report that the greatest potential for capturing carbon lies with agroforestry — a land-use system that integrates trees, plants, agricultural crops, and livestock. In addition, looking at Arabica coffee production in Ethiopia, Olivier Honnay et al. demonstrate that, apart from being beneficial to carbon storage, agroforestry also promotes biodiversity conservation.
Scientists Qiang Fang and Shuangquan Huang furthermore underline that biodiversity is essential to sustain pollination, i.e. the transmission of pollen between flowers. And since over 80% of wild plant species depend on insects for fruit and seed production, there would be much less carbon sequestration without pollination.
On a more microscopic level, some researchers have laid bare a linkage between the powerful greenhouse gas methane — it is 25 times more potent than CO₂ — and the gut microbiome composition of livestock. Bear in mind though that almost all the methane production from the livestock sector can be traced back to microbial digestive processes.
For instance, the comparative research between yak and cattle on the Qinghai-Tibetan Plateau by Juan Boo Liang et al. suggests that a more diverse gut community of methane-producing microorganisms (methanogens) might be related to a reduction of methane production. The study by Jagadish Padmanabha et al. comes to parallel conclusions.
In terms of genetic variability, Glenn Yannic et al. argue that animal species may generally narrow their living space as an adaptive response to climate change, thereby weakening their genetic diversity.
When it comes to the Madagascan endemic pea plant Delonix decaryi, Malin Rivers et al. have calculated to a similar extent that genetic diversity diminishes with an expanding range loss. To model the on-going habitat destruction, the researchers simulated range loss by relying on random loss of populations.
Besides these spatial aspects, Camilla Wikenros et al. point out in the context of their research on the population of wolves in Scandinavia that a shrinking species abundance impoverishes the genetic diversity and cultivates inbreeding, possibly resulting in species extinction.
Therefore, aside from mitigating climate change, investing in species abundance is key to strengthen individual fitness and avoid extinction, as genetic variation “is the most fundamental level of biodiversity […] and is crucial for maintaining the ability of species to adapt to new environmental conditions”, claim Glenn Yannic et al.
Even though the application of genetic data and tools in biodiversity conservation studies remains rather scarce, find Sean Hoban et al., some researchers are working on filling that gap, such as Malin Rivers et al. These scientists affirm that “genetic analyses could lead to a better understanding of the biological and ecological impacts of changes in population size and ranges”.
Turning our attention to nearshore habitats, Jonathan Lefcheck et al. demonstrate that, globally, marine biodiversity (compared to temperature variability and human influence) is one of the strongest predictors of fish biomass.
Reversing that relationship, Tim McClanahan and Catherine Jadot establish in the specific case of Madagascar’s coral reef systems that biomass (relative to coral cover, temperature, and water depth) is, in turn, the most significant forecaster of fish species richness.
Such interdependent connection between quantity and diversity becomes prominent in view of the findings of Jonathan Lefcheck et al. that a more diverse fish community is less susceptible to volatile temperatures, and hence more resistant to a changing climate.
Within the geographical confines of the Coral Triangle in Southeast Asia, Susan Williams et al. furthermore propose that multispecies seagrass restoration can stimulate plant survival and coverage.
Such research insights are crucial when considering that the coral reefs, seagrass meadows, and mangrove forests provide fisheries resources to the local people, shields the coastal areas against flooding (in itself a consequence of climate change), and enables carbon storage.
Notwithstanding these positive effects of biodiversity, some research studies, including those by Michaela Zeiter et al. and Aliny Pires et al., could not confirm that more diverse ecosystems are systematically better at protecting against the harmful repercussions of climate change.
Instead of then writing off biodiversity as an unsuccessful tool to counter climate change, perhaps such results rather serve as a pertinent reminder of the devastating potential of climate change.
In the words of Isabel dos Santos Silva et al., a pandemic describes “[a]n epidemic occurring over a very wide area, crossing international boundaries, and usually affecting a large number of people.” In the last 100 years, the frequency of pandemics has been on the rise, mostly due to “increased global travel and integration, urbanization, changes in land use, and greater exploitation of the natural environment”, according to Nita Madhav et al.
Indeed, as Dirk Schmeller et al. further explain, an “[i]ntensifying pathogen emergence can be attributed to climate change, biodiversity loss, habitat degradation, and an increasing rate of wildlife–human contacts.” What is more, 75% of emerging infectious organisms pathogenic to humans have an animal origin — that is, they are zoonotic.
More Is Less
The nature of the relationship between biodiversity and disease emergence is complex. One hypothesis, referred to as the dilution effect, stipulates that a substantial degree of biodiversity lowers the risk of disease infectivity (or, equivalently, that biodiversity loss amplifies the risk). Serge Morand, for one, establishes a link between threatened biodiversity and outbreaks of human infectious diseases.
One example that illustrates the dilution effect is the West Nile virus. For instance, Alex Byas and Gregory Ebel argue that, despite the high richness of viral hosts (bird species and mosquitos), major epidemics have been absent from Central and South America. Also, John Swaddle and Stavros Calos arrive at a similar outcome with regard to bird diversity in the eastern part of the United States.
Diversity Begets Diversity
Turning that relationship upside down, greater biodiversity might actually be conducive to disease emergence. Such are the findings by Denis Valle and James Clark: conservation efforts of forest cover in the Brazilian Amazon are related to a higher malaria incidence.
Other research studies corroborate this inverse association between biodiversity and disease. For example, Nathan Breit et al. assert that stronger mammalian richness and the presence of tropical forests — places of outspoken biodiversity — both boost the risk of pathogen emergence. Similarly, Michael Gavin et al. find that pathogen richness is positively correlated with the number of birds as well as mammals species (remember, correlation does not necessarily imply causation).
Still undiscovered mammalian viruses abound — one study by Jonna Mazet et al. estimates the number to be around 320,000 — which is why recurrent human contact with areas of high biodiversity could further inflate the chances of emerging zoonoses.
This is precisely what Amy Pedersen and Jonathan Davies have demonstrated. Studying wild primates in central Africa and Amazonia (South America), they conclude that these regions are hotspots for cross-species transmission incidents as a result of an elevated diversity of primate species. Moreover, in the wake of rapid human population growth and intensified primate-human contact, the authors maintain that these areas are therefore more likely to endure disease emergence.
In all fairness, the ‘diversity begets diversity’ hypothesis is more nuanced than what is described above. As a case in point, Richard Ostfeld and Felicia Keesing elucidate this higher complexity with the following logical chain of interdependent events: “High diversity of vertebrate hosts must result in high total diversity of pathogens […], which in turn must lead to high diversity of actual or potential zoonotic pathogens […], which in turn must increase the probability of new emergence events.”
In addition, they state that the data in support of this inverted line of thinking is rather unconvincing, compared to the strong empirical evidence available for the dilution effect.
Think Local, Act Local
In step with the dilution effect, it would thus be beneficial to globally invest in biodiversity conservation.
Regarding ecological diversity, this would make sense, since “[t]he environmental microbiome promotes ecosystem stability and the maintenance of biodiversity by preserving ecosystem health and contributing to important ecological functions”, as reported by Dirk Schmeller et al.
Such a wide-spread engagement would all in all not be an ill-considered decision especially in the knowledge that “[b]oth host and environmental microbiomes are interconnected and regularly exchange microorganisms”, the authors add.
After all, the dilution effect invites us to embrace the idea of the more the merrier, because it pushes the disease risk down.
Be that as it may, the study by James Jones et al. takes yet another angle to the biodiversity-disease discussion. Instead of resorting to generalizing trends of species biodiversity, they claim that zoonotic pathogen transmission risk is a local, idiosyncratic phenomenon.
That is not to say that the dilution effect could not hold in certain circumstances. The meta-analysis conducted by the authors rather emphasizes that “[a] focus on the local ecology and interactions of important hosts and vectors is more likely to reveal insights into how to reduce disease risk”.
So, more is still less, albeit not in a general way.
Taking One Step Back
But perhaps the entire current discussion of how biodiversity conservation impacts the rate of infection of reservoir hosts or pathogen transmitters (vectors) might come crumbling down if we humans continue to encroach upon the habitats of animals.
As a matter of fact, anthropological disturbances could very well change the behaviour of animal hosts, thereby affecting the odds of zoonotic transmission and the patterns of disease outbreak.
In particular, Jun Zeng et al. calculated that the avian influenza virus H7N9, which broke out in eastern China in 2013, did originally not resemble the influenza viruses capable of human-to-human transmission. In fact, the novel virus evolved over a period of 11.3 years in order to acquire such specific transmission properties.
At the same time, Timothy Robinson et al. associate the emergence of H7N9 with the rapid intensification of the poultry industry. As these economic activities occur in the midst of regions characterized by wetland agriculture and habitats of wild birds (waterfowls), increased contact with reservoirs of avian influenza viruses becomes inevitable.
Based on such views, we may be inclined to think that we need to withdraw ourselves from wildlife-rich areas as much as possible. Although there is certainly some truth to that against the backdrop of proliferating pathogen communities, it is equally sensible to remain mindful of the biodiversity hypothesis of health.
That hypothesis tells us that not only less contact with the natural environment but also a decline in biodiversity deprives us from exposure to a rich microbial diversity, pushing up in this way the risk of inflammatory diseases as our immune system subsequently degrades.
In the end, finding a stable, dynamic balance between the living spaces of humans, animals, and the natural environment is probably the most insightful route to take. Are biological processes in nature also not guided by a self-regulating, stability-providing mechanism called homeostasis?
Making the Circle Round
To a large extent, it appears that encouraging biodiversity reassures a healthier life with a minimal risk of disease emergence while alleviating pressure on our climate and natural environment.
Having said that, a global one-size-fits-all approach to biodiversity conservation does not cut the deal. Rather, fostering greater sensitivity to local conditions might be a more reasonable way to go about restoring a richer variety of life.
But if we truly wish to witness such greater biodiversity, we must ask ourselves the following questions:
What are we willing to do to save and sustain our biological hotspots, which prove crucial to the whole ecosystem of our planet? Are we willing to adopt more ecologically tenable and equitable ways of living our lives, for the sake of survival of ultimately all forms of life?
After all, we can’t have our cake and eat it too.