It’s a pretty loaded question, isn’t it? The suggestion that our species is the greatest threat to biodiversity is—perhaps bizarrely—a hugely polarising topic. Despite the vast extent of evidence to the contrary, many people still believe that humanity’s impact on the environment is nominal. I believe part of this disconnect is due to a lack of understanding. I’ve deciphered countless research papers, trudged through reviews, and collated all of the available evidence. Spoiler alert: it all points back to us.
I’m going to start by explaining a bit about what biodiversity actually is. Biodiversity encompasses the genetic variation within a species, the variety and abundance of species within an ecosystem, and all of the habitats within a given landscape. I believe biodiversity holds an intrinsic value; each species and habitat has a value and a right to exist, regardless of their meaning to humans. If that weren’t enough, biodiversity also has a quantifiable value; it is intimately linked with the delivery of ecosystem services, environmental processes which benefit us as a species. Despite its obvious value, biodiversity is declining on a global scale and shows no signs of stabilising.
The past five hundred years have witnessed a wave of extinction events and local population declines, comparable in speed and scale to the five previous mass extinction events in the Earth’s history. Just like the extinction events which have preceded it, this ‘sixth wave’ of extinction affects all taxonomic groups—but some species suffer more than others. In previous extinction events, a combination of environmental factors—for example, asteroid impacts or volcanic eruptions—have driven extinction. In this case, however, many of the driving factors for species loss have anthropogenic roots. Exploitation, habitat degradation, climate change and invasive species threaten the future of life as we know it. In this blog, I want to investigate the role these factors play in biodiversity fluxes, and how much of this effect can be attributed to humanity.
Biodiversity Losses: Scale and Drivers
In order to assess the impact humanity has on global biodiversity, we need to understand the magnitude of declines and the processes which drive them: do biodiversity losses occur naturally on this scale, or is humanity responsible for the global extinction of species?
The long-established drivers of biodiversity loss in the modern age—overexploitation, habitat loss, habitat degradation and invasive species—occur on a grand scale. In fact, there is evidence to suggest that these processes have increased in magnitude and speed during recent years. In addition, more recently identified threats such as climate change and disease are rapidly increasing, impacting biodiversity around the globe.
Biodiversity levels can and do fluctuate naturally. The extinction of species is a natural process which has occurred throughout history: for every million species, 0.1 become extinct annually as part of a background extinction rate. However, based on this estimate, the current rate of extinction is around 1000 times higher than the natural rate—and could grow to 10,000 times higher in the future. This clearly indicates that recent developments have triggered the acceleration of biodiversity losses on a massive scale.
The Living Planet Report, published annually by the World Wildlife Fund, measures biodiversity abundance levels based on 14,152 monitored populations of 3,706 vertebrate species (as ‘Living Planet Index’/LPI). During the period of 1970 to 2012, the LPI has fallen by 58%. According to this trajectory, by 2020 we should except to see a decrease of 67% compared to biodiversity levels in 1970. These figures genuinely make my stomach turn; we have already lost so much of the Earth’s biodiversity, and if we don’t change, things will only get worse.
How does Climate Change Affect Biodiversity?
The Living Planet Report cites climate change as one of the main threats to biodiversity—and research has suggested that it could surpass habitat destruction in the next few years, becoming the greatest threat to biodiversity on Earth. I won’t talk extensively about the effects of climate change, otherwise we’d be here all night. Check out fig. 2 for a summary of the effects global warming is likely to have.
Climate change is likely to result in world-wide alteration of mean temperatures. Projected temperature increases are likely to exceed the habitable limits for many species, resulting in the extinction, reduction or relocation of entire species. The oceans will also be affected by temperature increases; thermal expansion and glacial melting have caused sea levels to rise for the past century. Not only does this threaten the habitats of coastal plants and animals, it also threatens the animals which rely on glaciers for one or more of their life stages.
Relative to levels prior to the industrial revolution, surface oceanic waters are now 0.1 pH units more acidic as a result of anthropogenic carbon emissions. This acidification will have severe biological consequences; marine organisms such as coral will be unable to function properly. This has the potential to alter ecosystems beyond repair, resulting in the loss of species on a huge scale.
The scientific community is in almost unequivocal agreement that global warming is a direct result of anthropogenic activity. As a result, we must conclude that the biodiversity losses which will inevitably occur as a result of climate change are directly attributable to the human race.
How do Invasive Species Reduce Biodiversity?
An unfortunate side-effect of tourism, migration and trade in our increasingly globalised society is the unrestricted movement of species away from their native habitats, resulting in their introduction into new ecosystems (see fig. 3). In many cases, these creatures—known as invasive species—perform better in novel habitats than in their native region, often due to a lack of their natural enemies or competitors. This enables invasive species to take over new ecosystems, resulting in the displacement and loss of native species—either through predation, disease or competition. For this reason, invasive species are now considered to be one of the most prominent threats facing global biodiversity. Despite this, the full impact of invasive species is difficult to quantify; most data is compiled at local or national levels and the existing data is inconsistent in terms of format and definition, preventing comparison across regions.
Although invasions are not a novel process, nor are they driven exclusively by humans, the scale and rate of invasion has grown massively as a result of globalisation. For this reason, we can conclude that the vast majority of losses as a result of invasive species can be attributed to human influence.
Exploitation as a Driver of Biodiversity Loss
Exploitation refers to the harvest of natural resources and organisms at an unsustainable rate. This process can be direct— for example, through practices like hunting or fishing—or indirect, through the unintentional destruction of species by other practices—for example, as bycatch in fisheries. Exploitation can account for as much as 24% of the decline in freshwater organisms, 25% of the decline in terrestrial species, and 42% of the decline in marine species; these figures demonstrate just how devastating exploitation can be. For the purpose of this essay, the exploitation of fish stocks and bycatch will be used as a benchmark for exploitation.
Overfishing is a hugely damaging form of exploitation; an increasing number of fish-stocks are overexploited, depleted or even collapsed. Fig. 4 demonstrates the huge proportion of fish stocks which are exploited. Unsustainable fishing practices—for example, bottom trawling and dynamite fishing—are damaging formerly biodiverse habitats such as coral reefs. In addition, inappropriate fishing methods result in the capture of huge amounts of unwanted fish known as by-catch. Some estimates suggest that by-catch could make up as much of 40% of total global catch, including 85,000 turtles and 600,000 marine mammals each year, with severe consequences for species conservation. Persistent overfishing can have severe effects on marine biodiversity; local extinction and population collapse occur on a fairly regular basis as a result over overexploitation. The extinction of species through this process has far-reaching effects; it also jeopardises the future of any species which interacted with the now extinct species.
Exploitation is a threat which can only be delivered by humans; we are the only species on Earth capable of harvesting resources to excess despite our ability to understand their finite nature. As a result, we can conclude that humans are responsible for biodiversity losses caused as a result of exploitation.
A Huge Threat to Biodiversity: Habitat Degradation, Change and Loss
Habitat change and degradation, combined with total habitat loss, account for 45% of species losses. The modification of landscapes for agriculture and urbanisation is responsible for a large proportion of habitat losses and change. Humans typically modify landscapes in the same way; native vegetation is shrunken, dissected, perforated, sub-divided and weakened by anthropogenic influence. This isolates population and removes flora vital to one or more of the life stages of a huge range of species.
Habitat degradation is the process by which a habitat gradually declines in quality, often leading to the reduced survival and/or reproductive success of species living there. It is often difficult to detect habitat degradation; some forms take a long time to manifest, and some species with long generation times may show presence in an area even if they are unable to breed. The factors which lead to habitat degradation vary between species; for example, grazing pressure or deforestation.
Although habitats can change as a result of entirely natural processes, anthropogenic activity has increased the speed and scale of these changes beyond comprehension. Unsustainable agriculture, deforestation, transportation, infrastructure development, energy production and mining all result in the destruction and degradation of habitats vital to the preservation of biodiversity on earth.
How can we Preserve Biodiversity?
In order to prevent further biodiversity losses, we must put measures in place which protect vulnerable species on a global and local scale. ‘Refugia’ are one of these measures. The term refugia refers to a microhabitat which provides protection in space and or/time from outside disturbances, as well as providing biotic advantages. In the past, species have contracted to—and persisted within—refugia when climates have become unsuitable. Refugia are not a cure-all solution; the greater the amplitude and the rate of climate change, the less likely it becomes that refugia will remain habitable. However, refugia could provide a number of species with a greater chance of survival under the effects of climate change. The identification and protection of both past and future refugia should therefore be pursued.
Ecological restoration aims to restore biodiversity and ecosystem services in habitats which have been damaged or disrupted. Action can be taken through a community approach, in which interventions which accelerate natural succession are utilised. Methods which bypass intermediate successional phases, such as planting late-successional tree species, can be used to speed up ecosystem recovery and restore biodiversity. An ecosystem approach, in which efforts focus on the restoration of ecosystem functions such as nutrient cycling and primary production, can also be used. Thus approach aims to restore abiotic conditions, facilitating the passive recolonisation of species. For example, reforestation of former woodland sites can alter soil composition, making the area habitable to a greater range of species.
In order to ensure the future of global biodiversity, law and policy-based changes must be enforced. In China, the recent development of an ambitious Ecological Civilisation plan could provide a solution to global biodiversity declines. Under this plan, China is divided into a number of sections, and specific sections are highlighted for their contribution to ecosystem function; for example, biodiversity conservation or flooding mitigation. The intensity of development in these zones is restricted, with the aim of reducing human impacts on ecosystem function—including biodiversity.
Is Humanity the Greatest Threat to Global Biodiversity?
Humanity occasionally contributes to the diversification of gene pools and the emergence of new traits. In addition, there are threats facing biodiversity which are beyond our control—as shown by the previous extinction events which occurred in a pre-human era. Despite this, the threats repeatedly identified as causing the greatest amount of damage to biodiversity worldwide are intimately linked to human activity. For this reason, we can conclude that humanity is the greatest threat to global biodiversity.