5) Terrestrial colonisation

A direct driver of global biodiversity loss is land-use change, in particular, habitat loss, fragmentation and degradation. Modification of land combined with other direct drivers (Figure 1) have altered ecosystems. Between 1970 - 2014, human populations doubled whilst wild vertebrate populations halved. Our impact is no longer sustainable and is affecting Earth system processes including surface energy balance, carbon cycle, water cycle and thus - species survival.

Figure 1. Infographic showing the direct drivers of biodiversity loss, and associated impacts on terrestrial and aquatic ecosystems

Where the wild things are (no longer)

In 1700, half of the terrestrial biosphere was once wilderness, untouched by humans. Today, over 75% of land has been modified into anthropogenic biomes, known as anthromes.

Definition: Anthromes are globally significant ecological patterns created by sustained interactions between humans and ecosystems, including urban areas, villages, cropland, rangeland and pasture. Similar to biomes, which are formed by global patterns of climate, anthromes are formed by global patterns in human populations and their use of land over long time periods. 

Globally, anthromes are typically concentrated in the regions of higher biodiversity and agricultural productivity (Figure 2), thus, exposing more species to anthropogenic activity.

Figure 2. A global map of anthromes and wildlands: the diagram shows general patterns in human population densities, land use within and across anthromes, and their relationship with emerging novel habitats and biotic communities. (Ellis, 2013)

The spread of anthromes into wildlands have caused species extinction through fragmentation and transformation of species' native habitats. Even conservation efforts that protect biodiversity hotspots by designating IUCN protected areas have disturbed ecosystems. Drawing borders to national parks habitually cut off wildlife corridors, vital for sustaining species' populations. The extent of 'used' anthromes neatly maps over the urbanisation and transportation system; our connectivity penetrates terrestrial biodiversity (Figure 3).

Figure 3. The global transportation system: a visualisation of the global roads, shipping routes and air networks that connect urban areas.

The masquerade of tree planting

Global tree cover has increased by 7.1% (2.24 million km ²) since 1982 (Figure 4), this respite however, is at mid-latitudes caused by reforestation efforts in China and Africa. The decline of tree cover in tropical dry forest biomes in South America and Central Africa continues, driven by agricultural intensification. These regions are biodiversity hotspots, rich in named and unnamed taxa, both under threat from habitat conversion. For example, a new law authorised road construction through TIPNIS, Bolivia, one of Earth's most biodiverse regions. The trend of human-wildlife conflict caused by prioritising economic gains over conservation, is rising throughout Amazonia.

Figure 4. A satellite-based record of global TC, SV and BG cover from 1982 to 2016. Both mean and change estimates are expressed as per cent of pixel area at 0.05° × 0.05° spatial resolution. Pixels showing a statistically significant trend (n = 35, two-sided Mann–Kendall test, P < 0.05) in either TC, SV or BG are depicted on the change map. Circled numbers in the colour legend denote dominant change directions: 1, TC gain with SV loss; 2, BG gain with SV loss; 3, TC gain with BG loss; 4, BG gain with TC loss; 5, SV gain with BG loss; and 6, SV gain with TC loss. Credit: Nature (2018)

Whilst reforestation efforts help carbon capture, the impact on local biodiversity is challengeable. The value of native species as supporters of other species and providers of ecosystem services is greater than that of non-native species. Therefore, planting new trees may not complement the existing community. The locals will recognise the stranger in their neighbourhood!

Local but global: seeing the bigger picture

Focusing on the health of local biomes risks skirting over global biodiversity trends. Whilst biodiversity trends in local populations (α-diversity) remains non-decreasing, changes in β-diversity occur as geographical ranges become disrupted. Evidence shows a net loss of species globally whilst local losses remain constant over time.

Definition: A biome is a community of plants and animals that have common characteristics for the environment they exist in. They can be found over a range of continents. Biomes are distinct biological communities that have formed in response to a shared physical climate. Biomes a broader term than habitat; any biome can comprise a variety of habitats.

Another caveat: assessment of global biodiversity decline measures the richness in vertebrate data, thus are not reflective of the broader ecosystem. How local ecological assemblages respond to land modification remains unclear. Temporal analysis that compares α-diversity/β-diversity may underrepresent land use transitions. Evident is the complexity of measuring the relationship between land use-change and biodiversity trends. Nevertheless, scientists confirm that:

Human-dominated areas are inferred to have lost much more local diversity than have regions where more natural vegetation remains. The worst-affected cells showed a 31% reduction in average local richness—probably enough to alter ecosystem functioning substantially. - Newbold et al, 2015

Aerial view of deforestation in Nascentes da Serra do Cachimbo Biological Reserve in Altamira in Brazil’s state of Pará and the Amazon basin on August 28, 2019. Credit: Joao Laet

Summary of ideas:

• Global tree cover has increased due to reforestation, but deforestation of tropical biomes is rising, thus threatening global biodiversity hotspots.

• Local biodiversity remains constant over time but globally, vertebrate species are declining.

• Critical further research is required to understand how changes in local biodiversity influence the health of larger ecosystems.

For humanity's sake

Land modification creates a superficial dichotomy between humans and nature. In reality, we thrive from biodiversity because ecosystem services function best with diverse biomes. Habitat modification has the potential to drive functional ecosystems beyond tipping points, at which fundamental services such as biomass production and pollination become disrupted.

The Earth's resources and resilience are finite, exploitation in the name of development has been costly to the biosphere. Species loss remains rampant as we continue to extract from nature. The higher the species diversity in an ecosystem, the more resilient the ecosystem.

Maintaining diversity will provide greater security in the face of climate change.

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See you soon!

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