Anthropogenic climate change and land use change in the form of agricultural expansion (‘habitat conversion’ – a sweet description for deforestation) act as synergistic drivers of biodiversity loss – in a Costa Rican environmental experiment – literally drying out the natural diversity of species, bird species at least.

Land use change turning natural forest to agricultural land has a similar effect on neotropical biodiversity as climate change-induced droughts, new study in Global Change Biology shows.

If you just want to know ‘the news’, the above sums up the conclusions from an interesting new study performed by a team of six researchers led by conservational biologist Daniel Karp. Below we cover one single detail that we just personally think is also valuable to add to the broader conversation about climate & ecology – which comes from our interest (possibly background) in geography.

‘Climate change and habitat conversion homogenise nature’

The new study reached headlines shortly after its publication in Global Change Biology last week. In line with the press release by the University of California – Davis (where Karp holds a seat) these read ‘Climate change and habitat conversion combine to homogenize nature’.

Well, as specialised science journalists we really have a very easy job – a comfortable one. We don’t need to go deep in these mosquito-filled tropical jungles, we don’t actually have to understand the computer models or all the science that’s behind them – our task is to keep things simple, and the general direction (of media coverage) as close as possible to the truth. And that’s ‘truth’ as we think we see it, quite literally from our armchair that we have nicely positioned on a sunny balcony after we first drank a homebrew cup of (cheap) coffee. Please interpret the above as a general disclaimer to what’s to follow:

Just a short commentary from our side: two types of synergy – both bad, but different

We think it’s definitely the case that all ecological stressors are –by definition– synergistic drivers of extinction. That’s because biodiversity needs biodiversity to survive (there is ecosystem interdependence to name one thing) – and therefore there are likely positive feedbacks beyond a certain biodiversity ‘safe limit’ (threshold).

But whether or not drivers are synergistic because they both contribute to overall biodiversity decline – or because they literally act in the same direction, hurting the same specific species – that’s a different story, and there geographic differences may become important to bring into the equation.

Climatically all tropical rainforests are the same (sort of). Ecologically they’re different

The authors in the title of their publication (‘Agriculture erases climate-driven β-diversity in Neotropical bird communities’) therefore clearly refer to the geographical setting of the study. The neotropics are one of 8 official ‘biogeographical realms’ (or ecozones) on Earth – a subdivision that’s somewhat broader than biomes (which are essentially climate zones) – but then also differentiated based on the actual endemic flora and fauna diversity.

Therefore tropical rainforest of Asia and America belong to the same biome, but not to the same biogeographical realm. ‘Neo-‘ in ‘neotropics’ stands for the old description of the ‘New World’ – so it’s used to refer to the tropical regions of America, that includes Central America and the entirety of South America (even temperate ecoregions in the South, that are not tropical).

Rainforest in neotropics biogeographical realm. Cartography by Jon Lerner, Peter Dana, and Robert Dull (2008).

A possible effect of climate change on tropical rainforest biomes is disturbance of the daily rain cycle, even to the point where periodic droughts may occur – as is already witnessed on a very large scale in the Amazon rainforest – that as a direct consequence of anthropogenic climate change suffered big droughts in 2010 and again in 2015, causing record-high CO2 emissions from associated forest fires.

The effects of land use change, converting natural habitat to agricultural land has a very similar effect – the research team showed, again favouring a portion of local biodiversity that can survive in dry conditions, while putting extra pressure on species that depend on wet conditions.

In the end it’s always best to let the actual researchers explain it themselves:

“[…] we found that local land-use determined community shifts along the climate gradient. In forests, bird communities were distinct between sites that differed in vegetation structure or precipitation. In agriculture, however, vegetation structure was more uniform, contributing to 7%–11% less bird turnover than in forests. In addition, bird responses to agriculture and climate were linked: agricultural communities across the precipitation gradient shared more species with dry than wet forest communities. These findings suggest that habitat conversion and anticipated climate drying will act together to exacerbate biotic homogenization.”

Climatic complexity: two disturbances promoting drought: rain cycle – and ITCZ

Key to understanding the specific local impacts on biodiversity of climate change, especially understanding drought risk, is apart from disturbances of the daily rain cycle (that is also linked with vegetation density, therefore deforestation for agriculture) understanding possible changes in the Intertropical Convergence Zone (ITCZ) or monsoon. At least over the Amazon basin this change can be big, as local ITCZ climate response is associated with boreal warming that may drag the centre of rainfall towards the Caribbean, possibly exacerbating Amazon rainforest drought.

In any case it is quite clear what we humans have to do, if we want to prevent escalation of the Holocene-Anthropocene Mass Extinction – no matter the direction of the synergy and no matter which driver is actually worse: if we want to protect life on Earth we’ll have to stop both climate change and direct habitat destruction through agricultural expansion. And both challenges are bigger than any single other mankind has faced before.

© Rolf Schuttenhelm | www.bitsofscience.org