Climate change could become the main driver of biodiversity decline by mid-century

Global biodiversity has declined between 2% and 11% during the 20th century due to land-use change alone, according to a large multi-model study published in Science. Projections show climate change could become the main driver of biodiversity decline by the mid-21st century.

The analysis was led by the German Centre for Integrative Biodiversity Research (iDiv) and the Martin Luther University Halle-Wittenberg (MLU) and is the largest modelling study of its kind to date. The researchers compared thirteen models for assessing the impact of land-use change and climate change on four distinct biodiversity metrics, as well as on nine ecosystem services.

GLOBAL BIODIVERSITY MAY HAVE DECLINED BY 2% TO 11% DUE TO LAND-USE CHANGE ALONE

Land-use change is considered the largest driver of biodiversity change, according to the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES). However, scientists are divided over how much biodiversity has changed in past decades. To better answer this question, the researchers modelled the impacts of land-use change on biodiversity over the 20th century. They found global biodiversity may have declined by 2% to 11% due to land-use change alone. This span covers a range of four biodiversity metrics1 calculated by seven different models.

"By including all world regions in our model, we were able to fill many blind spots and address criticism of other approaches working with fragmented and potentially biased data," says first author Prof Henrique Pereira, research group head at iDiv and MLU. "Every approach has its ups and downsides. We believe our modelling approach provides the most comprehensive estimate of biodiversity trends worldwide."

MIXED TRENDS FOR ECOSYSTEM SERVICES

Using another set of five models, the researchers also calculated the simultaneous impact of land-use change on so-called ecosystem services, i.e., the benefits nature provides to humans. In the past century, they found a massive increase in provisioning ecosystem services, like food and timber production. By contrast, regulating ecosystem services, like pollination, nitrogen retention, or carbon sequestration, moderately declined.

CLIMATE AND LAND-USE CHANGE COMBINED MIGHT LEAD TO BIODIVERSITY LOSS IN ALL WORLD REGIONS

The researchers also examined how biodiversity and ecosystem services might evolve in the future. For these projections, they added climate change as a growing driver of biodiversity change to their calculations.

Climate change stands to put additional strain on biodiversity and ecosystem services, according to the findings. While land-use change remains relevant, climate change could become the most important driver of biodiversity loss by mid-century. The researchers assessed three widely-used scenarios -- from a sustainable development to a high emissions scenario. For all scenarios, the impacts of land-use change and climate change combined result in biodiversity loss in all world regions.

While the overall downward trend is consistent, there are considerable variations across world regions, models, and scenarios.

PROJECTIONS ARE NOT PREDICTIONS

"The purpose of long-term scenarios is not to predict what will happen," says co-author Dr Inês Martins from the University of York. "Rather, it is to understand alternatives, and therefore avoid these trajectories, which might be least desirable, and select those that have positive outcomes. Trajectories depend on the policies we choose, and these decisions are made day by day." Martins co-led the model analyses and is an alumna of iDiv and MLU.

The authors also note that even the most sustainable scenario assessed does not deploy all the policies that could be put in place to protect biodiversity in the coming decades. For instance, bioenergy deployment, one key component of the sustainability scenario, can contribute to mitigating climate change, but can simultaneously reduce species habitats. In contrast, measures to increase the effectiveness and coverage of protected areas or large-scale rewilding were not explored in any of the scenarios

MODELS HELP IDENTIFY EFFECTIVE POLICIES

Assessing the impacts of concrete policies on biodiversity helps identify those policies most effective for safeguarding and promoting biodiversity and ecosystem services, according to the researchers. "There are modelling uncertainties, for sure," Pereira adds. "Still, our findings clearly show that current policies are insufficient to meet international biodiversity goals. We need renewed efforts to make progress against one of the world's largest problems, which is human-caused biodiversity change."

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How does biodiversity change globally? Detecting accurate trends may be currently unfeasible

Existing data are too biased to provide a reliable picture of the global average of local species richness trends. This is the conclusion of an international research team led by the German Centre for Integrative Biodiversity Research (iDiv) and the Martin Luther University Halle-Wittenberg (MLU). The authors recommend prioritising local and regional assessments of biodiversity change instead of attempting to quantify global change and advocate standardised monitoring programmes, supported by models that take measurement errors and spatial biases into account. The study was published in the journal Ecography.

The global loss of biodiversity has been recognised by society and politicians as one of the most urgent challenges facing humanity in the coming generations. At the World Biodiversity Conference COP15 that recently took place in Montréal, the member states of the UN Convention on Biological Diversity (CBD) adopted new goals and rules accordingly to slow down and eventually reverse this decline. In order to be able to measure the successes of this new agreement, one of these targets calls for improved biodiversity monitoring to record and evaluate trends.

While there are many different ways to measure biodiversity, the most common is species richness at the local scale. However, although species are being lost at alarming rates at the global level, this does not always reflect what is occurring at the local scale. Previous global syntheses have indicated conflicting results on the extent and even direction to which local species richness is changing. "There has been a heated debate on the scientific community on why major global syntheses so far have not found negative trends of local species richness," states Prof Henrique Pereira, head of the Biodiversity and Conservation Research Group at iDiv and MLU and last author of the study. "We show that the declines in local species richness are likely to be much smaller than many anticipated and that, in those conditions, even minor spatial biases and errors in monitoring lead to the lack of detection of global trends."

In order to create a global picture of what is occurring at the local scale, all available observation data must be compiled and evaluated across time. "The occurrence of species is recorded locally all over the world by many different people and organisations," says first author Dr Jose Valdez, a postdoctoral researcher at iDiv and MLU. "The problem with the data is that they were and are recorded under completely different conditions and mostly not under standardised rules. If you then pile them together, the errors and deviations add up, making the result very inaccurate."

The researchers were able to show that the monitoring results are significantly influenced by various factors, such as the time intervals between sampling, the size of the sampling sites, or small errors in counting the number of species at a site. A significant problem in recording global biodiversity trends is also the regional imbalance. For example, most of the data is collected in world regions such as Europe and the United States, particularly habitats such as temperate deciduous and mixed forests. The underrepresentation of the tropical regions and habitats, areas with the highest species richness and also the largest losses, can lead to a significantly distorted impression of the global biodiversity status.

To find out whether and how these biases can be compensated for, the researchers simulated thousands of monitoring networks that varied in the above-mentioned factors. The basis for this was provided by the PREDICTS projections of local species richness trends, based on a model developed with a globally comprehensive compilation of data from over 32,000 sites worldwide and over 51,000 species. The researchers found that global changes in biodiversity could theoretically be determined in hundreds of perfectly sampled sites within a decade and thousands of sites within a 3-year period.

Changes in species richness on a global scale only detectable with unrealistically many sampling sites

However, perfect sampling does not exist in reality. Studies show that monitoring data typically contain 10% to 30% errors due to missing or misidentifying species during sampling. By just adding very small measurement errors of up to 5%, the researchers found that it drastically reduced the ability to detect any global change. With more realistic errors and further imprecision factors, detecting the average global trend may simply be impossible.

"Our results demonstrate that capturing accurate trends in local species richness would require monitoring an unfeasibly large number of perfectly sampled sites," adds Jose Valdez. "However, the question is whether this would even be useful or meaningful for effective and responsive biodiversity conservation. Conservation strategies and measures are coordinated and implemented not on a global level, but at local and national scales. Measuring biodiversity trends at these smaller scales is not only more practical but also helps in understanding the drivers of biodiversity loss and assessing the progress of conservation policies."

"A substantial increase of biodiversity monitoring is needed, combined with analysis that uses models to fill in data gaps," says Henrique Pereira. The authors advise establishing a representative network of sampling sites around the world that provides independent, integrated, and regularly updated biodiversity data. Such an approach is currently being developed for the European Union with the EuropaBON project.

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