The value of long-term studies

It can take years to notice the effects of changes in nature. Long-term research is therefore essential. If there are far fewer great tits or winter moths in a year, is that an exception or a new trend due to climate change? The long haul needed to measure this year-on-year is a big challenge. The National Institute for Ecological Research NIOO-KNAW, which celebrates its 70th anniversary this month, does this kind of research. What does this research teach us?

'If you really want to understand where changes come from, you have to take measurements. And that can soon take years,' explains animal ecologist Marcel Visser. 'This is the only way we get to know ecosystems, on which we can base predictions and insights for nature management. Such as how to prevent calcium deficiency in animals or the best moment in spring is for mowing the grass while preserving the insects.'

At the Netherlands Institute of Ecology (NIOO-KNAW), Visser leads the research on birds breeding in nest boxes, with the great tit as the main star. That research is experiencing its 70th field season with a dubious record: the earliest laid great tit egg in seventy years, on 23 March to be precise. The previous record stood at weeks later, in April. Besides the work on birds, there is more research for which the ecologists have been collecting data in the same way for years: from soil development on former agricultural plots to the effects that light pollution has on nature.

Interactions

Ecology centres on the interactions between organisms and their environment. That ecological realm is dynamic. It is therefore quite a scientific challenge to grasp the ecological processes and patterns in the world around us, and to understand how they change naturally or through human influence. Predicting what our environment will look like in the future is the ultimate challenge, since the number of tools to investigate that is limited.

Usually an experiment under controlled conditions gives you insights into parts of an ecosystem, but not the whole. And causes of changes in nature are often difficult to determine from isolated observations. Long-term research puts us in a better position to track the effects on biodiversity of large-scale environmental changes. Soil ecologist Wim van der Putten has years of experience with such research. 'For nature management, it is important to know what they can do to ensure species survival and sustainability of ecosystems, and to know to what extent changes can be mitigated or how to move with the changes.'

Birds in the boxes

Back to the birds. The great tit research started at the Plant Protection Services (Plantenziektekundige Dienst). The idea was to attract insect eating birds like great tits by hanging nesting boxes, which would then control insect plagues in the forestry sector. In 1912, a trial of nesting boxes began on the Oranje Nassau's Oord estate near Wageningen. The idea soon turned out not to work: the nesting box inhabitants ate a lot, but not all pest insects. Nevertheless, the research was followed up. In 1955, the IOO, one of the forerunners of the NIOO, based a much larger project on it. Cabinets were hung at four locations around the country, which are still monitored today. All tits are known individually, with complete records of their 'marital status'. Worldwide, this is now one of the longest-running monitoring series.

The research studies how bird populations change when the environment changes. What happens to their numbers? How many eggs do they lay, when and with how much success? The great tit proved to be a valuable model species, providing insight into the evolution of bird behaviour and reproductive strategies. Visser: 'The tit populations already provided us with a lot of information on the effects of urbanisation, calcium deficiency (thinner egg shells due to acid rain) and adaptation of their behaviour to changing environmental conditions.'

Best known is how the food chain of songbirds is disrupted by climate change. In the database of the many thousands of great tits, the changes are finely documented. Visser already showed this at the end of previous century: climate change causes caterpillars of the winter moth to develop much quicker in spring. The caterpillars are the favourite food of young tits, but in a warmer world they hatch too late for that. Great tits pay much more attention to day length for breeding. This made the Dutch great tit missing out on food for its young a global example of an ecological mismatch.

The next surprise was that you don't immediately see the effects of climate change in the following year. Visser: 'The birds are breeding late, resulting in dead young in the nest, yet the following year you see the same number of breeding birds again. How is that possible?' After puzzling for a while, the explanation was found. 'It's like a lottery: with more lottery tickets, you have less chance of winning. Similarly, individual young birds are more likely to survive the winter if there are fewer 'competitors' left to share the food with.' This buffer ensures, temporarily, that the effects of climate change are not immediately noticeable.

Into the soil

In the middle of the Veluwe, close to De Mossel, Van der Putten has been conducting research on decommissioned farmland for about 25 years. The original question was how to transform those soils into semi-natural ecosystems. 'We learned that it can take up to 10 to 20 years before you get soil life with a more natural composition and food web structure,' Van der Putten summarises his experiences. 'With soil transplantation, where you add the soil community from good soil from elsewhere, this period can be shortened to five to 10 years or even less.' In Het Nationale Park De Hoge Veluwe, a soil transplantation trial is now under way to develop 'climate-proof' forests.

Understanding of how common ragwort can be tamed by natural soil diseases was also gained at De Mossel. Van der Putten: 'All the accumulated knowledge about plant-soil and underground/above-ground interactions we now apply to make agriculture more sustainable. We have also gained a lot of knowledge that we use to advise on the new European soil monitoring law.' Namely: what makes sense to measure?

A new offshoot in the same area is the work of Ciska Veen. As part of the international Nutrient Network with some 100 sites, she studies the effect of fertilisation with nutrients such as nitrogen and phosphate in grasslands, combined with grazing. 'These are the two main drivers of biodiversity, stability and the occurrence of exotic species.' The seventh year has just started. Veen wants to use the results of the study to predict what will happen to the grasslands of the future. Her Veluwe-research just yielded an interesting effect of nitrogen fertilisation. Veen: 'It reduces plant and insect diversity, but as long as you graze the grassland, those effects on biodiversity are not too bad.'

Light on nature

Twelve years ago, light ecologist Kamiel Spoelstra placed rows of lampposts with different colours of light at eight locations of natural forest edges. There, he observes the effects the light has on the natural environment. Do species disappear, are new species gained, do the density of and interactions between species change, does the activity of animals change and do they suffer from stress?

'Only after five years did it become clear that great tits were more likely to breed near white and green lights: the colder the spring, the greater the effect of light at night. An increase in moths in the first year was followed by years of decline. Continued attraction to the light and subsequent mortality eventually the moths 'ran out' in the vicinity of the lights. 'Only through multi-year research can we establish these kinds of effects,' Spoelstra explains. 'The research results are used directly in national and international lighting policy, you don't want to give the wrong advice on that.

What is long?

How long is long? That depends on the species and processes you are studying. Species can take years before they reach a tipping point in their populations. Visser: 'The thing about many things in ecology is: as soon as you look at something with an annual cycle, it quickly becomes a long study. From about 10 years onwards, the study actually only starts to discover patterns. That's much longer than a standard research duration.'

Research on populations is better measured in generations than in years, Visser believes. Because that is what species adaptation is all about: does the next generation do better in the changing circumstances? In a species with a generation time of a few days, you find out much faster than if you can only do one measurement every year. In rotifers, which NIOO also studies, real changes are visible after just a few months.

One year is cold and wet, the next hot and dry. Another important reason for research over several years is the influence of many variables in nature. How else do you know what is 'average' or 'abnormal' and what an effect is caused by. 'If we look at acid rain, manure problems and climate and biodiversity change, you actually need measurement series of at least forty years,' Van der Putten argues. 'That requires structural support for long-term research.'

'In short, you can rarely determine effects with just a few measurements. And this applies to a greater extent to species with long population cycles such as lemmings and mast years,' Visser concludes. And if you think you know the pattern, sometimes you are still in for a surprise. The winter moth always had an 11-year cycle, but it is unclear whether it still has one. 'Since 1992, we have been counting females sitting on oak trunks in winter waiting for males. This time, the expected increase in numbers failed to materialise: an exception? You'd actually want a hundred years of monitoring data then.'

Citizen science

The Dutch Centre for Avian Migration and Demography is also part of the NIOO. In the Netherlands, bird ringing for research, policy and protection began in 1911. It is the oldest form of citizen science at the institute. More than a century later, the database contains a wealth of information. As of April this year, there are 15 million birds registered that have ever been ringed (over 12 million) or recaptured.

It turns out: bird migration is dynamic. Gradual changes but sometimes surprisingly rapid adjustments have been recorded in the unprecedentedly long measurement series. Head of the Centre for Avian Migration Henk van der Jeugd: 'Many birds nowadays migrate less far, or stay the whole year like blackbirds. Eurasian blackcaps, on the other hand, have found new wintering grounds.' With standardised ringing projects, the Bird Migration Station tracks demographics. 'We clearly see that the survival of young birds in many species is under pressure, and structurally too low to sustain populations.' A new branch is infectious diseases. Bird ringers have already found three viruses of tropical origin for the first time in the Netherlands, such as the West Nile virus, which is also dangerous for humans.

The future

If you have such long-term measurement and really learn to understand the natural system, you can start using that as a model to make predictions and test them out. We have to be careful with that though: you can only predict patterns, not details. Take a question from nature management like: which trees should we plant next? Then you have to predict which tree will do well in 50 years. We cannot pinpoint the exact number of oak trees, but we can tell whether the oak will decrease or increase compared to the pine, for example. It is good to realise that ecological predictions are more like a climate prediction than a weather forecast.

We need the knowledge from the past to make better forecasts. We can use artificial intelligence to discover patterns and to link datasets. Van der Putten is now using data from soil food webs, which involve thousands of species of organisms, to predict how best to make a transition to sustainable and multifunctional soils.

Another development is that of digital twins. For example, the new programme LTER-LIFE will build a digital version of the Veluwe and the Wadden Sea, using and linking all available measurements from those areas. 'Preferably, we will also add measurements from an entire food chain instead of individual species, such as for the great tit. In this way, we hope to soon be able to better predict how these ecosystems respond to change,' Visser says. It is therefore crucial that data will become even more public and better available, making them reusable for initiatives that can help nature and society of the future.

A quick glance at NIOO-KNAW

The Netherlands Institute of Ecology conducts research on biodiversity, climate change and sustainable use of land and water across the country and far beyond: from Spitsbergen to Ethiopia and Brazil. With over 250 staff members and guest researchers, national research facilities and a sustainable building in Wageningen, it is one of the largest institutes of the Royal Netherlands Academy of Arts and Sciences (KNAW). 

On 26 June 2024, it was exactly 70 years since the KNAW founded the Institute for Ecological Research (IOO); the first of three ecological research institutes to professionalise ecology in the Netherlands. The IOO (Arnhem/Kampen/Oostvoorne/Heteren) was followed three years later by the Delta Institute for Hydrobiological Research (Yerseke) and the Limnological Institute (Nieuwersluis/Oosterzee). In 1992, the institutes joined forces in the NIOO. Long-term research has been NIOO's strength from the start. 

Want to read more? Find the lustrum page here https://nioo.knaw.nl/en/70