It is a sad but evident fact that the expansion of agricultural areas over the past centuries has led to an enormous loss of natural habitats worldwide. However, it is perhaps less well known that the unprecedented intensification of agriculture in the decades following the Second World War has also caused the disappearance of many weed species traditionally associated with extensively managed farmland. These rare weeds used to inhabit arable fields, oldfields, and pastures, but many have now become endangered.
Formerly widespread weed species have declined due to multiple factors: changes in landscape structure (due to land consolidation), altered tillage systems (for example, changes in the timing of ploughing), the increasing use of chemicals in agro-ecosystems, the disappearance of fallows and oldfields, and the abandonment or overgrazing of pastures. Importantly, rare weeds do not threaten crop yields. Instead, they contribute to agro-biodiversity and provide valuable ecosystem services. Their preservation is therefore important both for nature conservation and, indirectly, for agricultural production.
One promising refuge for rare weeds is kurgans – ancient burial mounds scattered across Central, Southeastern and Eastern Europe and beyond. When embedded in agricultural fields and covered with grasslands of various ages, these small, elevated sites often escape intensive management and host diverse vegetation.
In an intensive field survey covering approximately 21,500 km² in Eastern Hungary, we set out to identify which site- and landscape-specific factors (such as geographic position, habitat area, soil properties, vegetation age, and environmental heterogeneity) influence the occurrence and species richness of rare weeds. Altogether, we surveyed 216 kurgans: 24 covered with ancient grasslands and 192 with successional grasslands, ranging in age from one to 144 years. All were out-of-production sites surrounded by agricultural fields.
We recorded 38 weed species that have become rare in the region in recent decades, including 15 red-listed and two legally protected species. Rare weeds were present on more than half of the kurgans (50.9%), showing that even small, abandoned patches of land can provide important refuges for species once common in arable lands, oldfields, and pastures.
Several ecological factors shaped their occurrence. Environmental heterogeneity – linked to the topographic variation of kurgans – proved to be the strongest predictor of whether rare weeds were present at a site. And species richness of rare weeds was also considerably higher on mounds with larger environmental heterogeneity. Species richness was generally lower in northern sites, which reflects the Mediterranean origin of many rare arable weeds and their association with warmer climates in southern regions. High soil CaCO₃ content supported species-rich weed communities. While high soil phosphorus content suppressed them, likely by increasing the level of competition in nutrient rich environments. Sites with more diverse vegetation harboured more oldfield and grassland-related rare weeds, probably due to reduced competitive pressure typical of species rich grassland assemblages.
Interestingly, we also found that smaller kurgans hosted more rare weed species. At first glance, this seems paradoxical: for habitat specialists, larger areas usually support more species because they contain larger undisturbed core zones. In contrast, as rare weeds depend on a certain level of disturbance, the high edge-to-core ratio of small kurgans appears to provide precisely the slightly disturbed conditions they need. As was found in our previous study, the presence of burrowing mammals (such as red foxes) on kurgans can also support the existence of disturbance tolerant species that require open gaps for establishment.
In summary: our study shows that small, island-like out-of-production sites on kurgans can play an outsized role in maintaining rare weed populations in intensively managed agricultural landscapes. Preserving such landscape features – a form of land sparing – can therefore make a major contribution to conserving biodiversity in farmland.
The full study, published in Agriculture, Ecosystems & Environment, provides further insights into factors supporting the existence of rare weeds on kurgans holding pristine or secondary grasslands.
The citation of the paper:
Deák, B., Bede-Fazekas, Á., Süveges, K., Tölgyesi, Cs., Kelemen, A., Bede, Á., Borza, S., Godó, L., Valkó, O. (2025): Ancient mounds, modern refuges: Out-of-production sites on kurgans support rare weeds in agricultural landscapes. Agriculture, Ecosystems & Environment, 394: 109902.
The paper is open access and can be downloaded from here:
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