La Tranquera Reservoir, Zaragoza. Photo: Diego Delso. Wikimedia Commons

A comprehensive analysis of waterbird communities across the Iberian Peninsula shows that reservoirs are not only alternative refuges in the face of wetland loss, but also support greater functional diversity and a more pronounced trajectory of diversification than natural lakes. This is one of the main conclusions of a study by researchers at the Doñana Biological Station of the Spanish National Research Council (CSIC), which has compared waterbird communities in natural lakes and reservoirs across the Iberian Peninsula over several decades.

The study shows that, although species richness has followed a similar trajectory over time in both types of wetland, there are important differences in the ecological functions performed by the species that use them. In particular, reservoirs have greater functional diversity than natural lakes, and this diversity has increased markedly over time. Natural lakes, by contrast, have shown much smaller changes in functional diversity.

The high functional diversity of reservoirs

One of the key findings of the study is that the increase in functional diversity observed in reservoirs is not simply due to there being more species, but rather to the fact that some of the species that have increased their presence bring ecological traits that are very different from those of species already present. These include large fish-eating birds such as the great cormorant and great crested grebe, which use the deep waters of reservoirs to feed. Large wading birds such as the white stork, black stork and Eurasian spoonbill have also become more prominent.

“The picture is different in Mediterranean natural lakes. Many are shallow and temporary, and are subject to major fluctuations in water levels and, in some cases, salinity. These environmental conditions act as a filter that restricts the ecological strategies that can thrive in them,” says Vicente García-Navas, the study’s author.

Functional diversity does not necessarily correlate with species richness

Another key finding of the study is that species richness and functional diversity do not tell exactly the same story. In both types of wetland, the number of species increased from the mid- to late 1980s through the 1990s and subsequently tended to level off. The authors link this increase, among other factors, to the protection of wetlands that survived decades of transformation, stricter environmental legislation and growing interest in nature conservation in Spain.

However, this relatively similar trajectory in species numbers contrasts with the differences observed in functional diversity. The study therefore shows that gaining species does not necessarily mean adding new ecological functions. A community may become richer by accumulating species with similar traits, while the arrival or increase of just a few ecologically distinct species can produce a much greater change in its functional diversity.

Reservoir management that takes biodiversity into account

The relevance of these findings is particularly important in the Mediterranean region, where many natural wetlands have disappeared or undergone severe degradation. “This does not mean that reservoirs can ecologically replace natural lakes. Dam construction profoundly alters river ecosystems, and natural wetlands retain ecological values that cannot be replaced by artificial systems,” says García-Navas. However, the findings show that reservoirs have acquired an important role as habitat and refuge for waterbirds, and that this role may have increased over time.

In this context, the researchers consider it important for reservoir management to take their biodiversity functions into account as well. Water planning and decisions on water management can directly affect the availability and characteristics of habitats used by birds. Measures such as creating islands and shallow-water areas, encouraging the development of shoreline vegetation or avoiding abrupt changes in water levels during the breeding season could increase their value for birds. Management practices that are compatible with conservation are particularly relevant in a context of environmental change and growing pressure on water resources.