Preliminary typology (en)
The need for a pond typology
Ponds are small standing freshwater bodies about one metre square to a few hectares in surface area, and from a few centimetres to a few metres in depth. Natural processes have created ponds throughout geological time. Examples include topographical depressions created following periods of glaciations or by weathering of rocks, floodplain backwaters, ponds created by tree falls or by animals. For the last few thousand years, human activity has also artificially created ponds for industry, agriculture, fish production, or to provide beauty in the landscape. Today, ponds are increasingly being created for ecosystem services (protection against fire hazard, nutrient retention, etc.) and leisure activities, but also as tools in education to natural history and experimental research.
Recent ecological studies have revealed the importance of ponds for the conservation of biodiversity because, despite their small size, they disproportionately contribute to regional diversity when compared to other freshwater systems such as rivers or lakes. A key objective of the current European research on ponds is to develop the knowledge-base and tools to enable integrated monitoring and assessment for the conservation of freshwater biodiversity. Exploration of fundamental ecological patterns and definition of a typology of European ponds is needed, to cover the range of habitats found along broad geographical, altitudinal and environmental gradients. Indeed, by knowing what biological diversity of plant and/or animals should be like in a given area, we may determine the degree to which human activities have altered it, and we should be able to provide useful information for conservation and management framework directives.
During the last few decades, there have been many attempts to produce reference schemes for freshwater systems across Europe (mostly streams). These typologies were either based on fish, diatoms, aquatic plants or invertebrates. While great emphasis has been laid on the fauna and flora of running waters, little is known about the distribution patterns of pond animals and plants. There are, however, millions of ponds in Europe and elsewhere, rich in biodiversity and supporting a wide range of rare and endangered plants and animals. Therefore, a biological typology of ponds is much needed, as it will help define the main pond types in Europe for subsequent conservation issues.
In order to establish a preliminary typology of European ponds based on biological diversity, we used data on the distribution of pond invertebrates across 4 biogeographic regions (Mediterranean, Atlantic, Alpine, Continental). To emphasize the biological distinctness of the pond fauna across a latitudinal gradient ranging from Northwestern to Southwestern Europe, the typology was based both on species distributions and on specific biological attributes (e.g., food, reproduction, dispersion modes, etc.).
Published studies provided the database on invertebrate diversity and species biology in ca. 100 ponds, distributed along a North-South gradient from UK to southern Spain (Fig. 1). Advanced statistical methods were used to classify ponds according to the similarity of their fauna (species occurrence), and to the biology of invertebrate species (reproduction, locomotion, dispersal mode, food ingested, etc.). Our objective was to interpret differences in pond ecosystems in terms of species assemblages and biological traits of their aquatic communities.
Ponds classified according to the diversity and biology of aquatic invertebrates formed three subsets in our analyses: Mediterranean, Atlantic and Alpine, and Continental. These subsets corresponded to the North to South distribution of the ponds, i.e. to a latitudinal gradient of underlying variables probably related to climatic, topographic and geologic conditions. The Atlantic and Mediterranean regions had the highest invertebrate diversity (145 and 160 taxa, respectively). Thirty and 53 taxa were found to occur in the Alpine and Continental regions.
Mediterranean ponds hosted the richest, and the more distinct fauna – 34 taxa were exclusively recorded in this region. Among these typically Mediterranean invertebrates, the most common were Anostraca fairy shrimps, the tadpole shrimps Triops cancriformis and Lepidurus apus (Fig. 2), the beetle Cybister lateralimarginalis (Fig. 3), the backswimmer Anisops sardea, the medicinal leech Hirudo medicinalis (a species listed in Annex II of the Habitats Directive and in the IUCN Red List), or the snail Physella (Fig. 4).Most ponds in the Mediterranean Region are extreme sites, characterized by an altered salinity due their vicinity with the sea, and/or high evaporation rate related to arid climate and fluctuating hydrology. There is a dominance of organisms with short life-span and/or asexual reproduction and small size in Mediterranean climate areas, as these traits promote survivorship in unstable conditions. Mediterranean pond species allocate much energy to reproduction and resistance (formation of cocoons). These characteristics, and others such as small body size and the feeding habits based on fine particulates and microorganisms, suggest that populations are selected by unstable habitats or by habitats fluctuating in an unpredictable way. Importantly, we found a higher proportion of species with narrow thermal ranges in the Mediterranean region. It is worth noting that the Mediterranean region has the highest taxonomic richness for pond invertebrates (160 out of 204 European taxa), and that 20% of these taxa were exclusive to this region.
Among the 53 invertebrates recorded for this region, only one species (the dragonfly Epitheca bimaculata, Fig. 5) was exclusively recorded in the Continental region. Continental ponds were characterized by invertebrates with short life cycles (<1 year), and higher proportions of rather large (20 – 80 mm) predatory species (e.g. dragonflies, diving beetles) that fed on other macroinvertebrates, or even on small vertebrates (tadpoles). The traits of pond invertebrates in Continental areas suggested that few energy was allocated to reproduction and dispersal, whereas habitat use was favoured by larger body size, short-lived organisms (thus enabling many temporally segregated populations to utilize small ecosystems), and a higher diversity of feeding modes. These characteristics suggest high resilience capabilities, and the occurrence of strong competitors. Those populations are selected by more stable and structured habitats, where resource can be partitioned through the spatial and temporal segregations of species. Atlantic and Alpine ponds Twenty-eight taxa out of 145 were exclusive to the Atlantic region. The commonest “Atlantic pond taxa” were mostly caddisflies such as Phryganea (Fig. 6), Anabolia (Fig. 7), Glyphotaelius pellucidus and Trianodes (Fig. 8), but also the crustacean Crangonyx, the Heteroptera Callicorixa, the snail Armiger crista, or the flatworm Dendrocoelum lacteum. Only one taxa (the coleopteran genus Limnius) was exclusively recorded in the Alpine region. Atlantic and Alpine pond invertebrates were mostly medium-sized detritivores or herbivores (2.5-20 mm), with a higher proportion of grazers and filterers. Despite similarities in their biological traits, invertebrates of the Alpine and Atlantic regions showed large differences in terms of number of taxa (30 and 145 taxa, respectively). Alpine ponds may represent harsh environments that are physically stressful (rapid and wide daily temperature fluctuations, intense UV, strong wind, abundant precipitation and low nutrient concentrations), so that a smaller proportion of potential colonizers could become established. Conclusions and Perspectives Throughout Europe and elsewhere, environmental policies aiming at preserving the biological diversity of terrestrial and/or aquatic ecosystems heavily rely on action plans for the delineation of zones of ecological interest (which concentrate rare and/or threatened species, or which have patrimonial values), and environmental managers ask for explicit schemes such as distribution patterns that allow them to identify areas at greater ecological risk. With the richest fauna, the highest number of distinct invertebrates, but also the most vulnerable organisms, Mediterranean ponds are probably the most threatened in the international context. With high proportions of invertebrates with narrow thermal ranges, threats associated with climate change clearly align with zones of ecological interest, and this should add new impetus to calls for freshwater conservation policies in the area. While we focused on broad scale patterns of pond biodiversity, regional typologies (within each regions) are now needed to specify the role of microclimatic, landscape, and pond habitat variables in selecting sets of species from the regional pool, in relation to biological traits. Within-region variability in invertebrate diversity and biological traits was apparent in our analyses, and this was certainly related to the peculiarities of the sub-areas from which the data were collected. In the southernmost areas, the annual hydrologic cycle is an important variable differentiating pond communities. Along a gradient ranging from ephemeral pools to semi-permanent and permanent ponds, periodic drying is a major constraint to invertebrates. Future analyses, in addition to giving a vital understanding of local to large-scale patterns, are also intended to reveal gaps in existing knowledge. The very first, and probably more problematic gap is the lack of accessible data. We must unfortunately acknowledge that only a very small fraction of the existing information on pond biodiversity is available. Although we could gather information on about one hundred ponds supposed to represent various biogeographic regions, we are still unable to meet our attempts to obtain information on a much larger set of ponds that would be evenly distributed across Europe. These data certainly exist, but the heterogeneity of the existing formats (e.g. data per sample units, aggregated data) and low accessibility (e.g. unpublished data be found with literature databases, not disclosed by authors) remain a problematic issues. There is still, therefore, a pressing need for the incorporation of high quality national data sets into a standardized database so that they can be further analyzed in an integrated European-wide manner.