This paper critically analyzes and review an article by Penny Williams, Mericia Whitfield, Jeremy Biggs, Simon Bray, Gill Fox, Pascale Nicolet, and David Sear, “Comparative biodiversity of rivers, streams, ditches and ponds in an agricultural landscape in Southern England”, focusing on the study of the biodiversity of water bodies in the specified UK territory and examining the role of the small water bodies in the regional landscape.
Authors argue that understanding biodiversity of various bodies of water is essential to forming a proper idea about the UK conservational needs. Although a number of studies were conducted on the man-nature interaction, including reports and researches on major rivers and lakes in the country, little has been written on the subject of smaller water bodies. Williams et al. (2004) made a comparative study of biodiversity of rivers, streams, ditches and ponds located in Southern England. The study covered an area of approximately eighty square kilometers.
Authors found that water bodies had dissimilar influence on the diversity of macrophytes and macroinvertebrates in the region. For example, majority of rivers analyzed in the study had similar types of species living in them. At the same time, different ponds showed varying degrees of species inhabitation, with some ponds showing the same level of species richness as the largest river sections. Other ponds hardly included any species at all. Nevertheless, authors concluded that ponds contributed most to biodiversity. This type of water bodies contained more species, including exclusive species and more rare species than all other water bodies combined.
Williams et al. (2004) also concluded that streams showed low level of species inhabitation, significantly lagging behind rivers and ponds in terms of species rareness and uniqueness. It was also found that most ditches have seasonal nature. The contained the fewest number of species among other water bodies, yet uncommon species, such temporary water invertebrates lived in this specific water type.
Finally, multivariate analysis showed that permanence, depth, flow and altitude served as critical factors contributing to species richness and number in any particular water body. Authors concluded that such small water bodies as ponds and ditches play a significant role in the regional biodiversity; and that more studies on the governmental and regional levels are needed in order to analyze the effects these small water bodies have on the ecosystem.
According to Williams et al. (2004), ponds include a wide range of man-made and natural water bodies. In Southern England, they have been defined as small water bodies between 1 m2 and 2 ha in area, which hold water for four months of the year or more. Authors argues that ponds are often thought of as artificial habitats but the geological record shows that they have always been a natural feature of landscapes; however, human activity has added a variety of new ways in which ponds can be formed. Consequently, it is informative to think of ponds as a natural habitat type, which has often been recreated by human activity. It should be noted that, even before landscapes were widely influenced by anthropogenic processes, the pond environment was probably very common, with a wide range of natural processes capable of creating ponds.
Until recently, comparatively little has been known about the ecology of ponds and ditches. It seems likely that the small size of these water bodies has led to them being regarded as generally unimportant ecologically. Increasingly, however, it is becoming clear that small water bodies are biologically rich, and recent data have been valuable in altering scientists’ perspective on the ecology of ponds, ditches, and streams.
Data collection and richness of individual samples
The study conducted by Williams and colleagues (2004) provides clear evidence of the biological richness of small water bodies. For example, their comparisons of aquatic macroinvertebrate species richness in rivers and ponds in Southern England show ponds to support more species, and more uncommon species of conservation importance, than rivers (Williams et al. 2003). Authors argued that just over 300 lowland ponds accounted for 50% of Britain’s wetland plants. In addition, more freshwater species are found in ponds than either rivers or lakes (Williams et al., 2003).
World-wide, small water bodies occur in all landscape types from mountain tops to deserts. In the highly modified British landscape ponds are most numerous in woodland and in the grassland dominated west of the country. Pond densities in arable dominated landscapes are lower (Williams et al., 2003). Although enormous losses of ponds are thought to have occurred in Britain (in the order of 75% of those existing at the end of the nineteenth century), ponds are still common. Indeed, in Britain, the majority of standing waters are ponds; at least 97% of all discrete water bodies are less than 2 ha in area and even by area small water bodies probably represent some 15% of the total area of standing waters (Williams et al., 2003).
This study has shown that, in Southern Britain, small water bodies remain numerous in the landscape. However, this survey has also provided the first evidence of the very widespread impairment of ecological quality of rivers, streams, ponds, and ditches. For instance, ponds in the lowland landscape were found to support on average only half the expected number of wetland plant species, when compared to minimally impaired ponds in similar landscape types. Aquatic (i.e. submerged and floating leaved) plant assemblages were particularly degraded, with the average pond in the countryside supporting only one-third the number of species expected. The study by Williams et al. (2004) also provided evidence of the extent to which non-native species have entered the British semi-natural flora; one in six wetland submerged plants recorded in the survey were non-native species, a local example of the world-wide phenomenon of erosion of biodiversity by alien species.
Catchment’s aquatic biodiversity analysis
Williams and colleagues (2004) found that small water bodies are very varied, and may be seasonal or permanent. Indeed, water permanence in rivers is one of the most important environmental variable shaping community types, with only pH-related variation being more significant. In this survey, the primary grouping is into acid and base-rich sites; then, ditches are grouped according to whether they are permanent.
Perhaps the single most important hydrological feature of small water bodies (especially those fed mainly by surface runoff) is that many have very small catchments. This means that, unlike most rivers and lakes in other parts of the UK, many water bodies in Southern England have catchments wholly protected from the impacts of intensive rural and urban land use, and the pollutants associated with intensive land use. Consequently it is possible to find streams, ponds, and ditches in non-intensively managed landscapes which are amongst the least impacted by human activities of all freshwater ecosystems. However, small water bodies that are impacted by pollutants are even more vulnerable than large rivers and lakes because their small volumes provide little ability to dilute the impacts of pollutants (Williams et al., 2003).
Authors argues that although it is quite simple to protect small water bodies from the effects of many degrading impacts derived from intensive land use (point source effluents and diffuse pollutants), many ditches and small streams are exposed to atmospheric deposition. In landscapes vulnerable to acidification, both upland and lowland, it is likely that very large numbers of small water bodies are impaired by acidification. At present, authors state, there are few systematic data for small water bodies to assess this impact, although it seems likely that the same trends as those shown in the extensive and detailed studies of acid lakes will be apparent. No data are currently available on the effects of nutrient-enriched rainfall on the ecology of small water bodies (Williams et al., 2003).
Lack of information about the ecology of small water bodies, and their popularity and familiarity, has led to the development of a wide variety of misconceptions about their ecology. This has been most apparent in the management of ponds and ditches for nature conservation, which has been largely dominated by the belief that maintaining open water by the physical removal of colonizing vegetation and silt was the primary need for all small water bodies. In fact, the most important factor (as with all other freshwaters) is to ensure that water quality is maintained, usually by protecting the catchment, with physical, ‘fine-tuning’ management ponds in specific situations (Williams et al., 2003).
Author further state that in Britain, the widespread neglect of small water bodies once used for watering farm livestock has led to the general assumption that they have no uses in the modern landscape. However, the this study illustrates that a considerable proportion of ponds have modern amenity uses (for fishing and shooting for example, as well as providing visual diversity in the landscape), some of which are also compatible with protection of biodiversity. In addition, increasing numbers of ponds are being created to balance flood flows from urban areas, and to assist in ‘source control’ of pollutants.
The primary threats to small water bodies in Britain, including Southern England, come from two main sources: pollution and in-filling (Williams et al., 2003). Climate change also seems likely to pose a particular risk for seasonal ditches, which often support exceptional assemblages of specialized plant and animal species, many of which could completely disappear with slight shifts in climate (Williams, et al., 2003). In pristine landscapes rich in natural water bodies, loss of seasonal water bodies would probably be compensated for by permanent water bodies becoming seasonal. In the fragmented British landscape, this process is not likely to operate effectively. For example, seasonal water bodies in the New Forest in southern England, which are exceptionally important freshwater ecosystems supporting a wide variety of rare and vulnerable species, could disappear.
Characterizing flood regimes, given the diversity of generating mechanisms and variety of catchment types, is a difficult task. Catchment geology and relief are important controls, spates being more common in mountainous, impermeable catchments than in rivers draining more subdued permeable catchments. One current area of research making a valuable contribution focuses on the seasonality of flooding; often flood generating mechanisms are associated with a particular seasonal signature. Williams an colleagues (2004) describe the diversity of flood seasonality present amongst South English rivers, identifying links with soil moisture, rainfall seasonality, snowmelt and other factors. In most regions, flooding is mainly a winter phenomenon; soil moisture deficits substantially mitigate the flood risk in most catchments during the summer. Authors have observed a general pattern of mean flood date occurring later in the autumn/winter with distance from west to east across Britain. However, in urban catchments, especially those in the southeast, convectional storms in the summer and early autumn can be the primary cause of floods.
Overall, the study by Williams et al. (2004) is an invaluable research contributing to the scientists’ realization of the small water bodies’ importance in the modern landscape, biodiversity, and ecosystem. Careful attention is needed to be paid to these small water bodies in order to save the nature and keep the preservation of the water at a desirable level.