iSimangaliso water beetles face uncertain future as drought intensifies - study

2016-06-20 10:32
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Cape Town - iSimangaliso Wetland Park and UNESCO World Heritage Site has long been associated with a plethora of described species of fauna and flora - a list that continues to grow as new species are discovered by researchers undertaking a multitude of natural and social science projects in this fertile land.

One such project, a study of predaceous water beetles (Coleoptera and Hydradephaga) residing in the Lake St Lucia system, has brought to light the uncertain future for the beetles, mainly due to climate change.  

According to Professor Renzo Perissinotto, leader of this project, “Water beetles are one of the dominant macroinvertebrate groups in inland waters and are excellent ecological indicators, reflecting both the diversity and composition of the wider aquatic community." 

The predaceous water beetles (Hydradephaga) make up around one-third of known aquatic Coleoptera and, as predators, are a key group in the functioning of many aquatic habitats. 

A study of the beetles revealed that the Lake St Lucia system and its associated wetlands support at least 68 species of Hydradephaga. Comparatively, it is currently estimated that around 410 species of Hydradephaga occur in southern Africa as a whole, meaning that almost 20% of the known fauna of this biodiverse region occur in the wetlands of the St Lucia system. 

Photos: Lynette Clennell
There appears to be more diversity of life per unit water volume in the small freshwater puddles and ponds surrounding Lake St Lucia than anywhere else in the park, and it comes in the form of water beetles. A total of 32 sites covering the entire spectrum of water body types were sampled over the course of three collecting trips. 

Smaller, surrounding wetlands play critical role 

The study, which took place between 2013 and 2015 provided the first biodiversity census for this important aquatic group in the iSimangaliso Wetland Park, a UNESCO World Heritage Site within the Maputaland biodiversity hotspot.

It was the first study to highlight the importance of temporary depression wetlands, emphasising the need to maintain a variety of wetland habitats for aquatic conservation in this biodiverse region. 

Only three species (Cybister vulneratus Klug, 1834, Hydaticus servillianus Aubé, 1838 and Derovatellus cf. natalensis Omer-Cooper, 1965) were found in the margins of Lake St Lucia itself, while the overwhelming majority of species being associated with small wetlands in the park. 

The False Bay sites supported relatively distinctive beetle assemblages, including species which were not recorded elsewhere, whereas the fauna of the eastern and western shores largely overlapped.

Photos: Matthew Bird and David Bilton
According to Perissinotto and his co-workers, Dr Matthew Bird of NMMU (Port Elizabeth) and Prof David Bilton of the University of Plymouth (UK), “Five species of Hydradephaga found during our surveys (pictured above) are apparently new to the fauna of South Africa.”

Photo: Matthew Bird
According to Perissinotto, “It is highly likely that the Peltodytes found during our surveys (above) is currently undescribed but cannot be either positively identified or described at present, in the absence of males.”

Intensifying drought 

Due to unprecedented drought conditions in the region and increasing pollution due to human activity, there have been significant changes in the Lake St Lucia system in recent decades. 

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The estuary mouth closed in 2002 and large-scale desiccation of the lake basins began in 2004. At the peak of these events, over 80% of the lake bottom sediments became exposed to the air and hypersaline conditions dominated the lake system, except in the Narrows and at the mouth. 

Alternation of dry and wet cycles are not new to this estuary, as can be seen for instance in historic records showing the regular occurrence of four to 10 year cycles of either droughts or anomalous wet conditions since at least the early 1900s. 

But projections of climate change for the next 50-100 years indicate that this situation will persist and possibly intensify, with the most likely scenario being an alternation of extreme droughts followed by floods, making iSimangaliso’s current Global Environmental Facility (GEF) – the World Bank restoration of Lake St Lucia's hydrology through strengthening the flow of the Umfolozi river into the Lake St Lucia - even more critical to ensure future resilience. 

SEE: PICS: Lake St Lucia forcibly choked as drought grips iSimangaliso

Predaceous water beetle biodiversity in the at Lake St Lucia system is concentrated in small natural water bodies, rather than the main lake system - a worrying predicament if considered the intense droughts dry up the smaller, surrounding wetlands first. 

"With the most likely scenario being an alternation of extreme droughts followed by floods, and the consequences of such changes, the beetles' futures remain unclear, and may depend on the degree to which changes in the lake system cascade through the wider wetland complex," iSimangaliso’s Research Manager, Nerosha Govender says. 

These ephemeral or temporary wetlands are vulnerable to prolonged drought such as that currently being experienced.
Photos: Lynette Clennell

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