Conserving the function of freshwater ecosystems in production landscapes
* This is the second of a series of articles by Dr Lize Joubert-van der Merwe sharing key findings of on-going research conducted by the Mondi Ecological Network Programme (MENP) at Stellenbosch University. The first article introduced ecological networks (ENs) as a viable conservation approach in production landscapes. Read part one here.
Freshwater is the basis for life on land. It is a vital variable that makes food or fiber production possible. For many of us, the color green is synonymous with the prevalence of surface water. However, surface water constitutes only a tiny fraction of freshwater on earth. Much lies beneath the soil surface in large aquifers.
Aquifers are semipermeable layers of soil through which water moves. They are located deep beneath the earth surface where they are protected from evaporation. This makes them especially suited to store water for longer periods of time.
The issue of water raises two concerns: quality and quantity. With the majority of the country suffering under the effects of the worst El Niño event on record, the lack of water is foremost in people’s minds. But what would you say if I told you that many of the management practices needed to maintain water quality are similar to those needed to replenish underground water sources. Infiltration of water through the soil profile is needed to fill up sources of groundwater during wet periods so that people can access that water (e.g. through boreholes) during times of drought. However, infiltration can only occur when water has time to soak into the earth i.e. when the vegetation layer slows down flow rate of surface run-off, when plant roots hold on to the fertile top soil layer and prevent it from being washed away, and when water moves as a sheet across the landscape (and not in furrows).
It makes intuitive sense that degraded land will not fulfill this necessary sponge-like function. Rapid surface run-off over bare patches of land, accumulation of water in furrows, and confluence of these to form ditches and dongas are not conducive to water infiltration. As a matter of fact, dongas drain water tables and so dry out the immediate surroundings. This makes degraded landscapes even more susceptible to fluctuations in rainfall.
As mentioned in the first article of this series, well-managed forestry plantations are divided into commercial and non-commercial zones, with the latter tasked with the conservation of biodiversity and ecosystem processes, including hydrology (Figure 1 above). The commercial zone (e.g. eucalypt or pine compartments) have been criticized for their water use, and justifiably so. Alien trees do indeed use more water than natural vegetation types, such as grassland, and have deeper roots, which mean that they can access deeper water sources. In addition, forestry plantations often occur in upper catchment areas, which means that they influence all downstream land users at least to some extent. Therefore, the forestry industry has a huge responsibility to regulate and mitigate their water consumption burden on the environment.
Wall-to-wall forestry was a massive legacy left over from the past, and has had major effects on hydrological processes as well as local aquatic biodiversity and local livelihoods. Fortunately, this has been largely corrected by retracting plantation trees from areas with hydromorphic soils in a process of wetland delineation (Figure 2 below), leaving conservation buffer zones > 30 m wide on either side of streams (Figure 3 below), and by actively controlling invasive alien plant species in riparian zones (Figure 4 below). Then, dragonflies are used as bio-indicators to measure the response of aquatic diversity to these management interventions (See Box 2 and 3 at the bottom of this post).
Negative effect of shading
The negative effects of forestry compartments and alien invasive trees on dragonflies relates mainly to dragonflies’ sensitivity to shading. Warmth (~ 20 °C) and sunlight play major roles in dragonfly ecology. If it is too cold, dragonflies cannot fly, which, in turn, means that they cannot hunt. On cooler days, larger dragonflies vibrate to generate heat, but smaller dragonflies have to bask in the sun or on hot rocks that radiate heat. When trees cast shade across riparian zones alongside streams, dragonflies that are used to sunny conditions gradually start to disappear. Fortunately, dragonflies are strong flyers and good dispersers, which means that dragonfly assemblages can recover rapidly after sunny conditions are restored. Generalist dragonfly species will start arriving as soon as trees are felled, while specialist endemic species take about 2 years (Samways and Pryke 2016). Historical dragonfly assemblages are restored within ~ 5 years.
In addition to shading by alien trees, the negative effects of plantation forestry on dragonfly assemblages mainly relates to issues of siltation and soil erosion. Siltation increases water turbidity and changes the type of substrate in stream bottoms. This leads to declines in dragonfly species richness, and changes to their assemblage composition (Kietzka et al. 2015). The endemic dragonfly species, i.e. those with localized distribution ranges, could suffer from siltation of headwaters.
Soil erosion and siltation
Siltation of freshwater ecosystems is the logical result of soil erosion, specifically sheet erosion i.e. the loss of top soil over an extensive area. (Sheet erosion can cause loss of productivity and should be addressed as soon as it appears to safeguard long-term sustainability of plantations). Sheet erosion often occurs directly after clear-felling a forestry compartment when the exposed soil surface is impacted by large rain drops and high water volumes typical of thunderstorms (Figure 5 below).
Mitigation measures to address soil erosion and resultant siltation of freshwater ecosystems include:
• Timely harvesting of compartments. For example, 1) avoid clear-felling of entire plantations at a time, and 2) harvest forestry compartments alongside streams in alternate years to lessen the silt load entering freshwater ecosystems.
• Judicious post-harvesting care of forestry compartments in terms of using fire to burn away timber remains to avoid loss of top soil.
• Proper road construction with appropriate drainage systems to reduce the velocity and volume of surface water run-off.
• Buffer zones of well-managed grassland (> 30m wide) alongside streams to capture silt particles that wash out of forestry compartments.
• These buffer zones should be burned every few years to maintain grassland vitality, but not in the year before or after harvesting adjacent to forestry compartments. They should also be kept free of alien invasive plants.
• Cattle grazing should be kept to moderate levels in these buffer zones to prevent excessive trampling and bare patches from appearing.
It is necessary to invest in management practices and infrastructure that helps land soak up water like a sponge. In other words, access to water is not only a function of precipitation, but also a question of good management.
Box 1: Six strategic approaches to water management in production landscapes
• Restore and manage all natural vegetation types in catchment areas to encourage water infiltration and recharge of subsurface water sources.
• Remove planted trees from soils indicative of subterranean water movement, such as wetlands and springs. Springs are found in places where the earth surface intersects with a flowing body of groundwater.
• Control invasive alien plants along streams.
• In plantations, do not clear-fell very large areas all at once. Rather, harvest compartments alongside streams in alternate years to prevent huge silt loads from entering freshwater systems.
• Leave > 30 m wide buffer zones of well-managed natural vegetation along streams to avoid decreases in stream flow and to prevent shading of sun-dependent biota, such as dragonflies.
• Do not burn buffer zones in the one year period before or after clear-felling forestry compartments adjacent to streams.
Box 2: Using dragonflies as bio-indicators
Dragonflies are the lions and eagles of the insect world. They are apex predators, which means that they hunt down and eat insects smaller than themselves. Dragonflies even have serrated teeth, which is aptly described by the scientific name for the dragonfly and damselfly order Odonata, which means ‘toothed one’. Although dragonflies will bite when provoked, their teeth cannot pierce human skin. Hence, there is no need to be afraid of them. Because dragonflies have a high trophic position on the food chain, they are sensitive and respond rapidly to changes in the ecosystem.
The majority of a dragonfly’s life cycle is actually spent under water, first as an egg and then as a larvae. During this time, which can take up to two years, they live in the substrate making up the bottoms of streams, and feed on various small animals including tadpoles, mosquito larvae and even other dragonfly larvae and small fish. In turn, the predators of dragonfly larvae include fish and frogs. When a dragonfly reaches the end of its larval stage, the larvae climbs onto a partially emerged stem, twig, rock or sand bank, pumps itself up by swallowing air until the exoskeleton cracks, and then emerges. The fully-formed dragonfly that emerges has to wait a few hours for its wings to dry and harden. But once all the creases have been ironed out and the dragonfly takes off, very few other insects can trump it in terms of flight speed and agility. It can hover in one place, fly vertically up and down like a helicopter, and race across the landscape at incredible speeds ranging from 0 (when hovering) to a record speed of 60 km/h, depending on the species. It also has extremely good eye sight in most directions, which makes it a formidable hunter. In fact, a single dragonfly can eat hundreds of mosquitos per day, which in itself is a good enough reason to warrant their conservation.
In turn, adult dragonflies are eaten by insectivorous birds, such as swallows and bee-eaters. Compared to the aquatic life stages, adults are short-lived i.e. varying from a few weeks to about six months. However, it is during this life stage that they are used as indicators of stream health, integrity and resilience i.e. the ability of a system to ‘bounce back’ to its original state after disturbances.
Box 3: The Dragonfly Biotic Index
The Dragonfly Biotic Index (DBI) is used to monitor ongoing changes in freshwater ecosystems, and to prioritize conservation action based on special species occurring in certain streams (Samways and Simaika 2016). The DBI per species is built upon three cornerstones: threat status on the IUCN Red List, sensitivity to environmental change, and geographic distribution. Each dragonfly species has a sub-score (ranging from 0 to 3) for each of these categories (Samways 2008), and these sub-scores are added together for each species. This means that any one species can have a score ranging from 0 to 9. High scores (9 or close) go to dragonflies that are highly threatened, extremely sensitive to changes in their immediate environment, and localized in occurrence (i.e. narrow range endemics). At the other end of the spectrum, a dragonfly species that is common, not bothered by environmental disturbances, and that occurs throughout the country will score very low (with 0 the lowest). Based on all dragonflies present, a DBI score per site can be calculated i.e. sum of DBI values for all species divided by the number of species. In summary, the DBI score per site gives an indication of the ‘quality’ of a site in terms of its particular suite of species. For example, when a site is restored, such as through removal of invasive alien trees, the DBI score per site can increase substantially.
The DBI is not the only method used to monitor stream health. The South African Scoring System (SASS) is based on the presence of different families of aquatic macro-invertebrates living in stream bottoms. The DBI and SASS are highly correlated, which means that they show similar responses to environmental disturbances. This makes sense, because dragonflies spend a substantial proportion of their life cycle living underwater in the substrate of stream beds with these other macro-invertebrates. However, unlike SASS, the DBI is based on the occurrence of easily-spotted adult dragonflies. Hence, there is no need for exhaustive sampling of the stream bed, as for SASS. The sensitivity and ease-of-use of the DBI will probably cause it to become the monitoring method of choice within the foreseeable future.
Kietzka GJ, Pryke JS, Samways MJ (2015) Landscape ecological networks are successful in supporting a diverse dragonfly assemblage. Insect Conserv Divers 8:229–237. doi: http://dx.doi.org/10.1111/icad.12099
Samways MJ (2008) Dragonflies and damselflies of South Africa. Pensoft, Sofia – Moscow. Available from http://entomologia.net/L_Odonata/Odonata_Dragonflies%20and%20Damsels%20of%20South%20Africa.pdf
Samways MJ, Pryke JS (2016) Large-scale ecological networks do work in an ecologically complex biodiversity hotspot. Ambio 45:161–172. doi: 10.1007/s13280-015-0697-x
Samways MJ, Simaika JP (2016) Manual of freshwater assessment for South Africa: Dragonfly Biotic Index. SANBI, Pretoria
*First published in SA Forestry magazine, April 2016