Individual study: Effects of barley straw and stone addition to ponds to enhance invertebrate food supply for wading birds, Upper Waitaki Basin, South Island, New Zealand
Sanders M.D. (2000) Enhancing food supplies for waders: inconsistent effects of substratum manipulations on aquatic invertebrate biomass. Journal of Applied Ecology, 37, 66-76
The abundance of wetland birds is often correlated with aquatic invertebrate prey availability and wetland managers may therefore aim to increase the value of wetlands to birds by maximizing production and accessibility of aquatic invertebrates. The potential of substratum manipulations as a technique for enhancing food supplies for waders was investigated in 98 constructed ponds at six sites in the Upper Waitaki Basin, South Island, New Zealand. Substrata were manipulated by using different construction methods, and by adding barley straw or stones to some ponds. Total invertebrate biomass and chironomid biomass were measured 3 and 15 months after ponds were constructed.
Study area: Experiments were undertaken at six sites representing a range of conditions in the Upper Waitaki Basin, a large intermontane basin, 350–700 m above sea level. .
Substratum manipulations: Between three and six substratum manipulations were investigated in newly constructed ponds (up to 500 m apart) at each site, and each manipulation was applied to between three and six replicate ponds at each.
Ponds were constructed in late winter by excavation or by building low dams and flooding. As the experimental aim was to provide information on aquatic invertebrates, rather than provide wader habitat, ponds were constructed to provide a realistically large-scale habitat for invertebrates. Excavated ponds were 7–15 m in diameter and 10–30 cm deep, whereas the size and depth of flooded ponds depended on the microtopography of each site. At two predominantly stony sites, the substratum in some ponds was raked back and forward with the teeth of an excavator bucket to bring coarser material to the top. Three construction methods were used: 'excavated', 'excavated and raked' and 'flooded'.
Two substratum additions were investigated: barley straw and stones. Straw was added to five ponds at all sites except one, soon after construction. At the latter site, straw was added 12 months after construction as the ponds were not full until then. Barley straw was added as it has been used successfully to increase invertebrate biomass in gravel pits in England (Street 1983), and it is a cheap and widely available. A thick layer (equivalent to about 2 kg/m²) was spread over each pond, after 10 days it formed a buoyant open matrix from the bottom of the ponds to the surface.
A 10 cm deep layer of stones (5–20 cm long) was added to ponds at three sites with silty substrata. Stones were added to test whether they would mitigate the negative effects of silt. Ponds to which stones were added were initially dug deeper so that the final water depth was similar to that of the others.
At five of the six sites, shallow-water wetlands already existed, these were also sampled.
Invertebrate sampling: Four invertebrates samples were taken in each pond; a 0.1 m² stainless steel cylinder was placed in the pond at c. 15 cm depth, and rotated to cut 5 cm into the substratum. Samples were sieved (0.8 mm mesh) in the water during collection. Sampling depth (5 cm) was chosen to encompass typical foraging depths of most waders. In the laboratory, the invertebrates were dried and weighed. Many samples contained high numbers of chironomid larvae Chironomus zealandicus. Because of their potentially high value as wader food, chironomids were weighed separately, and additional chironomid-only analyses were undertaken. Molluscs and stony-cased caddis flies were also weighed separately, so that their biomass could be adjusted to exclude shells and cases.
Aquatic invertebrates: Potential wader invertebrate food items in many of the new ponds contained large proportions of chironomid larvae. However, in some various nektonic invertebrates dominated, and one site was dominated by molluscs. Three months after construction most ponds contained similar or greater total invertebrate biomass (average: 1.20, range: 0.10–7.66 g/m² dry mass) and greater chironomid biomass (0.66, 0–5.41 g/m²) than nearby pre-existing wetlands (1.79, 0.39–6.18 g/m² total dry mass; 0.01, 0–0.08 g m² chironomid dry mass). Ponds with stony substrata contained relatively low total invertebrate and chironomid biomass.
Effect of substratum manipulation: Substratum significantly affected either chironomid or total invertebrate biomass, or both, at all sites. However, newly constructed ponds to which straw was added did not contain significantly greater chironomid or total invertebrate biomass than ponds with a similar substrate but lacking straw.
Differences in biomass between substrata were large at some sites (e.g. an excavated pond + straw and excavated + stones differed in chironomid biomass by 2.7 g/m², and in total invertebrate biomass by 3.4 g/m²) indicating that substratum manipulations have good potential for increasing food supplies for wetland birds.
Few consistent effects of substratum were evident among sites. One consistent negative effect was evident in the new ponds with stones added or those that were excavated at stony sites. In these ponds, invertebrate biomass was never greater, and usually lower, than that in other new ponds at the same site.
Conclusions: The newly constructed ponds rapidly developed a high biomass of aquatic invertebrates. However, despite the sometimes large effects of substratum manipulations on potential wader food supply, no technique achieved consistently positive results. Ponds to which barley straw was added did not contain significantly greater chironomid or total invertebrate biomass than ponds with a similar substrate but lacking straw. The effects of any given substratum manipulation varied greatly among sites, indicating that, although substratum manipulations can be worthwhile, they need to be evaluated on a site by site basis.
Street M. (1983) The use of waste straw to promote the production of invertebrate foods for waterfowl in man-made wetlands. Managing wetlands and their birds. Proceedings of the Third Technical Meeting on Western Palearctic Migratory Bird Management (ed. D.A. Scott), pp. 98-103. International Waterfowl Research Bureau, Slimbridge, UK.
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