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Seasonal Variation in Waterbird Assemblage in the Mahango Game Reserve, North-Eastern Namibia

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Seasonal Variation in Waterbird Assemblage in the Mahango Game Reserve, North-Eastern Namibia Seasonal variation in waterbird assemblage in the Mahango Game Reserve, north-eastern Namibia Grzegorz Kopij1,2 and Mark Paxton3 1Department of Integrated Environmental Science, University of Namibia, Ogongo Campus, Namibia 2Department of Vertebrate Ecology, Wroclaw University of Environmental & Life Sciences, ul. Kozuchowska 5b, 51-631 Wroclaw, Poland 3Shamvura Camp, Rundu, Namibia ABSTRACT There are few African studies on seasonal changes in the waterbird assemblages in river ecosystems. Tropical freshwater ecosystems are regarded as more stable habitats over the year than terrestrial ones. The same could be expected in regard to the bird fauna associated with such ecosystems. In order to test if waterbird asembalges followed the stable ecosystem paradigm a study was undertaken in the Okavango River, in the Mahango Game Reserve, north-eastern Namibia. Counts of all waterbirds were conducted in wet and dry seasons during 2001–2006. In total, 88 waterbird species were recorded. In wet seasons the number was slightly lower (N = 75) than in dry seasons (N = 78) (chi-square test: χ2 = 0.06; p > 0.05). The total number of individuals of all resident species recorded in wet seasons was unexpectedly much lower in wet (N = 9 979) than in dry seasons (N = 15 501) (chi-square test: χ2 = 1 196; p < 0.01). In wet seasons, Collared Pratincole, Blacksmith Lapwing, African Openbill Stork and African Darter dominated (58.8%), while in the dry season: White-faced Duck, Spur-winged Goose, African Skimmer, African Reed Cormorant, African Darter and Squacco Heron contributed 60.8% to the overall abundance. Diversity indices were similar in both seasons. Keywords: population trends, wetlands, Namibia INTRODUCTION There are very limited studies from Africa on seasonal changes in the waterbird assemblages in riverine ecosystems. In comparison with terrestrial habitats, tropical freshwater The water regime in rivers in the tropical and subtropical ecosystems are regarded as more stable over the year, as regions of Africa changes significantly over the years, one of the most important ecological factors, water, is not a influencing the nutrient content of water and consequently limiting factor in this case. The same could be expected in also bird fauna. In the wet season, a number of Palearctic and regard to the bird fauna associated with such ecosystems, intra-African species are expected to occur in such habitats. It but waterbirds are known to be seasonally highly mobile is therefore interesting to investigate their impact on the local under such conditions (Cumming et al., 2012). Some are waterbird fauna. Other possible drivers of waterbird seasonal resident throughout the year, others are intra-African variation can be the prevailing weather conditions (especially migrants or nomads; still others are African or Palearctic amount of rainfall) in the neighbouring regions, which may non-breeding visitors; and some are vagrants which appear force some waterbirds to vacate their habitats during prolonged only occasionally. There is not enough information, however, droughts and occupy the areas with perennial rivers. On the about waterbird communities in tropical ecosystems, other hand, exccesive flooding may force the local resident especially in arid areas, to be able to detect any pattern of birds to vacate their territories and look for rivers with lower changes (Cumming et al., 2012). water levels. Human activities may further contribute to Kelejta-Summers et al. (2001) studied year-to-year and the variation, for example, crop production (maize, wheat, seasonal changes in waterbird communities at the Cape groundnuts), fishing, water extraction for irrigation in the dry Flats Sewage Treatment Works in Strandfontein near Cape season, and influx of fertilizers and sewage to the river. Town. Swanepoel et al. (2009) studied year-to-year changes The aim of this study was to compare the species in waterbirds in an impoundment, the Theewaterskloof composition, diversity, abundance and dominance of Dam in the Western Cape, South Africa. Mundava et al. waterbird species occurring in a subtropical river in wet and (2013) investigated year-to-year and seasonal changes in the dry seasons. In addition to these, changes in main ecological composition and abundance of waterbirds in two artificial lakes guilds of waterbirds in these two seasons were investigated. (Chivero and Manyame) near Harare, Zimbabwe. Cumming et This will help to identify possible drivers of waterbir species al. (2013) studied seasonal changes in waterbirds in a natural composition and their abundance. We predict higher diversity lake and wetlands at Barberspan, South Africa. All of these and abundance of waterbirds in dry season, as we consider the standing and mostly artificial freshwater ecosystems constitute, river as a refugium for waterbirds in the dry season, given the however, quite different ecological settings in comparision with surrounding arid environment. a large perennial river. STUDY AREA * To whom all correspondence should be addressed. The study was conducted in the Mahango Game Reserve, which e-mail: [email protected] is situated in the panhandle of the Okavango River Delta in the Received 9 December 2017, accepted in revised form 3 April 2019. Kavango East region, north-east Namibia (18°12′S, 24°41′E). https://doi.org/10.4314/wsa.v45i2.13 Available on website http://www.wrc.org.za ISSN 1816-7950 (Online) = Water SA Vol. 45 No. 2 April 2019 Published under a Creative Commons Attribution Licence 259 The Mahango Game Reserve was established in 1988 and has an Counts were conducted by a team of 2 to 5 persons. All seen area of 245 km2. It consists of a vast floodplain and associated and heard birds were identified down to species level and riparian forest. The reserve is stocked with diverse ungulate counted. Birds were counted separately in each encountered species. There are 200 hippopotami Hippopotamus amphibius, group. All records were kept in notebooks, following the AWC 300 elephants Loxodonta africana, 500 buffalos Syncerus caffer, guidelines (http://www.wetlands.org). and about 10 antelope species (Mendelsohn and El Obeid, 2004; The following parameters were used to describe the own observ.). The flood reaches heights of 3 to 4 m above the waterbird assemblages: N – number of individuals recorded; low water level in November. The mean annual rainfall in the %N – dominance expressed as the proportion (%)contributed study area is 550–600 mm, with approx. 80% falling between by a given species to the total number of all individuals of all December and March (Mendelsohn and El Obeid, 2009). Annual species recorded. rainfall varies substantially from year to year. The seasonal difference between the total number of The river forms a meandering channel, 2–6 km wide, in the particular species in different years was tested with the Chi- reserve. The riverbanks are covered with extensive marshlands square test. All species for which the expected value was lower comprising tall grasses (Poaceae), sedges (Juncidae), reeds than 5 were excluded. Seasonal differences in the number of Phragmites australis (in the form of reedbeds a few hundreds particular species in each year over the period 2001–2006 were of meters in width) and papyrus Cyperus papyrus. The riparian tested with the Wilcoxon’s matched-pairs signed rank test. forest is composed mainly of Garicinia, Sclerocarya, Diospyros, This was used only when the number of matched pairs was at Vachellia/Senegalia, Grewia, Pterocarpus, Ricinodendron, least 6 and the difference between the numbers was not zero. Ziziphus, Baikiaea, Baphia, Phoenix and Adansonia. Bethune Regression analysis was applied to test the population trends (1991) recorded over 869 vascular plant species from 88 families over the years 2001–2006. This analysis was performed only for in the Okavango Valley. The river valley forms wetlands the more common species, the numbers for which were high connected to the main channel with numerous small channels enough to allow for application of the Wilcoxon’s Test. For each well vegetated with papyrus, rushes and sedges. species dominance was calculated separately for the wet and The annual flood cycle is the most prominent feature of the dry seasons. Okavango River (Mendelsohn and El Obeid, 2009). It influences Dominance was expressed as the percentage contributed the nutrient content in the water and subsequently all life in the by the total number of individuals of a given species recorded river and the valley. The total volume of water passing in Mukwe in the whole period 2001–2006 to the total number of all ranges from appox. 5 607 × 106 m3 to approx. 15 354 × 106 m3 individuals of all species recorded over the same period. per annum (Mendelsohn and El Obeid, 2003, 2004). The highest A ’dominant’ species is defined as that which comprised river flow is from January to March, the lowest from September 5% or more of all individuals of all species recorded, with to October. In November the water level is at its lowest, and ‘subdominant’ species defined as those comprising ≤ 4.99%. in April at its highest; on average there is a 3 or 4 times height The following guilds were distinguished: difference between November and April but this can reach 6 m • Foraging: OW – foraging outside wetlands, SV – short during extreme flooding events. The long-term average (1948– vegetation (including grass) and mud, EV – emergent 1998) is 9 594 × 106 m3. Flows are much more stable in the dry vegetation (including reed, rush and lilies), SW – shallow than in the rainy season. There is also extensive flooding in the water, DW – in or over deep water, A – aerial feeders. valley (Bethune, 1991; Cumming et al., 2012). • Diet: F – frugivorous, V – vegetarian, I – insectivorous, P – carnivorous, VI – vegetarian and insectivorous, PI – METHODS carnivorous and insectivorous. • Migration: R – resident (present throughout the year), RN The main channel surveyed was 50–200 m in width and – resident during breeding, otherwise nomad, LM – resident approx.
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