How important are water birds? Are they necessary? Do they serve a purpose? These are just some of the questions I asked myself when I started my research. I have been working on wading birds and their population trends which looked at Zandvlei Estuary Nature Reserve. The trends I analysed covered an average of thirty years.
I thought I knew my bird species before doing this research, however if you think waders are ducks like I did….. You’re wrong!
The research that I did for my Btech degree was concerned with their population trends over a 42 year period. The research was conducted at Zandvlei Nature Reserve to determine whether the wading bird species were decreasing or increasing.
What are waterbirds.
Waterbirds are bird species that are completely dependent on wetlands for many of their activities such as looking for food, nesting, loafing and moulting. Waterbirds are bio indicators of wetland habitats due to the fact that they respond to any vegetation disruptions as well as water level fluctuations better than any other animal. Waterbirds include many groups of species comprising of anseriformes, pelecaniformes and podicipedformes etc. Waders display many adaptations in order to exploit wetland habitats. These adaptations include bill length and size, bill lamellae distance, neck and leg length and body size. All these adaptations allow these different species to feed at different depths as well as on different food types. Water depth is thus very important in determining waterbird densities and establishing whether or not the habitat is accessible.
Resident versus migratory birds
Most scientists define resident birds as a species not prone to migration, remaining in the area all year-round. It is important to understand the population trends of these birds as they can be very sensitive to changes in the environment caused naturally or by humans. The majority of resident birds breed in the inland wetlands in South Africa then travel to the coast in the summer in order to avoid drought, however some resident birds increase during winter months such as the Dabchick Tachybaptus ruficollis. Migration is the consistent seasonal movement of species normally from a breeding location to a non-breeding location and back again. Migratory waterbirds are an important feature of coastal and estuarine habitats, ecosystems and conserved under the Ramsar Convention.
Migration is a seasonal use of ecosystems when migratory birds utilise the abundant but temporary resources enabling an increase of offspring production. Migration also contains significant costs associated with energy consumption, subject to predators or unpredictable food shortages, which can drop non breeding survival of migratory birds unlike resident birds. Scientists stated that adult and juvenile migrating birds show clear detachment before migrating so as to limit intraspecific competition. Increasing seasonality suggests an uneven temporal allotment of food resources. This affects the time period that birds will stay outside their breeding region. The time will be as short as environmental state allows. This is due to early activity of breeding areas which improves reproductive success. Migrating waders, with their readiness to accumulate in large numbers at wetlands and other water bodies, make them very important components of global biodiversity. There are 28 critically endangered waterbird species globally, however 83% of wading species are declining with only 62% of waterbird species having long term trend data available.
Human activity and it’s affects on wetlands.
Over 50% of wetlands around the world have been destroyed in the past century, while the remaining wetlands have become degraded in different aspects due to the adverse human activities which include agriculture, development and recreation. The degradation and loss of these wetlands has affected the waterbirds which rely heavily on these wetland ecosystems. Providing suitable habitats for waterbirds through good quality management is a prevalent problem in waterbird conservation. The use of synthetic wetlands used for conservation purposes is an added strategy managers are thinking of due to South Africa being a water scarce country.
Migrations are among the greatest phenomenon of the natural world, yet they are increasingly imperilled by human causes. Migratory animals must undertake energetically taxing migrations covering thousands of kilometres between breeding and non-breeding sites, and many interrupt their passage to rest and refuel at stopover sites along the way.
The data showed that 25 bird species had increased, 30 bird species had decreased and 29 bird species showed no significant trend over the 42 year period of bird count. Some of the birds increasing are the Kelp Gull (Larus dominicanus), Egyptian Geese (Alopochen aegyptiaca) and Red-knobbed Coot (Fulica cristata ) While the birds decreasing include the Curlew Sandpiper (Calidris ferruginea), Little Stint (Calidris minuta) and the Kittlitzs Plover (Charadrius pecuarius).
Water levels in wetlands tend to fluctuate depending on location, precipitation and incoming water resources. Single wetlands may not have attractions to waders continuously throughout the year or for many years. This is caused by unpredictable water level fluctuations which cause aquatic vegetation changes. The investigation of wader population fluctuations in wetland ecosystems is of high importance in order to determine the waders’ community structure and population status of the species occurring and dwelling in those areas. Wader communities can experience seasonal and annual variations in abundance on a local and regional scale due to births, deaths as well as water body size, physical and chemical conditions of those water bodies. Abundance also depends on availability of food, reproduction and nesting sites.
Causes of wader population declines
Potential causes of bird population declines could be due to the urbanised areas that surround Zandvlei Nature Reserve including light to heavy industry, agriculture and forestry. An industrial area opposite the Keysers River discharges its effluent into Zandvlei Nature Reserve. This could possibly impact the wetland and estuary negatively forcing the birds to leave. Habitat destruction and fragmentation is also an increasing concern. Habitat fragmentation in the Netherlands almost caused the extinction of the Sedge Warbler Acrocephalus schoenobaenus. Smaller fragments can have sub optimal habitat quality which leads to less reproduction. Habitat destruction affects breeding and dispersal success rates negatively as well as certain behavioral aspects which could affect foraging success. Waterbirds have a diverse feeding range including; seeds, leaves, tubers, rhizomes and some invertebrates like fish and amphibians. If these sources of food are taken away due to habitat destruction, the bird populations decline. Natural landscapes that maintain the required ecosystem functioning should produce more biodiversity. Habitat destruction such as deforestation in Canada caused a large reduction of migrant birds in the area as they need extensive forest area during breeding season.
Climate change is seen to be a long term problem which will affect the environment for many decades or centuries to come. The effects of climate change are known worldwide, with biological responses varying among species and taxa responding against potential climate related extinctions. Waders react to these changes by either staying or moving to find more suitable habitats. Scientists stated that environmental components can affect bird populations in many different spatial scales from micro habitats to macro habitats. Hydrological change linked to climate change could possibly affect surface water which ultimately could affect water birds directly or affect their food source. Many indicators on biodiversity show growing trends in recent decades on the impact of climate change on bird populations
The birds that are increasing are generalists which can adapt to changes in their environment, while the birds that are decreasing are specialists that can not adapt to human pressure. The human factors causing the decline of some waterbird species are urbanization, alien species and degradation of wetlands which aid migrating species on their routes or for stop overs. The first step in conserving waterbirds worldwide is by limiting their degradation and destruction. This is not an easy task in regard to the expanding human population as well as the increased threat of global warming. Thus intensive management is required for waterbirds and their ecosystems.
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