Short-term effects of natural disturbance on wetland bird abundance
A damaging heavy rainfall accompanied with strong gusty with winds struck Thalawathugoda, Sri Jayawardenepura Kotte on 9th December 2017, affecting the area severely with floods and other damages. Natural disturbances such as this provide a unique and rare chance to study how the impacts affected wildlife species. I investigated the short-term effects on abundance of wetland bird species of Diyasaru wetland park. With survey data of collected on five consecutive days before and after the heavy rainfall and strong winds, the total bird abundance was calculated, and the results presented that after abundance was lower than abundance before the storm. However, considering the species for each species showed a mixed abundance response, suggesting disturbance in abiotic and biotic factors of the wetland affected each species differently.
Ecological factors account for species abundance in an ecosystem, particularly abiotic factors rainfall, temperature, habitat area, latitude, and altitude can construct for an ultimate difference in abundance of species in an ecosystem. And biotic factors, for example, vegetation types also play a part in determining abundance and diversity of species of an ecosystem (Stroud et al. 2015). Many studies have shown a high ratio relationship between ecological factors and species abundance.
Wetland is important among the most productive ecosystems in the world, comparable to rainforests and coral reefs (EPA.2013). A monumental variety of species living organisms are a part of the wetland ecosystem. Climate, topology, geology and movement and abundance of water help to determine the flora and fauna that inhabit each wetland ecosystem. And therefore, the reason in this report I want to find how a natural disturbance such as heavy precipitation and strong winds can affect the abundance of birds in a wetland ecosystem.
1.1. Wetland definition
It is an interface between land and water (Association of State Wetland Managers.2015), and therefore wetlands are directly depending on precipitation to maintain its biotic and abiotic factors such as plants, animals and hydrology, temperature respectively. Wetlands also play an inherent role in the ecology of the watershed. By maintaining shallow water, high levels nutrients and primary productivity brings about ideal environment for development of organisms that form the base of the food web and many fish and amphibians as well as many species of mammals and birds especially on their breeding seasons rely on wetlands for their shelter, food and water (Environmental Protection Agency .2013). Therefore, due to extreme rainfall and strong winds can cause a shift in biological communities in wetlands. Floods and decrease the level of food supply and primary productivity of the ecosystem by sediment deposition and habitat destruction (Pandit & Qadri, 1990). In turn, this affects rest of the food web via the fish and amphibians that consume the aquatic plants and occupy the habitat. This causes a loss of populations of aquatic birds and desynchronization of migration events and, breeding patterns according to the availability of a food supply at a time (Association of State Wetland Managers.2015).
1.2. Defining Disturbance
A disturbance befalls when potentially damaging forces affect a habitat space of an ecosystem (Lake. 2000). The disturbance could be defined by the nature of the abiotic dame to the properties such as the intensity and forms of forces, other parameters (e.g., frequency, predictability and temporal distribution). This characterization of disturbance was advocated by Lake (1990) and Poff (1992), however, others have defined disturbance by their effect on biota and biotic response (White and Pickett.1985). In other areas of disturbance ecology (e.g.: floods), disturbances are characterized by season of occurrence, intensity, frequency, type and extent and patchiness (Whelan 1995). Stream ecologists characterize disturbance as drought or floods with realistic comparisons made among times, sites, and rivers (Poff and Ward 1989, Resh et al. 1988).
On 2017, 9th December as predicted by the Department of Meteorology a heavy rainfall average exceeding 168 mm was received throughout the country including Thalawathugoda, Sri Jayawardenepura Kotte, of Western Province accompanied by strong gusty winds of 70 -80 kmph (Holiday weather.2017, ‘Rainfall exceeds 150 mm’. 2017). Due to the heavy rainfall and strong gusty winds disruption of the equilibrium of the ecosystem of both abiotic and biotic elements of the Diyasaru Wetland park was seen (Lake. 2000). However, with careful planning and management, the impact on the habitat of wetland species could be mitigated promiscuously.
The best ways to do this is to participate in cross-cutting interest groups engaged in addressing similar effects of climatic changes on wetland habitats, and other land issues (Association of State Wetland Managers.2015). Ensuring wetland managers to recognize the likely consequence of impacts on wetlands prior to such climatic changes including direct effects of climate change and be ready to mitigate.
The aim of this report is to analyse the effects of the heavy rainfall and strong winds on the wetland bird abundance of Diyasaru wetland park in Sri Jayawardenepura Kotte, Sri Lanka (Figure 1) regarding before and after disturbance using transect count method to collect data and discuss the management resource strategies for the Wetland Park and mitigation of resource damage due to the store and finally, take the outcome of this research to aid in management and possible future recommendations to further improving the Wetland Park.
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Figure 1: Google Earth Map view of Diyasaru Wetland Park, Sri lanka (Google Earth Maps, 2018)
2.1. Study Area
The study was carried out in a wetland area of Diyasuru Park (Thalawathugoda Biodiversity Study Park) within an approximate 20km radius of site located in highly urbanized area in proximity to Parliament complex in Sri Jayawardenepura Kotte, Sri Lanka; (6.880016°N, 79.930402°) had not been officially (Lockwood, 2016). The Diyasaru Park comprises of artificially created ponds and lakes and marshlands, a three-storied observation deck, wooden walkways, a picnic patch, benches, wooded areas, a rush and a reed pond dinghy boats that can be used to explore the lakes (Mihipidia,2018). As a wetland, park serves home to many different animals and marshy vegetation, especially resting places for migratory birds and to more than 40 butterflies’ species, more than 80 species of wetland bird and many other types of dragonflies, fishes, mammals, amphibians, reptiles, and other terrestrial and aquatic species denotes a high biodiversity value states the values of urban wildlife (Wetland Link International, 2015).
Figure 2: Diyasaru Biodiversity Wetland Park of Thalawathugoda, Sri Jayawardenepura Kotte, Sri Lanka (Lockwood. I,2018)
2.4. Avian Surveys
The first data set was collected from 4th to 8th December 2017. For every five minutes at a slow rate and steady pace of ca. 0.8 km/hr counted every bird heard or seen and measured the distance between each five-minute interval using a measuring tape. The following day and night as predicted in the weather report heavy rainfall exceeding average 168 mm and strong gusty winds of 70 -80 kmph (Holiday weather.2017, ‘Rainfall exceeds 150 mm’. 2017). swept across the country. The second set of data was collected from the following day 10th to 14th December. Subtotals were made every fifteen minutes (approximately 200 m). Photographs were taken of birds counted and later cross-referenced them to made sure they were not the same bird through the differences of their physical appearance. No bird was knowingly counted twice (Dawson ; Bull,1975). Each day surveys began by 1600hr and concluded by 1700hr.
2.6. Data analysis
2.6.1. Two sample dependent t-test and Species abundance
I calculated species abundance as the maximum number of counts for five consecutive days. I divided this by the total number of counts to get a mean abundance. I resolved the pre-and post-storm species abundance after collecting each data set on the fifth day of surveying (Faccio. 2003). I used the average values for before storm and after storm total abundance of birds to calculate the standard deviation. Thereafter, using 0.05 as the alpha value the confidence intervals were calculated. And finally, a two-sample dependent t-test was used to test difference among the before the storm and after storm total abundance of birds (Faul et al., 2007).
The abundance of bird species before the rain storm was greater than the abundance of birds after (Figure 3). Two bird species out of the sixteen species surveyed for bird abundance/ha had shown an increase in abundance after the storm. For Spotted-billed Pelican (Pelecanus philippensis) abundance before the storm was 2.4 and after storm 6.3, whereas for the Purple Swamp hen (Porphyrio porphyrio) before abundance was 8.7 and after abundance was 12.6. From this, it can be observed that abundance for both Purple swamp hen and Spotted-billed Pelican had been increased by a difference of 3.9 abundance/ha. Whereas White-breasted water hen (Amaurornis phoenicurus), Red-Wattled lapwing (Vanellus indicus), Purple Heron (Ardea purpurea) and Eastern Spotted dove (Spilopelia chinensis) abundance after the storm has decreased compared to the abundance before the storm. Rest of the species had not shown a significant difference in the number of abundance/ha before and after the storm.
Figure 3: Bird species abundance before and after rain storm
However, considering the average of the total bird abundance, the before storm abundance (7.49) was greater than the abundance after the storm (6.26) (Table 1 and Figure 4) and had a significant difference in the abundance of birds before and after the storm.
Table 1: Total bird abundance values of before and after
Bird Abundance n Mean Standard Deviation Confidence intervals P value Degree of freedom
Before 16 7.49 5.12 0.50 0.05 30
After 16 6.26 5.09 0.50 0.05 30
Figure 4: Difference in abundance of birds before and after the storm
Many studies have been done on the long-term consequences of environmental impact on wetland bird abundance (Masero. 2003) and in most of them have investigated upon significant negative effects of habitat destruction, breeding site reduction, and disturbance in flow regimes on bird abundance (Knopf. ; Sedgwick.1987, Nilsson. ; Dynesius. 1994). In this report discuss how short-term natural disturbance could impact on the bird abundance of the wetland.
4.1. Effects of disturbance
In flooding wetlands, large volumes of precipitous moving water with shear forces which suspend sediments, and redistribute the bottom materials, remove vegetation (e.g., microscopic algae to macrophytes), snags and debris streambeds and maim, kill and displace all forms of biota (Lakes,2000). Floods could be predictable or unpredictable (Poff. 1992) and could vary from high water events which entrain fine sediments and moves patches of streambeds to infrequent disastrous events. (Costa and O’Connor 1995). The strong gusty wind that accompanied the heavy rainfall further destroyed the tall vegetation by uprooting them, damaging the public goods such as information boards, bridges, boardwalks, pathways, birdwatch hideouts, and bird watch tower. There are many factors that influence the storm damage to structures: Storm surge, Wind direction, waves, velocity, and building quality. Moreover, the effects of waves, storm surge, and wind usually interact in a nonlinear way that might vary with the vegetation and local topography (Howes et al., 2010; Wamsley et al.,2010)
4.2. Changes in relative bird abundance
The two-sample dependent t-test approach (Zuckerberg et al. 2009) I used to analyse changes in relative bird abundance from before to after rain storm disturbance provided a basis for attributing the observed changes to the rain storm (Smucker. 2005). The main trend in decline in abundance (Figure 3) among them was following species of birds. The White-breasted Waterhen (Amaurornis phoenicurus) were seen foraging along the edge of the waterbody mainly on the ground and low vegetation. They probed their bill into shallow water and sometimes mud picking insects, small fish and aquatic invertebrates by sight (Thai National Parks. 2018). Due to the change in their usual habitat by an increase in water levels of the wetland park and decline of food availability of on the feeding grounds might have been a reason to decline of the abundance of White-breasted Waterhen (Figure 3). Similarly, Red-wattled lapwing (Vanellus indicus) were seen bathing in waterbodies, resting on the ground with the tarsi laid flat and resting on one leg (Thai National Parks. 2018). They feed on beetles, aquatic insects, molluscs, worms and crustaceans (BirdLife International. 2016). Additionally, to the main reasons for the decline of birds after storm another reason might have been that some of Red Wattled lapwings that inhabited the wetland park had migrated to a lower altitude (Wiersma and Kirwan 2016). The Purple heron (Ardea purpurea) were seen waiting motionless for fish, feeding in shallow water of wetland (Oiseaux bird. 2018) and their main diet comprises in small fish, frogs, invertebrate small reptiles (BirdLife International. 2016). However, due to the disturbances to the vegetation and large quantities of uprooted vegetation and debris in the waterbody of the wetland might have played a role in the decline in abundance of purple herons. And final species that had shown a decline in the study (Figure 3) were Eastern Spotted dove (Spilopelia chinensis). Eastern Spotted doves were mostly seen in pairs and small flocks or perched on trees of wetland park. Their diet comprised of grass seeds, small seeds, and termites on dry land (Frost, P.G.H. 2013). Although the Eastern spotted doves are not wetland birds due to the disturbance of the habitat might be a reason for the decline of their abundance. The increase in abundance of Spot-billed Pelican (Pelecanus philippensis) and Purple Swamphen (Porphyrio porphyrio) might be their similar behaviour and ecology such as inhabitation of deep and shallow wetlands of variety. Their similar diet comprises of mollusc, leeches, small crustaceans, insect larvae, fish, fish eggs, frogs, and frog spawn (del Hoyo et al. 1996, BirdLife International. 2017). In addition, the list of species of the wetland ecosystem affected had an overall equal before storm abundance to after storm abundance and it can be observed that the different bird’s species showed a mixed response of abundance to the flooding and other damages to the wetland (Figure 3) (Kotliar et al. 2002).
However, considering the overall abundance of birds before and after storm the abundance after had significantly declined compared to the abundance before (Table 1, Figure 4). could be seen due to several factors such as debris from the flooding left on the ground, the decrease of vegetation cover, reduction of food availability on the surface of the wetland floor due to floods (Wenny et al. 1993; Waide.1991).
4.3 Importance of pre and post bird abundance surveys to heavy rainfall and strong wind
The results were noteworthy in a manner it showed that different species of birds had a mix response in abundance before and after the storm (Figure 3) (Kotliar et al. 2002). This result is coherent with other researches, that have found that bird communities vary among forest types and before and after forest fires (Kirk et al. 1996; Welsh and Lougheed 1996; Kotliar et al. 2002). Additionally, this gave insights identification and monitor birds that visit the wetland, advocacy and policy development to promote conservation of wetland birds and biodiversity, local community involvement in wetland management programmes, species recovery programmes, and expand and strengthen organisations to conserve birds and wetlands (Bibby et al., 2000)
To make this research project more successful in future studies I suggest in conducting the survey for month and make comparison with the change in bird abundance weekly analysis for a month to conclude on how storm disturbance resulted in bird abundance and how long it takes for bird abundance to recover and investigate the vegetation present in the ecosystem. In addition, I would like to suggest an immediate start on mitigations of the wetland park by consulting the regulations under the Land Reclamation and Development Cooperation (Kulathilaka. 2012). Modify the outlet of wetland and reroute the excess water from wetland park to Diyawanna lake across the road via underground tunnels, use level spreaders (Pekarek. 2011) to diffuse the flow of water across the landscape to trenches leading to the butterfly garden and Diyawanna lake. Also, organise a campaign event with the help of locals and homeowners include them in the mitigation of the wetland (Homer. 2000) such as replanting of destroyed vegetation, riparian vegetation, add artificial support such as strong wooden poles to windblown vegetation, and clean up the debris came from the flood. Also conduct a weekly observation routine for management planning and monthly repair of main structures of wetland such as observation tower, stormwater diffusion system, bird watch hideouts, boardwalks, pathways and bridges to maintain the Diyasaru wetland park for the future generations
Overall, this project was a success because as my aim for the project was analysed the effects of heavy rainfall and strong winds and the compare bird abundance of Diyasaru wetland park before and after and, I found that there was a slow trend in the recovery of bird abundance. This shows that even the short-term disturbances that could be mitigated by proper management could cause a change in the recovery of bird abundance of the wetland park. Also, by discussing and suggesting recommendations for immediate management of the wetland park after the storm I was able to fulfil my objective of taking the outcome of this research to aid in future management and further improving the wetland park.