Research Article |
Corresponding author: Amaël Borzée ( amaelborzee@gmail.com ) Academic editor: Christoph Knogge
© 2022 Godfred Bempah, Martin Kobby Grant, Changhu Lu, Amaël Borzée.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Bempah G, Kobby Grant M, Lu C, Borzée A (2022) The direct and indirect effects of damming on the Hippopotamus amphibius population abundance and distribution at Bui National Park, Ghana. Nature Conservation 50: 175-201. https://doi.org/10.3897/natureconservation.50.87411
|
Landscape changes resulting from human activities have resulted in range restrictions and substantial reductions in population sizes of most animals. The construction of hydroelectric dams has the same effect on species, but the study of their impact on semi-aquatic megafauna species is limited. We examined the response of a Hippopotamus amphibius population to the inundation of their habitat after the construction of a hydroelectric dam in Bui National Park, Ghana. We conducted an abundance and distribution survey of H. amphibius and compared the population size from our results with a pre-dam construction survey to determine changes in the abundance and distribution of the species within the focal area. Furthermore, we conducted a landscape analysis to estimate land cover before and after the dam construction and determined if the changes in land cover were related to the changes in population of H. amphibius. Finally, we conducted selected interviews to understand additional threats to the species perceived by the local population, as indirect effects of the dam construction. Contrary to our original hypothesis on an increase in the abundance of H. amphibius in the medium term (within a decade) through population recovery after the disturbances caused by the construction of the dam, we found lower numbers of H. amphibius after the dam construction, compared to the pre-dam results. The results indicated a reduced abundance from 209 H. amphibius individuals in 2003 to 64 H. amphibius individuals in 2021. Some individuals may have migrated to areas outside the reserve during damming when their habitat was disturbed. The amount of land covered by water increased from 0.41% before damming to 19.01% after damming, which flooded the resting and grazing sites of the H. amphibius. We conclude that the abundance and distribution of H. amphibius significantly and negatively decreased after the construction of the dam at the Bui National Park. We tentatively relate this decrease to the species’ semi aquatic ecology and sensitivity to changes in both the terrestrial and aquatic environment. The activities of human settlement encroachment such as poaching, as well as associated land cover changes, affected the stability of the H. amphibius population. However, as the species can survive in the medium to long term when effective management plans are implemented, we recommend H. amphibius to be given high conservation priorities by enhancing strict laws for habitat protection.
Abundance, flooding, habitat destruction, hydroelectric dam land use, semi-aquatic mammals
The spatial distribution and abundance of animals is an important factor to understand population changes, and a requirement to explain and predict interactions between species and habitat (
Reliance on hydroelectric sources incurs several negative effects to the environment (
Changes in land-use because of human activities have resulted in range restriction in H. amphibius, leading to a substantial reduction in population size (
The population of H. amphibius in Africa is around 155,000 individuals (
Hippopotamus amphibius is a semi-aquatic species which selects areas with sufficient grass for grazing and water for thermoregulation (
The study area, Bui National Park, covers about 1821 km2 is in north-western Ghana (between 8.167°N–8.75°N and 2.083°W–2.45°W, Fig.
We first carried out a reconnaissance survey to establish the presence of H. amphibius in the BNP along the periphery of the Black Volta River.
The study area was divided into five shorelines, each with a team conducting survey simultaneously twice a week for two weeks in July 2020 covering a total survey shoreline length of about 124 km (Appendix
Ground counts survey techniques (land- and boat-based counts) provide comparatively accurate results to record changes in H. amphibius numbers and distribution. Hippopotamus amphibius census during this study were carried out at different periods of the year to determine the seasonal differences in population sizes (
The study was therefore concentrated within the reserve and along the entire periphery of the river (Fig.
To avert double counting during the survey, we classified the 66 km river shore into segments and counting along the shore was carried out simultaneously (
Local migration or cyclical pattern of H. amphibius at a particular site is common and it explains their movements during a particular period, including immigration and emigration (
Rate (%) = PtMA1 – PvMA2 / PvMA2 × 100%
where PtMA1 is present month abundance and PvMA2 is previous month abundance. A negative variation in percentage represents emigration and positive variation in percentage represents immigration.
We evaluated the impact of the dam construction on H. amphibius population by comparing this current study to a pre-dam construction study (
We performed a land cover analysis for the period 2000–2020 to understand how habitat types were linked to the impacts of the dam construction on abundance and distribution of H. amphibius. We acquired land cover matrixes from the Geographic Information System (GIS; https://glovis.usgs.gov/). For the pre-dam period (2000), during dam construction period (2010) and post-dam period (2020), we obtained cloud-free cover Landsat 7 Enhance Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager (OLI) images (Table
Satellite | Sensor | Level of Processing | Tile Number | Number of Bands | Date |
---|---|---|---|---|---|
Landsat 7 | Enhance Thematic Mapper Plus (ETM+) | L1T | 195 054 | 9 | 02-02-2000 |
9 | 12-11-2010 | ||||
Landsat 8 | Operational Land Imager (OLI) | 11 | 01-02-2020 |
Land cover | Description |
---|---|
Close forest | Dense woodland with trees canopy cover of more than 60% |
Open forest | Open woodland with trees canopy cover between 15%–59% |
Water | Rivers and Lake |
Grassland | Shrubs, tree grass mosaic |
Built-up | Building, cleared areas, mining sites, farms, bare land and rocky surfaces |
We collected brief information on the perceptions and knowledge regarding the impact of the dam construction on wildlife by interviewing participants who were regularly active on the Black Volta River in the Bui National Park. The focal group interview was aimed at finding out if the population of H. amphibius had changed following the construction of the dam and what could have accounted for the changes. We used open-ended questions and a conversational style interview (
As part of routine law enforcement and patrol activities, rangers of Bui National Park record a number of illegal activities occurring in the reserve to guide management. This data was accessed from the office of the Bui National Park to analyze the occurrence of illegal activities between 2012 and 2021, the period for which data is available. The illegal activities were classified into poaching (all poaching related activities such as poachers arrested, poachers observed, cartridges found and snares found), cattle herding, charcoal burning, tree felling and illegal mining. The percentage composition of the illegal activities was estimated by dividing the number of each illegal activity by the total number of all illegal activities multiplied by 100%. To test for any significant differences in the number of recorded illegal activities, we performed a Kruskal Wallis chi-square test.
We calculated the average numbers of H. amphibius per site for each month. We used a one-way ANOVA to test for significant differences in the number of H. amphibius between the pre-dam (2003) and post-dam periods (2021). The ANOVA test was appropriate because the normality test for the response variable (abundance) showed a normal distribution. We then tested for differences in H. amphibius abundance for; 1) the refuge sites where the study was done, 2) the various months during which data collection was done, 3) seasonal differences between the study sites and 4) rates of migration between the study sites using a Kruskal Wallis chi-square test. We used a Kruskal Wallis chi-square test, a non-parametric test, because the normality test for the response variable (abundance) showed a non-normal distribution and no data transformation restored normality. We performed a linear regression to determine the relationship between H. amphibius population and the years of study. We also performed a linear regression to determine the relationship between the population of H. amphibius and changes in landcover at Bui National Park for the pre-and post-dam construction periods. The study assumed p < 0.05 to be statistically significant. The statistical analysis was done using R (version 4.0.3) statistical tool (
We determined that the population of Hippopotamus amphibius sharply declined in Bui 10 years after the inundation of the area during the dam construction (Fig.
Population trend for Hippopotamus amphibius over the sampling period in Bui National Park, Ghana.
Number and variation in the number of Hippopotamus amphibius individuals counted in the Bui National Park for the years 2003, 2018 and 2021.
Reference | Year of study | Number | % of change from 2003 |
---|---|---|---|
|
2003 | 209 | – |
|
2018 | 94 | -55.02 |
This study | 2021 | 64 | -31.91 |
Between 2003 and 2021 | -69.38 |
The distribution of H. amphibius changed after the dam construction, and most of the population in the southern section of the Bui National Park disappeared after the dam construction, leaving only one sustainable population at 4 O’clock (Fig.
Spatial distribution of Hippopotamus amphibius and land cover in the Bui National Park before the dam construction (2003, Pre-dam), during (2010) and after the-dam construction (2020, post-dam).
During the current study (post-dam), the maximum abundance of H. amphibius was recorded at “Herimankuna”, followed by “4 O’clock”, while “Didipreko” had the lowest number (Fig.
Rates of immigration and emigration of H. amphibius did not vary between sites (Chi-square test; χ2 = 0.369; p = 0.984). We found a significant seasonal difference in migration of H. amphibius (Chi-square test; χ2 = 7.914; p = 0.005), and the rate of emigration was the highest in the month of June (-168.08%) while the highest immigration rate was recorded in December (512.12%, Table
Monthly differences in migration rates (%) of Hippopotamus amphibius at the various refuge sites in Bui National Park.
Month | Sites | ||||
---|---|---|---|---|---|
4 O’clock | Herimankuna | Nyameraga upwards | Didipreko | Agave patase/Gareba | |
August | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
September | -6.66 | 0.00 | 50.00 | -100.00 | -33.33 |
October | -14.26 | 33.33 | -33.33 | 0.00 | 50.00 |
November | -8.33 | -8.33 | 50.00 | 100.00 | 100.00 |
December | 36.36 | 109.09 | 166.67 | 100.00 | 100.00 |
January | -6.66 | 0.00 | 0.00 | 0.00 | -8.33 |
February | 7.14 | -4.35 | 0.00 | 0.00 | 9.09 |
March | 0.00 | 4.54 | 0.00 | -16.67 | -8.33 |
April | -6.66 | -39.13 | -37.5 | -40 | -36.36 |
May | -7.14 | -7.14 | -20.00 | -66.67 | -14.29 |
June | -0.77 | 7.69 | -25.00 | -100.00 | -50.00 |
July | 16.67 | -42.86 | -66.67 | 100.00 | 0.00 |
During the interview, the respondents emphasized that the human pressures in the Bui National Park increased during and after the dam construction. The answers detailed the increase in economic activities including illegal gold mining, increased fishing activities, cattle herding, charcoal production and settlement/farming in the area. The answers provided data for eight major gold digging sites, especially in the northern section of the reserve, with settlements, cattle herding and farming along the bank of the river (Figs
General trend of H. amphibius population and perceived causal factors at the Bui National Park suggested by respondents in a focus group interview.
Trend of H. amphibius population | Number of respondents | Percentage (%) | Perceived cause of changes in trend | Number of respondents | Percentage (%) |
---|---|---|---|---|---|
Declining | 25 | 83.3 | poaching | 15 | 50 |
Stable | 3 | 10 | Poaching/ inundation | 5 | 16.7 |
Do not know | 2 | 6.7 | inundation | 3 | 10 |
Do not know | 5 | 16.7 | |||
Retaliatory killing | 2 | 6.6 |
Among the illegal activities recorded in the reserve, poaching related activities was the highest (70%), followed by illegal mining (11%) and cattle herding (9%, Fig.
Explaining the effects of hydroelectric dam construction on biological diversity and the response of species is essential for the management and conservation of species and their habitat (
The land cover analysis revealed significant changes in the Bui National Park (BNP) over the study period, covering 2000–2020. The decrease in forest cover (close and open), flooding of riparian grassland and an increase in “built up” areas were obvious during and after the dam construction at BNP. Between the period of 2008–2010, permits were given to contractors to fell trees in the flood zones along the southern section of the Black Volta River before the inundation. However, ineffective monitoring of contractors and other illegal settlers led to indiscriminate felling of trees in the national park. Tree felling continued after the completion of the dam, leading to a significant decrease in forest cover. In addition, the deployment of heavy machines during this process altered the grassland cover. The study revealed a decline in grassland cover between 2010 and 2020, during and after the dam construction. The observed increase in land use during and after the damming is attributed to the establishment of illegal settlers involved in illegal mining, uncontrolled cattle grazing, and charcoal production. This likely resulted in the decline of grassland and an increase in built up area during this period. The results of our study are confirmed by other studies reporting a decline in grassland after damming (
The results of the interviews showed that participants expected the population of H. amphibius to have declined following the construction of the dam. The participants mentioned the flooding of the area where the animals graze as well as the increased volume of water of the resting sites of the H. amphibius as the key elements. The results of our land cover analysis demonstrated the veracity of this expectation as it showed an increase in water cover from 0.41% before damming to 19.01% after damming, as exemplified by similar result (
Differences in H. amphibius abundance between the pre-dam and post-dam period revealed a decrease in the number of individuals, as suggested by the respondents of the interview. This decrease is likely to be driven by the displacement and assumed death of young individuals that were not able to escape from the flooding, as demonstrated in other studies in National Zoological Park Washington, USA and Australia (
The decline in abundance was associated with distribution changes as the population size of H. amphibius significantly dropped in the affected floodplains. Our results are in line with several studies relating to dam construction and significant negative decline in semi-aquatic species (
Other effects on wildlife accompanying floods during damming were outbreaks of wildlife-related diseases. Outbreaks of diseases were predicted during hydromechanical projects (
The results of our study show a higher number of H. amphibius recorded during the dry season than during the wet season. The variation in numbers can stem from the difference in count accuracy between seasons. Observations during the wet season show H. amphibius mostly submerge in pools, therefore reducing visibility and researchers’ ability to detect and count them. Several studies have confirmed this observation challenge during H. amphibius census in wet seasons, suggesting higher counts during the dry season than the wet season when water levels are higher (
Our results from the interviews attributed illegal hunting of H. amphibius to the illegal settlers intruding into the Bui National Park during and after the dam construction. This is supported by the analysis of the data from the Bui National Park for which poaching related activities constituted about 70% of illegal activities recorded in the reserve (Fig.
An observed builtup area in the reserve for the pre-dam period (2003, Fig.
The decline in the number of H. amphibius temporally matched with the influx of illegal mining, nomadic herdsmen and fishing village settlements where poaching occurs. For example, at Virunga National Park in the Democratic Republic of the Congo, fishing settlers supported and participated in poaching of wild fauna including that of H. amphibius (
The results of our study demonstrate that the population of Hippopotamus amphibius has declined drastically with the construction of the dam because of direct effects such as the flooded area, and indirect effects such as poaching facilitated by the construction of the dam. Even though H. amphibius is considered a highly mobile species, it is highlighted from the land cover analysis that flooding affected the riparian grasses which served as food for the H. amphibius and as a major factor regulating the population density of H. amphibius. The co-resultant degradation of suitable grass and increased builtup areas caused additional stress on H. amphibius, further impacting the species. The H. amphibius population could possibly be displaced by the combined effects of flooding and anthropogenic activities of encroached settlements and associated land cover changes. Illegal mining activities and human settlers who become poachers are currently a major challenge towards conservation efforts and could cause further decline in the H. amphibius population. Effective law enforcement actions with commitment from all levels of government and stakeholders should be implemented to avoid further invasion of the protected area by illegal actors that affect species and their habitat. We also recommend establishing a consultative process to evacuate all illegal settlers in the Bui National Park, especially those along the Black Volta River.
We wish to thank the staff and management of Bui National Park, Ghana for their assistance during the post-dam survey.
Funding was received from the Priority Academic Program Development of Jiangsu Higher Education Institutions. AB was supported by the Foreign Youth Talent Program (QN2021014013L) from the Ministry of Science and Technology of the People’s Republic of China.