Impact of climate change and biotic factors on avifauna diversity and spatial distribution in the mid-Zambezi Valley, Zimbabwe.

Abstract

Construction of Lake Kariba on the Zambezi River restructured the biodiversity of the Zambezi Valley. However, studies on how the avian community of Kariba has changed since construction of the lake are scant. This study looked at trend in aquatic avifauna of Kariba from the period prior to damming to contemporary period and related this trend to changes in hydrology. The study also developed a habitat guild structure for aquatic avifauna of Lake Kariba. The developed guild structure categorised birds into four distinct functional habitat guilds. Wilks Lambda statistics revealed significant differences among the avian guilds (P< 0.05). Canonical discriminant function revealed that lake output, lake level and lake evaporation were the three most important variables influencing the avian guild structure in Lake Kariba. Correlation analysis revealed significant negative relationship between species richness and river inflow into Lake Kariba, lake level and total outflow from Lake Kariba (p<0.05: r = -0.60786; r = -0.68859; r = -0.67551 respectively) and significant positive relationship between species richness and atmospheric temperature and lake evaporation (p<0.05: r = 0.465; r = 0.45594 respectively). This study conclusively revealed that the avian community of Kariba exhibited apparent continuous temporal changes in composition in response to hydrology. In context of the observed continuous temporal changes in avian community composition of Lake Kariba in response to hydrology, integration of conservation issues in the management of Lake Kariba is essential to conserve the aquatic habitats and biodiversity in the lake. Construction of Lake Kariba on the Zambezi River channel was associated with apparent biodiversity changes in the submerged areas of Kariba and sections downstream of the lake. These changes were a result of impact of damming on the hydrology and ecology of the aquatic ecosystems of the Zambezi Valley. No empirical studies however, were ever conducted to compare avian diversity of Lake Kariba with that of the Zambezi River upstream and downstream of the lake. The present study used Czekanowski Coefficient, species richness, Simpson’s Diversity Index and evenness to compare contemporary avifauna of Lake Kariba with that of the Zambezi River upstream and downstream of the lake more than half a century after construction of Lake Kariba. Three sections of the Zambezi Valley were covered in this study namely: (i) Lake Kariba which was a lentic ecosystem (lake environment) that was created on the Zambezi River channel in the 1950s; (ii) Zambezi River upstream of Lake Kariba which was a lotic ecosystem (river environment) and whose flow regime was presumed not altered by construction of Lake Kariba; and (iii) Zambezi River downstream of Lake Kariba which was a lotic ecosystem and whose flow regime was altered by construction of Lake Kariba. Surveys involved distance sampling point count method to establish the winter and summer avian community composition in the three sections of the Zambezi Valley. Spatial community comparisons showed low similarity (Czekanowski Coefficient < 45%) in avian community composition between Lake Kariba and both sections of the Zambezi River upstream and downstream of the lake in both winter and summer. Spatial community comparisons also showed low similarity (Czekanowski Coefficient < 45%) in avian community composition between the upstream and downstream sections of the Zambezi River in both winter and summer. Results showed significant differences (p<0.05; Kruskal-Wallis test) in avian species richness, diversity and evenness among the three sections of the Zambezi Valley in winter and in summer. Aquatic avian species diversity was higher in the lotic environs of the Zambezi River than in the lentic environs of Lake Kariba in winter and in summer. Mann-Whitney pairwise comparison tests showed significant differences (p<0.05) in species richness between Lake Kariba and both sections of the Zambezi River as well as between the upstream and downstream sections of the Zambezi River in both winter and summer. Mann-Whitney pairwise comparison tests also showed significant difference (p<0.05) in species diversity and evenness between the lentic environs of Lake Kariba and lotic environs of the Zambezi River both upstream and downstream of the lake in winter and in summer. However, species diversity and evenness did not vary significantly (p>0.05; Mann-Whitney pairwise comparison test) between the upstream and downstream habitats on the Zambezi River in winter and in summer. Results showed more heterogeneity in avian species abundances on the lotic habitats than on the lentic habitat. This was shown by higher species evenness values that were observed on the Zambezi River compared to those observed on Lake Kariba. Species evenness ranged between 0.90 and 0.97 on the Zambezi River upstream of Lake Kariba and between 0.83 and 0.98 downstream of the lake across all sampling months. The lowest species evenness was observed on Lake Kariba where values ranged between 0.57 and 0.85 across all the sampling months. In conclusion, this study revealed that contemporary aquatic avian species diversity of Lake Kariba significantly differed from the avian species diversity on the lotic habitats of the Zambezi River upstream and downstream of the lake. Conservation of all the three aquatic sections of the Zambezi Valley is thus, important in order to preserve the unique avian communities in these areas. Reports indicate a decline in spatial distribution, reproduction and population size of the African Skimmer (Rynchops flavirostris Vieillot, 1816) in the Zambezi Valley, Zimbabwe following construction of Lake Kariba. The objective of the present study was to assess contemporary distribution and breeding status of R. flavirostris in Lake Kariba and upstream and downstream of the lake in Zimbabwe. The study also developed habitat suitability criteria for the dominant substrates, soil moisture levels, soil temperature, potential risk and food abundance for R. flavirostris in lentic and lotic environs of the Zambezi Valley, in order to establish the appropriate habitat conditions for the species. A survey method was used to determine R. flavirostris distribution and breeding status within the valley. Soil moisture level was measured by categorising levels into wet, damp, and dry soils and the category with the highest frequency was used to define habitat substrate moisture levels. Dominant substrate was visually estimated basing on proportion of soil particle sizes. Habitat potential risk was assessed on the basis of potential risk from anthropogenic and natural factors. Seine net fishing was conducted to estimate food abundance at sampling sites in Lake Kariba. Seine net fishing to estimate food abundance was not conducted in the lotic environs because of dangers of crocodiles and hippos on the Zambezi River. Significant differences in sample means of measured habitat attributes between sites occupied by R. flavirostris and those unoccupied by the species were tested by two sample t tests with separate variance. One-way ANOVA was used to test for significant differences in habitat conditions among sampling sites. Three-way ANOVA was used to test for significant interactions of habitat conditions on R. flavirostris abundance in lentic and lotic environs. Preference ratios for habitat conditions were used to develop habitat suitability criteria. Results revealed that R. flavirostris was inhabiting and breeding in Lake Kariba and upstream and downstream of the lake. The species tolerance of habitat risk was observed to vary significantly (p<0.05, One-way ANOVA) among the three sections of the valley. Within the lake, R. flavirostris mostly preferred sandbars of moderate potential risk (0.4<R<0.7). Contrary, R. flavirostris showed absolute preference to sandbars with low potential risk (R<0.4) in habitats upstream and downstream of the lake. Significant differences (p<0.05; One-way ANOVA) in R. flavirostris abundance among sites of varying substrates were observed. Habitat Suitability Indices showed that R. flavirostris preferentially selected predominantly silty and sandy habitats in both lentic and lotic environs. Significant difference (p<0.05; One-way ANOVA) was also observed on R. flavirostris abundance among sites with different soil moisture content in the valley. R. flavirostris showed low preference for dry habitats in both lentic and lotic environs. No significant difference (p>0.05; One-way ANOVA) in temperature was observed on sandbars occupied by R. flavirostris within the lake and upstream and downstream of the lake. R. flavirostris preferentially selected sandbars with food abundances greater than 0.15fish/m2 within Lake Kariba. This study conclusively revealed that food availability, dominant substrate, soil moisture level and habitat risk emanating from both natural and anthropogenic factors at sandbars were important determinants of habitat suitability criteria for R. flavirostris in the Zambezi Valley. Sandbars that offer suitable habitat for R. flavirostris should thus, be prioritised when formulating conservation strategies for the species in Lake Kariba and on the Zambezi River. Intensifying conservation efforts on sandbars that offer suitable habitat for the species may have positive effect on the breeding and population size of the species in the Zambezi Valley. The African Skimmer (Rynchops flavirostris Vieillot, 1816) is an aquatic intra-African migrant species which is listed as near threatened owing to its small population size and ongoing poor breeding within its Southern African habitat. In Zimbabwe, the species is confined to the Zambezi Valley where it occupies habitats on Lake Kariba and on the Zambezi River upstream and downstream of the lake. Construction of Lake Kariba was reported to have adversely affected habitat quality for R. flavirostris in the Zambezi Valley. In this study, foraging budgets and foraging durations of R. flavirostris were used to compare habitat quality for the species in three sections of the Zambezi Valley namely: (i) Lake Kariba; (ii) the Zambezi River upstream of Lake Kariba; and (iii) the Zambezi River downstream of Lake Kariba. The focal animal sampling method was used to collect data on species foraging behaviour in the three sections of the Zambezi Valley. The study was developed under the hypothesis that R. flavirostris would display distinct foraging budgets and foraging durations in the three sections of the Zambezi Valley due to differences in habitat quality in these aquatic environments. The study predicted that R. flavirostris would spend less time foraging in habitats with more favourable conditions than those with less favourable conditions. The study also tested the interaction effect of flock size, sampling month and location on foraging budgets and foraging durations of R. flavirostris in the Zambezi Valley. Distinct differences (p < 0.05; Kruskal-Wallis test) were observed in foraging budgets and foraging durations of R. flavirostris in the three sections of the Zambezi Valley. Foraging budgets and foraging durations were significantly lower (p < 0.05; Mann-Whitney pairwise comparisons) in Lake Kariba than upstream and downstream of the lake on the Zambezi River, suggesting habitat conditions to be more favourable for the species in Lake Kariba than on the Zambezi River. The lowest foraging budgets were observed in Lake Kariba in October, where mean foraging budgets of 0.1207 ± 0.0242 and 0.1847 ± 0.0326 were recorded in the morning and late afternoon respectively. The highest foraging budgets were recorded on the Zambezi River upstream of Lake Kariba, where mean foraging budget of 0.55 ± 0.0471 was recorded in July in the morning and mean foraging budget of 0.4671 ± 0.038 was recorded in August in the afternoon. Lowest foraging durations were recorded on Lake Kariba in October where mean foraging duration of 6 707.44 ± 1 546.07 seconds was recorded. Highest foraging durations were recorded on the Zambezi River downstream of the lake in August, where mean foraging duration of 18 086 ± 1 184.84 seconds was recorded. Results of this research revealed that R. flavirostris foraging behaviour was not homogeneous in the two sections of the Zambezi River. Mann-Whitney pairwise comparison tests revealed significantly lower (p < 0.05) foraging budgets and foraging durations in the downstream section of the Zambezi River compared to the upstream section, indicating possible occurrence of more favourable habitat conditions for the species in the downstream section of the Zambezi River than in the upstream section. Rynchops flavirostris flock sizes varied significantly (p < 0.05, Kruskal-Wallis test) among the three sections of the Zambezi Valley. Test for interactions showed no statistically significant interaction between flock size, sampling month and location (p > 0.05, Three-way ANOVA) on foraging budgets and foraging durations of R. flavirostris. There was also no statistically significant interaction (p > 0.05, Three-way ANOVA) between flock size and sampling month and between flock size and location on foraging budgets and foraging durations of R. flavirostris. However, significant effect on foraging budgets was observed on the interaction between location and the sampling month (p<0.05, Three-way ANOVA). This study thus, conclusively revealed that habitat quality for R. flavirostris varied significantly among the three sections of the Zambezi Valley with Lake Kariba offering the best habitat quality for the species. Based on these results, water resources management decisions for Lake Kariba thus, have implications on habitat quality for R. flavirostris. Conservation of the species habitat needs to be taken into consideration when crafting management plans for the lake. Climate models project a hot and dry future for Southern Africa. In this research, Maximum Entropy was used to model the extent to which climate change, land cover and distance from water edges may influence current and future distribution of the African Skimmer (R. flavirostris) in the Middle Zambezi Valley. Global Biodiversity Information Facility (GBIF) data collected between the years 2000 - 2019 were used to model distribution of R. flavirostris in the Middle Zambezi Valley. Three models were built: one for current distribution, and two for future distribution under Representative Concentration Pathways (RCPs) 2.6 and 6.0. Results revealed that annual precipitation and distance from water edges were the most important predictors of habitat suitability for R. flavirostris under current and future climate. Temperature and land cover were least important in explaining current and future distribution of R. flavirostris in the Middle Zambezi Valley. The RCP 2.6 predicted future decrease of suitable habitat for R. flavirostris in the Middle Zambezi Valley, while RCP 6.0 predicted future increase in suitable habitat for the species. This research conclusively revealed that precipitation and distance from water edges were consistently key predictors of suitable habitat for R. flavirostris in the Middle Zambezi Valley. Predicted decrease in suitable habitat for R. flavirostris in the Middle Zambezi Valley under the RCP 2.6 is a pointer to possible future decrease of the species and loss of genetic diversity in the Middle Zambezi Valley. There is therefore need to enhance conservation efforts for R. flavirostris in the aquatic environs of the Zambezi Valley.