Competition, predation, and migration: individual choice patterns of Serengeti migrants captured by hierarchical models

Large-herbivore migrations occur across gradients of food quality or food abundance that are generally determined by underlying geographic patterns in rainfall, elevation, or latitude, in turn causing variation in the degree of interspecific competition and the exposure to predators. However, the role of top-down effects of predation as opposed to the bottom-up effects of competition for resources in shaping migrations is not well understood. We studied 30 GPS radio-collared wildebeest and zebra migrating seasonally in the Serengeti-Mara ecosystem to ask how predation and food availability differentially affect the individual movement patterns of these co-migrating species. A hierarchical analysis of movement trajectories (directions and distances) in relation to grass biomass, high-quality food patches, and predation risk show that wildebeest tend to move in response to food quality, with little attention to predation risk. In contrast, individual zebra movements reflect a balance between the risk of predation and the access to high-quality food of sufficient biomass. Our analysis shows how two migratory species move in response to different attributes of the same landscape. Counterintuitively and in contrast to most other animal movement studies, we find that both species move farther each day when resources are locally abundant than when they are scarce. During the wet season when the quality of grazing is at its peak, both wildebeest and zebra move the greatest distances and do not settle in localized areas to graze for extended periods. We propose that this punctuated movement in highquality patches is explained by density dependency, whereby large groups of competing individuals (up to 1.65 million grazers) rapidly deplete the localized grazing opportunities. These findings capture the roles of predation and competition in shaping animal migrations, which are often claimed but rarely measured

Ecohydrology as a tool for the survival of the threatened Serengeti ecosystem

The results of 10 years of monitoring water quality and quantity during the period 1996-2006 in the three rivers (Mbalageti, Grumeti and Mara) draining the Serengeti ecosystem are presented, together with river gauging data starting in 1948, rainfall data starting in 1960, and animal population data starting in 1960. Water quality remained unchanged in the Mbalageti and Grumeti rivers; these rivers are seasonal and they dry out during a drought. The Mara River is perennial and is vital to maintain the ecosystem during a drought. Its quality has changed, with increased contribution from groundwater, with higher pH and visibility and decreasing salinity. The flow rate during a drought has decreased by 68% since 1972. This is attributed to deforestation of its upper catchment in the Mau forest in Kenya and to extraction of water for irrigation in Kenya upstream of the ecosystem. Hydrological modeling suggests that the Mara River would now dry out for two months and one month respectively if the 1949-1952 and the 1972-1973 severe droughts occurred again. Ecohydrologic modeling suggests that this would in turn lead to the collapse of the herbivore population from the lack of drinking water. This model also suggests that providing drinking water to the animals at artificial water holes spread throughout the ecosystem would lead to decadal time-scale booms and busts of the herbivore population. The Serengeti ecosystem stability is maintained by the annual migration that partitions the ecosystem in seasonally used compartments. It is thus necessary to restore the natural hydrology of the Mara River in Kenya, and this requires remediation measures in Kenya. If that does not occur, disaster prevention measures are needed by providing water in weirs, dams, and artificial wetlands along the Mara River in the Serengeti National Park, as well as extending by 5 km the western edge of the park so as to reach Lake Victoria to provide acces to permanent water

The influence of wetlands in regulating water quality in the Seronera River, Serengeti National Park, Tanzania

The distribution of temperature, salinity, visibility and dissolved oxygen was sampled from 1996 to 2002 at sites along the Seronera River. The minimum temperature decreased with distance upstream. The salinity increased up-river where occasionally hypersaline conditions prevailed. Dissolved oxygen was highly variable spatially and temporally, depending on both the level of eutrophication by animal dung and the presence of wetlands that help filter the excess nutrients. During the study period, fringing, freshwater wetlands have generally been degraded and in some cases destroyed, and this has been accompanied by significantly decreased oxygen levels, sometimes nearing anoxic conditions. Also during this period, saltwater\ud wetlands have increased, and since wildlife impacted these wetlands little, dissolved oxygen levels remained high throughout. Visibility was highest in areas fringed by wetlands

The influence of wetlands, decaying organic matter, and stirring by wildlife on the dissolved oxygen concentration in eutrophicated water holes in the Seronera River, Serengeti National Park, Tanzania

The dissolved oxygen concentration (DO) was sampled during a diurnal cycle in three water holes heavily used by wildlife and with distinctive biological features along the Seronera River. The DO fluctuated widely (by up to 11.5 mg l−1) as a function of time, mechanical stirring and aeration by animals, and the presence of fringing wetlands. The DO cycle was successfully modeled (within 0.3 mg l−1) by assuming that the four dominant processes were photosynthesis and respiration by algae near the surface, trapping by wetlands, decomposition of dead organic matter on the bottom, and stirring/aeration by hippos. The rate of DO decline from the decay of dead organic matter was equal to the rate of DO removal by algal respiration at night

Raw climate station data for the southern slopes of Kilimanjaro, Tanzania

The climate station network was set up on the southern slopes of Kilimanjaro, Tanzania, in 2010 and presents the recorded characteristics of air temperature, air humidity, and precipitation from both a plot-based and area-wide perspectives. The station set-up followed a hierarchical approach covering an elevation as well as a land-use disturbance gradient. It consisted of 52 basic stations measuring ambient air temperature and above-ground air humidity and 11 precipitation measurement sites, with recording intervals of 5 min. With respect to precipitation observations, the network extended the long-term recordings of A. Hemp which has installed and maintained up to 117 multi-month accumulating rainfall buckets in the region since 1997

Monthly maps of air temperature and air humidity of the southern slopes of Kilimanjaro, Tanzania

Monthly maps of air temperature and air humidity of the southern slopes of Kilimanjaro, Tanzania. The dataset is part of our study on eco‐meteorological characteristics of the southern slopes of Kilimanjaro, Tanzania ([https://doi.org/10.1002/joc.4552]). (1) ta200_kriging.zip The dataset contains interpolate monthly air temperature maps using universal kriging with elevation, aspect, slope, sky‐view factor and mean monthly normalized difference vegetation index (NDVI) as external drift variables. This corresponds to step 5 in chapter 3.1 of [https://doi.org/10.1002/joc.4552]. (2) rh200_kriging.zip The dataset contains interpolate monthly air humidity maps using universal kriging with elevation, aspect, slope, sky‐view factor and mean monthly normalized difference vegetation index (NDVI) as external drift variables. This corresponds to step 5 in chapter 3.1 of [https://doi.org/10.1002/joc.4552]. (3) ta200_kriging_multi-year_average.zip and rh200_kriging_multi-year_average.zip The dataset contains multi-year monthly averages from (1) and (2) and a map of the multi-year annual mean air temperature and humidity. This corresponds to step 6 in chapter 3.1 of [https://doi.org/10.1002/joc.4552]. For the datasets (1) and (2), we used 5-min measurements between 2011 and 2014 of 52 climate stations that distributed across the southern slopes of Mt. Kilimanjaro. We aggregated the 5-min measurements to hourly observations and filled existing gaps of up to 1 year through multivariate regression using the five nearest stations. We aggregated the gap-filled hourly data to daily averages if at least 22 h of valid records exist for that day at a specific station. Finally, we aggregated the daily averages to monthly averages if a single month has at least 20 valid daily records

Processed climate station data for the southern slopes of Kilimanjaro, Tanzania

This dataset consists of processed (gap filled) climate station data. The data is obtained from the web interface of TubeDB which is an easy to operate software system for archiving, quality control, query and processing time series data in an efficient manner. Further details of Tube DB can be found in this article : Wöllauer S, Zeuss D, Hänsel F, Nauss T (2021) TubeDB: An on-demand processing database system for climate station data. Computers & Geosciences [https://doi.org/10.1016/j.cageo.2020.104641]

Competition, predation, and migration: individual choice patterns of Serengeti migrants captured by hierarchical models

Large-herbivore migrations occur across gradients of food quality or food abundance that are generally determined by underlying geographic patterns in rainfall, elevation, or latitude, in turn causing variation in the degree of interspecific competition and the exposure to predators. However, the role of top-down effects of predation as opposed to the bottom-up effects of competition for resources in shaping migrations is not well understood. We studied 30 GPS radio-collared wildebeest and zebra migrating seasonally in the Serengeti-Mara ecosystem to ask how predation and food availability differentially affect the individual movement patterns of these co-migrating species. A hierarchical analysis of movement trajectories (directions and distances) in relation to grass biomass, high-quality food patches, and predation risk show that wildebeest tend to move in response to food quality, with little attention to predation risk. In contrast, individual zebra movements reflect a balance between the risk of predation and the access to high-quality food of sufficient biomass. Our analysis shows how two migratory species move in response to different attributes of the same landscape. Counterintuitively and in contrast to most other animal movement studies, we find that both species move farther each day when resources are locally abundant than when they are scarce. During the wet season when the quality of grazing is at its peak, both wildebeest and zebra move the greatest distances and do not settle in localized areas to graze for extended periods. We propose that this punctuated movement in high-quality patches is explained by density dependency, whereby large groups of competing individuals (up to 1.65 million grazers) rapidly deplete the localized grazing opportunities. These findings capture the roles of predation and competition in shaping animal migrations, which are often claimed but rarely measured