Ecological and behavioral mechanisms of density‐dependent habitat expansion in a recovering African ungulate population

Abstract

Major disturbances can temporarily remove factors that otherwise constrain population abundance and distribution. During such windows of relaxed top-down and/or bottom-up control, ungulate populations can grow rapidly, eventually leading to resource depletion and density-dependent expansion into less-preferred habitats. Although many studies have explored the demographic outcomes and ecological impacts of these processes, fewer have examined the individual-level mechanisms by which they occur. We investigated these mechanisms in Gorongosa National Park, where the Mozambican Civil War devastated largemammal populations between 1977 and 1992. Gorongosa's recovery has been marked by proliferation of waterbuck (Kobus ellipsiprymnus), an historically marginal 200-kg antelope species, which is now roughly 20-fold more abundant than before the war. We show that after years of unrestricted population growth, waterbuck have depleted food availability in their historically preferred floodplain habitat and have increasingly expanded into historically avoided savanna habitat. This expansion was demographically skewed: mixed-sex groups of prime-age individuals remained more common in the floodplain, while bachelors, loners, and subadults populated the savanna. By coupling DNA metabarcoding and forage analysis, we show that waterbuck in these two habitats ate radically different diets, which were more digestible and protein-rich in the floodplain than in savanna; thus, although individuals in both habitats achieved positive net energy balance, energetic performance was higher in the floodplain. Analysis of daily activity patterns from high-resolution GPS-telemetry, accelerometry, and animal-borne video revealed that savanna waterbuck spent less time eating, perhaps to accommodate their tougher, lower-quality diets. Waterbuck in savanna also had more ectoparasites than those in the floodplain. Thus, plasticity in foraging behavior and diet selection enabled savanna waterbuck to tolerate the costs of density-dependent spillover, at least in the short term; however, the already poorer energetic performance of these individuals implies that savanna occupancy may become prohibitively costly as heterospecific competitors and predators continue to recover in Gorongosa. Our results suggest that behavior can provide a leading indicator of the onset of density-dependent limitation and the likelihood of subsequent population decline, but that reliable inference hinges on understanding the mechanistic basis of observed behavioral shifts.

Carbon, nitrogen, and oxygen stable isotopes in modern tooth enamel: A case study from Gorongosa National Park, central Mozambique

“…Today, numerous projects are underway in GNP. These studies use a combination of methods including motion-triggered cameras (Gaynor et al, 2018(Gaynor et al, , 2021Easter et al, 2019), biologgers (often including Global Positioning System units, triaxial accelerometers, and sometimes video recorders; e.g., Branco et al, 2019a,b;Becker et al, 2021;Bouley et al, 2021), aerial counts (Cumming et al, 1994;Dutton and Carvalho, 2002;Dunham, 2004;Stalmans, 2012;Stalmans et al, 2019), animal follows (Hammond et al, 2022), molecular studies (Martinez et al, 2019;Santander et al, 2022), and other field observation tools (e.g., Muschinski et al, 2019) to gain insights in the park's ecology. The δ 13 C enamel , δ 15 N enamel , and δ 18 O enamel datasets of Gorongosa's fauna presented here-the first stable isotopes result for any terrestrial fauna in Mozambique-will help us to better understand dietary patterns of the large-bodied animals which roam GNP today.…”

Diet composition and diurnal activity pattern of defassa waterbuck (Kobus ellipsiprymnus defassa, Ruppell, 1835) in Maze National Park, Southern Ethiopia

“…This may be attributed to a shortage of grass availability and nutritional quality (Copeland et al, 2008) in the dry season and could also be due to low nutritional value of the plant species. Becker et al (2021) reported that waterbuck routinely forage on forbs and subshrubs throughout their range. Kassa et al (2007) found waterbuck diet to be less diverse during the dry season and much more varied during the rainy season.…”

“…This is coherent with the findings of Okello et al (2015) in the Kenya‐Tanzania border where the authors reported that in the dry season about 32% of the 24‐h day was spent in browsing (eating leaves, small shoots, and fruits), whereas no time was spent on it during the wet season. Gutbrodt (2006) and Kassa et al (2007) reported that although the diet of waterbuck consists mainly of grasses, browse and forbs are also consumed while Becker et al (2021) found that waterbuck routinely forage on forbs and subshrubs.…”

“…As hunting and human disturbance are among the major threats, large populations of the species are being forced to use suboptimal habitats (Okello et al, 2015). Becker et al (2021) investigated waterbuck diet and activity budgets in Gorongosa National Park, Mozambique. Their study showed that waterbucks are being forced to use suboptimal habitats for a very different reason i.e.…”

Socio-economic factors strongly determine megafauna biomass in African savanna protected areas

“…These processes can regulate megafauna populations and influence their distribution within the broader landscape 22,23 . Megafauna, in turn, can have a significant impact on vegetation structure, composition 24 , and ecosystem processes and functioning, such as foraging, trampling, and nutrient/seed dispersal 25,26 . Megafauna conservation using (large) PAs is partly based on the assumption that all these natural processes will continue to drive megafauna population dynamics by preventing unsustainable human influences within PA borders 27 .…”