Ostrich population densities and temporal dynamics in coupled social-ecological systems: Suitable indicators for the ecological effectiveness of protected areas?

Assessing the ecological effectiveness of protected areas is a high priority in conservation biology and requires suitable indicators. Instead of using multiple species, single species can potentially be used as indicators to assess if conservation efforts are effective in maintaining species over time. Here, we assessed the suitability of ostrich (Struthio camelus massaicus) population densities and trends over time as indicators for the ecological effectiveness of four study areas (two national parks, a pastoral area, and a game-controlled area with manifold human impacts) in the Tarangire-Manyara ecosystem of northern Tanzania. Based on road transect surveys from 2011 to 2019, we estimated area- and season-specific population densities in a distance sampling framework and estimated temporal changes using generalized linear mixed models. Ostrich population densities and population trends over time were not associated with formal protection status. Ostriches in one national park occurred at highest densities and were constant over time, while densities were low in another national park and apparently declined over time. Ostriches in the pastoral area had the second-highest mean population density, while remaining constant over time. The study area with the highest human impact had lowest ostrich densities and a seemingly declining trend over time. Ostrich densities were positively correlated with time-matched population density estimates of nine out of ten ungulate species in the same study areas, suggesting that ostrich population densities broadly reflect those of large savanna mammals in this ecosystem. However, site-specific ostrich population trends over time were not closely correlated with trends of common large mammal species. Therefore, ostrich population densities appear as suitable management indicators to assess the broad ecological effectiveness of protected areas. However, ostrich population trajectories do not appear suitable as indicator surrogate to monitor trends of mammal populations over time.