Strong positive effects of termites on savanna bird abundance and diversity are amplified by large herbivore exclusion.

Vast areas of the African savanna landscapes are characterized by tree-covered Macrotermes termite mounds embedded within a relatively open savanna matrix. In concert with termites, large herbivores are important determinants of savanna woody vegetation cover. The relative cover of woody species has considerable effects on savanna function. Despite the potentially important ecological relationships between termite mounds, woody plants, large herbivores, and birds, these associations have previously received surprisingly little attention. We experimentally studied the effects of termites and large herbivores on the avian community in Lake Mburo National Park, Uganda, where woody vegetation is essentially limited to termite mounds. Our experiment comprised of four treatments in nine replicates; unfenced termite mounds, fenced mounds (excluding large mammals), unfenced adjacent savanna, and fenced savanna. We recorded species identity, abundance, and behavior of all birds observed on these plots over a two-month period, from late dry until wet season. Birds used termite mounds almost exclusively, with only 3.5% of observations occurring in the treeless intermound savanna matrix. Mean abundance and species richness of birds doubled on fenced (large herbivores excluded) compared to unfenced mounds. Feeding behavior increased when large mammals were excluded from mounds, both in absolute number of observed individuals, and relative to other behaviors. This study documents the fundamental positive impact of Macrotermes termites on bird abundance and diversity in an African savanna. Birds play crucial functional roles in savanna ecosystems, for example, by dispersing fruits or regulating herbivorous insect populations. Thus, the role of birds in savanna dynamics depends on the distribution and abundance of termite mounds.

Research note: Avian gut passage reduces seed exit costs in Sorbus aucuparia (Rosaceae) as measured by a diametral compression test.

Reduced seed exit costs have been suggested to explain advanced seedling emergence and increased seedling growth in Sorbus aucuparia L. (Rosaceae; rowan) following Turdus spp. (Turdidae; thrush) gut passage. In the present study, seed coat tension strength of Turdus merula L. (Turdidae; blackbird) gut-passed and non-ingested control seeds were tested with a diametral compression test. Both maximum load (N) and work (mJ) required for opening the seeds were reduced in gut-passed seeds, although scanning electron microscopy showed no visible differences in seed coat surface structure between treatments. In addition, gut passage increased dry seed weight by 0.64 mg (22%), weight of hydrated seeds by 0.62 mg (16%) and width of hydrated seeds by 0.1 mm (9%). Absorption causing the seed coat to yield more easily to the germinating seedling is proposed as an explanation for the reduction in seed exit costs. For the emerging seedling, the time and force needed to open the seed coat was reduced, thus lowering dispersal and germination costs of S. aucuparia.

Corrigendum to: Avian gut passage reduces seed exit costs in Sorbus aucuparia (Rosaceae) as measured by a diametral compression test.

Reduced seed exit costs have been suggested to explain advanced seedling emergence and increased seedling growth in Sorbus aucuparia L. (Rosaceae; rowan) following Turdus spp. (Turdidae; thrush) gut passage. In the present study, seed coat tension strength of Turdus merula L. (Turdidae; blackbird) gut-passed and non-ingested control seeds were tested with a diametral compression test. Both maximum load (N) and work (mJ) required for opening the seeds were reduced in gut-passed seeds, although scanning electron microscopy showed no visible differences in seed coat surface structure between treatments. In addition, gut passage increased dry seed weight by 0.64mg (22%), weight of hydrated seeds by 0.62mg (16%) and width of hydrated seeds by 0.1mm (9%). Absorption causing the seed coat to yield more easily to the germinating seedling is proposed as an explanation for the reduction in seed exit costs. For the emerging seedling, the time and force needed to open the seed coat was reduced, thus lowering dispersal and germination costs of S. aucuparia.