Abstract
Insect communities vary seasonally with changing climatic conditions and related changes in resource availability, strength of competition, or pressure by natural antagonists. But seasonal dynamics, particularly in tropical mountain ecosystems, are not well understood. We monitored cavity-nesting Hymenoptera communities on Mt. Kilimanjaro, Tanzania, to analyse temporal patterns of nest-building activity, ecological rates, and life-history traits in relation to seasonal climatic variation and elevation. We installed trap nests on 25 study sites in natural and disturbed habitat types covering the colline (<1,300 m) and submontane zones (≥1,300 m a.s.l). We analysed patterns of seasonality in the cavity-nesting ecology of Hymenoptera at three different trophic levels –bees, caterpillar-hunting wasps and spider-hunting wasps– over a complete annual period, covering two rainy and two dry seasons. Nest-building activity showed strong seasonal trends in all three investigated trophic levels and peaked at the end of the short rainy season at low elevations. Nest-building activity was considerably higher and seasonal trends were better synchronised between the different trophic levels in the colline zone at low elevations. We also detected seasonal patterns for parasitism and natural mortality rates, sex ratio, and development time, which varied with trophic level and between elevation levels. Climate and flower abundance were important predictors for seasonal patterns in nest-building activity, ecological rates and life-history traits. These results reveal that seasonal trends in nest-building activity of lowland Hymenoptera seem to be linked to changes in climate and resource availability that reflect the seasonal patterns in plant growth and flowering documented in lowland savanna ecosystems. Higher resource availability also increased the sex ratio in bees towards the more costly females and enhanced their survival rates. These spatiotemporal links between climate, resources, ecological rates, and life-history traits indicate high sensitivity of plant-host-antagonist interactions to environmental changes.
Highlights
Insect communities vary spatially and often temporally in their abundance, diversity, and community composition
We dissected 6,852 nests of cavity-nesting Hymenoptera, of which 5,973 could be identified in detail for life-history traits and ecological rates and 87% of them could be assigned to bees, caterpillarhunting wasps, and spider-hunting wasps
We found seasonal patterns in nest-building activity, ecological rates, and life-history traits, presented in detail below, which differed between trophic levels and between the colline and submontane zone (Table 1)
Summary
Insect communities vary spatially and often temporally in their abundance, diversity, and community composition. For the purpose of survival, they have to synchronize their activity phase with suitable environmental conditions (Tauber and Tauber 1976). Abiotic factors such as temperature, humidity, and rainfall are suggested to drive these temporal variations (Frith and Frith 1990, Novotny and Basset 1998, Wagner 2001, Chen et al 2009, KishimotoYamada and Itioka 2013). Climatic factors can only account for a fraction of seasonality in tropical communities. Climatic conditions in the tropics are largely favourable for insects (Denlinger 1986) and in the lowland tropics neither temperature nor precipitation seem to be limiting factors. The degree to which tropical insects show seasonality in activity, reproduction, and traits is little understood (Maicher et al 2019)
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