Temporal changes in food resource availability between two karst bat assemblages

Abstract

The temporal availability of arthropods varies in response to changing quality of vegetation which is influenced by climatic conditions. Climate in the Savanna and Grassland biomes is defined by wet and dry cycles. Bats (Chiroptera) are major consumers of nocturnal flying arthropods and respond to temporal fluctuations of their arthropod prey as well as climatic conditions. My study is undertaken in two karst landscapes within two biomes, Savanna (Meletse) and Grassland (Cradle of Humankind World Heritage Site-CoH WHS). The migratory Miniopterus natalensis utilizes Madimatle cave (Meletse –Thabazimbi) for maternity roosting (October-February/March) whilst caves in the CoH WHS provide hibernacula (winter) roosts (April-September) for this population. Migration in bats has been linked to changing climatic conditions for temperate species and food availability in tropical species. The factors driving the migration of Min. natalensis population are not yet understood. The temporal changes of bat assemblages and their arthropod prey from dry to wet season climatic regimes are of interest. To test this, I collected arthropods and recorded bat echolocation calls from dry to wet season (June-February), using malaise traps and Anabat SD2 bat detectors, respectively. Different bat species are adapted to forage within different levels of vegetation clutter. Traps and bat detectors were therefore spatially placed across different levels of vegetation clutter at the study sites. Arthropods were caught, measured (size and biomass) and capture success (CS=individuals/trap-hours) were calculated monthly. An Acoustic Activity Index (AAI) of recorded bats was also used to obtain monthly relative activity. My results show that bat activity corresponds to arthropod availability on a temporal basis with wet season highs and dry season lows, at both study sites. When comparing Grassland and Savanna sites, arthropod CS was significantly higher (P<0.01) at the grassland (mean CS=0.248 ind./trap) whilst biomass was greater at the savanna site. In addition, the Grassland had higher CS with lower biomass for the arthropods Diptera and Lepidoptera, whilst the larger biomass taxa, Coleoptera and Hemiptera, had higher CS at the savanna. The frequency of larger arthropods was higher in November and corresponded with peak biomass measured that month. At Meletse, significant increases (P<0.05) for bat activity and arthropod CS were recorded in August/September (late dry season) and November (wet season). Species level bat activity increases corresponded with increases in arthropod CS and biomass. The matching between bat activity patterns and arthropod availability is important for the maternity season when pregnant females have to meet increased energy demands. Indeed, the maternity season matched with periods of highest food resources expressed as increased arthropod abundance and biomass. Greater bat activity during the maternity period places Meletse as an important food resource reservoir both supporting resident and the migratory species. As bats are major predators of several taxa of nocturnal flying arthropods amongst which are agricultural pests, the ecological and economic implications of maintaining healthy bat assemblages remain of importance. Careful and effective monitoring of bat assemblages and arthropod prey will further increase understanding of the temporal dynamics between predator activity and prey abundance within habitats. Knowledge on the effects of land-use change on habitats will develop a better understanding of impacts on species and broader ecological systems.