The interaction between bird predation and plant cover in determining habitat occupancy of darkling beetles

Tenebrionid beetles in the Negev Desert exhibit size-related habitat segreg ation, with larger species found in denser cover. Size-dependent predation by birds has been suggested as the mechanism behind this habitat segregatio n. Two predictions of this hypothesis were tested: (1) plant cover reduces the predation efficiency of birds upon large tenebrionids. and (2) birds pr efer larger species. Both predictions were supported: plant cover reduced p redation rate by the most common spring and summer predatory birds: white s torks (Ciconia ciconia) and stone curlews (Burhinus oedicnemus), in cage ex periments. Results from preference experiments suggest that tenebrionid spe cies can be divided according to their profitability as prey. Large species are the most profitable, medium-sized species are less profitable but stil l acceptable and small species are unprofitable and therefore ignored. Field observations demonstrated that the well-vegetated wadi habitats are d ominated by large and small species whereas acceptable, medium-sized specie s are under-represented in this habitat. The results of the cage experiment s indicate possible apparent competition between the large profitable and t he medium acceptable tenebrionid species in the wadis. Aggregative response of predators in the profitable habitat is suggested as the mechanism leadi ng to truncated distribution of prey species. Large profitable species are refuge-dependent. medium-sized acceptable species use enemy free space and small species are predator independent.