Feeding-season production in the desert millipede Orthoporus ornatus (Girard) (Diplopoda)

Abstract

Production during an assumed 131-day feeding season in 1974 was estimated for Orthoporus ornatus between 4.0 and 12.0 mm in midsegment width at Tornillo Flat, Big Bend National Park, Texas. A conservative density estimate in 1973 of 1,302 millipedes ha^-1 involved daily specimen removal from three, 929-m² plots for a month. Each plot typified a different aspect of local desert vegetation; most specimens came from the plot with greatest plant diversity and relatively high (20%) cover. Production calculations using 1973 density estimates were based on increase in size-class specific dry weight (minus gut contents) between 14 May and 21 September, 1974. Production ha^-1 of cuticle and tissue was estimated at 0.85 kg (1972 kcal), while that of tissue alone came to 0.29 kg (1971 kcal). Orthoporus ornatus from Albuquerque, New Mexico increased in dry weight during 92 days in 1974 more rapidly and to a greater extent than comparable size classes at Tornillo Flat. An estimated feeding-season energy budget based on ash-free values of shrub food eaten at Tornillo Flat indicated ingestion ha^-1 of 3,434 g (13,712 kcal) and defecation of 3,181 g (9,187 kcal). An independent estimate of ingestion based on known ingestion rates was 8,851 g ha^-1. Considering probable net primary production at Tornillo Flat, local O. ornatus exert a trophic impact similar to that of other large invertebrate detritivores elsewhere.