How small is too small for small animals? Four terrestrial arthropod species in different-sized remnant woodlands in agricultural Western Australia

Island biogeography theory, and the 50/500 rule of genetics, have effective ly devalued small habitat fragments for species conservation. Metapopulatio n theory has given new value to small remnants but data on species persiste nce are scarce. This study examined the capacity of very small and sheep-gr azed remnants of eucalypt woodland in agricultural Western Australia to sup port remnant-dependent terrestrial arthropods. We surveyed 53 sheep-grazed remnants of wheatbelt wandoo Eucalyptus capillosa for the presence of four species of arthropod with different dispersal strategies (terrestrial versu s aerial) and diet (predaceous vs. herbivorous): the harvester and mound-bu ilding termite Drepanotermes tamminensis, the wood-eating and mound-buildin g termite Amitermes obeuntis, the predaceous and burrowing scorpion Urodacu s armatus and the predaceous 'bull' ant Myrmecia nigriceps. All species wit h the exception of the scorpion disperse aerially, and all construct above- ground structures that are easily recognized. Remnants ranged in size from 50 m(2) to 21 000 m(2) (mean 1791 m(2)), in spatial isolation (distance to the nearest vegetation remnant) from 10 m to 500 m (mean 123 m) and in a le ngth-to-width ratio (shape) from circular (mean ratio 1.0) to linear (mean ratio 4.0). Observations in small and grazed remnants were compared with ob servations made in six wandoo woodland sites within a large (1040 ha) and u ngrazed remnant. The total number of target species was highly correlated w ith remnant area (r = 0.68). Remnant isolation and remnant shape had no app arent influence on the total number of target species. The minimum area of grazed remnants in which individual species were recorded followed the larg e predator Urodacus armatus (4515 m(2)) > smaller predator Myrmecia nigrice ps (300 m(2)) > harvester termites Drepanotermes tamminensis (102 m(2)) > w ood-eating termites Amitermes obeuntis (50 m(2)). With the exception of U. armatus which occurred only in three of the four largest grazed remnants, t he occurrence of all other species increased from small to large grazed rem nants, suggesting a remnant-size effect for all species. Remnant isolation or remnant shape had no apparent influence on the occurrence of any one spe cies. The terrestrially dispersing scorpion persisted in remnants despite t heir isolation from other remnants from 200 m to 500 m. For both termite sp ecies, mound heights were significantly greater in large, ungrazed woodland s than in small and grazed woodlands. The incidence of mound abandonment in smaller and grazed remnants was considerably higher for harvester than for wood-eating termite colonies. This suggests differences in spatial require ments and possibly diet-related susceptibilities to fluctuations in food av ailability. The diameter of Myrmecia nigriceps nests showed no relationship with remnant size or isolation. This study demonstrated that even very sma ll remnant woodlands on farms may play an important role in sustaining smal l native animals, either as stepping-stones for dispersing individuals (ter mites, ants) or in providing adequate habitat to sustain populations for lo nger periods (all four species).