Influence of patch size and resource quantity on litter invertebrate assemblages in dry treeholes

Treeholes are dry or water-filled cavities that occur in the aboveground parts of forest ecosystems and support terrestrial and aquatic invertebrates. Compared with aquatic invertebrate assemblages in water-filled treeholes, little is known about how strongly patch size or resource quantity affects the terrestrial invertebrate assemblages in dry treeholes. Therefore, we examined the factors that affect terrestrial invertebrate assemblages in dry treeholes using a field experiment with artificial treeholes and by investigating natural treeholes, both in broad-leaved forests in central Japan. In the artificial treeholes, final litter density (final litter weight per unit treehole volume), which indicates the extent of resource concentration, was positively correlated with total invertebrate abundance and negatively correlated with the proportion of litter remaining at the end of the experiment. These results suggest that greater litter density results in an increase in both invertebrate abundance and litter decomposition. In natural treeholes, however, the total abundance of terrestrial invertebrates was not significantly correlated with litter density, litter weight, or treehole volume. The quantity of litter, which consisted mainly of fine particulate organic matter, increased with treehole opening size (area) and volume, suggesting that already decomposed litter was filled, depending on the treehole volume. This may have been attributed to the nonsignificant results concerning the abundance of terrestrial invertebrates in natural conditions. Thus, we suggest that both litter weight and density are important factors in determining the abundance of invertebrate in dry treeholes, even though their influence is weakened in natural treeholes by the effects of other factors.