Exploring co-occurrence of closely-related guild members in a fragmented landscape subject to rapid transformation

Quantifying biotic responses to landscape transformation is a major research focus. Most past studies have explored co-occurrence of entire communities of a given group (e.g. birds) within largely intact ecosystems or over a limited time-frame. By contrast, here we use data from a 15 yr experimental study, to explore intra-guild co-occurrence of six closely-related and functionally-similar sets of birds within 55 woodland fragments. Areas surrounding these remnants are undergoing transformation from grazed paddocks to Pinus radiata plantations, leading to a novel assemblage of forest and woodland birds. We sought to determine if the occurrence of a given species in a guild influenced the occurrence of other closely-related species in that guild, and through this relationship whether there was evidence of co-occurrence between species. After controlling for environmental and habitat variables which can affect species occurrence like time since commencement of landscape transformation, patch size and vegetation type, we found the occurrence of a given species was influenced by the occurrence of a closely-related species in the same guild. Co-occurrence varied among bird guilds and included: 1) positive co-occurrence in which occurrence of one species within fragments positively affected the occurrence of another closely-related guild member (e.g. eastern and crimson rosellas); and 2) negative co-occurrence in which the occurrence of one species was negatively associated with the occurrence of another within the same guild (e.g. willie wagtail and grey fantail). We also identified interactions between patch size and species recording frequency within members of two guilds. For example, modelling of conditional recording frequency revealed the eastern rosella increased with increasing recordings of the crimson rosella in large patches, but decreased with increasing recordings of the crimson rosella in small patches. Our results provide empirical evidence of co-occurrence among guild members and underscore the complexity of biotic responses to landscape transformation.