Urban development commonly leads to a reduction in native species richness and increased dominance of non-native, urban-adapted species, a process termed “biotic homogenization.” As urban regions expand, it is essential that we improve our understanding of how these changes in community structure driven by urbanization alter critical ecological functions such as plant-animal mutualisms. The Puget Sound region of Washington State has experienced significant urban expansion in recent decades, leaving numerous increasingly rare Garry oak (Quercus garryana) woodland habitat fragments embedded in an urban landscape. Oak acorns rely on a comparatively small number of animal species for dispersal and urban development is known to influence the distribution and abundance of these species. In this study, I ask: do acorn dispersal processes (removal, predation, and dispersal) differ depending on local forest cover and surrounding urban development conditions?
I placed experimental acorn plots in oak, conifer and open grassland habitats within urban and non-urban oak woodlands. I attached fluorescent flags to a subset of acorns to facilitate relocation of dispersed acorns without significantly altering dispersal behavior. I monitored acorn removal for all acorns, and predation and dispersal for flagged acorns. Data were analyzed using mixed-effects regression models (α ≤ 0.05).
I found significantly higher acorn removal from forest compared to open grassland plots. The likelihood of removed acorns being eaten did not differ depending on canopy type. Consequently, forest cover may facilitate dispersal activity. Acorn removal was significantly higher in urban (69%) compared to non-urban (54%) plots. More acorns were dispersed (moved > 0.1 meters) from urban (43%) than from non-urban plots (27%). However, more acorns were eaten in urban (55%) than non-urban plots (31%). In addition, of the acorns that were dispersed, urban acorns were significantly more likely to be eaten (44%) and moved significantly shorter distances (mean 3.6 meters, max 30 meters) than dispersed non-urban acorns (23% eaten and moved mean 9.6 meters, max 98 meters). Lastly, more acorns were buried intact from non-urban plots than from urban plots. This is significant because being buried but not eaten gives the acorn the best chance of germinating. These results suggest that dispersal services in urban oak woodlands are inferior to those provided in non-urban oak woodlands and that landscape type may significantly alter important plant-animal mutualisms. Dispersal activity was higher under forest canopy, suggesting that maintaining forest patches in urban landscapes may facilitate acorn dispersal.