Investigation of Pacific Northwest forest litter invertebrate communities and resilience along an urbanization gradient

Background/Question/Methods

Understanding urban ecological processes is of increasing importance as human populations and environmental modifications escalate, while developing the field of urban ecology and its conceptual framework carries social implications for education and conservation efforts. With large areas of highly modified land coverage in urban areas, urban forests play a critical role by harboring remnant hot spots of species abundance and diversity. These urban forests can exist as parks, recreation areas, or trees spread through neighborhoods, and are often highly fragmented as development expands, producing a gradient of types of forests related to urbanization. An essential process in these urban fragmented habitats is species’ abilities to colonize, and recolonize after disturbances that vary in frequency and intensity. As such, the role of resilience in shaping urban ecosystems is an important area of study for management, restoration, and conservation in urban areas. Urban-rural gradients provide a unique opportunity to study how metropolitan development affects ecological patterns, and forest floor litter invertebrates provide an ideal system to investigate effects of urbanization due to their abundance, diversity, accessibility, and role as indicators of ecosystem responses to disturbance. Our study examined the urbanization gradient among four forest parks in the Seattle metropolitan area ranked by percent impervious surface cover. We inspected the community composition of litter invertebrates between these sites, as well as across an array of environmental conditions. Disturbance was simulated by removing, de-faunating, and replacing leaf litter to identical sites of initial collection. When replaced, access to leaf litter was restricted in certain trials via wire mesh bags to manipulate recolonization and discern the effects of urbanization on community resiliency.

Results/Conclusions

More urban sites had lower invertebrate abundance, richness, and evenness, as well as higher soil temperatures and lower soil moisture levels, suggesting less favorable conditions with more development. Responses differed by species, suggesting that some invertebrates may be better able to withstand urban conditions, while others, typically rarer species, may be less tolerant. To test a potential mechanism responsible for these patterns of community assembly, we conducted a disturbance-recolonization experiment in the same forest parks. After disturbance, invertebrate abundance declined at the least urban site but was maintained at the most urban, reinforcing a potential role for urban-tolerant species. However, after disturbance richness was maintained at the least urban site but declined at the most urban site, suggesting fragmentation by urban development my limit the source pool of potential recolonizing species.