Ecology

The majority of humanity now lives in cities or towns, with this proportion expected to continue increasing for the foreseeable future. As novel ecosystems, urban areas offer an ideal opportunity to examine multi‐scalar processes involved in community assembly as well as the role of human activities in modulating environmental drivers of biodiversity. Although ecologists have made great strides in recent decades at documenting ecological relationships in urban areas, much remains unknown, and we still need to identify the major ecological factors, aside from habitat loss, behind the persistence or extinction of species and guilds of species in cities. Given this paucity of knowledge, there is an immediate need to facilitate collaborative, interdisciplinary research on the patterns and drivers of biodiversity in cities at multiple spatial scales. In this review, we introduce a new conceptual framework for understanding the filtering processes that mold diversity of urban floras and faunas. We hypothesize that the following hierarchical series of filters influence species distributions in cities: (1) regional climatic and biogeographical factors; (2) human facilitation; (3) urban form and development history; (4) socioeconomic and cultural factors; and (5) species interactions. In addition to these filters, life history and functional traits of species are important in determining community assembly and act at multiple spatial scales. Using these filters as a conceptual framework can help frame future research needed to elucidate processes of community assembly in urban areas. Understanding how humans influence community structure and processes will aid in the management, design, and planning of our cities to best support biodiversity.

Decomposition rates of leaf litter have been predicted from the leaves' lignin or nutrient (N or P) contents, the C:N ratio, and more recently the lignin:N ratio. But tests of these predictors have been based on groups of substrates each spanning only part of the natural range of lignin contents, and neglecting low—lignin (<10%) species. We allowed leaf litter from eight species of tree, shrub, or herb, ranging in lignin content from 3.4 to 20.5%, to decompose in laboratory microcosms for up to 4 mo (equivalent to 1.5—2 yr decay in the field) to test two hypotheses: (1) that the lignin: nitrogen ratio would have a better correlation with mass loss rates than would the C:N ratio, nutrient content, or other substrate quality indexes, and (2) that correlations of mass loss with initial N content would decrease, while correlations with lignin content would increase, as decay proceeded. Contrary to the first hypothesis, nitrogen content and the C:N ratio were the best predictors of mass loss rate, and were substantially better than the lignin:N ratio. We could find no better predictor of decomposition rate than the C:N ratio, and no better regression model than the simple linear one. However, when regressions were tested using pine needles (lignin content 26.2%), the C:N ratio and N content badly underestimated mass remaining (by 10—16%), while lignin content and the lignin:N ratio overestimated it by <2%. In accordance with the second hypothesis, regressions of initial lignin content or lignin:N ratio on mass remaining improved (higher R²) from 2 to 4 mo decomposition, while those of N content grew worse, illustrating succession of nitrogen to lignin control of decomposition rate. Reported correlations of the lignin: N ratio with decomposition rate for some litter types arise as a special case of this two—phase mechanism of control by nutrients and lignin. For substrates low in lignin, or where a broad range of lignin contents is being considered, the C:N ratio is a better predictor of decomposition rate than the lignin:N ratio.

Element dynamics (C, N, P, Ca, Mg, Mn, Fe, and Al) were examined along a decay continuum from freshly fallen litter to soil organic matter in a red spruce ecosystem in Maine. The continuum was defined using previously reported data on litterfall chemistry and the early stages (i.e., first 24 mo) of decay, combined with new studies on an additional 33 mo of decay and an evaluation of forest floor organic matter at the same site. Carbon concentrations decreased over time as fresh litter was transformed into soil organic matter; N, Fe, and Al concentrations increased; and P, Ca, Mg, and Mn concentrations showed variable patterns of increased and decrease. In general, homogeneity of the litter increased as fresh litter of different initial chemistries was transformed into chemically more uniform soil organic matter. Ecosystem budgets indicated that after 57 mo of decay, litter had lost 63% of its original mass and had released C (65%),N (17%), P (79%), Ca (50%), Mg (80%), and Mn (74%), and had accumulated Al (431%) and Fe (353%). Evidence is presented indicating that the retention of Al (and Fe) is primarily controlled by abiotic adsorption onto litter exchanges sites. Overall results are consistent with a two—phase model of decay: in the first phase (0 to 3—4 yr), mass loss fits a negative exponential model of decay, an inverse linear relationship exists between N concentration and mass loss, and rapid changes are observed in litter chemistry; the second phase (3—4+ yr) is characterized by markedly slower changes in mass loss and litter chemistry.