TWEL Dan Shustack Dissertation

Impacts of urbanization on forest bird communities
 
Dan P. Shustack, PhD
Advisor: Amanda Rodewald
 
 
We live on an urbanizing planet where development profoundly impacts ecological communities, including those within protected natural areas. Within this context, effective conservation of biodiversity ultimately requires that ecologists understand both the patterns and mechanisms of urban-associated influences on native plant and animal communities. While a growing body of literature probes the effects of development on community structure and composition, more subtle behavioral and demographic consequences of urbanization remain generally overlooked. One key example of a neglected consequence of urbanization is seasonal timing of life cycle events, particularly the tendency of many urban birds to breed earlier in the spring than their rural-breeding counterparts. The overall objectives of this dissertation were to (1) describe the phenological and biological differences in avian reproduction in urban and rural forests, (2) identify the underlying ecological mechanisms responsible for observed patterns, and (3) identify the demographic consequences of phenological shifts in reproduction. To do this, I studied common plants and two focal bird species, Northern Cardinals (Cardinalis cardinalis) and Acadian Flycatchers (Empidonax virescens), in riparian forests embedded within landscapes along a rural-to-urban gradient in central Ohio, USA between 2001-2007.
 
As a first step to understanding avian reproductive phenology, I examined vegetation phenology and, specifically, evaluated two hypotheses that could account for advanced green-up in forests along an urban to rural gradient: advanced phenology within individual species or differences in woody plant community. I was particularly interested in spring leafing phenology of Aesculus glabra Willd. (Ohio buckeye), Lonicera maackii (Rupr.) Herder (Amur honeysuckle), and Acer negundo L. (box elder) in eleven forests spanning an urban to rural gradient in central Ohio, USA. These three species are well represented in all study sites, are relatively early leafing species, and are used as nesting substrates by the focal bird species. From February-April 2006, I monitored these species, recorded woody plant composition, and documented daily minimum and maximum temperatures at each site. There was an overall pattern of advanced phenology (e.g., bud break, leaf elongation) within species in more urban landscapes which might be associated with temperatures. More importantly, there was evidence for shifts in woody plant communities along the urbanization gradient, mainly driven by the abundance of L. maackii, an invasive exotic species, in the more urban forests. Both because leaves of L. maackii emerge weeks earlier than native woody species and the shrub is very abundant in urban forests, the earlier green-up of urban forests may be partly a consequence of invasion by this species.
 
Despite several published accounts describing patterns of advanced reproductive phenology in urban versus rural populations of birds, the causes and consequences of altered reproductive phenology associated with urbanization are not well understood. Studying Northern Cardinals in urban and rural forests in central Ohio, USA between 2004-2007, I found that the earliest dates of both nesting activity and clutch initiation were about seven to ten days earlier in urban versus rural sites. Temperature accumulations in March best predicted the timing of breeding, whereas vegetation phenology, site-level survival, and the availability of nest sites failed to explain variation in breeding phenology of cardinals. There was no apparent benefit in terms of number of fledglings produced over the breeding season based on initiation of the first nesting attempt. On a per nest basis, the expected number of fledglings produced per successful nest remained constant over the course of the season at ~1.7 fledglings per successful.