OIKOS 93: 213 – 220. Copenhagen 2001 Changes in transmission of Baylisascaris procyonis to intermediate hosts as a function of spatial scale L. Kristen Page, Robert K. Swihart and Kevin R. Kazacos Page, L. K., Swihart, R. K. and Kazacos, K. R. 2001. Changes in transmission of Baylisascaris procyonis to intermediate hosts as a function of spatial scale. – Oikos 93: 213–220. Physical changes in landscapes alter the abundance and distribution of species. Higher-order effects can occur when changes in ecological processes result in altered interspecific interactions and subsequent changes in a species’ abundance or persis- tence. Baylisascaris procyonis, a roundworm parasite of raccoons (Procyon lotor ), is pathogenic to numerous small vertebrates that serve as intermediate hosts, including white-footed mice (Peromyscus leucopus ). Raccoons have responded positively to agriculturally induced changes in landscapes, with potential consequences for inter- mediate hosts of B. procyonis. We examined white-footed mice from a homogeneous, predominantly forested landscape in south-central Indiana and a heterogeneous, predominantly agricultural landscape in northwestern Indiana for presence of larval B. procyonis. We compared prevalence of infection, intensity of infection, and average number of larvae per mouse between the landscapes, and among forest patches within the highly fragmented agricultural landscape. Prevalence, intensity of infection, and average number of larvae were significantly higher in the highly fragmented landscape. Within the agricultural landscape, regression models predicted probability of infection, intensity of infection, and average number of larvae per mouse per patch as functions of forest patch area and isolation. We conclude that positive responses of raccoons to agriculturally induced fragmentation of forests have resulted in increased encounter rates between white-footed mice and infective stages of B. procyonis between and within landscapes, resulting in increased transmission of the parasite to intermediate hosts. L. K. Page and R. K. Swihart, Dept of Forestry and Natural Resources, Purdue Uni., West Lafayette, IN 47907, USA (present address of LKP: Dept of Biology, Wheaton College, Wheaton, IL 60187, USA [kristen.page@wheaton.edu]).– K. R. Kazacos, Dept of Veterinary Pathobiology, Purdue Uni., West Lafayette, IN 47907, USA. Agriculture and urbanization have been primary agents of landscape change, resulting in extensive fragmenta- tion of native habitat (Iverson 1988, Anderson et al. 1996). As a process, habitat fragmentation has two components, habitat loss and insularization (Wilcox and Murphy 1985, Noss and Csuti 1994). If physical changes in landscapes alter species’ abundance, distri- bution, and the resulting spatial structure of popula- tions (Saunders et al. 1991, Noss and Csuti 1994), ecological processes within the landscape may also be altered (Gilpin 1987, Hanski and Gilpin 1991). In addi- tion, higher-order effects can occur when changes in ecological processes result in altered interspecific inter- actions and subsequent changes in a species’ abundance or persistence (e.g., Taylor and Merriam 1996). These effects may be direct, with one species affecting another through predation, herbivory, or parasitism, or indirect as a result of the presence of a third species (Strauss 1991, Wootton 1994). Raccoons (Procyon lotor ) have responded positively to changing land-use practices. They are abundant in landscapes characterized by a diversity of habitat types (Oehler and Litvaitis 1996), especially in areas that have been moderately to highly fragmented by agriculture Accepted 21 December 2000 Copyright © OIKOS 2001 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 93:2 (2001) 213