Movement response of small mammals to burn severity reveals importance of microhabitat features

Abstract

Disturbance events are increasing at a global scale, with cascading impacts to ecosystems and residents therein that include fragmentation and altered vegetation structure and composition. Such changes may disproportionately impact small mammal movements, risk perception, and community dynamics as smaller species perceive such changes at finer spatial scales. We examined movement response to burn severity, vegetation structure, and composition in Mexican woodrats (Neotoma mexicana), a common but understudied small mammal species. The study was conducted on Mt. Graham in southeastern Arizona, United States, following a fire that burned over 19,400 ha. We measured path tortuosity of woodrats translocated over patches of different burn severity. Tortuosity can indicate microhabitat selection, foraging behavior, and perceived predation risk-features affecting population-level processes that changes in community composition alone cannot fully demonstrate. We captured woodrats, released them 50 m away from their midden, and used fluorescent powder to track woodrat movement paths through areas of low-severe burn severity. We analyzed features of the resulting powder trails including straightness, average step length, fractal dimension, and squared displacement. We also compared used versus expected vegetation structure and composition along movement paths across burn severities. Analyses indicated shorter step length with increased bare ground, as well as higher squared displacement in areas with more logs. Vegetation analyses likewise showed that logs were heavily used in low-burned areas, whereas dense vegetation was avoided in highly burned areas. Burn severity alone did not have a direct effect on movement parameters, rather its influence on vegetative composition and structure appears to be most important. Selection for logs and avoidance of dense vegetation may be attributed to auditory concealment and ease of travel. With projected increases in wildfire extent and severity, this work represents an understudied approach to understanding these disturbances and their effects on ecological communities.

Keywords: Neotoma mexicana; animal movement; burn severity; disturbance ecology; fluorescent powder tracking; path tortuosity; postfire response; small mammal.

Fig. 1.

Study area overview in the Pinaleño Mountains of southeastern Arizona, Graham County, Coronado National Forest, United States. Left panel shows study area in detail, where individual Mexican woodrats (Neotoma mexicana) were released (crosses) and their subsequent movement paths (filled circles; identified via fluorescent powder tracking) within a matrix of burn severity (semitransparent colored overlay).

Fig. 2.

A visual representation of movement trajectories of Mexican woodrats (Neotoma mexicana) following translocation across a mosaic of burn severity and ground cover in the Pinaleño Mountains of southeastern Arizona, United States.

Fig. 3.

Predicted relationships from univariate models between (A) straightness and bare ground, (B) step length and bare ground, (C) straightness and logs, and (D) step length and logs for movements of Mexican woodrats (Neotoma mexicana) in a mosaic of burned mixed conifer forest in the Pinaleño Mountains of southeastern Arizona, United States. A straightness value closer to 1 indicates a straight line.