Time Lags and Niche Shifts in a Biological Invasion of Hummingbirds

• Published in: bioRxiv
• Main Author: Battey, C J
• Format: Paper
• Language: English
• Published: Cold Spring Harbor Cold Spring Harbor Laboratory Press 30.05.2018
• Subjects: Acclimation; Breeding seasons; Calypte anna; Carrying capacity; Climate change; Colonization; Indigenous species; Introduced species; Invasive species; New records; Niches; Population growth; Range extension; Pacific Northwest
• DOI: 10.1101/329615

Shifts in a species' realized niche can lead to rapid population growth by increasing the carrying capacity of local habitats and allowing colonization of new areas. This process is well known in "invasive" species introduced to novel ranges by humans, but can also occur when native species expand their range. In these cases expansions may be driven either by a shift in the available environment or by a shift in the species' use of existing niche space, but identifying the specific environmental or behavioral changes involved is often hindered by time lags in the process of colonization. Here I document a century of range shifts in the Anna's Hummingbird (Calypte anna) and show that recent abundance in the Pacific Northwest is the product of a series of range and niche expansions that started in the early 20th century following the spread of garden cultivation and introduced plant species in California. Demographic trends in the northwest have tracked simple models of exponential growth since populations became established in the 1960's and 70's, and nest records suggest that the species has delayed the beginning of the breeding season by at least 18 days in the north. Niche models trained on historic climate and occurrence data fail to predict the modern range, suggesting that climate change is not the primary cause of the expansion. Range expansions in the Anna's Hummingbird thus closely track the dynamics of an invasive species spreading across a novel range, and were made possible by a mix of introduced plants, phenological acclimation, and an expansion of the realized climatic niche. Footnotes * Revision 1 (29 May 2018). Summary of changes: - Figure 1: Dropped outlier localities (< 2 neighbors within 100km), increased concavity, switched to 1940-2010 range with 10-year bins. Switched picture, moved to top right, and added state borders. - Figure 2: Added interior southwest sites to CBC model plots. - Expanded introduction section on nesting phenology. - Moved description of niche similarity test to methods, removed review paragraph on niche model comparisons. - Revised discussion paragraph on Greig et al 2017 to discuss evidence for different mechanisms, including historical evidence for introduced trees supporting higher populations. Added summary paragraph reviewing likely cause of early expansions.