Influence of invasion history on rapid morphological divergence across island populations of an exotic bird

Abstract

There is increasing evidence that exotic populations may rapidly differentiate from those in their native range and that differences also arise among populations within the exotic range. Using morphological and DNA‐based analyses, we document the extent of trait divergence among native North American and exotic Hawaiian populations of northern cardinal (Cardinalis cardinalis). Furthermore, using a combination of historical records and DNA‐based analyses, we evaluate the role of founder effects in producing observed trait differences. We measured and compared key morphological traits across northern cardinal populations in the native and exotic ranges to assess whether trait divergence across the Hawaiian Islands, where this species was introduced between 1929 and 1931, reflected observed variation across native phylogeographic clades in its native North America. We used and added to prior phylogenetic analyses based on a mitochondrial locus to identify the most likely native source clade(s) for the Hawaiian cardinal populations. We then used Approximate Bayesian Computation (ABC) to evaluate the role of founder effects in producing the observed differences in body size and bill morphology across native and exotic populations. We found cardinal populations on the Hawaiian Islands had morphological traits that diverged substantially across islands and overlapped the trait space of all measured native North American clades. The phylogeographic analysis identified the eastern North American clade (C. cardinalis cardinalis) as the most likely and sole native source for all the Hawaiian cardinal populations. The ABC analyses supported written accounts of the cardinal's introduction that indicate the original 300 cardinals shipped to Hawaii were simultaneously and evenly released across Hawaii, Kauai, and Oahu. Populations on each island likely experienced bottlenecks followed by expansion, with cardinals from the island of Hawaii eventually colonizing Maui unaided. Overall, our results suggest that founder effects had limited impact on morphological trait divergence of exotic cardinal populations in the Hawaiian archipelago, which instead reflect postintroduction events.

Rapid morphological change of nonnative frugivores on the Hawaiian island of O'ahu*

“…In these studies, inter‐island divergence was thought to be the largely a result of selection acting on the populations after founding (Mathys and Lockwood ; Valentin et al. ). However, founder effects and other nonadaptive processes were not able to be ruled out for many species (Mathys and Lockwood ).…”

“…Across a different suite of bird species, including several seedeating species, previous work found significant divergence in traits among the Hawaiian Islands with, in many cases, O'ahu bird populations having shorter wings than the archipelago mean (Mathys and Lockwood 2011;Valentin et al 2018). In these studies, inter-island divergence was thought to be the largely a result of selection acting on the populations after founding (Mathys and Lockwood 2011;Valentin et al 2018). However, founder effects and other nonadaptive processes were not able to be ruled out for many species (Mathys and Lockwood 2011).…”

“…; Valentin et al. ) divergence among the main Hawaiian Islands. However, these previous studies focused on only a single species or differentiation among the Hawaiian Islands.…”

“…In fact, recent work has found that bird species introduced to the main Hawaiian Islands showed significant morphological (Mathys and Lockwood ; Valentin et al. ) and genetic (Foster et al. ; Valentin et al.…”

“…The Hawaiian archipelago presents a unique opportunity to examine rapid morphological change because of the extremely high number and prevalence of nonnative species and the extinction of native species (Vitousek et al 1997;Foster 2009). In fact, recent work has found that bird species introduced to the main Hawaiian Islands showed significant morphological (Mathys and Lockwood 2011;Valentin et al 2018) and genetic (Foster et al 2018;Valentin et al 2018) divergence among the main Hawaiian Islands. However, these previous studies focused on only a single species or differentiation among the Hawaiian Islands.…”