Species hybridization can lead to fitness costs, species collapse, and novel evolutionary trajectories in changing environments. Hybridization is predicted to be more common when environmental conditions change rapidly. Here, we test patterns of hybridization in three sympatric tree finch species (small tree finch Camarhynchus parvulus, medium tree finch Camarhynchus pauper, and large tree finch: Camarhynchus psittacula) that are currently recognized on Floreana Island, Galápagos Archipelago. Genetic analysis of microsatellite data from contemporary samples showed two genetic populations and one hybrid cluster in both 2005 and 2010; hybrid individuals were derived from genetic population 1 (small morph) and genetic population 2 (large morph). Females of the large and rare species were more likely to pair with males of the small common species. Finch populations differed in morphology in 1852-1906 compared with 2005/2010. An unsupervised clustering method showed (a) support for three morphological clusters in the historical tree finch sample (1852-1906), which is consistent with current species recognition; (b) support for two or three morphological clusters in 2005 with some (19%) hybridization; and (c) support for just two morphological clusters in 2010 with frequent (41%) hybridization. We discuss these findings in relation to species demarcations of Camarhynchus tree finches on Floreana Island.
Parasites place their hosts under strong selection for adaptive traits that increase parasite resistance. The initial impact of invasive parasites has rarely been observed and can be particularly strong on naïve hosts with limited prior exposure to parasites.Philornis downsiis an introduced fly to the Galapagos Islands whose parasitic larvae cause high mortality in nestlings of Darwin's finches. We used a within-nest camera system and nest monitoring data to examine this new host–parasite interaction in the wild. ManyP. downsiflies entered finch nests with incubated eggs or nestlings but only when parent finches were not present. ParasiticP. downsilarvae were observed to emerge from the nest base at night to feed both internally and externally on nestlings. Adult and nestling Darwin’s finches exhibit grooming and avoidance behaviours in the presence ofP. downsiparasites. Specifically, in nests with high parasite intensity, nestlings increased self-preening behaviour, ate larvae and stood on top of one another. Female finches probed into their nestling’s nares (first instar larvae reside in the nares) and probed into the nest base (second and third larvae reside in the nest base during the day). These findings shed light on the emergence of anti-parasite behaviour as well as host–parasite relationships after recent parasitism in a naïve host.
Darwin's medium tree finch (Camarhynchus pauper) meets the 2009 International Union for Conservation of Nature Red List criteria for a critically endangered species because it has ''a very small range on a single island'' and is ''declining rapidly owing to the effects of the parasite Philornis downsi'', habitat degradation, and introduced predators. The medium tree finch is only found in patches of remnant highland forest on Floreana Island, where it co-exists with breeding populations of small and large tree finches (C. parvulus and C. psittacula). Here, we examine the intensity of P. downsi in nests of small, medium, and large tree finches on Floreana. We expected that parasite intensity would increase with finch body size, and with greater rainfall, and would also correlate with increased nestling mortality. We found a trend in the expected direction for parasite intensity and rainfall. Combined meta-analytically with data from a previous study, the overall trend for the two studies was significant. We also found a significant linear trend in parasite intensity with finch body size. In addition, the medium tree finch exhibited a somewhat higher parasite intensity than would be expected based on body mass alone. Of 63 active medium tree finch nests, 17 nests had nestlings: all of which were infested with P. downsi. Only 25% of medium tree finch nestlings fledged, 28% were depredated, 41% died due to P. downsi parasitism, and 6% died for other reasons.
The conservation behavior framework is useful to identify key linkages between behavior and conservation practice. We apply this framework to a novel host-parasite system on the Galapagos Islands and ask if there have been changes in parasite oviposition behavior and host mortality patterns across the first decade (2004-2013) of its known association. The Dipteran parasite Philornis downsi was first discovered in Darwin’s finch nests in 1997 and is the biggest threat to the survival of Galapagos land birds. Host mortality has increased over the past decade. In Dipterans, pupation and pupae size are determined by access to host resources. Here, we test the hypothesis that P. downsi flies are laying eggs in finch nests earlier in the nestling phase to maximize larval feeding time and therefore chance of pupation success before host death. The results show fewer 1st instar larvae later in the host nesting cycle in support of earlier egg laying behavior by female flies. Between 2004 and 2013, parasite intensity increased from ~28 to ~48 parasites per nest, host mortality increased from ~50% to ~90%, and host age at death decreased from ~11 to ~5 days. The earlier age at host death was correlated with fewer pupae (from ~50% to ~20%) and smaller pupae size (~10% decrease). Changes in parasite behavior reveal new fitness costs to both the parasite and Darwin’s finches. These findings underscore the need for urgent conservation action to save Darwin’s finches from extinction due to a novel, lethal and introduced parasite [Current Zoology 60 (4): 542–550, 2014].
The consequences of hybridization for biodiversity depend on the specific ecological and evolutionary context in which it occurs. Understanding patterns of gene flow among hybridizing species is crucial for determining the evolutionary trajectories of species assemblages. The recently discovered hybridization between two species of Darwin's tree finches (Camarhynchus parvulus and C. pauper) on Floreana Island, Galápagos, presents an exciting opportunity to investigate the mechanisms causing hybridization and its potential evolutionary consequences under conditions of recent habitat disturbance and the introduction of invasive pathogens. In this study, we combine morphological and genetic analysis with pairing observations to explore the extent, direction and drivers of hybridization and to test whether hybridization patterns are a result of asymmetrical pairing preference driven by females of the rarer species (C. pauper). We found asymmetrical introgression from the critically endangered, larger-bodied C. pauper to the common, smaller-bodied C. parvulus, which was associated with a lack of selection against heterospecific males by C. pauper females. Examination of pairing data showed that C. parvulus females paired assortatively, whereas C. pauper females showed no such pattern. This study shows how sex-specific drivers can determine the direction of gene flow in hybridizing species. Furthermore, our results suggest the existence of a hybrid swarm comprised of C. parvulus and hybrid birds. We discuss the influence of interspecific abundance differences and susceptibility to the invasive parasite Philornis downsi on the observed hybridization and recommend that the conservation of this iconic species group should be managed jointly rather than species-specific.
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