Abstract
Variation in relative fitness of competing recently formed species across heterogeneous environments promotes coexistence. However, the physiological traits mediating such variation in relative fitness have rarely been identified. Resting metabolic rate (RMR) is tightly associated with life history strategies, thermoregulation, diet use, and inhabited latitude and could therefore moderate differences in fitness responses to fluctuations in local environments, particularly when species have adapted to different climates in allopatry. We work in a long‐term study of collared (Ficedula albicollis) and pied flycatchers (Ficedula hypoleuca) in a recent hybrid zone located on the Swedish island of Öland in the Baltic Sea. Here, we explore whether differences in RMR match changes in relative performance of growing flycatcher nestlings across environmental conditions using an experimental approach. The fitness of pied flycatchers has previously been shown to be less sensitive to the mismatch between the peak in food abundance and nestling growth among late breeders. Here, we find that pied flycatcher nestlings have lower RMR in response to higher ambient temperatures (associated with low food availability). We also find that experimentally relaxed nestling competition is associated with an increased RMR in this species. In contrast, collared flycatcher nestlings did not vary their RMR in response to these environmental factors. Our results suggest that a more flexible nestling RMR in pied flycatchers is responsible for the better adaptation of pied flycatchers to the typical seasonal changes in food availability experienced in this hybrid zone. Generally, subtle physiological differences that have evolved when species were in allopatry may play an important role to patterns of competition, coexistence, or displacements between closely related species in secondary contact.
Highlights
A major predicted response to climate change is that species may expand or contract their distribution ranges (e.g., Tayleur et al, 2015)
We found that collared flycatcher nestlings that survived to 12 days old gained an average of 0.971 ± 0.2 g/day and pied flycatcher nestlings gained an average of 0.948 ± 0.2 g/day
We found that 8-day-old collared flycatcher nestlings had an average mass of 12.95 ± 1.6, and average Resting metabolic rate (RMR) of 1.54 ± 0.7 ml/ min and that 8-day-old pied flycatcher nestlings had an average mass of 12.53 ± 1.1 and average RMR of 1.58 ± 0.6 ml/min
Summary
A major predicted response to climate change is that species may expand or contract their distribution ranges (e.g., Tayleur et al, 2015). Many recently genetically diverged species will come into secondary contact, after a period of allopatry, when they still have overlapping niche requirements. Theoretical models suggest that competitive exclusion can be avoided when there is environmental heterogeneity favoring different species in different times or space (Amarasekare & Nisbet, 2001; Chesson & Warner, 1981; Chesson & Huntly, 1997). It is expected that at least some divergence in niche use between young species is required to cause such different fitness responses across heterogeneous environments, which allow for coexistence. The identification of key characteristics that underlay differences in niche requirements of young species is important if we want to understand and predict how biodiversity in communities will be affected when species come into secondary contact
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