ABSTRACT Aim Resolution of the topological branching patterns, the timing of demographic events and the effective population size changes associated with major demographic events. Location Midwestern (eastern North Central) and Northeastern USA. Taxon Blacklegged tick, Ixodes scapularis (Say, 1821). Methods Using three independent genomic datasets, single‐nucleotide variants were analysed for demographic inference. Maximum likelihood topologies and prior ecological knowledge were used to generate nested demographic hypotheses. The best‐fit scenario and the associated demographic parameter estimates were determined using approximate Bayesian computation under a random forest statistical model. The topologies and parameters supported in the three independent datasets were compared to generate insights into the demographic history of blacklegged ticks in the region. Results The emergence of extant northern populations of blacklegged ticks began between 10 and 15 kya (thousand years ago), with independent population splits from the common ancestor during the Early‐Mid‐Holocene, and never more recent than 4 kya. All populations sustained moderately large population sizes without bottlenecks, with Michigan as the exception. Michigan appears to have an uncertain placement that depends on sampling, reflecting its admixed origin. Main Conclusions There are multiple populations of northern blacklegged ticks that have persisted independently, as deglaciated regions in the northern US were recolonized following the Last Glacial Maximum (26.5–19 kya). The current ecological expansions across the northern US are likely seeded by separate relictual populations with distinctive genomic ancestry rather than a range expansion from a single source, with important implications for vector‐borne disease management.
ABSTRACT Aim Polyploids are widely expected to be more frequent at high latitudes, where polyploid vigour may confer advantages under stressful or highly seasonal climates. This expectation, however, rests largely on evidence from seed plants. We tested this prediction in Asplenium , a species‐rich, nearly cosmopolitan fern genus, using global cytogeographic data. Location Global. Taxon Asplenium (Aspleniaceae). Methods We integrated over 80,000 georeferenced records with curated cytotype data and a time‐calibrated molecular phylogeny to quantify latitudinal gradients in polyploid frequency and cytotype diversity and reconstruct the timing and phylogenetic placement of ploidy transitions. We then tested how these patterns relate to environmental conditions, geographic range structure, and lineage diversification. Results The latitudinal polyploidy gradient in Asplenium was reversed relative to expectations from seed plants. Polyploid frequency, cytotype diversity, and maximum ploidy peaked in the tropics and declined with latitude. Ploidy transitions were largely recent, occurred across all latitude bands, and varied markedly among clades, with median transition ages decreasing towards higher latitudes. With increasing latitude, polyploid occurrences became more closely spaced, whereas occupancy increased in diploid and mixed‐ploidy taxa. Polyploid frequency was positively associated with a diversification proxy, but character‐independent models were favoured over cytotype‐dependent models, providing little support for ploidy‐dependent diversification. Main Conclusions The reversed latitudinal polyploidy gradient in Asplenium is better explained by a formation – persistence framework than by polyploid vigour. Formation appears to occur across latitudes, whereas persistence may be favoured by climatic stability. This stability may allow the tropics to serve as reservoirs of polyploid diversity, whereas extratropical polyploids may be retained less often despite continued opportunities for formation. Latitudinal polyploidy gradients may therefore be lineage‐dependent, reflecting clade‐specific biological traits and evolutionary and biogeographic histories.
ABSTRACT Aim The climatic variability hypothesis (CVH) predicts that ectotherms exposed to greater climatic variability evolve broader physiological tolerances, but its application to biogeography has been dominated by thermal traits. Here, the author argue that the CVH empirical testing warrants expansion beyond the thermal axis, using an existing mechanistic niche framework. Location Global. Major Taxa Studied Terrestrial ectotherms. Main Conclusions Evidence for thermal tolerance breadth is extensive and generally supports CVH predictions, but exceptions exist, largely driven by taxon‐specific responses and climatic extremes rather than variability. Evidence for hydric and energetic traits remains more limited but shows that water balance and metabolic traits vary across geographic gradients. These axes interact mechanistically, that is, dehydration can reduce thermal tolerance and performance, while water limitation can restrict the activity and foraging needed to meet metabolic costs. Testing these interactions at the distributional level requires models that capture them, yet current correlative species distribution models do not incorporate these interacting constraints. Mechanistic and hybrid models provide a way for integrating physiological data into forecasts of range limits and vulnerability to climate change. The major empirical gap is the scarcity of datasets that measure thermal, hydric and energetic traits simultaneously in the same groups of organisms across environmental gradients. A multidimensional CVH provides a testable framework for improving mechanistic understanding and prediction of terrestrial ectotherm responses to global change.
ABSTRACT Aim We tested how climate–growth relationships, temporal coherence and responses to compound climate extremes vary across a regional elevational gradient occupied by Pinus flexilis . Location Greater Yellowstone Ecoregion, northwestern Wyoming, USA. Time Period Dated tree‐ring material spans 514–2020 ce , with detailed analyses focused on the past millennium. Major Taxa Studied Limber pine ( Pinus flexilis James). Methods We developed three millennial‐length ring‐width chronologies from high‐, mid‐ and low‐elevation sites. We compared radial growth with instrumental and reconstructed climate, quantified shared and elevation‐dependent growth patterns using PCA and moving‐window analyses, and evaluated growth responses to compound hot‐dry and cool‐wet extremes. Results Growth at all elevations was moisture sensitive and declined during warm summer conditions, indicating that heat amplified water limitation across the gradient. The chronologies shared a broad hydroclimatic signal, but the high‐ and low‐elevation sites often differed in growth magnitude and in the timing of growth responses. High‐elevation Wolf Knob temporarily decoupled from the dominant growth pattern during cool Little Ice Age conditions, whereas low‐elevation Outlaw Ridge showed the most persistent growth suppression following hot‐dry extremes. Main Conclusions Across the sampled ecological amplitude in the GYE, P. flexilis growth reflects both shared hydroclimatic variability and elevation‐specific constraints. Future warming may reduce cold limitation near upper treeline in some years, but is likely to intensify moisture limitation and drought legacy effects at lower elevations.
ABSTRACT Aim Newly documented records are widely used as historical evidence for species redistribution, especially where continuous monitoring is unavailable. However, they are often summarized as range expansion or directional shifts, without testing whether they deviate from random additions to incomplete biodiversity baselines. Here, we ask whether newly documented bird records reveal structured distributional knowledge accumulation, shaped by prior distributions, species traits and habitat context. Location China. Taxon Birds. Methods We compiled seven national bird checklists spanning 1955 to 2023 and analysed 2258 newly documented subregional records. Using nullmodel‐based metrics, we compared the proximity and northward displacement of new records with random expectations. We then related these spatial patterns to species traits and tested whether new records occurred in land‐cover contexts matching species' primary habitat preferences. Results New records accumulated unevenly across taxa, time and zoogeographical regions. They occurred significantly closer to prior distributions than expected, and this proximity was stronger in species with restricted distributions and narrower habitat breadths. Migratory species were distributed farther north than expected under the null model during most of the study period, especially among species with broader distributions and greater ecological flexibility. While resident species were generally distributed farther south than expected. Habitat analyses further showed that new records were more often located in land‐cover contexts matching species' primary habitats, both in current landcover composition and temporal land‐cover change. Main Conclusions Long‐term changes in documented bird distributions in China cannot be reduced to simple range expansion or poleward displacement. Repeated national checklists reveal spatial, trait and habitat patterns in new records, highlighting a structured process of distributional knowledge accumulation in an incomplete biodiversity baseline. Our framework helps distinguish random additions to incomplete biodiversity baselines from structured distributional knowledge accumulation, especially where continuous standardised monitoring is limited.
