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The range of hosts a pathogen can infect is a key trait influencing human disease risk and reservoir host infection dynamics. Borrelia burgdorferi sensu stricto (Bb), an emerging zoonotic pathogen, causes Lyme disease and is widely considered a host generalist, commonly infecting mammals and birds. Yet the extent of intraspecific variation in Bb host breadth, its role in determining host competence and potential implications to human infection remain unclear. We conducted a long-term study of Bb diversity, defined by the polymorphic ospC locus, across white-footed mice, passerine birds, and tick vectors leveraging long-read amplicon sequencing. Our results reveal strong variation in host breadth across Bb genotypes, exposing a spectrum of genotype-specific host-adapted phenotypes. We found support for multiple niche polymorphism maintaining Bb diversity in nature and little evidence of temporal shifts in genotype dominance, as would be expected under negative frequency-dependent selection. Passerine birds support the circulation of several human invasive strains in the local tick population and harbor greater Bb genotypic diversity compared to white-footed mice. Mouse-adapted Bb genotypes exhibited longer persistence in individual mice compared to non-adapted genotypes and infection communities infecting individual mice preferentially became dominated by mouse-adapted genotypes over time. We posit that intraspecific variation in Bb host breadth and specificity helps maintain overall species fitness in response to transmission by a generalist vector. Because pathogen genotypes vary in host breadth and result in diverse human disease manifestations, our findings indicate that a more nuanced definition of ‘host competence’ incorporating local genotype frequency is warranted. 

Ecosystems across the United States are changing in complex and surprising ways. Ongoing demand for critical ecosystem services requires an understanding of the populations and communities in these ecosystems in the future. This paper represents a synthesis effort of the U.S. National Science Foundation-funded Long-Term Ecological Research (LTER) network addressing the core research area of “populations and communities.” The objective of this effort was to show the importance of long-term data collection and experiments for addressing the hardest questions in scientific ecology that have significant implications for environmental policy and management. Each LTER site developed at least one compelling case study about what their site could look like in 50–100 yr as human and environmental drivers influencing specific ecosystems change. As the case studies were prepared, five themes emerged, and the studies were grouped into papers in this LTER Futures Special Feature addressing state change, connectivity, resilience, time lags, and cascading effects. This paper addresses the “connectivity” theme and has examples from the Phoenix (urban), Niwot Ridge (alpine tundra), McMurdo Dry Valleys (polar desert), Plum Island (coastal), Santa Barbara Coastal (coastal), and Jornada (arid grassland and shrubland) sites. Connectivity has multiple dimensions, ranging from multi-scalar interactions in space to complex interactions over time that govern the transport of materials and the distribution and movement of organisms. The case studies presented here range widely, showing how land-use legacies interact with climate to alter the structure and function of arid ecosystems and flows of resources and organisms in Antarctic polar desert, alpine, urban, and coastal marine ecosystems. Long-term ecological research demonstrates that connectivity can, in some circumstances, sustain valuable ecosystem functions, such as the persistence of foundation species and their associated biodiversity or, it can be an agent of state change, as when it increases wind and water erosion. Increased connectivity due to warming can also lead to species range expansions or contractions and the introduction of undesirable species. Continued long-term studies are essential for addressing the complexities of connectivity. The diversity of ecosystems within the LTER network is a strong platform for these studies. 

The transient climate response (TCR), defined to be the warming in near‐surface air temperature after 70 years of a 1% per year increase in CO₂, can be estimated from observed warming over the nineteenth and twentieth centuries. Such analyses yield lower values than TCR estimated from global climate models (GCMs). This disagreement has been used to suggest that GCMs' climate may be too sensitive to increases in CO₂. Here we critically evaluate the methodology of the comparison using a large ensemble of a fully coupled GCM simulating the historical period, 1850–2005. We find that TCR estimated from model simulations of the historical period can be much lower than the model's true TCR, replicating the disagreement seen between observations and GCM estimates of TCR. This suggests that the disagreement could be explained entirely by the methodology of the comparison and undercuts the suggestions that GCMs overestimate TCR.