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Abstract Understanding mechanisms that generate range limits is central to knowing why species are found where they are and how they will respond to environmental change. There is growing awareness that biotic interactions play an important role in generating range limits. However, current theory and data overwhelmingly focus on abiotic drivers and antagonistic interactions. Here we explore the effect that mutualists have on their partner's range limits: the geographic “footprint” of mutualism. This footprint arises from two general processes: modification of a partner's niche through environment‐dependent fitness effects and, for a subset of mutualisms, dispersal opportunities that lead suitable habitats to be filled. We developed a conceptual framework that organizes different footprints of mutualism and the underlying mechanisms that shape them, and evaluated supporting empirical evidence from the primary literature. In the available literature, we found that the fitness benefits and dispersal opportunities provided by mutualism can extend species' ranges; conversely, the absence of mutualism can constrain species from otherwise suitable regions of their range. Most studies found that the footprint of mutualism is driven by changes in the frequency of mutualist partners from range core to range edge, whereas fewer found changes in interaction outcomes, the diversity of partners, or varying sensitivities of fitness to the effects of mutualists. We discuss these findings with respect to specialization, dependence, and intimacy of mutualism. Much remains unknown about the geographic footprint of mutualisms, leaving fruitful areas for future work. A particularly important future direction is to explore the role of mutualism during range shifts under global change, including the promotion of shifts at leading edges and persistence at trailing edges. 

Abstract Mutualism benefits partner species, and theory predicts these partnerships can affect the abundance, diversity, and composition of partner and non‐partner species. We used 16 years of monitoring data to determine the ant partner species of tree cholla cacti (Cylindropuntia imbricata), which reward ants with extrafloral nectar in exchange for anti‐herbivore defense. These long‐term data revealed one dominant ant partner (Liometopum apiculatum) and two less common partners (Crematogaster opuntiaeandForelius pruinosus). We then used short‐term characterization of the terrestrial ant community by pitfall trapping to sample partner and non‐partner ant species across ten plots of varying cactus density. We found that the dominant ant partner tended a higher proportion cacti in plots of higher cactus density, and was also found at higher occurrence within the pitfall traps in higher density plots, suggesting a strong positive feedback that promotes ant partner occurrence where plant partners are available. Despite the strong association and increased partner occurrence, ant community‐wide effects from this mutualism appear limited. Of the common ant species, the occurrence of a single non‐partner ant species was negatively associated with cactus density and with the increased presence ofL. apiculatum. Additionally, the composition and diversity of the ant community in our plots were insensitive to cactus density variation, indicating that positive effects of the mutualism on the dominant ant partner did not have cascading impacts on the ant community. This study provides novel evidence that exclusive mutualisms, even those with a strong positive feedback, may be limited in the scope of their community‐level effects. 

Abstract The effects of climate change on population viability reflect the net influence of potentially diverse responses of individual‐level demographic processes (growth, survival, regeneration) to multiple components of climate. Articulating climate–demography connections can facilitate forecasts of responses to future climate change as well as back‐casts that may reveal how populations responded to historical climate change.We studied climate–demography relationships in the cactusCyclindriopuntia imbricata; previous work indicated that our focal population has high abundance but a negative population growth rate, where deaths exceed births, suggesting that it persists under extinction debt. We parameterized a climate‐dependent integral projection model with data from a 14‐year field study, then back‐casted expected population growth rates since 1900 to test the hypothesis that recent climate change has driven this population into extinction debt.We found clear patterns of climate change in our central New Mexico study region but, contrary to our hypothesis,C. imbricatahas most likely benefitted from recent climate change and is on track to reach replacement‐level population growth within 37 years, or sooner if climate change accelerates. Furthermore, the strongest feature of climate change (a trend towards years that are overall warmer and drier, captured by the first principal component of inter‐annual variation) was not the main driver of population responses. Instead, temporal trends in population growth were dominated by more subtle, seasonal climatic factors with relatively weak signals of recent change (wetter and milder cool seasons, captured by the second and third principal components).Synthesis. Our results highlight the challenges of back‐casting or forecasting population dynamics under climate change, since the most apparent features of climate change may not be the most important drivers of ecological responses. Environmentally explicit demographic models can help meet this challenge, but they must consider the magnitudes of different aspects of climate change alongside the magnitudes of demographic responses to those changes. 

Abstract Heritable symbionts are often observed at intermediate prevalence within host populations, despite expectations that positive fitness feedbacks should drive beneficial symbionts to fixation. Intermediate prevalence may reflect neutral dynamics of symbionts with weak fitness effects, transient dynamics of symbionts trending towards fixation (or elimination), or a stable intermediate outcome determined by the balance of fitness effects and failed symbiont transmission. Theory suggests that these outcomes should depend on symbiont‐conferred demographic effects and vertical transmission efficiency, which may both depend on environmental context.We established experimental populations of winter bent grassAgrostis hyemalisacross a range of prevalence of the heritable fungal endophyteEpichloë amarillans. Using irrigation, we elevated the precipitation for half of the populations, which we hypothesized would weaken the benefits of symbiosis. Across two annual transitions, we assayed 5,485 individuals to determine prevalence and censused 954 individuals for demographic (survival, flowering, reproduction and recruitment) and vertical transmission data. We used hierarchical Bayesian models to infer long‐run equilibria from short‐term changes in symbiont prevalence and estimated demographic vital rates to link individual‐level effects to population‐level outcomes.We found evidence for all three proposed mechanisms for intermediate symbiont prevalence, but the outcome differed qualitatively across years and precipitation treatments. In the first year, symbionts trended towards fixation under drought conditions but drifted neutrally under elevated precipitation. Fixation likely arose from symbiont‐conferred recruitment benefits outweighing reproductive costs under the drought conditions, while elevated precipitation tempered these effects. In the second transition year, we inferred stable intermediate prevalence across both precipitation treatments, which indicated a balance between symbiont conferred recruitment benefits that allowed low‐prevalence populations to increase and imperfect transmission that caused high‐prevalence populations to decrease.Synthesis. We find support for neutral, transient and stable mechanisms underlying symbiont prevalence, indicating that symbiont prevalence is often pushed and pulled in different directions by the composite outcome of symbiont effects on demographic rates and transmission efficiency, and the way in which these processes respond to environmental context. 

Abstract Understanding the effects of climate on the vital rates (e.g., survival, development, reproduction) and dynamics of natural populations is a long‐standing quest in ecology, with ever‐increasing relevance in the face of climate change. However, linking climate drivers to demographic processes requires identifying the appropriate time windows during which climate influences vital rates. Researchers often do not have access to the long‐term data required to test a large number of windows, and are thus forced to makea priorichoices. In this study, we first synthesize the literature to assess currenta priorichoices employed in studies performed on 104 plant species that link climate drivers with demographic responses. Second, we use a sliding‐window approach to investigate which combination of climate drivers and temporal window have the best predictive ability for vital rates of four perennial plant species that each have over a decade of demographic data (Helianthella quinquenervis,Frasera speciosa,Cylindriopuntia imbricata, andCryptantha flava). Our literature review shows that most studies consider time windows in only the year preceding the measurement of the vital rate(s) of interest, and focus on annual or growing season temporal scales. In contrast, our sliding‐window analysis shows that in only four out of 13 vital rates the selected climate drivers have time windows that align with, or are similar to, the growing season. For many vital rates, the best window lagged more than 1 year and up to 4 years before the measurement of the vital rate. Our results demonstrate that for the vital rates of these four species, climate drivers that are lagged or outside of the growing season are the norm. Our study suggests that considering climatic predictors that fall outside of the most recent growing season will improve our understanding of how climate affects population dynamics. 

Abstract Despite a global footprint of shifts in flowering phenology in response to climate change, the reproductive consequences of these shifts are poorly understood. Furthermore, it is unknown whether altered flowering times affect plant population viability.We examine whether climate change‐induced earlier flowering has consequences for population persistence by incorporating reproductive losses from frost damage (a risk of early flowering) into population models of a subalpine sunflower (Helianthella quinquenervis). Using long‐term demographic data for three populations that span the species’ elevation range (8–15 years, depending on the population), we first examine how snowmelt date affects plant vital rates. To verify vital rate responses to snowmelt date experimentally, we manipulate snowmelt date with a snow removal experiment at one population. Finally, we construct stochastic population projection models and Life Table Response Experiments for each population.We find that populations decline (λ_s < 1) as snowmelt dates become earlier. Frost damage to flower buds, a consequence of climate change‐induced earlier flowering, does not contribute strongly to population declines. Instead, we find evidence that negative effects on survival, likely due to increased drought risk during longer growing seasons, drive projected population declines under earlier snowmelt dates.Synthesis.Shifts in flowering phenology are a conspicuous and important aspect of biological responses to climate change, but here we show that the phenology of reproductive events can be unreliable measures of threats to population persistence, even when earlier flowering is associated with substantial reproductive losses. Evidence for shifts in reproductive phenology, along with scarcer evidence that these shifts actually influence reproductive success, are valuable but can paint an incomplete and even misleading picture of plant population responses to climate change. 

Species' distributions and abundances are shifting in response to climate change. Most species harbor microbial symbionts that have the potential to influence these responses. Mutualistic microbial symbionts may provide resilience to environmental change by protecting their hosts from increasing stress. However, environmental change that disrupts these interactions may lead to declines in hosts or symbionts. Microbes preserved within herbarium specimens offer a unique opportunity to quantify changes in microbial symbiosis across broad temporal and spatial scales. We asked how the prevalence of seed-transmitted fungal symbionts of grasses (Epichloe endophytes), which can protect hosts from abiotic stress, have changed over time in response to climate change, and how these changes vary across host species' ranges. Specifically, we analyzed 2,346 herbarium specimens of three grass host species collected over the last two centuries (1824 -- 2019) for the presence or absence of endophyte symbiosis, and evaluated spatial and temporal trends in endophyte prevalence. We found that endophytes increased in prevalence over the last two centuries from ca. 25% prevalence to ca. 75% prevalence, on average, across three host species. We also found that changes in prevalence were associated with observed changes in seasonal climate drivers; notably increasing precipitation corresponding to each host species' peak growing season and changes in off-peak season variability in precipitation. Our analysis performed favorably in an out-of-sample predictive test with contemporary data, however we identified greater local-scale variability in endophyte prevalence in contemporary data compared to historic data, suggesting that model fusion may be an important step moving forward. Our results provide novel evidence for a cryptic biological response to climate change that may contribute to the resilience of host-microbe symbiosis through context-dependent benefits that confer a fitness advantage to symbiotic hosts under environmental change. 

This project was designed to understand the demographic effects of vertically transmitted fungal endophytes (Epichloë spp.) on their grass hosts. The experiment includes seven host-symbiont taxonomic pairs: Agrostis perennans - E. amarillans, Elymus villosus - E. elymi, Elymus virginicus - E. elymi or EviTG-1, Festuca subverticillata - E. starrii, Poa alsodes - E. alsodes, Poa sylvestris - E. PsyTG-1, Schedonorus arundinaceus - E. coenophiala. Experimental plots were established at the Indiana University Lilly-Dickey Woods Research and Teaching Preserve in south-central Indiana, USA in 2007. For each species, 5-10 plots were planted with naturally symbiotic (S+) hosts, and 5-10 plots were plated with hosts that were disinfected of fungal endophytes by heat treatment (S-). Over 15 years (2007-2022) we collected demographic data on the survival, growth, reproduction, and recruitment of all plants in all plots. Beginning in 2018 we also collected data on the locations of all plants in every plot.