Biology subjects

Hargreaves, A. L.

Publications and source records attributed to Hargreaves, A. L..

14 recordsLinked to original sources

bioRxiv2026

A modified transplant design reveals that habitat quality and quantity both limit a species' range

Species range edges provide excellent arenas for testing which ecological constraints prevent expansion into new environments. Theory predicts that ranges can be constrained by declines in the quality or amount of habitat, but their relative impact is unknown because empirical studies are seldom designed to quantify habitat amount. Here, we propose a simple modification of across-the-range-edge transplant experiments that enables tests for declines in both habitat quality and amount. Using this design, we show that quality and amount of suitable microhabitat both decline across the high-elevation range edge of the herb Rhinanthus minor. Using simulation models parameterized with field data, we show that either decline is sufficient to impose range limits, and both declines contribute to limiting R. minors high-elevation range. We end with three simple suggestions for the design and presentation of across-the-range-edge transplant experiments that would clarify how often and severely declines in habitat amount limit species ranges.

ecology↗

bioRxiv2026

Pollination ecology of a threatened lupine from the core to northern edge of its geographic range

Lack of sexual reproduction limits the fitness and long-term viability of many plant populations. This may pose a particular problem for populations at the edges of species ranges, which are often small and isolated and therefore may be less likely to attract pollinators. But despite the fact that many range-edge populations are of significant conservation concern and value, there is often little information about which visitors are effective pollinators, and few explicit tests of whether range-edge populations experience reduced pollination. Here, we assess which visitors are effective pollinators of sundial lupine (Lupinus perennis), a legume that is threatened in much of its range, and whether pollination success varies between populations in the range core and those at the species northern range edge. Across six populations in the northern USA and southern Canada (Ontario), sundial lupine was visited almost exclusively by bees, but only large bees (Bombus, Xylocopa) could be confirmed as effective pollinators in single-visit experiments. While seed production varied significantly among populations, visitation rates did not. Neither pollinator visitation, pollen receipt, nor seed production declined at sundial lupines northern range edge. We therefore found no evidence that pollination success constrains either performance of at-risk populations of sundial lupine or the species northern range limit.

ecology↗

bioRxiv2025

Urbanization weakens the latitudinal diversity gradient in birds

The latitudinal diversity gradient of increasing species richness toward the tropics is Earths most ubiquitous and ancient macroecological pattern, but may not be immune to modern anthropogenic change. Urbanization represents a particularly severe and globally-distributed ecological change; while urbanization generally reduces local diversity, whether it affects large- scale diversity patterns is unclear. Analyzing global databases including >10,000 bird species on five continents, we show that the avian latitudinal diversity gradient is 3x weaker in urban than natural areas. Urbanization reduces diversity across latitudes, but tropical cities lose disproportionately more species, partly because tropical bird assemblages contain more specialists and specialists are particularly excluded by urbanization. Our results show that urbanization is disrupting macroecological patterns, and suggest that specialization mediates both latitudinal diversity gradients and urbanization impacts.

ecology↗

bioRxiv2023

Latitudinal gradients in predation persist in urban environments

Urbanization can profoundly disrupt local ecology. But while urban areas now stretch across latitudes, little is known about urbanizations effects on macroecological patterns. We used standardized experiments to test whether urbanization disrupts latitudinal gradients in seed predation, a macroecological pattern that shapes community assembly and diversity. Using >56,000 seeds, we compared predation in urbanized and natural areas across 14,000 km of latitude, spanning the Americas. Predation increased 5-fold from high latitudes to the tropics, and latitudinal gradients in predation persisted in urban areas despite significant habitat modification. Urbanization reduced predation by vertebrates, but not invertebrates, and seemed to increase ant predation specifically. Our results show that macroecological patterns in predation intensity can persist in urbanized environments, even as urbanization alters the relative importance of predators. One-Sentence SummaryAcross 56,000 seeds and 112{degrees} of latitude, latitudinal gradients in seed predation are equally strong in natural vs. urban areas

ecology↗

bioRxiv2023

Clay larvae do not accurately measure biogeographic patterns in predation

AimSpatial variation in predation can shape geographic patterns in ecology and evolution, but testing how predation varies across ecosystems is challenging as differing species compositions and defensive adaptations can mask underlying patterns. Recently, biogeography has borrowed a tool from ecology -clay prey models. But clay models have not been adequately tested for geographic comparisons, and a well-known problem -that clay prey only appeal to a subset of potential predators- could lead to inaccurate detection of geographic patterns whenever the relative importance of predator guilds varies among sites. Here, we test whether clay larvae accurately capture geographic differences in predation on real larvae. Location90{degrees} of latitude and >2000 m elevation across the Americas Taxonvertebrate and invertebrate predation on superworms (Zophobas larve) MethodsAcross six sites that vary dramatically in latitude, elevation, and biome, we quantified predation on live, dead, and clay larvae. We physically excluded vertebrate predators from some larvae to distinguish total predation and invertebrate-only predation. ResultsPredation on live superworms almost doubled from our high-elevation high-latitude site to out low-elevation tropical site. Geographic patterns were highly consistent among live and dead larvae, but clay larvae missed extremely high predation at some sites and therefore mismeasured true geographic patterns. Clay larvae did a particularly bad job at capturing geographic patterns in predation by invertebrates. Main conclusionsClay larvae are inappropriate for large-scale tests of predation, and should be abandoned for biogeographic studies. Biogeographic experiments should instead employ realistic baits, and clay prey should be reserved for comparisons within, rather than across, predator communities.

ecology↗

bioRxiv2023

Snow Buntings, an arctic cold-specialist passerine, risk overheating under intense activity even at low air temperatures

Birds maintain some of the highest body temperatures (Tb) among endothermic animals. Often deemed a selective advantage for heat tolerance, high Tb also limits the capacity to increase Tb before reaching lethal levels. Recent thermal modelling suggests that sustained effort in Arctic birds might be restricted at mild air temperatures (Ta) during energetically demanding life history stages, which may force reductions in activity to avoid overheating, with expected negative impacts on reproductive performance. Consequently, understanding how Arctic birds will cope with increasing Ta has become an urgent concern. We examined within-individual changes in Tb in response to an experimental increase in activity in outdoor captive Arctic cold-specialised snow buntings (Plectrophenax nivalis), exposed to naturally varying Ta from -15 to 36 {degrees}C. Calm buntings exhibited a modal Tb range from 39.9 - 42.6 {degrees}C. However, we detected a dramatic increase in Tb within minutes of shifting birds to active flight, with strong evidence for a positive effect of Ta on Tb (slope = 0.04 {degrees}C/{degrees}C). Importantly, by Ta of 9 {degrees}C, flying buntings were already generating Tb [≥] 45{degrees}C, approaching the upper thermal limits of organismal performance (i.e., Tb = 45 - 47 {degrees}C). Under scenarios of elevated Tb, buntings must increase rates of evaporative water loss and/or reduce activity to avoid overheating. With known limited evaporative heat dissipation capacities, we argue buntings operating at peak energy levels will increasingly rely on behavioral thermoregulatory strategies (i.e., reducing activity) to regulate Tb, at the potential detriment to nestling growth and survival.

ecology↗

bioRxiv2021

Mapping nationally and globally at-risk species to identify hotspots for (and gaps in) conservation

Protecting habitat of species-at-risk is critical to their recovery, but can be contentious. For example, protecting species that are locally imperilled but globally common (e.g. species that only occur in a jurisdiction at the edge of their geographic range) is often thought to distract from protecting globally-imperilled species. However, such perceived trade-offs are based on the assumption that threatened groups have little spatial overlap, which is rarely quantified. Here, we compile range maps of terrestrial species-at-risk in Canada to assess the geographic overlap of nationally and globally at-risk species with each other, among taxonomic groups, and with protected areas. While many nationally-at-risk taxa only occurred in Canada at their northern range edge (median=4% of range in Canada), nationally-at-risk species were not significantly more peripheral in Canada than globally-at-risk species. Further, 56% of hotspots of nationally-at-risk taxa were also hotspots of globally-at-risk taxa in Canada, undercutting the perceived trade-off in their protection. Hotspots of nationally-at-risk taxa also strongly overlapped with hotspots of individual taxonomic groups, though less so for mammals. While strong spatial overlap across threat levels and taxa should facilitate efficient habitat protection, <7% of the area in Canadas at-risk hotspots is protected, and more than 70% of nationally and globally-at-risk species in Canada have <10% of their Canadian range protected. Our results counter the perception that protecting nationally vs. globally at-risk species are at odds, and identify critical areas to target as Canada strives to increase its protected areas and promote species-at-risk recovery.

ecology↗

bioRxiv2021

Warming in the land of the midnight sun: breeding birds may suffer greater heat stress at high- vs low-Arctic sites

Rising global temperatures are expected to increase reproductive costs for wildlife as greater thermoregulatory demands interfere with essential breeding activities such as parental care. However, predicting the temperature threshold where reproductive performance is negatively impacted remains a significant hurdle. Using a novel thermoregulatory polygon approach, we predicted the threshold temperature at which an Arctic songbird-the snow bunting (Plectrophenax nivalis)-would need to reduce activity and perform below the 4-times basal metabolic rate (BMR) required to sustain nestling provisioning to avoid overheating. We then compared this threshold to operative temperatures recorded at high (82{degrees}N) and low (64{degrees}N) Arctic sites to estimate how heat constraints translate into site-specific impacts on sustained activity level. We predict buntings would become behaviourally constrained at operative temperatures above 11.7{degrees}C, whereupon they must reduce provisioning rates to maintain thermal balance. Low Arctic sites had larger fluctuations in solar radiation, producing consistent daily periods when operative temperatures exceeded 11.7{degrees}C. However, high-latitude birds faced entire, consecutive days where parents would not be able to sustain required provisioning rates. These data indicate that Arctic warming is likely already disrupting the breeding performance of cold-specialist birds, but also suggests counterintuitive and severe negative impacts of warming at high-latitude breeding locations.

ecology↗

bioRxiv2021

Does pollen limitation limit plant ranges? Evidence and implications

Range limits often involve declines in sexual reproduction, reducing fitness, dispersal, and adaptive potential at range edges. For plants, sexual reproduction is frequently limited by inadequate pollination. While case studies show that pollen limitation can limit plant distributions, the extent to which pollination commonly declines toward plant range edges is unknown. Here, we leverage global databases of pollen-supplementation experiments and plant occurrence data to test whether pollen limitation increases toward plant range edges, using a phylogenetically controlled meta-analysis. While there was significant pollen limitation across studies, we found little evidence that pollen limitation increases toward plant range edges. Pollen limitation was not stronger toward the tropics, nor at species equatorward vs poleward range limits. Meta-analysis results are consistent with results from targeted experiments, in which pollen limitation increased significantly toward only 14% of 14 plant range edges, suggesting that pollination contributes to range limits less often than do other interactions. Together, these results suggest pollination is one of the rich variety of potential ecological factors that can contribute to range limits, rather than a generally important constraint on plant distributions.

ecology↗

bioRxiv2021

Biotic interactions are more important at species' warm vs. cool range-edges: a synthesis

Predicting which ecological factors constrain species distributions is a fundamental question in ecology and critical to forecasting geographic responses to global change. Darwin hypothesized that abiotic factors generally impose species high-latitude and high-elevation (typically cool) range limits, whereas biotic interactions more often impose species low-latitude/low-elevation (typically warm) limits, but empirical support has been mixed. Here, we clarify three predictions arising from Darwins hypothesis, and show that previously mixed support is partially due to researchers testing different predictions. Using a comprehensive literature review (886 range limits), we find that biotic interactions, including competition, predation, and parasitism, influenced species warm limits more often than species cool limits. At cool limits, abiotic factors were consistently more important than biotic interactions, but temperature contributed strongly to cool and warm limits. Our results suggest that most range limits will be sensitive to climate warming, but warm limit responses will depend strongly on biotic interactions. O_QD"When we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favored, as in this one being hurt. (Whereas)... the number of species, and therefore of competitors, deceases northwards; hence in going northward or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate" -Darwin 1859 C_QD

ecology↗

bioRxiv2020

Climate warming weakens local adaptation

Anthropogenic climate change is generating mismatches between the environmental conditions that populations historically experienced and those in which they reside. Understanding how climate change affects population performance is a critical scientific challenge. We combine a quantitative synthesis of field transplant experiments with a novel statistical approach based in evolutionary theory to quantify the effects of temperature and precipitation variability on population performance. We find that species average performance is affected by both temperature and precipitation, but populations show signs of local adaptation to temperature only. Contemporary responses to temperature are strongly shaped by the local climates under which populations evolved, resulting in performance declines when temperatures deviate from historic conditions. Adaptation to other local environmental factors is strong, but temperature deviations as small as 2{degrees}C erode the advantage that these non-climatic adaptations historically gave populations in their home sites. One sentence summaryClimate change is pulling the thermal rug out from under populations, reducing average performance and eroding their historical home-site advantage.

evolutionary biology↗

bioRxiv2020

Expansion dynamics and marginal climates drive adaptation across geographic ranges

Every species experiences limits to its geographic distribution. Some evolutionary models predict that populations at range edges are less well-adapted to their local environments due to drift, expansion load, or swamping gene flow from the range interior. Alternatively, populations near range edges might be uniquely adapted to marginal environments. In this study, we use a database of transplant studies that quantify performance at broad geographic scales to test how local adaptation, site quality, and population quality change from spatial and climatic range centers towards edges. We find that populations from poleward edges perform relatively poorly, both on average across all sites (15% lower population quality) and when compared to other populations at home (31% relative fitness disadvantage), consistent with these populations harboring high genetic load. Populations from equatorial edges also perform poorly on average (18% lower population quality) but, in contrast, outperform foreign populations (16% relative fitness advantage), suggesting that populations from equatorial edges have strongly adapted to unique environments. Finally, we find that populations from sites that are thermally extreme relative to the species niche demonstrate strong local adaptation, regardless of their geographic position. Our findings indicate that both nonadaptive processes and adaptive evolution contribute to variation in adaptation across species ranges.

evolutionary biology↗

bioRxiv2020

Spatial distribution and hotspots of mammals in Canada

High-latitude countries often contain the polar range edge of species that are common farther south. The more peripherally a species occurs in a country, the smaller its national range will be and the more its national range will consist of range-edge populations, which are often predicted to be relatively small, isolated, and unproductive. Together, this may focus national conservation efforts toward peripheral species whose global conservation value is controversial. However, if range-edge taxa occur where overall diversity is also high, there would be fewer trade-offs in protecting them. Using 153 of the 158 terrestrial mammal species in Canada, we tested how species distributions relate to their national conservation status and total mammal richness. Half of Canadian mammals had <20% of their global range in Canada. Range area in Canada was strongly associated with national threat status; mammals considered at-risk in Canada had 42% smaller Canadian ranges than mammals considered secure. However, after accounting for range area, being more peripheral (smaller proportion of global range in Canada) did not increase the likelihood that a taxon was considered at-risk. We overlaid the 153 maps to calculate mammal diversity across Canada, divided into 100x100 km grid cells. We found that hotspots of at-risk mammals (cells with >4 at-risk taxa) and hotspots of range-edge mammals (cells with >12 taxa with [≤]20% of their range in Canada) were about twice as species rich as non-hotspot cells, containing up to 44% of Canadian mammal diversity per grid-cell. Our results suggest that protecting areas with the most at-risk or range-edge mammals would simultaneously protect habitat for many species currently deemed secure.

ecology↗

bioRxiv2018

Local adaptation primes cold-edge populationsfor range expansion but not warming-induced range shifts

According to theory, edge populations may be the best suited to initiate range expansions and climate-driven range shifts if they are locally adapted to extreme edge conditions, or the worst suited to colonize beyond-range habitat if their offspring are genetically and competitively inferior. We tested these contrasting predictions by comparing fitness of low, mid, and high-elevation (edge) populations of the annual Rhinanthus minor, transplanted throughout and above its elevational distribution under natural and experimentally-warmed conditions. Seed from low-quality edge habitat had inferior emergence across sites, but high-elevation seeds were also locally adapted. High-elevation plants initiated flowering earlier than plants from lower populations, required less heat accumulation to mature seed, and so achieved higher lifetime fitness at high elevations. Fitness was strongly reduced above the range, but adaptive phenology enhanced the relative fitness of high-elevation seeds. Experimental warming improved fitness above the range, confirming climates importance in limiting R. minors distribution, but eliminated the advantage of local cold-edge populations. These results provide experimental support for recent models in which cold-adapted edge populations do not always facilitate warming-induced range shifts. The highest fitness above the range was achieved by a super edge phenotype from a neighboring mountain, suggesting key adaptations exist at the regional scale even if absent from local edge populations. Our results demonstrate that assessing the value of edge populations will not be straightforward, but suggest that a regional approach to their conservation, potentially enhancing gene flow among them, might maximize species ability to respond to global change.\n\nSignificanceIndividuals from range-edge populations are the most likely to disperse to habitat beyond the species current range, but are they best suited to colonize it? Our multi-year transplant experiment throughout and above the elevational range of an annual herb in the Canadian Rocky Mountains found that adaptive flowering phenology enhanced the fitness of high-edge seeds above the range, outweighing detrimental effects of poor seed quality. However, only one edge population maintained its advantage over central populations under experimental warming. While edge populations were most likely to drive range expansion, adaptation to cold climates may not help them initiate range shifts in response to climate warming, unless superior genotypes spread among isolated edge populations.

ecology↗