Anna Lesley Hargreaves

The striking increase in species richness toward the tropics is perhaps Earth’s most ubiquitous macroecological pattern, but even such fundamental patterns can be vulnerable to anthropogenic change. Urbanization is a particularly severe and globally distributed change that often reduces local diversity; whether such local effects scale up to alter global biodiversity patterns is unclear. We present three hypotheses for how urbanization might affect latitudinal gradients in diversity, and test them using community science observations of >10,000 bird species on five continents. We find that the avian latitudinal diversity gradient is 3× weaker in urban areas compared to natural areas. Urbanization reduced bird diversity across latitudes (by 32% on average), but tropical cities lost a higher proportion of species, an effect that was consistent across hemispheres, seasons, and methodological choices. This disproportionate diversity loss from tropical cities seems driven partly by latitudinal gradients in specialization, as tropical bird assemblages contained more specialists and specialists were disproportionately excluded by urbanization, and partly by more intense urbanization toward lower latitudes. Our results show that urbanization is disrupting macroecological patterns that have defined global biodiversity for millennia, and suggest that both community traits and urban planning mediate urbanization’s local and global impacts.

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. minor's 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.

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 lupine’s 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.

The latitudinal diversity gradient of increasing species richness toward the tropics is Earth's 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.

The idea that species interactions are more ecologically and evolutionarily important toward lower latitudes underpins seminal theories in ecology and evolution. Recent global studies have found the predicted latitudinal gradients in interactions, particularly predation. However, latitudinal patterns alone do not reveal why interactions vary geographically and so do not provide strong predictions in space (e.g., for specific ecosystems) or time (e.g., forecasting responses to global change). Here, I review theory to identify a clearer, mechanistic, and testable framework for predicting geographic variation in the importance of species interactions. I review competing metrics of importance, proximate mechanisms that can increase interaction importance, and environmental gradients that could generate predictable geographic patterns (climate extremes and stability, warmer temperatures, productivity, and biodiversity). Strong empirical tests are accumulating thanks to the rise of global experiments and datasets; renewed focus on testing why interactions vary spatially will help move the field from identifying latitudinal patterns to understanding broader mechanisms.

Urbanization is creating a new global biome, in which cities and suburbs around the world often resemble each other more than the local natural areas they replaced. But while urbanization can profoundly affect ecology at local scales, we know little about whether it disrupts large-scale ecological patterns. Here we test whether urbanization disrupts a macroecological pattern central to ecological and evolutionary theory: the increase in seed predation intensity from high to low latitudes. Across 14,000 km of latitude spanning the Americas, we compared predation intensity on two species of standardized experimental seeds in urbanized and natural areas. In natural areas, predation on both seed species increased fivefold from high latitudes to the tropics, one of the strongest latitudinal gradients in species interactions documented so far. Surprisingly, latitudinal gradients in predation were equally strong in urbanized areas despite significant habitat modification. Nevertheless, urbanization did affect seed predation. Compared with natural areas, urbanization reduced overall predation and vertebrate predation, did not affect predation by invertebrates in general, and increased predation by ants. Our results show that macroecological patterns in predation intensity can persist in urbanized environments, even as urbanization alters the relative importance of predators and potentially the evolutionary trajectory of urban populations.

Aim: Long‐standing theory predicts that the intensity of biotic interactions increases from high to low latitudes. Studies addressing geographic variation in predation on insect prey have often relied on prey models, which lack many characteristics of live prey. Our goals were to explore global latitudinal patterns of predator attack rates on standardised live insect prey and to compare the patterns in predation on live insects with those on plasticine prey models.Location: Global forested areas.Time Period: 2021–2023.Major Taxa: Arthropods, birds.Methods: We measured predation rates in 43 forested locations distributed across five continents from 34.1° S to 69.5° N latitude. At each location, we exposed 20 sets of three bait types, one set per tree. Each set included three live fly larvae (maggots), three live fly puparia and three plasticine models of the puparia. We used glue rings to isolate half of the sets from non‐flying predators.Results: Arthropod attack rates on plasticine prey decreased linearly from low to high latitudes, whereas attack rates on maggots had a U shaped distribution, with the lowest predation rates at temperate latitudes and the highest rates at tropical and boreal latitudes. This difference emerged from intensive predator attacks on live maggots, but not on plasticine models, in boreal sites. Site‐specific attack rates of arthropod predators on live and plasticine prey were not correlated. In contrast, bird attack rates on live maggots and plasticine models were positively correlated, but did not show significant latitudinal changes.Main Conclusions: Latitudinal patterns in predation differ between major groups of predators and between types of prey. Poleward decreases in both arthropod and combined arthropod and bird predation on plasticine models do not mirror patterns of predation on our live prey, the latter likely reflecting real patterns of predation risk better than do patterns of attack on artificial prey.

Foraging niches become more specialized toward bird range limits

Aim: Spatial 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 bias detected 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.Location: 90° of latitude and >2000 m elevation across the Americas.Taxon: Vertebrate and invertebrate predation on ‘superworms’ (Zophobas larvae).Methods: Across 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.Results: Predation on live superworms almost doubled from our high‐elevation high‐latitude site to our low‐elevation tropical site. Geographic patterns were consistent among live and dead larvae, but clay larvae missed extremely high predation at some sites and therefore mis‐measured true geographic patterns. Clay larvae did a particularly bad job at capturing geographic patterns in predation by invertebrates, although sample sizes for invertebrate predation were small.Main Conclusions: Clay larvae are inappropriate for comparing predation rates across sites. They should be abandoned for biogeographic studies and reserved for comparisons within, rather than across, predator communities.

Urbanization can profoundly disrupt local ecology. But while urban areas now stretch across latitudes, little is known about urbanization’s 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 Summary Across 56,000 seeds and 112° of latitude, latitudinal gradients in seed predation are equally strong in natural vs. urban areas