Seed Predators, not Herbivores, Exert Natural Selection onSolidagospp.in an Urban Archipelago

Abstract

The effects of urbanization on biodiversity are well established, as a growing city will reduce the size and diversity of patches of native plants. Recolonization of old patches and discovery of new ones by arthropod herbivores should occur as predicted by island biogeography theory. Although colonization represents an increase in biodiversity, such arrivals may exert new forms of natural selection on plants through herbivory and seed predation. Using a single species of old-field aster (Solidago altissima L.), we found that the level of natural selection by seed predators and herbivores follows patterns of island biogeography, with lower amounts of damage on smaller islands, where there are fewer species, and hypothetically smaller populations of arthropods. We also found that in an urban system, levels of herbivory are far below the tolerance levels of Solidago, and that seed predators are likely to be the only arthropod to cause reduced fitness. The pattern seen also implies that as a patch of Solidago grows through clonal expansion, it will come under higher selective pressure.

Decreased losses of woody plant foliage to insects in large urban areas are explained by bird predation

“…Our study has demonstrated that urbanization generally decreases the background damage of woody plants by leaf‐eating insects across multiple cities, tree species and herbivore feeding guilds. This pattern contrasts with the long‐standing belief about higher herbivory in urban habitats (Dreistadt et al., ) but agrees with more recent work, which frequently has reported a decrease in herbivory with urbanization, especially in terms of plant damage imposed by defoliating insects (Bode & Gilbert, ; Nuckols & Connor, ; but see Cuevas‐Reyes et al., ). The abundance of leafminers increased more frequently with urbanization when compared to the increase in defoliators (Raupp et al., ), but this pattern may differ between insect species (Rickman & Connor, ), thereby yielding a zero net effect (Nuckols & Connor, ).…”

“…Patches of habitats suitable for wildlife tend to become smaller and more isolated with increases in urbanization (Luck & Wu, ), and the loss of habitat area generally results in a decrease in plant damage by insects (Bode & Gilbert, ; Rickman & Connor, ). Therefore, the possibility remains that habitat fragmentation has also contributed to the observed decrease in background insect herbivory in urban habitats.…”

“…The rapid growth in numbers of publications addressing different aspects of insect-plant relationships in urban areas (reviewed by Raupp, Shrewsbury, & Herms, 2010) reflects the increasing appreciation of the importance of services provided by urban plantings for human well-being (Pataki et al, 2011). These services can be deteriorated by plant-feeding insects (Dale & Frank, 2014;Dreistadt, Dahlsten, & Frankie, 1990;Raupp, Shrewsbury, & Herms, 2012), but the existing case studies provide conflicting evidence, showing either increased or decreased herbivory in urban habitats relative to rural habitats (Bode & Gilbert, 2016;Cuevas-Reyes, Gilberti, Gonz alez-Rodr ıguez, & Fernandes, 2013;Dreistadt et al, 1990;Matter et al, 2012;Nuckols & Connor, 1995;Rickman & Connor, 2003). No general pattern in the effect of urbanization on insect herbivory has yet emerged and the sources of variations in outcomes of individual studies remain unclear.…”

Plants in the city: understanding recruitment dynamics in urban landscapes

“…Urban production limitation can be affected at multiple stages, from the number of flowers produced to pre‐dispersal seed predation (eg Bode and Gilbert ), but research has mainly focused on the impact of urban drivers on pollen limitation. Pollen limitation may be due to changes in pollen quality and quantity, resulting in unfertilized flowers or self‐fertilization, which increases the risk of inbreeding depression.…”

“…Seed removal and predation rates are reported to be higher in urban plant communities than in rural sites (Bowers and Breland ), though such trends may be species dependent (Pufal and Klein ). What drives these changes is less clear, but there is evidence that light pollution, fragmentation and patch size, and biotic invasion may alter the intensity and spatial patterns of seed predation (eg Bode and Gilbert ; Guiden and Orrock ). Other urban drivers, including pollutants such as nitrate and ammonium, may also modify seed bank dynamics and drive compositional shifts in seed bank populations (Pellissier et al .…”