Urbanization creates a hostile environment for native trees

Background/Question/Methods

As urbanization proceeds at the fastest rate in history there is a desire for cities serve a dual role as habitat for people but also for plants and animals that provide ecosystem services and connect urban-dwellers with nature.  To this end, interest in using native plants in urban forests and landscapes has increased in hopes of enriching urban habitat for dependent invertebrate and vertebrate animals.  Unfortunately, many native plants and animals find urban areas difficult places to live due to low plant species diversity, poor habitat connectivity, complexity, and hostile microclimates.  However, as in other human-altered systems with these characteristics, such as agricultural fields, many herbivorous arthropods thrive.  Our hypothesis is that native herbivorous arthropods find enemy-free space in urban areas which promotes rapid population growth but that herbivore abundance and damage is greater on native than introduced tree species.  To test this hypothesis we sampled the free-living herbivore and natural enemy communities on three native and two exotic Acer spp. and three native and two exotic Quercus spp. in urban North Carolina.  We also intensively sampled the abundance and diversity of sedentary scale insects on each of the plant species.

Results/Conclusions

Native trees had greater herbivore abundance and diversity than introduced species.  However, the abundance natural enemies, such as spiders, lady beetles, and parasitoid wasps, was not correlated with that of herbivores.  This suggests top-down control of herbivores in urban trees is weak.  Scale insects showed an even stronger response to native trees such that native species could have over 100 times the scale abundance of introduced trees.  For example, introduced Norway Maple, A. platanoides, averaged only 0.32 scale insects per 30 cm branch compared to the native Red Maple, A. rubrum, which had 30.2.   Our results indicate that native trees provide quality habitat for native herbivores, particularly scale insects.  However, due to depauperate natural enemy communities in urban areas, scale and other herbivores often lead to severe damage and death of native trees.  In contrast, introduced tree species, as novel hosts for herbivores, find enemy-free space in urban habitats.  Preservation of native tree species in hostile urban environments may require considerable anthropogenic inputs that outweigh their purported benefits.