This is a guest post by MS graduate student Christina Mitchell
Most people think of a forest as any habitat with trees. But what really makes a forest? Is it the size, number, or species of tree? Is it the ability to sequester carbon, or release oxygen, or filter water? Maybe a forest is defined by the organisms that live within there. Sometimes certain species, called indicator species, can define an ecosystem. For example, wetlands are characterized by plants, like bald cypress, that can tolerate extremely wet conditions. But what about forests, do they have indicator species? Organisms ranging from large mammals to tiny hexapods can affect how forests function, but what defines a forest?
As a Masters’ student in the Frank lab, I study relatively small organisms called ground beetles. Ground beetles, in the family Carabidae, are fast moving beetles that typically travel along the forest floor in search of food and mates. Ground beetles are used to monitor forest health because they are common, vary in food and habitat requirements, and are sensitive to human-caused disturbances.
Urbanization is one of the most common types of human-caused disturbance. One definition of urbanization is a profound modification of original habitat with the loss of most original plant and animal species, often accompanied by replacement of native species by nonnative ones (Elek and Lovei 2007). Urbanization also includes gray infrastructure, or impervious surfaces such as buildings and roads. Urban forests are often smaller in size, surrounded by more impervious surface, and more invaded by nonnative species, which can negatively affect their quality for native species.
For my research, I am interested in how the increase of nonnative plant species in urban forests might affect the communities of ground beetles. I hypothesized that forest invasion by nonnative plant species would increase the amount of ground beetles that prefer open habitats, because the nonnative plants change the structure and function of forests. To test this hypothesis, I sampled the vegetation and ground beetle communities in 12 forest fragments around Raleigh, NC and Newark, DE.
Christina installing a pitfall trap for catching ground beetles (photo: R. David Mitchell) and an example of a pitfall trap made from a plastic cup set in the ground so the lip is level with the soil (photo: J. Christina Mitchell). The cup is filled with a nontoxic solution (50/50 water and propylene glycol) for catching and preserving the ground beetles. The cup is covered with plywood to prevent debris and rainwater from getting into the cup, while allowing the beetles to walk underneath.
Urban forests have different characteristics than rural forests. For example, urban forests usually have less leaf litter, which is a crucial component of ground beetle habitat. We found that where the number of nonnative plant species is high, there is significantly less leaf litter.
Invaded by nonnative plants, urban forests have a dense understory. This blocks native tree saplings from growing, which causes urban forests to also have less canopy cover. Invaded urban forests have more ground beetle species that prefer an open habitat, like grasslands, and are not true forest species. We also caught less beetles, and less predatory species, in urban forests compared to more rural forests.
This study is important because it quantifies the relationship between nonnative plant species and their effect on native ecosystems. I am using carabid beetles as indicator species to understand the complex relationship between landscape effects and urban forest function. If a forest is defined by the organisms that live in them, this research shows that nonnative plant species change that definition. These findings support the idea that improving remnant habitats in urban areas, by removing nonnative species, may help to conserve forest-specialist ground beetles and other native plants and animals.