Investigating Effects of Invasive Species on Plant Community Structure

Abstract

Ever wondered what was meant by the old adage Can't see the forest for the trees? For my students beginning their study of ecology, the expression takes a new turn: Can't see the trees for the forest. They explain that when they step into a forest they see a big interrupted by brown tree trunks. Bringing the green (or what Wandersee and Schussler [1999] call blindness) into focus is one of the driving forces behind the following set of activities. Is the blur one species of plant or many? What, if anything, is influencing the number and distribution of the plants? How does one distinguish between randomness and organization in plant communities? Those questions become the basis for a series of laboratory periods devoted to a field study exploring factors influencing forest community structure. Students have the chance to learn local plant species, investigate issues of invasive species, use the line-intercept method of plant sampling, gain experience in experimental design, practice summarizing data with descriptive statistics and, depending on their level, learn more elaborate statistical tests. When I began planning a field project to introduce undergraduate biology students to ecological concepts, processes, and methodology, I wanted a real problem that would engage them. Fortunately, wild lands and suburban landscapes all over the United States are ripe with just such a problem. Escaped horticultural plants and introduced European and Asian species like English Ivy or Kudzu vine have come to dominate many unmanaged habitats. Invasive species, as they are known, are defined variously in the literature as non-native or introduced species that become established outside of their home range and typically displace or reduce populations of native species (c.f. Campbell & Reece, 2005). The United States Department of Agriculture (USDA, 2008) defines them as: a species that is 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Worldwide, an estimated 80% of endangered species could suffer losses due to competition with or predation by invasive species (Pimentel et al., 2005). This begs the question, what impact will invasive species have on ecosystem services. Healthy ecosystems supply valuable services to humans such as cleaning our water supply, stabilizing topsoil, and supplying agricultural and timber products to our economy. (Vitousek, 1990; Zavaleta & Hulvey, 2006). The disruption of these crucial services could result in large economic losses. Infact, the estimated damage and control cost of invasive species in the U.S. alone amount to more than $138 billion annually (Pimentel et al., 2005). It is in this context that students begin the first of three labs. Lab 1: Preliminary Field Trip to the Study Site The first three-hour laboratory session begins with a short walk to the study site. It is a small (approximately one acre) forested area located on the campus of Bryn Mawr College in southeastern Pennsylvania. The unmanaged woodland is surrounded by residential gardens and, as a consequence, has become a sanctuary for escaped horticultural plants as well as accidentally-introduced plants; the most pervasive of which are Pachysandra and English Ivy. After an initial trip to the study site to learn the dominant plants and qualitativelydescribe the area, we discuss--first in small student teams and then as a class--all the possible factors influencing plant community structure at this site. Moisture and light gradients are the abiotic factors that the students postulate most affect the forest structure. Biotic factors that might influence the forest plant community are a bit more elusive for most students. However, if I ask specifically about plant-animal interactions that might be affecting structure, many will respond that deer-grazing could be a factor. …