context. The pitfall trap activity we describe below is a cheap, simple,
and weeks-long investigation that provides an opportunity for students
to engage in QR-C while they are exposed to several ecological concepts and two data analysis techniques.
Within the topic of Conservation and Biodiversity in the IB Biology
course guide (International Baccalaureate Organization, 2014), students are expected to understand the biogeographic variables that
affect biodiversity, including edge effect, and be able to analyze
and compare “the biodiversity of two local communities using
Simpson’s reciprocal index of diversity” (p. 131). Although the
AP Bio CED does not mention edge effect specifically, students
are expected to understand that species composition and diversity
are used to measure and describe community structure. A study
of the edge effect phenomenon serves well to fulfill this objective
(College Board, 2015). Moreover, Learning Objective 4.12 in the
AP Bio CED suggests that teachers should provide students with
an opportunity to apply quantitative reasoning within the context
of an ecological phenomenon such as edge effect: “The student is
able to apply mathematical routines to quantities that describe
communities composed of populations of organisms that interact
in complex ways” (p. 87). An investigation of edge effect on diversity also allows students to employ several of the science practices
described in the AP Bio CED. Thus, the edge effect study described
here not only emphasizes an important focus on QR-C, but also
fulfills learning objectives in the IB Biology course guide, the AP
Bio CED, and likely most any college general biology lab course.
What Is Edge Effect?
Habitat edges are abrupt transition zones where communities from
adjacent and distinct habitats meet and interact (Holland & Risser,
1991). Edge effect theory, first popularized by Eugene P. Odum in
the first edition of his widely used textbook Fundamentals of Ecology
(Odum, 1953), states that due to the impact of biotic and abiotic
factors from multiple habitats, species diversity is expected to spike
at an edge along a transect that moves from one habitat to another
(Magura et al., 2001). Edge characteristics can also create distinct
microclimates that provide valuable resources to insects and other
small animals (Lö vei & Sunderland, 1996), which can contribute
to overall ecosystem function.
Human-created edges have been increasing for decades as a
result of habitat destruction and fragmentation (Saunders et al.,
1991; Haddad et al., 2015), and the resulting edge effect can negatively influence species survivorship and community stability.
Fragmentation increases the proportion of edge to habitat area
and can separate or isolate existing communities, disrupting gene
flow and genetic diversity (Sarre et al., 1995). While edges and
the interior habitats they outline tend to have distinct species
assemblages (Saunders et al., 1991), survivorship of habitat generalists is often favored due to increased adaptability (Elton, 1958;
Gibbs & Stanton, 2001). Thus, the study of edges has become
progressively more important to the field of conservation biology,
particularly with regard to the preservation of biodiversity.
The goal of the activities described below is to authentically and
effectively expose biology students to a multitude of ecological phenomena and quantitative reasoning opportunities in the context of
habitat edges and edge effect.
Teaching Diversity Indices
Quantitative Reasoning in Context (QR-C)
Nearly 100 years ago, scientists began applying mathematical models
in an attempt to describe and predict community interactions such as
predator–prey population dynamics (see Lotka, 1920). By the middle
of the 20th century, ecologists began to tackle the problem of describing and comparing community characteristics like diversity, especially
in the context of increasing human disturbance to ecosystems. Inventing new mathematical models and adapting existing models was necessary, and Simpson’s and Shannon’s diversity indices emerged as
useful quantitative solutions in the context of species diversity.
The species richness of a community is simply a count of the total
number of different species found in an area and is an easy concept for
most students to grasp. However, as teachers present the concept of
species diversity to students, they must keep in mind that species richness in a community can easily be mistaken as an indication of species
diversity, but richness does not account for the fact that the individuals among species in most communities are distributed unevenly.
Indeed, communities can be dominated by individuals from just a
few species while all other species are rare. Therefore, species diversity
indices account for both richness and evenness.
Have students consider the data in Table 1, which compares two
hypothetical communities of wood warblers, a type of songbird.
While both communities have the same species richness ( i.e., 10
wood warbler species), the individuals in community A are more
evenly distributed among the 10 species than the individuals in community B. In fact, the individual yellow-rumped warblers in community B occupy a greater proportion of the community than all other
species combined. Ask students about which community is more
diverse and why. To elicit ideas, teachers can ask students to also
think about and discuss in what ways the two communities might
function differently – for example, how competition for food might
look different in each community. Finally, before presenting the
Table 1. Distribution of individuals among 10
species of wood warbler in two hypothetical
Community A Community B
Yellow-rumped warbler 38 128
Palm warbler 24 8
Magnolia warbler 24 8
Canada warbler 20 6
Blackburnian warbler 18 6
Nashville warbler 18 4
Yellow warbler 16 4
Orange-crowned warbler 6 2
Wilson’s warbler 6 2