Ecological field techniques such as transect surveys are long-used, integral means of immersing students in field exercises to illustrate ecological measures. Many species of crayfish create conspicuous burrows within or near aquatic habitats. Such burrows can easily be identified and measured by students during an ecological field exercise. In this article, we describe a field activity we developed as part of a college-level course in which students utilized transects and calipers to collect counts and measurements of crayfish burrows in order to evaluate their distribution and size among different substratum types along a small stream. This field exercise could be incorporated, with or without modification, into an applicable high school or introductory/intermediate college biology course as a means of illustrating ecological concepts, sampling technique, and/or behavioral biology.
Ecological field techniques such as transect surveys are essential, integral means of immersing students in simple but meaningful ecological field exercises that can provide students at any level of education with hands-on activities to illustrate concepts such as species distribution or general abundance (Brown, 1959; Hamilton, 1988). Additionally, ecological field exercises can be an excellent vehicle to inexpensively challenge high school students with Next Generation Science Standards objectives such as the integration of core concepts and themes (Craven et al., 2019). Many species of crayfish create some form of burrow (Hobbs, 1981; Berrill & Chenoweth, 1982) that can be easily identified and measured by students in order to collect data to illustrate ecological concepts and techniques. Crayfish are typically classified among three categories in regard to burrowing ecology: primary, secondary, and tertiary burrowing species. Primary and secondary burrowing crayfish construct intricate burrow systems (e.g., see Hobbs, 1981, fig. 10), which these species may occupy for a portion or for nearly all of the year. These burrows typically terminate in a conspicuous “chimney” (Figure 1A) consisting of excavated sediment in the form of pellets surrounding the burrow entrance. Tertiary burrowing crayfish species live in open waters and typically create simple burrows (Figure 1B) within the benthic substratum or along the banks of ponds, streams, and rivers (Hobbs, 1981). Although these species may only burrow during certain environmental conditions or life history events (e.g., in winter or during summer drought, or when brooding eggs; Hobbs, 1981; Berrill & Chenoweth, 1982), burrows typically remain present and can be observed/measured outside of these times of active occupancy.
Although crayfish have long been used as a model organism in the biology laboratory (e.g., Strayer, 1969; Rop, 2010) we are not aware of any published work describing the integration of crayfish burrow measurement into a field exercise as a component of a biology course. However, such an exercise could be usefully and practically integrated into biological coursework, particularly in regions in which crayfish are diverse and common, such as eastern North America. Additionally, studying local natural history phenomena such as crayfish burrows is an excellent means of engaging students in place-based biological education (Smith, 2002). Here, we describe a field exercise that we developed and implemented within a college-level invertebrate zoology course. It could be assimilated and modified as a component of zoology, ecology, or introductory biology coursework.
Preparation & Materials
Prior to conducting the field exercise with students, the instructor should identify a location in which crayfish burrows occur in sufficient densities for measurement by students. The instructor might consult regional texts or primary literature regarding crayfish species within their region to aid in selecting a suitable site for students to measure crayfish burrows. For our course, we selected a small stream near our institution in which the common crayfish (Cambarus bartonii bartonii), a tertiary burrowing species that typically constructs simple burrows both in streams and along stream banks (Figure 1B; Ortmann, 1906), was determined to be present. To select our site, we conducted cursory surveys of several locations and ultimately chose this site because of its accessibility (i.e., public access) and its abundance of burrows. Instructors should consider regional regulations and the conservation status of crayfish species within their region when selecting a site. For instance, because our selected site was within an area managed by a state agency, we needed to submit a brief project proposal to this agency prior to visiting the site with our students to collect data.
For this field exercise, we split our students into several groups consisting of three students each. Each group was provided a 50 m transect tape, 1 m ruler, clipboard, pencil, and paper for recording data, as well as a set of calipers for measuring burrow diameter and depth. Given the nature of our study site (along a small stream) and the nature of the burrows studied (those of a tertiary burrowing crayfish), our students sampled burrows within 0.5 m of either side of a transect line placed adjacent to the stream. However, depending on the objectives of the instructor, the characteristics of the study site, and the burrowing classification of the crayfish species studied, quadrats could be used in addition to or in lieu of transects for estimating densities of crayfish burrows.
We note that because crayfish burrows tend to occur within or near aquatic environments and soft (muddy) substratum, students should be instructed to wear shoes and clothing that can get wet/dirty and to wear boots or other wading footwear if available/applicable. Additionally, although this exercise is minimally invasive toward the crayfish, note that some caution must still be undertaken by students and instructors to minimize any impacts on crayfish habitat, particularly because crayfish burrows may also be utilized by other organisms, such as certain herpetofauna (Hulse et al., 2001). For instance, prior to taking students to the site, we reminded them to step carefully and to be mindful to avoid incidentally stepping on crayfish burrows or riparian plants, and to reduce sedimentation by avoiding stepping in muddy areas adjacent to the stream.
Following a brief introductory talk on site regarding an overview of crayfish burrowing ecology and behavior, we led students along the stream and casually pointed out crayfish burrows to familiarize students with their sampling subject. We organized students into groups consisting of three members each and assigned group members tasks: one student assumed the role of a data recorder and the other two students were responsible for counting/measuring burrows. We then provided instruction/demonstration regarding transect setting and burrow measurement. Students subsequently selected a stretch along the stream that did not overlap with that of another group to measure burrows and subsequently laid out a 50 m transect along the stream, immediately parallel to the water’s edge (Figure 2). Students then walked along the transect and tallied all burrows observed within a 0.5 m area perpendicular to each side of the transect line. Burrows were tallied in terms of the number of burrows occurring within the stream or on the stream bank (Table 1), as well as the total number present among three substratum types (rocky substrate, in mud/undercut soil, or within vegetation; Table 1). Students then measured the diameter and depth of burrow openings to the nearest millimeter with calipers.
|Burrow no. .||Location .||Burrow Substrate .||Burrow Diameter (mm) .||Burrow Depth (mm) .|
|Burrow no. .||Location .||Burrow Substrate .||Burrow Diameter (mm) .||Burrow Depth (mm) .|
Assessment, Feedback & Suggestions
We provide examples of data that were collected during our study (Table 1) as a template for instructors wishing to adopt a similar exercise to the one we describe here. However, we note that this exercise can easily be modified in terms of different variables that could be collected to investigate different aspects of crayfish burrow biology (for example, water depth, velocity, or other variables could be measured at submerged burrows), depending on the objectives of the instructor and characteristics of the habitat in which the crayfish burrows occur (e.g., stream vs. wetland habitat). We recommend that instructors visit the site prior to having a preliminary discussion with students to determine which variables they deem appropriate to measure.
Ideally, following data collection, students would perform analysis and interpretation of data collectively as a class, in a manner that depends on the scope and objectives of the activity in relation to the course. For instance, our students performed summary statistics of their data and simple statistical tests, such as a chi-square test of goodness-of-fit to examine whether burrow counts differed among substrate types. Our students also performed statistical tests to examine whether mean burrow sizes (diameter or depth) differed among substrate types and whether burrow diameter and depth were statistically correlated. Students could also examine other factors, such as assessing the distribution of burrows (i.e., clumped, random, or uniform) in an ecological context. Following data analysis, we then had our students interpret their results in the form of a group laboratory report as well as a group poster presentation, which was disseminated at a local scholarly meeting. Our students were highly engaged in the activity and demonstrated an understanding of the sampling technique during a mid-laboratory discussion in which they discussed the benefits and limitations of the sampling methods used in the field exercise. Students also demonstrated success in placing the activity in the broader context of their coursework (in invertebrate zoology) by asking relevant questions and facilitating brief discussions regarding the burrowing habits of crayfish and other organisms.
Minus travel time, this activity took approximately two hours to complete and could potentially be done within the span of a typical undergraduate biology laboratory period. However, this exercise could be completed during a field trip scheduled outside of regular class time if desired. Our group was small, consisting of approximately a dozen students. However, we believe that classes with higher numbers of students (e.g., up to 30) could easily accommodate this exercise, and even larger groups could be taken into the field in rotations, in subgroups with proper supervision, or in multiple trips to complete this activity. Some sites may easily lend themselves to students dispersing and sampling numerous adjacent locations. Other sites may have topography suitable only for small groups. Although we incorporated this field exercise as part of an upper-level invertebrate zoology course, it is relatively simple to complete and could easily be incorporated into a high school or college introductory biology/ecology course as a practical means of introducing ecological concepts and sampling technique regarding distributions and densities. Additionally, although the exercise described here is relatively simple, it could easily be modified by the instructor to achieve other desired learning objectives regarding ecological measures or other aspects of biology illustrated by measuring crayfish burrows. Techniques such as transect surveys and quadrats have long been a component of field exercises in biology courses, and the use of crayfish burrows as a study subject for laboratories, as described here, may be a useful component with which to achieve these lessons.
We thank the Bloomsburg University College of Science and Technology for support and our invertebrate zoology students for their enthusiasm during our pilot test of this activity. Crayfish burrow fieldwork described here was approved by the Pennsylvania Department of Conservation and Natural Resources (SFRA-1904).