Atlantic Forest

Exercise 12: Recurring Disturbances and the Risk of Egg Predation in Edge Habitats
Module 12: Disturbances - Geological and Contemporary

Your Questions

1. How do the ongoing disturbances in edge habitats place pressures on the species using the habitat in areas that have become fragmented?
2. More specifically, how does fragmentation and its associated edges influence egg survival?

Background

Human alteration of large areas of continuous habitat often results in fragments or patches of native habitat surrounded by edges. Species living in the remaining fragmented habitat can be negatively impacted by these edges due to increased contact with humans, elevated temperatures and drying from winds (explored in Exercise 9), and competition with or predation by edge-exploiting species. In addition, because the area of the habitat is reduced, population sizes are smaller and therefore more likely to go extinct in any particular patch.

These fragmented areas can also be thought of as having recurring disturbances, because the habitat is fragmented due to human activity. When we move around or live in an area, we bring with us a great diversity of non-native associated animals such as cats, dogs, rats, and a great diversity of non-native birds. These animals can be thought of as ongoing disturbances in that they greatly impact on the native flora and fauna.

Amongst the best-documented effects of fragmentation is the decline of songbird populations in formerly forested areas. The mechanisms by which this occurs are varied but one of the primary problems appears to be egg or nest predation along edges. This predation occurs when generalist edge-exploiting species (birds and mammals together) increase in numbers as a function of fragmentation and can act to severely limit recruitment of interior species inhabiting the patches. Similarly, the non-native species that are associated with humans are most active in these edges and often prey upon the native species that are least equipped to cope with the pressure. Songbirds that nest on the ground are particularly strongly impacted by these ground-dwelling predators.

Today’s activity is based on the idea that if you place many baits at different distances from an edge and monitor the portions of them that are consumed, these portions will constitute an index of the predation pressure from potential bird nest predators.

Your Assignment

Our goal is to determine if the processes associated with fragmentation such as habitat change, edge effects and borders within formerly continuous habitat influence the probability of predators finding a song bird nest.

This activity will require monitoring the artificial nests that have already been constructed by the TAs to monitor predation. The class will divide into several groups during the day before we do the experiment and each group will design their own experimental set-up after consulting the instructors and the current scientific literature. Examples of edges we can select include those caused by fire, development and construction in the forest, roads adjacent to the forest, parking lot construction, and edges along water. The design for the experiment that the instructors have followed will not be made available to these groups so that each group can come up with their own design. We will present and discuss these designs at in class the day before the activity.

The next day, the class will proceed to the disturbed and control sites that were set up by the TAs. Results and conclusions will be presented in class in the afternoon.

Some of the variables that should be considered are:

1. type of bait
2. location of study
3. number and distribution of bait
4. execution of study
5. placement of controls
6. analysis

Given the unique set-up of this activity, it is not possible to assign time to specific tasks. However, the bulk of your in-class time should be toward designing the experiment, and researching the question online. The actual monitoring of the nests on the day of the activity and checking for predation should not require a significant amount of time.

Evaluation will be based on:

1. Design and execution of experiment
2. Oral summarization of each groups’ analyses the day before.
3. Participation in discussion of other group presentations. Based on your analyses, do you agree or have additional insights?

Objectives

1. Familiarity with current literature on fragmentation effects on songbird populations.
2. Designing and conducting a field experiment.
3. Determining experimental variables, constraints and limitations.

Key Skills

1. Increased competence collecting field measurements.
2. Designing, carrying out, and analyzing the results of a field experiment

Timetable

1. Total elapsed time to perform the experiment: 2 weeks.
2. Total elapsed hands-on time by the class: 7 hours

• Lecture and discussion (2 hrs day before)
• Monitor egg survival (2 hrs)
• Analysis of results (2 hrs)
• Write-up and discuss results (1 hr).

Procedural Notes

1. In order to monitor predation through time, this activity requires that the TAs set up and monitor the nests two weeks before class. Students will be responsible for designing the field experiment in-class the day before. On the day of the activity, the students will present their results to the class and discuss their results.
2. This lab, based on Gibbs et al. (1998), is an opportunity for the students to design their own experimental system. How can this be done given the startup time needed (Gibbs suggests the experiment run 2 wks in order to get some results). Each group will present their design at the end of the day before's discussion - we'll discuss each design then and will then discuss how the experiment was set up.
3. We have found that even setting the eggs out for a week has been sufficient to get interesting results. If desired, the activity could be changed slightly so that groups of students determine what specific question that they wish to answer, then they put out the eggs, and collect the data one week later.

Materials Needed

1. Flagging to mark nests
2. Small paper bowls to act as nests
3. several dozen small bird eggs - probably from the codorna (a small gallinaceous bird like chickens - photos below include their eggs in the lower left and upper right of the photo on the right)
4. Record sheets