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, and competition from 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.
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 increase in numbers as a function of fragmentation and can act to severely limit recruitment of interior species inhabiting the patches.
Todays 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.
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 creating and monitoring artificial nests over several days to monitor predation. To accomplish this, we will conduct this activity concurrently with other activities. Following a lecture on fragmentation and edge effects on songbirds (1 August 2000), the class will divide into several groups. Each group will design their own experimental set-up after consulting the instructors and the current scientific literature. These designs will be presented and discussed on 7 August after which the groups will proceed to their disturbed and control sites and set up their experiments. 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. Results and conclusions will be presented on 17 August 2000.
Some of the variables that should be considered are:
Given the unique set-up of this activity, it is not possible to assign time to specific tasks. However, the bulk of your time should be toward designing the experiment, reading the literature handed out, placing the nests in the field and monitoring predation. The actual monitoring (checking for predation) should not require a significant amount of time.
Evaluation will be based on:
This activity involves a lecture regarding fragmentation, edge effects on songbird populations, and designing field experiments. This will take place in the afternoon. The students are then given approximately one week to design their experiment. The designs are reviewed in class and the students are then sent out to place the nests in the field.
Authors: A. Hoylman, M. Gompper, and heavily based on the Gibbs et al. 1998. Edge Effects: Designing a nest predation experiment.