INDIRECT EFFECTS OF LARGE HERBIVORES ON SNAKES IN AN AFRICAN SAVANNA

Overview of Study

• Title: Indirect Effects of Large Herbivores on Snakes in an African Savanna

• Authors: Douglas J. McCauley, Felicia Keesing, Truman P. Young, Brian F. Allan, Robert M. Pringle

• Published in: Ecology, 87(10), 2006, pp. 2657–2663

• Objective: To investigate how the removal of large herbivores influences the abundance of olive hissing snakes (Psammophis mossambicus) in savanna ecosystems.

Key Findings

• The removal of large herbivorous mammals leads to an increase in small mammal density, which subsequently affects predator populations.

• Snakes were more abundant in areas devoid of large herbivores due to higher rodent populations, not due to differences in predation pressure on snakes.

• This study represents the first empirical evidence of indirect effects of large herbivores on snake populations in African savannas.

Indirect Effects in Ecology

• Dominant and Keystone Species: Have significant impacts on community structure and dynamics. Their interactions can lead to direct and indirect effects.

• Indirect Effects: Occur when changes in one species affect a second species through one or more intermediary variables.

• Other studies on indirect pathways often focus on trophic cascades, where predator changes impact prey populations multiple levels down the food web.

Context of the Study

• Habitat: Eco-initiatives are critical in areas experiencing declines in large herbivores due to factors like poaching and habitat destruction.

• Specific focus on Psammophis mossambicus, a prevalent predator of small mammals in the region.

Hypotheses

• Snakes benefit from increased prey (rodents) when large herbivores are removed, resulting in higher snake abundance in areas without large herbivores.

• The research aims to measure snake populations and understand the mechanisms driving changes in abundance.

Study Site Details

• Conducted at Mpala Research Centre, Laikipia District, Kenya (geographical location: 0°82'N, 36°83' E).

• Eco-system features include Acacia drepanolobium savanna and varying rainfall patterns impacting vegetation and animal populations.

• Consists of plots that exclude large herbivores and control plots with unrestricted animal access.

Methods

• Snake Surveys: Monitored using cover boards and standard measures to track snake sightings and individual captures over various sessions.

• Small Mammal Sampling: Implemented trapping sessions, noting rodent densities, particularly for Saccostomus mearnsi.

• Vegetation Analysis: Measured understory cover to gauge habitat availability for snakes. No significant differences were noted between plots.

• Raptor Surveys: Evaluated potential predation impacts by monitoring raptor populations in both study plot types.

Results

• Snake Abundance: Plots without large herbivores reported nearly double the snake sightings and individuals

• Rodent Density: Higher in areas devoid of large herbivores during both study years correlated with increased snake populations.

• Vegetation Cover: No significant differences in cover were observed between plots with and without large herbivores, suggesting potential factors influencing snake populations were not due to habitat quality alone.

• Raptors: No significant differences in the presence or activity of snake predators were observed between different plots, negating predation pressure as the primary factor for snake abundance variance.

Discussion

• Mechanisms Considered: Three primary mechanisms discussed concerning increased snake populations:

  1. Reduction in trampling by large herbivores.

  2. Predation pressure differences.

  3. Increased prey availability (rodents) leading to higher snake densities.

• Indications suggest that heightened rodent populations are the significant factor driving increases in snake density.

Conclusion

• Removal of large herbivores could enhance prey availability, illustrating an indirect impact on predator populations (snakes) through trophic cascades.

• The findings contribute to a broader understanding of ecosystem dynamics and the role of large herbivores within savanna food webs.

Acknowledgments

• Thanked various Kenyan authorities, research centers, and individuals whose support was instrumental for this study.