A food web serves as a crucial conceptual framework, visually representing the intricate feeding relationships among different species within a community. By doing so, it elucidates species interactions, community structure, and the dynamic processes of energy transfer within an ecosystem.
Introduction to Food Webs
The food web is a fundamental concept in ecology, essentially illustrating the feeding relationships in a community (Smith & Smith, 2009). It depicts how food energy flows, originating from plants, through herbivores, and ultimately to carnivores (Krebs, 2009). Food webs typically consist of interconnected food chains. Each food chain represents a series of organisms where energy is transferred from one to another through feeding.
There are two primary types of food chains: grazing and detrital (Smith & Smith, 2009). Grazing food chains start with autotrophs (plants), while detrital food chains begin with dead organic matter. In a grazing food chain, energy moves from plants to herbivores and then to carnivores. A detrital food chain involves the decomposition of dead plant and animal matter by decomposers like bacteria and fungi, followed by its transfer to detritivores and then carnivores.
Food webs are essential for understanding ecological interactions, energy flow, and predator-prey relationships (Cain et al., 2008). Consider a simplified food web in a desert ecosystem (Figure 1). Grasshoppers consume plants, scorpions prey on grasshoppers, and kit foxes prey on scorpions. While this is a basic example, most food webs are complex, involving numerous species with varying degrees of interaction (Pimm et al., 1991). For instance, scorpions in a desert ecosystem may be preyed upon by golden eagles, owls, roadrunners, or foxes.
Alt text: Desert food web diagram showing energy flow from plants to grasshoppers, scorpions, and kit foxes.
Charles Elton first proposed applying food chains to ecology in 1927, recognizing their typical length of four or five links and their interconnected nature forming food webs. These feeding interactions have significant impacts on community species richness, ecosystem productivity, and stability (Ricklefs, 2008).
Types of Food Web Models
Food webs illustrate relationships among species, but these relationships vary in their importance to energy flow and species population dynamics. Robert Paine identified three types of food webs based on species interactions in a rocky intertidal zone: connectedness webs, energy flow webs, and functional webs (Ricklefs, 2008).
- Connectedness Webs (Topological Food Webs): Focus on feeding relationships, portraying them as links.
- Energy Flow Webs: Quantify energy flow between species, where arrow thickness indicates the strength of the relationship.
- Functional Webs (Interaction Food Webs): Represent the importance of each species in maintaining community integrity and reflect their influence on the growth rate of other species’ populations.
For example, limpets in a community might consume considerable food energy (energy flow web), but their removal may not significantly impact the abundance of their resources (functional web).
Alt text: Comparison of connectedness, energy flow, and functional food webs in a rocky intertidal zone.
Applications of Food Webs in Ecological Studies
Food webs are valuable tools with numerous applications in ecological research.
Describing Species Interactions
Food webs primarily describe feeding relationships, categorizing species into basal species (autotrophs), intermediate species (herbivores and intermediate-level carnivores), and top predators. These categories represent different trophic levels. Basal species are primary producers, converting inorganic chemicals and solar energy into chemical energy. Herbivores form the second trophic level as primary consumers, while carnivores at subsequent levels consume animals from lower levels. Grouping species into functional groups or trophic levels simplifies understanding species relationships.
Illustrating Indirect Interactions
Indirect interactions occur when species influence each other without direct contact, often mediated by a third species. Keystone predation, demonstrated by Robert Paine, exemplifies this. In an experiment in the rocky intertidal zone, Paine showed that the predator starfish Pisaster influenced competition among species. Removing the starfish led to a decrease in prey species due to the dominance of superior competitors like mussels and barnacles. This type of indirect interaction is called keystone predation.
Alt text: Rocky intertidal zone food web illustrating keystone predation by starfish on various invertebrate species.
Another study demonstrated indirect interactions in aquatic and terrestrial ecosystems. Knight et al. (2009) found that fish in ponds decreased dragonfly populations, which in turn reduced pollinators and seed production in nearby plants. This complex trophic cascade showed that adding fish to a pond could improve the reproductive success of land plants (Ricklefs, 2008).
Alt text: Interaction food web showing indirect effects of fish on terrestrial species through trophic cascades.
Studying Bottom-Up and Top-Down Control
Food webs also help study bottom-up and top-down control of community structure. Bottom-up control suggests that productivity and abundance at a trophic level are influenced by the level below. Correlations between consumers and their resources support this. Top-down control, on the other hand, occurs when a consumer controls the density of its resource. Trophic cascades, a type of top-down interaction, describe the indirect effects of predators.
Revealing Energy Transfer Patterns
Energy flow patterns differ in terrestrial and aquatic ecosystems. Food webs reveal these differences. Shurin et al. (2006) showed that the turnover rate of phytoplankton is much faster than that of grasslands and forests, leading to different carbon storage and consumption patterns. Detrital food chains dominate in terrestrial ecosystems with high biomass, while grazing food chains are more prevalent in deep-water aquatic ecosystems.
Alt text: Comparison of carbon flow pathways and pools in aquatic and terrestrial ecosystems, highlighting differences in biomass and consumption rates.
Conclusion
Food webs are a powerful diagrammatic tool for illustrating species interactions and testing research hypotheses. They continue to be invaluable in understanding the relationships between species richness, diversity, food web complexity, ecosystem productivity, and stability. By visualizing these complex relationships, scientists can better predict how changes in one part of an ecosystem might affect the entire system.
References
Cain, M. L., Bowman, W. D. & Hacker, S. D. Ecology. Sunderland MA: Sinauer Associate Inc. 2008.
Cebrian, J. Patterns in the fate of production in plant communities. American Naturalist 154, 449-468 (1999)
Cebrian, J. Role of first-order consumers in ecosystem carbon flow. Ecology Letters 7, 232-240 (2004)
Elton, C. S. Animal Ecology. Chicago, MI: University of Chicago Press, 1927, Republished 2001.
Knight, T. M., et al. Trophic cascades across ecosystems. Nature 437, 880-883 (2005)
Krebs, C. J. Ecology 6th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.
Marquis, R. J. & Whelan, C. Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects. Ecology 75, 2007-2017 (1994)
Molles, M. C. Jr. Ecology: Concepts and Applications 5th ed. New York, NY: McGraw-Hill Higher Education, 2010.
Paine, R. T. The Pisaster-Tegula interaction: Prey parches, predator food preferences and intertide community structure. Ecology 60, 950-961 (1969)
Paine, R. T. Food web complexity and species diversity. The American Naturalist 100, 65-75 (1966)
Paine, R. T. Food webs: Linkage, interaction strength and community infrastructure. Journal of Animal Ecology 49, 667-685 (1980)
Pimm, S. L., Lawton, J. H. & Cohen, J. E. Food web patterns and their consequences. Nature 350, 669-674 (1991)
Power, M. E. Top-down and bottom-up forces in food webs: do plants have primacy? Ecology 73, 733-746 (1992)
Schoender, T. W. Food webs from the small to the large. Ecology 70, 1559-1589 (1989)
Shurin, J. B., Gruner, D. S. & Hillebrand, H. All wet dried up? Real differences between aquatic and terrestrial food webs. Proc. R. Soc. B 273, 1-9 (2006) doi:10.1098/rspb.2005.3377
Smith, T. M. & Smith, R. L. Elements of Ecology 7th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.
