Chapter 17 - Lecture

Chapter 17: Species Interactions and Community Structure

Outline of Key Concepts

• Concept 17.1: A food web summarizes the feeding relations in a community.

• Concept 17.2: Indirect interactions between species are fundamental to communities.

• Concept 17.3: The feeding activities of a few keystone species may control the structure of communities.

• Concept 17.4: Mutualists can act as keystone species.

Introduction to Feeding Relationships

• Feeding relationships among organisms in a community are documented through food webs.

• Historically, the study of communities centered around identifying who eats whom, leading to the development of food web diagrams.

Community Webs

• Complexity of Food Webs: Early studies focused on simplified communities, but even in these, feeding relations are complicated.

• Comparative studies of diverse communities reveal that even limited species can exhibit complex feeding patterns.

Detailed Food Web Analysis

• Winemiller's research on tropical freshwater fish highlighted:

  • Representation of food webs that include only common species.

  • Exclusion of weak trophic links providing a clearer view but still yields complex webs.

  • Focus on strongest trophic links enhances understandability of food webs.

Strong Interactions and Food Web Structure

• Strong Interactions: Proposed by Paine, indicating that the feeding activities of a few species significantly influence community structure.

  • The strength of interactions should be recognized to accurately depict food webs.

Phragmites Food Web Study

• Tscharntke's research on wetland reeds detailed:

  • Interaction dynamics among the reeds, parasitic flies, and predator wasps.

  • Identification of weak versus strong interactions allows for pinpointing species with substantial ecological influence.

Indirect Interactions

• Distinction between Indirect and Direct Interactions:

  • Direct interactions (competition, predation) involve two species without intermediaries.

  • Indirect interactions occur when the effect of one species on another is mediated through a third species.

  • Examples include trophic cascades and indirect commensalism.

Indirect Commensalism Example

• Case Study: Beavers affect beetle growth through the creation of stump sprouts from felled cottonwood trees.

  • Beetles benefit from consuming sprouts, exhibiting growth advantages due to beaver activities.

Apparent Competition

• Definition: Negative interactions between two competitors facilitated by shared predators or herbiors.

  • Example: Brassica nigra (exotic) preventing native Nassella pulchra from thriving due to increased herbivory linked to predator dynamics.

Keystone Species

• Feeding activities of a few keystone species have significant impacts on community structure.

• Keystone species can help reduce competitive exclusion, promoting coexistence among different species in ecosystems.

Food Web Structure and Species Diversity

• Rising numbers of species in food webs increase represented predator proportions.

  • Greater predator numbers contribute to heightened predation pressure, boosting overall diversity within communities.

Experimental Insights on Keystone Species

• Paine's experiment involved the removal of the top predator, Pisaster, from intertidal zones:

  • Monitoring showed a decline in species diversity from 15 to 8 in removal plots, evidencing Pisaster’s role as a keystone species.

  • Community collapsed without Pisaster, demonstrating its crucial ecological function.

Grazing Effects on Algal Diversity

• Research Variables: Illustrated the relationship between intertidal snail populations (Littorina littorea) and algal diversity.

  • Herbivore preferences influence competitive dynamics of algal species, affecting their community structure.

Snails and Crabs Interactions

• High density of snails can lower populations of preferred algae, opening niches for less preferred species.

• Predation by green crabs affects juvenile snail populations, indicating a complex interplay among species.

Variations in Snail Density

• Density Effects:

  • Low Snail Density: Enteromorpha dominates.

  • Medium Density: Algal diversity increases.

  • High Density: Reduces diversity as snails consume preferred algae, thereby limiting mutual growth.

Fish Mass Impact on River Ecosystems

• Investigated the role of predatory fish (California roach and steelhead trout) in reducing algal densities, reinforcing their position as keystone species impacting food web dynamics.

Mutualistic Keystones

• Mutualistic species may also serve as keystones due to their significant impact on community structures despite low biomass.

A Cleaner Fish Case Study

• Coastal cleaner wrasse significantly affect coral reef ecosystems by removing parasites from fish:

  • Their absence can lead to a 24% decrease in fish species diversity; conversely, their presence may increase diversity by the same measure.

Seed Dispersal by Ants

• Christian’s research on seed dispersal illustrated the advantageous behavior of native ants in shrubland ecosystems:

  • Displacement by Argentine ants can lead to reduced plant recruitment and diversity by preying on seeds.

Review Summary

• Key concepts discussed include feeding relationships, community webs, indirect interactions, keystone species, and the significance of mutualists in maintaining ecological balance.