Animals and Food Webs

Introduction to Species Distribution in Ecosystems

The exploration of animal presence in various ecosystems, particularly in streams, begins with the question of what controls species distribution within these systems. A pivotal paper describes several filters and scales that affect species placement, emphasizing various factors at play in determining their locations.

Factors Controlling Species Distribution

Species distribution in an ecosystem is influenced by a combination of both abiotic and biotic factors. The following points elaborate on these:

1. Abiotic Factors

Definition: Non-living chemical and physical elements in the environment that influence the potential range of species.
Examples: Climate significantly affects habitat range, which species may occupy based on historical biogeographic conditions and defines the regional species pool.
Key Points:
- These factors are primarily responsible for controlling species distribution at larger scales.
- Important elements include temperature, dissolved oxygen levels, pH, and salinity.

2. Biotic Factors

Definition: Living components of the environment and their interactions that affect community structure.
Examples: Competition, predation, and community dynamics significantly influence species presence at local scales.
Key Points:
- Within a given region, local diversification occurs.
- The influence of predation and competition on community dynamics varies relative to the abiotic factors.

Filters and Scales Influencing Species Presence

The understanding of species distribution can be illustrated through a series of filters:

Super Coarse Filter: All fish species.
Freshwater Filter: Narrowing down to only freshwater fish.
Geographical Filter: Further filtering concerning geographical limitations.
Regional Filter: Identification of the regional community.
Physiological Screening: Factors like oxygen levels and acidity that further refine which species can thrive in this community.
Community Fauna: Final step recognizing the local community composition.
- The conceptual framework allows consideration of various scales, whether it's an individual stream reach or an entire stream network.

Importance of Range and Filters in Research

The concept of constraining ranges in research data is elaborated upon:

Condition Constraining: When the range of conditions evaluated for a species' response variable is too narrow, significant responses may not be observed.
End Members: Identifying conditions at the extremes of a distribution becomes crucial in continuous variable assessments, influencing the understanding of biological responses.

Understanding Niches

The niche concept is essential in determining "where species are and why". Four major definitions of ecological niches are:

Grinnellian Niche (1917): Defines the niche as the habitat or environment a species can occupy based purely on its biological needs.
Eltonian Niche: Considers the species' role within the community context.
Hutchinson's Niche: A widely used definition that includes both physical and biological conditions necessary for stable or increasing populations. It highlights that just existing in an area does not mean the site meets the full niche requirements.
McArthur and Levins' Definition: Views niche in terms of spatial resource use by species within the landscape.

Hutchinson's Niche and Competitive Exclusion

Hutchinson’s insights include:

Concept of Competitive Exclusion: No two species can occupy the same niche simultaneously. Competition and predation are vital influencing factors on how species coexist in similar environments.

Distinction Between Fundamental and Realized Niches

Understanding the difference between these niches adds depth to the discussion:

Fundamental Niche: The theoretical range of environmental conditions a species can occupy in the absence of interspecific interactions (concept related to Grinnell's niche).
Realized Niche: The actual conditions in which a species finds itself, shaped by competition, predation, and other biotic interactions.
- Examples show that the realized niche is often more limited than the fundamental niche due to interspecific competition and environmental constraints.

Case Study: Chinook Salmon

Chinook salmon serve as a compelling example to illustrate these niche concepts:

Historic Range: Originally only found in saltwater.
Introductions: Introduction to the Great Lakes demonstrated their ability to thrive outside their assumed fundamental niche, which was initially much broader than previously understood.
- This observation highlights how human influence complicates the understanding of species niche dynamics.

Summary on Potential and Realized Niches

The potential niche illustrates the ideal conditions for a species' survival, and various factors constrain this potential niche down to the realized niche.

Food Web Dynamics in Stream Ecosystems

Once the species distribution is understood, the next step is identifying where species lie within stream systems:

1. Dominance in Stream Habitats

Kurt Fowcher's Work: His research suggests that dominant fish establish clear hierarchies based on size and hierarchy relative to energy gains.

2. Experimental Setup and Findings

Experimental Design: Fowcher set up an experimental channel featuring riffles and pools, introducing a food source (insects) to observe fish placement based on energetic profitability.
Energy Considerations: The relationship between water velocity, energy availability, and distance from food sources highlights where fish can optimally position themselves for growth.

Conclusion

The research showcased by Fowcher elegantly illustrates the balance between energetic costs and food resource availability, revealing how fish populations select habitats with maximum growth potential. This understanding of species distribution, niche dynamics, and food web interactions in stream ecosystems is vital for ecological studies and conservation efforts.