6.2 Niche Theory and Resource Partitioning

Fundamental vs Realized Niches

Niche theory explains how species coexist by using different resources or occupying different environments. Understanding niches is central to explaining why certain species live where they do, how communities are structured, and what happens when conditions change.

Resource partitioning is the process by which competing species divide up resources in space, time, or type, allowing them to live side by side with reduced direct competition.

Niche Concepts and Definitions

A species' fundamental niche is the full range of environmental conditions and resources it could potentially use if no competitors, predators, or other limiting factors were present. Think of it as the theoretical maximum.

The realized niche is what actually happens. It's the narrower range of conditions and resources a species ends up occupying once you factor in competition, predation, and other ecological constraints. The gap between fundamental and realized niches often results from competitive exclusion, where a stronger competitor pushes a weaker one into only a portion of its potential niche.

• Hutchinson's n-dimensional hypervolume conceptualizes niches as multidimensional spaces, with each axis representing an environmental variable or resource (temperature, food size, humidity, etc.)
• Niche breadth describes how wide a species' niche is along one or more of those dimensions. A broad niche means the species is a generalist; a narrow niche means it's a specialist.
• Niche overlap occurs when two or more species use similar resources or occupy similar conditions, which can intensify competition between them.

Niche Dynamics and Ecological Implications

Niche theory does more than describe where species live. It helps predict how species will respond to environmental changes like climate shifts or habitat loss, and it explains patterns of community composition and ecosystem functioning.

• Niche partitioning allows multiple species to coexist by reducing direct competition. For example, different bird species in a forest may occupy distinct vertical layers: warblers feeding in the canopy, thrushes in the understory, and sparrows on the ground.
• Niche complementarity contributes to ecosystem stability and productivity by maximizing how thoroughly available resources get used. In grasslands, plants with different root depths access water and nutrients at various soil levels, so the community as a whole captures more resources than any single species could alone.

Resource Partitioning for Coexistence

Resource partitioning is the differential use of resources by competing species. It reduces the intensity of direct competition and is one of the main mechanisms that allows biodiversity to persist within a habitat.

Types of Resource Partitioning

Spatial partitioning occurs when species use different areas within a habitat.

• Reef fish occupy different depths or coral zones on the same reef.
• Savanna herbivores graze in distinct areas based on vegetation height preferences: giraffes browse treetops, wildebeest eat mid-height grasses, and warthogs feed close to the ground.

Temporal partitioning involves species using resources at different times.

• In deserts, some rodent species are active during the day while others forage at night, reducing encounters and competition for seeds.
• Migratory bird species may use the same breeding grounds but arrive and nest during different seasons.

Dietary partitioning means species consume different food types or different parts of the same resource.

• Herbivorous insects specialize on different plant parts: some feed on leaves, others on stems, and others on roots.
• Grazing ungulates have varying mouthparts adapted for different vegetation types, so zebras crop tall, tough grasses while gazelles select shorter, more nutritious shoots.

Evolutionary and Ecological Consequences

Over time, resource partitioning can drive evolutionary change. Character displacement occurs when competing species develop increasingly distinct traits in areas where they coexist, reducing niche overlap. A classic example: on Galápagos islands where two finch species live together, their beak sizes diverge more than on islands where each species lives alone. This allows them to specialize on different seed sizes.

• Resource partitioning increases biodiversity and ecosystem stability by allowing more species to pack into a given environment.
• The degree of partitioning often correlates with how intense interspecific competition is and how diverse the available resources are.
• Partitioning promotes efficient use of resources across the ecosystem. For instance, different pollinator species visiting flowers at various times of day ensures more comprehensive pollination.

Interspecific Competition and Niche Differentiation

Competition Principles and Outcomes

Interspecific competition occurs when individuals of different species compete for the same limited resources, potentially reducing the fitness of one or both species.

The competitive exclusion principle (Gause's principle) states that two species competing for the exact same limiting resource cannot coexist indefinitely. One will either outcompete the other, driving it to local extinction, or the weaker competitor will shift its niche through differentiation.

• Ecological character displacement is the evolutionary result of this pressure: species develop distinct traits to reduce competition. Body size differences in weasel species, for example, allow them to target different prey sizes.
• The "ghost of competition past" refers to cases where current niche differences between species were shaped by historical competition that is no longer directly observable. The species look like they don't compete now, but their separation is evidence that they once did.
• Interspecific competition also shapes species abundance distributions within communities, often producing competitive hierarchies with dominant and subordinate species. In plant communities, tall canopy trees may suppress shade-intolerant species below them.

Competitive Strategies and Community Dynamics

• Competition-colonization trade-offs influence community dynamics. A species that's a superior competitor is often an inferior colonizer, and vice versa. Fast-growing weedy plants colonize disturbed areas quickly but get outcompeted over time by slow-growing, competitively dominant trees.
• The strength and outcome of competition vary depending on environmental conditions, resource availability, and the presence of other interacting species.
• Apparent competition is an indirect form: two species don't compete for resources directly, but they share a common predator or parasite. If one prey species boosts the predator's population, the other prey species suffers increased predation as a result.
• Facilitation can sometimes emerge from competitive interactions. One species may indirectly benefit another through its competitive effects on a third. Nurse plants in arid environments provide shade and improve soil moisture, enabling other species to establish in spots they otherwise couldn't survive.

Niche Theory in Community Assembly

Species Distribution and Biogeography

Niche theory provides a framework for predicting where species can and cannot live, based on their environmental requirements and tolerances.

• Species distribution models use niche concepts to forecast potential ranges. Climate envelope models, for instance, project how species' ranges may shift as temperatures and precipitation patterns change.
• Niche conservatism is the tendency of species to retain their ancestral ecological characteristics over evolutionary time. This helps explain biogeographical patterns: tropical plant families often maintain similar climatic niches even across different continents, because they haven't evolved tolerance for drastically different conditions.

Community Assembly Processes

Community assembly rules describe the processes that determine which species end up coexisting in a given environment. Niche-based models emphasize two key filters:

1. Environmental filtering removes species that can't tolerate local conditions. Alpine plant communities, for example, are assembled from species with cold tolerance and adaptations to high altitude.
2. Limiting similarity prevents species that are too ecologically similar from coexisting, because competition between near-identical niches is too intense.

Not everyone agrees that niches explain everything. Neutral theory of biodiversity challenges niche theory by arguing that dispersal ability and random demographic events (births, deaths, immigration) can explain many community patterns without invoking niche differences. Species-area relationships in island biogeography, for example, can be partly explained by neutral processes.

• Niche theory also helps explain invasive species dynamics. Zebra mussels successfully invaded North American freshwater ecosystems in part because they filled a niche that native species weren't fully exploiting.
• Metacommunity theory integrates niche concepts with spatial dynamics, explaining community structure across multiple scales. In pond networks, for instance, local environmental conditions sort species into suitable habitats, while regional dispersal connects populations across the landscape.