14  Ecological Niche

“The total of the adaptations of an organismic unit.”

14.0.1 Definition

Ecological niche is a term for the position of a species within an ecosystem, describing both the range of conditions necessary for persistence of the species, and its ecological role in the ecosystem.

• Subsumes all of the interactions between a species and the biotic and abiotic environment.
• Effected by food, space, and time.

14.0.2 Biological Meaning

Data on niches can be used to:

• Make comparisons of the composition and organization of communities.
• Examine shifts in the behavior or ecology of one species in response to another species.
  • Niche shifts are commonly used to study interspecific competition (see Gause’s Principle of Competitive Exclusion)

14.0.2.1 Hutchingson’s model of niche as a “hypervolume”

Niches can be described or defined by relating fitness or utilization to environmental variable (aiotic and biotic).

• A range if conditions defines by many axes, which defines the set of conditions under which the animal can survive and reproduce.
• We can easily illustrate a niche described by up to 3 variables, but for fourth axes and onward, the result becomes a hypervolume.

Be aware:

• not all niche axes are environmental - some niche axes are behaviroal (e.g. noctural vs. diurnal)
• not all axes can be ordered linearly (e.g. types of antipredator behavior)

14.0.2.2 Fundamental niche vs Realized niche

• Fundamental niche (FN) is the entire set of conditions under which an animal (a population, a species) can survive and reproduce.
• Realized niche (RN) is the set of condition actually used by given animal (a population, a species) after interactions with other species have been taken into account.
• FN ≥ RN
• RN may differ in populations, because of differences in competitors and predators.

14.0.2.3 Niche breadth

• Specialists have narrow niches, generalists have broad niches. These are relative terms.
• Describe the endpoints of continuous variation in the degree of specialization in resource use, behavior, and physiology.

LEVINS (1966) - Niche breadth quantification

\[B = \frac{1}{\sum{p_i^2}}\]

• \(p_i\) = proportion of individuals that use resource \(i\)
• Species that use many resources will have large value of \(B\), reflecting a generalist pattern of resource use.

14.0.2.4 Between or within phenotypes

• Given known niche breadth of a population, it is not necessarily clear how individuals within population use resources
  • All individuals use entire niche of population, within-phenotype component of niche breadth is large
  • Each individuals uses a narrow part of population’s niche, between-phenotype component of niche breadth is large.
• Individual’s niches may change dramatically during lifetime (e.g. metamorphosis, tadpoles are herbivores, frogs are carnivores)

14.1 Niche overlap

14.1.1 Types of overlap

• Coextensive, included, reciprocal overlap, asymmetric overlap
• Non-overlapping: abutting, disjunct

14.1.2 Jacobs (1974) - Electivity index

Electivity indices measure the utilization of food types in relation to their abundance or availability in the environment (lechowicz1982?).

\[ D = \frac{r-p}{r+p-2rp} \]

• \(R\): proportion of item used; \(P\): proportion of item available.
• Modified from forage ratio and Ivlev’s index (1961), Fig. 12 in Jacobs (1974).
• Jacob’s index is independent of the relative abundance of food types in the environment.
• Also purposing \(log Q\) index, both should be tested when a relation is studies between food selection and variables such as the relative abundance of food, light conditions, etc.
• Ranges from -1 to 1, where 0 as separator of positive or negative selection.

14.1.3 Pianka (1973) - Pianka’s niche overlap index

Modification of one presented in MacArthur & Levins (1967).

\[ O_{kj} = \frac{\sum p_{ij}p_{ik}}{\sqrt{\sum p^2_{ij}\sum p^2_{ik}}} \]

This formula is symmetric, i.e. \(O_{jk} = O_{kj}\)

• \(P\): proportion of item \(i\) used in relation to the total of items used by species \(j/k\).

14.1.4 Winemiller and Pianka (1990) - Randomization algorithm

Winemiller and Pianka (1990) explained:

[!mini] Standard statistical tests cannot be applied to comparisons involving three or more species because dietary overlaps are not independent.

The idea is to:

[!note] … generate different replicates of a randomized real system (i.e. pseudo-community, the null model) that can be constructed for comparison with an observed prototype. If say, 95 or 99 of 100 randomized pseudo-communities have mean niche overlaps greater than the observed, results are significant at the P < .05 or P < .01 level, respectively.*

The significance indicates that:

• Observed community exhibits a non-random pattern of resource utilization.
• Species in observed community are utilizing resources in a way that minimized competition.
• Evidence of niche differentiation or resource partitioning among the species in community.
• Evidence for competitive interactions shaping community structure or for past evolutionary adaptations.

4 different algorithm:

	Relaxed niche breadth (Sum value is redistributed)	Retained niche breadth (Values are shuffled, but not redistributed)
Zeros eliminated (categories with 0 are also reassigned with value)	RA1	RA3
Zeros retained (categories with 0 are reassigned)	RA2	RA4

14.2 Something else to read

In Memory of Eric Pianka, “The Lizard Man”: He Thought, He Cared, And He Tried

Jacobs, Jrgen. 1974. “Quantitative Measurement of Food Selection: A Modification of the Forage Ratio and Ivlev’s Electivity Index.” Oecologia 14 (4): 413–17. https://doi.org/10.1007/BF00384581.

LEVINS, RICHARD. 1966. “The Strategy of Model Building in Population Biology.” American Scientist 54 (4): 421–31. http://www.jstor.org/stable/27836590.

Pianka, Eric R. 1973. “The Structure of Lizard Communities.” Annual Review of Ecology and Systematics 4 (1): 53–74. https://doi.org/10.1146/annurev.es.04.110173.000413.

Winemiller, Kirk O., and Eric R. Pianka. 1990. “Organization in Natural Assemblages of Desert Lizards and Tropical Fishes.” Ecological Monographs 60 (1): 27–55. https://doi.org/10.2307/1943025.