Niche Expansion and the Niche Variation Hypothesis: Does the Degree of Individual Variation Increase in Depauperate Assemblages?

Species in transformed habitats, frequently labeled as environmental generalists, tend to show broader niches than species in natural habitats. However, how population niche expansion translates into changes in the niches of individual organisms remains unclear, particularly in the context of habitat transformation. Niche expansion could be a product of individuals having broader niches, greater distances among individuals’ niches, or a combination of both processes. This would challenge the traditional conceptions on niche dynamics, which emphasize the role played by individual specialization (IS). Here, using stable isotopes, we computed total niche width (TNW), its within- and between-individual components (WIC and BIC), and IS (the ratio WIC/TNW), in 13 populations of 6 bird species and 8 populations of 3 frog species in natural and transformed habitats. We confirmed that species had broader niche width in transformed than in natural habitats, yet population niche expansion across habitats was mainly a product of increased distance between individuals. Within each habitat type, increases in TNW were linked to increases in WIC for all habitat types, while relationships between TNW and BIC were found in transformed but not in natural habitats. Hence, both increased individual niche width and increased distance among individuals were apparent within habitats, particularly in transformed ones, where increases in WIC dominated. Neither across or within habitats was niche expansion associated with increasing IS. Therefore, our results overturn traditional conceptions associated with the niche variation hypothesis and illustrate that niche expansion is not invariably associated with increased IS, because the distance between individual’s niches (BIC) can increase, as well as the breadth of those niches (WIC).

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Principles of niche expansion

Proceedings of The Royal Society B Biological Sciences ◽

10.1098/rspb.2018.2603 ◽

2018 ◽

Vol 285 (1893) ◽

pp. 20182603 ◽

Cited By ~ 10

Author(s):

Henrik Sjödin ◽

Jörgen Ripa ◽

Per Lundberg

Keyword(s):

General Theory ◽

Evolutionary Model ◽

Resource Acquisition ◽

Short Term ◽

Trade Off ◽

Niche Expansion ◽

Niche Variation ◽

Wide Range ◽

Niche Variation Hypothesis ◽

Resource Diversity

Niche expansion is attained by adaptations in two generalized phenotypical traits—niche position and niche width. This gives room for a wide range of conceptual ways of niche filling. The niche variation hypothesis reduces the range by predicting that expansion occurs by increasing variation in niche position, which has been debated on empirical and theoretical grounds as also other options seem possible. Here, we propose a general theory of niche expansion. We review empirical data and show with an eco-evolutionary model how resource diversity and a trade-off in resource acquisition steer niche evolution consistent with observations. We show that the range can be reduced to a discrete set of two orthogonal ways of niche filling, through (1) strict phenotypical differentiation in niche position or (2) strict individual generalization. When individual generalization is costly, niche expansion undergoes a shift from (2) to (1) at a point where the resource diversity becomes sufficiently large. Otherwise, niche expansion always follows (2), consistent with earlier results. We show that this either–or response can operate at both evolutionary and short-term time scales. This reduces the principles of niche expansion under environmental change to a notion of orthogonality, dictated by resource diversity and a resource-acquisition trade-off.

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Feeding limitations in temperate anurans and the niche variation hypothesis

Amphibia-Reptilia ◽

10.1163/15685381-00003131 ◽

2017 ◽

Vol 38 (4) ◽

pp. 473-482 ◽

Cited By ~ 2

Author(s):

Carl S. Cloyed ◽

Perri K. Eason

Keyword(s):

Individual Variation ◽

Niche Breadth ◽

Rana Catesbeiana ◽

Strong Support ◽

Diet Breadth ◽

Niche Variation ◽

Gape Size ◽

Dietary Niche ◽

Niche Variation Hypothesis ◽

Traditional Approaches

The niche variation hypothesis (NVH) states that populations with wider niches are more phenotypically variable. The NVH has important ecological and evolutionary implications but has been controversial since its inception. Recent interpretations have supported the NVH by directly comparing among-individual diet variation with population dietary niche breadth. Traditional studies of the NVH focused on morphological traits as proxies of niche variation, with contradictory results. Gape-limited predators may be relatively likely to show effects of morphological variation on diet breadth because gape size can strongly limit diet. We used five anurans to test NVH predictions, including three true frogs, Rana catesbeiana, R. clamitans, and R. sphenocephala, and two toads, Anaxyrus americanus and A. fowleri. We combined recent and traditional approaches by comparing both individual variation in diet and variation in gape width with dietary niche breadth. We found support for the NVH within two species of the three true frogs but not for either toad species, a difference likely driven by greater strength of the feeding limitation caused by gape width in the frogs. Toads had higher gape width to snout-vent length ratios, reducing the strength of the feeding limitation imposed by gape width. We found strong support for the NVH among species; species with more among-individual variation in diet and species with more variation in gape width had broader niches. Our results highlight the circumstances under which the NVH is applicable and demonstrate an example in which the NVH is supported through both traditional and recent interpretations.

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