Ontogenetic niche shift (abbreviatedONS)[1] is anecological phenomenon where an organism (usually an animal) changes itsdiet orhabitat during itsontogeny (development).[2] During the ontogenetic niche shifting anecological niche of an individual changes its breadth and position.[3] The best known representatives of taxa that exhibit some kind of the ontogenetic niche shift arefish (e.g.migration of so-calleddiadromous fish betweensaltwater andfreshwater for purpose of breeding[2]),insects (e.g.metamorphosis between different life stages; such aslarva,pupa andimago[2]) andamphibians (e.g.metamorphosis fromtadpole to adult frog[2]).[4] A niche shift is thought to be determinedgenetically, while also being irreversible.[5] Important aspect of the ONS is the fact, that individuals of different stages of apopulation (e.g. of various age or size) utilize different kind of resources and habitats.[6][7] The term was introduced in a 1984 paper by biologistsEarl E. Werner andJames F. Gilliam.[1][8]
The ontogenetic niche shift is thought to be determinedgenetically, while also being irreversible.[5] In complex natural systems the ONS happens multiple times in lifetime of an individual (in some examples the ontogenetic niche shifting can occur continuously).[4] The ontogenetic niche shift varies across species; in some it is hardly visible and gradual (for example a change in diet or in size inmammals andreptiles), while in others it is obvious and abrupt (the metamorphosis of insects, which often results in changing habitat, diet and other ecological conditions).[5][9] One of the studies suggests that differences in the ONS across species could be (at least to some degree) explained by diversity of traits and functional roles of a species. As a consequence differences in ontogenetic niche shifting are thought to follow some general patterns.[10]
It is thought that almost every organism shows some kind of ontogenetic niche shift. The ONS, which is responsible for causing a noticeablephenotypic variation among individuals of the same species, plays important role in structuringcommunities and influencing their inside dynamics.[4] In some cases individuals undergoing the ONS, in which they change their habitat, become a (mobile) link between two different communities (for example via flow ofenergy,matter andnutrients).[11] A stage structure of apopulation can result in various stages interacting with different representatives of a community or even with individuals of other communities,[2][12] thus having a distinct ecological role from other life-history stages of the same population.[13] Theoretical models, where communities are stage-structured, propose the ontogenetic niche shifting of studied organisms is influencing the whole community (especially its resilience and disturbance responses).[4]
The most apparent consequence of the ontogenetic niche shifting is a reduction ofcompetition between different stages of the same population. Because of the ONS individuals of different age or size do not compete for food, materials and other habitat resources.[6] Different stages of the same population also have differenttrophic effects onfood web of a community.[7] A division of a population on distinct life-history stages is useful and evident, when there is a lack of resources for one stage (for example when juveniles do not get enough resources for themselves). In that case a lacking stage will have highermortality rate.[6]
The ONS is of great importance for survival of populations. Researchers noticed that many species exhibit the ontogenetic niche shifting at different times and in a lot of examples the ONS occurred as a response to variousabiotic andbioticenvironmental factors. It is thought that the ontogenetic niche shift could be an adaptive response to changing conditions in individual's habitat. Authors of thelife history theory predicted that organisms can affect the time of their ontogenetic niche shifting. While individuals living in favorable conditions would usually delay their ONS to successive ecological niche, organisms living in a niche with poor conditions typically advance to a further niche.[2]
Understanding the ontogenetic niche shifting in different species and its impact on the whole community is important when studying abiodiversity andecosystem functioning.[4] It is thought to be useful when dealing with populations threatened by anthropogenic disturbances[4][13] andenvironmental changes.[10][13]
Even though the occurrence of ontogenetic niche shifting is thought to be widely distributed, the best known representative taxa with extensively studied ONS are insects and a few groups ofvertebrates, especially fish and amphibians, where individuals often change theirhabitat as well as a lot of other aspects of their niche during the development. The less pronounced ontogenetic niche shifting can be seen in many other taxa, where their habitat stays the same. Usually the ONS in those species is evident, when looking at resources being used by organisms of the same species but various ages or size classes (for example a change in their diet).[2]
The ontogenetic niche shifting, which is connected with extreme habitat changes, can be seen amonginsects.[2] Individuals of taxon Insecta are known to exhibit one of the various types ofmetamorphosis, the best studied beinghemimetabolism (where an insect passes three life stages;egg,nymph andimago) andholometabolism (characterized with four life stages of an insect; egg,larva,pupa and imago).[14] Nutritional niches and their shifting during a ontogeny can be accurately measured by using a stableisotopic signature of animals.[15][16] Such method has been used in studying the ONS ingastropods, such as fieldslugs.[15]
The ONS similar to that among insects happens in amphibian taxa,[2] the best known beingfrogs, which start as an egg and then hatch into alarval stage called thetadpole.[17] Tadpoles exhibit many differences that distinguish them from an adult stage of a frog; most species' tadpoles areaquatic, they usually possessexternal gills and primarily feed withplant material (even though there are some exceptions that consume dead animal flesh or mixed diet).[18] Another well studied example of the ONS occurs in fish, that exhibitdiadromousbehaviour. Diadromous fish species drastically change their habitat, when they set out on a journey fromsea (saltwater) torivers (freshwater) and vice versa.[2] A lot of freshwater fish species show the ONS in their diet, when they switch from preying onplankton to performingbenthivory.[5]
The ONS may not be so visible inreptiles, even though these vertebrates do utilize it. The ontogenetic niche shifting was studied in American alligator (Alligator mississippiensis), which is ideal for studying ecological aspects of ONS because of many distinct size stages in a population. Alligators were switching their habitat niche between hydrologically isolated, seasonalwetlands and riverine systems. The study has shown that riverine systems were populated primarily with adults and subadults of bothsexes, that used the area as a non-nesting habitat. On the other hand, juveniles and adult females were found on seasonal wetlands, which served as anursery andnesting sites respectively.[11]
Good example of the ONS inbirds are bigseabirds, such asalbatrosses, which spend some of their time as fully oceanic birds and whensexually mature begin to visitbreeding grounds. Immature juveniles usually stay in subtropical water, where they occupy hightrophic levels. Researchers noticed that young birds progressively direct towards lower trophic positions when they are coming closer to sexual maturity. After time they take on an isotopic niche of an adult bird.[19]
The ontogenetic niche shifting is a concept widely studied inpaleontology andpaleozoology. Largenon-avian dinosaurs are known to have had exhibited one of the most intensive ontogenetic niche shifting, as they were hatched from an egg and had to experience big size shifts during their ontogeny.[20] One of the problems, connected with understandingMesozoic dinosaurfauna was lack of so-calledmesocarnivores. It is predicted the ontogenetic niche shift is an answer, becausecarnivorous dinosaurs started out as smallhatchlings and progressed towards adult size, while occupying different successive niches and limiting trophic species diversity. Juvenile individuals ofmegatheropods are thought to occupy mesocarnivore niche.[21]
The ontogenetic niche shifting is primarily studied in animals, but there are some studies that deal with the ONS inplants.[3][22][23] One of the ONSs studied in plants is changing of a regeneration niche. Authors of the paper noticed that during the ontogeny theregeneration niche ofAcer opalus, the Italian maple, had shrinked. It is thought such ontogenetic niche shift was mainly a consequence ofherbivory, the depth of the litter layer and presence of other plants (especially adult trees and shrubs).[23]