Individual specialisation and niche variation in aquatic vertebrates: theoretical expectations and empirical evidence

Individual specialisation and niche variation in aquatic vertebrates: theoretical expectations and empirical evidence | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 29 April 2025 V1 Latest version Share on Individual specialisation and niche variation in aquatic vertebrates: theoretical expectations and empirical evidence Authors : Gabriela Piriz 0009-0004-4673-5237 [email protected] , Edwin Niklitschek , and Karin Maldonado Authors Info & Affiliations https://doi.org/10.22541/au.174592842.25844020/v1 382 views 211 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Recent research underscores the ecological importance of individual niche variation within populations, challenging traditional niche theory that primarily addresses species coexistence at the population level. This article reviews theories and hypotheses about individual specialisation (IS) and niche variation, focusing on their implications for coexisting species within the same guild. By emphasising predictions regarding resource partitioning under increased competition, we explore how these mechanisms may operate in coexisting species at both population and individual levels. A meta-analysis of empirical data from aquatic vertebrates revealed a positive correlation between population niche overlap and IS, alongside a negative relationship between niche overlap and total niche width (TNW). We found highly variable responses of TNW and IS to increased competition. TNW increased, decreased or remained stable in 13, 61 and 26 % of the cases. Moreover, TNW was positively correlated with IS and the between-individual component (BIC) but negatively with the within-individual component (WIC). These findings challenge established niche theory and highlight the role of IS in facilitating species coexistence. Adopting multidimensional quantitative methods and standardised metrics is essential for further disentangling the complex interactions between niche overlap, IS, TNW, and their components (BIC and WIC). Individual specialisation and niche variation in aquatic vertebrates: theoretical expectations and empirical evidence Individual specialisation and niche variation Gabriela Piriz 1 , Edwin J. Niklitschek 2 & Karin Maldonado 3 1. Universidad de Los Lagos, Programa de Doctorado en Ciencias, mención Conservación y Manejo de Recursos Naturales, Chinquihue km. 6, Puerto Montt, Chile. [email protected] 2. Universidad de Los Lagos, Centro i~mar, Chinquihue km. 6, Puerto Montt, Chile. [email protected] 3. Universidad Adolfo Ibañez, Departamento de Ciencias, Facultad de Artes Liberales, Diagonal Las Torres 2640, Santiago, Chile. [email protected] Statement of authorship: Gabriela Piriz : Conceptualisation; writing – original draft preparation; data curation; formal analysis; methodology ; writing – review and editing. Edwin Niklitschek : Conceptualisation; methodology; formal analysis; writing – review and editing. Karin Maldonado : Conceptualisation; writing – review and editing. Data accessibility statement: The data supporting this study’s findings are openly available in Figshare at https://doi.org/10.6084/m9.figshare.28879115.v1. Keywords : niche theory, optimal foraging theory, niche variation hypothesis, niche overlap, niche width, coexistence. Type Article : Synthesis - 191 words in the abstract, 6146 words in the main text. 74 references. Four figures and one table. - Gabriela Piriz Millar. Puerto Montt, Chile. +56995212497. [email protected] Summary (200 words) Recent research underscores the ecological importance of individual niche variation within populations, challenging traditional niche theory that primarily addresses species coexistence at the population level. This article reviews theories and hypotheses about individual specialisation (IS) and niche variation, focusing on their implications for coexisting species within the same guild. By emphasising predictions regarding resource partitioning under increased competition, we explore how these mechanisms may operate in coexisting species at both population and individual levels. A meta-analysis of empirical data from aquatic vertebrates revealed a positive correlation between population niche overlap and IS, alongside a negative relationship between niche overlap and total niche width (TNW). We found highly variable responses of TNW and IS to increased competition. TNW increased, decreased or remained stable in 13, 61 and 26 % of the cases. Moreover, TNW was positively correlated with IS and the between-individual component (BIC) but negatively with the within-individual component (WIC). These findings challenge established niche theory and highlight the role of IS in facilitating species coexistence. Adopting multidimensional quantitative methods and standardised metrics is essential for further disentangling the complex interactions between niche overlap, IS, TNW, and their components (BIC and WIC). Introduction Species coexistence emerges from an interplay of multiple factors and time scales, including competition, resource availability, niche partitioning and fitness, which shape community structure and stability (Chase & Leibold 2003; Gause 1934; Hutchinson 1957; Pianka 1981). While coexistence has traditionally been studied at the population or species level, increasing research highlights the ecological importance of individual variation within populations (IV) across diverse taxa and niche dimensions (Araújo et al. 2011; Araújo & Costa-Pereira 2013; Bolnick et al. 2003; Costa et al. 2008). Such IV studies, however, have led to mixed and sometimes conflicting views of the role of intraspecific variation (IV) in coexistence. For instance, while some research indicates that IV can facilitate coexistence by alleviating interspecific competition and enhancing niche differentiation (Violle et al. 2012), other studies suggest that increased intraspecific variation in traits influencing competitive abilities can hinder coexistence by strengthening the dominance of the superior competitor (Barabás & D’Andrea 2016; Hart et al. 2016; Uriarte & Menge 2018). To reach a better understanding of the ecological implications that IV may have on species coexistence, it is necessary to go beyond the conventional analysis of trait variability effects on fitness and explore the broader set of mechanisms that, operating at different time scales and organisation levels, facilitate resource partitioning, both within and between species. The concept of individual specialisation (IS) may play a relevant role here as it allows for integrating all consistent differences in resource use patterns among individuals within a population over time (Bolnick et al. 2003; Fridley & Grime 2010; Hubbell 2005; Jung et al. 2010; Messier et al. 2010; Pfennig & Pfennig 2012). This perspective links trait variability and individual resource use variability to niche differentiation, resource partitioning, and interspecific interactions, thereby providing a fresh viewpoint on species coexistence mechanisms (Clark et al. 2010; Violle et al. 2012). Costa-Pereira et al. (2018), for example, found significant variation in IS among frog populations of the same species and suggested that such IS modulation allowed for coexistence with competitors across different communities. Species coexistence has often been investigated through the lens of interspecific competition, niche differentiation and resource partitioning at the population level, overlooking the crucial role of IS (Costa-Pereira et al. 2018; Costa-Pereira & Araújo 2024). Thus, despite offering valuable insights, dominant theories like optimal foraging theory (OFT) and niche theory fail to address individual-level processes and their implications. Similarly, individual-level theoretical developments such as the niche variation hypothesis (NVH, Van Valen 1965) and the concept of “ecological release” (Bolnick et al. 2010; Roughgarden 1972) tend to neglect the broader implications of individual processes for species coexistence. Expanding and integrating existing frameworks like OFT, NT, and NVH to include IS predictions seems critical for linking individual-level resource use patterns with population and community dynamics (Carscadden et al. 2020; Costa-Pereira & Araújo 2024). By providing new insights into species coexistence and community stability, this integration may contribute to improving conservation strategies in a changing world (Valladares et al. 2015). For instance, niche theory suggests that reducing niche overlap facilitates coexistence (Chesson 2000; MacArthur & Levins 1967; Tilman 1982). Niche segregation can be achieved either at the population level, through uniform changes in resource use patterns across individuals, or at the individual level, through increased heterogeneity among individuals. Theoretical and empirical studies are needed here to examine further the interconnections between niche overlap, IS, total niche width (TNW), and its components (within- and between-individual components, WIC and BIC). In summary, we believe that integrating IS into ecological models is a transformative step toward understanding coexistence mechanisms, advancing conservation practices, and inspiring future empirical research. As a result, this review aims to (i) systematise the literature on IS, coexistence, and niche variation at both individual and population levels; (ii) formulate integrative hypotheses on the relationships between IS, niche overlap, TNW, and its components; and (iii) meta-analyse data from aquatic vertebrates, particularly coexisting guild members, to test these hypotheses. Aquatic vertebrates were chosen due to the greater availability of relevant data, minimising confounding environmental factors and enhancing the statistical power of our analyses. Theoretical concepts Individual specialisation and competence: main theoretical frameworks The coexistence of competing species is often explained by niche segregation, a well-established concept in traditional niche theory (Chesson 2000; Gause 1934; MacArthur & Levins 1967; Tilman 1982). This segregation can occur by reducing the population niche width and/or increasing the divergence between population niches. This population-level principle can also be extended to individual niches (Chase & Leibold 2003; Pianka 1974). Increased intraspecific competition may enhance niche segregation among individuals and increase IS. The effects of increased interspecific competition on IS can be, on the other hand, more complex and context-dependent (Araújo et al. 2011). An increase in IS may occur through a decrease in average individual niche width (WIC) and/or an increase in the distance or divergence between individual niches (BIC). Hence, IS within populations should increase as the within-individual component (WIC) decreases or as divergence among individuals (BIC) increases. Van Valen (1965) pioneered research linking intrapopulation variation to population niche width and interspecific competition in some bird species whose population niche width differed regionally. He proposed the NVH, which predicts that low interspecific competition increases intrapopulation variation in resource use patterns, leading to greater population niche widths. This relationship is attributed to adaptive advantages derived from the optimal use of a wider range of available resources through an enhanced number of specialised phenotypes. Conversely, the NVH suggests that higher interspecific competition constrains the populationś total niche width, selecting against divergent phenotypes and, therefore, against IS (Van Valen 1965). Following the NVH, Roughgarden (1972) proposed a mathematical model for the evolution of niche width in a population. Considering individuals whose phenotypes are specialised in exploiting specific segments of a resource axis (e.g., different prey sizes for predators), he proposed two ecological responses to a theoretical release from interspecific competition. The first one referred to what he called the between-phenotype component (BIC), where population niche width expanded by incorporating new individuals with specialised phenotypes, increasing the diversity of resource use across the population (Roughgarden 1972). A second one is where competition release would favour an enlargement of the within-phenotype component (WIC), i.e., broadening the range of resources used by each phenotype. Bolnick et al. (2002) took a step further than their predecessors by equating phenotypes to individuals, thus providing a novel perspective for analysing behavioural responses to the release from interspecific competition within the evolutionary frameworks established by Van Valen (1965) and Roughgarden (1972). Building on this foundation, Bolnick et al. (2010) expanded these ideas by coining the concepts of ”parallel release” and ”individual release”. The parallel release describes a simultaneous expansion of individual (WIC) and total (TNW) niche widths, reducing IS to some extent. Individual release corresponds to the case where TNW remained the same as the expansion of individual niches (WIC) is cancelled out by a greater convergence among them (BIC), reducing IS (Bolnick et al. 2010). Beyond niche and competition theory, Optimal Foraging Theory (OFT; Emlen 1966; MacArthur & Pianka 1966) provides an alternative framework for understanding predator behaviour. It posits that predators adjust their foraging strategies and diet to maximise net energy gain, offering insights distinct from those of traditional niche-based approaches (Rooney et al. 2006; Townsend & Winfield 1985). Although traditionally focused on predicting collective population-level responses to disturbances in ecological opportunity, the OFT rationale can also be applied to predict individual niche variation under such disturbances. For instance, under favourable conditions —characterised by large ecological opportunity (i.e., abundant highly-profitable prey, low competition, and favourable handling/search conditions; Araújo et al. 2011) — individual niches would be predicted to narrow around prey that resulted energetically optimal for each individual (Sheppard et al. 2018; Stephens & Krebs 1986), leading to a reduction in both individual (WIC) and population (TNW) niche widths. On the other hand, individual niche segregation (BIC) may decrease or remain stable depending on the degree of intrapopulation variation in phenotypic traits shaping optimal foraging behaviours. Theoretical responses to increased competition According to classic niche theory (MacArthur & Levins 1967; Tilman 1982), the population niche width should be reduced (TNW) to allow for coexistence under increased interspecific competition. Such a reduction would require a decrease in either the individual niche width (WIC), the individual niche divergence (BIC) or both. Since the first response (a reduction in WIC) represents the opposite process of the concept of “parallel release” proposed by Bolnick et al. (2010), we refer to it here as “parallel constraint”, i.e., the simultaneous reduction of TNW and WIC. This mechanism would mitigate the increased intraspecific competition through increased IS (Table 1; Response #1). The second response (a decrease in BIC) would also reduce the TNW but at the cost of increased intraspecific competition. This response reduces IS, aligns with expectations from the NVH (Table 1, Response #2), and enhances intraspecific competition. A combined reduction in both BIC and WIC (Table 1, Response #3), on the other hand, would alleviate both interspecific and intraspecific competition; the relative change in IS would depend on the relative change experienced by the two niche components. In contrast to predictions derived from niche theory, OFT predicts an expansion of TNW if the availability of optimal resources (e.g., preferred prey) decreases due to increased interspecific competition. This TNW enlargement may result from i) an expansion of individual niches (increased WIC) where individuals became more generalist and moved on to exploit sub-optimal (less profitable) but more abundant shared resources (OFT WIC , Table 1, Response #4), ii) an increase in individual niche segregation (increased BIC) where individuals moved away from other population members to exploit specific fractions of a newly expanded range of sub-optimal resources (OFT BIC , Table 1, Response #5) or iii) an increase in both components (Table 1, Response #6). Thus, OFT predicts that IS might decrease (Response #4) or increase (Response #5) under enhanced competition, depending on the dominant mechanism used to optimise individual foraging strategies. It is interesting to consider that TNW may also remain unchanged under enhanced competition if a decrease in WIC, as predicted by the niche theory, was fully compensated by an increase in divergence between individual niches (BIC), as expected under the OFT (Table 1, Response #5). This would reflect an “individual constraint” response, consistent (although in reversal) with the “individual release” process described by Bolnick et al., (2010). Here, the cost of greater interspecific competition would be compensated by reducing the intraspecific competition through an increased IS. Moreover, high individual niche divergence (increase in BIC) might create a complementary mosaic of individual niches across species, which could somewhat alleviate interspecific competition. If individual niche width was increased, as predicted by OFT Response #4, and this increase was cancelled out by an equivalent reduction in niche divergence (BIC), as expected by the NVH, the TNW would also remain unchanged. Still, IS would be reduced (Table 1, Response #6) as individuals would be forced to move into exploiting a broader range of shared sub-optimal resources. Up to this point, it is clear that available theory leads to contradictory predictions about population and individual niche responses to increased competition (Table 1). Thus, BIC should be reduced following the NVH and the parallel constraint hypothesis, but might be enlarged following the OFT. WIC should be diminished following the parallel or individual constraint hypotheses, but might be enlarged following the OFT. This apparent contradiction reflects opposing forces operating simultaneously at different time scales (evolutionary and ecological time) and organisation levels (guild, population and individuals). On the one hand, interspecific competition might force niche segregation by constraining population and/or individual niche widths within guilds. The extent of this constraint would depend on the competitive abilities and phenotypic (behavioural) plasticity of each species and individual (Maldonado et al. 2019). On the other hand, individuals may respond to intra-guild and intraspecific competition by expanding or shifting their niches to exploit less profitable but more abundant resources. The degree of this response would be influenced by the population’s or individual’s ability to adjust resource use in varying environmental conditions, a phenomenon referred to as ”niche plasticity” (Agrawal 2001; Sultan 2000). Evolutionarily, niche plasticity is expected to be greater in species exposed to larger and predictable variability in ecological opportunity, as well as to density-dependent and interspecific competition pressures. Such plasticity would be smaller at the individual than at the population level, given the genetic, developmental and phenotypic constraints that shape the fundamental niche of the individual and the spatio-temporal constraints, ecological interactions and learned behaviours that shape the realised individual niche (Takola & Schielzeth 2022; Trappes et al. 2022). In summary, species with lower individual niche plasticity would be expected to respond primarily to increased interspecific competition by reducing their individual niche widths, thereby minimising overlap with competitors. Conversely, species with higher individual niche plasticity would likely respond by increasing divergence among individual niches. Interestingly, while population niche width responses would differ—contracting for less plastic species and expanding for more plastic ones—IS would increase in both cases, enabling optimised resource partitioning among individuals. Empirical evidence in aquatic vertebrates Overall, the contradictory predictions emerging from available theory about population and individual niche responses to increased competition have not faced systematic empirical testing. This is partly because of the difficulties of controlling so many confounding variables and partly because of the lack of an integrative framework summarising all available theories to guide more transformative (hypothesis-driven) empirical efforts. Acknowledging these limitations, we attempt in the following section to obtain some insights from a meta-analysis of reported research on population and individual niche variation of coexisting aquatic vertebrates. Tested hypotheses Niche overlap and individual specialisation in coexisting competitors Available data (see below) were unsuitable for testing the hypothesis that individual and population niche responses to long-term competition are a function of individual niche plasticity, which has not been measured in most reviewed studies. Instead, we tested the following two derived hypotheses: (H1) that populations exhibiting higher levels of IS can endure greater population niche overlap with competitors, and (H2) that increased segregation between individual niches is the dominant mechanism explaining larger IS in species subjected to larger interspecific competition (enhanced niche overlap). Population and individual niche variation under increased competition Given a rather limited dataset suitable for contrasting population and individual niches under increased competition scenarios, we quantified empirical evidence supporting each of the eight response types depicted in Table 1, which reflect, in turn, predictions from NVH, parallel and individual constraint and OFT hypotheses. Population niche, individual niche and individual specialisation in marine vertebrates The available theory allows us to formulate three main nested hypotheses about the relationship between TNW, IS and TNW components, which we tested using the published studies in aquatic vertebrates described below: (H3) populations utilising a broader range of resources (higher TNW) exhibit greater levels of IS. This is mainly explained by (H4), a positive relationship between IS and BIC, which is stronger than the expected negative and weaker relationship between IS and WIC. Therefore, (H5) most variability in TNW is explained by variability in BIC. Meta-analysis methodology Literature search A systematic search of two peer-reviewed scientific literature databases, Web of Science (https://www.webofscience.com/wos/woscc/basic-search) and Google Scholar (www.scholar.google.com), was conducted to identify peer-reviewed articles on IS in aquatic vertebrates. The search terms used (across article title, abstract, and keywords) were: ‘individual speciali(z)sation’, ‘individual niche speciali(z)sation’, ‘individual diet varia*’, ‘individual varia*’,‘TNW’, ‘WIC’ and ‘BIC’. Selection criteria differed between hypotheses. For H1 and H2, articles were included if they (i) provided measures of population niche width (e.g., TNW, SEA, Levin’s index), niche overlap (e.g., Pianka’s, Schoener’s indices, % SEA overlap), and IS (e.g., Psi, RINI, WIC/TNW); (ii) focused on two or more species of aquatic vertebrates of the same guild that coexisted; and (iii) employed techniques relating to the trophic dimension of the ecological niche (e.g., stomach content analysis, stable isotope analysis, prey size measurements). For H3-H5, the inclusion criteria were: (i) provision of TNW and WIC measures; (ii) focus on aquatic vertebrates; and (iii) calculation of niche components using stable isotope analysis ( δ ¹³C, δ ¹⁵N, or both). Data compilation For hypotheses H1 and H2, we selected and compiled data from 14 articles containing measures of population niche width, overlap, and IS in coexisting competitors (Table S1). For hypotheses H3–H5, we built a second dataset that included data from 25 articles measuring and reporting TNW, WIC, BIC and IS (Table S2). Data were obtained directly from the main text, its supplementary material, or, whenever necessary, from its graphic material. For each dataset, the following information was also recorded: author, species (including order and family), data source (e.g., stomach content, stable isotopes, prey size) and indices used to calculate each variable of interest. Quantitative methods for individual specialisation IS quantifies the divergence between individual niche widths and those of the population. While the concept is rooted in comparing these two scales, the literature presents diverse metrics to operationalise this relationship. The foundational framework, proposed by Roughgarden (1972), partitions the total niche width (TNW) into two additive components: the within-individual component (WIC), representing the average variance in resource use within individuals, and the between-individual component (BIC), capturing variation in mean resource use across individuals. This framework accommodates both continuous data (e.g., prey size, stable isotope ratios) and discrete data (e.g., taxonomic resource categories). For discrete resources, metrics such as the Shannon-Weaver diversity index quantify TNW and its component (Roughgarden 1979), while the proportional similarity index (PS i ) measures niche overlap between individuals and the population. Population-level IS is then derived as the average PS i across individuals, with the complementary index V=1−PS i providing an intuitive scale where higher values denote greater specialisation. Although IS is traditionally defined as WIC/TNW, this ratio has a counterintuitive interpretation: smaller values correspond to higher specialisation (Bolnick et al. 2002). To resolve this, we adopt the inverted metric IS=1−WIC/TNW (Rosenblatt et al. 2015), ensuring that values scale directly with specialisation intensity. All data in this analysis were standardised to this formulation for consistency. To provide flexibility while selecting published studies and ensuring consistency in interpreting results across different studies, we accounted for the potential effects of the IS metric used in each study as a random factor. Nonetheless, Bolnick et al. (2002) suggest all existing metrics for quantifying IS are comparable as this is a relative measure, not directly related to the absolute widths of individual or population niches. It is important to acknowledge that higher levels of IS do not necessarily imply narrower individual niches (Costa-Pereira & Araújo 2024). Data description Fourteen studies contributed to testing H1 and H2 as they provided quantitative data on IS, TNW, and population overlap (Table S1). Repeated measures from the same populations were averaged, encompassing 36 species across 40 observations. The most frequently represented taxonomic classes were Actinopterygii (21 observations), Chondrichthyes (8), Amphibia (4), Mammalia (4), and Aves (3). Various methods and metrics were used to calculate IS. Total niche width (TNW = WIC + BIC) was most commonly calculated from either continuous data (e.g., stable isotopes, prey size; 15 studies) or categorical data (e.g., stomach content; 8 studies). The PS i index (Bolnick et al. 2002; Schoener 1968) was also frequently employed (8 studies) to assess individual-level specialisation. Similarly, several methods were used to calculate niche overlap: the SEA index (19 studies), the Schoener index (14 studies), and the Pianka index (5 studies). Hypotheses H3-H5 were tested using a dataset of 25 studies and 33 species utilising continuous data. Repeated measures from the same populations were averaged, incorporating 96 observations that utilised stable isotopes of carbon ( δ ¹³C, 31 observations), nitrogen ( δ ¹⁵N, 39 observations) or both (26 observations) in populations of aquatic vertebrates (Table S2). The species represented across these studies included 55 observations from Mammalia, 14 from Reptilia, 12 from Aves, eight from Amphibia, six from Actinopterygii and one from Chondrichthyes. The 25 articles that reported metrics for total niche width and its components exhibited large variability in population niche width, ranging from 0.08 to 26.18 ‰. This variability reflected considerable differences among methods, metrics, species, communities, and ecosystems, including a range of typically unassessed competition levels, ecological opportunity variations, and potential edge effects (ecotones). Given these differences, we deemed it essential to standardise the data according to the isotopes used, ensuring that our statistical analyses are more robust and reliable. Statistical Analysis We employed a multi-model inference approach (Burnham & Anderson 2004) using generalised linear mixed models (GLMMs; Bolker et al. 2009) implemented in R through the ’glmmTMB package’ (Brooks et al. 2017). This approach and package accommodated non-normality, non-linearity and potential random effects from the author and calculation methodology (IS, overlap, TNW). Global, null and alternative models were compared for each response using the second-order Akaike Information Criterion (AICc; Akaike 1973) and its relative weight (AICc-weight), interpreted as the probability of a competing model to be the most informative one within the tested set of competing models (Burnham & Anderson 2004). Niche metrics BIC, WIC and TNW were standardised to account for scale differences between axis types ( δ ¹³C, δ ¹⁵N, or both). Given the nature of the response variables, a beta distribution was used for species overlap, and IS (H1, H2 & H3), incorporating a zero-inflated beta distribution to account for a few zeros in species overlap. A Gaussian distribution was used for TNW (H4, H5). Main findings Niche overlap and individual specialisation in coexisting competitors Our meta-analysis revealed a linear and positive relationship between niche overlap and IS, while a negative relationship was observed between niche overlap and population niche width (Figure 1). The model incorporating additive main effects from both terms was the most informative, with an AIC weight of 0.613, outperforming all alternative models (ΔAIC > 2.89, Table S4). The marginal contributions of IS and TNW accounted for 36.4% and 56.5% of this model’s sum of squares, respectively. These findings supported Hypothesis 1 (H1), i.e., increased IS enhances species tolerance to greater niche overlap, at least among coexisting aquatic vertebrates. IS was found to increase with TNW (Figure 1), as expected from our second hypothesis (H2), suggesting that greater IS was primarily associated with enhanced divergence between individual niches (BIC, Table S5). Thus, the negative relationship found between niche overlap and TNW was not due to population or individual niche contraction. Instead, it supports the idea that niche overlap increases between competitors that exploit a more limited range of resources. Therefore, population responses to long-term coexistence might combine two non-excluyent processes: (i) between-individual niche segregation to reduce intraspecific and/or intraguild competition, as predicted by the NVH, and (ii) population niche expansion to reduce overlapping through the incorporation of a broader range of (less optimal) resources, as predicted by the TFO. Variation in population and individual niche under increased competition Only seven studies we reviewed measured IS and TNW under increased competition scenarios involving 17 species. These scenarios considered seasonal variations (Neves et al. 2018, 2021), breeding periods (Kernaléguen et al. 2015), changes in resource availability (Costa-Pereira et al. 2018), and the presence/absence of competitors (Costa-Pereira et al. 2018; Gray et al. 2005; Kernaléguen et al. 2015; Prati et al. 2021). The observed responses were highly variable (Figure 2, Table S3), even within the same population, and showed considerable sensitivity to the measured axis (isotope) (Kernaléguen et al. 2015). TNW decreased under increased competition in 14 out of 23 cases, remained unchanged in 6 cases and increased in 3 cases. The reductions in TNW were accompanied by a decreased distance between individuals (decreased BIC) and, therefore, in IS, in three cases, aligned with predictions from the NVH (Table 1, Response #1). In the other three cases, this reduction was produced by a reduction of WIC, increasing IS, as predicted by the parallel constraint hypothesis (Table 1, Response #2). In the other three cases, there was a reduction in both components, WIC and BIC, consistent with both the NVH and the individual constraint hypotheses (Table 1, Response #3). Due to the metrics used (mean distance to centroid), it was impossible to determine the mechanisms behind the decrease in IS in five other populations (Figure 2, Table S3). In the three cases where TNW increased (Figure 2), there was also a reported increase in IS (Neves et al. 2018, 2021), attributed to a rise in BIC, which was consistent with the OFT (Table 1, Response #5). However, the metrics used (mean distance to centroid) made it impossible to ascertain whether this increase in BIC was accompanied by a corresponding rise in WIC (Table 1, Response #6). Finally, in instances where TNW remained unchanged despite increased competition, most populations (4 out of 6 cases) exhibited an increase in WIC, as expected under the Optimal Foraging Theory, alongside a reduction in BIC predicted by the NVH, leading to a decrease in IS (Table 1, Response # 8). Only one population showed an increase in IS, resulting from a reduction in WIC and an increase in BIC (Kernaléguen et al. 2015), consistent with the combination of OFT and individual constraint process (Table 1, Response #7). Again, the metrics provided (mean distance to centroid) prevented determining component changes for one case (Figure 2, Table S3). Individual specialisation, population niche width and its components Individual specialisation Individual niche segregation (BIC) was the most informative single explanatory variable for IS variability across articles and species (AICc = -65.473, AICw = 0.995; Table S6). It was followed by TNW (AICc = -41.57) and WIC (AICc = -39.343) with AICc weights of <0.001 in both cases. While linear relationships were more informative than smoothed ones for both TNW (ΔAIC = 2.23) and WIC (ΔAIC = 2.23), a non-linear (cubic spline) relationship was more informative than a linear one between IS and BIC (ΔAIC = 10.48). The most informative model (AICc = -104.808, AICw = 0.744) included additive effects from both BIC (non-linear) and WIC (linear) and accounted for 36.2% of IS deviance (Table S7). Marginal contributions of BIC and WIC reached 92.9 and 56.3 % of the model sum of squares, respectively. The IS response to BIC followed a positive asymptotic relationship that plateaued around IS = 0.9 as BIC exceeded 2 SDs (Figure 3). The IS response to WIC followed a negative linear relationship (slope = -0.67 ± 0.108 SE, p>t of IS deviance (Figure 3), showing a positive linear effect (slope = 0.42 ± 0.131 SE, p> t = 0.0013). These findings align well with the NVH’s expectations that variability in individual niche segregation (BIC) is the primary force driving TNW and IS (Bolnick et al. 2007; Franco-Trecu et al. 2022; Van Valen 1965). It must be acknowledged, however, that a limited ecological opportunity may restrict individual niche divergence (BIC), thus constraining both IS and TNW (see next section). Total niche width A nonlinear and positive relationship was observed between TNW and its two components (Figure 4). However, BIC was the most informative explanatory variable of TNW variability (AICc= 179.4, AICw = 1, Table S8), outcompeting WIC and accounting for 46% of TNW deviance. Although the evidence supporting WIC effects upon TNW was much weaker (AICc= 230.4, AICw = 0), this variable still explained 24% of TNW deviance. These results further support the hypothesis that the width of the population niche is more closely related to the divergence or distance between individual niches (BIC) than their width (WIC), indicating that IS is more influenced by variability in the BIC than the WIC (Van Valen 1965). Discussion and conclusions. More than a century after the first definitions of the ecological niche (Johnson 1910), there is a growing recognition that population niche properties mainly originate at the individual level, whose heterogeneous resource use patterns shape what we observe at the species or population level. Thus, differences in how individuals use resources can scale up to affect ecological interactions and community structures (Carscadden et al. 2020; Costa-Pereira & Araújo 2024; Takola & Schielzeth 2022; Trappes et al. 2022). Recent advances in technology and analytical methods have made it possible to measure this variation and understand its role in broader ecological patterns. Our review of definitions, theories, and hypotheses related to IS and niche variation highlights two main theoretical frameworks available to explain variability and relationships between individual and population niches: niche theory (Chase & Leibold 2003) and OFT (Sheppard et al. 2018; Stephens & Krebs 1986). We considered the NVH (Van Valen 1965) and the concept of ecological release (Bolnick et al. 2010; Roughgarden 1972) as part of niche theory applied at the individual level. Both concepts capture how individuals respond to competition, predation risk, and ecological opportunity across ecological and evolutionary timescales (Bolnick et al. 2010; Chase & Leibold 2003; Schoener 1974). Since IS is often calculated as the ratio between individual niche width and population niche width, it is crucial to recognise that this is not an absolute index (i.e., it does not represent an independent or standardised measure but rather one relative to the context) and that varying scenarios of niche segregation and population’s total niche width can yield the same level of IS. Therefore, higher levels of IS do not necessarily imply that individuals have or shift to narrower niches (Costa-Pereira & Araújo 2024). Moreover, the population niche width, its two components, and the IS hold significant ecological interests on their own. Therefore, we recommend examining these emerging properties separately as well as in conjunction at different organisational levels, from individuals to guilds or communities. Additionally, it is essential to consider IS as a continuum (Costa-Pereira & Araújo 2024), avoiding dichotomic, but at the same time arbitrary, definitions or thresholds that categorise the population as ”individual specialists” or ”individual generalists”. This perspective would allow a more nuanced understanding of ecological roles and resource utilisation within diverse populations. Despite its potential significance for understanding species coexistence, limited theoretical and empirical research has explored the relationship between niche overlap, IS, and the population’s total niche width, particularly in aquatic vertebrates. Consistently with some theoretical frameworks, we found that increased IS would facilitate coexistence by enabling populations to endure greater niche overlap, likely due to the optimisation of individual-level resource partitioning (Violle et al. 2012) that promotes niche complementarity and reduces intra- and interspecific competition between individuals that specialise in exploiting distinct subsets of available resources (Bolnick et al. 2003). This strategy also aligns well with OFT, given the predicted population niche expansion to exploit additional suboptimal resources is mainly attributed to individual niche divergence (OFT BIC ). However, it is crucial to note that if this individual-based expansion leads to divergent specialisation between coexisting species, this would also increase overall niche width and reduce interspecific overlap (Costa-Pereira et al. 2019). Despite being supported by several theoretical frameworks and empirical studies, the positive effects of IS upon coexistence remain contentious since other authors argue that heightened intraspecific variation in competitive traits might instead exacerbate dominance hierarchies, destabilising coexistence (Barabás & D’Andrea 2016; Hart et al. 2016). Moreover, while efficient exploitation of narrow resource subsets could reduce direct competition, aggregated specialisations might inadvertently expand population niche width, intensifying interspecific niche overlap. Resolving this paradox requires distinguishing between scenarios where IS acts as a stabilising force (e.g., through fine-grained resource partitioning) or a driver of competitive exclusion (e.g., via resource monopolisation), particularly in aquatic vertebrates where these dynamics remain underexplored. Our findings suggest that species coexistence is possible despite high population-level niche overlap. This seems to occur because individual niche differentiation decouples population-level overlap from direct competitive interactions, i.e. the apparent overlap at the population scale masks fine-grained resource partitioning among individuals. Consequently, traditional measures of niche overlap may systematically overestimate actual competition pressures (Murray et al. 2023). Thus, while these findings seem to challenge classical ecological models that equate population-level overlap with competitive exclusion, such contradiction may fade away if individual niche variation is considered. Chelotti & Gadig (2022) present a striking example of this, involving two sympatric fishes from the same genus, exhibiting>85% population niche overlap yet coexisting. IS here is pronounced, and diet analysis shows that individuals effectively partition resources at the micro-scale, displaying a range of distinct prey types and foraging strategies that minimise interspecific encounters and reduce competition intensity. The negative relationship we observed between niche overlap and population niche width—where broader population niches correlate with reduced niche overlap— challenges predictions from traditional niche theory that posit that interspecific competition drives niche packing to minimise overlap (MacArthur 1965; Schoener 1974). Our results suggest an alternative response: individual niche segregation, which expands the population niche width to access novel resources while reducing direct competition. This mirrors findings in predatory fish, where niche overlap persisted despite competition, likely due to intraspecific partitioning. This aligns with the concept of compensatory competition dynamics (Ågren et al. 1984; Keddy 1989), where symmetric competitive abilities permit overlap when niche axes diverge at finer scales (e.g., individual microhabitat use). Thus, niche width and overlap are not inherently antagonistic, as IS allows for decoupling these variables by enabling coexistence through resource partitioning at subpopulation levels. Reported observations indicating that niche width overlap can vary non-linearly with resource availability (Gabler & Amundsen 2010; Keddy 1989) further complicate our previous narrative. Species and individuals may opportunistically overlap to utilise shared resources when abundant, reduce overlap and show increased specialisation when resources become moderately limited and converge again to exploit the fewer resources available during periods of extreme scarcity (Schoener 1983; Wiens 1993). Thus, the relationship between niche overlap and species coexistence is not static but influenced by seasonal and stochastic variability in environmental conditions and resource availability (Costa-Pereira et al. 2019). This underscores the necessity of considering the complexity of competitive interactions and resource dynamics in future studies. Understanding these nuances can provide deeper insights into the mechanisms supporting biodiversity and ecosystem stability under natural and anthropogenic pressures, such as recent and ongoing biological invasions (Barrero et al. 2024; Prati et al. 2021). Our research adds to the growing evidence highlighting individual niche specialisation as a critical factor in driving the coexistence of species within complex ecological communities (Chen et al. 2020; Costa-Pereira et al. 2018; Hausch et al. 2018). Nonetheless, the large variability we detected in methods and metrics makes it necessary to call for standardising analytical and reporting approaches. This is crucial to enhance comparability and clarify conclusions, particularly when integrating information across studies. Care must be taken, for instance, regarding the time frame for calculating IS, as methods like stomach content analysis reflect short-term dietary habits, capturing variability within a single meal rather than long-term patterns (Hyslop 1980). This approach may overrepresent ephemeral foraging choices or rare prey items, potentially inflating niche width estimates if not contextualised with complementary methods. In contrast, stable isotope analyses provide a proxy for trophic niches over varying periods depending on the tissues analysed (Costa-Pereira et al. 2018). Adopting advanced quantitative methods (e.g., maximum likelihood and Bayesian approaches) represents another potential improvement to the assessment of IS in a multivariate context, which results fully consistent with the multidimensional definition of niche (Bonnet-Lebrun et al. 2020; Costa-Pereira & Araújo 2024; Dehnhard et al. 2020; Ingram et al. 2018). Consistent application of the same metrics used for assessing IS at the intra-population level to community or guild-level analyses will enable the quantification of both interspecific niche segregation (as opposed to mere overlap) and the degree of species-specific specialisation (TNW as a proportion of the total community niche width) (Carscadden et al. 2020; Clark et al. 2010; Devictor et al. 2010; Ingram et al. 2018). Beyond the need to standardise and update methodological approaches, individual niche variation and specialisation remain a relatively new research topic, which still requires much conceptualisation, hypothesis formalisation and empirical testing (Costa-Pereira et al. 2019). Notably, references to temporal scales (e.g., short-term vs. evolutionary dynamics) are often implicit or absent in reviewed studies. An integrative perspective suggests evolution has shaped species with varying degrees of phenotypic niche plasticity, enabling flexible responses to ecological interactions—particularly competition and predation—within ecological timescales. This plasticity may help explain transient individual-level niche shifts observed in many studies, even when evolutionary dynamics are not explicitly considered (Araújo et al. 2011). Therefore, we may still need to shape a genuinely unified niche theory suitable for predicting or understanding individual and population variability at different spatial and temporal scales. In conclusion, our synthesis underscores the critical role of individual niche specialisation in shaping patterns of species coexistence. It may have practical implications for conserving and managing biological diversity within complex ecosystems. Given the interplay between individual-level resource use and population-level niche dynamics, there is a clear need to update and expand some traditional views that overlook the importance of intraspecific variation. Such integrative effort is paramount to enhance our understanding of community assembly and stability and, therefore, to obtain more accurate and precise predictions about species responses to environmental change and anthropogenic pressures. 4. References Agrawal, A.A. (2001). 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Keywords coexistence individual variation niche overlap niche theory niche variation hypothesis optimal foraging theory Authors Affiliations Gabriela Piriz 0009-0004-4673-5237 [email protected] Universidad de los Lagos - Campus Puerto Montt View all articles by this author Edwin Niklitschek Universidad de los Lagos - Campus Puerto Montt View all articles by this author Karin Maldonado Universidad Adolfo Ibáñez View all articles by this author Metrics & Citations Metrics Article Usage 382 views 211 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Gabriela Piriz, Edwin Niklitschek, Karin Maldonado. Individual specialisation and niche variation in aquatic vertebrates: theoretical expectations and empirical evidence. Authorea . 29 April 2025. DOI: https://doi.org/10.22541/au.174592842.25844020/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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