Biotic Interactions

A substantial amount of ecological theory and empirical work has focused on how antagonistic interactions (such as competition and predation) exclude species from habitats or resources that they could otherwise occupy, resulting in realized NB being smaller than fundamental NB (e.g., Leibold 1995). For example, the classic study by Connell (1961) showed that the barnacle Chthamalus is restricted to the high zone of rocky intertidal shorelines because it is outcompeted by the barnacle Balanus in the otherwise more favorable lower intertidal zone. The interaction between competition and NB is thought to be broadly important for structuring communities, particularly when multiple species draw upon the same abiotic or biotic resources (“shared preferences”; McGill et al. 2006). Coexistence among species with shared preferences can arise from contrasting mechanisms. Competitive species with broad fundamental niches can shift to use alternative forms of a shared resource (e.g., ammonium versus nitrate forms of nitrogen), increasing niche partitioning and coexistence (Ashton et al. 2010). Alternatively, a tradeoff between competitive ability and NB can allow poor competitors to persist because they tolerate the lowest levels of shared resources or endure the least preferred conditions (e.g., Tittes et al. 2019; Bohner and Diez 2020).

In contrast to the extensive work examining how competition drives discrepancies between the fundamental and realized niche, far fewer studies have considered the influence of mutualism and facilitation on NB. Mutualisms can have contrasting effects on host NB. Like competition, mutualisms could reduce host NB. In an obligate aphid-bacterium mutualism, association with a certain bacterial genotype lowered aphid thermotolerance (Dunbar et al. 2007). Moreover, many mutualistic associations involve costs, like provisioning partners with resources (Bronstein 2001) that could limit their investment in strategies that increase their range of environmental tolerances. Positive interactions can also expand the realized niche of a species (Bruno et al. 2003). For example, associations with some fungal endophytes increase drought tolerance and allow range expansion of host plants (Afkhami et al. 2014). Hence, mutualisms can create mismatches between fundamental and realized niches by expanding or constraining host NB, depending on the nature of the tradeoffs and the explicit benefits provided by mutualistic partners. Further, although mutualisms may expand realized NB along some axes, they might contract it along others (e.g., Afkhami et al. 2014), again highlighting the need for research that simultaneously considers multiple niche axes and their interactions.

Species’ Abundance and Dispersal Limitation

A species’ abundance and dispersal ability can shape its likelihood of encountering different species, resources, and conditions. Limited opportunities generate a narrower interaction network (e.g., fewer plant species visited by a focal bee) and thus a narrower realized NB. Indeed, null models based on species’ relative abundances have predicted the NBs of freshwater fish and mammalian host-parasite systems reasonably well (Vázquez et al. 2005; Canard et al. 2014). Similarly, low-abundance or dispersal-limited species may be restricted to a few habitat types simply because they have fewer opportunities to sample the full environmental spectrum (“habitat selection”), although this idea has not been supported by many studies (Gaston et al. 1997). Nevertheless, it highlights an important conundrum: is narrow NB in a species a cause or consequence of its abundance? Only the former causal pathway implies narrow NB species have an “intrinsic degree of specialization” (Vázquez et al. 2005:952), rather than their NB arising solely due to their opportunity to interact with different habitats and species (Vázquez et al. 2005). Therefore, studies that empirically quantify fundamental NB are invaluable for differentiating between NB as a cause or consequence of relative abundance. Such studies could include physiological experiments evaluating growth rates across a range of temperatures or feeding trials assessing growth when an herbivore is fed different plant species. Robust quantification of NB using performance or growth rates, rather than resource usage as a proxy, should help here. Studies using logical tests and null models to illuminate the direction of causality also provide a path for understanding process from pattern (e.g., Sheth et al. 2014; Fort et al. 2016).

The Scale-Dependence of Factors Shaping Realized Niche Breadth

The effects of dispersal limitation and biotic interactions on the realized niche likely vary across spatial scales. At large geographic scales, dispersal limitation can reduce a species’ ability to colonize environments or resources, thus excluding those conditions from its realized niche (Holt 2009). In contrast, the Eltonian noise hypothesis suggests that biotic interactions primarily influence species’ distributions and niches at local scales, while abiotic factors are mostly responsible for species occurrences at the landscape scale (Soberón and Nakamura 2009). The Eltonian noise hypothesis has been used to suggest that discrepancies between a species’ fundamental and realized NB should be greater at local scales (e.g., habitat boundaries) than at landscape scales (e.g., across broad climatic gradients). However, this hypothesis has received mixed support (Araújo et al. 2014; Fraterrigo et al. 2014). Since dispersal limitation should have the opposite effect—by decoupling fundamental and realized NB at landscape scales—the scale-dependence of factors shaping the realized niche likely hinges on the relative magnitudes of biotic interactions and dispersal limitation in a system.

Niche Breadth and Species Richness at the Local Scale

Niche theory provides a mechanistic explanation that helps inform predictions about coexistence when experimental tests are infeasible. There is a long history of interest in how niches influence community diversity (e.g., Pielou 1972; Schoener 1974). However, the impact of NB on local diversity is less studied. If an organism disperses into a community, its niche characteristics determine whether it can tolerate the local environment and biotic interactions (Weiher and Keddy 1995; Violle et al. 2012; Cadotte and Tucker 2017). That is, upon arriving at a site, an individual’s niche optimum and niche overlap with resident species impact competitive interactions and the likelihood that the individual successfully establishes there. Often it is implicitly assumed that niche optimum and overlap are captured by phylogenetic or functional trait distances within a community (e.g., Mouquet et al. 2012), and community assembly mechanisms are inferred from phylogenetic or functional trait distributions (i.e., overdispersion or clumping; Webb et al. 2002; Cadotte et al. 2011). NB is a component of niche overlap, but its independent contributions to the assembly and maintenance of diverse communities is underexplored.

We outline three possible (but conflicting) relationships between species’ resource-use NB and community species richness: wide NB increases niche overlap among species and decreases species richness via competitive exclusion; wide NB increases niche overlap and increases species richness (via neutral drift and slowing of competitive exclusion); and NB is mechanistically unrelated to species richness. Under the first scenario, the theory of limiting similarity implies that extensive niche overlap among species precludes coexistence (MacArthur and Levins 1967; Chesson 2008), and thus more narrow NB species can coexist in a community (“niche packing”). This idea is supported by a strong negative relationship between species richness and NB (whether based on habitat, climate, diet, or other species interactions) reported in a recent meta-analysis (Granot and Belmaker 2020). This is consistent with “stabilizing” niche differences, which reduce resource competition, allowing more species to coexist (Chesson 2000). Niche differences have been linked to coexistence in a number of experimental systems (e.g., Gravel et al. 2011b; Narwani et al. 2013; Kraft et al. 2015), but often underpredict coexistence. The second scenario predicts a positive relationship between average NB and community richness. Le Bagousse-Pinguet et al. (2014) found that within-species (as opposed to among-species) trait variation best explained the distribution of species richness in calcareous grassland communities. They reported that within-species variation in plant height likely increased trait overlap among species and reduced the ability of taller species to exclude other species through shading. Species with high intraspecific trait diversity likely span a broader range of environments (Darwin 1859; Sides et al. 2014) so, in the above study, it is thought that trait diversity and overlap translate to broad, overlapping niches between species. Trait similarity between species also minimized competitive differences, so Le Bagousse-Pinguet et al. (2014) concluded that “equalizing” mechanisms reduced fitness differences between species and would enable species to persist together through neutral processes (Chesson 2000). However, equalizing mechanisms alone, without stabilizing niche differences, are insufficient for stable long-term coexistence (Chesson 2008). This suggests that species richness could decrease in the long term, and in turn reduce the positive correlation between NB and species richness. Under the third scenario, NB is not a primary determinant of community richness. Instead, other factors (e.g., propagule pressure or disturbance) largely shape community composition (Lockwood et al. 2005).

Consistent trends have yet to emerge from tests of a direct relationship between NB and coexistence. Instead, insights into community assembly likely lie at the intersection of NB, niche optimum, and performance under specific conditions. This tripartite approach will be especially productive if there are tradeoffs between NB and individual performance (jack of all trades, master of none; Futuyma and Moreno 1988). Narrow NB species will be more likely to coexist if they utilize resources that are not used by other narrow NB species, or if they outperform wide NB species where they overlap. Alternatively, species with broader NB might, on average, be more likely to tolerate a given local environment (Kraft et al. 2015) and successfully integrate into an assemblage. In addition, if local temporal heterogeneity in environmental conditions precludes narrow NB species from consistently maintaining a local fitness advantage, broad NB species could be competitively superior and stabilize community composition. However, if narrow NB species can avoid unfavorable years (e.g., in dormant seeds) and capitalize in suitable years, they can coexist by partitioning niches through time in variable environments (“storage effect”; e.g., Angert et al. 2009). Clarifying when these different proposed mechanisms of community assembly are likely to apply may aid restoration efforts (de novo community assembly) and help predict how incoming species, such as potential invaders or species displaced due to environmental change, might interact with resident communities (Figure 3).

Niche Breadth in Biogeography

Fundamental NB acts in concert with landscape heterogeneity, dispersal, and interacting species to influence realized NB, species distributions, abundance, geographic range size, and occupancy (the fraction of suitable habitat that is occupied; e.g., Holt et al. 2004; see the section titled Disentangling Fundamental and Realized Niches: How Ecological Context Shapes Niche Breadth). That is, fundamental NB determines the suite of conditions under which a species could persist and is therefore useful for predicting range shifts in changing environments and the potential range for invasive species. However, realized NB is more closely related to present-day distributions and is often estimated using occurrence data across species distributions (see the section titled Quantifying the Niche).

A positive relationship between abundance and occupancy is one of the most consistent observations in macroecology (Holt et al. 1997; Gaston et al. 2000). Since locally abundant species usually produce more propagules and are less subject to stochastic extinctions than locally rare species, higher abundance should promote a species’ exploitation of its fundamental niche. Abundant species with high fundamental and realized NB should better withstand habitat heterogeneity because population fitness will be less sensitive to spatial environmental variation. In this case, the metapopulation dynamics that drive occupancy will be influenced largely by stochastic colonization and extinction processes rather than deterministic niche-based processes. Similarly, fundamental NB influences occupancy by determining the fraction of physical space in a heterogeneous landscape in which a species might persist (Brown 1984). Hence, NB may help explain variation in the abundance-occupancy relationship among taxa. For example, aquatic invertebrates with broad realized habitat NB were found to occupy more sites, while those with narrow realized habitat NB were more abundant at a given level of occupancy (Verberk et al. 2010). In the same study, the abundance-occupancy relationship fit the data better for species with broad habitat NB. Perhaps there was greater variance in the data for habitat specialists because specialists are more likely to be very abundant within their optimal habitats (Lesica et al. 2006) or have low abundance if the available habitat is outside their ideal conditions. Importantly, aquatic invertebrates with broader NB tended to also be better dispersers (Verberk et al. 2010), revealing part of the difficulty in distinguishing causal relationships among NB, dispersal ability, habitat heterogeneity, abundance, occupancy, and range size. Recent meta-analyses have shown that species’ geographic range size increases with NB for a wide range of taxa (Slatyer et al. 2013) at regional and global scales (Kamback et al. 2019). Yet, studies disagree whether low niche position (i.e., use of widely available resources or habitat) or high NB best explain species’ geographic distributions (e.g., Gregory and Gaston 2000; Sheth et al. 2014) and occupancy (Heino and Grönroos 2014). Together, these studies suggest that NB (fundamental and realized) interacts with habitat heterogeneity to drive key biogeographical patterns: species’ abundance, occupancy, and geographic distributions.

Niche Breadth Across Latitude: Species Richness and Rapoport’s Rule

The decline of species richness with latitude is remarkably consistent across taxa and scale of diversity (e.g., alpha, beta) considered (reviewed in Willig et al. 2003). One of many proposed explanations for this latitudinal species richness gradient involves underlying gradients in species’ climatic NB. Janzen (1967) hypothesized that reduced seasonal temperature extremes in the tropics would promote the evolution of species with narrow thermal tolerances compared to higher latitude species. This spurred the idea that narrow NB in tropical regions might lead to higher tropical species richness (e.g., Huey 1978; Cadena et al. 2011).

Some explanations for the latitudinal gradient invoke a key role of NB along biotic axes instead (Schemske et al. 2009). Dobzhansky (1950) proposed that stable, moderate tropical climates should render biotic interactions more important than abiotic tolerances in structuring communities and driving specialization and diversification in tropical regions. Biotic interactions could drive coevolution, adaptation, and speciation in the tropics (Schemske et al. 2009). Forister et al. (2015) found that the breadth of host plants used by herbivorous insects is narrower in the tropics, with many tropical insect species feeding on a single plant family or even species. Other work on herbivorous insects shows that narrow dietary breadth is associated with increased diversification (Hardy and Otto 2014; see the section titled Niche Breadth and Macroevolution). Thus, the narrow NB of tropical species could help explain why tropical regions are so species-rich. However, studies have also documented increased interaction breadths in the tropics (e.g., within mutualistic networks; Schleuning et al. 2012) or no gradient at all (Vázquez and Stevens 2004).

The hypothesis that NB is narrower in the tropics carries several often-untested assumptions. As outlined by Vázquez and Stevens (2004), this hypothesis relies on reduced environmental variability in the tropics, which begets stable populations and thus allows species to become more specialized. However, their cross-taxon analysis revealed that tropical populations are subject to substantial fluctuations in precipitation and population size, even though temperatures are more stable in the tropics (Vázquez and Stevens 2004). Thus, tropical species may have narrower thermal but wider precipitation NB compared to temperate species (Figure 2). This idea is now supported across hundreds of species of plants and animals (Liu et al. 2020). Therefore, the relationship between NB and latitude depends on the niche axis considered. Moreover, whether there is a causal relationship between NB and species richness is unclear (see the section titled Niche Breadth and Species Richness at the Local Scale). In the context of interaction breadths, for example, competition could lead to specialization and diversification, or high species richness could increase competition and drive selection for specialization over time (Fischer 1960). Similarly, Vázquez and Stevens (2004) proposed that narrower NB in the tropics is a product, rather than a driver, of the latitudinal species richness gradient.

A competing hypothesis proposes that climate actually fluctuates more in low-latitude than high-latitude areas, fragmenting species’ ranges and increasing rates of allopatric speciation in the tropics (see Rangel et al. 2018; Saupe et al. 2019). Recent modeling studies have suggested that climate heterogeneity through space and time drives diversification (Rangel et al. 2018); specifically, variation in precipitation at low latitudes increases speciation rates, and extinction rates across latitudes increase with precipitation and temperature fluctuations (Saupe et al. 2019). A relatively bare-bones model of speciation and extinction rates and climatic variability (i.e., species had different but fixed NB and dispersal abilities, biotic interactions not included) generated a latitudinal diversity gradient closely resembling our own, including a preponderance of narrow NB species at low latitudes (Saupe et al. 2019). In another model, intermediate levels of niche evolution maximized diversification and helped the model match realistic species richness gradients across a complex landscape (Rangel et al. 2018). Niche conservatism led to high extinction rates, as species failed to adapt to changing climates, and very rapid niche evolution resulted in a few widespread species and little diversification, since climate variation was tolerable and insufficient to isolate populations (Rangel et al. 2018).

Rapoport’s rule posits that species’ latitudinal range size increases with latitude (Stevens 1989). This rule may provide a more proximate explanation for the latitudinal richness gradient, since species’ distributions across latitudinal climatic gradients can be explained by NB variation. Rapoport’s rule has received mixed empirical support. Many taxa, particularly those in the tropics and Southern Hemisphere, do not follow Rapoport’s rule (Gaston et al. 1998; Willig et al. 2003). Although Rapoport’s rule is typically used to explain the latitudinal richness gradient, Beaugrand et al. (2013) instead suggested the reverse: that richness may help explain why certain taxa do not follow Rapoport’s rule. They modeled plankton thermal tolerances and geographic distributions and found that species richness of broadly thermotolerant species increased with latitude. But, since richness of narrow NB species peaked both at the poles and tropics, certain clades showed no discernible NB-latitude gradient.

Latitudinal variation in NB is central to several hypotheses for the latitudinal richness gradient, but single variables are unlikely to adequately predict the distribution of species richness. Synthetic hypotheses (e.g., simultaneously considering NB in temperature and precipitation and other factors that covary with latitude) should provide important insights (Willig et al. 2003).

Niche Breadth and Microevolution

NB hierarchical structure arises when processes shaping niche overlap among individuals or populations influence NB at higher biological scales. Intraspecific competition can generate disruptive selection, favor divergently specialized individuals, and ultimately increase phenotypic variation at the population or species level (Bolnick 2001). Similarly, competition among species within a community can lead to character displacement and niche differentiation (e.g., Zuppinger-Dingley et al. 2014). For example, niche differentiation in resource (seed size) and habitat use in desert rodent species increased with rodent community diversity, suggesting that competition drives resource partitioning (M’Closkey 1978). Studies with sticklebacks confirmed that competition can drive niche divergence (Schluter 1994). At larger spatial scales, contrasting selection pressures across a heterogeneous landscape can cause populations to diverge, increasing species-level NB (e.g., Linhart and Grant 1996).

Like competition, positive biotic interactions can promote niche differentiation among populations. However, these interactions have received relatively little attention in this context. Positive biotic interactions that increase organismal performance in new or marginal environments (broadening species’ realized NB or allowing new niche optima to emerge) could provide the initial ecological opportunity for niche differentiation (e.g., Liancourt et al. 2012; Weber and Agrawal 2014). For example, in the mutualistic interaction between lycaenid butterflies and ant species, ants protect caterpillars from predation in return for nectar rewards secreted by the caterpillars (Forister et al. 2010). Attendant ants were found to increase larval caterpillar survival on a novel host plant. Therefore, they could lead to evolution of increased dietary breadth in lycaenid butterflies (Forister et al. 2010). In another system, fungi provide both food and defense for midge larvae developing in plant galls, and this symbiosis with fungi likely spurred expansion of host-plant resource use for gall-inducing insects (Joy 2013). Symbiotic insect lineages used a breadth of host plant taxa seven times greater than asymbiotic lineages (Joy 2013). In some cases, this increased NB through positive biotic interactions can allow subsequent niche divergence, reproductive isolation, and ecological speciation in diverging lineages, as discussed below (Hawthorne and Via 2001; Nosil 2012).

As competitive inequalities or positive biotic interactions spur populations to explore new niche space, the balance of coexistence mechanisms (stabilizing niche differences and fitness-equalizing factors; Chesson 2000) should shape successive NB evolution. Fitness-equalizing mechanisms might relax both stabilizing selection and selection against less fit genotypes, thus maintaining NB. In contrast, stabilizing mechanisms could reduce opportunities for further niche expansion for species that are tightly packed along a niche axis. In this case, species at the ends of the niche axis (i.e., that use less contested or more marginal resources) could increase their NB if more extreme environments are available. These ideas would benefit from experimental tests.

Niche Breadth and Macroevolution

Studies of NB and macroevolution have included analyses of how NB impacts diversification (speciation - extinction), and macroevolutionary transitions between specialists and generalists. In vertebrates, studies have found negative relationships between temperature NB and diversification (Rolland and Salamin 2016) and positive relationships (after including precipitation and niche position; Gómez-Rodriguez et al. 2015). In herbivorous insects, two contrasting mechanisms by which NB can influence diversification have been proposed. The oscillation hypothesis (Janz and Nylin 2008) suggests that lineages fluctuate between specialization and generalization over evolutionary time (analogous to Figure 1.a.ii). If breadth comes at the cost of lower performance in any single environment, generalists are expected to give rise to specialists as lineages adapt to that environment and diverge. Over time, specialist lineages evolve into generalists as they disperse across a landscape and broaden their host plant usage. Alternatively, the musical chairs hypothesis (Hardy and Otto 2014) suggests that host switching helps drive insect diversification. Host switching entails changes in niche optimum and/or position that broaden a clade’s NB without necessarily changing mean species-level NB (analogous to Figure 1.a.i). Consistent with this idea, diversification of phytophagous butterflies increased with host switching rates, not changes in species’ NB (Hardy and Otto 2014).

Much of the literature linking NB and macroevolution has focused on the hypothesis that specialists originate from generalist ancestors, have limited potential for further evolutionary change, and are “evolutionary dead ends” destined for extinction (Cope’s Law of the Unspecialized; Cope 1896; Futuyma and Moreno 1988). This idea has received only mixed empirical support (Mayr 1963; Van Valen 1973). There are examples of specialists giving rise to generalists (e.g., Armbruster and Baldwin 1998; Nosil 2002), but other studies suggest that the transition from generalist to specialist is indeed more common (e.g., Kelley and Farrell 1998; Stephens and Wiens 2003). Tip-ratio bias (when two trait states—such as specialization and generalization—are unevenly represented among taxa on a phylogenetic tree), the number of taxa sampled, and how completely a phylogeny is sampled could all impact inferred transitions between generalization and specialization (Day et al. 2016).

There are many potential avenues for future work to explore the contexts in which NB promotes or precludes diversification. As one example, positive interactions can increase species-level NB, which may either maintain gene flow and hinder speciation across an environmental gradient or, alternatively, create opportunities for divergent selection and speciation (e.g., Liancourt et al. 2012). Association with ants increased lycaenid butterfly dietary NB, which is thought to have promoted butterfly diversification (Forister et al. 2010). Similarly, the enhanced species-level NB of symbiotic lineages of gall-inducing insects likely raised diversification rates: symbiotic lineages were 17 times more diverse (Joy 2013).

Changing Species Distributions

One current focus in conservation biology is understanding changes in species’ geographic ranges—for example, expansion of invasive species and movement of range edges as species track climatic conditions. Climatic NB may be particularly relevant for understanding current and future changes in species’ ranges. Species distribution models (e.g., Guisan and Zimmermann 2000; Peterson et al. 2011) built from climatic data are often used to predict the spread of invasive species (e.g., Peterson 2003) and the future distribution and persistence of species under climate scenarios (e.g., Thomas et al. 2004). This approach uses species’ current climatic distributions to predict their future spread and occurrence, but there is debate about how effective this approach is for these purposes (e.g., Pearman et al. 2008; Araújo et al. 2011; Petitpierre et al. 2012; Early and Sax 2014; Atwater et al. 2018). Most importantly, this approach assumes that climatic niches will remain broadly similar over time (e.g., Peterson 2003). However, examples of both niche conservatism and rapid niche evolution exist in the literature (e.g., Wiens and Graham 2005; Pearman et al. 2008). In addition, this approach may not accurately predict changes in a species’ distribution when its fundamental climatic NB is much greater than its realized climatic NB. If species overcome dispersal limitation or biotic factors change, species can rapidly shift or expand their realized climatic niches (e.g., Wiens et al. 2019). These rapid changes in realized niches can allow species to invade regions with very different climates and potentially persist under rapid climate change (e.g., Petitpierre et al. 2012; Atwater et al. 2018; Wiens et al. 2019). Climate change and species introductions are also expected to expose species to “nonanalog conditions,” including unique combinations of climatic variables and biotic factors not currently present in their native ranges (e.g., Jackson and Overpeck 2000; Reu et al. 2014). This possibility underscores the value of understanding which factors actually set species range limits (and potential correlations among these factors). Below we further discuss how NB can shape species distributions, in the contexts of biological invasions and climate change.

Response to Climate Change

Current work indicates that NB, its underlying hierarchy, and correlations in breadth among niche axes all influence species’ responses to climate change. Most studies using NB to infer sensitivity to climate change focus on thermal or other abiotic axes (e.g., precipitation, growing degree days). Empirical evidence suggests that species with narrow climatic NBs might be more sensitive to climate change (e.g., Thuiller et al. 2005) if climate shifts outside of their small range of tolerances and they cannot track preferred conditions over space (e.g., Loarie et al. 2009). In contrast, species with wide climatic NB are expected to be relatively robust to climate change (e.g., Schwartz et al. 2006).

The idea that species with wide NB will better withstand climate change (versus narrow NB species) is implicitly rooted in the idea that individuals and populations of those species are themselves broadly tolerant. However, the mechanisms and consequences of a NB advantage under climate change likely depend on the underlying hierarchy of NB (Figure 4). Given the same climatic NB, a species composed of broad NB individuals and/or genetically diverse populations is expected to have a higher probability of persisting in situ than one composed of highly specialized, differentiated populations, especially under dispersal limitation (Etterson and Shaw 2001). From an evolutionary perspective, if species-level NB results from high genetic variation among individuals, wide NB species could instead appear relatively insensitive to climate change not because they possess broader tolerances, but rather because they have the propensity to evolve to local conditions (Thomas et al. 2012). However, climatic NB is generally uncorrelated with rates of climatic niche evolution (Liu et al. 2020). Furthermore, niche change might not keep pace with projected rates of climatic change (e.g., Román-Palacios and Wiens 2020), especially given widespread local extinctions already observed (Wiens 2016).

Climate change could have contrasting effects on narrow NB species, depending on their niche position, dispersal ability, and how climate change impacts their communities. If the conditions or resources a species requires become more available with climate change, the species could become more abundant or widespread. For example, migratory songbirds inhabiting mixed deciduous forests are predicted to show range expansion as this habitat spreads in southern Ontario (Naujokaitis-Lewis et al. 2013). Climate change could also offer respite to narrow NB species by altering communities and species’ interactions. For instance, changing interaction networks could lead to a novel mutualistic interaction that ameliorates abiotic stress (Hoffmann and Sgrò 2011). Conversely, climate change could adversely affect narrow NB species. For many species, dispersal limitation could prevent them from escaping unfavorable conditions by colonizing new habitat patches or tracking preferred conditions (e.g., Thomas et al. 2004). A recent study of over 500 plant and animal species found that most are unlikely to disperse quickly enough to stay in their past thermal niches, based on their recent dispersal rates (Román-Palacios and Wiens 2020). Additionally, species declines from climate change often stem from changes in species interactions (Cahill et al. 2013; Ockendon et al. 2014). Climate change could precipitate species declines through novel competitive environments, intensified pathogen infections, and weakened mutualistic associations (Tylianakis et al. 2008; Alexander et al. 2015), which may disproportionately impact narrow NB species.

Even if species have narrow climatic NB, broad tolerances along nonclimate niche axes could buffer the effects of climate change (Figure 2). If climate change triggers range shifts, species that can use a wider range of resources (e.g., broad dietary NB) may be less impacted because they can meet their requirements in new habitats (Gravel et al. 2011c). Simulated diet expansion substantially reduced predicted extinction rates of alpine butterflies under climate change, even when just a few closely related host plant species were added (Descombes et al. 2015). Thus, predictions of species’ responses to climate change might differ between studies that consider nonclimatic niche axes versus those that focus on climate alone.

Niche Breadth and Conservation Planning

NB, abundance, and geographic range size interact to generate different forms of rarity (Figure 5), which influence how robust a species is to different types of environmental change (Rabinowitz 1981). Although these ecological characteristics are often positively correlated (see the section titled Niche Breadth in Biogeography), species can have, for example, narrow NB but high abundance (e.g., Juniperus cedrus) or broad NB but restricted geographic ranges (e.g., Acacia sciophanes; Figure 5; Rabinowitz 1981; Espeland and Emam 2011). Depending on their form of rarity, species could be more vulnerable to certain threats (Figure 5). Species that have low abundance but wide NB, for instance, if they occupy multiple habitat types but are not particularly competitive, should be more susceptible to diffuse perturbations such as nonpoint source pollution or regional climate change. In contrast, species that are both scarce and have a narrow range of habitat affinities will be more vulnerable to acute perturbations such as point-source pollution, a species’ invasion, or local land-use change (Sattler et al. 2007). Thus, the first type of species will be best protected by regional management plans dealing with land-use change and connectivity among habitats, for example, while the second type of species will be better served by local restoration efforts such as removal of invasive species or remediation of degraded habitat.

A fundamental goal of conservation biology is to determine which characteristics make species vulnerable to extinction. Narrow NB has been linked to extinction risk in more than 80% of studies included in a cross-taxon literature review (Colles et al. 2009). Estimates of fundamental climatic NB of 55 species of island-endemic conifers revealed that 3.6–23.6% of these species (depending on the climate change model) are expected to be in environments that fall outside of their fundamental niches within the next 50 years, with narrow NB species the most affected (Rosenblad et al. 2019). By teasing apart areas within species’ realized, fundamental, and tolerance (i.e., survival but no reproduction) niches, Rosenblad et al. (2019) made targeted conservation recommendations: prioritize conservation of areas within realized niches, protect microrefugia within fundamental niches, and enhance recruitment for species that will only have conditions within their tolerance niche remaining.

NB is seldom considered in most widely used vulnerability criteria, such as the IUCN Red List (http://www.iucnredlist.org). Rather, these criteria generally focus on geographic range size, population trends, and population viability (Yu and Dobson 2000). Viability assessments sometimes factor in habitat requirements where these data exist. The IUCN Red List is a globally recognized index of species’ vulnerability, grounded in ecological data that is applicable across taxa and informative for conservation planning (Rodrigues et al. 2006). Indeed, variables such as geographic range size may be reasonable surrogates for NB data in many cases, since NB is positively correlated with range size across a broad range of taxa (Slatyer et al. 2013; Kambach et al. 2019). However, in insectivorous bats, for example, narrow dietary NB was associated with extinction risk and was independent of geographic range size (Boyles and Storm 2007). Furthermore, by comparing traits of butterfly species listed as threatened and nonthreatened by the IUCN, Kotiaho et al. (2005) found that threatened taxa often had narrow larval dietary NB, narrow adult habitat NB, poor dispersal ability, and high niche position (i.e., the larvae specialize on geographically restricted plant species). By considering different forms of rarity and identifying the traits underlying vulnerability, beyond the ecological data used to construct the IUCN listing, the authors predicted that several species listed as nonthreatened would likely be at risk of extinction. More generally, if species ranges have been recently reduced by human activities, range size may be an underestimate of NB, so species may be more resilient than they appear based on their current distributions (Faurby and Araújo 2018). Since rarity is multifaceted, and species that exhibit multiple forms of rarity are most vulnerable (Davies et al. 2004), conservation assessments that include NB and other dimensions of rarity when available will provide a more comprehensive view of vulnerability while also pointing to possible solutions.

Conservation practitioners that wish to include explicit NB data in vulnerability assessments would first need to determine which (and how many) niche axes to consider. To this end, it would be valuable to know if and when breadth is correlated among niche axes and the functional or environmental constraints underlying that correlation (see the section titled Correlations in Breadth among Niche Axes). If NB is largely independent across niche axes, then species that are vulnerable to one threat, like higher temperatures, might nevertheless be robust to others, such as drought. Instead, if positive correlations in NB are common, a species’ vulnerability to habitat modification or climate change could be predicted by its NB along a single representative axis. In this case, species that have narrow breadth along one axis will likely have narrow breadth along others, increasing their extinction risk. An important additional step is to assess whether the environmental axes likely to be most affected (e.g., by invasive species or land-use change) are also the axes along which NB is most restricted.

Some ways of estimating NB for conservation assessments rely on data that are already collected for vulnerability assessments or existing information. The variables most often considered in vulnerability assessments, such as geographic range size and population trends, are derived from occurrence data or repeat counts over time. These same occurrence data could be used to estimate realized NB of potentially at-risk species. Existing information from experts could also be included. Floras and online databases often list habitat affinities. If, for example, a species is threatened by disappearing wetland habitat, knowledge of its fundamental NB could explain whether the species is intrinsically specialized on that habitat type or if conservation efforts that included assisted dispersal could allow it to establish in other habitats. However, estimating fundamental NB requires experimentation or parameterization of mechanistic models, which is not likely to suit all conservation contexts.

Niche Breadth and Biodiversity-Ecosystem Functioning Relationships

Ecosystem function is the measurable stocks, flows, and stability of energy and nutrients moving through an assemblage (e.g., primary production, nutrient cycling, support or control of other trophic levels; see, for example, Duffy et al. 2007). NB is thought to have important implications for how changes in biodiversity influence the magnitude and stability of ecosystem functioning (Figure 3; Duffy et al. 2007). The positive impact of biodiversity on ecosystem functioning occurs via complementarity among functionally diverse forms or via a numerical sampling effect by which diverse communities are more likely to contain single highly productive species (Loreau and Hector 2001). We couch our discussion of NB within these proposed mechanisms for the biodiversity-ecosystem functioning relationship.

Complementarity among species, and therefore ecosystem functioning, should be greatest in an assemblage of species that each specialize on different resources (Figure 3; Poisot et al. 2013; Turnbull et al. 2013). Theoretically, species with narrower NB along a resource axis will overlap less along that axis (on average) than those with broader niches and contribute in more distinct ways to system functioning. Consequently, a large proportion of narrow NB species within communities could lead to strong species richness–ecosystem functioning relationships (e.g., Gravel et al. 2011a). Even if species have broad NB along a resource axis, if they are composed of genotypes specialized on different parts of that axis (Cook-Patton et al. 2011), this could generate a similar level of functional diversity as a community of specialists and thus increase ecosystem functioning.

If biodiversity primarily increases ecosystem functioning through sampling of a high-performing species, rather than complementarity among species, the impact of NB on the biodiversity-ecosystem functioning relationship should depend on the availability and heterogeneity of resources. If resource availability is fairly homogenous, a narrow NB species with low niche position (in the biogeographic sense; see the section titled Quantifying the Niche) should perform best, assuming a NB performance tradeoff. However, on average, broad NB species should contribute relatively more to ecosystem functioning because many narrow NB species could have high niche position (i.e., specialize on marginal resources) and be ill-suited to the existing conditions, and the “best-performing” narrow NB species might not be present in each community (Huston 1997). In species-poor assemblages with high resource heterogeneity, broad NB species should best increase ecosystem functioning (e.g., Gravel et al. 2011a). As community species richness increases, this benefit of broad NB species would dissipate if narrow NB species are added that best exploit the available resources.

NB can have contradictory influences on the stability of ecosystem functioning in temporally heterogeneous systems. Ultimately, the effect of NB on ecosystem stability will depend on how NB influences niche overlap and competition, as well as how well community NB encapsulates temporal environmental variation. Highly overlapping niches among species within a community should reduce the stability of ecosystem functioning because strong competition among species with overlapping niches has been shown to be destabilizing (Loreau and de Mazancourt 2013). Although it is traditionally thought that competition stabilizes system functioning by creating asynchronous population dynamics, Loreau and de Mazancourt (2013) show that compensatory dynamics among asynchronous populations can be offset by increased population variability that can also be produced by competition. Given this, the NB mechanisms that promote coexistence, especially if they allow greater niche partitioning, should also result in greater stability. These ideas apply for biotic or abiotic niche axes: even if species stably coexist via niche partitioning under a set of conditions, changes to prey availability or climate, for example, can impact species’ fitness and ability to coexist (Cadotte and Tucker 2017), thus altering the stability of ecosystem functioning. Further, the interaction between species- and community-level NB could dictate how assemblages respond to environmental change. As local conditions fluctuate, performance of narrow NB species may be more variable as conditions deviate from their optima. Therefore, scaling up, ecosystem functioning should fluctuate more for assemblages with more narrow NB species, if species that can quickly compensate are not available in the system (Thébault and Loreau 2005). Community-level NB should predict the ability of compensatory dynamics to maintain stability of ecosystem functioning in changing environments.

Mapping traits to NB can help generate testable predictions about the mechanistic relationship between NB and ecosystem functioning. For example, on a given focal niche axis, the idea that variable performance of narrow NB species would translate into variable ecosystem functioning in changing environments (discussed above) seems most applicable when the traits that respond to the environment and that shape NB (“response” traits) also drive ecosystem functioning (“effect” traits; Lavorel and Garnier 2002). However, the overlap between these response and effect traits remains an area of active research (Funk et al. 2017; Refsland and Fraterrigo 2017). Ecosystem functioning depends on the relative abundances of effect traits, so we envision two general pathways by which traits, NB, and ecosystem functioning interact. First, response traits directly influence fundamental NB. NB in turn shapes the species’ survival and abundance in an area and, therefore, relative abundances of the effect traits that drive ecosystem functioning. Second, response traits could influence ecosystem functioning based solely on their impact on species’ abundances. In this scenario, realized NB arises from abundance and is not a driver of ecosystem functioning; rather, the correlation between realized NB and ecosystem functioning is due to their common cause (see the section titled Species’ Abundance and Dispersal Limitation). Linking traits to NB could also identify which niche axes are likely to be correlated with one another due to reliance on shared traits (e.g., Figure 2c where trichome hairs influence tolerance to both drought and herbivore pressure). Moreover, if we determine how traits influence NB in a given system (e.g., Lennon et al. 2012), we can gain ecological insight into other systems with similar traits (McGill et al. 2006). For example, we could predict that trichomes may generally make plants more drought tolerant and increase NB (Figure 2c), if trichomes do not also reduce plants’ ability to withstand nondrought conditions. Hence, identifying traits that underpin NB across specific environmental gradients may suggest pathways by which environmental changes are likely to modify ecosystem functions, including those functions that affect human well-being (i.e., ecosystem services).

Despite the conceptual outline above, the role of NB in modulating biodiversity-ecosystem functioning relationships has seldom been directly empirically tested (but see Gravel et al. 2011a). Thus, additional experimental tests are needed. Such empirical studies could productively be extended to consider multiple ecosystem functions and evaluate how negative correlations in NB across axes affect the stability of these functions.