1. Introduction
Dominant species are those species that excel at exploiting and sequestering resources [
1] thereby affecting the behavior and population dynamics of other species and potentially the structure of communities. Janzen [
2] and Connell [
3] famously argued that if populations of dominant species were not kept in check by predators or parasites, they would become overabundant and exclude other species, leading to a decrease in species richness. In relation to this hypothesis, several studies have documented a unimodal relationship between the richness of species in a local community and the abundance of dominant species [
4,
5]. The most common explanation for this unimodal relationship is that at low levels of dominance, the abundance of both dominant and non-dominant ants increases as environmental stress decreases. Then, as dominant species become more abundant, they competitively exclude non-dominant species leading to the descending portion of the hump-shaped curve [
4,
5]. An alternative explanation is that the abundance of dominant and non-dominant species peaks at a different point along the abiotic stress gradient. One approach for teasing apart these alternative explanations is to test predictions regarding the influence of competition with a dominant species across levels of organization, from individuals to entire communities.
In animals, dominant species are those with high abundance and biomass, which may be attained through exploitative and interference competition. Dominance can be attained via exploitative competition or interference competition. In ants, dominant species often display aggressive behavior enabling them to defend territories and monopolize food resources. Aside from the unimodal relationship between species richness and the abundance of dominant species [
4,
5], evidence that dominant species have community-wide effects on other species includes the effects of competitively dominant exotic species on the diversity [
6,
7,
8,
9], spatial arrangement [
10] and phylogenetic structure of native ant communities [
11]. Additional evidence stems from the effects dominant ants have on patterns of co-occurrence in arboreal assemblages of the tropics [
12,
13,
14,
15,
16,
17] and the dominance-species richness relationship [
4,
5,
18,
19]. However, a recent global study suggests that exotic dominant ants are more likely to exert community-wide effects than are native dominant ants [
20]. Moreover, experimental removals of dominant species have yielded mixed results [
21,
22,
23]. In sum, whether or not native dominant ants affect other members of the community deserves further exploration.
Several studies have documented the effects of dominant ant species on the behavior, resource use and fitness of non-dominant ant colonies [
24,
25,
26,
27,
28], while others have documented their effects on community structure [
4,
18,
29,
30,
31,
32,
33]. Examining the outcome of interspecific interactions across organization levels is key to elucidating the mechanism by which competition might shape community structure [
34]. However, few studies [
22,
23,
33] have linked the effects of a single dominant ant species on individual colonies, populations or communities of competitively inferior ants. In ants, community-level effects, relate to the richness or other multi-species metrics of diversity (including composition). Population-level effects relate to the abundance of a species at a site (i.e., number of colonies or workers). Individual effects relate to characteristics of the colonies of a given species. Worker-level effects relate to the behavior of individual ant workers when confronted with workers of another species at a given food source.
Community-level effects of dominant ants on non-dominant ant species are well documented but much less is known regarding the individual-level effects (i.e., colony-level effects) of dominant ants on non-dominant ants. It is well established that dominant ants often interfere with resource exploitation by non-dominant ant workers via negative behavioral effects [
8,
29,
35,
36,
37,
38]. Moreover, it is often assumed that by interfering with the foraging activities of non-dominant species, dominant ants also affect rates of resource acquisition and sequestration at the individual colony-level (but see [
27]). If that were the case, the effect of dominant ants should be observed at the individual-level by decreasing the productivity, size and/or fitness of colonies of non-dominant ants [
26,
31]. However, whether the negative effect of dominant species on colony success of non-dominant ants is the rule rather than the exception, and whether colony-level effects always translate into community-level effects, is unclear [
39].
Effects of dominant species in the genus
Formica (
rufa group) on non-dominant ant species have been well documented in a series of seminal studies in ant assemblages in Finland [
26,
27,
28,
32,
33].
Formica ants in the rufa group form large colonies and are aggressive but are uncommon in temperate forests of North America. However,
Formica ants in the
fusca group, which form smaller colonies and are less aggressive, are ubiquitous. Here, we examine how one of the most numerically and behaviorally dominant species in a low-elevation temperate forest in the eastern US,
Formica subsericea (
fusca group) Say, affects non-dominant ant species. We first investigate community-level effects by testing whether the relationship between the richness of non-dominant species and the abundance of
F. subsericea best fits a unimodal or linear model [
4,
5,
18]. Then, to elucidate whether community-level effects of
F. subsericea concords with individual-level and behavioral effects, we asked the following questions: (i) the colony density and/or abundance of non-dominant ant species decreases with increasing abundance of
F. subsericea, (ii) the colony size and/or productivity of colonies of non-dominant ant species is lower near than far from the nest of
F. subsericea, (iii) resource use by non-dominant ants is lower near than far from
F. subsericea nests and (iv) non-dominant ant foraging activity is lower when
F. subsericea are active than when inactive.
3. Results
The incidence of the dominant F. subsericea varied from 1 to 8 (of 12) pitfall traps per site. The total abundance of non-dominant species (the sum of incidences across 12 pitfall traps at a given site) varied from 24 to 45 and total richness of non-dominant species varied from 4 to 12 species per site.
3.1. Species Richness and Dominance
When considering all sampled sites, there was only a marginally significant polynomial relationship between the abundance of
F. subsericea and species richness (r
2 = 0.53, r
2adjusted = 0.39,
n = 10,
p = 0.07, AIC = 17.90 for quadratic fit vs. r
2 = −0.26, r
2adjusted = −0.12,
n = 10,
p = 0.95, AIC = 23.40 for linear fit;
Figure 1). The relationship was significant only if one observation was removed (r
2 = 0.63, r
2adjusted = 0.51,
n = 9,
p = 0.05, AIC = 10.36 for quadratic fit vs. r
2 = 0.25, r
2adjusted = 0.15,
n = 9,
p = 0.16, AIC = 14.64 for linear fit). Note that adjusted r
2 values can be negative when the model fits the data poorly [
68].
3.2. Is the Pooled Abundance of Non-Dominant Species Negatively Related to the Abundance of the Dominant Species?
There was no relationship between the abundance of F. subsericea and the total abundance (incidences) of non-dominant species (r2 = 0.28, r2adjusted = 0.07, n = 10, p = 0.32, AIC = 39.77 for quadratic fit vs. r2 = 0.04, r2adjusted = −0.07, n = 10, p = 0.57, AIC = 39.00 for linear fit). This relationship was not different when the statistical outlier was removed (r2 = 0.17, r2adjusted = −0.10, n = 9, p = 0.57, AIC = 30.80 for quadratic fit vs. r2 = 0.08, r2adjusted = −0.05, n = 9, p = 0.47, AIC = 29.76 for linear fit). The relationship between the abundance of non-dominant ants and dominant ants did not depend on whether the non-dominant species were common (r2 = 0.30, r2adjusted = 0.10, n = 10, p = 0.29, AIC = 28.87 for quadratic fit vs. r2 = 0.30, r2adjusted = 0.21, n = 10, p = 0.10, AIC = 26.91 for linear fit) or rare (r2 = 0.37, r2adjusted = 0.19, n = 10, p = 0.20, AIC = 27.65 for quadratic fit vs. r2 = 0.00, r2adjusted = −0.12, n = 10, p = 0.94, AIC = 30.30 for linear fit).
3.3. Is Colony Size and/or Productivity of Non-Dominant Ant Species Negatively Influenced by Proximity to a Dominant Ant Colony?
Neither colony size (
Figure 2a) nor colony productivity (
Figure 2b) of
A. rudis depended on distance from
F. subsericea nests (
Table 3). Likewise, the colony size of
N. faisonensis did not depend on distance from
F. subsericea nests (
Figure 2a), whereas productivity was, on average, 2× higher near than far from
F. subsericea nests (
Figure 2b), though this result was marginally significant (
Table 3).
3.4. Is Resource Exploitation by Non-Dominant Ants Negatively Influenced by Proximity to a Dominant Ant Colony?
The mean number of
F. subsericea workers recorded at baits was 3× higher near (≤1 m) than far (≥5 m) from the focal nest (
Table 4,
Figure 3a). During the day, the number of workers of non-dominant species was 50% lower at baits near than far from focal
F. subsericea nests but the richness of non-dominant species did not differ between near and far baits (
Table 4). There were 4× fewer A. rudis workers on baits near than far from
F. subsericea nests during the day (
Table 4,
Figure 3a). However, there was no difference in the number of
N. faisonensis (
Table 4,
Figure 3a) or
M. punctiventris (
Table 4,
Figure 3a) workers on baits near and far from
F. subsericea nests during the day.
In only one instance did we find
F. subsericea workers foraging at night (i.e., one worker at one bait), suggesting that
F. subsericea is largely diurnal, at least in this system. At night, there was no difference in the number of workers of non-dominant species (
Table 4,
Figure 3b), nor in the number of non-dominant species (
Table 4,
Figure 3b) on baits near and far from focal
F. subsericea nests. Likewise, there was no difference in the number of
A. rudis (
Table 4,
Figure 3b) or
M. punctiventris workers (
Table 4,
Figure 3b) on baits near and far from
F. subsericea nests. There were, however, 2× more
N. faisonensis workers on baits near than far from
F. subsericea nests at night (
Table 4,
Figure 3b).
3.5. Are Temporal Patterns of Foraging Activity in Non-dominant Species Negatively Related to Those of the Dominant Species?
There were higher numbers of workers of non-dominant species foraging on baits at night than during the day (mean difference = −28.5 ± 9.60;
Table 4;
Figure 4a). Additionally, there were fewer species of non-dominant ants recorded on baits at night than during the day (mean difference = 1.33 ± 0.36;
Table 4;
Figure 4b). There were 2× more A. rudis workers recorded on baits at night than during the day (mean difference = −14.92 ± 5.83,
Table 5 and
Table 6). The same trend was seen in N. faisonensis, with 2× more workers on baits at night than during the day (mean difference = −8.50 ± 3.29,
Table 5 and
Table 6). There were 2× fewer M. punctiventris workers recorded on baits at night than during the day (mean difference = −2.92 ± 1.19,
Table 5 and
Table 6).
4. Discussion
Increases in the abundance of one species are often associated with decreases in abundance and richness of other species it competes with [
2,
3]. If these patterns resulted from competitive exclusion, one would expect negative impacts of the dominant species on the subordinate species to trickle up and down organization levels. Here, we found only weak support that the abundance of dominant
F. subsericea ants was related to the richness of non-dominant ant species across sites [
4,
5,
18,
19,
44]. However, the shape of that relationship was sensitive to the inclusion/exclusion of a highly influential observation. Moreover, except for a few exceptions, population-, individual- and worker level variables such as abundance, productivity and foraging behavior did not match the predictions of competitive exclusion and its impact on diversity maintenance. It might thus be that competitive interactions do not lead to the exclusion of non-dominant ants [
69,
70].
One mechanism by which dominant ant species might exert community-level influence on non-dominant species is by reducing the amount of resources available in the system (e.g., dominance-impoverishment-rule [
71] but see Reference [
19]. Although in our system dominant ant species do not appear to affect population-level impact on the abundance of non-dominant ants, they could nevertheless have individual-level effects. Specifically, by interfering with resource use and/or resource acquisition by non-dominant species [
29,
32], dominant species may negatively affect the size, productivity and fitness of colonies of non-dominant ants [
26]. For example, in Finnish ant assemblages colonies of non-dominant ants found inside the territory of dominant
Formica tend to be smaller than those found outside dominant
Formica territories [
26]. However, proximity to a
F. subsericea nest, the dominant ant species at our study sites, did not affect colony size in two of the most common non-dominant ant species (
A. rudis and
N. faisonensis). Gibb and Hochuli (2004) experimentally excluded dominant
Iridomyrmex purpureus and found only congeneric species to benefit from competitive release, suggesting that dominant species might have greater effects on closely related species than they do on those that are distantly related [
11]. In our system,
F. subsericea was the only representative of the genus and the principal competitors are quite smaller in size suggesting perhaps lower overlap in resource use under natural conditions.
While we did not observe any effect of dominant ants on colony size, we found evidence of positive effects on another individual-level variable. Colony productivity of
N. fasionensis tended to be higher in colonies near
F. subsericea nests relative to colonies that were far from these nests (note that
p = 0.06). Thus, contrary to our expectations, dominant ants did not negatively affect the size and productivity of subordinate colonies of these common species and may actually be associated with enhanced productivity in one species—
N. fasionensis. Such positive effects of a large-bodied dominant species on small-bodied non-dominant species can arise when the indirect effects of competition outweigh the direct effects [
72]. Davidson [
73] who was working with harvester ants in desert systems showed that despite dietary overlap between a large species (
Pogonomyrmex rugosus) and a small species (
Pheidole xerophila), the large species facilitated the small species indirectly by suppressing populations of an intermediate-size species (
Pogonomyrmex desertorum). It may thus be the case that in our system
F. subsericea (large size) suppress populations of
A. rudis (medium size), thereby facilitating
N. fasionensis (small). Apparent facilitation might thus be at play [
73].
In sum, we did not find strong community, population or individual-level evidence that dominant
F. subsericea negatively affects non-dominant ants. We did, however, limit our analyses to a subsample of the most common species in the assemblage. One possibility is that dominant ants do not affect the resource use of all of the non-dominant species equally. For example, although
F. subsericea interferes with foraging and resource use by
A. rudis, two non-dominant ant species (
N. faisonensis and
M. punctiventris) did not exhibit lower resource use near
F. subsericea nests relative to far. Thus, while
F. subsericea was dominant over
N. faisonensis and
M. punctiventris in aggressive encounters, it did not decrease the resource use of these species at transient food items. However, we did not estimate the amount of time spent at resources or the rate at which resources were removed, which might be more accurate estimates of resource use by ants [
74]. Nevertheless, the lower foraging among
A. rudis observed near than far from
F. subsericea nests did not translate to reduced colony size or productivity. Herbers [
30] noted that temperate forest ant assemblages can be limited by the availability of nest sites, rather than food resources. Thus, the benefit of nesting in a suitable patch might outweigh the cost incurred by increased competition for food resources when nesting near a dominant ant nest.
The seemingly weak influence of competitive interactions with dominant ants on non-dominant ants at the community, population and individual level suggests a need for further examination of the life strategies that allow these species to coexist. Previous work in southeastern temperate forests of USA showed that the structure of these ant communities are highly influenced by temperature filtering; from regional to micro-scale [
43,
75,
76]. Moreover, in these ant communities, trade-off between thermal tolerance and competitive ability provide a mechanism by which species prevent competitive exclusion. Non-dominant species indeed appear to forage at lower temperatures exploiting shaded parts of the forest floors while dominant ants appear to dominate open canopy habitats where sunlight reach the forest floor [
61]. Results from the present study further demonstrate that non-dominant ant species can forage during both the night and the day whereas dominant Formica only forage during the day, which suggests a strategy to avoid interference competition [
59,
77,
78]. In our system, interspecific overlap in seasonal activity is high, with most species reaching peak foraging activity in the warmest months of the year [
79]. However, our results suggest that on a daily basis, variation in activity levels may be important for coexistence. While there were more species active during the day when
F. subsericea forages than at night when
F. subsericea is not active, there were more workers of non-dominant ants foraging at night. Thus, colonies of some non-dominant ants might send more workers out to forage at night than during the day to avoid aggressive encounters with dominant species, a phenomenon that has been previously documented with dominant
Formica in Finnish forest ant assemblages [
28].
Our results suggest that, although dominant ants might sometimes play an important role in structuring ant communities, we find very little evidence of the negative effects of the dominant species in this system across levels of organizations. Note however that whereas much of the previous work on dominant species focused on extremely abundant and aggressive ant species, the dominance of our focal species here is more relative and less absolute. Moreover, our study, as well as most studies that have examined richness-dominance relationship in ants, are observational, which limits our ability to infer causality. Nevertheless, we conclude that although competition evidently occurs in ant communities, other coexistence mechanisms in place may be sufficient to prevent local extinctions [
69]. Moreover, there are multiple explanations for the commonly observed hump-shaped relationship between the abundance of dominant ants and species richness among ant communities. Variation in the abiotic environment alone could be driving this widespread pattern if the fitness of non-dominant and dominant species peak at different places along micro-environmental gradients. Further, our results indicate that some non-dominant species may actually benefit from nesting in the vicinity of dominant ant species [
80], as was evidenced by higher productivity and nocturnal foraging in a non-dominant species near than far from dominant ant nests. Disentangling the direct and indirect effects of dominant ants on non-dominant ants likely requires further field experiments that manipulate the density of dominant species (rather than just presence/absence) to assess colony-level as well as population- and community-level responses of non-dominant ants (e.g., References [
22,
23,
81,
82,
83]).