Introduction
A long-standing goal in ecology has been to determine the underlying
mechanisms that give rise to species coexistence in local communities,
especially in assemblages with multiple competing species (MacArthur
1958; Hutchinson 1959). Numerous mechanisms have been proposed for
maintaining species coexistence (Wright 2002; Silvertown 2004).
Interspecific competitive trade-offs, whereby the superiority of a
particular species in an environment or biotic condition is balanced by
the inferiority of other species, can lead to segregation among
species (Tilman 1994; Levine et al. 2004). These interspecific
interactions are thought to lead to the long-term stable coexistence of
ecologically similar species (Levins 1979; Holt et al. 1994; Chesson
2000; Bever 2003; Rudolf and Antonovics 2005), and may also be
characterized by dominance hierarchies. Dominance hierarchies have been
widely observed in a wide range of taxa, from vertebrates to
invertebrates (Chase and Seitz 2011). Species can be ranked into
dominant species (i.e. higher-ranked individuals) or subordinate species
(i.e. lower-ranked individuals) on the basis of aggression or ritual
displays (Drews 1993). Interspecific dominance hierarchies have been
used to understand patterns of local species coexistence in ecological
communities with as consequence that higher ranked individuals
monopolize resources resulting in fitness benefits (Morse 1974; Schoener
1983).
In ant communities, dominance hierarchies have been used to examine
interspecific tradeoffs to explain species coexistence patterns (Stuble
et al. 2013). These trade-offs include the discovery-dominance
trade-off, the discovery-thermal tolerance tradeoff, and the
discovery-colonization trade-off (Cerdá et al. 1998a ; Stanton et
al. 2002; Lebrun and Feener 2007; Stuble et al. 2013). In addition to
testing interspecific trade-offs, dominance hierarchies have been used
to understand the role of dominant species in structuring local
communities and composition, such as partitioning dominant and
subdominant species within guilds (Baccaro et al. 2010; Arnan et al.
2012). Dominant ant species play an important role in the structuring of
local communities. For example, Formica species dominating a
boreal ecosystem divert resources away from subdominant
competitors (Savolainen and Vepsäläinen 1988). In Mediterranean
ecosystems, subdominant species forage at nearly lethal environmental
conditions while dominant species reduce mortality risk by foraging at
more favorably temperatures (Cerdá et al. 1998c ). In tropical
ecosystems, competing arboreal ants can be structured into a dominance
hierarchy with higher ranked ant species having greater access to
nesting sites and extrafloral nectaries. However, levels of uncertainty
associated with outcomes of interspecific interactions are often not
quantified (Stuble et al. 2017). Furthermore, its remains unclear how
arboreal ants or tropical ants are structured at higher-order
interactions, such as when interspecific interactions are viewed as a
network.
In this study, we examine dominance hierarchies for a community of
arboreal twig-nesting ants in a coffee agroecosystem. Both arboreal and
ground-dwelling twig-nesting ants in coffee agroecosystems are nest-site
limited in terms of number (Philpott and Foster 2005a ), diversity
(Armbrecht et al. 2004; Gillette et al. 2015), and sizes (Jiménez‐Soto
and Philpott 2015) of nesting resources. Many studies focused on
interspecific dominance hierarchies lack a clear and consistent
definition of dominance. For twig-nesting ants, nest takeovers are
common and nest sites are often limiting, thus dominance in this system
is defined as competition for nest sites (Brian 1952), and in at least
one case has been experimentally demonstrated (Palmer et al.
2000a ). Although dominance studies use various methods to rank
species, they don’t typically account for uncertainty in rankings,
except for a few cases (Adler et al. 2007; Stuble et al. 2013). This
present study aims to develop dominance hierarchies for twig-nesting
ants living in hollow twigs in a Mexican coffee agricultural ecosystems
due to competition for nest resources. We adopt methods from network
analysis to infer dominance hierarchies from competitive interactions by
estimating uncertainties and steepness in rankings (Pinter-Wollman et
al. 2014; Shizuka and McDonald 2015; Sánchez‐Tójar et al. 2017).
Furthermore, by viewing twig-nesting ants as a network we estimate the
orderliness of the hierarchy within the community. Specifically, we
tested the hypothesis that tropical, arboreal twig-nesting ants form a
clear, dominance hierarchy for nesting sites in controlled environments.