Print PageA central goal of the field of evolutionary ecology is to understand the evolutionary relationships between resource use and the diversification of functionally-related phenotypic traits. “Ecosystem engineers” are organisms that produce persistent physical state changes in the environment, which serve secondarily as resources for other taxa. Although ecosystem engineering is acknowledged as a widespread ecological process, how the resulting resources influence trait diversification in lineages that depend on them remains largely unaddressed. Here, we address how cavity-producing wood-boring beetles may have influenced trait diversification in a lineage of obligate cavity-nesting ants. Our focal ant lineage, the genus Cephalotes, has striking diversity in a number of traits that are putative adaptations to cavity-nesting life. All data were collected in Brazilian cerrado, where the focal ant lineage is particularly diverse. The characteristics of beetle-produced cavities were quantified by dissecting stems of six common tree species. Ant morphology and cavity use were also quantified for a broad phylogenetic sampling of Cephalotes species found in the same habitat. Phylogenetic comparative methods were used to provide initial insights into the macroevolutionary relationships between cavity use and phenotypic traits related to cavity defense and occupation.
Results/Conclusions
The entrance holes of beetle-produced cavities ranged from less than 0.1cm to over 1cm in diameter. Entrance diameters from 0.1cm to 0.2cm accounted for over 60% of all entrances. There was no significant relationship between entrance diameter and either external stem diameter or internal cavity diameter, but stem diameter was a significant predictor of cavity diameter. These cavity characteristics allow for evolved preferences in entrance and stem diameter to be decoupled. Concordantly, some Cephalotes species shared similar entrance size preferences, but different stem use, and vice versa. No species overlapped in the niche space defined by entrance and stem diameters. Cephalotes soldiers have specialized heads that they use to close cavity entrances, while workers can be dramatically smaller and dorsoventrally flattened to maximize the occupancy of small cavities. Phylogenetically independent contrasts suggested a significant relationship between the maximum diameter of soldier heads and mean entrance diameter, as well as soldier size range and hole size range. They also revealed a significant positive relationship between mean worker size and mean stem size. Overall, these findings provide compelling correlative evidence for an important evolutionary relationship between the use of beetle-produced cavities and phenotypic diversification in Cephalotes ants.
