Inter-specific aggression generates ant mosaics in canopies of primary tropical rainforest

Abstract:

The ant mosaic is a concept of the non-random spatial distribution of individual ant species in trees built upon the assumption of interspecific behavioural associations. However, colony identity and environmental variance may also play a role in species distribution. Here we assess the presence of ant mosaics in a primary forest ecosystem and whether they are structured by species’ aggressive behaviours or by habitat filtering.

We sampled arboreal ants from vertically stratified baits exposed in 225 canopy trees in a 9-ha plot of primary lowland forest in Papua New Guinea, the largest forest area surveyed to detect ant mosaics. We performed behavioural tests on conspecific ants from adjacent trees to determine the territories of individual colonies. We explored the environmental effects on the ant communities using information on the plot vegetation structure and topography. Furthermore, we created a novel statistical method to test for the community non-random spatial structure across the plot via spatial randomisation of individual colony territories. Finally, we linked spatial segregation among the four most common species to experimentally assessed rates of interspecies aggression.

The ant communities comprised 57 species of highly variable abundance and vertical stratification. Ant community composition was spatially dependent, but it was not affected by tree species composition or canopy connectivity. Only local elevation had a significant but rather small effect. Individual colony territories ranged from one tree to 0.7 ha. Species were significantly over-dispersed, with their territory overlap significantly reduced. The level of aggression between pairs of the four most common species was positively correlated with their spatial segregation. Our study demonstrates the presence of ant mosaics in tropical pristine forest, which are maintained by interspecific aggression rather than habitat filtering, with vegetation structure having a rather small and indirect effect, probably linked to microclimate variability.

Tree and environmental properties:

The study was conducted in the CTFS-ForestGEO research site. The 50 ha CTFS Wanang plot has all trees with diameter at breast height (DBH) >1 cm tagged, their DBH measured, identified to species level, and coordinates mapped. The CTFS plot is structured into a grid of 20x20 m subplots. For each tree in each subplot, we used information about elevation (in m asl), the slope of the subplot (in %), and topography (convex/concave, calculated from the height of surrounding subplots). Finally, we have estimated canopy connectivity for each tree as the number of neighbouring canopy trees that are connected to the sampled tree by their branches or lianas.

Ant sampling: 

We sampled arboreal ants from vertically stratified baits exposed in a 9-ha plot of primary lowland forest in Papua New Guinea. We used the slingshot bait-line method for sampling the ants in the selected trees. Tuna honey baits were placed on a rope positioned along the tree trunk from the ground level to the canopy with baits regularly spaced at 5 m intervals. In total, 1127 baits were exposed to the ants across the 225 trees. We set up baits in the morning and left them exposed for 4–5 h, then carefully retrieved baits to ensure that individuals did not fall down, checked them for ant occupancy, and estimated the number of individuals of each ant species found on each bait. We collected several individuals of each species in vials with absolute ethanol for later species confirmation. Baits were placed only in sunny weather without rain.

Data description:

For each excel sheet, we will describe each column if necessary.

Baits_summary:

Each row is a single morphospecies of ant on a single bait (mostly a single row is a single bait). 

[TreeTag_ID] - unique value for each tree

[BaitCode] - combination of TreeTag_ID and height of bait

[Height(m)]- self explanatory

[Ant_Species] - Unique ant species code

[Estimated_Number_of_Individuals] - self explanatory

[MoreSpeciesOn1Bait] - TRUE/FALSE, simple flag to show that selected bait has multiple species on a single bait.

TreeData:

Part of the CTFS-ForestGEO database. Each row represents a single tree stem.

[TreeTag_ID] - unique value for each tree

[Quadrat] - unique value representing 20x20 m quadrat. Environmenatl properties were measured on this scale.

[Species_Code] - trees were taxonomically identified using specific unique codes. 

[DBH (cm)] - Diameter at Breast Height 

[quadrat_coord_x] - X coordinates in selected quadrat (m)

[quadrat_coord_y] - Y coordinates in selected quadrat (m)

[total_coord_x] - X coordinates in whole CTFS plot (m)

[total_coord_y] - Y coordinates in whole CTFS plot (m)

[focal_area_coord_x] - X coordinates in this study (m)

[focal_area_coord_y] - Y coordinates in this study (m)

TreeTaxonomy:

Taxonomic table for CTFS-ForestGEO database. Each row represents a single tree species.

[ID] - tree species unique value

[code] - Tree species unique code. This is linked with [Species_Code] from table TreeData.

[family] - self explanatory

[Genus] - self explanatory

[species] - self explanatory

[name] - Latin taxonomic name

Quadratd_data:

Table with topographical information for each quadrat in CTFS-ForestGEO database. Each row is one 20x20 m quadrat.

[Quadrat] - unique value representing 20x20 m quadrat. This is linked with [Quadrat] from table TreeData.

[Mean_elevation(m)] - self explanatory

[slope] - the slope of the quadrats (in %)

[convex] -convex/concave, calculated from the height of surrounding quadrats.

ColonyData:

Spatial distribution of individual colonies of canopy ants. Each row is one dominant species found on a high canopy tree. Mostly is just one ant species per tree.

[TreeTag_ID] - unique value for each tree

[x_coord] - X coordinates in this study (m) 

[y_coord] - Y coordinates in this study (m)

[DBH(cm)]- Diameter at Breast Height 

[Crown_Diam(m)] - diameter of the crown estimated from the DBH and the tree taxonomy.

[Ant_Species] - dominant species found on that tree.

[Colony_ID] - unique value that linked colonies same colonies of ants between multiple trees. Colony identity was confirmed by behavioral tests. 

AggresivityTests:

We conducted a series of interspecies behaviour tests, using pair-wise interactions among the four Spatially Dominant Species (CREM 003, ANON 001, OECO 001, PODO 001). For each test, we sampled 10–20 workers from each colony and left them in a 100 ml clean plastic vial with cotton soaked in honey-water overnight to habituate. Five randomly selected individuals from each colony were then kept inside the vial that was joined via its opening with another vial with five individuals from another species. In this arena, a paired test between the two species was performed. During each confrontation, we observed the frequency of four behavioural reactions between all individuals of the species-pair for two minutes.

Each row is a summary of all one-to-one species interactions per one test.

[ID] - unique value of summary of all one-to-one species interactions per one test.

[DateOfTest] - self explanatory

[TimeOfTest] - self explanatory

[TreeTag_A] - unique value for a tree for a species A

[TreeTag_B] - unique value for a tree for a species B

[Reverse_value] - We have measured the interaction of Species A -> Species B and Species B-> Species A at the same time. Therefore, these two measurements are lined with each other. 

[Ant_Species] - Observed Species (Species A)

[Observer] - self explanatory

[Interaction_Type_1] - frequency of interactions "individual runs away after an interaction"

[Interaction_Type_2] - frequency of interactions "individual does not change direction or speed after interaction"

[Interaction_Type_3] - frequency of interactions "individual assumes warning position"

[Interaction_Type_4] - frequency of interactions "individual attacks the other species worker(s)"

[Total_N_Interaction] - self explanatory

[Agression_Index(AI)] - Aggression Index (AI) - 

[Species_Combination] - species-species combination

[Temperature©] - the temperature in the room during the test.

[AI_tranformed] - Aggression Index (AI) rescaled to range between 1 (really aggressive) and 0 (mostly run away).

AntSpeciesTable:

Taxonomic table of ant species. Each row is one ant species

[Code] - Unique ant species code

[Number of occupied baits] - number of occupied baits in this study

[Number of occupied Trees] - number of occupied trees in this study

[Scored as dominant species] - TRUE/FALSE; for each tree, we defined dominant species according to the abundance of workers at baits and the

the number of baits per tree.

[Latin name] - self explanatory