Interspecific aggression is a complex interaction with important evolutionary and ecological implications. While it can be an adaptive response to reproductive interference, it bears costs including energy expenditure, loss of territory, and missed mating opportunities. Consequently, species may diverge in habitat preferences or exhibit spatial partitioning, which reduce the costs of fighting. However, empirical evidence of such shifts is lacking. I studied the relationship between interspecific aggression, habitat partitioning, and spatial segregation in interspecifically aggressive species of rubyspot (Hetaerina) damselflies.

In Chapter 1, I explored whether species differences in microhabitat use reduce the frequency of interspecific fighting in 25 sympatric population pairs. I found that almost all population pairs had lower observed rates of interspecific fighting relative to chance expectations. Reduced rates of interspecific fighting were explained by competitor recognition, species differences in microhabitat use, and spatial segregation. I also found strong positive correlations between heterospecific aggression and species differences in microhabitat use, likely explained by competitive displacement and/or agonistic character displacement, and between heterospecific aggression and spatial segregation. These correlations were explored further in Chapters 2 and 3.

In Chapter 2, I tested the competitive displacement hypothesis using removal experiments. If competitive displacement occurs, territory holders of the subordinate species should shift their microhabitat use to that of the dominant species in the absence of the dominant species. However, I found no evidence of such shifts. I therefore reject the competitive displacement hypothesis and conclude that species have likely evolved divergent microhabitat preferences through agonistic character displacement.

In Chapter 3, I examined the correlation between heterospecific aggression and spatial segregation. Spatial partitioning is common among competing species, but there are numerous mechanisms that can cause species to be spatially segregated. Conspecific attraction is a widespread habitat selection mechanism, but the potential for it to cause spatial partitioning between interspecifically aggressive species is unknown. I explored this question by comparing the clustering of territories to a model that simulates territory settlement from microhabitat availability. I found that both conspecific attraction and microhabitat preferences contribute to the spatial partitioning between interspecifically aggressive species.