Dynamic movements: investigating movement and space use in territorial marine herbivores

Background/Question/Methods

Parrotfishes are functionally important herbivores on coral reefs with complex social and mating systems. In many species, males defend their home ranges (HRs) as stable intraspecific territories. However, we know little about how parrotfishes directly interact with one another in shared space where their HRs overlap. These dynamic interactions could influence species abundances, distributions, and coexistence. We investigated the social and spatial ecology of five common parrotfishes tracked in June-July 2019 on the coral reefs of Bonaire, NL: Scarus iseri, Sc. taeniopterus, Sc. vetula, Sparisoma aurofrenatum, and Sp. viride. We computed HRs for several terminal phase male and initial phase female individuals of these species (n = 150 total) using movement-based kernel density estimation and quantified overlap of spatially co-occurring HRs (static interactions). We then analyzed dynamic interactions between simultaneously tracked, spatially co-occurring pairs of male Sp. viride and male Sc. vetula (interspecific, n = 9) or male and female Sp. viride (intraspecific, n = 9) to investigate whether they move together, independently, or avoid one another in shared space. We hypothesized that fishes in both pairings, but particularly fish in intraspecific pairs, would avoid each other to limit agonistic interactions over foraging substrates.

Results/Conclusions

For all species except Sc. taeniopterus, HRs were significantly smaller for females than for males (p < 0.001). Additionally, we found that spatial overlap of HRs was significantly lower for intraspecific male than for interspecific male pairs (p < 0.001). Despite substantial spatial overlap of male Sp. viride and male Sc. vetula HRs, males of these species moved independently of one another in shared areas. Aggressive interactions between these species are uncommon, perhaps due to sufficient resource partitioning that limits the need for avoidance. Similarly, male Sp. viride and haremic female Sp. viride moved independently of one another within their territories. Social interactions between male and female Sp. viride were common, but typically brief. We were unable to analyze dynamic interactions between neighboring male Sp. viride because of the limited spatial overlap of their HRs. However, social interactions between male Sp. viride occurred close to HR boundaries and were often more aggressive with non-territorial floaters than with neighbors (i.e., ‘dear enemies’), suggesting that neighbors may avoid conflict, and one another, at the periphery of their territories. Our data provide important links between social behavior and space use in these territorial grazers, with implications for their functional roles on coral reefs.