Data from: Territory size but not territorial defence varies with habitat quality and competitor density in a farming species

High-quality habitats are predicted to have higher densities of
competitors, smaller territory sizes, and increased defensive costs for
territory holders. However, defence is typically considered to be directed
predominantly towards conspecifics, with heterospecific intrusions
generally thought to contribute less towards defensive costs. Moreover,
individuals may benefit from the collective defence of resources in areas
with increased densities of territory holders. Together, these factors
could affect the costs of defence in low- versus high-quality habitats. In
this study, we investigated how habitat quality and conspecific density
influence the territorial defence of algal-farming yellowtail damselfish
(Stegastes arcifrons) in the Galapagos. We quantified territory
characteristics, performed behavioral observations, and conducted an in
situ experiment to test whether damselfish in higher quality, conspecific
damselfish dense habitats (characterized by increased algal turf
coverage), differ in their competitive intrusion frequencies and defensive
costs of chasing compared to those in lower quality areas. We found that
damselfish in poorer quality habitats with fewer conspecific neighbours,
had larger territories compared to those in higher quality, more
damselfish dense areas. Despite this, both high- and low-quality territory
holders experienced equivalent numbers of competitive intrusions. However,
the type of competitor varied: territory holders in high-quality areas
primarily defended against conspecifics, while those in lower-quality
areas faced more frequent heterospecific intrusions. We also found no
evidence that damselfish in high-quality habitats gained any collective
benefits of defence by having increased densities of conspecific
damselfish nearby. Thus, while higher conspecific density in higher
quality habitats might alter the risk from certain intruders, it does not
appear to confer direct collective defence benefits in terms of reducing
defence effort once intruded upon. Instead, territory holders appear to
adapt their defence to balance the defensive pressures posed by
conspecifics and heterospecifics, maintaining similar overall defensive
costs across differing habitat qualities.