DOI: 10.5061/dryad.4j0zpc874
title: Data from: Habitat size thresholds for predators: why damselflies only occur in large bromeliads

description: Predators are often more sensitive to habitat size than their prey, and
 frequently occur in only the largest habitats. Four explanations have been
 proposed for this pattern: (1) small habitats do not have enough energy to
 support higher trophic levels; (2) small habitats are less likely to
 contain particular prey required by specialist predators; (3) small
 habitats are risky for predators with slow life histories or large body
 sizes; (4) small habitats are numerically unlikely to be colonized by
 regionally rare species, such as predators. We critically examine these
 four hypotheses in relation to the predatory damselfly larva,
 Mecistogaster modesta Selys. (Pseudostigmatidae), which occurs almost
 exclusively in bromeliads &gt; 100ml in capacity. We synthesize
 multiple years of survey data and three manipulative experiments from the
 Área de Conservación Guanacaste, Costa Rica to conclude that damselflies
 do not occur in small bromeliads due to their higher risk of desiccation –
 not because of energetic limitation, trophic specialization, risk of
 terrestrial predation, or pure numerical effects. These results suggest
 that recent and predicted declines in precipitation in northwestern Costa
 Rica may further restrict bromeliad occupancy by damselflies, with
 cascading consequences for the rest of the aquatic food web.
description: We have studied, since 1997, the aquatic macroinvertebrates
 living in bromeliads in the pre-montane rainforest surrounding Estación
 Biológica Pitilla (Área de Conservación Guanacaste, 10°59'N,
 85°26'W, 700 m a.s.l.). (1) INVERTEBRATE SURVEYS
 We determined abundances of <i>M. modesta</i> larvae in
 bromeliads from the secondary forest between September and November in
 1997, 2000 and 2002 (n=20, 20 and 18 bromeliads, respectively). Prey
 communities in these bromeliads were also recorded in 1997 and 2002, but
 not 2000. Bromeliads were selected to represent a wide range of water
 capacity: 0.4 -1530 ml in 1997, 35 - 4690 ml in 2000 and 18 - 7132 ml in
 2002. In 1997 and 2000, both <i>Werauhia sanguinolenta</i> and
 <i>Guzmania</i> spp bromeliads were included. In 2002, only
 <i>W. sanguinolenta</i> bromeliads were examined.
 (2) BROMELIAD WATER DEPTHS To examine the effect of bromeliad
 size on desiccation risk, we measured water depth (± 1
 mm) in three leaves (central and two peripheral) of 30 bromeliads every
 two days from October 2012 to October 2013. Bromeliads represented a range
 in capacity (12-33ml; 6 bromeliads, 34-67ml: 6 bromeliads, 68-100ml: 10
 bromeliads, 100ml – 580 ml: 8 bromeliads). (3) EFFECTS
 OF BROMELIAD SIZE ON DAMSELFLY SURVIVAL AND GROWTH In October 2002, we
 collected 29 <i>M. modesta</i> larvae (10 - 12 mm long,
 excluding caudal lamellae) and marked these insects by amputating the
 right middle leg. .Each larva was placed in a leaf axil (one larva per
 bromeliad), near the center of a bromeliad in the secondary forest (14
 <i>Guzmania </i>spp., 15 <i>Werauhia</i> spp., 8 -
 2636 ml in volume). Twenty days later, we searched for marked larvae by
 dismantling each bromeliad leaf by leaf to search and measured bromeliad
 capacity. To measure larval growth, we weighed larvae to the nearest
 0.002g before the experiment (but after amputation) and at the end of the
 experiment, and calculated specific growth rate the change in mass as a
 proportion of the initial mass. (4) EFFECTS OF SPIDERS
 AND BROMELIAD SIZE ON DAMSELFLY SURVIVAL. We collected twenty bromeliads,
 suspended them in a comon garden, and placed one spider (Trechaleidae:
 <i>Cupiennius coccineus </i>F.O. Pickard-Cambridge) in each of
 twenty of them. After 24 hours, all bromeliads were dismantled leaf by
 leaf to collect the experimental spiders and damselflies, and damselfly
 survivorship was recorded.  (5) EFFECT OF DROUGHT ON
 DAMSELFLY SURVIVAL To examine the effects of drought on damselfly survival
 and growth, we manipulated drought length (defined as 0 mm water depth) in
 artificial microcosms, imposing droughts of 10, 15, 20, 25, 30 days in
 length. We also had an equal number of controls, which were treated
 identically, except that they were not subjected to drought. Each
 microcosm consisted of a 50 ml centrifuge tube sitting in a 207 ml plastic
 cup, and housed a single damselfly larva. Each drought length treatment or
 matching control was represented by two microcosms, except for the 30-day
 drought treatment with six microcosms. Microcosms began with 15ml water,
 but two days later we removed all water from half of the microcosms by
 emptying the cups to initiate drought. After completion of each drought
 length treatment, we added water to the cups to return to a volume of 15
 ml inside the tube. In the controls (which were never subjected to
 drought), 50% of water was replaced every two days to prevent hypoxia. Two
 days after the completion of the drought, we emptied the mesocosms of that
 drought treatment, as well as their corresponding controls and checked for
 damselfly survival.
description: Please see Readme.txt file