Organizational Unit:
School of Biological Sciences
School of Biological Sciences
Permanent Link
Research Organization Registry ID
Description
Previous Names
Parent Organization
Parent Organization
Organizational Unit
Includes Organization(s)
Organizational Unit
ArchiveSpace Name Record
Publication Search Results
Now showing
1 - 8 of 8
-
ItemFunctional morphology of intertidal seaweeds: adaptive significance of aggregate vs. solitary forms(Georgia Institute of Technology, 1984-08-15) Taylor, Phillip R. ; Hay, Mark E.Many intertidal seaweeds show a tremendous gradient of morphological form ranging from spatially separated thalli, to thalli that are aggregated into dense turfs. Aggregation of seaweeds into turfs decreases productivity per g organic weight due to crowding of thalli but increases resistance to desiccation. The intertidal distribution of the turf growth form is correlated with the intensity of desiccation stress. Also turfs transplanted into tide pools developed the non-turf morphology, while non-turfs transplanted to emergent substrate either developed the turf form or died. The turf growth form is energetically expensive; apparent productivity of turfs was 23 to 48 % less than that of individuals. Increasing light and nutrients available to turfs by separating the thalli increased apparent productivity by 36 to 113 %. We tested the hypothesis that these turfs minimize energetic costs of this aggregated growth form through the spatial partitioning of photosynthetic and respirative activity. The lower portions of turfs showed less apparent photosynthesis than the upper portions (reductions of 37 to 85 %); however, this spatial partitioning was also found in the individual forms (37 to 63 % reductions) of these relatively simple seaweeds. Spatial differentiation of respiration was similar in turfs and nonturfs. The seaweeds, examined are able to alter the extent of thalli compaction in accordance with varying levels of environmental stress. This phenotypic plasticity allows seaweeds to adopt morphological features that maximize fitness in a wide variety of habitats without being developmentally committed.
-
ItemCoral Reef Ecology: Have We Been Putting All of Our Herbivores in One Basket?(Georgia Institute of Technology, 1984-05-01) Hay, Mark E.
-
ItemPatterns of fish and urchin grazing on Caribbean coral reefs: are previous results typical?(Georgia Institute of Technology, 1984-04) Hay, Mark E.Strips of the seagrass Thalassia testudinum were used in a field bioassay to assess herbivory on 11 coral reefs scattered throughout the Caribbean. Patterns of herbivory on overfished reefs in Haiti and the United States Virgin Islands were compared to patterns on seven less fished reefs (United States Virgin Islands, Panama, Honduras, Belize, and the Bahamas). On the overfished reefs, the rate of Thalassia removal increased significantly with depth, urchin densities were high, and urchin grazing was equal to, or greater than, fish grazing in shallow (<10 m deep) habitats. On reefs subject to little fishing pressure, the rate of Thalassia removal decreased with depth, urchin densities were low, and herbivorous fishes were responsible for almost all Thalassia removal. Previous studies assessing the importance of urchin grazing in the Caribbean have been conducted on overfished reefs where urchin densities were unusually high and the density of grazing fish unusually low. It is doubtful that the intensity of urchin—algal and urchin—coral interactions observed on these heavily fished reefs occurs on reefs unaffected by humans.
-
ItemBetween-habitat differences in herbivore impact on Caribbean coral reefs(Georgia Institute of Technology, 1983-12) Hay, Mark E. ; Goertemiller, TimTransplanted sections of the seagrass Thalassia testudinum were used as a bioassay to assess between-habitat differences in herbivory on three Caribbean reefs. Consumption of Thalassia by herbivorous fishes on shallow (1-10 m) reef slopes was significantly higher than on deep (30-40 m) reef slopes or on shallow reef flats. Seaweeds typical of reef flat habitats were rapidly consumed when placed on shallow reef slopes. Seaweeds typical of either deep or shallow reef slopes were relatively resistant to herbivory and a high proportion of these species are known to contain secondary chemical compounds that appear to deter herbivorous fishes. Shallow reef flats provide seaweeds with a predictable spatial escape from major reef herbivores; algae characteristic of these habitats have evolved few, if any, characteristics that significantly reduce losses to herbivory.
-
ItemIs glue production by seeds of Salvia columbariae a deterrent to desert granivores?(Georgia Institute of Technology, 1983-08) Fuller, Patricia J. ; Hay, Mark E.With a few notable exceptions (Borchert and Jain 1978, Inouye et at. 1980, O'Dowd and Hay 1980, Hay and Fuller 1981), most ecological studies on seed-granivore interactions in arid communities have concentrated on the adaptations and ecology of the granivores and not on characteristics of the seeds that may deter granivory. Since a large portion of the annual seed crop of many desert ephemerals is lost to seed predators (Chew and Chew 1970, French et al. 1974, Nelson and Chew 1977, Brown et al. 1979), characteristics that significantly reduce losses to granivores should be strongly selected. In this paper we show that under natural conditions seeds of the desert annual Salvia columhariae produce a glue-like substance when wetted that strongly binds sand grains to the seeds, and that seeds thus covered by sand suffer significantly less loss to desert granivores.
-
ItemHerbivory, algal distribution, and the maintenance of between-habitat diversity on a tropical fringing reef(Georgia Institute of Technology, 1981-10) Hay, Mark E.The bases of coral reefs in the Caribbean often abut sandy plains covered by sea grasses (Randall 1965; Ogden et al. 1973b) or algae (Earle 1972; Dahl 1973). Interactions occurring at the border of reefs and sea grass beds have been studied on several occasions (Randall 1965; Ogden et al. 1973b; Ogden 1976; Parrish and Zimmerman 1977; Ogden and Lobel 1978), but little is known about those which occur between reefs and sandy plains dominated by algae. Unlike sea grasses, which root into the sand, many of the algal species that occur on sand plains require hard substrates (Dahl 1973) such as shells and coral fragments. Suitable attachment sites are uncommon on the sand plain at Galeta Point, Panama, and many are periodically buried or turned over during heavy seas. Paradoxically algal species that predominate on the sand plain tend to be rare or absent from the shallower reef slope where stable, hard substrate is abundant. The maintenance of such distinct distributional boundaries is often attributed to differential competitive abilities (Connell 1961; Holmes 1961) or to restrictive specialization to particular physical regimes (Doty 1946; Terborgh 1971). These factors have been hypothesized to be especially important (Dobzhansky 1950; Janzen 1967; Ashton 1969; Diamond 1975) in diverse tropical communities where specialization to narrow niches is thought to promote resource partitioning and allow increased coexistence. While such explanations are often consistent with observed patterns, they are seldom tested using controlled field experiments. Without such field manipulations it is impossible to adequately assess the relative importance of competition, predation, and physical stress in determining the distribution and abundance of species or the intensity of interactions that occur between them. In this paper I examine experimentally the factors maintaining these separate algal assemblages and contend that the sand-plain species (algae that are almost never found on the reef) would competitively exclude other species from the reef slope if they were not selectively removed by reef-associated grazers. Competition and physiological specialization appear to have no effect on excluding sandplain species from the reef slope. In the remainder of the paper I test the following hypotheses. (1) Low light and scarcity of attachment sites severely limit the growth of algae on the sand plain. (2) Sand-plain algae are not physiologically restricted to deep waters; they are most fit in the physical regime typical of shallower reef-slope habitats. (3) Reef-associated grazers are responsible for excluding sand-plain species from the reef slope. (4) In the absence of herbivores, sand-plain species competitively dominate reef-slope species. (5) Because of differential competitive ability, sand-plain genera are better adapted for temperate areas than are reef-slope genera.
-
ItemThe functional morphology of turf-forming seaweeds: persistence in stressful marine habitats(Georgia Institute of Technology, 1981-06) Hay, Mark E.Many seaweeds that occur in physically stressful habitats or habitats subject to moderate herbivory grow as colonial turfs rather than as spatially separated individuals. The turf growth form is energetically expensive (the net production per gram ash free dry mass of turfs being 33—61% lower than that of individuals), but turfs suffer less physiological damage during desiccating low tides and lose less biomass to herbivores. The upper portions of turf—forming species show significantly greater rates of apparent photosynthesis and dark respiration than do the lower portions. This spatial partitioning of photosynthetic activity decreases the energetic cost of the turf arrangement and may allow basal portions to function as persistent resting stages during periods of adverse conditions when uprights cannot be maintained. Turf—forming species are specialized for areas that are subject to moderate grazing pressure and physical stresses. They are dependent upon these factors to prevent their competitive exclusion by more productive, but less resistant, seaweeds. Damage to apical portions causes increased branching that results in a more tightly compacted turf. Algae that regenerate in this way can adjust their growth form in accordance with varying levels of disturbance encountered in different habitats and thus incur the minimal cost consistent with survival in that area. For seaweeds occurring in stressful habitats, selection has favored characteristics that increase persistence in space and time even though these involve considerable losses in competitive ability and productivity.
-
ItemMutualism between harvester ants and a desert ephemeral: seed escape from rodents(Georgia Institute of Technology, 1980-06) O'Dowd, Dennis J. ; Hay, Mark E.The harvester ants Veromessor pergandei and Pogonomyrmex californicus modify the highly localized seed shadow of the Sonoran Desert ephemeral Datura discolor (Solanaceae) through (1) attraction to food bodies attached to seeds released beneath the parent plant, (2) immediate transport of disapores to the ant nest, (3) subsequent removal of the food body in underground granaries, and (4) final expulsion of intact seeds onto the colony midden. D. discolor seed without food bodies are not attractive to ants and remain beneath the parent canopy. At Nude Wash, California, USA, diaspore removal distances are generally small, averaging 2.3 m away from the nearest D. discolor canopy in March—May 1977. Two lines of evidence indicate that this limited transport has a large effect on seed detection and utilization by rodents: (1) seeds placed in ant—proof dishes beneath the D. discolor canopy were about 10 times more likely to be removed by nocturnally foraging rodents than seeds placed in locations similar to those where ants locate their nests; (2) using D. discolor as bait, significantly more rodents were trapped beneath the plant canopy than in open areas where seeds were transported by ants. Manipulation of the density of D. discolor seed in these tests suggests that seed utilization by rodents is distance—responsive to the parent canopy rather than dependent on the density of seeds. Disapore analyses indicate that investment in the transport system is relatively small and represents <5% of the total investment in seeds.