Jeff G. Holmquist

Indirect Upstream Effects Of Dams: Consequences Of Migratory Consumer Extirpation In Puerto Rico

Large dams degrade the integrity of a wide variety of ecosystems, yet direct downstream effects of dams have received the most attention from ecosystem managers and researchers. We investigated indirect upstream effects of dams resulting from decimation of migratory freshwater shrimp and fish populations in Puerto Rico, USA, in both high- and low-gradient streams. In high-gradient streams above large dams, native shrimps and fishes were extremely rare, whereas similar sites without large dams had high abundances of native consumers. Losses of native fauna above dams dramatically altered their basal food resources and assemblages of invertebrate competitors and prey. Compared to pools in high-gradient streams with no large dams, pool epilithon above dams had nine times more algal biomass, 20 times more fine benthic organic matter (FBOM), 65 times more fine benthic inorganic matter (FBIM), 28 times more carbon, 19 times more nitrogen, and four times more non-decapod invertebrate biomass. High-gradient riffles upstream from large dams had five times more FBIM than did undammed riffles but showed no difference in algal abundance, FBOM, or non-decapod invertebrate biomass. For epilithon of low-gradient streams, differences in basal resources between pools above large dams vs. without large dams were considerably smaller in magnitude than those observed for pools in high-gradient sites. These results match previous stream experiments in which the strength of native shrimp and fish effects increased with stream gradient. Our results demonstrate that dams can indirectly affect upstream free-flowing reaches by eliminating strong top-down effects of consumers. Migratory omnivorous shrimps and fishes occur throughout the tropics, and the consequences of their declines upstream from many tropical dams are likely to be similar to those in Puerto Rico. Thus, ecological effects of migratory fauna loss upstream from dams encompass a wider variety of species interactions and biomes than the bottom-up effects (i.e., elimination of salmonid nutrient subsidies) recognized for northern temperate systems.

Benthic macroalgae as a dispersal mechanism for fauna: influence of a marine tumbleweed

Effective dispersal is problematic for benthic organisms without planktonic larvae; rafting and vertical migrations are mechanisms that can potentially be employed by such fauna, but these strategies entail considerable predation risk as well as other disadvantages. Unattached, but non-floating, “drift” algae harbor large numbers of fauna and may serve as an alternative dispersal mechanism in some systems. This paper reports field manipulations in Florida Bay, Florida, USA designed to determine (1) if such algae can disperse benthic animals, and (2) if dispersal efficiency varies as a function of two common substrata types: seagrass and bare sediment. A live immersion stain was used to mark faunal associates of Laurencia spp. algal clumps in situ. The fidelity of molluscs, decapods, ophiuroids, and fishes to stationary algal clumps was then compared with the fidelity of these animals to clumps that were forced to tumble over a given distance with a blower apparatus; these experiments were performed over both sand and seagrass substrata. Measurements of frequency, spatial extent, and rate of algal drift were made to aid in assessing the potential importance of benthic algae as a dispersal mechanism. Algal clumps often rolled in a manner similar to that of terrestrial tumbleweeds; mark-recapture work showed that algal clumps can move up to 0.5 km/day and that algal drift is a frequent phenomenon. The algal masses were effective transporters of benthic fauna, including mobile shrimps and fishes; dispersal was more efficient over sand than over seagrass. Dispersal of fauna via this mobile habitat should entail lower risk than other adult dispersal stratagems such as vertical migration or rafting; this mechanism would be most advantageous for brooding species or those with limited planktonic phases. Differential fidelity to clumps tumbling across seagrass versus sand suggests that the algae could facilitate exchange of fauna between isolated seagrass patches.

Utilization by fishes of shallow, seagrass-covered banks in Florida Bay: 2. Diel and tidal patterns

SynopsisDiel and tidal patterns in the occurrence of water column fishes were examined on four shallow banks in Florida Bay, using continuous 72h gillnet sets. Patterns in capture rates were presumed to indicate movement of fishes on and off the seagrass-covered banks. Species that were nocturnally active on the banks included Arius felis, Mugil gyrans, Opisthonema oglinum, Harengula jaguana, Elops saurus, Lutjanus griseus, and Bairdiella chrysoura. Diurnal species included Eucinostomus gula, Lagodon rhomboides and Mugil cephalus. Strongylura notata and Mugil curema showed no consistent patterns. At the two sites with significant tidal fluctuation in water level, different activity patterns on the bank relative to tidal stage were evident for several species. At extreme low tides, water column fishes apparently left the banks to avoid stranding. Cycles of fish utilization of the bank habitat are proposed to be related to both availability of prey (diel patterns) and water level (tidal patterns). These cycles in turn influence activity patterns of predators foraging on these fishes.

Utilization by fishes of shallow, seagrass-covered banks in Florida Bay: 1. Species composition and spatial heterogeneity

SynopsisSpecies composition and relative capture rates of water column fishes occurring on the shallow (<1 m), seagrass-covered mudbanks of Florida Bay were assessed using small-mesh gillnets. The fauna was largely temperate, with few tropical representatives, and was similar to the fish community in adjacent basins. There was a high variability in the catch across the Bay, reflecting heterogeneity in both the physical environment and various aspects of the seagrass canopy. The Gulf site, in the northwestern section of the Bay, had the highest species richness and highest capture rates of individual species, relative to other sites. Higher densities of potential prey, greater available foraging area, and organically rich, fine sediments are probably influential in the greater fish utilization of this bank. The greater exchange of western Florida Bay with open Atlantic or Gulf waters is proposed as a secondary factor influencing species richness; the probability of non-resident species occasionally appearing on western banks is greater than in isolated interior sections of the Bay.

Permeability of patch boundaries to benthic invertebrates: influences of boundary contrast, light level, and faunal density and mobility

This study empirically examines predictions concerning boundary permeability as a function of patch contrast (i.e., the magnitude of difference in measures across patch interfaces), light level, and faunal mobility and density, as well as relative rates of emigration and immigration. I assessed permeability using invertebrate transfer in a macroalgal-seagrass-sand landscape, with particular emphasis on the caridean shrimp Thor floridanus. Proportional emigration was estimated by staining algal fauna in situ using neutral red (a live immersion stain) and then later collecting all animals in the algal clumps and in the surrounding sand or seagrass. Permeability was in part a function ofboundary contrast in this milieu; macroalgal patches were more of a closed system in sand than in seagrass. Contrary to predictions, proportional emigration across both the algal-seagrass and algal-sand boundaries was inversely related to faunal density. Another unexpected result was that the rate of change in emigration with increasing density was unaffected by differential contrast among habitat types. The observed enhanced permeability for more motile fauna was consistent with predictions. A related finding was thatpermeability was greater at night than during the day, probably as a function ofincreased nocturnal vagility of benthic fauna. Emigration was not in equilibriumwith immigration at the temporal scale of this study (hours) for the majority ofspecies tested. Emigration-immigration disequilibria should be most likely whenthe temporal scale is small, and in such cases caution should be exercised whenextrapolating from one transfer parameter to the other.

Decapod and stomatopod assemblages on a system of seagrass-covered mud banks in Florida Bay

The latticework of seagrass-covered mud banks in Florida Bay, Florida Keys, USA, divides the bay into distinct subenvironments and supports a robust seagrass community subject to pronounced physical stress. Throw-trap sampling of decapods and stomatopods during 1984 and 1985 (December–April, May–August and September–November of each year) showed that bank sides exposed to turbulence had low abundance but similar species richness to that of bank tops and sheltered sides. The fauna was more Gulf-Carolinean than Antillean. The crustacean communities of the different subenvironments, however, were distinct, with both Antillean and temperate assemblages represented and with one isolated area markedly depauperate. The two subenvironments adjacent to the Atlantic Ocean and Gulf of Mexico had the greatest densities. Multiple-regression techniques suggested that vegetational habitat characteristics played a secondary role compared to various physical factors. We hypothesize that (1) restricted circulation (compounded by winter cold-fronts or other seasonal causes of density minima), (2) juxtaposed faunal provinces, and (3) the wider salinity range of the isolated regions of the bay were primarily responsible for the strikingly different communities of the various subenvironments.

The Functional Morphology of Gnathopods: Importance in Grooming, and Variation With Regard to Habitat, in Talitroidean Amphipods

ABSTRACT Six talitroidean amphipod species from terrestrial, semiterrestrial, fresh-water, and marine habitats were examined with scanning electron microscopy. A number of specialized and ornate grooming organs are described in which cuticular scales and setae have been modified into bristles, fans, combs, and brushes. Terrestrial, semiterrestrial, and aquatic groups possess grooming modifications quite distinct from each other, while individual species, within a single habitat group, differ much less. The structure and function of several nongrooming specializations is also discussed. Although sexual dimorphism of arthropod grooming structures is rare, such dimorphism is present in the Amphipoda. This dimorphism stems from the use of the major grooming appendages as precopulatory organs in the male. The cuticular structures described in this study are used to scrape and brush the body during grooming movements, presumably enabling these crustaceans to exploit better their environment.

Effects of a long-term disturbance on arthropods and vegetation in subalpine wetlands: manifestations of pack stock grazing in early versus mid-season.

Conclusions regarding disturbance effects in high elevation or high latitude ecosystems based solely on infrequent, long-term sampling may be misleading, because the long winters may erase severe, short-term impacts at the height of the abbreviated growing season. We separated a) long-term effects of pack stock grazing, manifested in early season prior to stock arrival, from b) additional pack stock grazing effects that might become apparent during annual stock grazing, by use of paired grazed and control wet meadows that we sampled at the beginning and end of subalpine growing seasons. Control meadows had been closed to grazing for at least two decades, and meadow pairs were distributed across Sequoia National Park, California, USA. The study was thus effectively a landscape-scale, long-term manipulation of wetland grazing. We sampled arthropods at these remote sites and collected data on associated vegetation structure. Litter cover and depth, percent bare ground, and soil strength had negative responses to grazing. In contrast, fauna showed little response to grazing, and there were overall negative effects for only three arthropod families. Mid-season and long-term results were generally congruent, and the only indications of lower faunal diversity on mid-season grazed wetlands were trends of lower abundance across morphospecies and lower diversity for canopy fauna across assemblage metrics. Treatment x Season interactions almost absent. Thus impacts on vegetation structure only minimally cascaded into the arthropod assemblage and were not greatly intensified during the annual growing season. Differences between years, which were likely a response to divergent snowfall patterns, were more important than differences between early and mid-season. Reliance on either vegetation or faunal metrics exclusively would have yielded different conclusions; using both flora and fauna served to provide a more integrative view of ecosystem response.

Does Long-term Grazing by Pack Stock in Subalpine Wet Meadows Result in Lasting Effects on Arthropod Assemblages?

Pack stock are often used in mountain environments and are grazed in uplands and wetlands, particularly subalpine wet meadows. Effects of pack stock on wetland invertebrates are unknown. Sequoia National Park, (Sierra Nevada, USA), was an ideal location for the study of lasting stock impacts on fauna, because a) there was an 18-year database of stock usage, b) there were meadows with little grazing that could be contrasted with grazed meadows, c) there is a long winter with no stock use, and d) the start of grazing for each meadow is controlled, so we could sample after greenup but just before stock arrived. We could thus address persistent conditions produced by many years of stock use in isolation from any potential short term impacts. We sampled terrestrial arthropods in paired “grazed” and “ungrazed” meadows across the Park and collected associated vegetation data. We found some negative effects of grazing on vegetation structure, but few lasting negative or positive effects of long-term stock grazing on arthropods in these wetlands. Although it appears that pack stock do not cause lasting damage to this arthropod assemblage, the extent of impact at the height of the grazing season remains unknown.

Terrestrial and Aquatic Macroinvertebrate Assemblages as a Function of Wetland Type across a Mountain Landscape

Abstract Fens and wet meadows are important mountain wetland types, but influences on assemblage structure of associated invertebrates are poorly understood compared with other aspects of the ecology of these habitats. We sought to determine the relative contributions of terrestrial and aquatic invertebrates to diversity and abundance in these wetlands, the extent to which terrestrial and aquatic invertebrate assemblages differ with wetland type, and to what degree the aquatic assemblages vary as a function of slow sheet flow. We compared assemblages in fens and wet meadows, with and without flow, at 80 backcountry sites dispersed across the 6200 km2 landscape of Yosemite, Sequoia, and Kings Canyon National Parks in the Sierra Nevada mountains of California, U.S.A., using standard aquatic and terrestrial sweep netting. Cicadellid leafhoppers, aphids, and thomisid crab spiders were the most abundant terrestrial taxa. Cicadellids, Lepidoptera, anthomyiid, muscid, chloropid, and ephydrid flies, and thomisids were more abundant in fens than in wet meadows. Only mirid leaf bugs were significantly more abundant in wet meadows than fens. Sphaeriid clams and chironomid midges dominated aquatic assemblages both with and without flow. Chloroperlid stoneflies, mites, clams, and flatworms were all more abundant in flow, and Hemiptera and mosquitos were significantly more abundant in quiescent water. Mosquitos were more abundant in wet meadows, but there were few other population differences as a function of wetland type. Terrestrial diversity was 1.1 to 2.0 times that of aquatic diversity, depending on metric and habitat. Fens had greater terrestrial abundance, richness, evenness, and diversity than wet meadows; there were fewer differences as a function of wetland type for aquatic fauna. Presence or absence of slow sheet flow had more effect on these aquatic assemblages than did wetland type. Cluster analyses, ordination, and multi-response permutation procedures were generally consistent with the univariate results. Vegetation-based wetland classifications should be extrapolated to faunal assemblages with caution, particularly for aquatic invertebrates.