Erica A. Garcia

REGIONAL COEXISTENCE AND LOCAL DOMINANCE IN CHAOBORUS: SPECIES SORTING ALONG A PREDATION GRADIENT

Variation in the intensity of predation across the well-known environmental gradient of freshwater habitats from small, ephemeral ponds to large, permanent lakes is a key factor in the development and maintenance of aquatic community structure. Here, we present data on the distribution and abundance of four species of Chaoborus (Diptera: Chaoboridae) across this environmental gradient. Chaoborus show a distinct pattern of species sorting when aquatic systems are divided into fish and fishless environments, and this pattern is consistent with species traits known to influence their vulnerability to fish predation (i.e., pigmentation, diel vertical migration [DVM] behavior, and body size). To test whether fish are the drivers of this pattern, we created a gradient in fish density by stocking bluegill sunfish (Lepomis macrochirus) into 15 experimental ponds in southwestern Michigan, USA, and then allowed Chaoborus species to colonize. There was clear evidence of species sorting along the predation gradient; Chaoborus americanus was most abundant in the fishless ponds, C. flavicans was neutral in response to fish, and C. punctipennis and C. albatus were most abundant at high fish biomass, a response consistent with their field pattern. Furthermore, prey preference experiments confirm that size selective predation and differences in Chaoborus species traits contribute to the pattern of Chaoborus abundance and distribution.

Fish reintroductions reveal smooth transitions between lake community states

Whether communities respond smoothly or discontinuously to changing environmental conditions has important consequences for the preservation and restoration of ecosystems. Theory shows that communities may exhibit a variety of responses to environmental change, including abrupt transitions due to the existence of alternate states. However, there have been few opportunities to look for such transitions in nature. Here, we examine the impact of a two-orders-of-magnitude decrease and then increase in planktivore abundance in Wintergreen lake (Michigan, USA), caused by the extinction and reintroduction of two dominant fish species (largemouth bass, Micropterus salmoides, and bluegill, Lepomis macrochirus). Over a 16 + yr period of slow change from high planktivory to low planktivory back to high planktivory, the zooplankton community changed smoothly and predictably between states. In years of low planktivory, the zooplankton assemblage was dominated by a single, large, cladoceran species, Daphnia pulicaria, whereas in years of high planktivory, D. pulicaria disappeared and was replaced by a suite of small-bodied cladocerans. We quantified the multivariate change in zooplankton community dissimilarity and found that community state smoothly tracked changes in planktivore density in both a forward and backward direction. Thus, there was little evidence of discontinuity in this system where transitions are strongly driven by planktivory.

A life-history account of Macrobrachium spinipes (Schenkel, 1902) (Cherabin) in a large tropical Australian River

Rivers in tropical Australia are largely pristine, and ecosystem connectivity is intact, but water resources in northern Australia are under increasing development pressure. A better understanding of the role and life histories of key migratory species is urgently needed to manage the risks posed by development. We investigated the life history of Macrobrachium spinipes (Schenkel, 1902) in the Daly River of the Northern Territory, Australia. We carried out a 13-mo trapping program (2011–2012) at 7 sites along the river between 92 and 405 km from the estuary. We provide the first detailed account of spatial and temporal changes in relative abundance and reproduction, and present information on length at first maturity and Fulton’s body condition factor. Reproduction was strongly seasonal and was restricted to the wet-season months of November to April, followed by a recruitment pulse 3 to 4 mo later during the early dry season. Length at first maturity declined as the reproductive season progressed but did not vary significantly with distance from the estuary. Condition was temporally variable and peaked for both sexes immediately after the wet season before declining throughout the dry season. Relative abundance patterns followed a similar pattern. Contrary to results for other Macrobrachium species in rivers of comparable length, we found an increase in the relative abundance of females throughout the river’s length during the reproductive season and reproductive effort occurring far upstream, over a restricted time period. This result highlights the importance of maintaining connectivity for reproductive success of this species and further recruitment into these upstream reaches.

Crab regulation of cross-ecosystem resource transfer by marine foraging fire ants.

Permeability of boundaries in biological systems is regulated by biotic and/or abiotic factors. Despite this knowledge, the role of biotic factors in regulating resource transfer across ecosystem boundaries has received little study. Additionally, little is known about how cross-ecosystem resource transfer affects source populations. We used experiments, observations and stable isotopes, to evaluate: (1) the proportion of intertidal-foraging black fire ant (Solenopsis richteri) diet derived from marine sources, (2) how black fire ant cross-ecosystem resource transfer is altered by the dominant bioengineer in the intertidal, a burrowing crab (Neohelice granulata), (3) the top-down impact of these terrestrial ants on a marine resource, and (4) the effect of marine resources on recipient black fire ants. We found that more than 85% of the black fire ant diet is derived from marine sources, the number of intertidal foraging ants doubles in the absence of crab burrows, and that ants cause a 50% reduction in intertidal polychaetes. Also, ant mound density is three times greater adjacent to marine systems. This study reveals that cross-ecosystem foraging terrestrial ants can clearly have strong impacts on marine resources. Furthermore, ecosystem engineers that modify and occupy habitat in these ecosystem boundaries can strongly regulate the degree of cross-ecosystem resource transfer and resultant top down impacts.

Importance of the natural flow regime to an amphidromous shrimp: a case study

Amphidromous shrimp inhabiting large river systems are thought to migrate downstream to hatch larvae close to the estuary or sea. Recent work on Macrobrachium spinipes in northern Australia challenged this hypothesis, with the discovery of abundant females in reproductive condition over 400km upstream. We investigated the early life history of M. spinipes by using a combination of field sampling, laboratory experiments and modelling. We determined the spatial and temporal production of larvae, larval salinity requirements, and, finally, the potential for transport of larvae to the estuary during the wet season. Larvae occurred in high abundances across a 400-km river reach. However, successful larval development was contingent on access, by drift, to saltwater within 7 days. This interval is longer than that found for the majority of amphidromous shrimp species and we propose that larvae in the study river are able to successfully moult through the early stages in freshwater. Large, wet-season discharge events were critical for transporting larvae from all sampled sites to the estuary. The present study confirmed the amphidromous life history of M. spinipes, highlighted the importance of natural flows for maintaining existing populations and provided evidence of a novel type of amphidromous life-history strategy.

Context dependency of top-down and bottom-up effects in a Northern Australian tropical river

Top-down and bottom-up forces (consumer and resource limitation, respectively) influence biomass of primary producers and primary consumers in natural food webs. Few investigators experimentally examine both in concert, especially in the tropics. Tropical systems probably are more sensitive than temperate systems to eutrophication and other disturbances, such as destruction of riparian canopy cover, because of wide windows of ecological opportunity. We experimentally examined the relative importance of top-down and bottom-up effects at 2 sites in the Edith River in the Australian wet–dry tropics during baseflow conditions. We used large fish-exclusion cages to examine top-down effects of fish, and we used nutrient-diffusing substrates to manipulate bottom-up effects of nutrients. Macroinvertebrates were significantly more abundant in fish-exclusion cages than in open control areas at both sites. At 1 site, chlorophyll a on rock substrate was 1.5× greater and ash-free dry mass was 1.4× greater in fish-exclusion than in open cages. Chlorophyll a was >3× greater in the high-N and -P nutrient treatment than in the unamended control at both sites. Top-down effects on the algae on nutrient-diffusing substrates were detected at only 1 site. Bottom-up factors seem to be relatively more important in controlling primary producer biomass compared to top-down factors, with no evidence of a trophic cascade. Bottom-up and top-down effects on macroinvertebrates were strong and consistent, whereas top-down effects on algae were context dependent and varied depending on benthic habitat, current speeds, and community structure. These results indicated spatial variation at the scale of tens of meters.

Continental-scale decrease in net primary productivity in streams due to climate warming

An increase in stream temperature leads to a convergence of metabolic balance, overall decline in net ecosystem productivity, and higher CO2 emissions from streams, according to analyses of temperature sensitivity of stream metabolism across six biomes.AbstractStreams play a key role in the global carbon cycle. The balance between carbon intake through photosynthesis and carbon release via respiration influences carbon emissions from streams and depends on temperature. However, the lack of a comprehensive analysis of the temperature sensitivity of the metabolic balance in inland waters across latitudes and local climate conditions hinders an accurate projection of carbon emissions in a warmer future. Here, we use a model of diel dissolved oxygen dynamics, combined with high-frequency measurements of dissolved oxygen, light and temperature, to estimate the temperature sensitivities of gross primary production and ecosystem respiration in streams across six biomes, from the tropics to the arctic tundra. We find that the change in metabolic balance, that is, the ratio of gross primary production to ecosystem respiration, is a function of stream temperature and current metabolic balance. Applying this relationship to the global compilation of stream metabolism data, we find that a 1 °C increase in stream temperature leads to a convergence of metabolic balance and to a 23.6% overall decline in net ecosystem productivity across the streams studied. We suggest that if the relationship holds for similarly sized streams around the globe, the warming-induced shifts in metabolic balance will result in an increase of 0.0194 Pg carbon emitted from such streams every year.

Disturbance Type Affects the Distribution of Mobile Invertebrates in a High Salt Marsh Community

Abstract Salt marshes are frequently exposed to storm overwashes resulting in large deposits of sand and wrack at the margin of the high marsh and sand dune communities. On Cape Cod, MA, these disturbance-generated areas are dominated by burrows of the crab Uca pugilator and by nest entrances of the ants Formica subsericea and Tetramorium caespitum. We mimicked the effects of storm deposits through additions of sand and wrack and examined their effects on the distributions of the biotic structures of these organisms. We found that while crabs responded negatively to sand deposition, ants did so positively. We suggest that soil temperature and moisture explain these patterns. Wrack deposits extend higher the zone of moist soil and decrease evaporative stress for marine organisms such as crabs, whereas sandy areas tend to be drier and preferred by terrestrial ants. We conclude that disturbance type influences the distribution of these marine and terrestrial organisms over the ecotone.

Ontogenetic shifts in habitat use during the dry season by an amphidromous shrimp in a tropical lowland river

Caridean shrimp have considerable effects on ecosystem processes and, thus, understanding their use of key habitats is important for determining their potential ecological effect. The present study examined the meso-habitat use of Macrobrachium spinipes, a large-bodied and important amphidromous species, in the Daly River, northern Australia. We examined shrimp abundance at four common meso-habitat types; sand bank, rock bars, undercut and structurally complex banks and mid-channel areas at five sites on three occasions during the dry season (May to October). We found that habitat use changed considerably first, with ontogeny, and, second, with the colonisation of habitats with algae and macrophytes as the dry season progressed. As juveniles, their habitat use was strongly associated with well structured bank environments early in the dry season. By the mid-dry season, juveniles were more abundant within sand habitats recently colonised with macrophytes and filamentous algae. Females showed little change in habitat use, whereas large-bodied dominant males generally favoured rock bars and heavily structured bank environments. The present study has provided significant insights into the changes in use of key riverine habitats throughout the dry season by an ecologically important species. This information will be of considerable value to the determination of environmental flow requirements and food-web investigations.