Indirect Effect of Pesticides on Insects and Other Arthropods

Pesticides released to the environment can indirectly affect target and non-target species in ways that are often contrary to their intended use. Such indirect effects are mediated through direct impacts on other species or the physical environment and depend on ecological mechanisms and species interactions. Typical mechanisms are the release of herbivores from predation and release from competition among species with similar niches. Application of insecticides to agriculture often results in subsequent pest outbreaks due to the elimination of natural enemies. The loss of floristic diversity and food resources that result from herbicide applications can reduce populations of pollinators and natural enemies of crop pests. In aquatic ecosystems, insecticides and fungicides often induce algae blooms as the chemicals reduce grazing by zooplankton and benthic herbivores. Increases in periphyton biomass typically result in the replacement of arthropods with more tolerant species such as snails, worms and tadpoles. Fungicides and systemic insecticides also reduce nutrient recycling by impairing the ability of detritivorous arthropods. Residues of herbicides can reduce the biomass of macrophytes in ponds and wetlands, indirectly affecting the protection and breeding of predatory insects in that environment. The direct impacts of pesticides in the environment are therefore either amplified or compensated by their indirect effects.

Nonlinear ecosystem responses to leaf litter subsidies in experimental ponds

Subsidies of matter between ecosystems can strongly affect trophic interactions in food webs, yet most experiments apply a binary (i.e., subsidy vs. no subsidy) approach. This emphasis on extremes implicitly assumes linear relationships and fails to capture potential nonlinear effects in ecosystem responses along gradients of subsidy amount seen in real systems. To examine the effects of such gradients, we manipulated leaf litter biomass across seven subsidy levels in 1000-L replicated experimental ponds. As our subsidy recipient, we used a simple food web that included southern leopard frog tadpoles (Rana sphenocephala) and spotted salamander larvae (Ambystoma maculatum) as the top vertebrates. We measured temperature, dissolved N and P; dissolved organic carbon (DOC); dissolved oxygen (DO); phytoplankton and periphyton biomass; and amphibian biomass export. Dissolved N, phytoplankton and periphyton biomass were linearly related to subsidy amount, but DOC, periphyton ash-free dry mass, and DO were nonlinearly related to subsidies. Frog biomass export increased as a function of subsidy amount, while salamander biomass export increased up to moderate subsidy levels before decreasing to very low levels at the highest subsidies, suggesting not only a nonlinear response, but a non-monotonic one. Our data suggest that under high subsidies, despite high nutrient levels, the basal resource supply may have switched from an algal to a detrital one. These data show that a dose-dependent approach is fundamental for understanding how subsidy amount affects ecosystems.

Variability in periphyton community and biomass over 37 years in Lake Tahoe (CA-NV)

Nuisance periphyton blooms are occurring in oligotrophic lakes worldwide, but few lakes have documented changes in biomass through periphyton monitoring. For decades periphyton has caused concern about oligotrophic Lake Tahoe’s nearshore water quality. To determine whether eulittoral periphyton increased in Lake Tahoe, measures of biomass and dominant communities at 0.5 m below lake level have been monitored regularly at nine shoreline sites starting in 1982, with up to 54 additional sites monitored annually at peak biomass. Lake-wide, this metric of periphyton biomass has not increased since monitoring began. Biomass decreased at many sites and increased at one. Periphyton biomass peaked in March and was low in the summer lake-wide. The northern and western shores had higher biomass than the eastern and southern shores. Biomass varied with lake level. High biomass occurred at sites regardless of urban development levels. As increasing periphyton at Lake Tahoe was first cited in scientific literature in the 1960s, it is possible that periphyton increased prior to our monitoring program. A dearth of published long-term monitoring data from oligotrophic lakes with reported periphyton blooms makes it difficult to determine the extent of this issue worldwide. Long-term nearshore monitoring is crucial for tracking and understanding periphyton blooms.

Influence of intra- and interspecific competition on periphyton biomass and growth performance of Holothuria scabra juveniles

The effects of sea cucumber (Holothuria scabra) juvenile density and interspecific competition from the opisthobranch Stylocheilus striatus (SS) on periphyton biomass and growth performance of H. scabra (HS) were assessed. The number of H. scabra juveniles (15, 50, 100) in net enclosures (treatments HS15, HS50, and HS100) was varied and feeding activity of S. striatus was assessed at a density of 15 individuals per net enclosure (treatment SS15), and in co-culture with H. scabra (treatment HS15/SS15). A control treatment without H. scabra or S. striatus was included in the 28-d experiment. Highest and lowest growth rates of H. scabra occurred in the lowest (HS15) and highest (HS100) stocking density treatments, respectively, on days 14 and 28. Growth of H. scabra juveniles in the HS15 treatment was more than double that of those in the HS15/SS15 treatment. Juvenile survival was significantly higher in the HS15 treatment on days 14 and 28, but did not differ significantly from that of juveniles in the HS15/SS15 and HS50 treatments on day 14. Grazing activity of sea cucumber juveniles and S. striatus resulted in a decline in periphyton biomass (AFDW) after day 7. Differences in food consumption (based on faecal production) among treatments were significant only on days 1 and 21 with the greatest consumption (17.5 mg periphyton ind−1 d−1) by S. striatus in the SS15 treatment. Holothuria scabra juveniles in the HS15 treatment consumed 4.9 mg ind−1 d−1, while those in the HS100 treatment consumed only 1.8 mg ind−1 d−1 after 24 hrs.

Impacts of sediment derived from erosion of partially-constructed road on aquatic organisms in a tropical river: The Río San Juan, Nicaragua and Costa Rica

Throughout the humid tropics, increased land disturbance and concomitant road construction increases erosion and sediment delivery to rivers. Building road networks in developing countries is commonly a priority for international development funding based on anticipated socio-economic benefits. Yet the resulting erosion from roads, which recent studies have shown result in at least ten-fold increases in erosion rates, is not fully accounted for. While effects of road-derived sediment on aquatic ecosystems have been documented in temperate climates, little has been published on the effects of road-induced sediment on aquatic ecosystems in developing countries of the tropics. We studied periphyton biomass and macroinvertebrate communities on the deltas of Río San Juan tributaries, comparing north-bank tributaries draining undisturbed rain forest with south-bank tributaries receiving runoff from a partially-built road experiencing rapid erosion. Periphyton biomass, richness and abundance of macroinvertebrates overall, and richness and abundance of Ephemeroptera, Plecoptera and Trichoptera were higher on the north-bank tributary deltas than the south-bank tributary deltas. These findings were consistent with prior studies in temperate climates showing detrimental effects of road-derived fine sediment on aquatic organisms. A Non-Metric Multidimensional Scaling (NMDS) analysis showed the impacted community on the south-bank deltas was influenced by poorly-sorted substrate with greater proportions of fine sediment and higher water temperatures.

Comparison of Periphyton Biomass on Coconut Coir and Bamboo Poles as Natural Substrates in Earthen Lined Pond

As a first step in assessing the viability of periphyton-based shrimp production in lined pond aquaculture systems, the effects of natural substrates (split bamboo poles and coconut coir) on development of periphyton were evaluated. Experiments were conducted in outdoor earthen lined pond to study periphyton biomass on two types of natural substrates such as split bamboo pole and coconut coir which was placed inside the earthen lined pond filled with seawater for duration of 45 days. The periphyton biomass was recorded at 0–40 cm depth, 40–80 cm depth and 80–120 cm for all the substrates. The periphyton biomass, in terms of dry matter (DM) from coconut coir was comparatively higher (3.4995±0.3120 mg cm-2) than the split bamboo poles (3.2011±0.3044 mg cm-2) during the experiment. ‘T’ test has affirmed that significant difference (p<0.05) was observed in the dry matter content of the periphyton among the substrates. ‘T’ test analysis of the data relating to periphyton biomass for split bamboo pole and coconut coir recorded at different depths indicated that upper 0–40 cm depth had higher values than the other depths. Furthermore, among the two substrates, coconut coir (3.4995±0.31201 mg cm-2) was observed to be a better substrate than the split bamboo pole for periphyton biomass production. Coconut coir substrate can be utilised by fin and shellfishes as natural food.