Exercise adapted from “The impact of insecticides and pesticides on the biodiversity
and productivity of aquatic communities” Rick A. Relyea, Ecological Application,
15(2), 2005, pp 618-627
One of the goals of ecology is to understand patterns of species abundance and
diversity in communities and ecosystems. Ecologists also try to understand how
human influences these ecosystems (anthropogenic influences). Many ecosystems
are degraded because of exposure to insecticides and herbicides (pesticides). These
chemicals can potentially affect the biodiversity present in an ecosystem, therefore
it is important to develop experimental systems to analyze their effects. The author
chose to investigate the effects of herbicides and insecticides on aquatic
communities.
Experimental protocol
In order to recreate an aquatic ecosystem, Dr. Relyea used outdoor aquatic
mesocosms. In this study the mesocosms were 1200 L plastic tank filled with 1000
L of well water. After a week, the author added 300 g of dry leaves and 25 g of
rabbit chow to each tank to serve as habitat structure and initial nutrient source. He
also added zooplankton and phytoplancton collected from local ponds.
Five days later, he began adding macro-organisms collected from natural habitat
either as mixture of egg masses that were previously hatched from wading pools, or
as larvae or adults dip-netted from ponds and wetlands. He finally added five
species of larval damselfly (predators on zooplankton), snail species, and the
remaining predators (larval dragonflies, predator of tadpoles and snails; larval
beetles, predators on tadpoles and zooplankton; hemipterans (backswimmer and
waterbugs), predators on tadpoles and snails and recently hatched spotted
salamander larvae, predators on zooplankton).
All these species naturally coexist and the authors used densities that were within
the range of natural habitat
Insecticide and herbicide treatments:
Five treatments were used: two insecticides carbaryl (Sevin) and malathion; two
herbicides glyphosate (Roundup), and 2,4-D; and one negative control (no
treatment). Each treatment was applied at the suggested manufacturer application
and concentration rate.
All treatments were performed immediately after all the taxa had been added to the
microcosms. The experiment was conducted for two weeks. At the end of the
experiment, the author examined the impact of the pesticides on total species
richness and biomass for the four major functional groups: predators (insects and
salamanders), large herbivores (snails and tadpoles), zooplankton, and periphyton
algae (phytoplankton).
Each treatment were replicated six times.
Results:
1) Effect of Pesticides on Species Richness
FIG. 1. The impact of four different pesticides on the species richness of
predators (insects and spotted salamanders), large herbivores (tadpoles and
snails), and zooplankton in aquatic mesocosm communities. Data are means +/SE.
2) Effect of Pesticides on Biomass
FIG. 2. The impact of four different pesticides on the biomass (or abundance)
of predators (insects and spotted sal- amanders), large herbivores (tadpoles and
snails), zooplank- ton, and periphyton in aquatic mesocosm communities. Data are
means +/- SE.
Questions:
1) Formulate the question the author wished to investigate. (What is the
question they are answering?)
2) Using a flow chart, present the experimental protocol used in this study.
3) Do you think that the experimental protocol is appropriately designed to
answer the question(s) the author wish to investigate? (Justify your
reasoning. Does the scientist use appropriate controls and experimental
design?)
4) Analyze the data presented in the study and answer the following questions
(make sure to include values from the graphs in your answers):
-
Did all the pesticides have the same effects when including all animal
species?
What group of pesticides are the most toxic for all animals, herbicides or
insecticides? Justify your answer. Did you expect these results?
When looking at the different groups, are all the pesticides equally toxic to
each group? Did you observe any differences between the different
pesticides?
Do you observe a correlation between the results on species richness and
those on biomass? Was this expected? Explain.
5) What general conclusion can you infer from that study about the safe use of
insecticides and herbicides? Write a brief paper summarizing the
question(s) asked, the results shown and the conclusion that can be drawn
from this study