Indicator Species Others doubt the explanation More recently a global pandemic has been implicated But what should suddenly trigger lethal outbreaks of disease in amphibians is unclear 255 have at least two sets of data on the particular indicator species in question, taken in the same way, at the same place(s), on two separate occasions More frequent sampling allows greater confidence in the direction of apparent trends and the detection of more subtle environmental changes Environmental Toxicology In all the examples so far, the organisms being used as actual or possible indicators of environmental health have been in their natural environment There is another related but quite separate way in which biologists use the sensitivity of organisms to set environmental standards, namely in the science of environmental toxicology, or ecotoxicology for short In many areas of human endeavor, the aim is to apply some beneficial technology with minimum environmental damage Crop spraying with pesticides is a good example, and so is the discharge of treated effluent from a factory Some environmentalists claim that these types of operations should not lead to any environmental contamination; factories should have zero discharges, and if we must use pesticides, they should be targeted to reach only the crop and the pest and not, for example, the soil, nontarget organisms, or adjacent watercourses However, zero discharges or precision pesticides, if they can be achieved at all, can often only be obtained at great economic cost The more pragmatic solution is to ask whether there are minimal levels of discharge, spray drift into watercourses, and so forth that cause no detectable environmental damage To provide answers to this admittedly difficult question, environmental toxicologists use a wide variety of laboratory bioassays with standard organisms Examples from freshwater include the alga, Chlorella vulgaris, the water flea, Daphnia magna, the amphipod shrimp, Gammarus pulex, and the rainbow trout, Salmo gairdneri The fundamental problem is to try and establish acceptable levels of contamination Defining ‘‘acceptable’’ obviously requires political as well as biological judgment However, traces of a compound in water, air, or soil that cause no detectable changes in the performance (growth, survival, or reproduction) of the test organisms are clearly more acceptable than doses that kill 50% of the population (so called LD50 levels) Basically, the bioassays seek to set environmental standards for levels of potential pollutants in soil, air, and freshwater, using a range of standard laboratory organisms as indicators (Shaw and Chadwick, 1998), but there can be no absolute standards about what is safe or acceptable The general trend in modern societies is for standards to gradually tighten Species as Indicators of Environmental Change If the amphibian decline (discussed in the previous section) is real, it is an example of a group of organisms acting as indicators not only of the state of the environment, but also as indicators of ongoing changes to the global environment, albeit of an unknown nature In other words, given that species are sensitive to the condition of their environment, monitoring organisms not only tells you about the current state of an environment, but repeated monitoring can tell you about changes in that environment To act as indicators of change rather than current environmental health, it is necessary to Not all Monitoring is about Environmental Degradation Not all monitoring of species seeks to record environmental degradation Increasingly after mining operations, for example, mine operators are required to restore spoil heaps and mine pits by sowing or planting native vegetation Monitoring selected groups of common animals on nearby undisturbed control sites and on the restored land can give a good indication of the recovery of the entire ecosystem and of the success of the restoration project For instance, when biologists monitored ant assemblages on abandoned, replanted bauxite mines in Australia, they found that the ants provided a good indication of the recovery of these ecosystems Even after 14 years there were still differences between the ant communities found in the natural Eucalyptus forest and the restored land Historical Records of Change Lake Acidification It may not always be necessary to sample in real time When anthropogenic acidification of lakes was first discovered, many people doubted that the phenomenon was real In particular, there was considerable opposition to the notion from the power-generating industry, because solving the problem (by burning low-sulfur coal, adding ‘‘scrubbers’’ to power station chimneys to remove sulfur dioxide, or switching to natural gas) was inevitably going to be expensive After all, there were few historic data on the state of the acidified lakes Perhaps they had always been that way? Resolving the problem required knowledge of the fact that lake phytoplankton (the tiny, unicellular plants that float in the upper layers of lakes) are extremely sensitive environmental indicators, because different species grow best in very different conditions determined by nutrient status and pH (acidity) When algae die, they sink to the bottom where their bodies and characteristic pigments are buried and some are preserved (incipient fossils), particularly the resistant, silicious outer cases of a group called diatoms An undisturbed core through the sediments records the history of a lake’s phytoplankton, with the oldest flora at the bottom Cores showed unequivocally that many Scandinavian lakes that are acid now were not acid before the Industrial Revolution; the oldest diatomsFspecies not found in acid lakesFare gradually replaced in the sample column by acid-tolerant species Diatoms are wonderfully sensitive indicators of environmental change (Figure 1) Plants and Carbon Dioxide Herbarium specimens (pressed plants collected for taxonomic purposes) and fossil leaves can also be used as indicators of past environmental change Another consequence of the rapid rise in the burning of fossil fuel since the Industrial Revolution has been an accelerating rise in the concentration