ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 2 pptx

Thoreau 1851 Organismal ecotoxicology explores toxicant effects to individuals and, where possible, links them to effects to populations and communities.. The focus of attention in this

Part II

Organismal Ecotoxicology

Conventionalities are as bad as impurities

Uncommon Learning (H.D Thoreau 1851)

Organismal ecotoxicology explores toxicant effects to individuals and, where possible, links them to effects to populations and communities Such exploration has been at the center of ecotoxicology and its predecessors (aquatic, wildlife, and environmental toxicology) since their inceptions Because of our proclivity toward study of toxicant effects to the soma—the body of the individual organism— much in this section should be comfortably familiar to the professional ecotoxicologist or advanced student What might not be as familiar will be the focus on fundamental principles and linkage of these effects to those at higher levels of biological organization

The preoccupation of ecotoxicologists with the soma emerges from the historical foundations of our new science It is obvious during even a cursory examination of the most popular ecotoxicology textbooks (e.g., Cockerham and Shane 1994, Connell et al 1999, Landis and Yu 1995, Newman

1998, Walker et al 2001) that many basic concepts and techniques blended into ecotoxicology come from mammalian toxicology, a field with a justifiable emphasis on the individual Still other concepts and techniques come from classic autecology Used with balance and insight, this offers several advantages to the field Ecotoxicologists can draw deeply from the mechanistic knowledge base of classic toxicology, a field focused on individuals This knowledge is directly useful for charismatic, endangered, or threatened species that are protected by prohibiting the taking of even a single individual It also provides a firm base at one level of biological organization from which to extend scientific insight upward to the next

The mechanistic and technological richness of classic toxicology and autecology comes at a price The paradigms around which phenomena are explored by ecotoxicologists are often those associ-ated with the soma Exploration of other important ecotoxicological phenomena are unintentionally addressed with less intensity or quietly dismissed as secondary The rich technology associated with organismal toxicology naturally draws practitioners to these tools The result is a rapid enrichment

of the field: an enrichment that also maintains the present imbalance Resolution of this incongru-ity requires application of concepts and technology in a way that does not foster any unintentional

neglect of higher levels of organization and with the intent of producing predictive insight about phenomena at higher levels of organization That is the intent of this section

REFERENCES

Cockerham, L.G., and Shane, B.S., Basic Environmental Toxicology, CRC Press, Boca Raton, FL, 1994 Connell, D., Lam, P., Richardson, B., and Wu, R., Introduction to Ecotoxicology, Blackwell Science Ltd.,

Oxford, UK, 1999

Landis, W.G., and Yu, M.-H., Introduction to Environmental Toxicology, CRC Press, Boca Raton, FL, 1995 Newman, M.C., Fundamentals of Ecotoxicology, CRC Press/Lewis, Boca Raton, FL, 1998.

Thorean, H.D., Uncommon Learning, Bickman, M (ed.), Houghton, Mifflin, Co., Boston, 1851.

Walker, C.H., Hopkin, S.P., Sibly, R.M., and Peakall, D.B., Principles of Ecotoxicology, Taylor & Francis,

New York, 2001

2 The Organismal

Ecotoxicology Context

2.1 OVERVIEW

The science of ecotoxicology has grown rapidly in 30 years and has brought together a vast body of facts around several explanatory systems Explanatory systems were borrowed in necessary haste from mammalian toxicology and ecology The immediacy of our environmental problems required this haste Required now is coherence among the clusters of explanatory hypotheses that are rapidly coalescing at each level of biological organization Together, these paradigms form the foundation for ecotoxicological theory.1If these paradigms are not made mutually supportive, the foundation

of ecotoxicology will not be adequate to support further knowledge accumulation and organization The field will break into semi-isolated scientific disciplines

Conceptual consilience is not an intellectual nicety: it is vital to the health of any science Without consistency among theories and facts, there is no way for the ecotoxicologist to choose from among many the explanation providing the best foundation for predicting pollutant effects

The requirement of consistency will be appreciated if one realizes that a self-contradictory system is uninformative It is so because any conclusion we please can be derived from it Thus no statement is singled out, either as incompatible or as derivative since all are derivable A consistent system, on the other hand, divides the set of all possible statements into two: those which it contradicts and those with which it is compatible This is why consistency is the most general requirement for a [scientific]

system if it is to be of any use at all.

(Popper 1959)

As articulated by Popper, sciences lacking self-consistency are not viable Ecotoxicological explanations need to be consistent among all levels of organization or the science of ecotoxicology will eventually fail to be useful Beyond this, efforts spent finding consistency have another desirable effect relative to scientific logic It can identify common causes for phenomena described at different levels of biological organization The identification of a common cause allows the overall number

of theories to be reduced Why have two distinct theories to explain the same thing? The parsimony resulting from theory reduction—that is, intertheoretical reduction (Rosenberg 2000)—enhances any science and is particularly warranted in ecological sciences (Loehle 1988)

A final reason exists for the emphasis on integrating explanatory systems from different levels

of biological organization Not doing so allows the current condition to remain in which an ecotox-icologist trying to describe and solve a particular environmental problem may present and defend findings based on contradictory explanations This diminishes the legal defensibility of arguments

1 Definitions of Rosenberg (2000) are being used in this discussion A set of explanatory principles or paradigms comprise the established scientific theory of a discipline, for example, evolutionary theory contains many explanatory principles such

as genetic drift or natural selection The paradigms have withstood rigorous testing and currently provide the best causal explanation of natural phenomena, for example, evolution theory explains genetic change in a population exposed to a toxicant.

13

calling for costly remediation It also increases the risk of pathological science, science practiced with an excess loss of objectivity (Langmuir 1989, Rousseau 1992)

It is a fault which can be observed in most disputes, that, truth being mid-way between the two opinions that are held, each side departs the further from it the greater his passion for contradiction

(Descartes, translated by Sutcliffe 1968)

Integration, combined with differentiation, is a major theme here because it fosters the identifica-tion of causal mechanisms that are consistent among levels of organizaidentifica-tion, is logically necessary in any healthy science, fosters resolution of environmental issues, and decreases the tendency toward pathological science

2.2 ORGANISMAL ECOTOXICOLOGY DEFINED

2.2.1 WHATISORGANISMALECOTOXICOLOGY?

Every species of plant is a law unto itself, the distribution of which in space depends upon its individual peculiarities of migration and environmental requirements. It grows in the company with any other

species of similar environmental requirements, irrespective of their normal associational affiliations

(Gleason 1926)

The scope of ecotoxicology is so necessarily encompassing that an ecotoxicologist can study fate or effect of toxicants from the molecular to the biospheric scales This book attempts to discuss this wide range of topics The focus of attention in this particular section is organismal ecotoxicology, the science of contaminants in the biosphere and their direct effects on individual organisms

The prominence of the organismal context is so long-standing and familiar to ecologists that it has its own name, autecology Autecology is the study of the individual organism or species, and its relationships to its physical, chemical, and biological environment The quote above from Gleason’s classic paper articulates the autecological framework

The boundaries of autecology are often vague Since its origins, autecology has been described

as either distinct from (e.g., Emmel 1973) or synonymous with (e.g., Reid 1961) population ecology

In reality, it overlaps with population ecology but tends to characteristically emphasize species requirements, physiological tolerances, means of adaptation, and life history traits, and how these influence success or failure in certain environs It emphasizes the soma and how it manages to survive For example, a wildlife manager concerned with a specific game bird species might take an autecological vantage to managing that particular species Another example of an autecological topic might be how the physiological tolerances of an estuarine crab relative to salinity and temperature influence its spatial distribution within an estuary A study by Costlow et al (1960) is a classic one of this sort (Box 2.1) in which tests of the physiological limits of individuals were used to suggest that salinity confines the spatial distribution of an estuarine crab The emphasis is plainly on qualities of individuals, not complex interactions among species or even the interactions among individuals in populations of this crab species.2

Several fundamental laws of ecology emerge from this context Liebig’s law of the minimum (Liebig 1840), first formulated to explain how nutrients limit plant standing crop, states that the factor

in the shortest supply of all required factors will limit the number or amount of individuals that a

2 In contrast to autecology, the subdiscipline of ecology focused on the integrated interactions of groups of individuals within an environment is called synecology Conventional topics of synecology are discussed principally in the last sections

of this book The population ecotoxicology section is the boundary between autecology and synecology, and covers a blend

of autecology with some synecology.

The Organismal Ecotoxicology Context 15

Box 2.1 Autecology of a Crab: Physiological Tolerances Determine Adult

Distribution

Costlow et al (1960) reared larvae of the estuarine crab, Sesarma cinereum, at different

combin-ations of salinity and temperature, hoping to gather enough information to explain the observed distribution of adult crabs in estuaries The assumption was simple: the physiological tolerances

of the larval stages, as reflected in survival rates, will determine the most likely part of the estu-ary in which the larvae will survive to become adults

Salinity strongly influenced survival and development time for all larval stages For example, the first zoea withstood higher salinities much better than lower salinities (Figure 2.1) Eggs hatched at all tested salinity and temperature combinations However, close to 100% mortality occurred at low (<12.5‰) and high (>31.1‰) salinities for most larval stages This suggested

that those larvae of any stage that were brought into intermediate salinity (and temperature) conditions would have the highest chances of survival Those hatching and staying in the low-est or highlow-est salinity waters would have the poorlow-est survival probability The optimum salinity and temperature for each larval stage were the following:

Larval Stage Salinity (‰) Temperature ( ◦ C)

minimum

minimum

Zoeal Stage 1

Megalops

Zoeal Stage 4

Zoeal Stage 3

20

35

30

5

25

Salinity (parts per thousand)

15

10 15 20 25 30 35 40

The shaded area is that

in which mortality is

approximately 25% or less

Zoeal Stage 2

FIGURE 2.1 Salinity and temperature tolerances

of Sesarma cinereum larvae The 25% mortality

contours were arbitrarily chosen to show the differ-ences in tolerances among stages (Modified from Figures 8–12 of Costlow et al 1960 and larval drawings rendered from Figures 1–5 in Costlow and Bookhout 1960.)

In contrast to the first and third zoea, the second zoea had high tolerances of salinity and temperature The first and second zoea had best survival at 21–31◦C and 23–28‰ The

last zoeal stage showed an increase in temperature tolerance (to 35◦C) and a salinity tolerance

down to 3‰ The megalops, the stage reached just before settling to the bottom, showed wide tolerances The authors concluded that completion of this crab’s life cycle to the adult depended primarily on the fourth zoea and that “the survival and molting to the megalops can only occur

in estuarine waters.” Any earlier stage larvae that were transported by water movements outside

of the estuarine conditions had very low probabilities of producing megalops Survival was less dependent on temperature or salinity once the megalops stage was reached So, the tolerances

of the larval stages determined the estuarine region in which the life cycle of this crab will

be successfully completed The weak links in the life cycle were the earlier larval stages

If the fourth zoeal larvae emerged under the appropriate salinity–temperature conditions, the relatively tolerant megalops would be produced, resulting in adults in that particular part of the estuary

particular habitat can sustain As an example, phosphorus might limit the standing crop of a nuisance blue-green algal species in a freshwater lake and, based on Liebig’s law, lake managers might focus on controlling phosphorus input to the lake Shelford’s law of tolerances (the tolerance of individuals of

a species over one or more environmental gradients determines the species’ geographical distribution

or abundance) (Shelford 1911, 1913) is another such law that is neatly illustrated by the Costlow

et al (1960) study

The ecological niche concept was formulated originally with emphasis on individual tolerances and requirements, and only later was enlarged to include biotic factors In fact, the niche concept theorizes that the organism occupies a realized niche in the presence of other species that is only a portion of its fundamental niche as defined by its organismal tolerances and requirements As a classic

example, the realized niche of the intertidal barnacle, Chthamalus stellatus, is strongly influenced

by desiccation at one extreme and interspecific competition for space with Balanus balanoides at

the other (Connell 1961)

In ecotoxicology, an autecological study might be conducted of the effects of a pollutant on individuals of a protected or threatened species An autecological approach might also be used if synecological aspects of a species’ niche occupation were thought to be unimportant or secondary Such an approach is also taken reluctantly if there was an absence of sound synecological information available relative to contamination

Much of ecotoxicology is done within an autecological context and is justified by the indisputable success of classic autecology (Calow and Sibly 1990) As successful as this approach might be for many situations, the autecological approach is often applied in ecotoxicology for situations in which a moment of introspection might reveal that crucial synecological factors are unjustifiably ignored.3

Reflecting the stage of ecology almost half a century ago, Costlow et al (1960) described how the physicochemical tolerances of a crab species contributed to its distribution in the coastal systems Twenty-five years later, when the understanding of contaminant effects became more and more essential, they approached the ecotoxicological consequences of drilling fluid discharge in the same way, implying whether populations would remain viable based on acute assays on the notionally most sensitive stages of a species’ life cycle (Box 2.2) This approach drew on a well-established autecological approach and, in this case, produced a reasonable conclusion They also adopted, with minimal adaptation, a technological paradigm from mammaliam toxicology—the LC50/LD50

3 Staying with a coastal marine theme, see Harger (1972) for further discussion of the importance of species interactions

in determining intertidal species distributions.

The Organismal Ecotoxicology Context 17

Box 2.2 Crab Autecotoxicology: Do Chromium Tolerances of Larvae Determine

Adult Fate?

Twenty-four years after the study described inBox 2.1, these researchers (Bookhout et al 1984) again described crab larval survival relative to environmental conditions In keeping with the emerging concern about anthropogenic chemicals, they focused on a pollutant this time.4The intent was very similar to that of their first paper—to determine the tolerances of the larval stages

to an environmental quality and, in doing so, to predict the likelihood of life cycle completion

in the presence of a specified intensity of that quality

They studied chromium used in drilling fluids to thin mud as it becomes dense Added as ferrochrome or chrome lignosulfonate, chromium was discharged during and after the drilling

At the time of this study, there was little information on whether its use was harmful to mar-ine species To explore the potential hazard, Bookhout et al (1984) exposed decapod larva to different concentrations of hexavalent chromium (as Na2CrO4) Results for mud crab,

Rhithro-panopeus harrisii, larvae are described here Figure 2.2 summarizes the cumulative mortality

experienced at different larval stages exposed to 0–58µg/L of sodium chromate Sodium

chro-mate concentrations from 7 to 29µg/L were considered to be sublethal concentrations because

10% or more of exposed larvae reached the first crab stage The lethal range was above 29µg/L

The LC50 for the complete hatch→ zoea → first crab life stages was 13.7 µg/L.5

After integrating this information with knowledge about drilling fluid distributions around points of discharge, the authors concluded that “it is probable that Cr in drilling fluids, whether

Cr3+or Cr6 +, is not likely to reduce the population of crab larvae and other planktonic organisms

in the area around oil wells except possibly in the immediate vicinity of the discharge pipes.”

Implied from these acute lethality tests on individual larvae was that the persistence of the mud

crabs population was not jeopardized, except in the immediate vicinity of a discharge This

reasoning was adapted from that used to define the fundamental niche, that is, application of the law of tolerances to predict the habitat that a species could occupy However, other aspects of the niche concept, such as interspecies competition, predation, and disease, were ignored This expedient neglect seems understandable in this particular application but is not always justified

58 µg/L

46

µg/L 41

µg/L

29 µg/L

15 µg/L

7 µg/L

100

0

25

75

Mud crab larval stage

Zoea

3 4 Megalops

50

1 µg/L

0 µg/L

FIGURE 2.2 Cumulative mortality of mud crab

exposure (Modified from Figure 4 in Bookhout

et al 1984.)

4The research described in Box 2.1 was published 2 years prior to Rachel Carson’s watershed book, Silent Spring (Carson

1962) As evidenced by the shift in Costlow et al.’s research, Carson’s book mandated that our research efforts become more focused on ecotoxicological questions.

5 Note that this last metric, the LC50, was borrowed from the mammalian toxicology literature to measure toxic effects here but was absent in the temperature–salinity study described in Box 2.1.

approach The approach taken by these authors and many others dominates ecotoxicology to this day (SeeChapter 9for details.) It is sufficient in many cases or useful in others for quickly identifying gross problems associated with contamination Despite its utility in such cases, this autecotoxicolo-gical approach is insufficient in many others and a synecotoxicoloautecotoxicolo-gical context is needed The key

to successful prediction of ecotoxicological consequences is being able to accurately discriminate between situations requiring autecological or synecological vantages, and being able to integrate information from both vantages into reliable predictions of exposure consequences

2.3 THE VALUE OF ORGANISMAL ECOTOXICOLOGY

VANTAGE

If the modes of action of toxicants are better understood, we could more accurately predict their effects

as pollutants; much knowledge already exists in medical sciences, and could be transferred

(Sprague 1971)

Although this discussion may appear hostile to single species toxicity testing efforts, it is not intended

to be Single species tests are exceedingly useful and are presently the major and only reliable means

of estimating probable damage from anthropogenic stress Furthermore, a substantial majority, perhaps everyone in this meeting is certainly aware of the need for community and system level toxicity testing How then does one account for the difference between awareness and performance?

(Cairns 1984)

Just as autecology is an essential component of ecology, organismal ecotoxicology— autecotoxicology, if you will—is an essential component of ecotoxicology Unfortunately, as exemplified in the quote by Cairns above, organismal ecotoxicology tends to overshadow equally crucial investigative vantages In the remainder of this chapter, the many appropriate and essential applications of organismal ecotoxicology will be highlighted

2.3.1 TRACTABILITY ANDDISCRETENESS

Organismal effects are generally the most discrete and tractable of ecotoxicological effects Few ecotoxicologists would disagree with this statement After agreeing, a good number of ecotoxico-logists would immediately identify this truism as a sad statement about the field, or point out that this condition may simply be a matter of the historical amounts of effort and thought that have gone into autecotoxicology and synecotoxicology Here, the follow-up to this statement will simply be

to demonstrate the important advantages of drawing on our comprehensive knowledge of organis-mal effects The ease with which organisorganis-mal effects or exposure can be assessed will be described first Next, the relatively effective extrapolation among individuals will be detailed Organismal information also contributes to our abilities to do reasoned extrapolations of effect to populations and communities, and to predict toxicant transfer within communities

2.3.2 INFERRINGEFFECTS TO OREXPOSURE OFORGANISMS WITH

SUBORGANISMALMETRICS

Sprague, as quoted above, stated correctly that knowledge of suborganismal modes of action greatly improves predictions of toxicant effects to individuals A current example is endocrine system mod-ulation by xenobiotics Our newfound understanding of this mode of action for diverse classes of xenobiotics, such as 17β-estradiol from oral contraceptives, 4-nonylphenols, and polychlorinated

biphenyls (PCB), improves prediction of similar effects of new chemicals or from new sources of existing xenobiotics (Brown et al 2001, Hale and La Guardia 2002, Schultz 2003) Knowledge of the

The Organismal Ecotoxicology Context 19

suborganismal mode of action also provides a means by which diverse phenomena can be linked by a common thread As an example, Bard (2000) opined that multixenobiotic resistance in aquatic organ-isms can be explored in the context of the multidrug resistance phenomena The important insight

is that aquatic toxicologists could greatly advance their understanding of multixenobiotic resistance

in exposed populations by exploring the extensive literature on the role of P-glycoprotein overex-pression in determining antitumor drug resistance of cancer cells Transmembrane P-glycoproteins tend to inhibit the transport of xenobiotics into cells, reducing the concentration of xenobiotics at intracellular sites of action The same is true whether the xenobiotic is a cancer drug or a contamin-ant Environmental xenobiotic resistance and anticancer drug resistance share a common theme of adaptation by P-glycoprotein overexpression

Suborganismal qualities also provide evidence of contaminant exposure or effects As a sur-prising example, mesopelagic fish sampled at 300–1500-m depth in the open Atlantic Ocean show evidence of exposure to aryl hydrocarbon receptor antagonists, e.g., polynuclear aromatic hydro-carbons (PAHs) and coplanar-halogenated aromatic hydrohydro-carbons (Stegemen et al 2001) Elevated cytochrome P450 1A also suggests exposure at considerable distance from coastal sources of aryl hydrocarbon receptor antagonists

2.3.3 EXTRAPOLATING AMONGINDIVIDUALS: SPECIES, SIZE, SEX,

ANDOTHERKEYQUALITIES

Often predictions require extrapolation from data in hand to some less well-defined situation.6 Suter (1998) describes two typical examples: extrapolation from LC50 values of Salmoniformes

to those for Perciformes, and prediction of carbamate pesticide LD50 based on a test species weight Ellersieck et al (2003) provide a computational means for extrapolation of toxicant effects among species Although challenging and error prone, interpolation among individuals within a species

is perhaps the most credible of ecotoxicological interpolations As an example, Newman (1995) describes interpolation among mosquitofish sexes, genotypes, and sizes relative to survival during acute mercury exposure

2.3.4 INFERRINGPOPULATIONEFFECTS FROM

ORGANISMALEFFECTS

Sound inference about population effects is possible based on effects on individuals as has already been discussed in our treatment of autecology The population ecotoxicology section of this book also explores many instances of such reasoning These instances can be rendered as the following general statements:

Population Genetics. (1) Genotypic and phenotypic qualities of individuals sampled from a study population can be used to document population consequences of toxicant exposure (2) Tox-icants can influence the germ line of a population and, in so doing, influence the phenotypes present

in the population (3) Differences in individual genotypes’ fitnesses in critical life stages can be used

to suggest key selection components acted on by toxicants

Population Demographics. (1) Vital rates derived by sampling individuals from populations can be used to document current or to project future conditions of a population These vital rates include rates of mortality, growth, natality, and migration (2) Toxicants that lower an individual’s fitness can influence the demographics of an exposed population

Metapopulation Biology. (1) Differences in vital rates of individuals occupying habitat patches that differ in their capacity to maintain the species can produce differences in metapopulation

6 See Suter (1998) for a general discussion of ecotoxicological extrapolation.

dynamics and persistence (2) Spatial distributions of individuals relative to the distribution of toxicant in habitat patches will influence metapopulation dynamics and vitality

Epidemiology. (1) Disease prevalence, incidence, and distribution in a population can be defined by measuring disease state in sampled individuals (2) Causal knowledge derived from the suborganismal or organismal levels reinforce epidemiological inferences

Life History. (1) In the presence of phenotypic plasticity, an organism will experience a shift

in its life history traits if stressed Ideally, such a trade-off in life history traits will optimize the individual’s Darwinian fitness under the environmental conditions it finds itself (2) Toxicants can change the life history traits of individuals in predictable ways and, in so doing, also influence the population demographics of exposed populations (3) Phenotypic expression by individuals can involve reaction norms or polyphenisms

It is also true that the abundance of individual-based data tempts intelligent and well-intended ecotoxicologists to make flawed inferences about population consequences from individual-based effects data As an important example, Newman and Unger (2003) identify the weakest link incon-gruity: the inappropriate prediction of the most sensitive quality relative to population persistence based on the most sensitive life stage of an individual Often, toxicity testing is done for all life stages of a species and the most sensitive life stage is identified as that stage with the lowest NOEC

or LC50 The incorrect extension of such an approach is to falsely infer from this that the most sensitive life stage relative to individual fitness (i.e., survival, growth, or reproduction) is also the most crucial or sensitive relative to population persistence or vitality Although there are cases in which this approach is adequate, it would be inconsistent with the foundation concepts of popula-tion ecology (Hopkin 1993) to assume that it is always adequate It is demonstrably false in some cases (e.g., Kammenga et al 1996, Petersen and Petersen 1988) Another important example is the assumption that individual-derived effect metrics are accurate, albeit conservative, predictors of concentrations that will adversely affect important population qualities Forbes and Calow (2003) found that this was sometimes the case, but in general, individual-based metrics of adverse effect were not reliable predictors of concentrations adversely impacting populations More information was needed to accurately infer population effects

2.3.5 INFERRINGCOMMUNITYEFFECTS FROM

ORGANISMALEFFECTS

If done cautiously, potentially useful inferences about community consequences can be made from the effects of contaminants on individuals These are detailed in the community ecotoxicology section A quick review of that section reveals that many community metrics are generated with counts of individuals for the community of interest Presence of individuals of key or indicator species is also crucial to many of the community-oriented methods Species-specific sensitivity

of individuals to toxicants can be used to develop biotic indices for implying toxicant effect to communities Colonization or succession theory draws on individual life history qualities for its causal foundation The autecologically oriented laws of Liebig and Shelford are used to describe the transition in community types along environmental gradients Rapoport’s rule relating species richness and latitude (or elevation) is also based on individual species’ tolerances

Ambiguously useful applications of individual-based metrics to predictions of community-level consequences are also present in ecotoxicology A current example is the emerging species sensitivity distribution method The LC50 (or NOEC) values are collected for all relevant species and used to produce a curve that describes the distribution of toxicant sensitivities of the tested species The curve is used to compute the LC50 or NOEC concentration associated with only the lowest 5% (or 10%) of species Only the most sensitive 5% of test species would have an LC50 (or NOEC)

at or below that concentration This HCp is then used to imply a concentration below which all but a small percentage of species in a community will be protected from the adverse effects of the