Sampling to differentiate between pulse and press perturbations ...

SAMPLING TO DIFFERENTIATE BETWEEN PULSE AND PRESS PERTURBATIONS T. M. GLASBY* and A. J. UNDERWOOD Institute of Marine Ecology, Marine Ecology Laboratories, A l l University of Sydney, N.S.W. 2006, Australia

(Received: July, 1995; revised: October, 1995)

Abstract. There is great inconsistency in the use of the terms 'pulse' and 'press' when describing types of perturbations. This is due primarily to a failure to distinguish between the cause and the effect of the perturbation in question. The cause and effect may be either short- or long-term and clearly one may be short-term and the other long-term. Distinction between these two types of disturbance is crucial for management to prevent further impact. Thus, it is important to describe separately these two aspects of a perturbation. Here, we define a protocol for sampling perturbations which enables the cause and effect to be distinguished between short- or long-term. Existing (i.e., already established) assemblages and newly-established assemblages are sampled and compared among disturbed and control locations. Existing assemblages may have been affected by past (pulse) disturbances and/or ongoing (press) disturbances, whereas the establishment of new assemblages can only be influenced by ongoing disturbances. We describe the procedures for assessing impacts of estuarine marinas as an illustration of the issues to be considered in any habitat. Settlement plates and defaunated sediment are suggested for sampling the establishment of new assemblages in aquatic environments.

1. Introduction and Terminologies Disturbance has long been recognised as important in structuring aquatic and terrestrial assemblages (Watt, 1947; Dayton, 1971; Connell, 1978; Thistle, 1981; Barrett and Rosenberg 1981; Sousa, 1984; Pickett and White 1985; McGuinness, 1987; D e t e n b e c k et al., 1992). As a result, there is a large volume o f literature on disturbance theory (e.g. Sutherland, 1981; Bender et al., 1984; Gerritsen and Patten 1985; Petraitis et al., 1989; Pickett et al., 1989; Underwood, 1989; Lake, 1990). T h e r e is, however, relatively little consistency in the terminology used by different authors and consequently a plethora o f definitions has emerged. Two terms that have been used extensively and in many different ways in recent literature are 'pulse' and 'press'. These terms were originally used by B e n d e r et al. (1984) to distinguish between two types of experimental perturbation in analyses o f responses o f populations to disturbance. A pulse perturbation was defined to be short-term and causing a sudden change in numbers o f species from which the assemblage recovers once the disturbance has ceased. A press perturbation was defined as a continuous disturbance causing the abundance or density o f species to be permanently changed. These types of perturbations are analogous to 'Type II' and 'Type III' perturbations, respectively, as defined by Suthefland (1981). T h e difference is that Bender et al. (1984) did not differentiate between the cause and * To whom corrspondence should be addressed Environmental Monitoring and Assessment 42: 241-252, 1996. (~ 1996 Kluwer Academic Publishers. Printed in the Netherlands.

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the effect of the perturbation, while Sutherland (1981) only referred to the effect (which he considered to be related to the population's 'stability'). As noted by Rykiel (1985) and Gerritsen and Patten (1985), in order to avoid ambiguity, it is necessary to distinguish between cause and effect when describing perturbations. This may explain why the terminology developed by Bender et al. (1984) has been used in so many different ways by various authors. At this point, it is necessary to define some terms to avoid confusion. We shall use the word p e r t u r b a t i o n to describe the combination of cause and effect, as used by Bender et al. (1984). Thus, a perturbation is a process whereby a disturbance of sufficient magnitude elicits a response in terms of altered densities or composition of species in a population or assemblage. In order for a perturbation to occur, a disturbance must be large enough to overcome the inertia (sensu Underwood, 1989) of at least one of the populations in an assemblage. Disturbance will therefore be restricted in use to describe the cause of a perturbation. The effect (or impact or stress; Selye 1973) will be referred to as the response by species to a disturbance. Using this terminology, the two types of perturbation described by Bender et al. (1984) can be partitioned into five categories of disturbance and response (Table I). The first of these is not a perturbation as defined here, because there is, in fact, no response. The population or assemblage being disturbed is inert to the disturbance. This is what Sutherland (1981) called a 'Type I perturbation' and will not be considered further. The names we have used for the four types of perturbation (Table I) are necessary only to make discussion of this topic easier. We do not necessarily recommend their use in the future. Rather, we suggest that perturbations be described simply in terms of cause and effect. When classifying perturbations, it is important to consider the time-scale of the disturbance and the response in relation to longevity or turn-over of the populations (Frank, 1968; Auerbach, 1981; Connell and Sousa, 1983). These parameters must be clearly stated when using the terms pulse and press. For example, a disturbance lasting 50 years would be deemed to be a pulse for organisms living 100s of years, but would be considered a press for organisms with generation times of only a few years. For these reasons, the terms pulse and press are probably most useful for describing anthropogenic perturbations or experimental manipulations for which time-scales often can be easily defined. In the following discussion, the words 'continuous' and 'permanent' are used only in the context of some hypothetical, predefined time-scale and do not necessarily denote an everlasting effect of a disturbance.

2. The Problem

The division of perturbations into only two types, either pulse and press (Bender et al., 1984) or Types II and III (Sutherland, 1981) assumes too much about the possible responses to disturbance. As noted by Underwood (1993, 1994), a

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Table I Classifying perturbations according to the cause (type of disturbance) and effect (response by the system) Type of disturbance Cause

Response by by system Effect

Perturbation name Comment

Pulse or press none

Not a perturbation because no effect. Sutherland'sa Type I

Short-term (pulse)

Short-term (pulse)

DiscretePulse

Equivalent to Bender et al.'sb pulse and Sutherland's Type II

Short-term (pulse)

Continued (press)

ProtractedPulse

Equivalent to Sutherland's Type III

Continuous (press)

Continued (press)

ProtractedPress

Equivalent to Bender et al.'s press and Sutherland's Type III

Continuous (press)

Short-term (pulse)

DiscretePress

Uncommon; equivalent to Sutherland's Type II

a Sutherland (1981); b Bender et al. (1984).

population or assemblage subjected to a short-term (pulse) disturbance may show either a short-term, or a continuous (press) response. For example, a short-term spill of toxic chemical (a pulse disturbance) may have a short-term influence on numbers in a population. Many individuals are killed, but the chemical is then denatured, diluted to be no longer toxic, or cleaned up. The population recovers. So there was a discrete response (a pulse occurred in the abundances). This is Bender et al.'s (1984) pulse disturbance and Suthedand's (1981) Type II disturbance. We have termed this a discrete pulse perturbation (Table I). In contrast, the pollutant may have exactly this effect but the relatively sudden removal of large numbers of a species results in space and other resources being freed. Other species can colonize, pre-empt the resources and prevent the recovery of the original population, causing a permanent reduction in numbers even though the disturbance has gone. This sort of 'press response' to a 'pulse disturbance' can occur by a demographic change within a population. If, for example, the original population consists of sparse old trees, a short-term disturbance may kill them, allowing rapid growth of numerous saplings released from shading by the previous canopy (Runkle, 1985). The response to a pulse disturbance would then be a permanent increase in abundance of trees within the area. To distinguish this from the previous example, we have termed this a protracted pulse perturbation (Table I). Kay and Keough (1981) also provided an example that supports this scenario. They created patches of bare space on pier pilings and found that vegetative growth of surrounding epifauna accounted for most of the recolonization of the bare sub-

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stratum. Thus, species that are able to survive a pulse disturbance (such as an oil spill) may have the opportunity to colonize patches created in the disturbed area, thereby excluding previously abundant species. As a result the composition of the assemblage in the disturbed area may be permanently changed from what it was before the disturbance (because the densities of several component species were permanently reduced). Protracted pulse perturbations are also likely in soft sediment habitats where facilitation is commonly the primary mechanism of succession (Lake, 1990). In these habitats, the presence of individuals of some species have been found to influence the development of macrobenthic assemblages within a patch (Gallagher et al., 1983; Ambrose, 1984; Probert, 1984). In areas where these species were removed (due, for example, to a pulse disturbance) the assemblages that develop may be very different from those in areas where the species are present. Perhaps a more commonly described perturbation is what Bender et al. (1984) termed a press perturbation and what we refer to here as a protracted press (Table I). That is, a perturbation that is caused by a continuous disturbance and results in a continued response by an assemblage. An example of this would be the introduction of an exotic species that causes the abundance of a native species to be reduced. The exotic species may out-compete or consume the natives, thereby permanently decreasing their abundance. Clearly, however, there may be another response by the native species. Their abundance may be only temporarily reduced after the introduction of the exotic species. The native population may then recover as the individuals adjust to the presence of the exotic species. That is, the population may recover (i.e. a pulse response), despite the persistence of the disturbance (i.e. a press disturbance). This is an example of a discrete press perturbation (Table I). This type of perturbation is probably uncommon and may be difficult to identify. It is important to note that any disturbance may elicit either a pulse or press response by an assemblage depending on the conditions within the component populations at the time. That is, the response may depend on the current condition of a population or synergisms or antagonisms that may occur at the time of the disturbance. Thus, a specific disturbance may not always have the same effect on a particular species.

3. How to Sample Environmental Perturbations A common procedure for investigating environmental impacts is to compare assemblages in the putatively impacted area to those in a similar undisturbed area (Green, 1979). It is increasingly recognised that sampling must be done at several, ideally randomly-chosen, times (Bernstein and Zalinski 1983; Stewart-Oaten et al., 1986). Any comparison with only one control location is, however, spatially confounded (Underwood, 1989; Lincoln Smith, 199 l). To avoid this, multiple control locations should be used so that the abundance of organisms in the disturbed location is compared to that in a set of control locations (Underwood, 1992). Moreover, the

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locations must be sampled repeatedly to ensure that possible time x space interactions can be properly interpreted (Underwood, 1992). It is also necessary to have adequate spatial replication within (as well as among) locations to ensure reliable estimates are obtained (Underwood, 1981, 1993; Hurlbert, 1984; Andrew and Mapstone, 1987; Morrisey and Underwood, 1992). With this design, it can be argued more coherently that the disturbance has had an impact if either the composition or developmental trajectories of assemblages in the disturbed location are different from or outside the range of those in the control locations. Alternatively, if it is known that a disturbance is going to take place (for example, the construction of a marina), then the location to be disturbed and multiple controls can be sampled repeatedly before, during and after the disturbance using the 'beyond BACI' designs proposed by Underwood (1991, 1992, 1993, 1994). Often, the purpose of detecting or measuring the magnitude of an impact is to identify its cause so that effects (either current or future) can be mitigated or removed by preventing or managing the disturbance. Clearly, this will only be practicable where the cause of the impact is properly understood. Frequently, to detect environmental perturbations, existing assemblages are sampled in the putatively impacted area and in controls (e.g., Lenihan et al., 1990; Schroeter et al,. 1993; Osenberg et al., 1994). So, whatever is currently living in the study sites is sampled. This cannot, however, provide information about the type of disturbance that has occurred. For example, to determine whether an established marina is having a significant impact on abundances of populations or structure of assemblages in surrounding soft sediments, replicate samples may be taken at random intervals from around the marina and in nearby control areas in similar habitats. If differences are found among the marina and control samples, they could be due either to a current press perturbation (caused, for example, by the operation of the marina) or to a protracted pulse perturbation (originally caused by disturbance during the construction of the marina). The latter may have occurred many years previously. Similarly, to investigate whether a sewage outfall is currently having an effect on concentrations of pollutants in surrounding sediments, it may not be sufficient simply to measure concentrations around the outfall and at nearby controls. If, in the past, the discharge of the outfall had been different from the current effluent (because of changed practices), then the concentrations of pollutants in the sediment may be due either to past or present discharges. Sampling must therefore be designed to unconfound the history of a place from the comparison of locations under current conditions. In the case of a sewage outfall, this may be done by introducing clean sediments around the outfaU and control locations. Then only the current discharge of the outfall can have any influence on the concentration of pollutants in these new sediments. So sampling new sediments at random intervals will reveal any current impact. When measuring the abundance of organisms (such as in the marina example), the only way to differentiate between a protracted pulse and some form of press perturbation (without manipulative experiments) is to sample repeatedly the estab-

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lishment of new assemblages at the putatively impacted location and at multiple controls. There are two general ways of studying the establishment of assemblages. One is to remove existing organisms to create patches (and also create a large buffer zone to ensure that surrounding organisms do not have any influence on the establishment of new assemblages by colonizing patches by vegetative growth). The other is by introducing new surfaces for organisms to recruit onto (e.g., settlement plates or defaunated sediment). The composition of new assemblages at the impacted location can then be compared with the composition of the new assemblages at the controls. It should be noted that the new assemblages need not develop to be similar to the existing assemblages at any location (although ideally they should be left for as long as practicable). What matters is whether new assemblages developing at the impacted location are similar to or different from those developing at the control locations. If the latter occurs, then it is important to determine whether these differences are equivalent to the differences among existing assemblages. The possible outcomes involving these comparisons are described here for a marina, but are, of course, applicable to a number of other situations, such as a sewage outfall. To determine whether a marina is having an impact on soft sediment infauna, replicate trays of defaunated sediment could be set up at the marina and at multiple controls and sampled many times. If the new assemblages that develop at the marina are consistently different from new assemblages at the controls and if a comparable difference consistently occurs between the existing marina assemblages and the existing control assemblages, then a protracted press perturbation would be implicated (Figure la). That is, the disturbance is still operating and affecting new assemblages. In contrast, if existing assemblages differ and the new assemblages that develop at the marina are similar to the assemblages that develop at the controls, then it could be concluded that a protracted pulse perturbation caused the existing assemblages at the marina to be different from controls (Figure lb). That is, the disturbance at the marina is no longer operating, but its effects on existing assemblages are still evident. Existing assemblages at the marina will, therefore, continually be different from those at the controls. If the new assemblages that develop are consistently similar at the marina and at the controls, then this means that there is no impact at the time of sampling. If there are also no differences between existing assemblages at the marina and the controls, then clearly it must be concluded that currently the marina is having no impact. So assuming that in the past, existing assemblages had been found to differ, then a discrete pulse perturbation must have occurred at the marina and the assemblages have subsequently recovered (Figure lc). Estimating how long assemblages may take to recover from a disturbance will always be difficult, although various methods have been suggested (e.g., Sutherland, 1981; Yodzis, 1988). The fourth possible perturbation is a discrete press. As previously mentioned, this may be an uncommon perturbation which will probably only involve species

SAMPLING PULSE AND PRESS PERTURBATIONS a.

Protracted press

: ~

w

J

m

I,,=,

~ t i n g

existingimpacted

247

c. Discrete pulse

impacted ,~ new control

control

> (D o3 cO

d. Discrete press

b. Protracted pulse

I1) rr

Time,. Figure 1. Comparison of the effects of a) protracted press, b) protracted pulse, c) discrete pulse and

d) discrete press perturbations. Different lines represent patterns in abundances of species or structure of existing assemblages at the impacted location (thick black line) and a control (thick speckled line) as well as the establishment of new assemblages at the impacted location (thin black line) and a control (thin speckled line). For simplification, only one control is shown. The solid bar on the x-axis identifies the duration of a disturbance - the vertical dotted lines delineate the start and finish of the disturbance.

that can adjust to new conditions. It is, therefore, difficult to conceive how newly developing assemblages may respond to a discrete press. Nevertheless, a graphical illustration of what would theoretically occur has been included (Figure ld). In this situation, the existing assemblages at the marina and control locations must have been found to differ in the past, but are currently not different (because the assemblages at the marina have somehow adjusted to any disturbance). New assemblages at the marina would therefore start to develop differently from those at the controls, but would later converge (Figure ld). Another possible explanation for such an outcome would be that some new disturbance began during the time that the new assemblages developed and this would have to be investigated. Practically, it may not always be necessary to distinguish between a discrete press and a discrete pulse as they are, for all intents and purposes, the same. That is, in both situations the system recovers and so it may not matter whether or not the disturbance is still operating. We maintain, however, that a protracted pulse

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(that is a situation in which a system is briefly disturbed, but the effects of this are permanent) must be considered when sampling. Thus, by using the two sampling methods it is possible to differentiate between various types of perturbation. It should be noted, however, that in some circumstances, the interpretation of the results may not be straightforward. In the situation described for a protracted press, new assemblages at the marina developed to be different from new assemblages at the controls. In most situations, this would imply that some ongoing disturbance is operating. It is, however, also possible that a protracted pulse has led to certain species being abundant at the marina and that these now continue to recruit in large numbers into the new sediment. Thus, the original disturbance has gone, but its effect is perpetuated. This would only be important to consider when dealing with species that may recruit locally (and therefore directly into the new sediments). It would not be an issue if all the species had an extended dispersive phase in their life-cycles. The only difficulty with sampling the establishment of assemblages is that sufficient time must be available for new assemblages to develop. With regards to environmental monitoring, however, it may be rare that sufficient time is ever available for designing, let alone conducting suitable monitoring programmes (Hollick, 1986; Fairweather and Lincoln Smith 1992). Information about the type of perturbation that has led to differences between impacted and control locations is useful for two reasons. First, for designing experiments to determine the exact cause(s) of the differences. Knowing the type of perturbation gives vital information about its cause and thus makes it easier to decide upon the sorts of studies necessary to understand the disturbance. Second, for managerial purposes. To continue with the marina example, if a protracted pulse has caused assemblages at a marina to differ markedly from those at controls, changing the methods of construction of future marinas may prevent future disturbances. If, in contrast, a protracted press perturbation was implicated, aspects of the operation of the established marina will need to be studied so that the cause of the response can be removed. Ideally, to investigate the exact causes of disturbances (for example, to differentiate between a protracted press disturbance caused by the operation of a marina or some other ongoing process), assemblages should be studied using the beyond BACI designs mentioned earlier. In this way, it should be possible to differentiate between different pulse disturbances or between different press disturbances because assemblages would be studied before, during and after a disturbance. Such sampling opportunities are, however, rare. Therefore, studying the establishment of assemblages and doing manipulative experiments will often be the best way to identify the nature of a problem and its solution, Even if sufficient time is available, in certain habitats, it may not always be possible to examine the establishment of new assemblages. For example, in terrestrial habitats, it may be impossible to remove existing assemblages of small mammals or reptiles and then monitor the development of new assemblages. Even

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in some marine habitats such as the plankton, these methods would also commonly be impractical. An alternative, therefore, may be to do a manipulative experiment, such as translocating assemblages from undisturbed to disturbed areas and comparing these over time to appropriate controls. There are, however, many marine examples where existing assemblages have been removed and the establishment of new ones monitored (soft sediment infauna: Grassle and Grassle 1974; DeWitt and Levinton 1985; Miller, 1993; epibiota: Murray and Littler 1978; Kay and Keough 1981; Fairweather, 1990). These methods would presumably also be applicable to, or could be developed for, numerous other habitats. Settlement plates may also be used to study the establishment of epifaunal or fouling assemblages (e.g. Osman, 1977; Sutherland and Karlson 1977; Russ, 1980; Butler, 1991). Thus, if it is possible and whenever it is useful to differentiate between a protracted pulse and a protracted press perturbation, the establishment of new assemblages should be studied.

4. Conclusions In order to prevent or ameliorate the effects of anthropogenic disturbances, or to rehabilitate disturbed areas, it is necessary to be able to distinguish between cause and effect when describing environmental perturbations. As defined by Bender et ai. (1984) and commonly used by others, the terms pulse and press combine and confuse cause and effect and therefore can only be used to describe two specific types of perturbation. Clearly, it is necessary also to consider perturbations caused by a pulse disturbance and resulting in a press effect on a system (what we have termed a 'protracted pulse perturbation'). Similarly, a press disturbance to which a system has a pulse response (i.e. a 'discrete press') is also theoretically possible, although may not be common. Given the various types of perturbation that are possible, if it is necessary to distinguish between them, sampling must be designed to do so. Detection of a protracted pulse perturbation requires repeated assessment of the establishment of new assemblages at the putatively impacted location and at multiple controls. The frequency and period of sampling need to be planned carefully with respect to the type of disturbance and the life-histories of relevant organisms. The majority of environmental studies to date have sampled existing assemblages only and consequently could not distinguish between various types of perturbation. It will not always be possible to plan or complete the appropriate experimentation necessary to distinguish different reasons for impacted locations appearing different from controls. Good judgement, well-developed understanding of the relevant natural history and experience in the pitfalls of quantified experimentation remain crucial. Nevertheless, in many cases, it will be important to distinguish between historical and ongoing causes of impacts before appropriate managerial control will be guaranteed to help solve the defined problem. The kind of sampling

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protocol described here will provide information about the magnitude and type o f disturbance that has occurred and the responses to it in the perturbed system.

Acknowledgements The preparation o f this paper was assisted by an Australian Postgraduate Award (to T M G ) and grants from the Australian Research Council (to AJU) and by funds from the Institute o f Marine E c o l o g y and School o f Biological Sciences o f the University o f Sydney. We thank Drs A.J. Butler, C.J. Glasby, M. Haddon, G.A. Skilleter and T. Crowe, T. Minchinton and M. Schreider for useful discussion and an a n o n y m o u s referee for useful comments. V. Mathews helped with the figure.

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