local habitat restoration in streams: constraints on the effectiveness of restoration for stream...

ECOLOGICAL MANAGEMENT & RESTORATION VOL 4 NO 3 DECEMBER 2003 193

P E R S P E C T I V E

Local habitat restoration in streams: Constraints on the effectiveness of restoration for stream biotaN. R. Bond and P. S. Lake

Summary The restoration of physical habitat has emerged as a key activity formanagers charged with reversing the damage done by humans to streams and rivers, andthere has been a great expenditure of time, money and other resources on habitat restorationprojects. Most restoration projects appear to assume that the creation of habitat is the key torestoring the biota (‘the field of dreams hypothesis’). However, in many streams where newhabitat is clearly required if populations and communities are to be restored, there may benumerous other factors that cause the expected link between habitat and biotic restorationto break down. We discuss five issues that are likely to have a direct bearing on the success,or perceived success of local habitat restoration projects in streams: (i) barriers tocolonization, (ii) temporal shifts in habitat use, (iii) introduced species, (iv) long-term andlarge-scale processes, and (v) inappropriate scales of restoration. The purpose of the studywas primarily to alert ecologists and managers involved in stream habitat restoration to thepotential impacts of these issues on restoration success. Furthermore, the study highlightsthe opportunities provided by habitat restoration for learning how the factors we discussaffect populations, communities and ecosystems.

Key words dispersal barriers, field of dreams, habitat structure, introduced species,restoration ecology, scale.

Introduction

here has been a concerted effort inrecent years to begin reversing the

tremendous damage that has been exactedon freshwater ecosystems by human beings.Much of this effort has been targeted atrestoring physical habitat, on the presump-tion that this will engender a desirableecological response (Palmer et al. 1997;Lake 2001b). Coined the ‘field of dreamshypothesis’ (Palmer et al. 1997) (i.e. ‘if youbuild it, they will come’), this approach isimplicitly based on the observation that inmany ecological systems, biotic diversity ispositively correlated with habitat heteroge-neity (Bell et al. 1991; Rosenzweig 1995).

Arguably, because habitat heterogeneityhas been lost in so many aquatic systems asa result of both instream (e.g. desnagging,dredging and straightening) and riparianzone activities (e.g. vegetation removal),there is good reason to believe that a lack ofhabitat is an important aspect of the overalldegradation that has occurred. Conse-quently, if this damage is to be reversed, therepair and creation of depleted habitat willbe a necessary component of restoration.

However, as pointed out in a number ofreviews (Frissell & Nawa 1992; Kauffmanet al. 1997; Roni et al. 2002), local habitatmanipulations may be necessary, but notsufficient, to restore key aspects of eco-system structure and function – a factexemplified by several recent meta-analyses (Smokorowski et al. 1998; Larsonet al. 2001). For example, in a recentreview (Smokorowski et al. 1998) of 78habitat rehabilitation (or enhancement)projects targeting increases in riverine andwetland fish populations, just four (5% ofthe 55 completed projects) were able todemonstrate an increase in fish produc-tion, even though 98% achieved theirhabitat targets. Low success rates in achiev-ing biological targets have also beenreported by Beschta (1994) and Larsonet al. 2001) for fish and invertebrate fauna,respectively.

In addition to the apparently lowsuccess rate, few attempts at habitat resto-ration are properly evaluated to determineeither their success or reasons for theirfailure (Lake 2001a). Among those that areevaluated, few meet the basic design prin-ciples that allow a thorough scientific and

statistical assessment (Chapman & Under-wood 2000; Downes et al. 2002). Othersare evaluated only over short time scalesdue to budgetary constraints (Osborneet al. 1993), and thus cannot accommo-date lag effects in the response of the biota.Hence, very few restoration attemptsactually contribute significantly to ourknowledge about how to go about streamrestoration successfully (Minns et al. 1996;Michener 1997; Smokorowski et al. 1998;Chapman & Underwood 2000). Clearly,this situation must be reversed if restor-ation ecology is to grow as a science (Hobbs& Norton 1996; Lake 2001a).

The conceptual aspects of stream resto-ration (goal setting, monitoring, method-ologies, etc.) have been discussed in theliterature, and several strategies for settingpriorities and planning restorationprograms have been proposed (e.g. Hobbs& Norton 1996; Lake 2001b; Roni et al.2002). There has, however, been muchless consideration of the ecological proc-esses (e.g. dispersal, recruitment, speciesinteractions, etc.) at work as a systemresponds to physical habitat manipulation,and importantly, how these factors might

T

Nick Bond undertook this research in collab-

oration with Professor Sam Lake in the School of

Biological Sciences, Monash University (Clayton,

VIC 3800, Australia. Tel. (+61 3) 9905 5606. Email:

[email protected]) while involved in

a habitat restoration trial jointly funded by the

Natural Heritage Trust (NHT), and the Cooperative

Research Centres for Freshwater Ecology (CRCFE)

and Catchment Hydrology (CRCCH), and the Goul-

burn Broken Catchment Management Authority

(GBCMA).


194 ECOLOGICAL MANAGEMENT & RESTORATION VOL 4 NO 3 DECEMBER 2003

limit the biotic response to any physicalchanges. While there are many suchfactors that could be identified, here wediscuss five broad issues: (i) barriers to dis-persal of biota, (ii) temporal changes inhabitat use, (iii) introduced species, (iv)long-term or large-scale driving processes,and (v) inappropriate scales of restoration(Table 1), that together encompass manyof the more important factors that willinfluence the biotic response following res-toration, potentially causing restoration tofail (or be perceived to fail) in achievingdesired ecological goals.

In applying habitat restoration instreams, it seems that most of these issuesare commonly overlooked; a problem thatwe suspect comes in part from failing toconsider localized habitat restorationwithin a broader spatial and temporalcontext (or scope). We therefore close byhighlighting the potential for insightsgained from landscape ecology, whichexamines regional processes, to providevaluable information for planning andguiding restoration.

While we focus our attention onstreams and rivers, the points made mayapply equally to other ecological systems(both aquatic and terrestrial) in whichhabitat is restored locally in order to seek abiotic response. It should be stressed thatdespite the problems that can beset habitatrestoration, we do not believe that habitatrestoration is unnecessary; this is far fromthe case. What we do suggest is thathabitat modifications will likely meet withhigher success rates, and the causes offailure be more readily identified when thefactors we discuss are considered.

Barriers to colonization

In the absence of active introductions,colonization of restored habitat is entirelyreliant on the dispersal of organisms fromextant populations. Even where local pop-ulations still exist, dispersal can still beimportant. For example, Riley and Fausch(1995) found that 2 years on from habitatrestoration, dispersal, rather than greatersurvival or recruitment, was responsiblefor increases in abundance of three speciesof salmonid, even though each species wasinitially present at restored sites.

Rates of colonization will generallydepend largely on the distance betweenthe restored areas and potential sourcepopulations (Kareiva 1990) as well as thedispersal abilities of the relevant species(Skalski & Gilliam 2000). For manyriverine organisms, dispersal is constrainedto movements along the stream channel,making longitudinal connectivity critical(Wiens 2002), while at the same timedramatically increasing travel distancesfar beyond the overland or straight linedistance between points within a streamnetwork (Fagan 2002). Despite famousexamples of individuals of some taxa dis-persing over huge distances (e.g. taggedGolden Perch Macquaria ambigua(Richardson) have been recaptured thou-sands of kilometers away from theirrelease point; Koehn & O’Connor 1990),it is clear that such long range move-ments are relatively rare, and that individ-uals of most taxa move only shortdistances over their lifetime (Bunn &Hughes 1997; Rodriguez 2002). Thisimmediately constrains the probability

that restored habitat will be colonized,particularly in the short term.

In addition, in many catchments con-nectivity of stream networks has been lostdue to instream barriers, thereby constrain-ing the likelihood of colonization evenfurther (Ward & Stanford 1995; Stanfordet al. 1996). We distinguish here between‘hard’ and ‘soft’ barriers to connectivity.Hard barriers represent actual physicalstructures such as dams and weirs thatblock the channel, thereby making disper-sal physically impossible without humanintervention, particularly in an upstreamdirection (Ward & Stanford 1995). Theinstallation of fish ladders is one way inwhich this type of barrier is being over-come, but this is of little help to most taxa(including some fish) with low mobilities(Harris 1984; Jungwirth 1996). There is theadditional problem that in large reservoirs,the ability of animals to find their way islost in the absence of any directional flow(McCully 1996). Perhaps the best exampleof this comes from rivers such as theColumbia River in North America, wherejuvenile salmon are transported acrosslarge dams each year in the holds of bargesso that they can make their migration tosea (Maule et al. 1988).

Soft barriers are associated with the iso-lation of restored habitat, whether this isthrough the sheer distance from potentialsource populations (e.g. Fuchs & Statzner1990; Lafferty et al. 1999), or becauseintervening stream sections present con-ditions through which organisms areunlikely to disperse, even when withintheir dispersal range. There is, for example,a number of studies demonstrating that the

Table 1. Important ecological questions to consider in the planning and target setting stages of habitat restoration

1. Are there barriers to colonization?• What and where are the source populations?• How can potential barriers be overcome?

2. Do the target species have particular habitat requirements at different life stages?• What are these requirements?• How should these habitats be arranged spatially?

3. Are there introduced species that may benefit disproportionately to native species from habitat restoration?• Can colonization of these organisms be restricted?

4. How are long-term and large-scale phenomena likely to influence the likelihood, or timeframe of responses?• Will these affect the endpoints or just the timeframe of responses?• How will this affect monitoring strategies, and can monitoring strategies be adjusted to deal with this?

5. What size habitat patches must be created for populations, communities and ecosystem functions to be restored?• Is there a minimum area required?• Will the spatial arrangement of habitat affect this (e.g. through the outcomes of competition and predation)?



movements of pool-dwelling fish areenhanced when intervening habitats areshorter (Lonzarich et al. 2000), deeper(Gilliam & Fraser 2001), or have lowervelocities (Schaefer 2001). It is worthnoting that high flows may also play a keyrole in overcoming both hard and softbarriers, both by passively redistributingorganisms and by inundating physical‘hard’ barriers to active movement (Harris1984; Unmack 2001).

The movement of organisms, bothactively and passively, is also influenced bychannel complexity and the distribution ofhabitat patches (Bond et al. 2000; Klinget al. 2000; Gilliam & Fraser 2001). Intheir modelling of how metapopulationsrespond to habitat addition in terrestrialsystems, Huxel and Hastings (1999) foundthat restoring habitat close to remainingsource populations could significantlyreduce the time lag of response whencompared to restoration of habitat patchesat random. This seems intuitive for terres-trial and aquatic systems alike, but field-testing these predictions is yet to be done.

The effects of soft barriers are notrestricted to instream dispersal. Patterns ofterrestrial dispersal by adult insects (mostof whom are weak flyers) will be affectedby patterns of air movement (Baldwinet al. 1975; Downes & Keough 1998). Forthese taxa, instream habitat restorationmay require similar action outside of thechannel if population dynamics are to befully restored.

Temporal changes in habitat use

In general, habitat restoration efforts (par-ticularly, for example, those associatedwith fish) are based largely on the restor-ation of what is believed to be good habitatfor a particular (usually adult) life-stage,without considering the age at whichdemographic processes are most limiting.There are good examples in which lack ofhabitat for a particular life-stage causes sig-nificant population bottlenecks (e.g. Beck1995), but the importance of such habitatsis not always obvious. For example, localabundances of semiaquatic insects maydepend more on factors affecting the

adults, such as the availability of suitableoviposition sites (Peckarsky et al. 2000;Speirs et al. 2000; Reich in press) than onthe specific characteristics of eitherjuvenile or adult habitat, but these issuesare poorly understood. Other taxa undergoshort- and/or long-range movements (e.g.to spawn), or utilize distinct habitats asjuveniles and adults (Kocik & Ferreri1998), in which case spatially isolatedhabitats may require concurrent restor-ation (habitat complementation; Schlosser1995), and be free of the types of barriersdiscussed above.

As well as ontogenic habitat shifts, manyspecies utilize novel habitats (refugia)during periods of environmental stresssuch as floods and droughts (ecologicaldisturbances; Sedell et al. 1990; Schlosser1995). For example in lowland streamsystems that intermittently are dry, organ-isms must disperse into, or local residentssurvive within, the few remaining pools ifpopulations are to persist (Schlosser 1995;Labbe & Fausch 2000; Lake 2003). Thereare strong parallels here with ontogenichabitat shifts (Sedell et al. 1990), butbecause disturbances are often infrequentand unpredictable, these intermittentrefugia may be substantially more difficultto identify, or may have been lost from thesystem altogether because of human activi-ties. Similarly, in streams subjected to largevariations in discharge, identifying, pro-tecting and attempting to restore refugehabitats used by the biota should be animportant consideration in restoration,even where, on average, other habitatmodifications appear successful.

Introduced species

For many of the invasive species in ourwaterways, little is actually known abouthow they interact with native plants andanimals. There are some well-documentedexamples in which exotic species haveresponded rapidly to habitat restoration,out-competing native biota (Zedler 2000;Klotzli & Grootjans 2001). For example,the highly invasive plant Purple Loosestrife(Lythrum salicaria Linnaeus) has come todominate many restored wetlands andstream margins in parts of North America

(Cole 1999). Even in the absence of strongevidence for negative interactions, manyinvasive species (almost by definition)display traits that favour them takingadvantage of newly created habitat (Rej-manek 2000). Thus, the arrival of exoticbefore native species might indicate somedegree of success (D. Crook, pers. comm.,2001) because habitat has clearly beencreated. The challenge then is to deter-mine whether habitat restoration canoccur in such a way as to favour nativeover exotic taxa. One example, comingfrom our own work, is the creation ofscour pools (through the introduction ofcoarse woody debris) in several lowlandstreams from which habitat has been lostfollowing the formation of erosion-inducedsand slugs (Davis & Finlayson 2000).Despite initial disappointment at the smallsize of the pools that were formed, thesehave been colonized by small native fishMountain Galaxias (Galaxias olidusGunther), but are too small to support alarger-bodied exotic invader CommonCarp (Cyprinus carpio Linnaeus), which iscommon within larger pools in unsandedsections of these streams. This examplerepresents simple good luck, but, ingeneral, preventing exotic invasions ofnewly created habitat is a clear challengein stream habitat restoration.

Long-term and large-scale processes

Above all else, it is important to place local-ized habitat restoration within the contextof appropriate scales of space and time. Inspace, most degradation has occurredacross large areas of the landscape, oftenwhole catchments. Yet most efforts athabitat restoration are pitched at muchsmaller scales, typically individual sites orstream reaches. As a consequence, thelegacies of past disturbances and theimpacts of on-going disturbances operatingat larger (possibly catchment-wide) scalescan compromise works done at individualsites or reaches. One example would beefforts to restore habitat in rivers affectedby large dams, where altered flow regimesmay override the benefits of restoringhabitat such as large woody debris. The



central problem is that habitat restorationis, with few exceptions, limited to smallspatial scales because of logistical andresource constraints, and because ecolog-ical values must be balanced with theeconomic and social values derived fromongoing human pursuits. However, ifbroad-scale disturbances that, as historic,ongoing or impending processes, are likelyto influence the outcomes of localizedhabitat-restoration, then this must beaddressed early in the planning phase ofrestoration. Bohn and Kershner (2002)provide an excellent example in whichdeclines in populations of Bull Trout(Salvelinus confluentus (Suckley)) andCutthroat Trout (Oncorhynchus clarkilewisi Suckley) were initially believed toresult from a lack of suitable habitat, butwere ultimately linked to sediment inputsto the stream network, which were occur-ring throughout the catchment. Thus, awatershed analysis directed initial manage-ment actions toward minimizing sedimentinputs from outside of the stream, ratherthan tackling the issue of habitat limitationdirectly, thereby changing the nature andtiming of management actions that wereadopted in addressing observed fishdeclines. Ecological goals and endpointsset within this broader context willperhaps become less ambitious and thenecessary management actions perhapsmore difficult to achieve, but in the longerterm such an approach may be far moreeffective.

As with broad-scale spatial processes,temporal trends in processes such asrainfall can have significant consequencesfor restoration, particularly at the short(< 10 years) time scales over whichoutcomes from restoration are typicallysought. Both floods and droughts affectpopulation processes such as mortality andrecruitment, and thus can slow rates ofpopulation growth and increase the fre-quency of localized extinctions. This,combined with other rate-limiting factors,such as dispersal, will mean that popula-tion, community and ecosystem responsesto the addition of habitat will often takeconsiderable lengths of time (Mitsch &Wilson 1996; Lake 2001b). Of course,these sorts of delays do not cause restor-ation to fail, but instead, may push

response times beyond those over whichmonitoring is typically funded.

In the longer term, phenomena such asglobal warming and species invasions willpotentially cause significant directedchanges in aquatic ecosystems (Sala et al.2000), and this may require the resettingof restoration targets for both communitystructure and ecosystem function. Dealingeffectively with large-scale spatial andtemporal processes in localized restor-ation efforts will require a great deal ofeffort, in the planning phase of restor-ation, to determine realistic goals and real-istic timeframes over which these goalsmight be reached (Hobbs & Norton 1996;Lake 2001a).

Inappropriate scales of restoration

The spatial extent of restoration is rarelyset from the perspective of target speciesor communities, instead being drivenmore often by human perceptions ofimportant scales, or by issues of economicand social convenience. This type ofscoping problem (Schneider 1994) canresult in great mismatches between therequirements of individual animals (interms of foraging behaviour, home rangesize, etc.), and the areas of habitat that areactually created (Minns et al. 1996). Yet formany species the viability of local popula-tions is highly contingent on patch size(Dunham & Rieman 1999; Sand-Jensenet al. 1999). Therefore, even if we restorethe correct types of habitat, doing so at thewrong scale can cause restoration to fail,and at the same time cause potentiallyuseful restoration techniques to be aban-doned. Although there appears to havebeen no direct research into the impor-tance of habitat-patch size in the streamrestoration literature, evidence fromstudies examining habitat fragmentationsuggests this is an important issue toconsider (Dunham & Rieman 1999; Harig& Fausch 2002).

There are some physical properties ofstreams such as water temperature andrates of nutrient uptake for which thesescaling rules are better understood. Forexample, Storey & Cowley (1997) foundthat in three second-order streams in New

Zealand, water temperature began tostabilize only after passing through severalhundred meters of vegetated riparian zone.Nutrient levels required double this dis-tance. Similar predictions come from thetheoretical work of Collier et al. (1998).Unfortunately, however, these scalingissues are generally much less tangiblebecause the spatial scales of environmentalpattern to which different organismsrespond are poorly understood (MacNally1999). Thus, while the question of patch-size can be reconciled for some physicalproperties of streams and for taxa aboutwhich we know sufficient biology (includ-ing home-range size, dispersal ability, etc.),a concerted effort should be made toinclude these questions in future restor-ation experiments.

Another critical question is how theeffects of large-scale processes on ecolog-ical communities (see previous section) aremitigated as the size of the area beingrestored is enlarged. Finally, a number ofauthors (Minns et al. 1996; Bell et al. 1997;Palmer et al. 1997; Collier et al. 1998) havestressed the need to better understand howthe spatial arrangement of restored areaswill affect the response in terms of rates ofrecolonization as well as aspects of eco-system structure and function.

Conclusions

There is no question that the reversal ofhabitat loss must form an important com-ponent of ecological restoration. What isclear though, is that there are a number offactors, other than the presence of habitat,which can influence local populationdynamics, and these may operate at differ-ent scales in space and time. Successfulrestoration will only occur where thesefactors are considered in unison.

At the most basic level, the key issueswe have discussed should be consideredearly in the planning and goal setting stagesof restoration, and framed as simple ques-tions, as in Table 1, and potentially incor-porated into a more holistic frameworksuch as that proposed by Roni et al.(2002). In some cases the answers to thesequestions will highlight importantknowledge gaps. Where this is the case,the necessary information should be



gathered before full-scale restoration begins(e.g. through pilot studies), or the restor-ation and monitoring strategy modified sothat relevant questions can be testedexplicitly. These aspects of planning couldgreatly increase the rate at which we learnhow to best restore.

At a more general level, however, weconcur with the idea that progress inrestoration can be maximized throughlinkages to landscape ecology (Schlosser1991, 1995; Bell et al. 1997). Althoughdescribed in many ways, landscapeecology has two important foci: pattern(e.g., habitat patch structure in the land-scape), and process (e.g. recruitment anddispersal), and how these factors interact(Wiens 2002). The first of these (pattern)has obvious consequences for habitatrestoration because habitat creation repre-sents a direct manipulation of landscapestructure. Process in this case is concernedwith issues such as dispersal, colonizationand local extinction, which drive theresponses to restoration. Thus, there arestrong parallels between the questions ofinterest to landscape and restoration ecolo-gists alike, and many lessons that might belearned from taking a landscape perspec-tive (Bell et al. 1997; Fausch et al. 2002).In return, restoration provides uniqueopportunities to manipulate habitat acrosslarge spatial scales. The similarities andcross-benefits of these two fields are beingincreasingly recognized – for example, inthe shift toward watershed, or wholecatchment, restoration (Roni et al. 2002).It is our hope that as this occurs, there willbe growth in the opportunities andsupport for research and experiments ofthe sort that will aid our understanding andability to deal with some of the limits onhabitat restoration outcomes highlightedhere.

Acknowledgements

This work was funded by an Australiangovernment Natural Heritage Trust (NHT)grant. Thanks to Ralph Mac Nally, NicoleMcCasker, Rhonda Butcher and tworeviewers for informative discussions anduseful comments on drafts of the manu-script.

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local habitat restoration in streams: constraints on the effectiveness of restoration for stream...

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