Beaver-Created Habitat Heterogeneity InfluencesAquatic Invertebrate Assemblages in Boreal Canada
Glynnis A. Hood & David G. Larson
Received: 6 November 2012 /Accepted: 26 August 2013# Society of Wetland Scientists 2013
Abstract Habitat heterogeneity and wetland area play impor-tant roles in aquatic biodiversity; however, other factors areequally important in the composition and distribution of eco-logical communities. Over a 3-year period, including a year ofdrought, we demonstrate how beavers physically altered iso-lated shallow-water wetlands in Miquelon Lake ProvincialPark, Canada, which then influenced aquatic invertebratesdiversity and abundance of functional feeding groups andtaxa. Digging channels by beavers extended aquatic habitatsover 200 m into the upland zone and created unique aquatichabitats, which became hot-spots for predaceous aquatic in-vertebrates. Some taxa (e.g., Gerridae and Gyrinidae) werefound exclusively in beaver ponds, while Culicidae wereprimarily in wetlands without beavers. Amphipoda werestrongly associated with beaver ponds in drought and post-drought years. During extreme drought in 2009, species rich-ness, diversity and abundance declined dramatically, but re-covered quickly in 2010. Although species richness was as-sociated with wetland area, increased niche availabilitythrough active maintenance of wetlands by beavers playedan important role in aquatic invertebrate diversity and distri-bution. Understanding the role of common, but seldom sur-veyed within-wetland habitats in boreal wetlands expands ourability to understand aquatic biodiversity, the importance ofhabitat heterogeneity and the role of other taxa in speciesassemblages.
Keywords Aquatic invertebrates . Beaver . Biodiversity .
Boreal wetlands .Castor canadensis . Drought . MiquelonLake Provincial Park
In its early form, niche theory (Hutchinson 1957) suggestedthat habitat heterogeneity and consequently, niche partitioning,are main factors influencing ecological communities, and thatniche availability and species diversity are positively associat-ed. More recently, however, the concept of ecological nicheconstruction (Odling-Smee et al. 1996) has been used to de-scribe how organisms can create physical changes in the envi-ronment through ecosystem engineering (Jones et al. 1994;Wright and Jones 2006), which in turn creates new habitatsfor other species. This alteration of habitats allows taxa withvery little niche overlap to interact indirectly through habitatmodifications by one or both taxa (Stachowicz 2001).
However, a wide range of abiotic and biotic factorsalso characterize wetland habitats and can result in eachwetland developing a distinct aquatic invertebrate com-munity (Jeffries 2011) dominated by specific functional feed-ing groups (Cummins 1973; Merritt et al. 2008). Key factorsare directly and indirectly hydrological in nature and areassociated with hydroperiod (Jeffries 1994; de Szalay andResh 1996; Gathman and Burton 2011), water chemistry(Angradi and Jicha 2010) and vegetation communities (deSzalay and Resh 1996; de Szalay and Cassidy 2001; Kratzerand Batzer 2007). Surrounding landscape (Batzer et al. 2004),specific structure and composition of aquatic vegetation(Christensen and Crumpton 2010; Gathman and Burton2011), and the presence of woody debris (Braccia andBatzer 2001) are additional influences. Under particular con-ditions, these environmental factors can shape the structure ofaquatic invertebrate communities (Van de Meutter et al. 2007)
Electronic supplementary material The online version of this article(doi:10.1007/s13157-013-0476-z) contains supplementary material,which is available to authorized users.
G. A. Hood (*) :D. G. LarsonDepartment of Science, Augustana Faculty, University of Alberta,4901-46 Avenue, Camrose, AB T4V 2R3, Canadae-mail: email@example.com
and the relative abundance of functional feeding groups(Hawkins and Sedell 1981). For example, such factors mightaffect nutrient and energy cycling of food web dynamics(Hann 1995) through an increase in habitat structural com-plexity (Van de Meutter et al. 2008).
While environmental factors can directly affect aquaticinvertebrate diversity and abundance, fluctuations in naturalabiotic and biotic factors often produce weak (less than 20 %of the variation), or even no response (Batzer et al. 2004).Resistance to variable habitat condition has been attributed todominance of habitat generalists in these communities (Batzeret al. 2004). Evolutionary adaptations in these taxa such asresistance to desiccation (Batzer and Wissinger 1996) andpassive or active dispersal (Van de Meutter et al. 2007) aidin survival during unpredictable fluctuation in wetland habi-tats (Bazzanti et al. 2010). Factors such as immigration, hab-itat suitability and reproductive rates also influence stability ofsuch ecological communities (Kadman and Allouche 2007).
As ecosystem engineers, species such as beaver (Castorcanadensis Kuhl) can also alter aquatic invertebrate commu-nities (McDowell and Naiman 1986; France 1997). By creat-ing and maintaining wetlands, beavers can impact temporaland spatial dynamics of pond hydrology (Westbrook et al.2006; Hood and Bayley 2008) and composition and extent ofriparian vegetation communities (Hood and Bayley 2009). InOntario, Canada, France (1997) determined that addition ofcoarse woody debris and sediment by beavers increased spe-cies richness and abundance of some taxa of benthic inverte-brates adjacent to beaver lodges. Likewise, McDowell andNaiman (1986) determined that the shift of lotic ecosystems tolentic environments by beaver impoundments changed thecomposition of functional feeding groups from collectorsand predators to shredders and scrapers. However, little isknown about aquatic invertebrate responses to physical alter-ations of wetlands through digging of channels and alterationof pond bottoms by beavers.
Because changes to pond structure, vegetation architecture,and hydrology are known to affect aquatic invertebrate diver-sity (McDowell and Naiman 1986; Heino 2000; Hornung andFoote 2006), activities of beavers in a wetland likely affecttaxa richness and abundance at the microhabitat scale. Borealwetlands of Miquelon Lake Provincial Park (MLPP) at thesouthern extent of the Cooking Lake Moraine (CLM) in east-central Alberta, Canada provide a unique site for examiningrelationships between aquatic invertebrates and beavers be-cause of the lack of lotic systems in the park and the isolatednature of the wetlands. MLPP also has a well-establishedbeaver population (Bromley and Hood 2013), which hasextensively modified the landscape through the digging ofchannels, cutting of trees, and increasing the depth and extentof wetlands.
The purpose of our study is to examine how the modifica-tion of open-water wetlands by beavers (as an ecosystem
engineer) influences the diversity, composition and within-pond distribution of aquatic invertebrates. We also investigatewhether abiotic factors (i.e., wetland area, depth, precipitationand water chemistry) and biotic factors (i.e., habitat type andpresence or absence of beavers) play a role in aquatic inver-tebrate assemblages. Specifically, our study tests the followinghypotheses: (H1) that there will be habitat-specific speciesassemblages and functional feeding groups that are specificto wetlands with beavers, (H2) that aquatic invertebrate rich-ness and abundance will be higher in wetlands with beaversthan in those without beavers present, and (H3) that beaver-created features such as channels will provide more heteroge-neous habitat and consequently, higher diversity and abun-dance of aquatic invertebrates than wetlands lacking beavers.
Miquelon Lake Provincial Park (MLPP; 5321N 112 55W)is at the southern extent of the Cooking Lake Moraine in east-central Alberta, Canada (Fig. 1). The CLM is an isolated pocketof the dry mixed-wood boreal region, surrounded by the AspenParkland ecotone (Achuff 1994). MLPP (~13 km2) consists ofknob and kettle topography, which is characterized by isolatedshallow open-water wetlands. With an average total annualprecipitation of 457 mm (Environment Canada, http://www.weatheroffice.gc.ca), the park has a continental climate that ischaracterized by warm summers and cold winters. Total annualprecipitation was measured using the hydrologic year (1November to 31 October). Total precipitation differed amongthe three survey years. In the 2007 to 2008 (2008) hydrologicyear, total precipitation was 327.9 mm. The 2008 to 2009(2009) year had total precipitation of 224.7 mm, and the 2009to 2010 (2010) year had total precipitation of 516.6 mm. Lowerprecipitation in 2009 resulted from a drought from July 2008through 2009.
Common emergent vegetation in MLPP consists of cattail(Typha latifolia L.), celery-leaved buttercup (Ranunculussceleratus L.), sedge (Carex spp.), creeping spike-rush(Eleocharis palustris L.), common great bulrush (Scirpuslacustris L.) and bulrush (Scirpus spp.). Apart from commonduckweed (Lemnaminor L.), star duckweed (L . trisulca L.) andpondweed (Potamogeton spp.), floating-leaved plants are un-common. Hornwort (Ceratophyllum demersum L.) and spikedwater milfoil (Myriophyllum spicatum L. var. exalbescens) aredominant submergent species.
Although game fish were present historically in the park,only brook stickleback (Culaea inconstans Kirtland) is stillfound inMiquelon Lake (Mitchell 1990). No fish were presentin the wetlands used in this study, as determined by minnow
trapping, over 5 years of aquatic surveys and visual observa-tions for other ongoing research projects.
Despite their extirpation frommuch of the CLM during the1800s (Hood and Bayley 2008), beaver populations havesince recovered and almost all wetlands in the park havecurrent or historical use by beaver (Bromley and Hood2013). In 2008, beaver density in MLPP was four activelodges per km2 (Bromley and Hood 2013), but numbers havefluctuated since the 2009 drought. Trapping is limited tomanagement purposes.
Streams in the park are rare and those that exist are inter-mittent; however, beaver channels play a similar role byconnecting many wetlands. Unlike more lotic systems, bea-vers in MLPP do not build extensive dams to create wetlandhabitats, although small dams do exist at some sites. Instead,beavers appear to expand territory through extensive channelsystems dug perpendicularly from the wetland edge to accessadjacent wetlands or upland foraging areas. As well as beingquite long (~100 to 200 m), these channels are often over70 cm deep. Because beavers use channels to transport woodymaterial back to their lodge, channel walls are quite steep andlack the gradual shorelines of other littoral habitats in thewetland.
We selected eight wetlands with active beaver colonies(active sites) and eight wetlands without recent beavercolonies (inactive sites).Wetlands were roughly comparablein size. Beaver activity was determined through winter lodgesurveys using methods outlined in Hood and Bayley (2008)
and Bromley and Hood (2013). We originally chose wetlandsthat consistently either had beavers present (active) or lackedbeavers (inactive) since January 2008. We maintained thesame selection criteria when we were required to choose re-placement sites due to drought. Sixteen wetlands were sur-veyed in 2008, 14 in 2009 (three wetlands were not surveyeddue to low water levels and one new wetland was added), and15 in 2010 (two were still dry and the one added in 2009 wasretained).
Pond Metrics and Water Chemistry Parameters
To obtain wetland areas (ha) for the original 16 study wetlandsselected in 2008, we used an orthophoto with a 0.25 m reso-lution in an ArcMap 9.3 Geographic Information System(ESRI 2006). We could not obtain comparable images toconduct the same analyses for wetland areas in 2009 and2010. In 2008, we also measured water depths (cm) with afolding ruler and hand-held sonar (calibrated to the ruler)along a 10 m10 m grid across the entire wetland. Waterdepths in beaver channels, which are maintained by beaver inactive sites, were also measured. We were confident that ourmeasurements were accurate to within 1 cm. Finally, weanalyzed pond characteristics such as depth and area to deter-mine differences due to beaver activities.
When collecting aquatic invertebrates, we also measuredvarious environmental parameters including air and water sur-face temperature (C), pH ( Hanna Instruments HI 98128),
Fig. 1 Miquelon Lake ProvincialPark in Alberta, Canada
electrical conductivity (EC; Eutech Instrument ECTestr11),dissolved oxygen (LaMotte Dissolved Oxygen 5860 Kit), andwater clarity (LaMotte Secchi Disk 0171-CL). These data wereused in analyses to compare environmental conditions amongyears and between active and inactive ponds.
Aquatic Invertebrate Sampling
We sampled aquatic invertebrates over the 3-year period(2008, 2009, and 2010) at the active and inactive sites.Aquatic invertebrate surveys commenced shortly after ice-off each year to ensure a comparable sample of early springaquatic invertebrates. Depending on ice-off dates and snowcover, survey times varied slightly from year to year, but allcommenced by early to mid-May and were completed over a 2to 3-week period before mid-June.
Invertebrate use of within-pond habitats weremeasured withfive samples from vegetated-edge and open-water habitats and,after 2008, in beaver channels as well. At vegetated-edgehabitats (VE) along the waters edge, we took four samples atthe four cardinal directions and then one sample at a randomlyselected location chosen between 0 and 360. In open-waterhabitats (OW) we randomly selected sample sites in the mainbody of each of the 16 wetlands (one sample at the pond centerand four samples at each of the four cardinal directions). Duringthese surveys, we noticed that beaver channels (BC) appearedto be distinct from the vegetated-edge or open-water habitats.We did not sample these areas in 2008, but in 2009 and 2010,we added five samples from beaver channels in the active andinactive wetlands. This addition produced a total of 15 samplesper pond from all habitats, except in 2009, when we could nottake the full complement of samples in channel and open-waterhabitats due to drought effects on water levels. Accordingly, forboth 2009 and 2010 we sampled a total of six habitat combi-nations: 1) vegetated edge in active wetlands, 2) vegetated edgein inactive wetlands, 3) open water in active wetlands, 4) openwater in inactive wetlands, 5) beaver channel in active wet-lands, and 6) beaver channel in inactive wetlands. Beaverchannels in inactive wetlands were remnants of historic beaveractivity and were not currently maintained by beavers.
All aquatic invertebrate samples were obtained using aD-net (0.07 m2, 500 m mesh). Each sample consisted ofa 1-m sweep that entered the water column, lightly contactedthe surface of the be...