The environmental impact of crayfish biopedoturbation on a...
Transcript of The environmental impact of crayfish biopedoturbation on a...
Landform Analysis, Vo/. 3: 35-40 (2002)
The environmental impact ofcrayfish biopedoturbation on a floodplain:
Roanoke River, North Carolina Coastal Plain, D.S.A.
David R. Butler
The James and Marilyn Lovell Center forEnvironmental Geography and Hazards ResearchDepartment a/Geography. Southwest Texas State UniversitySan Marcos. TX 78666-4616, U.S.A.e-mail: [email protected]
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AbsfrflCI: A terrestrial crustacean. the crayfish, creates widespread fine-scale landfonns (mounds or "chimneys'")on the floodplain oflhe Roanoke River in eastern Nonh Carolina, U.S.A. These mounds are typically 12 cm highand 8 cm in diameter. and are composed of extremely high ooncenlrations ofday, Non-crayfish-affectcd soils onthe noodplain, regardless of coarser-scale Iandform type, are dominated by sand. illustrating that crayfish an: I
primary mechanism for concentrating clay and creating spatial heterogeneity on the Ooodplain.
Kq words: zoogeomorpbology, biopedoturbation. crayfish, Roanoke River, North Carolina
Introduction
Zoogeomorphology is the study of the role ofanimals as geomorphic agents (Butler, 1995). Recent years have seen a surge of interest in tbe methods by wbich animals geomorpbically influence theirenvironment. Examples include several papers thatexamine tbe gcomorphological role of dam-building and excavations by beavers in orth Americaand Europe (Butler & Malanson, 1995; Bums &McDonnell, 1998; Gumell, 1998; Hillman, 1998;Mcentemeycr el al., 1998; Meentemeyer & Butler,1999); burrowing by ground squirrels (Andersen,1996), arctic foxes (Anthony, 1996), pocket gophers(Inouye el al., 1997), rabbits (pickard, 1999), andvoles (G6mez-Garcia et al., 1999); grazing and digging for food (Boeken et aJ., 1995; MaUick el al.,1997; Manson, 1997; Mauson & Reinhart, 1997;Evans, 1998; Tardiff & Stanford, 1998); tramplingby large berbivores such as bison, cattle, and sheep(Govers & Poesen, 1998; Fritz et al., 1999; Hallet al., 1999); geophagy (soil-eating) (Mahaney etal., 1996a, 1996b); and the effects of fish on sedimentation paltcrns in tropical streams (Flecker,1996).
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In spite of this recent interest in tbe role of largeranimals as geomorphic agents, however, it isnoteworthy that little attention has been paid to tbegeomorphic impact of invertebrates, althoughinvertebrates are among the most widespread andcommon ofaU animals on the planet (Butler, 1995).Exceptions include works that have examined thegeomorphic and hydrologic effects of earthwonns(Haria, 1995), ants (Hululgalle. 1995; Butler, 1996;Green el al., 1999), scorpions (Rutin, 1996, 1998).snails and isopods (Shachak el al., 1995), river crabs(Onda and Itakura, 1997), and salt-water mud shrimp(Ziebis et al., 1996). This paper describes thegeomorphology of crayfish mounds ("chimneys"),and examines the mound particle-size texture,compared to non-crayfisb·afTected soils on portionsof the floodplain oftbe Roanoke River, on the CoastalPlain of eastern North Carolina, USA.
Background
Crayfish are terrestrial crustaceans found in lowlying terrain throughout the GulfCoast region of thesouthem United States, as well as in such diverse
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David R. Butler Crayfish biopedoturbation, flood plain, Roanoke River, u.s.A.
The study area
The lower Roanoke River floodplain is located ineastern NQrth Carolina, on the Coastal Plain physiographic province of the southeastern United States(Fig. 2). The floodplain contains a mosaic ofalluvialbottomland forests that have developed as a consequence ofcQmplex and interrelated gcomorphje, hydrologic and ecological processes. Floodplaintopography is sublle, but small elevational differences can result in major differences in drainage, soils,and vegetation community composition. The annual flooding cycle constitutes the dominant geornorphic process influencing floodplain topography. Geomorphic features of the floodplain include oxbowlakes, natural levees, meander scrolls (ridge and
of crayfish in "mixing the surface of a very poorlydrained soil seems comparable to that of largeearthworms such as Lumbricus terrestris L (aloneor together with burrowing predators)". Theseconclusions about the hydrological effects of crayfishecho those presented by Onda and ltakura (1997)concerning river crabs, and illustrate the profoundeffects thal terrestrial crustaceans can have onlhe geomorphology and hydrology onow-lying floodplains around the world.
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Roanoke River Soil Sample Sites
< Ridge,Levee.,Terrace
D Bluff
Fig. 2. Map of the soil sampling sites, Roaooke River. North Carolina. The location of the BroadneckSwamp intensive study area is labeled
10 a tower more than 30 cm high". The most detaileddescriptions of crayfish mounds were provided byHobbs and Whiteman (1991). They reported densitiesof2,730 mounds per hectare from onc site in easternTexas, with average mound heights of 12 cm anddiameters of28 cm. They reported that >17.000 kgofsoil and 40 kg ofsodium per hectare were broughtto the surface each ycar, such that the entire surface"must have exhibited some stage ofmound construclion or erosion over a period of <3 years" (p. 128).At six other cast Texas study sites described by Hobbsand Wbiteman (1991), the estimated number ofmounds per hectare was 44,124 (averaging 7 cm taUand 15 cm in diameter), 63,505, 31,214, 62,676,61,775 (eQvering 15% of the surface area), and 42,470(with >40,000 kg of soil moved per hectare). ]ndividual mounds examined weighed up to 40 kg, allhoughI I-25 kg was the normal range.
The density of crayfish burrows and tunnels inareas oflhe southern GulfCoast ofeast Texas, westernMississippi, and eastern Alabama is in fact so grealthat crayfish are a primary factor in the hydrology ofthe region. Stone (1993, p. 1,099) stated that, in poorlydrained areas, "[w]hen water tables are near thesurface, however, the network of large-diametercrayfish tunnels [... ] allows lateral flow at ratesunlikely to be equaled by the activity of any otherburrowing organism", and noted that the effectiveness
and chamber develQpment, and spend much time outof lhe burrow (Hobbs, 1981; Hasiotis & Mtt~hell,
1993); and• Tertiary: Crayfish spend most oftheir lives in openwater, burrowing only 10 reproduce or to escapedesiccation (Hobbs, 1981).
The digging ofburrows by crayfish results in theexcavalion and subsequent deposition ofsurface sediment. These deposits typically lake the form ofsteep, conical mounds also known as crayfish "chimneys" or "turrets" (Fig. 1). The chimneys arc surface entry points to a tunnel system frequently over1 m in depth and chiefly 4-8 cm in diameter (Stone,1993). In many eases the mounds, reportedly com~
posed of silt and/or clay, become case-hardened bythe sun SQ that they are an impediment to farm machinery. Burrows may so penneate Ihe subsurfacethat farm machinery and livestock break through thesurface and collapse into the underlying burrow system (Hobbs & \Vhiteman, 1991; Stone, 1993). Thepcdoturbational activities in burrow excavation andmound emplacement also, in some cases, bring deleterious levels of sodium salts to the surface, harming agricultural usage of the area (Hobbs & Whiteman, 1991).
The morphology and density ofernyfish chimneyshave been described in some detail. Hobbs (1884)reponed a typical mound size as ]5 cm wide and 10cm high, and Grow and Merchant (1980, p. 231)reported mounds ranging in height from "a low mound
habitats as the midwestern United States, easternCanada, and Australia (Hobbs. 1981; Butler, 1995).Many crayfish species dig deep vertical and complexshafts leading to long-lasting underground burrows.Crayfish burrowing has occurred as a geomorphic andpedoturbational activity since at least the Triassicperiod (Hasiotis & Mitchell, 1993; Hasiotis et al.,1993).
The significance ofcrayfish as agents of zoogeomorphological landscape modification was first dcscribed in detail by Tarr (1884). His illustrations ofcrayfish mounds and burrows remain the standard,as witnessed by their ulilization in a modified fonnby Grow (1981) in her examination of the burrowingbehavior of crayfish (Figs. 2-4).
Crayfish burrows arc cQmplex and may be as IQngas 4-5 m (HQbbs, 1981; Hasiotis, Mitchell & Dubiel,1993). Hobbs (1981) identified three categories ofcrayfish burrowers and burrows based Qn the amount oftime spent in the burrow, ilS architecture, :lIld its connection to open water:• Primary: Crayfish construct burrows with complex architecture, unattached to open water, andspend the majority of their tife in the burrow. Burrows are vertical, obtain great length, and branchout horizontally below the local water table (Hobbs& White man, 1991; Hasiotis & Mitchell, 1993;Stone, 1993);• Secondary: Crayfish construct burrows that arca"ached to open water and exhibit some branching
Fig. I. Typical crayfish mound on the Roaookc Rivcr floodplam. lens cap is 49 mm in diameter
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David R. Butler Crayfish biopedoturbation, flood plain, Roanoke River, U.S.A.
swale topography), point bars, sloughs (arcas ofstagnant or sluggish water in meander scroll depressions), backswamp deposits, and terraces (Townsend& BUller, 1996).
Metbods
Crayfish construct conspicuous entrance mounds,and these chimneys are particularly ubiquitous inbackswamps of the lower Roanoke River, typicallyassociated with the low-lying swalcs between adjacent"swells" or ridges. High numbers of mounds arelocated in the areas of Broadneck Swamp, and soilswere collected from 30 crayfish mounds within a 10m x 30 m plot in this area. An additional 13 moundswere randomly selected from a variety of additionalfloodplain sites, in order to detenninc the degree ofspatial heterogeneity of crayfish mounds. Moundswere also photographcd in the field, and the diameterand height were determined. An additional 102 soilsamples, taken from four characteristic landformtypes of the floodplain, were also collected forcomparison with crayfish-affected soils. Particle sizcdistribution of mounds and non-mound floodplainsoils was determined using hydromcler analysis.
Results
The morphology ofcrayfish mounds in the studyarea arc broadly similar to thosc reported for crayfish elsewhere in the literature, about 12 cm average height and 8 cm average diameter (N = 30 inboth cases). These mounds are larger in diameterthan those reported in the literature for other burrowing crustaceans. Onda and ltakura (1997) reported that river crab mounds are about 5 cm in diameter(no height data was given); and Ziebis et al. (1996)described the conical mounds produced by mudshrimp as averaging 4 cm in height (9 cm maximum).Although Ziebis et af. did not provide an averagemound diameter. extrapolating from !.heir Fig. 2 suggests an average ofabaut 6-8 cm, flaring to 10 cmat the base where particles collect at the angle ofrepose.
Soils comprising crayfish mounds in the Broadneck Swamp area showed strong within·site similarity (Table 1). The mounds contain extraordinarily highlevels of clay, which when dry, becomes case-hard·ened and extremely difficult to break apart. Al!.houghthe mounds were collected wi!.hin a backswamp areawhere clay concentration would naturally be somewhat high, non-crayfish soil samples from through·out the backswamp laodform (Table I) do not begin
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Table I. Textural Data, Crayfish Mound Soils vs.Non-Mound Soils
s... ,... c.,u.-dr_
1%1 1%' 1%'Crayfish MOIIOds,Broadntck SwamP
(N-30) 4.8 (2.5)- 4.601) 90.6 (5.1)
Cl'1vflSb Mounds. Olhcr Sila(N- 13) 31.4 (18.4) 121 (IOJ) 56.4 (23.3)
IlIlIIf>(N'" 27) 62.5 (17.6 13.9 (9.2) 23.6 (15.4
:l<IIJw(N-") 61.6 (10.0 14.6 (8.1) 23.7 (9.4)
Lma(N-7) 63.3 (16.8 20.5 (7.1) 16.2 (10.8
Bad.swamp Ridges and SWJI"(N- 20) 59.0 (J 1.2) 16J (7.0) 1',.7 (122
10 approach these levels of clay conceDlration. Thepedoturbational activities of the crayfish clearly concentrated the clay in extremely elevated levels thatare statistically significantly different (alpha = .01)than the levels ofnon·mound soils. These results alsocorroborate those ofGrow and Merchant (1980), whoexamined 40 crayfish mounds for particle size distribution, and classified all of those mounds as clay orsilty clay in tcxturc.
Between-site variability existed for crayfishmounds sampled from other locations (Tablc 1).These mounds were collected from a more diverselandforms assemblage, although primarily fromwithin meander·bend, low-lying swales. The spatial hctcrogeneity of the crayfish mounds is a reflection of the diversity of soil textures on similarlandforms along the length of tbe study reach.Even so, however, the crayfish mounds spatiallyconcentrate clay in statistically significantly greateramounts than in any non-crayfish·affected soils inthe study area, and are more similar to otber crayfish soils (i.e., the Broadneck Swamp crayfishmound soils) tban to non-crayfisb soils. Noncrayfish soils contain high amounts ofsand. in spiteof the fact that those soils come from a variety ofcoarser landform types, some of which (e.g .•backswamp ridges and swales) provide the primarysurfaces upon which the crayfish mounds are located The concentration of high levels ofclay, andthus the creation of spatial heterogeneity at a finescale, is thus clearly attributable to the moundbuilding activities of the cra.yfish.
Conclusions
Crayfish mounds are comprised largcly of claysize particles, and are typically about 12 cm high and8cm in diameter in the study area. These dimensions,the first reponed from the North Carolina CoastalPlain, place the Roanoke River crayfish moundswithin a typieal range for such landfonns. They are adominant fine-scale landform imposed on the coarserscale landforms of the floodplain. Given that thebackwater sites of the Roanoke River floodplainclearly qualify as locations wbere the water table isnear the surface, crayfish clearly must play an integralrole in lateral througbflow and soil aeration in thisenvirorunent. Crayfish may be a keystone species ininducing spatial heterogeneity of soil characteristicsin an environment where soils of great similarityotherwise exist regardless of landform type.
Acknowledgments
This study was funded by a grant from The NatureConservancy to Stephen J. Walsh, David R. Butler,Charles E. Konrad 11, and Robcrt K. Peel. Assistancein the field for crayfish mounds and soil data collectionwas provided by Ms. Marilyn Wyrick and Mr. JobnVogler. Ms. Wyrick and Mr. Michael Brown providedassistance in laboratory analysis of the non-crayfishsoils data, and Mr. Vogler assisted with the laboratoryanalysis of the crayfish mounds.
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