Influence of Beaver Dams on Benthic Trophic Structure

Julia McMahon

Dickinson College, Carlisle PA

Mentor: James Nelson

2

Abstract

Beavers are one of the most widely known ecosystem engineers with the potential

to change benthic invertebrate communities such as the one at Cart Creek in

Massachusetts. They alter the abundance and biomass of the species inhabiting the newly

created pond and the water downstream of the dam. By creating ponds they also change

the amount of different primary producers available to the taxa inhabiting the ponds and

streams. Altering the amount of primary producers allows a change in what drives the

benthic food web. Aquatic primary producers are most utilized in the impounded sites

while the streams are reliant on detritus. This shift in primary producer utilization allows

species to shift trophic levels and therefor shift their diet based on where they are.

Overall, beaver-altered systems change the dynamics of the aquatic food web in Cart

Creek, Massachusetts.

Keywords

Benthic Invertebrates, Trophic Structure, Stable Isotopes

Introduction

Beavers have the potential to influence the riparian and aquatic communities in

their surrounding area, with a particularly pronounced impact on the benthic invertebrate

communities. Beavers alter the aquatic environment by building dams, transforming a

lotic community structure into a lentic community structure. By creating a larger aquatic

3

habitat, this beaver activity can also change the amount of aquatic primary production

available for different species, which in return can influence the type of taxa present.

Alterations in habitat also differ with types of sediment, pond/stream size, and

times of disturbance. Along with the different taxa present, beaver activity can also alter

the amount and importance of different functional groups. The beaver dam creates a

pond, referred to as an impoundment, with retention of sediment and organic matter. It

also creates a larger surface area with differences in nutrient cycling and rates of

decomposition. These impounded sites often exhibit properties that are more typical of

ponds than streams, such as a high rate of adult insect production. The effects of beavers

on invertebrate community structure have been looked at from different views, including

energetic studies that examined the emergence of adult insects from beaver

impoundments. Sprules (1940) was one of these researchers and found that adult

assemblages in impoundments differ temporally, while Naiman and colleagues (1984)

found a spatial difference. One more recent examination looked at the differences in

benthic communities seasonally in Quebec (McDowell and Naiman 1986) and found

more diversity in riffle sites along with more mayflies and chironomid larvae in riffle

non-impounded sites.

The main inquiry I pursued was how beaver activity affects the trophic dynamics

of aquatic food webs along a stream gradient. I hypothesized that beavers have altered

the Cart Creek, MA benthic trophic structure by changing the amount and biomass of

different organisms, increasing diversity in beaver impoundments, and shifting the food

sources.

4

Methods

Data collection

Aquatic invertebrates were collected from Cart Creek in Massachusetts on

November 7th

– 9th and November 13

th – 16

th 2014 using an Ekman Grab. From Cart

Creek, five sampling sites were chosen along different points of the stream. The first site

includes two sub-sites: the actual beaver dam in a large beaver impoundment, referred to

as “dam”, and the other site from the same large beaver impoundment, referred to as

“large pond.” “Stream 1” is located downstream of the large pond and occurs after a

debris dam. The “small pond” is located in the second beaver impoundment, which is

smaller than the first large beaver impoundment. “Stream 2” is located near the end of

the creek after other debris dams. Samples of invertebrates were collected from all five

sites in order to determine abundance and biomass of the invertebrates inhabiting the

benthic habitat. In the field, after each Ekman Grab, the samples from each site were

placed in plastic bags and taken back to the lab for counting and sorting. The samples

were inspected to sort the different invertebrates by taxonomic genera in the Ekman

Grab. Later, when determining which invertebrate taxa to select for running isotopes, the

taxonomic family was used to combine organisms.

Two assumptions were made when calculating biomass. First, the dry weight of

the benthic organisms was assumed to be 15% of the wet weight of individuals. Second,

the percent carbon content was assumed to be 50% of the dry weight of the individual

(Water, 1977).

5

Once sorted, based on abundance, biomass, and weight of sample, sixteen

samples were chosen for running carbon, nitrogen, and sulphur isotopes. The isotope

results were analyzed with the program R, which ran a Bayesian statistical mixing model

(Parnell et al. 2008).

Results

Abundance and Biomass

Abundance and biomass values varied from different sites for different organisms,

as shown in Figures 1 - 5. The abundance and biomass were both utilized to determine

which organisms’ isotope data would be analyzed. When comparing the dam’s

abundance and biomass, caecidotea had the largest abundance while macromia had the

largest biomass, and therefore their taxonomic families were utilized for running stable

isotopes (asellidae and macromiidae respectively). Since this site had many more taxa

found, three more organisms were picked to analyze using stable isotopes, including the

taxonomic family glossiphonia (genus marvinmeyeria, glossiphonia, and helobdella),

planorbidae (genus planorbdella), and dytiscidae (genus liodessus).

In the large pond, caecidotea again had the largest abundance while planorbdella

had the largest biomass. Both values were utilized for picking taxonomic families for

running stable isotopes (asellidae and planorbidae respectively). The other families

picked to run stable isotope analysis for this site were gammaridae (genus gammarus)

and macromiidae (genus macromia).

6

In Stream 1, the taxa with the largest abundance and biomass were caecidotea

(family asellidae). Based on the other values, the other taxaonomic family chosen for

isotope analysis was podonominae (genus paraborochlus and boreochlus).

In the small pond, caecidotea (family asellidae) again had the largest abundance

while paraboreochlus (family podonominae) had the largest biomass. The other

taxonomic family chosen was chironomidae, referred to as chrionomid #2 since the exact

genus was unknown.

Lastly, in Stream 2, the caecidotea (family asellidae) and chironomus (family

chironomidae) both had the highest abundance and biomass and therefore both were

chosen to run stable isotope analysis.

Diversity

The diversity values showed differing results. The abundance diversity shows a

slight decrease while traversing Cart Creek downstream, while the biomass diversity

shows a slight increase using the same traversal path (Figure 6).

Stable Isotope Analysis

Stable isotope analysis helped to determine the trophic level of different

organisms and the relative level of primary source within their tissue, helping identify

what drove the food webs in the pond and stream. Based on the results, the pond is

driven by aquatic primary producers and the stream is based on detritus (Figures 10 and

12). These same patterns are also visible through percentages as shown in Figures 11 and

13.

There was a large range of δ13

C values (-74 to -28) from the organisms selected

for running isotopes, as shown in Figures 7 and 8. However, there was not a large range

7

of δ15

N values (-3 to 6). Asellidae, which was found in each site, differed in trophic level

and shifted their diet from different sites (Figure 9).

From the ranges, a new source was added, since none of the existing sampled

sources could explain it. The organic matter, which produces methane, contributes to the

very different δ13

C value (Hornibrook et al. 2000). Methane was found to contribute at

least 66% of the tissue in the chironomid #2 larvae (small pond) and 47% in the

chironomidae larvae from stream 2 (Table 1). Methane was suggested to be the source

based on previous literature (Grey 2002).

Discussion

Abundance and biomass values could differ between sites due to the dams created

by the beavers. If the dams create a new habitat, this new habitat is likely to support

different kinds of species from those that were prevalent prior to the dam construction.

When the beaver dams are created, they increase the surface area of the water, creating a

pond. This pond is not only deeper but also wider than the original stream, and allows

more aquatic primary production to occur, which is a better source of food than terrestrial

inputs. Different organisms have different preferences for where they reside. For

example, leeches may only be found in the large pond because this location potentially

provides larger organisms on which they can feed, such as turtles, although none were

observed. This could also explain the diversity differences. If there are fewer species

downstream, there is less competition resulting in organisms being able to attain a greater

biomass.

8

Overall, the results of this investigation support my hypothesis that beaver dams

cause a difference in benthic trophic structure. They alter the abundance and biomass of

different species shown by different species dominating different habitats. They decrease

the abundance diversity downstream while increasing the biomass diversity downstream.

Asellidae shift trophic levels and they also shift which primary sources comprise their

tissue, which is representative of their diet shift. More samples should be taken to see if

this occurs in other species downstream. There was also a large number of chironomidae

found in the Cart Creek beaver impoundments, which stands in contrast to the analysis of

other beaver dam sites reported from Quebec (McDowell and Naiman 1986). I found

that chironomidae larvae are fueled by methanotrophs, which are utilizing methane, and

therefore beaver-altered sites could be small sources of methane and should be

monitored, since beavers are no longer being trapped in Plum Island, MA.

Works Cited

Grey, J. 2002. A Chironomid Conundrum: Queries Arising from Stable Isotopes. Verh.

Internat. Verein. Limnol. 28: 1-4.

Hornibrook, E. R. S., F. J. Longstaffe, and W. S. Fyfe. 2000. Factors Influencing Stable

Isotope Ratios in CH4 and CO2 Within Subenvironments of Freshwater Wetlands:

Implications for delta signatures of emissions. Isot. Environ.Health Strud 36(2):

151-176.

9

Naiman R. J., D. M McDowell, and B. S. Farr. 1984. The Influence of Beaver (Castor

Canadensis) on the Production Dynamics of Aquatic Insects. Verh Internat

Verein Limnol 22: 1801-1810.

ParnellA., R. Inger, S. Bearhop, and A. L. Jackson. 2008. SIAR: Stable Isotope Analysis

in R. R Package Version, 3.

Sprules W. M. 1940. The Effect of a Beaver Dam on the Insect Fauna of a Trout Stream.

Trans Am Fish Soc 70: 236-248

10

Figures and Tables

A)

B)

Figure 1. Abundance and Biomass information for taxa found in the dam in the large pond. Data was used to determine taxa to run isotope samples on A) Abundance data with caecidotea having the largest abundance. B) Biomass data with macromia having the largest biomass.

0

100

200

300

400

500

600

700

Gen

us

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

ma

rus

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

ab

ore

och

lus

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Ab

un

da

nce

(in

div

idu

als

/m2)

Genus

Dam

0

100

200

300

400

500

600

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

mar

us

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Bio

ma

ss (

gC

/m2)

Genus

Dam

11

A)

B)

Figure 2. Abundance and Biomass information for taxa found in the large pond. Data was used to determine taxa to run isotope samples on A) Abundance data with caecidotea having the largest abundance. B) Biomass data with planorbdella having the largest biomass and macromia having the second highest biomass.

0

50

100

150

200

250

300

350

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

de

ssu

s

Ep

hem

erel

la

Gam

ma

rus

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Ab

un

da

nce

(In

div

idu

als

/m2)

Genus

Large Pond

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

mar

us

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Bio

ma

ss (

gC

/m2)

Genus

Large Pond

12

A)

B)

Figure 3. Abundance and Biomass information for taxa found in the stream 1. Data was used to determine taxa to run isotope samples on A) Abundance data with caecidotea having the largest abundance. B) Biomass data with caecidotea having the largest biomass and boreochlus having the second highest biomass.

0

100

200

300

400

500

600

700

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

de

ssu

s

Ep

hem

erel

la

Gam

ma

rus

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Ab

un

da

nce

(in

div

idu

als

/m2)

Genus

Stream 1

00.10.20.30.40.50.60.70.80.9

1

Cae

cid

ote

a

Ca

enis

Pal

po

my

ia

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Ga

mm

aru

s

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Bio

ma

ss (

gC

/m2)

Genus

Stream 1

13

A)

B)

Figure 4. Abundance and Biomass information for taxa found in the small pond. Data was used to determine taxa to run isotope samples on A) Abundance data with caecidotea having the largest abundance and paraboreochlus having the second largest. B) Biomass data with paraboreochlus having the largest biomass and chironomid #2 having the second highest biomass.

0

100

200

300

400

500

600

700

800

900

1000

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

mar

us

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

ab

ore

och

lus

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Ab

un

da

nce

(in

div

idu

als

/m

2)

Small Pond

0

20

40

60

80

100

120

Ca

ecid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

mar

us

Hel

ob

de

lla

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Bio

ma

ss (

gC

/m2)

Small Pond

14

A)

B)

Figure 5. Abundance and Biomass information for taxa found in the stream 2. Data was used to determine taxa to run isotope samples on A) Abundance data shows caecidotea and chironomus having the largest abundance B) Biomass data shows chironomus having the largest biomass.

0

50

100

150

200

250

300

350

400

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

ma

rus

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Ab

un

da

nce

(in

div

idu

als

/m

2)

Genus

Stream 2

0

1

2

3

4

5

6

7

8

Cae

cid

ote

a

Cae

nis

Pal

po

myi

a

Ch

iro

no

mu

s

Isch

nu

ra

Ep

ico

rdu

lia

Ch

auli

od

es

Co

ryd

alu

s

Lio

des

sus

Ep

hem

erel

la

Gam

mar

us

Hel

ob

del

la

Pla

cob

del

la

Glo

ssip

ho

nia

Mar

vin

mey

eria

Mac

rom

ia

Pla

no

rbd

ella

Pro

clad

ius

Bo

reo

chlu

s

Par

abo

reo

chlu

s

Lim

no

dri

lus

Ch

iro

no

mid

#2

Aga

bu

s

Pis

sid

um

Bio

ma

ss (

gC

/m2)

Genus

Stream 2

15

Figure 6. Abundance and biomass diversity based on the Shannon’s Diversity Index. Abundance diversity decrease while biomass diversity increases indicating differing trends.

Figure 7. Biplot of δ13C and δ15N values for 8 different taxonomic families (shown with different colored dots) using R to run statistical analysis based on isotope data. Five primary sources were utilized including litter, grass, algae, detritus, and moss.

0

0.5

1

1.5

2

2.5

3

3.5

4

Dam Large Pond Stream 1 Small Pond Stream 2

Sh

an

no

n's

Div

ers

ity

In

de

x

Abundance Diversity

Biomass Diversity

16

Figure 8. Biplot of δ13C and δ15N values for 6 different taxonomic families (shown with different colored dots) using R to run statistical analysis based on isotope data. The remaining two-outlier taxonomic families were excluded in order to examine possible trends closer up. Five primary sources were utilized including litter, grass, algae, detritus, and moss.

17

Figure 9. Histograms depicting the percent contribution of each source (litter, grass, algae, detritus, and moss) to the caecidotea in five different locations. A) caecidotea sampled from the large pond and the dam relying on primarily litter and is on trophic level 1. B) caecidotea sampled from stream 1 relying on a homogenized diet of all 5 sources increasing to trophic level 2. C) caecidotea sampled from the small pond relying on mostly algae and grass and remaining on trophic level 2. D) caecidotea sampled from stream 2 relying on litter and grass and remaining at trophic level 2.

18

Figure 10. The minimum required aquatic, detrital, and terrestrial inputs to support the observed Pond food web. This was back calculated based on percentages from R and abundance and biomass data. The Pond food web is dependent on aquatic primary producers.

19

Figure 11. Percentages based on figure 10 for how much each trophic level utilizes each primary producer. The pond food web utilizes 64% aquatic primary producers.

20

Figure 12. The minimum required aquatic, detrital, and terrestrial inputs to support the observed stream food web. This was back calculated based on percentage of primary sources contributing to the specified organisms tissue from R and abundance and biomass data. The stream food web is dependent on detritus.

Figure 13. Percentages based on figure 12 for how much each trophic level utilizes each primary producer. The stream food web utilizes 64% detritus as a source.

21

Table 1. Taxa with more than 4% methane contributing to their tissue.

Family and Location Percent Methane

Asellidae (Dam) 4.7%

Macromiidae (Dam) 4.2%

Asellidae (Large pond) 4.9%

Macromiidae (Large Pond) 4.1%

Chironomid #2 (small pond) 66.8%

Chironomid #1 (Stream 2) 47.5%

Dytiscidae (Stream 2) 5.6%