Influence of Beaver Dams on Benthic Trophic Structure. Julia McMahon. Dickinson College, Carlisle PA. Mentor: James Nelson

Transcription

1 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 2

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 (194) 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. 3

4 Methods Data collection Aquatic invertebrates were collected from Cart Creek in Massachusetts on November 7 th 9 th and November 13 th 16 th 214 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 5% of the dry weight of the individual (Water, 1977). 4

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. 28). 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). 5

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 1 and 12). These same patterns are also visible through percentages as shown in Figures 11 and 13. There was a large range of δ 13 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 6

7 of δ 15 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 δ 13 C value (Hornibrook et al. 2). 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 22). 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. 7

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. 22. 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. 2. Factors Influencing Stable Isotope Ratios in CH 4 and CO 2 Within Subenvironments of Freshwater Wetlands: Implications for delta signatures of emissions. Isot. Environ.Health Strud 36(2):

9 Naiman R. J., D. M McDowell, and B. S. Farr The Influence of Beaver (Castor Canadensis) on the Production Dynamics of Aquatic Insects. Verh Internat Verein Limnol 22: ParnellA., R. Inger, S. Bearhop, and A. L. Jackson. 28. SIAR: Stable Isotope Analysis in R. R Package Version, 3. Sprules W. M The Effect of a Beaver Dam on the Insect Fauna of a Trout Stream. Trans Am Fish Soc 7:

10 Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Biomass (gc/m 2 ) Genus Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Abundance (individuals/m 2 ) Figures and Tables A) Dam Genus B) Dam Genus 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. 1

11 Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Biomass (gc/m 2 ) Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Abundance (Individuals/m 2 ) A) Large Pond Genus B) Large Pond Genus 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. 11

12 Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Biomass (gc/m 2 ) Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Abundance (individuals/m 2 ) A) Stream 1 Genus B) Stream 1 Genus 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. 12

13 Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Biomass (gc/m 2 ) Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Abundance (individuals/m 2 ) A) Small Pond B) Small Pond 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. 13

14 Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Biomass (gc/m 2 ) Caecidotea Caenis Palpomyia Chironomus Ischnura Epicordulia Chauliodes Corydalus Liodessus Ephemerella Gammarus Helobdella Placobdella Glossiphonia Marvinmeyeria Macromia Planorbdella Procladius Boreochlus Paraboreochlus Limnodrilus Chironomid #2 Agabus Pissidum Abundance (individuals/m 2 ) A) Stream 2 B) Genus Stream 2 Genus 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. 14

15 Shannon's Diversity Index Abundance Diversity Biomass Diversity Dam Large Pond Stream 1 Small Pond Stream 2 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 δ 13 C and δ 15 N 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. 15

16 Figure 8. Biplot of δ 13 C and δ 15 N 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. 16

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. 17

18 Figure 1. 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. 18

19 Figure 11. Percentages based on figure 1 for how much each trophic level utilizes each primary producer. The pond food web utilizes 64% aquatic primary producers. 19

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. 2

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% 21