The influence of beaver dams on communities of streams and

surrounding riparian zones

Audrey Rowe

University of Chicago

Adviser: Linda Deegan

Abstract

Beaver dams tend to increase habitat heterogeneity by breaking up streams into lentic and lotic sections. I investigated the impact of beaver dams on macroinvertebrates in and near Cart Creek in Newbury, MA. I found that the high spatial density of dams on this small creek suppresses aquatic diversity, but the ponds might be increasing the species diversity of terrestrial macroinvertebrates. I also found that significant trophic subsidies are present between aquatic and terrestrial organisms, with terrestrial predators feeding on aquatic and aquatic herbivores and detritivores feeding on leaf litter and other detritus fallen into the creek from the land.

Key words: beavers, benthic macroinvertebrates, species richness, trophic subsidies

Introduction

Due to hunting and habitat loss, the North American beaver (Castor canadensis) was extirpated from much of New England in the late 18th century (Jackson and Decker). Beavers returned to Massachusetts in the early 20th century, and have once again become populous throughout much of the state. Their unique behavior as ecosystem engineers often makes them problematic in areas inhabited by people, as they are destructive to trees and can cause flooding of manmade structures. However, beavers may be important to maintaining healthy stream ecosystems. Beavers alter free flowing streams by constructing dams. A series of dams in a stream forms a system of impoundments of lentic water upstream of the dams connected by shallow, lotic streams downstream of the dams, increasing habitat heterogeneity. The differences in flow velocity directly affect the populations of stream invertebrates and fish that prefer particular velocities. (Arndt and Domdei 2011). Additionally, the dams cause a host of differences in physical characteristics of the stream, including changes to depth, water chemistry, and temperature. The sediment that accumulates in the ponds stores vastly more carbon and nitrogen than the flowing stream (Naiman and Melillo 1984). This affects species composition where certain organisms are well adapted to the nutrient conditions that the beaver dams create. Collen and Gibson (2011) discuss how dams warm the water of the stream and thus allow some fish species to occupy streams that would otherwise be too cold for them. Beavers can even tip the scales in competitive interactions, as Collen and Gibson show that one of two cohabiting species can become the dominant competitor when nutrient or temperature conditions change as a result of a dam. Alternatively, beaver dams might be detrimental to certain species that occupy streams because the ponds promote hypoxic conditions (Arndt and Domdei 2011). The impact of beaver dams can radiate throughout the trophic web of the riparian zone. Nummi et al (2011) found that the presence of beavers can benefit bats because the impoundments act as fertile breeding grounds for emergent insects and thus cause a local increase of this important food source for the bats. Fish in different parts of the stream can feed on different species of insects, affecting fish species composition as well, which can in turn affect waterfowl in the system (Nummi 1992). Benthic macroinvertebrate taxonomic richness is often used as a measure of stream health, most frequently by measuring the diversity of Ephemeroptera, Plecoptera, and Trichoptera, which are three aquatic insect orders that are particularly sensitive to water quality (Lenat 1993). A study by Arndt and Domdei (2011) of brooks in central Europe found that EPT taxa were reduced in beaver impoundments compared to free flowing streams, probably due to low dissolved oxygen content. This, along with Naiman and Melillo’s findings (1984), suggests that impoundments tend to be eutrophic. Though this can be interpreted as evidence that the dams have a negative impact on benthic macroinvertebrates, Rolauffs et al (2001) argue the opposite. They also found that the EPTC diversity (including Coleoptera) was lowered in ponds and flowing streams when counted separately, as Arndt and Domdei did. However, when the stream system is viewed as a whole so that the dam and stream EPTC species are summed, the EPTC diversity was higher overall in streams with beaver dams, as the species represented in the ponds were different than those in the adjacent free flowing streams. The dams thus contributed to greater species diversity in the stream system by increasing habitat heterogeneity. Now that beavers are making a full resurgence in the area, I aimed to examine how beaver dams affect animal diversity of riparian zones in New England. I investigated how beaver dams affect macroinvertebrate populations in a stream, comparing both species richness and composition upstream and downstream of the dams to see how the ponds and free flowing streams might differ. Using those data, I then estimated water quality using the EPT index, comparing the upstream and downstream portions to potentially corroborate findings about whether abandoned beaver dams can act as natural water filters (Como 2015). I also wanted to investigate how important the aquatic macroinvertebrates are to the diets of terrestrial animals in the nearby riparian zone, how differences between beaver ponds and streams cross over into the terrestrial habitat, and how far into land their effect extends. To do this, I sampled the creek and the nearby terrestrial habitats using various methods to collect data on the species composition and abundance of small animals in the riparian zone.

Methods

Sampling took place in November and early December of 2015. I sampled three consecutive dams on Cart Creek in the Martin H. Burns Wildlife Management Area in Newbury, MA. My field partner and I originally scouted six dams on the creek, and I chose to use the second, third, and fourth dams from upstream (referred to as Dams 2, 3, and 4 throughout this paper) because they were relatively intact and were easiest to access from the road. Our investigation of Cart Creek revealed that the water level is currently low. The flow rates of the impoundments and some sections of the free flowing streams were too slow to be measured. This means that the stream only trickles through the dams rather than over them. The water of the creek was quite shallow and lentic at every dam site, eliminating the usefulness of collection methods like Surber sampling. Moreover, with the fieldwork conducted in November, there were too few emergent insects to collect using a net. Thus, I only sampled benthic macroinvertebrates in the creek using a 15cm x 15cm Ekman grab. At each dam, I sampled at three points: just upstream of the dam (in the pond), 2m downstream of the dam, and approximately 10m downstream of the dam. I calculated the water quality index in each benthic sample using the “Key to Macroinvertebrate Life in the River” developed by the University of Wisconsin. I sampled the adjacent terrestrial ecosystem of the creek using a series of barrier pitfall traps, vacuum samples, and live mousetraps. I sampled with a macroinvertebrate vacuum on 0.5mx0.5m quadrats at 1m, 5m, and 10m away from the creek, both adjacent to the pond and the downstream sections of each dam site. I set up the barrier pitfall traps perpendicular to the creek at the same intervals and left them out for two days. I set 12 mousetraps at each dam site: 6 adjacent to the pond and 6 adjacent to the downstream section, moving in a transect away from the creek at 2 meter intervals. I clipped small samples of fur from the mice for isotopic analysis before releasing them. I selected a total of 33 samples of both terrestrial and aquatic specimens for stable isotopic analysis of carbon and nitrogen to examine feeding habits and compare trophic relationships.

Results

The benthic macroinvertebrate species richness measured did not change significantly moving across Dams 3 or 4 (Figure 1). An exception to this is Dam 2, where the pond had a much higher species richness than the downstream portions. The water quality index scores show similar trends, with Dams 3 and 4 being fairly uniform and ranging from ratings of “Fair” to “Poor” (Figure 2). Again, Dam 2 was exceptional: the rating was “Excellent” in the pond, and dropped to “Fair” in the 2m downstream section and “Poor” in the 10m downstream section. This is due both to the large species richness of the Dam 2 pond and its unique species composition, as the sample included 3-point caddisfly and mayfly taxa, as well as several 2-point taxa. Oligochaetes (thought to be tubifex) were very abundant 2m downstream of Dams 3 and 4, and comparatively low in number or not found at all in the pond and 10m downstream samples of these dams (Figure 3). Dam 2 had low abundances of these worms at all three sampling points. This trend might also be loosely seen with fingernail clams, which were very abundant 2m downstream of Dams 2 and 4 and less abundant in the pond and 10m downstream (Figure 4). However, Dam 3 shows a different trend, where the clams are most abundant 10m downstream of the dam. The number of species identified in vacuum samples tended to be greater in the samples adjacent to the ponds (Figure 5). This was especially evident in the samples taken 5m away from the creek. I calculated the percent dependence on aquatic food sources of each of the terrestrial species analyzed for stable isotopes by using the millipede as a purely terrestrial signal and the water scorpion as a purely aquatic signal. The dependence on aquatic food sources seems to generally decrease with increased distance from the stream (Figure 6). The correlation is loose, but it is worth consideration because it is likely thrown off by many of the terrestrial herbivores that do not feed from any aquatic species. The small Pisauridae collected in the vacuum sample 10m from the stream had a δ13C value over 3‰ higher than those of the Pisauridae that were found closer to the creek, indicating that the spiders of the same species near the creek rely on aquatic food sources for a significant portion of their diets (Figure 7). This suggests that even small spatial differences might considerably alter the diets of macroinvertebrates. The shrew, fishing spider, centipede, and Pisauridae collected in the 1m downstream vacuum sample all had a very similar δ15N values, indicating that the organisms are at a similar trophic level. This reflects the similar feeding strategies of these species, as they all mainly eat insects. The shrew is an interesting contrast to the deer mouse. Both are similarly sized rodents, but the mouse is an omnivore, while the shrew tends to eat small animals. It has a much more terrestrial δ13C value and is at a lower trophic level, both of which are perhaps indicative of the importance of terrestrial plants to its diet. The fishing spider had a distinctly low δ13C value, suggesting that it might be feeding directly from the water. The stonefly, collected as an adult in a terrestrial sample, has a strongly aquatic signal, reflecting the longest stage of its life cycle as an aquatic nymph.

Discussion

The species richnesses of the benthic macroinvertebrates were opposite of typical expectations for beaver dam-influenced rivers. Ponds tend to be less diverse than the free-flowing streams (e.g., Arndt and Domdei 2011); however, in Cart Creek, the beaver ponds were generally not different from or, in one case, more diverse than the sections downstream of the dams. This may be due to the physical characteristics of the creek itself. Cart Creek is quite lentic throughout its entire length. This is exacerbated by the beaver dams, which are so close together that the creek has essentially become a series of beaver ponds rather than a system of alternating ponds and free flowing streams. Thus, the sediment of the creek is a fairly uniform mud throughout the creek, as evidenced by the similar organic matter percentages measured in the sediments at different points near Dam 2 (Table 1). With both similar water velocities and substrates upstream and downstream of the beaver dams, it appears that the high spatial density of the dams is homogenizing the creek and thus suppressing species diversity. The benthic macroinvertebrates throughout the creek are thus similar in composition to lentic streams with detritus- dominated bottoms. This can be illustrated by comparing the data to a river not impacted by beaver dams. The species compositions I found in Cart Creek were very different from those found in much of the Mashpee River, which had a sandy bottom and was richer in EPT taxa (Seiz 2011). However, samples from more detritus- dominated sections of the river were more similar in composition to the Cart Creek samples. The oligochaetes and clams tended to be most abundant just downstream of the dams. An exception to this trend for clams was Dam 3, where the 2m downstream section had a depressed number of clams in comparison to the other dams. This section also had the highest number of oligochaetes of all of the benthic grabs. Together, this might indicate that the stream is particularly high in organic matter and low in oxygen immediately downstream of Dam 3. Tubifex worms can live in very hypoxic conditions due to their high red blood pigment (Thorp and Covich 1991). This might make them better suited for the conditions in this section of the stream, allowing them to flourish in the absence of competition. The species richness of terrestrial samples tended to be higher adjacent to the pond than adjacent to the downstream sections of the dam sites. However, the aquatic species richness was similar in the pond and downstream sections, meaning that the differences in terrestrial richness are probably not driven by a difference in aquatic species richness or composition. Perhaps this might be caused by a sheer difference in abundance of aquatic organisms between the different sections of the dam sites. Though the ponds are similar in composition to the streams, they are much larger in area and thus provide a larger habitat to support more biomass of insects that emerge in adulthood and feed the terrestrial animals. However, there was no significant difference in the abundance of predators between the terrestrial samples adjacent to the stream and adjacent to the pond. Perhaps these differences might thus be caused by factors that I did not study, such as differences in vegetation among the sampling sites. Many of the terrestrial species with middling δ13C values, indicating a mix of aquatic and terrestrial food sources in their diets, were predators near the top of the food web for the system. However, most of these species do not have life history strategies that involve feeding directly from the stream. Instead, this pattern is probably a result of aquatic larvae that emerge from the stream as terrestrial adults and potentially feed the terrestrial predators. One such example is the stonefly, which was caught in a vacuum sample as an adult, but had the most strongly aquatic δ13C signal of any of the terrestrial samples, as stoneflies spend most of their lives as aquatic nymphs. Conversely, the aquatic species with middling δ13C values were herbivores and detritivores. This may be due to allochthonous inputs of leaf litter from the nearby terrestrial sphere making up a significant portion of their diets.

Conclusions

Cart Creek system is different from a typical beaver impacted stream system found in literature because of the currently high spatial density of the dams, which form a series of stagnant ponds that suppress species diversity. However, these ponds might be benefiting nearby terrestrial species diversity by creating a wider wetland habitat that supports abundant macroinvertebrate life. Unfortunately, I do not have biomass data to support this idea; perhaps a future project can investigate this. The system shows clear trophic subsidies between aquatic and terrestrial organisms, particularly terrestrial predators feeding on emergent aquatic invertebrates, and aquatic herbivores and detritivores feeding on organic matter falling into the creek from the terrestrial sphere. However, a deeper investigation of this would require identification and stable isotopic analysis of the base of the food web, which I was not able to do. Another direction for further research might thus be to examine how aquatic and terrestrial plants differ upstream and downstream of the dams, and how this might be impacting the food web.

Acknowledgments

I would like to thank my adviser Linda Deegan for being the driving force behind the ideas in my project, and for introducing me to bugs and beavers in the first place. I would like to thank Rich McHorney and Angela Como for their enormous amount of help and time spent at the Plum Island LTER, as well as Kendall Kotara and David Tian for volunteering to spend one last night there to help me catch mice.

Literature Cited

Arndt, E. , Domdei, J. 2011. Influence of beaver ponds on the Macroinvertebrate benthic community in Lowland Brooks. Polish Journal of Ecology. 59: 799-811

Collen P, Gibson RJ. 2001. The general ecology of beavers (Castor spp.) as related to their influence on stream ecosystems and riparian habitats, and the subsequent effects on fish - a review. Reviews in Fish Biology and Fisheries. 10: 439-461

Como A. 2015. Do abandoned beaver dams act as natural water filters? SES Project.

Hirabayashi K, Fu Z, Yoshida N, Yoshizawa K and Kazama F. 2012. A comparison of results from previous and present investigations of benthic macroinvertebrates in the small and shallow Shoji, Fuji Five , Japan. Fauna norvegica. 31: 47-54.

Jackson, S., Decker, T. Date unknown. Beavers in Massachusetts: Natural History, Benefits, and Ways to Resolve Conflicts Between People and Beavers. University of Massachusetts, United States Department of Agriculture, Massachusetts Division of Fisheries and Wildlife and Massachusetts counties cooperating. Accessed 21 Dec. 2015.

Lenat D. R. 1993. A biotic index for the southeastern United States: derivation and list of tolerance values, with criteria for assigning water-quality ratings. Journal of the North American Benthological. 12: 279-290

Nummi P. 1992. The importance of beaver ponds to waterfowl broods: an experiment and natural tests. Annales Zoologici Fennici. 29: 47-55

Nummi P, Kattainen S, Ulander P, Hahtola, A. 2011. Bats benefit from beavers: A facilitative link between aquatic and terrestrial food webs. Biodiversity and Conservation. 20: 851-859

Pinna M, Marini G, Mancinelli G, Basset A. 2014. Influence of sampling effort on ecological descriptors and indicators in perturbed and unperturbed conditions: A study case using benthic macroinvertebrates in Mediterranean transitional waters. Ecological Indicators. 37: 27–39

Seiz, A. 2011. Benthic Invertebrate Community Composition in Four Streams across a Restoration Intensity Gradient. SES Project.

Rolauffs, P., Hering, D., Lohse, S. 2001. Composition, invertebrate community and productivity of a beaver dam in comparison to other stream habitat types. Hydrobiologia 459: 201-212

Thorp, J., Covich, A. 1991. Ecology and Classification of North American Freshwater Invertebrates. Page 407. Academic Press, Inc. San Diego, California, USA.

Author unknown. Date unknown. Key to Macroinvertebrate Life in the River. Developed by the University of Wisconsin– Extension in cooperation with the Wisconsin Department of Natural Resources.

Figures and Tables

16

14

12

10 Dam 2 8 Dam 3

6 Dam 4 number number species of

4

2

0 pond 2m downstream 10m downstream

Figure 1. Species richness in benthic grabs.

25

20

15 Dam 2

Dam 3 Score 10 Dam 4

5

0 pond 2m downstream 10m downstream

Figure 2. Water quality index scores in benthic grabs.

180

160

140

120

100 dam 2

abundance 80 dam 3 dam 4

60 Tubifex 40

20

0 pond 2m downstream 10m downstream

Figure 3. Tubifex abundance of benthic grabs.

60

50

40 dam 2 30 dam 3 dam 4

20 clam clam abundance

10

0 pond 2m downstream 10m downstream

Figure 4. Clam abundance in benthic grabs.

14

12

10

Dam 2 adjacent to pond 8 Dam 2 adjacent to stream Dam 3 adjacent to pond 6 Dam 3 adjacent to stream

Dam 4 adjacent to pond Species Richness 4 Dam 4 adjacent to stream

2

0 1m 5m 10m

Figure 5. Species richness, upstream adjacent to pond vs adjacent to stream.

0.80

0.70

0.60

0.50

0.40 adjacent to pond adjacent to stream % aquatic % aquatic diet 0.30

0.20

0.10

0.00 0 2 4 6 8 10 12 distance from creek (m)

Figure 6. Dependence of terrestrial species on aquatic feeding, by distance from creek

Figure 7. Stable isotopic values for selected terrestrial and aquatic samples.

Section Pond Dam Below Dam 6m 20m downstream downstream % organic 16 11 20 14 25 matter Table 1. Content of organic matter in sediment from Dam 2 (Como 2015).