The Influence of Beaver Dams on Animal Communities of Streams and Surrounding Riparian Zones Audrey Rowe University of Chicago A
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The influence of beaver dams on animal 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 species 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 animals 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 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.