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The effects of beaver ponds on community structure in a forest ecosystem
Jessie Moravek
Northwestern University
Mentor: James Nelson
The Ecosystems Center, Marine Biological Laboratory
Collaborators: Julia McMahon, Delaney Gibbs, Mairi Possion
Abstract:
Aquatic and terrestrial environments are connected by the exchange of production and resources
across ecosystem boundaries. The strength of this connection depends on the characteristics of
both ecosystems. As such, heterogeneous aquatic environments, such as those in streams
colonized by beavers (Castor Canadensis), are a valuable study site for analyzing how variation
in aquatic systems can impact plant and animal communities in the forest. The focus of this study
was to define the effect of beaver ponds on tree size and species distribution, and to analyze the
importance of aquatic production in terrestrial food webs. Overall, it was found that beaver activity
decreases the number of small trees next to beaver ponds, and increases the relative abundance of
red maple trees. In terms of food webs, higher trophic levels such as spiders and other insect
predators rely heavily on inputs from the aquatic production pathway, while lower trophic levels
are more dependent on terrestrial production. Aquatic production makes up about 60% of the total
primary production required to support terrestrial food webs. This might be attributed to the higher
nutritional value of aquatic sources, making them better able to support mobile secondary
production. Additionally, the influence of aquatic production in the diets of terrestrial insects
extends up to 60 meters into the forest, and reliance on aquatic production pathways does not vary
between pond and stream sites. This could be attributed to the mobility of both insect predators
and emergent aquatic insects, which act as vectors to move aquatic production into the forest.
Key Words: Beavers, Food Webs, Trees, Isotopes, Cart Creek, Aquatic Production
Introduction:
The exchange of resources across ecosystem boundaries forms a vital link in both
terrestrial and aquatic food webs. Inputs from land to water in the form of sediments, leaf litter,
and terrestrial insects are well characterized contributors to aquatic productivity (Gary A. Polis
1997). The reciprocal case is more difficult to define, since terrestrial resource-use is widely
varied and spread over a large area (Mary E. Power 2004). However, several studies have
demonstrated clear evidence of aquatic inputs to terrestrial food webs. For example, nutrients
from dead salmon provide important nitrogen inputs to riparian habitats (M. Ben-David 1998),
and lizard growth rates in Northern California riparian areas were found to be seven times
greater when aquatic insects were part of their diets (J.L. Sabo 2002). Based on these
observations, the connectivity between aquatic and terrestrial ecosystems is crucial to the
productivity and function of both environments. As such, changes to one ecosystem should have
corresponding impacts on the other. In this project, I attempted to characterize the degree to
which the variation in aquatic ecosystems affects terrestrial habitats.
For this study, I analyzed riparian areas of streams recently re-colonized by beavers
(Castor Canadensis). Historically, beavers were extremely prevalent throughout New England
until the fur trade decimated their populations in the 18th and 19th centuries (R. J. Naiman
1986). However, a 1996 ban on leg-hold traps in Massachusetts has allowed populations to
recover, and beavers have recolonized streams where they had been absent for at least 50 years
(2014). As a keystone species, beavers dramatically alter aquatic systems. Dam construction
backs up stream water, which changes physical parameters such as water flow, sediment
deposition, and nutrient cycling, and also biological production and trophic interactions (R. J.
Naiman 1986). By building lodges and dams in several locations along a stream, beavers also
create a heterogeneous aquatic environment where flow, depth, and production vary with
proximity to beaver impoundments. The variation in stream structure caused by beavers is the
perfect opportunity to study how terrestrial systems are affected by aquatic change.
Beavers and Trees
The most obvious way that beavers influence terrestrial environments is how they affect
tree communities surrounding their ponds. Beavers often eat the cambium, or soft inner bark, of
small saplings 3-5 cm in diameter (Fryxell and Doucet 1993). They also use larger trees, up to
50 cm in diameter, for building dams and lodges. Beavers gnaw and fell trees, which they drag
to the proper location and patch with mud to create a watertight seal (Denney, Game et al. 1952).
Though beavers use what trees are available to them for food and building materials, they prefer
certain tree species over others (Denney, Game et al. 1952). Especially for feeding, they
typically choose soft wooded trees such as birch, alder, or willow. Harder woods like maple and
oak are avoided because they are more difficult to digest (Fryxell and Doucet 1993). Some trees
also have anti-herbivory compounds that deter beavers, including conifers and red maples
(Denney, Game et al. 1952, Doucet, Walton et al. 1994).
Based on this information, I expected to see changes in tree size and species distribution
next to beaver ponds and near streams with no beavers. Specifically, since beavers prefer to eat
small saplings, I expected to find fewer small trees next to beaver ponds. Also, since beavers do
not like to eat red maple trees, I predicted that a higher proportion of trees near beaver ponds
would be red maples.
Beavers and Forest Food Webs
The second part of this study focused on the less visible, but still important, impact of
beavers terrestrial food webs. By altering their water-based habitat, beavers change aquatic
production (R. J. Naiman 1986). As mentioned previously, inputs from aquatic systems often
move on to land via emergent aquatic insects like caddisflies, mayflies, and mosquitoes (Mary E.
Power 2004). These organisms start their lives in water, where they depend on aquatic
production pathways. At maturity, they fly out of water into terrestrial habitats and provide food
for terrestrial consumers. Prior studies have suggested that the presence of these emergent
aquatic insects alters the feeding behavior and habits of insect predators such as spiders (J.L.
Sabo 2002, Chika Kato 2004, Mary E. Power 2004). For example, filmy dome spiders in
California change their web shape and level of activity depending on how close they are to water
(Mary E. Power 2004).
I suspected that by impacting the diet and behavior of secondary consumers at higher
trophic levels, these aquatic inputs would affect the entire terrestrial food web. As such, I tried
to identify which terrestrial trophic levels relied on aquatic inputs, and how important those
inputs were for sustaining the overall food web. Additionally, since the movement of aquatic
inputs is mainly dependent on the movement of aquatic insects (Mary E. Power 2004), I
expected to see a decrease in reliance on aquatic inputs farther away from the water.
Furthermore, since beaver ponds alter aquatic systems by slowing down water movement, I
thought that aquatic production would be higher in the ponds, meaning reliance on aquatic
production would be greater in terrestrial environments around the beaver ponds.
Methods
Study Site:
I studied beaver activity in Cart Creek, a small freshwater stream located on the Martin
H. Burns Wildlife Management Area in Newburyport, Massachusetts. Cart Creek is a small
freshwater tributary to the Parker River, which eventually flows into the Plum Island Estuary and
out to sea (Figure 1). The creek was recolonized by beavers sometime in the past 5-10 years. I
sampled in the forest next to two pond sites with heavy beaver activity and two stream sites with
no beavers (Figure 2). These four sites form a rough lateral gradient parallel to the stream,
which allowed me to compare how terrestrial impact varied with differences in stream structure.
At each site, I extended a 60m transect perpendicular from the creek, which allowed me to
analyze how the terrestrial habitat changed with distance from water.
Tree Methods
At each site, I created two 10x20 meter plots 0 to 10m and 10 to 20m from the water. I
also made one 20x20 meter plot 40 to 60m from the water. In these plots, I identified and found
the diameter at breast height (DBH) of all trees above 3 cm in diameter.
Food Web Methods
At each site, I threw 3 to 4 quarter meter quadrats in order to quantitatively collect insects
at six stepped distance intervals along the 60 meter gradient. I collected insects from 0-2m, 2-
5m, 5-10m, 10-20m, 20-40m, and 40-60m. I weighed and identified all insects and spiders, then
froze to kill. I dried the insects for 4 days in a 50oC oven and weighed them again to obtain dry
weights. Using allometric equations for the production to biomass ratios of invertebrates, I
calculated the total secondary production of each trophic level at each site (K. Banse 1980).
By creating rarefaction curves for biomass and number of individuals, I determined
which insects contributed most to the overall biomass and number of individuals at each site. I
chose to run stable isotopes on these insects, in addition to as many spiders as possible. All dried
samples were ground by hand with a glass stir rod for stable isotope analysis.
I ran carbon 13, nitrogen 15, and sulfur 34 isotopes on a total of 16 insect and spider
samples (Otter 2014). I also ran the same isotopes on three aquatic primary production sources
(semi-aquatic moss, benthic algae, and aquatic detritus) and two terrestrial primary production
sources (leaf litter and understory grasses). Using the computer program R with a SIAR
Bayesian mixing model, I determined the proportion of every insect or spider’s diet that relied on
each production source (Parnell 2010). With this dietary information, I used the production of
higher trophic levels to calculate the total terrestrial and aquatic primary production necessary to
sustain the food web, assuming a 10% transfer efficiency between each trophic level. The isotope
analysis also provided me with the percent nitrogen and carbon of all sources and consumers.
Results:
Tree Results:
In the 10x20 meter plots from 0-10m, I found that the stem density of trees with a
diameter less than 10cm was lower by the beaver ponds than by the streams, especially at pond
one. Trees in the 10x20 meter plots from 10 to 20m followed the same pattern, although less
pronounced. The 20x20 meter plots between 40 and 60m from the water did not show any
regular trend between the pond and stream sites (Figure 3).
Red maples made up a greater relative proportion of trees by the beaver ponds than by the
streams in the 10x20 meter plots from 0-10m (Figure 4). Again, this trend was stronger at pond
1 than pond 2. The pattern was not observed 10-20m or 40-60m away from the stream.
Food Web Results:
Based on the isotope analysis from R, I identified two major groups of organisms (Figure
5). At the bottom of the food web were detritivores, such as earthworms (Lumbricus terrestris),
cranefly larvae (Tipula spp.), and millipedes (Ophyiulus spp.). This group of organisms
processes dead plant and animal material on the forest floor, and most are on the first trophic
level. However, several earthworms are on trophic level two since they often eat parts of dead
animals.
The highest trophic level in this system were the insect predators. These organisms
occupied the third trophic level, and consisted of spiders including the six spotted fishing spider
(Dolomedes triton), Carolina Wolf spider (Hogna carolinensis), and the Hobo spider (Eratigena
agrestis). With trophic information from the detritivores and insect predators, I developed a
generalized food web for the system (Figure 6).
Using the isotope data and SIAR Bayesian mixing model, I analyzed the diets of
detritivores and insect predators. I found that on average, organisms in higher trophic levels
relied more heavily on aquatic production sources than lower trophic levels. The insect
predators based an average of 62% of their diets in the aquatic production pathway, while the
detritivores used aquatic production for 15% to 48% of their diets. Additionally, 60% of the total
amount of primary production required to support terrestrial food webs came from aquatic
production pathways (Figure 7).
I also looked for a spatial difference between food webs close to and far away from the
water, and for a difference by beaver ponds and by streams (Table 1). However, I did not see a
spatial pattern. Especially in the insect predator level, insects close to and far away from the
water had similar levels of aquatic input, and there was little difference in food webs by ponds or
by streams (Table 1). Within the insect predator trophic level, there were differences in reliance
on aquatic input based on species: For instance, fishing spiders relied more heavily on aquatic
production than wolf spiders.
From the isotope data, I was able to compare the nitrogen content of terrestrial and
aquatic producers. I found that aquatic producers contain almost twice the amount of nitrogen as
terrestrial producers (Figure 8).
Discussion:
Trees:
The patterns I observed in tree size and species distributions were what I expected. First
of all, there were fewer small trees by the beaver ponds than by the streams. I hypothesize this
occurred because beavers generally eat saplings less than 10cm in diameter, and by doing so
beaver activity decreased small tree density by the ponds. I did not observe this pattern 40 to
60m away from the ponds, most likely because beavers do not forage that far from a body of
water.
Moreover, there was a higher proportion of red maples by the ponds than by the streams.
I predict this is because beavers avoid red maple trees and eat around them, thus reducing the
number of other tree species and therefore increasing the relative proportion of red maple trees
by the ponds compared to the streams. I did not see this trend in plots far away from the pond.
This again indicates that beavers do not have a selective influence on trees past 20m from the
pond, probably because they do not forage that far into the forest.
Both the trend in tree size and species distribution between the ponds and streams were
stronger at pond 1: there were fewer small trees and a higher proportion of red maples at pond 1
than pond 2. I hypothesize this occurred because pond 1 is a more well-established beaver pond.
The water at pond 1 covers about 2 times more surface area than pond 2; trees in the middle of
pond 1 are dead, while those at pond 2 are in the process of dying; and the dam is at pond 1 is
about twice as tall as the dam at pond 2. These clues indicate that of the two beaver dams, pond
1 has been maintained for a longer period of time. I predict the stronger trend in tree size and
species distribution in pond 1 compared to pond 2 occurred because beavers have been active in
that area longer, and have thus had a greater impact on the tree community.
Food Webs:
The isotope data demonstrated that organisms in highest trophic level were more reliant
on aquatic production than terrestrial production. Since these organisms were insect predators,
they must have been feeding on insects that relied on aquatic production themselves. This
intermediate link in the food chain most likely came from emergent aquatic insects. Because of
the time of year, I did not catch any of these insects and could not analyze them directly, but I
can infer their presence based on the diet of the insect predators.
Differences in how much each trophic level relies on aquatic inputs is most likely
attributed to the different methods of food acquisition. Deritivores process dead material, the
majority of which is terrestrial plant matter like leaf litter. A small portion of this dead material
might consist of dead aquatic insects or other life forms with an aquatic signal, therefore
contributing slightly to the diet of detritivores, but most of their food is from land based
environments. Insect predators, on the other hand, eat other insects. If emergent aquatic insects
are available as prey, these spiders will subsist on them, thus acquiring their aquatic signal.
Additionally, it is possible that the slight aquatic signal in detritivores could magnify up the food
chain, and if insect predators directly eat many detritivores, they will obtain a greater aquatic
signal than any one detritivore itself.
I speculate that the strong overall reliance of the terrestrial food web on the aquatic
production pathway is connected to the nitrogen content of terrestrial and aquatic producers. On
average, aquatic primary producers have almost twice as much nitrogen as terrestrial primary
producers. Assuming nitrogen is the limiting nutrient in both the aquatic and terrestrial systems,
this indicates that aquatic food is more nutritious than terrestrial food. I hypothesize that because
of this, aquatic production can support more primary consumers than the equivalent amount of
terrestrial production. Primary consumers directly support insect predators, so a higher relative
abundance of aquatic compared to terrestrial primary consumers should mean they make up
more of the diet of insect predators. As such, the diets of insect predators are more strongly
based in the aquatic production pathway, and aquatic production is more important than
terrestrial production in supporting the food web overall.
The differences in reliance on aquatic production between different spider species makes
sense based on the life history of those species. For instance, the six-spotted fishing spider
generally relies more heavily on aquatic production than the wolf spider, which is intuitive based
on the name of the species. However, when looking at all species of spiders at all distances at
each site, there is a lack of spatial difference in how much they rely on aquatic production.
Though the fishing spiders were found close to the water and based the majority of their diets off
the aquatic pathway, a hobo spider found 60m away from the stream also depended on aquatic
production for over 60% of its diet. Contrary to my hypothesis, this indicates that the reliance on
aquatic input does not decrease with distance from the water. This raises a new question: how
did aquatic production move from the stream 60m into the forest?
In analyzing potential vectors for the movement of aquatic production, I again turned to
the life history of the terrestrial spiders. None of the spiders I analyzed build webs. Instead, they
hunt on the ground, either lying in ambush for other insects or actively seeking out their prey. In
either case, these spiders are highly mobile. As such, they may act as more of a vector for
moving aquatic inputs on to land than we might expect. That being said, the most important and
effective vector for moving aquatic input from the water into the forest might be emergent
aquatic insects, who can cover more distance when they fly (Mary E. Power 2004).
I did not find a difference in aquatic inputs between beaver ponds and streams. I believe
this might be related to the mobility of both insect predators and emergent aquatic insects. Even
if aquatic production and insect emergence is different between ponds and streams, if the insects
can move up to 60 meters into the forest, they can certainly move just as far downstream. This
would muddle the differences between pond or stream terrestrial food webs. The season during
which I sampled prevented me from directly analyzing this hypothesis, so I would like to
perform the same procedure again in the summer, when there are more aquatic insects present.
Conclusion:
I found that there are fewer small trees and more red maples next to beaver ponds than by
streams, as expected based on usual beaver activity. I also found that this pattern was not present
40-60m away from the stream, suggesting that beavers do not forage that far away from the
water. My food web data indicates that aquatic input is more important to higher terrestrial
trophic levels than lower ones, which I attributed to the presence of emergent aquatic insects and
trophic magnification. Additionally, aquatic production makes up the majority of total
production required to support terrestrial food webs, which I predict is due to the high nutritional
value of aquatic primary producer sources and their ability to support more aquatic primary
consumers. Furthermore, aquatic production moves at least 60 meters into the forest, probably
via a combination of movement by insect predators and emergent aquatic insects.
This study demonstrates how variation in aquatic ecosystems affects the surrounding
forest. Primarily aquatic mammals such as beavers utilize and manipulate the terrestrial
environment surrounding their ponds, changing the nature and composition of the tree
community. Additionally, production from the aquatic system is a major component of the total
production required to support terrestrial food webs, and makes up over 60% of the diets of
higher level organisms. Furthermore, this aquatic input extends at least 60 meters into the forest,
as demonstrated by the diets of insect predators found at that location. No variation was
observed between forest food webs by the beaver ponds or by the streams, possibly because of
the mobility of both insect predators and emergent aquatic insects. Overall, this analysis is a
good demonstration of how aquatic and terrestrial systems are connected, both in tree
communities and animal food webs.
A future study will further probe the lack of variation in terrestrial food webs between
ponds and streams. Interesting information about changes in aquatic production and movement
onto land might be found in emergent aquatic insects, which need to be sampled in late summer
when the insects mature. Emergent aquatic insects are an important vector for the exchange of
resources across the aquatic and terrestrial boundary, and most likely contribute to the lack of
difference in terrestrial food webs between pond and stream sites, and the movement of aquatic
production 60 meters into the forest. This question continues to explore the link between aquatic
and terrestrial systems, and comes closer to the goal of characterizing the reciprocal influences of
these two ecosystems.
Acknowledgements:
I would like to thank Jimmy Nelson for his support, guidance, and instruction in isotope
analysis. I appreciate him hiking around the woods even though it was cold. I want to also thank
the TIDE project for the use of their facilities and resources. Thanks to Rich McHorney,
Marshall Otter, and John Schade for assisting with portions of this project, and to Fiona Jevon,
Nick Barrett, and Tyler Messerschmidt for answering questions and assisting with lab analysis.
Lastly, I would like to thank the Beaver Group, Julia McMahon, Delaney Gibbs, and Mairi
Possion, for an educational and productive collaboration.
References:
(2014). Managing Beavers. E. a. E. Affairs, Commonwealth of Massachusetts.
Chika Kato, T. I., Eitaro Wada (2004). "Prey use by web-building spiders: stable isotope analyses of
trophic flow at a forest-stream ecotone." Ecological Research 19(6): 633-643.
Denney, R. N., C. Game and F. Dept (1952). A Summary of North American Beaver Management,
1946-1948, Colorado Game and Fish Department.
Doucet, C. M., R. A. Walton and J. M. Fryxell (1994). "Perceptual cues used by beavers foraging on
woody plants." Animal Behaviour 47(6): 1482-1484.
Fryxell, J. M. and C. M. Doucet (1993). "Diet Choice and the Funcional Response of Beavers." Ecology
74(5): 1298-1306.
Gary A. Polis, W. B. A. a. R. D. H. (1997). "Toward an Integration of Landscape and Food Web
Ecology: The Dynamics of Spatially Subsidized Food Webs." Annual Review of Ecology and
Systematics, 28: 289-316.
J.L. Sabo, M. E. P. (2002). "River-Watershed Exchange: Effects of riverine subsidies on riparian lizards
and their terrestrial prey." Ecology 83(7): 1860-1869.
K. Banse, S. M. (1980). "Adult body mass and annual production/biomass relationships of field
populations." Ecological Monographs 50(3): 355-379.
M. Ben-David, T. A. H. a. D. M. S. (1998). "Fertilization of Terrestrial Vegetation by Spawning Pacific
Salmon: The Role of Flooding and Predator Activity." Oikos 83(1): 47-55.
Mary E. Power, G. P., Gary R. Huxel (2004). Food Webs at the Landscape Level. Chicago and London,
University of Chicago Press.
Otter, M. (2014). Effect of Beaver Dams on Forest Communities. MBL STABLE ISOTOPE
LABORATORY, The Ecosytems Center - Marine Biological Laboratory.
Parnell, A. C., Inger R., Bearhop, S. & Jackson, A.L (2010). "Source partioning using stable isotopes:
coping with too much variation." PLoS ONE 5(3): e9672.
R. J. Naiman, J. M. M., J. E. Hobbie. (1986). "Ecosystem Alteration of Boreal Forest Streams by Beaver
(Castor Canadensis)." Ecology 67(5): 1254-1269.
Appendix Contents:
Figure 1: Map of Cart Creek watershed
Figure 2: Map of study sites along Cart Creek
Figure 3: Tree Stem density, <10cm DBH
Figure 4: Tree Species Distribution, 0-10m
Figure 5: Secondary Consumer R Data, 15N and 13C
Figure 6: Terrestrial Food Web
Figure 7: Percentages of aquatic inputs to each trophic level
Figure 8: Terrestrial and Aquatic primary producer nitrogen content
Table 1: Percent of aquatic/terrestrial inputs to diet of each trophic level in each location
Figure 1: Map of Cart Creek Watershed, Parker River, and Plum Island Estuary.
Figure 2: Map of Cart Creek area of study, with Pond and Stream study sites.
Figure 3: Stem Density of trees less than 10cm DBH in 10x20m plots from 0-10m and 10-20m
from water, and 20x20m plot from 40-60m from water.
Figure 4: Species Distribution of trees in 10x20m plot from 0-10m from water, % abundance in
terms of number of individuals. Red Maples are the red bar at the bottom of the graph.
Figure 5: Isotope byplot of 15N and 13C, generated in R. Note that the information in this
byplot also includes the 34S isotope, which makes up a third axis. For ease of viewing, only
nitrogen and carbon isotope data are shown here.
Figure 6: Generalized food web of the terrestrial ecosystem
Figure 7: Percent of aquatic and terrestrial inputs to the diet of each trophic level. Percentages
are based on dietary information from isotope analysis in R, and are taken as an average of all
individuals from each trophic level.
Figure 8: Percent Nitrogen content of terrestrial and aquatic producers, taken as an average of
terrestrial and aquatic sources used for isotopic analysis.
Table 1: Percent of diet based in terrestrial or aquatic production in each trophic level at each
location. Some sites do not include all trophic levels, because insects from those levels were not
caught at that location.
Site Interval
Trophic
Level
% Terrestrial
Production
% Aquatic
production
Pond 1 0 to 5 1 0.85 0.15
2 0.33 0.67
3 0.38 0.62
5 to 20 1 0.87 0.13
2 0.86 0.14
3 0.39 0.61
20 to 60 1 0.80 0.20
Stream 1 0 to 5 1 0.86 0.14
2 0.41 0.59
3 0.37 0.63
5 to 20 1 0.86 0.14
2 0.82 0.18
20 to 60 1 0.85 0.15
3 0.37 0.63
Pond 2 0 to 5 1 0.90 0.10
2 0.33 0.67
3 0.39 0.61
5 to 20 1 0.87 0.13
2 0.33 0.67
3 0.41 0.59
20 to 60 1 0.92 0.08
2 0.33 0.67
3 0.39 0.61
Stream 2 0 to 5 1 0.86 0.14
2 0.92 0.08
3 0.43 0.57
5 to 20 1 0.86 0.14
2 0.49 0.51
3 0.38 0.62
20 to 60 2 0.63 0.37
3 0.48 0.52