Effects of habitat complexity on intraguild predation and
cannibalism in an assemblage of size-structured predators
Shannon K. Carter
Austin, Texas
Baylor University
Waco, Texas
Class of 2014
Research Experience for Undergraduates
Research Project Proposal
Mentor: Dr. Patrick Crumrine
University of Virginia
Blandy Experimental Farm
Summer 2013
SUMMARY
Predator-prey interactions are among the most widely studied relationships in community
ecology, but traditional understanding of energy flow represented in food webs fails to fully
explain the complex interactions that take place at the community level. Intraguild predation and
cannibalism are two common interactions with substantial effects on community dynamics that
are poorly understood and not accounted for in most food web studies. To add even more
complexity, both of these interactions are influenced by size-structure and physical habitat
composition. Feeding choices of generalist predators are determined more by size of prey than
by species identity and densely vegetated habitats provide sites of refuge for prey, thus
diminishing the hunting success of these predators. My experiment will examine the effects of
habitat complexity on size-structured predator-prey interactions between two size classes of
larval aquatic beetles (Cybister fimbriolatus) and larval dragonflies (Anax junius), both of which
act as top predators in fishless ponds. These four categories of organisms will be crossed
pairwise and resulting predaceous interactions will be measured at high and low habitat
complexity. I expect that instances of IGP and cannibalism will decrease with the introduction
of habitat complexity. Results of this experiment will further elucidate complex predator-prey
interactions in aquatic ecosystems. The project will be shared with the general public at Blandy
Experimental Farm and Baylor University.
INTRODUCTION AND CONCEPTUAL BACKGROUND
Predator-prey interactions in aquatic communities can be very complex because
populations are often comprised of individuals that differ in body size. However, food web
diagrams often simplify this complexity by ignoring size structure and considering each species
as a single entity (Miller and Rudolf 2011). Intraguild predation (IGP) and cannibalism are two
common size-structured interactions which influence energy flow through a system, but the
impact of these interactions on community structure is underappreciated (Fox 1975a, Polis et al.
1989). Habitat complexity can also vary from one system to another and may influence the
strength of these interactions.
Theory predicts that IGP should be rare, but IGP is common in aquatic, marine, and
terrestrial communities (Polis et al. 1989, Holt and Polis 1997, Crumrine et al. 2008, Arim and
Marquet 2004). IGP occurs when two predators competing for the same resource prey on one
another (Polis et al. 1989). The interaction can be either symmetrical, signifying that each
predator preys on the other (though not necessarily in equal amounts) or asymmetrical, meaning
one predator always preys on the other (Polis et al. 1989). Asymmetric IGP is more common in
nature; here, the two predaceous species are distinguished as top predator and intermediate
predator (Polis et al. 1989). Theoretical models suggest that species engaged in IGP should only
coexist when three conditions are met: the intermediate predator is more successful at exploiting
the shared resource than the top predator, the top predator acquires great energetic benefits from
the intermediate predator, and the shared prey is available at intermediate quantity (Holt & Polis
1997). Despite these limitations, IGP is a common interaction in many communities (Arim and
Marquet 2004). In order to explain the commonality of IGP despite restrictions outlined by the
mathematical models, it is necessary to consider other contributing variables such as sizestructure within a population.
Most IGP interactions result from larger individuals consuming smaller heterospecifics
(Crumrine et al. 2008). Wissinger (1992) examined size-structured IGP in an assemblage of
larval odonates and found that even as seasonal progression reduces competitive overlap,
opportunity for IGP increases because of increasing body size difference between species. More
recent studies show that size-structured predator-prey interactions are related to the size range
among predators, rather than mean size of these predators (Rudolf 2012). Empirical evidence
supporting the hypothesis that greater difference in body size between predators leads to IGP is
also available for marine (Armsby & Tisch 2006) and terrestrial (Okuyama 2007) systems. In
addition to influencing IGP, size-structure at the population level can promote cannibalism.
Cannibalism adds complexity to community interactions and could contribute to coexistence
between top predators and intermediate predators more generally than simple mathematical
models predict (Rudolf 2007).
Cannibalism occurs frequently among aquatic organisms and is most common in sizestructured populations (Fox 1975a). Fox (1975b) found that in the freshwater predator
Notonecta hofmanni, cannibalism occurred when different-sized individuals were in close
proximity, even if another food source was available. Cannibalistic behavior can be a limiting
factor on predator populations that may facilitate the survival of other species (Fox 1975b).
Size-selective generalist predators that feed on anything smaller than themselves, regardless of
species identity, are among the most likely cannibals in aquatic systems (Yee 2010). Larval
odonates are voracious generalist predators that commonly live in resource-limited habitats
(Corbet 1999). These factors contribute to the indiscriminate consumption of conspecifics by
larval odonates, despite the potential maladaptive implications of cannibalism (Crumrine et al.
2008). Cannibalism in top predators allows top predators and intermediate predators to coexist
even in cases where the top predator exploits the shared resource more successfully because it
redirects a portion of the top predator’s energy needs away from the intermediate predator
(Rudolf 2007). Effects of prey cannibalism on community dynamics, especially related to IGP in
size-structured assemblages, have not been adequately explained by empirical data. One study
suggests that cannibalistic behavior in the prey can have a significant effect on predator-prey
interactions, but the underlying mechanisms are poorly understood (Rudolf 2008).
Existing empirical studies have only begun to address the influence that habitat
complexity could have on the coexistence of species in IGP systems. A short-term study by
Janssen et al. (2007) demonstrated that habitat complexity could promote the persistence of IGP,
but size-structure was not considered. Size-structure is important to take into account because it
is so influential in the foraging behavior of these voracious predators. Several studies (Yee
2010, Persson and Eklov 1995, Finke and Denno 2006, Swisher et al. 1998) have found that
habitat complexity offers refuge for prey and thus may diminish predation rates in an IGP
system. Reichstein et al. (2013) suggests that when the intermediate predator was able to take
refuge in vegetation, the top predator could not attain adequate nutrition from hunting and died.
Additionally, habitat complexity may partially counteract effects of size-structured interactions
in communities, creating conditions which may promote IGP (Reichstein et al. 2013). However,
robust empirical data addressing the effects of habitat composition on IGP and cannibalism is
rather limited (Reichstein et al. 2013).
A predator’s behavior influences its response to habitat complexity, and these responses
may change the dynamics of community interactions. Hunting strategies of aquatic organisms
span a continuum from sit-and-wait to active predators (Crumrine et al. 2008). Sit-and-wait
predators take refuge and ambush nearby insects. Habitat complexity could facilitate this
foraging strategy, thus increasing the predation rates of sit-and-wait predators (Delclos and
Rudolf 2011). Active predators have more opportunities to feed, but also may be more
vulnerable to predation themselves (Crumrine et al. 2008). Studies have shown that predation
rates of these predators decrease with habitat complexity because their prey take refuge
(Reichstein et al. 2013, Persson and Eklov 1995), but Delclos and Rudolf (2011) observed no
effect on predation rates. If rates of cannibalism and IGP are affected by feeding strategy, which
is highly dependent on habitat complexity, it would logically follow that habitat complexity is an
important variable to consider when studying these complex predator-prey interactions.
Cybister fimbriolatus and Anax junius are two important predators in fishless aquatic
communities. C. fimbriolatus is a highly voracious dytiscid commonly used in predator-prey
studies with 3 developmental instars proceeding maturation. C. fimbriolatus is known as an
active hunter (Norwood 2012), but can also feed passively, via the sit-and-wait strategy (Yee
2010). A. junius is the common green darner dragonfly with 13 developmental instars. Because
they exhibit many instars, A. junius are common subjects in size-structure research (Borror &
White 1970). A. junius is predatory but utilizes more passive feeding strategies than C.
fimbriolatus, which is a predator of A. junius (Oquendo 2011, Grandinetti 2012).
I propose a study to examine the effects of habitat complexity on intraguild predation and
cannibalism in an assemblage of size-structured top predators. The top predator of this system
will be C. fimbriolatus and the intermediate predator will be A. junius. This will expand on past
studies in this system with related objectives and methods. Kawecki (2010) found that prey
mortality was directly related to top predator survival and that cannibalistic and IGP interactions
increased prey survival. Oquendo’s (2011) study suggests that size structure of the prey is
influenced by the type and size of top predator in the system. Finally, Grandinetti (2012) found
cannibalism to be much more frequent than IGP among these predators (2012).
My proposed study will address the following questions: (1) Does habitat complexity
affect the frequency of cannibalism and IGP among A. junius and C. fimbriolatus? (1a) Do A.
junius and C. fimbriolatus respond similarly to changes in habitat complexity? (1b) Will the
introduction of habitat complexity shift IGP interactions between A. junius and C. fimbriolatus
from highly asymmetrical to more symmetrical? In other words, will A. junius become a more
effective predator on C. fimbriolatus? (2) Could habitat complexity facilitate coexistence
between A. junius and C. fimbriolatus?
Hypotheses for the questions raised are as follows: (1) Introduction of habitat complexity
will reduce occurrences of cannibalism and IGP because prey will be able to seek refuge. (1a) A.
junius’s foraging strategy will be more affected by the introduction of habitat complexity
because of its sit-and-wait feeding strategy. C. fimbriolatus is a more active species so the
introduction of refuge will not significantly alter its behavior. (1b) It is predicted that A. junius
will be a more effective predator on C. fimbriolatus in a more complex habitat because
vegetation promotes the foraging strategy of A. junius. (2) While this short-term experiment will
reveal some principle effects of habitat complexity on community structure, long-term studies
are necessary to prove coexistence.
PROJECT DESCRIPTION
The experiment will use larvae of the dytiscid water beetle C. fimbriolatus, the green darner
dragonfly A. junius, and the blue dasher dragonfly Pachydiplax longipennis. All specimens will
be collected from fishless ponds in and around Blandy Experimental Farm. All instars of C.
fimbriolatus will be collected from ponds and individuals in first instar will be reared in the lab
until they molt and are large enough to use in the experiment. A wide range of instars of A.
junius will be collected for use in the experiment. If P. longipennis is not abundant in sampled
ponds, another prey species will be selected based on availability.
The experiment will consist of a 2 x 2 factorial treatment design with two size classes of C.
fimbriolatus (3rd instar and 2nd instar) crossed with two size classes of Anax junius (12th instar
and 10th instar). To examine size-structured cannibalism I will also include treatments with the
two size classes of each predator resulting is a total of six predator combinations (Table 1). Ten
P. longipennis will be present in every treatment as a source of shared prey. Figure 1 illustrates
the projected predator-prey interactions between these organisms. Each predator combination
will be tested at low and high habitat complexity, creating 12 treatments. Habitat complexity
will be manipulated by changing the density of submerged vegetation. Anchored segments of
nylon rope will represent vegetation. Habitat complexity treatments will be 52 stems/m2 and 279
stems/m2. These figures are based on the range of abundance of pondweed at shallow depths
(Sheldon 1977). To improve statistical power while maintaining manageable workload, each
treatment will be replicated two times across three temporal blocks for a total of six replicates
per treatment.
Fig.1: Arrows indicate predicted predation in the food web. This model represents many predatorprey interactions including intragulid predation and cannibalism. Red arrows illustrate the six
groupings used in the experiment.
Treatment
High Habitat
Complexity
Low Habitat
Complexity
Grouping 1
Large C.
fimbriolatus
Large
A. junius
Large C.
fimbriolatus
Large
A. junius
Grouping 2
Large C.
fimbriolatus
Small
A. junius
Large C.
fimbriolatus
Small
A. junius
Grouping 3
Large C.
fimbriolatus
Small C.
fimbriolatus
Large C.
fimbriolatus
Small C.
fimbriolatus
Grouping 4
Large
A. junius
Small
A. junius
Large
A. junius
Small
A. junius
Grouping 5
Large
A. junius
Small C.
fimbriolatus
Large
A. junius
Small C.
fimbriolatus
Table 1: Treatments designed to elucidate effects of habitat complexity on intraguild
predation and cannibalism in C. fimbriolatus and A. junius.
Grouping 6
Small C.
fimbriolatus
Small
A. junius
Small C.
fimbriolatus
Small
A. junius
The experiment will be conducted in plastic mesocosms (22 cm x 15 cm x 7 cm). All
mesocosms will be maintained in the Oak Grove at Blandy Experimental Farm and covered with
window screen mesh to prevent wildlife interference and colonization by amphibians and aquatic
insects. Mesocosms will be filled with filtered pond water inoculated with phytoplankton and
zooplankton. Before being placed in the mesocosm, the instar of each larva will be recorded. As
an additional measure of size, headwidth of each A. junius and C. fimbriolatus will be measured
with NIH ImageJ. One individual of the appropriate predator type outlined in Table 1 will be
added to the mesocosms at the start of the trial. These predator and prey densities reflect natural
conditions in fishless ponds (Corbett 1999; Crumrine personal observation). Each trial will last
for three days, and will be monitored twice daily. At the end of each trial, surviving specimens
will be removed and size measurements will be taken again.
During each observation session, predator-prey interactions will be apparent by residual
carcasses of consumed prey. C. fimbriolatus and A. junius utilize different feeding strategies
which leave prey remains in a condition which reveals the identity of the predator. An empty but
complete exoskeleton indicates that C. fimbriolatus was the predator while a torn exoskeleton
indicates that A. junius was the predator. Using this knowledge, the predator of each prey will be
determined. Prey survival across all predator treatments will be analyzed using 2-factor
ANOVA. The proportion of replicates that result in cannibalism or IGP will be determined and
analyzed using G-tests.
It is hypothesized that large C. fimbriolatus will be the most voracious predator and will
consume all other predator and prey present. As indicated by Figure 1, large C. fimbriolatus is
predicted to be the top predator in the system. I expect large A. junius will be the second most
effective predator, feeding on everything except large C. fimbriolatus In replicates with
conspecifics of different sizes, I expect the larger specimen to cannibalize the smaller. In the
final grouping, it is hypothesized that the more voracious small C. fimbriolatus will prey on
small A. junius. However, this interaction has less experimental evidence and could be more
symmetric than hypothesized. These predicted interactions illustrate IGP within the system.
IGP will occur between C. fimbriolatus and A. junius, with C. fimbriolatus as the top predator,
competing with its prey A. junius for the shared resource. It is also predicted that IGP and
cannibalism will be less prevalent as habitat complexity increases because prey will be able to
take refuge in vegetation, decreasing the frequency of encounters between predators and prey,
and thus decreasing frequency of predaceous interactions. Thirdly, I expect that predaceous
interactions by C. fimbriolatus will be less affected by habitat complexity than those by A. junius
because C. fimbriolatus is a much more active predator. Lastly, since A. junius’s foraging
strategy is promoted by refuge sites, it is predicted that it may be a more successful predator on
C. fimbriolatus when habitat complexity is introduced.
SIGNIFICANCE AND BROADER IMPACTS
This experiment will elucidate a portion of the complex predator-prey interactions that
occur within aquatic ecosystems. It will examine the effects of size structure on IGP and
cannibalism and demonstrate how these interactions are affected by habitat complexity. The
results will show the complexity of aquatic community interactions and the potential
vulnerability of these aquatic ecosystems to changes in habitat structure. Knowledge of these
interactions can be applied to terrestrial ecosystems and may be particularly useful in developing
effective biocontrol strategies in agriculture or to conserve wildlife that may be susceptible to
IGP. The research will be performed at the Blandy Experimental Farm in the State Arboretum of
Virginia so that the design and results of the experiment can be shared with interested visitors.
Results will also be disseminated to K-12th students at a summer camp hosted at Blandy
Experimental Farm. Sharing my project with these budding learners will expose them to the
scientific process and give them a brief introduction to community interactions. At the end of
the summer, results will be shared in a formal presentation at a public conference in Berryville,
Virginia. Furthermore, I will bring this project back to Baylor University by presenting it at the
Undergraduate Research and Scholarly Achievement Scholar’s Week and sharing it with
Baylor’s chapter of BBB. Hopefully, I will also present the project at the annual BBB Biological
Honor Society regional conference in Oklahoma. Finally, if significant results are obtained, the
research will be published so that it is accessible to the greater scientific community.
PROJECTED TIMETABLE

June 3rd- June 24th—Find local fishless ponds rich in desired larvae. Most ponds will be
within 30 miles of Blandy, but it may be necessary to go farther if larvae are not
abundant. Collect study organisms, with back-up specimens in case of death, emergence,
or growth outside of desired instar. Obtain all equipment and begin to prepare
experimental set-up for trials.

June 24th- July 21st—Trials begin in five day increments, repeating trials in three
temporal blocks (2 replications per block) through the following 3-5 weeks. A fourth
temporal block will be added if time permits. The following outlines daily schedule
during data collection weeks:
o Monday—collect
o Tuesday—collect/set up
o Wednesday— begin observation
o Thursday—continue observation
o Friday—complete observation, tear down

July 21st—Data collection complete, begin writing and preparing presentation

July 30th – Project Presentation
MATERIALS AND BUDGET
Item
Waders
36 Tablecraft Tote Boxes
Nets and Net Bags
Clear shoebox containers
Nylon Rope
Mesh screen (1 100-ft roll)
Staple Gun
TOTAL
Price
$172.12
$295.85
$59.51
$27.04
$554.52
Vendor
LL Bean
Food Service Direct
Provided
Provided
Provided
Home Depot
Lowe's
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