ASSESSMENT OF NICHE PARTITIONING IN CO-OCCURRING SILVERSIDES FROM
THE ALBEMARLE AND PAMLICO SOUNDS OF NORTH CAROLINA.
by
W. Robert Cope
A Senior Honors Project Presented to the
Honors College
East Carolina University
In Partial Fulfillment of the
Requirements for
Graduation with Honors
by
W. Robert Cope
Greenville, NC
May 2015
Approved by:
Dr. Joseph J. Luczkovich
Department of Biology, Thomas Harriot College of Arts and Sciences
Abstract
The niche partitioning hypothesis predicts that two species cannot compete for the same limited
resources and that one species should differentiate (genetically, morphologically or
behaviorally), so that the two species are filling separate niches. To test this hypothesis, we
collected two co-occurring silverside species, the Atlantic silverside Menidia menidia and Inland
silverside Menidia beryllina, from the Albemarle and Pamlico Sounds of North Carolina.
Approximately 1,100 fish were collected through multiple beach seining trips from 20 sites in
the Pamlico Sound and 8 sites from the Albemarle Sound throughout the months of August to
November of 2013. The fish from each site were positively identified as being Menidia menidia
or Menidia beryllina and their total length was measured (to the nearest mm). Length frequency
analysis showed that M. menidia were larger at any given time than M. beryllina in both bodies
of water. Food habit analysis showed that silversides in the Albemarle Sound were preying upon
dipteran insects and chironomid larvae while silversides in the Pamlico Sound were preying
upon copepods. Niche overlap analysis was run using a Pianka Index in EcosimR. Index
analysis showed high overlap values between M. menidia and M. beryllina in both the Albemarle
and Pamlico Sounds. These data do not support the niche partitioning hypothesis. It is possible
that when food availability changes and becomes limited, overlap in diet may change as well,
increasing food competition. More data (including food availability data and diet analyses
during additional time periods or years) are needed to confirm results.
Introduction
The Competitive Exclusion Principle is the ecological axiom that when two species
compete for the same critical resources within an environment, one of them will eventually
outcompete and displace the other (Gause 1932) (Hardin 1960). This means that species which
are competing for the same food or habitat should not be able to coexist in the long term, because
one of the species would outcompete the other species and drive them to extinction. However,
there is a phenomenon that allows closely related species to both coexist in the same areas. This
is known as niche partitioning. Niche partitioning occurs when closely related species are
occupying the same area, but are not competing for the same resources because each species is
actually fulfilling its own ecological niche, either by exploiting the resources in a different
manner or exploiting different resources altogether.
The Atlantic Silverside (Menidia menidia) and the inland silverside (Menidia beryllina)
are two closely related estuarine fish species that live along the Atlantic coast of North America.
These two species co-occur in both the Albemarle and Pamlico Sounds of North Carolina. In
these areas, the competitive exclusion principle dictates that these two closely related species
should be competing for the same resources and, therefore, unable to live together in these areas.
Because the two species are co-occurring, my hypothesis is that some form of niche partitioning
is taking place between the two species, allowing for them to survive together. The two most
common resources that are partitioned are habitat and food (Schoener 1974). In this study, I
conducted a diet analysis to determine if food partitioning is the reason for the ability of these
two species to co-occur. I predicted that different foods would be consumed by the two silverside
species, and little or no diet overlap would be observed for fish of the same size in the same
habitat.
Methods
Fish Sampling
During the months of September - November 2013, the North Carolina Division of
Marine Fisheries (NCDMF) sampled 25 sites in the Pamlico Sound and 8 sites in the Albemarle
Sound. Included in this sampling was beach seining. During the beach seine sampling the
NCDMF collected approximately 1100 silversides consisting of both M. menidia (Figure 1) and
M. beryllina (Figure 2). These fish were then transferred to East Carolina University and were
placed in 10% formalin solution.
Figure 1: Menidia menidia
Figure 2: Menidia beryllina
Length-Frequency Distribution
Proper identification of the silverside species began and each fish was positively
identified as being either M. menidia or M. beryllina and its total length was measured in mm.
This process was completed for approximately 400 samples. Once total lengths had been
recorded, a length frequency distribution was created to determine the dominant size classes of
the species that were sampled. Dominant size classes were created in 10-mm increments and
were broken down into four categories: Menidia menidia Pamlico, Menidia menidia Albemarle,
Menidia beryllina Pamlico, and Menidia beryllina Albemarle. Ten fish were selected from each
dominant size class from each category and composited into a single sample for diet analysis.
This gave a total sample size of 100 fish, 50 fish from the Pamlico Sound and 50 fish from the
Albemarle Sound.
Gut Content Analysis
To analyze the gut contents, I used the sieve-fractionation method of Carr and Adams
(1972). The entire alimentary canal from the esophagus to the anus was removed from each fish
and the contents from the alimentary canal were removed. Gut contents were stored in a vial of
10% formalin until ready for processing. Once a full sample of ten fish was ready for analysis,
the gut contents were passed through sequential sieves with mesh sizes 2000, 850, 425, 250, 125,
and 75 µm. Prey items trapped on each sieve fraction were then observed under a microscope
and prey items were categorized and counted (Figure 3). The contents from each sieve fraction
were then placed in a drying oven for 48 hours and once removed were weighed to the nearest
0.00001 g. Prey category counts for each fraction were divided by total dry weight from each
sieve fraction giving the each prey category’s biomass to the nearest 0.00001 g. Total biomass of
a prey type from all the sieve fractions was calculated by adding dry weights across sieve
fractions. Total percent dry mass for each prey category could then be calculated.
PREY DESCRIPTION
COPEPOD
DIPTERA
CHIRONOMID
UNKNOWN GUT
MATTER
CRUSTACEAN
PARTS
FROG EGGS
SAND
GREEN ALGAE
PICTURE
Figure 3: Prey Descriptions with Pictures
Statistical Analysis
Once all the data were collected, niche overlap was analyzed. The Pianka Index for Niche
Overlap (1973) was used to determine the overlap of diets between the two species. The index
describes niche overlap values using the formula:
𝑂12 = 𝑂21 =
∑𝑛𝑖=1 𝑃2𝑖 𝑃1𝑖
√∑𝑛𝑖=1( 𝑃2𝑖 2 )(𝑃1𝑖 2 )
Where P1i is the resource utilization of species one and P2i is the resources utilization of species
two. In this case, P1i refers the diet habits of one species of silverside and P2i refers to the diet
habits of the other silverside species. The index is symmetrical so that Overlap of species 1 and 2
(O12) is the same as the overlap as species 2 and 1 (O21). Index values range from zero to one,
where a value of zero indicates that there is no overlap between the two species and one indicates
that there is total overlap between the two species. I computed the Pianka Index in the software
EcosimR (Gotelli and Ellison 2013).
Results
Our length frequency distribution (Figure 4) showed that M. menidia had a larger size
distribution from each sound. Dominant size classes for M. menidia were 50-59, 60-69, and 7079 mm in the Albemarle Sound and 60-69, 70-79, and 80-89 mm in the Pamlico Sound. M.
beryllina had smaller size classes with dominant classes at 40-49 and 50-59 mm for both the
Albemarle Sound and Pamlico Sounds.
Figure 4: Length Frequency Distribution of Menidia menidia and Menidia beryllina in
Albemarle and Pamlico Sounds
Prey consumption percentages (Figure 5) varied between the two bodies of water, but
there was little variation between the diets from the different species in the same body of water.
In the Albemarle Sound, both M. menidia and M. beryllina preyed upon mostly adult stage
dipteran insects and chironomid insect larvae, with these two categories composing 61.9% of the
M. menidia’s diet and 47.7% of the M. beryllina’s diet. In the Pamlico Sound, M. menidia and M.
beryllina both preyed upon various species of copepods, composing 46.7% of the M. menidia
diet and 28.7% of the M. beryllina diet.
Figure 5: Stacked bar graph with prey compositions (% dry mass) for each size class sample
separated by species and sound
Niche overlap analysis in EcosimR showed that the two species had extremely high
overlap values. The Pianka Index (Table 1) showed that there was high overlap between the two
species in both the Albemarle and Pamlico Sounds. The observed Pianka value for the
Albemarle Sound was 0.88, showing high overlap between M. menidia and M. beryllina.
Pamlico Sound had similar results with a Pianka value of 0.77, also showing high overlap values.
One thousand simulations in EcosimR of data expected under the null hypothesis of no overlap
in diet (Gotelli and Ellison 2013) provided a Pianka value of 0.44, a much lower value than the
observed values. Pamlico Sound also had a much lower simulated null hypothesis Pianka value
of 0.27. These Pianka null values provide an estimate of the probability of obtaining an overlap
as large as, or larger than the observed value, if the value was drawn from the null distribution.
In the case of Albemarle Sound fish, the probability of getting an observed Pianka value as large
as or larger than the one I observed under the null hypothesis was P = 0.049, a difference that
would not be expected by chance. In the case of Pamlico Sound fish, the probability of getting
an observed Pianka value as large as or larger than the one I observed under the null hypothesis
was P = 0.061, also a result that would be unexpected due to chance alone.
Table 1: Pianka Niche Overlap Analysis for the M. menidia vs. M. beryllina in both sounds using
EcoSim R and 1000 null model simulations.
Niche Overlap Parameters
Albemarle fish
Pamlico Fish
Observed Pianka Index
0.88
0.77
Mean of simulated Null Model
index
0.44
0.27
P(Obs >=null)
0.049
0.061
Discussion
I found that length differences between M. menidia and M. beryllina could likely be
attributed to the growth patterns of the two species. Bengston (1984), who studied M. menidia
and M. beryllina for a full year in Rhode Island estuaries, found that M. menidia had an earlier
spawning season and faster growth rate than M. beryllina. Therefore, in Rhode Island at autumn
sampling times, such as was done in this study, same aged M. menidia were larger than M.
beryllina. Our results were consistent with the Bengston (1984) findings, in that in both bodies of
water, M. menidia were larger than M. beryllina at the time of sampling in fall 2013.
Differences in prey consumption by the two Menidia species between the two bodies of
water can most likely be explained by differences in the prey availability in the areas. Because
copepods and dipteran insects were present in the stomach contents of fishes in both the Pamlico
and Albemarle sounds, and if one may assume that the fish are eating whatever prey are most
abundant, then diptera are likely to be the most abundant prey in Albemarle Sound and copepods
the most abundant prey in Pamlico Sound. More sampling for prey availability is required to
understand if feeding preferences, rather than prey availability alone, could explain the
differences in prey consumed by each species in the two sounds.
The Pianka index analysis showed considerable overlap between the diets of M. menidia
and M. beryllina in each of the bodies of water. This finding does not support the niche
partitioning hypothesis. In such a case as this, M. menidia and M. beryllina should show little to
no overlap between prey items because competition for the food source would lead to one
species out competing the other. These high overlap values show that M. menidia and M.
beryllina are not directly competing for food resources, as both species are able to thrive in the
same areas and consume a mixture of the same prey items. However, size class comparisons
were not taken into account in the niche overlap analysis and if size was accounted for, there
may have been less niche overlap. Menidia beryllina in the Albemarle Sound showed a switch
from dipteran insects to copepods as size class increased; in Pamlico Sound, both M. menidia
and M. beryllina showed no ontogenetic change in feeding patterns throughout the size classes.
Even though such differences size classes may exist, the prey consumed by all sizes taken
together were similar enough to give extremely high Pianka overlap values. I believe that these
high overlap values are due to the availability of prey items in the area. Prey availability must
have high enough to support high feeding rates for both species. Bengston (1984) found similar
results when studying niche partitioning on M. menidia and M. beryllina in Rhode Island
estuaries. In times of high prey abundance, he observed extremely high overlap values, showing
common feeding between the two species. However, when prey abundance was limited, overlap
values decreased substantially and were almost non-existent. Given more sampling opportunities
or opportunities to sample the prey populations at periods of low prey abundance, I would expect
to show similar patterns of niche partitioning. Less-preferred prey types may have to be
consumed in times of foot shortage. Even though copepods and dipteran insects were being
consumed in large amounts in the fish examined here, different preferences may arise in times
when these prey items are not as abundant.
However, niche partitioning seems to be a commonly observed phenomenon among
different species of fish. Two studies have shown low niche dietary overlap: Dineen, Harrison,
and Giller (2007), looking at brown trout and Atlantic salmon (Salmonidae) and using a
proportional diet similarity index, showed low diet similarity between the species; and Fanelli, et
al. (2011), examining two species of Sparidae using Levin’s index of niche overlap, have shown
clear niche partitioning in these co-occurring species. In contrast, two studies, one by Henrique
et al. (2015), looking at two species of grunts (Haemulidae), and one by Wheeler and Allen
(2003), looking at two species of basses (Centrarchidae) have shown a lack of niche partitioning,
with high Pianka index values for dietary niche overlap. The reasons for these differences are
unclear, but may also have to do with periods of low prey availability leading to diet
specialization, or diet overlap in period of great prey abundance.
In studies that failed to demonstrate food resource partitioning, habitat use seems to be a
mitigating factor. Wheeler and Allen (2003) found a difference in habitat used, but few
differences in food habits, when comparing the Shoal Bass (Micropterus cataractae), which
lived in shoal microhabitats, and Largemouth Bass (Micropterus salmoides), which lived in pool
microhabitats in the Chipola River, Florida. Schoener (1974) found that habitat dimensions are
more important than food and temporal dimensions when analyzing niche partitioning in many
ecological communities. As noted earlier, Bengston (1984) found that habitat and temporal
partitioning were main the reasons that M. menidia and M. beryllina could coexist, as they high
high dietary overlap. In Rhode Island, Menidia menidia’s faster growth rate allowed them to
migrate to different habitat areas before M. beryllina could move there, which avoided
competition. This allowed for the two species to consume similar prey items, especially in times
of abundance. I think this habitat partitioning hypothesis should be investigated for the two
species of silversides in Albemarle and Pamlico Sounds. One way to do this would be to
characterize any microhabitat differences (different use of submerged aquatic vegetation, sandy
areas, marshes, and wooded swamp habitats) that the silversides might exhibit and examine their
habitat use in the two sounds temporally.
In summary, I did not find any dietary niche partitioning between M. menidia and M.
beryllina in the Pamlico and Albemarle Sounds. These findings do not support the niche
partitioning hypothesis and the competitive exclusion principle. However, more research is
needed to examine other potential niche axes that could be partitioned, such as available habitats,
substrate types, vegetation, temperature, salinity and depth zones. Habitat sampling, year-round
fish sampling, and prey availability sampling would all lead to more conclusive results on the
niche partitioning or overlap between the two species. Additional sampling may also show that
just because niche partitioning is not occurring in the food dimension, there may be partitioning
in the habitat or temporal dimension. In the future, I would like to examine growth rates
between the species, determine if there are changes in prey preferences as they grow, and further
examine the potential for niche overlap or partitioning between the Menidia species.
Acknowledgements: Thanks are given to NC Division of Marine Fisheries, who supplied these
specimens for examination, Dr. Anthony Overton, who suggested the project, and Dr. Joe
Luczkovich, who provided the laboratory space, helped with the stomach content analysis,
statistical analysis and final editing of the paper.
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