# Long-Term Bird Assemblage Trends in Areas of High and Low Human Population Density

Authors: Kyle Barrett; Christina M. Romagosa; Matthew I. Williams
Journal: Research Letters in Ecology
 (2008)

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## Abstract

Urban areas are expanding globally, and the impact of high human population density (HHPD) on bird species richness remains unresolved. Studies primarily focus on species richness along an urban-to-rural gradient; however, some studies have analyzed larger-scale patterns and found results that contrast with those obtained at smaller scales. To move the discussion beyond static species richness patterns, we analyzed the effect of HHPD on bird assemblage dynamics (year-to-year extinction probability, turnover, changes in species richness) across the United States over a 25-year period. We found that bird assemblages in both high and low human population density areas changed significantly over the period of record. Specifically, bird assemblages increased in species richness on average. Assemblage change in areas of HHPD was not significantly different from assemblage change in areas with LHPD. These results suggest that human population density alone does not alter the persistence of avian assemblage patterns.


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## Body

## 1. Introduction

Urbanization is
known to alter the species richness and diversity of many communities and/or
assemblages [1, 2], and many researchers have
chosen to focus on the response of bird species richness to urbanization [1, 3–5]. Birds have been the focus of so many studies
because they are sensitive to changes in habitat structure [1] and may serve as indicators
of the ecological consequences that can accompany urban development.
Additionally, many years of data on species richness and abundance exist
through standardized bird surveys such as the North American breeding bird survey
(BBS) in the United States,
by which long-term changes in bird assemblages can be monitored.Studies
of how anthropogenic factors impact bird species richness have offered several
contradictory perspectives. First, many
studies that assess richness along an urban-to-rural-gradient have found that
richness is generally negatively [1, 4], or unimodally (i.e., widely
distributed species are added at the expense of native species with moderate
urbanization) [6] related to urban
development. Second, several recent
studies have suggested that many areas of high human population density (HHPD)
are also areas with high avian species richness [5]. This positive correlation is
generally thought to result from the positive impact that high energy inputs (and
thus high primary productivity) have on both human population density and avian
richness. Finally, many organisms have
demonstrated a pattern of decreasing global species richness, while local
richness increases via species invasions [7]. A more complete description
of the impact HHPD has on bird assemblages can be achieved by examining
assemblage dynamics over long time periods.We studied bird
assemblage dynamics in counties with high and low human population densities (LHPDs) across the U.S. to
determine if assemblage dynamics differ over a 25-year period as a function of
human population density.Our analyses
use statistical techniques that account for the fact that not all species
present are necessarily detected, which is an important consideration in long-term
studies [8].


## 2. Methods

We evaluated all
U.S. ecoregions [9] and selected those for study
containing pairs of BBS routes with one route in a county of LHPD (<65
persons/km2) and one route in a county with HHPD (>700 persons/km2) based on data from the 2000 United States census. We only evaluated routes with at least 20 of
25 years of available BBS data. Twelve
of the 68 ecoregions within the United
States
contained LHPD and HHPD routes that met
all of our criteria (Figure 1).Figure 1
To illustrate our route
selection process, the main map shows portions of two ecoregions containing breeding
bird survey (BBS) routes used in this study. Within each ecoregion, we selected
one BBS route in an LHPD area and one in an HHPD area (see Methods for
definitions). Shaded polygons on main map indicate county population density.
Inset shows all 12 ecoregions used for the study.We compared assemblage
change over the past 25 years between HHPD and LHPD sites using measures of the
rate of species increase, local extinction rate, turnover, and the number of colonizing
species (mathematical definitions and derivations are provided in [8, 10]). The rate of species
increase is a measure of an overall change in species richness between two
points in time within a site. Local
extinction rate is the probability that a given species becomes locally extinct between the two sampling periods. Species
turnover is the probability that a
given species colonized the focal area since the previous sampling period.The number of colonizing species is an
estimate of the number of species new
to the assemblage relative to a previous sampling period.Collectively, these four measures are
hereafter referred to as assemblage properties. We calculated assemblage properties
using Program COMDYN [10]. This software allows for the
calculation of species’ detection probabilities, rather than relying only on
the species observed to generate assemblage properties.To test for an
effect of HHPD on bird assemblages, we compared mean year-to-year assemblage
properties using Hotelling’s multivariateT2-test [11]. By calculating the mean
year-to-year values for our assemblage properties, we estimated the persistence
of an assemblage, where persistence was defined as the maintenance of
assemblage composition over time. We first used two one-sample T2-tests to examine the null
hypotheses that bird assemblages in HHPD and LHPD areas experienced no assemblage
change over the past 25 years. Next, we
used paired T2-tests to examine
the prediction that average year-to-year assemblage parameters were higher in HHPD
habitats relative to LHPD habitats. When
an overall test was significant, we tested for pairwise significance between
each pair of variables in the overall test using Bonferroni simultaneous
confidence intervals. To examine the species composition that contributed to
any observed changes in assemblage parameters, we compared the relative numbers
of area-sensitive and nonarea-sensitive species [12] present between past (species
list from earliest 5 years of data) and present (species list from latest 5
years of data) using a paired t-test.We used
least-squares linear regression to test for an effect of human population
density increase on bird assemblage properties. We plotted the difference
between past (mean from earliest 5 years of data) and present (mean from latest
5 years of data) bird assemblage properties against the difference in past
(1975) and present (2000) human population density. This approach tested the
assumption that areas with greater increases in human population density over
time will have greater impacts on bird assemblages. The test for each COMDYN-estimated property
was performed using the pooled dataset of rural and urban BBS routes. All
analyses were performed using Minitab 13.1. We consideredP-values <.05 to be significant for all tests.


## 3. Results

We found evidence
for assemblage change over the past 25 years in both HHPD and LHPD areas (T2=527.0,P<.005 and T2=331.3,P<.005, resp.). In
each area, the probability of local extinction, turnover, rate of species
increase, and the number of colonizing species were significantly greater than
the null hypothesis of no change (Table 1).
Mean turnover at HHPD and LHPD sites was slightly higher than extinction
rates (although confidence intervals overlapped), which suggests that on
average the colonization of a route by a species absent in a previous year is
more likely than the local extinction of a species.Rate of species increase from one year to the
next was significantly greater than 1, which suggests a slow increase in
species richness over the span of years we examined. Finally, the number of
colonizing species was 8.3 in HHPD sites and 7.9 in LHPD sites. Both of these values (and associated
confidence intervals) indicated that along a route in an average year,
approximately eight species appeared that were not present in the previous
year. Our examination of species composition revealed 75% of both LHPD and HHPD
routes examined had increases in species richness. The mean number of area-sensitive
species added along a route was not significantly different from the number of
nonarea-sensitive species (t=0.57,P=.58). We found no evidence to
support the hypothesis that bird assemblages in HHPD habitats had higher assemblage
parameters (i.e., were more dynamic) than bird assemblages in LHPD habits (T2=9.86,P>.05).Table 1
Estimated mean year-to-year properties for
bird assemblages in HHPD and LHPD areas over a 25-year period throughout the United States.
All assemblage properties were first compared to the null expectation of no
change, and then each individual assemblage property was compared between HHPD
and LHPD areas. Rate of species increase is defined as a ratio between species
richness in year 1 and species richness in year 2, thus if there is no change
in assemblage structure (the null expectation), this value equals 1.
PropertyNo changeMeanCI lowerCI upperUrbanLocal extinction probability00.0660.0450.088Turnover00.0700.0520.088Rate of species increase11.0141.0081.020Number of colonizing species08.3326.8559.809RuralLocal extinction probability00.0750.0440.106Turnover00.0800.0400.119Rate of species increase11.0171.0031.032Number of colonizing species07.8786.9728.783The increase in
human population density within an area did not predict the magnitude of change
in any of the assemblage properties over the 25-year period (allP-values
>.30). This was true for both HHPD and LHPD regions, despite the fact that
the mean increase in human population density between 1975 and 2000 for HHPD
areas was 477 people/km2, but only 7 people/km2 for LHPD
habitats.


## 4. Discussion

For both LHPD and HHPD
areas, the mean values for rate of species increase and number of colonizing
species indicated that on average species richness increased over time (Table1).
A similar pattern of increasing species richness was reported by La Sorte and
Boecklen [13]. While their study tracked
several diversity measures overall BBS routes within the United States,
they did not directly compare HHPD and LHPD habitats or incorporate species
detection probability into their analysis. No mechanism has been identified
empirically to explain thepattern
of increasing species richness described here by La Sorte and Boecklen [13]. Because only two nonindigenous bird species
colonized our routes during the dates evaluated and since observer bias is
accounted for by our analysis, we suggest that biotic homogenization (i.e., increasing
assemblage or community similarity between two sites) may be acting across native
bird assemblages in the United States. As land continues to undergo
development, bird species may be driven from preferred habitat into other areas,
where they were previously absent. This process could lead to increased local
diversity in either urban or rural assemblages [14]. In addition, common species
that benefit from anthropogenic activity are colonizing new habitats [15], while species particularly
sensitive to human development are likely to go locally or globally extinct. Along
the routes, we examined both area- sensitive and nonarea-sensitive species were
added to species lists in more recent years; however, we caution that only a
small percentage of the total species richness along our routes has been
classified into one of these two categories. As a result, a more thorough
classification of all species will be necessary before a definitive statement
can be made regarding the expansion (on a continental scale) of
nonarea-sensitive species.The absence of a
significant difference in assemblage properties between areas of high and low
human population density was surprising, as several previous studies documented
a negative relationship between urban development and avian species richness [4, 16]. Differences between our findings
and others may result because we included all observed bird species rather than
a selected group
such as neotropical migrants [4]. It is also possible that we
did not detect a difference between assemblage dynamics in high and low areas
of human population density because land use effects such as fragmentation from
agricultural or urban development [17] (rather than human population
density per se) drive bird assemblage dynamics. More data are needed to test
this idea directly.Future studies
modeling large-scale assemblage or community dynamics with the mark-recapture
approach used here and by others [8, 16–18] should consider two as of yet
unaddressed aspects of the analysis. First, it is important to explicitly
consider the relationship between mean year-to-year assemblage dynamics (e.g.,
local extinction rate) and ecological concepts such as community persistence or
stability. An increase in the fluctuations of assemblage or community dynamics
over time can signal a decrease in the overall stability of the assemblage or
community [19]. Thus, we believe that measures
of mean year-to-year assemblage dynamics offer a suitable, easily measured
proxy to stability and persistence. A second analytical issue is the assumption
that the null expectation for change within a reference assemblage (i.e., LHPD areas)
is zero. Most assemblages and communities are not static with respect to
species composition over time [20], but in the absence of good
information on what degree of species change should be expected, we used no
change as our null expectation. We hope the study we present here provides a
baseline from which such null expectations might be developed, enabling one to distinguish
assemblage composition shifts due to natural fluctuations from those caused by
anthropogenic factors.



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