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Forest mediated light regime linked to amphibian distribution


Abstract and Figures

The vegetation in and around the basins of ephemeral wetlands can greatly affect light environments for aquatic species such as amphibians. We used hemispherical photographs to quantify the light environment in terms of the global site factor (GSF), the proportion of available solar radiation that actually strikes the wetland. We compared GSF to the distribution and performance of two amphibian species (Pseudacris crucifer and Rana sylvatica) within 17 ephemeral wetlands in northeastern Connecticut, USA. We found that P. crucifer is restricted to lighter wetlands (GSF >0.34) and that its abundance is proportional to GSF. By contrast, R. sylvatica is found across the light gradient and its abundance is unrelated to GSF. For both species, GSF is a strong predictor of larval developmental rate. In addition, P. crucifer growth rate is higher in lighter wetlands. Through thermal effects, changes in resources, or other influences, light appears to be an important predictor of the distribution and performance of amphibians. Because the structure of canopies can change rapidly, and because amphibians can be strongly impacted by these changes, vegetation mediated effects on wetland light environments may be critical to understanding the dynamics of amphibian populations within forested biomes.
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Oecologia (2003) 134:360–364
DOI 10.1007/s00442-002-1136-9
M. A. Halverson · D. K. Skelly · J. M. Kiesecker ·
L. K. Freidenburg
Forest mediated light regime linked to amphibian distribution
and performance
Received: 17 May 2002 / Accepted: 5 November 2002 / Published online: 9 January 2003
! Springer-Verlag 2003
Abstract The vegetation in and around the basins of
ephemeral wetlands can greatly affect light environments
for aquatic species such as amphibians. We used hemi-
spherical photographs to quantify the light environment in
terms of the global site factor (GSF), the proportion of
available solar radiation that actually strikes the wetland.
We compared GSF to the distribution and performance of
two amphibian species (Pseudacris crucifer and Rana
sylvatica) within 17 ephemeral wetlands in northeastern
Connecticut, USA. We found that P. crucifer is restricted
to lighter wetlands (GSF >0.34) and that its abundance is
proportional to GSF. By contrast, R. sylvatica is found
across the light gradient and its abundance is unrelated to
GSF. For both species, GSF is a strong predictor of larval
developmental rate. In addition, P. crucifer growth rate is
higher in lighter wetlands. Through thermal effects,
changes in resources, or other influences, light appears
to be an important predictor of the distribution and
performance of amphibians. Because the structure of
canopies can change rapidly, and because amphibians can
be strongly impacted by these changes, vegetation
mediated effects on wetland light environments may be
critical to understanding the dynamics of amphibian
populations within forested biomes.
Keywords Amphibian · Development · Distribution ·
Global site factor · Growth
Amphibians resident in small freshwater wetlands have
become a model system for ecologists (Wilbur 1997;
Werner 1998; Morin 1999). Despite this attention,
important aspects of the mechanisms underlying the
distribution and abundance of natural populations remain
poorly understood (Wellborn et al. 1996; Skelly 1997,
2001; Alford 1999; Skelly and Kiesecker 2001). Recently,
long-term surveys have uncovered dramatic changes in
the distribution of amphibian species over time. While
some of these dynamics have occurred in the context of
population declines and species extinction (reviewed by
Alford and Richards 1999), it appears that even healthy
amphibian populations can undergo rapid changes (Sjo-
gren-Gulve 1994; Skelly et al. 1999; Carlson and
Edenhamn 2000). Preliminary evidence suggests that
some observed changes in amphibian distributions could
result from alterations in the vegetation canopy above
small wetlands (Skelly et al. 1999, 2002; Werner and
Glennemeier 1999).
The light environments of nonpermanent wetlands are
apt to be particularly susceptible to the effects of
vegetation. Because they tend to be relatively small, the
crowns of trees and other vegetation can overtop ephem-
eral wetlands. Because they are often shallow, many
nonpermanent wetlands can support rooted vegetation
within their basins. Where succession, fire, timber
harvest, beaver (Castor canadensis), or other disturbances
result in temporal changes in vegetation structure, corre-
sponding changes in light environment may be expected
(Skelly et al. 1999; Skelly and Freidenburg 2000).
Ecologists studying the role of gap formation in terrestrial
forests and other systems have shown that the timing,
size, and spatial distribution of such dynamics can have
an overriding effect on the distribution of species (e.g.,
Paine and Levin 1981; Whitmore 1989; Pacala et al.
1996). The purpose of this study is to evaluate the
relationship between vegetation and amphibians. We
hypothesize that relationships among vegetation, solar
radiation, temperature, and other factors are likely to have
M. A. Halverson (
) · D. K. Skelly
School of Forestry and Environmental Studies,
Yale University,
370 Prospect Street, New Haven, CT 06520 USA
Fax: +1-203-4323929
J. M. Kiesecker
Department of Biology,
Pennsylvania State University,
University Park, PA 16802 USA
L. K. Freidenburg
Department of Ecology and Evolutionary Biology,
University of Connecticut,
Storrs, CT 06265 USA
strong effects on the present day distribution and perfor-
mance of amphibians across a light gradient.
We used hemispherical photographs of the canopy
cover to quantify the light regime over 17 temporary
ponds in Connecticut, USA. This technique has been
largely developed by plant ecologists who have long
recognized the importance of solar radiation and the need
for a method with which to analyze it at relatively fine
scales and over long time periods (Evans and Coombe
1959; Anderson 1964). We used 3 years worth of survey
and sampling data to evaluate what effect this factor has
on the distribution, abundance, and performance of two
amphibian species: wood frogs (Rana sylvatica) and
spring peepers (Pseudacris crucifer).
Materials and methods
This study was conducted at the 3,800-ha Yale-Myers Forest in
northeastern Connecticut, USA. Timber harvesting, as well as the
activities of beaver (Castor canadensis), have promoted the
development of a diversity of light environments within wetlands
at the Yale-Myers Forest. The 17 ephemeral wetlands used in this
study range from heavily shaded to relatively open. Maximum
surface areas range from 40 to 5,200 m
and maximum depths were
between 25 and 250 cm. They were selected for study during 1998
or earlier because they were known amphibian-breeding sites and
were nonpermanent.
We quantified the light environment at each wetland in terms of
the global site factor (GSF) (Anderson 1964). This ratio estimates
the amount of light that actually strikes a point over the amount of
light that would strike the same point if there were no overhead
obstructions. The GSF changes depending both on the length of
time considered and the time of year. We estimated GSF using
hemispherical photographs of the forest canopy. With such
photographs, it is possible to estimate the amount of diffuse and
direct beam radiation that would strike the photograph point during
a given time interval (Evans and Coombe 1959; Anderson 1964).
This method has been used by plant ecologists for many years, and
estimates of photon flux based on hemispherical photographs have
been shown to closely correspond to average direct sensor
measurements (Chazdon and Field 1987; Becker et al. 1989; Rich
et al. 1993). We analyzed the photographs and estimated GSF with
Hemiview (Delta-T 2001). Necessary parameters for this program
were estimated from the 1960–1990 data in the National Solar
Radiation Database (2001) for Hartford, Connecticut (approxi-
mately 40 km from the study site).
Photograph poin ts w ere laid out in agridpatternwith5-mintervals
bounded by the high water mark of the wetlands in 1999. Two
photographs were taken at each point. “Leaf-on” photographs were
taken during the summer of 1999 and“leaf-off”photographswere
taken during the winters of 1999 and 2000. We used these two sets of
photographs to estimate the average GSF from April to August, the
embryonic and larval periods of our focal species at this site. As leaf
emergence at the study site typically occurs during mid-May, we
based our calculations on leaf-off photographs prior to and leaf-on
photographs after 15 May. The average GSF for a wetland thus
estimated is ref erred to simply as GSF in the remainder of this paper.
We measured natural patterns of distribution, abundance, and
performance of two amphibian species in each of the 17 wetlands:
wood frogs ( Rana sylvatica) and spring peepers (Pseudacris
crucifer). These species are the most common anurans in nonper-
manent wetlands at Yale-Myers Forest. From 1998 to 2000, every
wetland was visited in March or April and at least three more times
during discrete sampling periods between early May and early July
(I: first half of May; II: second half of May; III: first half of June;
IV: first half of July). Sampling intervals usually lasted 2 days or
less and never lasted more than 1 week. During March and April
wetlands were visually inspected for eggs. During May through
July sampling visits, depth was measured, wetlands were visually
inspected for adult and larval amphibians and eggs, and timed
continuous dip-net sampling (between 5 and 15 person minutes
depending on surface area) was conducted throughout the wetland.
Up to 20 amphibian larvae of each of the focal species were
collected and stored in 70% EtOH. The larvae were later measured
(snout-vent length, SVL), and staged (Gosner 1960) under a
dissecting scope.
We used logistic regression to determine the relationship
between GSF and the presence-absence distribution of spring
peepers and wood frogs. A species was considered present if eggs
or larvae were ever found in a wetland between 1998 and 2000.
Linear regression was used to relate larval density of each species
to GSF. Density was measured as the number of larvae encountered
per person-minute averaged across the three dip-net sampling
rounds (I, II and III). In addition, we analyzed the relationship
between GSF and larval performance. For each sampling period
during which a species was present in at least three wetlands, we
related GSF to the average body size (mm SVL) and developmental
stage (Gosner stage, Gosner 1960) of the larvae from a given
wetland using linear regression. Results from 2000 are presented
here; results from prior years were comparable. Finally, we used
multiple linear regression to determine whether the relationship
between size and developmental stage varied with GSF. In these
regressions, the response variables were the average sizes of
tadpoles from each developmental stage from each wetland.
Predictor variables were GSF and Gosner stage. A significant
effect of GSF in this regression suggests that tadpoles at a given
Gosner stage vary in size in a manner that is correlated with GSF.
Water temperature was measured in 11 of the study wetlands
using a Hobo temperature logger suspended 10 cm below the surface
at the site of maximum wetland depth. Temperature was recorded
every hour during the month of April. Temperature is probably lower
during this month than at any other time during the larval period and
is thus probably more important at this time as a developmental
constraint. In addition, the embryos and larvae are less developed
during this month and are thus less able to compensate for pond
temperature variation by seeking out different spots within the pond
(L.K. Freidenburg, unpublished data). To estimate the association
between water temperature and light environment we calculated a
regression of GSF against the average April temperature from each
of the wetlands for which we had such data.
Wood frogs were present in each of the 17 wetlands, thus
their distribution was independent of GSF. By contrast,
distribution of spring peepers was strongly correlated with
wetland light environment (logistic regression: df=1,
=5.83, P=0.016). These frogs were found in most of
the high light wetlands and in none of the darkest
wetlands (GSF<0.34).
The density of wood frog larvae was unrelated to GSF
(Fig. 1a; linear regression: R
=0.001, MS=0.06, F
1, 15
P=0.889) while density of spring peeper larvae was
positively related to GSF (Fig. 1b; linear regression:
=0.44, MS=1.89, F
1, 15
=11.98, P=0.003). The increase
in spring peeper density with increasing GSF persisted
when the analysis was restricted to the eight wetlands
with nonzero densities (linear regression: R
MS=1.62, F
1, 6
=9.12, P=0.023).
Larval developmental stage of both species tended to
be more advanced on a given date within lighter wetlands
(Table 1, Fig. 2). The relationship between GSF and
developmental stage was small or insignificant during
early sampling and became strong and significant by early
June (Table 1, Fig. 2).
Body size of spring peepers tended to show a pattern
similar to that observed for development: a strong positive
relationship between body size and GSF became evident
as the season progressed (Table 1, Fig.2). This was not the
case for wood frogs. While there was some evidence of a
positive relationship between light and body size for
wood frogs early in the season, this relationship tended to
disappear as the season progressed.
Multiple linear regression showed that wood frog
larvae at a given stage of development tended to be larger
in darker wetlands (1998: slope ="3.038, SE=0.819,
="3.7071, P<0.001. 1999: slope="1.288, SE=0.587,
="2.19, P=0.031. 2000: slope ="1.387, SE=0.5062,
="2.740, P=0.008). There was no significant relation-
ship between GSF and size of spring peepers at a given
stage of development (1998: slope="1.854, SE=1.354,
="1.3688, P=0.2134. 1999: slope ="414, SE=0.4942,
="0.838, P=0.4125. 2000: slope="0.351, SE=0.583,
="0.602, P=0.552).
Among the 11 wetlands in which water temperature
was measured (April 2000), wetlands with higher
GSF tended to be warmer (linear regression: MS=2.26,
1, 9
=6.43, R
=0.35, P=0.032).
Incoming solar radiation is apt to be particularly variable
in ephemeral wetlands. Tree crowns often extend entirely
across smaller wetlands and some species such as red
maple (Acer rubrum) and buttonbush (Cephalanthus
occidentalis) can root within shallow basins. As a result,
the light environments in our darkest wetlands are
comparable to those found in the understory of mature
forests in the northeastern United States (Canham et al.
1994; Finzi and Canham 2000).
This gradient in light among wetlands created by
variation in vegetation structure is strongly associated with
the distribution and performance of amphibian species. In
the case of one species, spring peepers, we identified a
light boundary (GSF=0.34) below which the species was
consistently absent. In those wetlands in which spring
Fig. 2 Performance of wood frogs (a, c) and spring peepers (b, d)
during 2000 as a function of light level within a wetland (indexed
as global site factor, GSF). Performance was measured as the
average Gosner (Gosner 1960) developmental stage (a, b), or body
size (c, d). Samples were collected during three sampling periods:
early May (filled circles), mid May (hollow diamonds), and early
June (crosses). Lines of best fit from least squares regression are
presented for each sampling period (Table 1). The standard error
for average body size (mm SVL) and average Gosner stage was
generally less than 5% of the mean
Fig. 1 Density of a wood frog and b spring peeper larvae across 17
wetlands as a function of light level. Light level is indexed as
global site factor (GSF). Larval density is estimated as the number
of tadpoles encountered per person minute while dip-netting. Each
point represents an average calculated across visits during up to
3 years. The bars represent 1 SE
Table 1 Results from least squares regression of average tadpole
size and developmental stage as a function of light environment
(indexed by global site factor, GSF). Responses for wood frogs and
spring peepers were collected during 2000 while sampling 17
wetlands at the Yale-Myers Forest in northeastern Connecticut.
There were three sampling periods: early May (I), late May (II) and
early June (III)
Species Response Period N Slope R
Gosner stage I 3 "0.20 0.11 0.789
II 6 4.38 0.52 0.107
III 8 13.90 0.78 0.003
Body size I 3 "1.05 0.87 0.239
II 6 5.02 0.57 0.082
III 8 9.94 0.62 0.020
Gosner stage I 15 3.23 0.56 0.001
II 14 5.40 0.55 0.107
III 13 11.48 0.56 0.003
Body size I 15 2.65 0.33 0.025
II 14 3.10 0.12 0.227
III 13 1.93 0.02 0.685
peepers were present, their density was proportional to
GSF (Fig. 1b). Despite their reliance on forested terrestrial
environments after metamorphosis (Delzell 1958), spring
peepers appear to be excluded from wetlands under heavy
forest cover during embryonic and larval phases. Given
that closed-canopy forests grow throughout much of the
range of this species, this means spring peepers necessarily
depend on wetlands that are too large to be overtopped by
tree crowns, where shoreline vegetation has recently been
disturbed, or where a higher light environment is main-
tained through some other means.
What prevents spring peepers from using heavily
shaded wetlands? Spring peeper larvae grow and develop
more slowly in darker wetlands. During June, when
spring peepers in the lightest wetlands were nearing
metamorphosis, conspecifics in the darker wetlands were
still in early stages of development (Fig. 2b). We
documented up to three and fourfold differences in body
length during late season. Field transplant experiments
have shown similar decreases in growth and development
in low light environments (Werner and Glennemeier
1999; Skelly et al. 2002). Such reduced performance may
lead to an increased likelihood of larval mortality from
drying events in nonpermanent wetlands (Skelly 1995).
While reduced growth and development could explain
the restricted distribution of spring peepers as well as
decreased density in darker wetlands, amphibians also are
known to be selective in their choice of oviposition sites
(Hopey and Petranka 1994; Resetarits and Wilbur 1989;
Resetarits and Wilbur 1991; Kiesecker and Skelly 2000).
The results of a pilot experiment in which the canopy was
removed from a heavily shaded wetland at the Yale-
Myers Forest are consistent with such an effect. Spring
peeper larvae were absent from this wetland during the
3 years prior to canopy removal and have been present
during each of 3 years since (D.K. Skelly, unpublished
data). While darker wetlands may act as demographic
sinks for spring peepers, it also is possible that adults
avoid placing their offspring in these environments.
Unlike spring peepers, wood frogs breed in wetlands
across the light gradient and their larval density was not
significantly related to GSF (Fig. 1a). Neither was there any
indication that light was related to wood frog growth patterns
(Fig. 2c). While wood frogs seem able to maintain relatively
dense populations of rapidly growing larvae even in heavily
shaded wetlands, their development rate was correlated with
GSF (Fig. 2a). Thus, for both species there is a performance
cost associated with living in shaded wetlands.
Solar radiation could affect amphibian performance
through a variety of mechanisms. Light affects the
composition and abundance of the primary producers in
aquatic ecosystems (Feminella et al. 1989; Wetzel 2001).
Periphyton is an important food source for tadpoles
(Kupferberg et al. 1994; Hill et al. 1995) and can be less
abundant and less diverse in shaded wetlands (Skelly et
al. 2002). In addition, there is evidence that spring
peepers and wood frogs are less able to assimilate organic
matter available in benthic substrates from shaded
wetlands (Skelly and Golon, unpublished data).
The performance patterns we observed also are
consistent with the effects of temperature on ectotherms.
In this study, darker wetlands tended to be colder.
Controlled temperature laboratory experiments show that
both growth and development rates of amphibian larvae
can be limited by temperature (e.g., Smith-Gill and
Berven 1979; Berven and Gill 1983). Even when provided
abundant, high quality food resources, modest decreases
in ambient temperature can cause large reductions in
growth and development of amphibians (Newman 1998).
These reductions are typically asymmetric; for a given
decrease in temperature, larvae will suffer greater reduc-
tions in development rate versus growth rate (Ray 1960;
Smith-Gill and Berven 1979). Consequently, larvae
sampled from colder environments tend to be larger at a
given developmental stage compared with conspecifics
from warmer environments. The asymmetry of tempera-
ture effects is widely reported in ecotherms but the
mechanisms remain debated (Atkinson 1994, 1995; Van
der Have and de Jong 1996). Whatever the reason, the
existence of this asymmetric pattern among wood frogs is
strongly suggestive of an impact of temperature on
growth and development patterns. Its absence among
spring peepers suggests that this species may respond
differently to temperature change, that it is not temper-
ature limited in its performance, or that its restriction to
relatively lighter wetlands precluded observation of the
asymmetric temperature effect.
The links established here between wetland light
environment and amphibian performance and distribution
are particularly meaningful in the context of vegetation
dynamics. The forest cover on the Yale-Myers Forest and
central New England has changed dramatically over the
last 300 years. After settlement reduced the forest cover in
some parts of New England to as low as 20% by the middle
of the eighteenth century, many farms were abandoned and
forests allowed to regenerate. Today, as in pre-settlement
times, forest covers more than 90% of the Yale-Myers
Forest and central New England (Meyer and Plusnin 1945;
Foster 1992; P.M.S. Ashton, unpublished data). Distur-
bances from such things as timber harvest, beaver and fire
can also cause the canopy above wetlands to change
rapidly (Skelly et al. 1999; Skelly and Freidenburg 2000).
When allowed to regrow, forest can encroach and overtop
a small wetland within a few decades or less (Skelly et al.
1999). This study suggests amphibians may undergo
changes in distribution, abundance and performance as a
result of such changes in vegetation.
The ability of wood frogs to persist in spite of canopy
closure could be related to their ability to maintain
relatively rapid growth and development across different
light environments. There also is some evidence that this
species may evolve rapidly in response to thermal
changes that accompany change in canopy (Skelly and
Freidenburg 2000). Regardless of origin, the persistence
of wood frogs across the light gradient means that this
species is able to utilize a much higher fraction of
ephemeral wetlands in our study area than spring peepers.
The distribution of spring peepers appears be at least
partly determined by the pattern of recent disturbance
around smaller wetlands (e.g., Paine and Levin 1981;
Pacala et al. 1996; Batzer et al. 2000).
Acknowledgements Thanks to C. Apse, C. Burns, J. Golon, G.
Jones, J. Morton, E. Sagor, R. Schiff, S. Price, M. Urban and J.
Virdin for help with field work. P.M.S. Ashton generously provided
the use of his camera. H. zu Dohna, E. Palkovacs, C. Burns and M.
Urban provided helpful comments on previous drafts of the
manuscript. This research has been supported by the National
Institutes of Health and the National Science Foundation (Ecology
of Infectious Diseases Grant 1R01ES11067–01) and by a gift to
Yale University from Mrs. E.S. Dwyer.
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... One environmental variable that could cause fine-scale geographic differences in phenology between amphibian populations is the degree of forest canopy cover over wetlands. Open-and closed-canopy wetlands differ in many factors, including community structure, amphibian growth and development, and even fine scale genetic differentiation [32,[34][35][36], but no work has investigated differences in breeding phenology between these types of wetlands. Thus, it is an open question as to whether the phenology of geographically isolated populations of amphibians in close proximity are typically synchronous with one another or exhibit substantial local idiosyncrasies. ...
... For example, earlier breeding in wood frogs leads to earlier metamorphosis [15], which in turn affects post-metamorphic growth and fecundity [40,41]; thus, wetlands with consistently early breeding could produce a disproportionate number of recruits into a metapopulation. In these populations, canopy cover has been demonstrated to affect amphibian community structure, larval growth and development, and local adaptation [32,[34][35][36]. Although the effects of canopy cover on amphibian larval ecology are well-studied, potential effects of canopy cover on breeding phenology have not been explored. ...
... Geographic variation in phenological responses on relatively small spatial scales can be caused by factors including spatial variation in microclimate as well as phenotypically or genetically based differences in how individuals respond to the same environmental cues [9,78]. Despite the ecological differences in canopy cover [55,79] and its effect on amphibian growth and development [34,36,80], there was no effect of canopy cover on average breeding date. There were some years in which ponds did differ in phenology, such as 2011, when there was an approximately 10-day difference in average breeding date between the most extreme wetlands. ...
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A mechanistic understanding of phenology, the seasonal timing of life history events, is important for understanding species’ interactions and the potential responses of ecological communities to a rapidly changing climate. We present analysis of a seven-year dataset on the breeding phenology of wood frogs (Rana sylvatica), tiger salamanders (Ambystoma tigrinum), blue-spotted salamanders (Ambystoma laterale), and associated unisexual Ambystoma salamanders from six wetlands in Southeast Michigan, USA. We assess whether the ordinal date of breeding migrations varies among species, sexes, and individual wetlands, and we describe the specific environmental conditions associated with breeding migrations for each species/sex. Breeding date was significantly affected by species/sex identity, year, wetland, and the interactions between species/sex and year as well as wetland and year. There was a great deal of variation among years, with breeding occurring nearly synchronously among groups in some years but widely spaced between groups in other years. Specific environmental triggers for movement varied for each species and sex and changed as the breeding season progressed. In general, salamanders responded to longer temperature lags (more warmer days in a row) than wood frogs, whereas wood frogs required longer precipitation lags (more rainy days in a row) than salamanders. Wood frogs were more likely to migrate around the time of a new moon, whereas in contrast, Ambystoma salamander migration was not associated with a moon phase. Ordinal day was an important factor in all models, suggesting that these amphibians require a latency period or similar mechanism to avoid breeding too early in the year, even when weather conditions appear favorable. Male wood frogs migrated earlier than female wood frogs, and male blue-spotted salamanders migrated earlier than female A. laterale and associated unisexual females. Larger unisexual salamanders migrated earlier than smaller individuals. Differences in species’ responses to environmental cues led to wood frogs and A. laterale breeding later than tiger salamanders in colder years but not in warmer years. This suggests that, as the climate warms, wood frog and A. laterale larvae may experience less predation from tiger salamander larvae due to reduced size differences when they breed simultaneously. Our study is one of few to describe the proximate drivers of amphibian breeding migrations across multiple species, wetlands, and years, and it can inform models predicting how climate change may shift ecological interactions among pond-breeding amphibian species.
... proportion of aquatic patches invaded) and (B) identify the ecological factors promoting the invasibility of habitats considering features of the breeding patches (ponds), landscape composition and connectivity, as incorporating multiple scales is recommended for a clear understanding of distribution patterns (Denoël et al. 2019;Cox et al. 2021). Several factors could favor occupancy by invasive Pelophylax through different processes as shown in water frogs and other amphibians: (1) deep ponds could offer shelter and (2) have less probability to dry, thus offering a permanent habitat for adults and allowing tadpoles to complete development until metamorphosis (Buskirk 2003); (3) large ponds could host more resources (Hoffmann et al. 2015); (4) aquatic vegetation could provide shelter and increase food resources (Bounas et al. 2020); (5) complex natural substrate of ponds could provide shelter and resources (Zamora-Marín et al. 2021); (6) sun exposure could allow basking and benefit pond productivity and tadpole growth (Halverson et al. 2003;Ficetola and De Bernardi 2004); (7) the presence of fish could affect the survival of tadpoles (Buskirk 2003). At the landscape scale, (8) high pond density should facilitate connectivity (Bounas et al. 2020;Vimercati et al. 2020); (9) forests, despite serving as shelters for terrestrial life in various stages could negatively affect connectivity (Wells 2007;Cordier et al. 2021); (10) areas with more human settlements can benefit invasive frogs . ...
... However, this task is challenging, as invaders may also cause shifts in the composition of invaded communities. Sunny ponds favor basking, a behavior of water frogs that are often described as thermophilous anurans (Ficetola and De Bernardi 2004), and increase the growth of tadpoles (Halverson et al. 2003). Vegetation promotes the complexity of aquatic microhabitats, can increase the availability of prey (such as insects) and can provide shelter to predators (Briggs et al. 2019;Hamer et al. 2021). ...
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Determining the invasibility of habitats by alien species is crucial for understanding their spread potential, the habitats the most at risk and to implement adequate management actions. This is urgent for introduced taxa that show high invasion potential across broad geographical scales. We here assess these processes in invasive Pelophylax water frogs which are widespread colonizers across Western Europe and for which the invasibility of habitats remains to be quantified. Specifically, we used hierarchical occupancy models in a Bayesian framework to identify local- and landscape-scale features that can enhance occupancy of the most common invasive water frog, the marsh frog (P.ridibundus), in southern France. Water frogs were highly detectable and showed high occupancy across the invaded landscape. The invaders expressed a very broad habitat tolerance for both local- and landscape-scale variables while their invasion was facilitated by the occurrence of deep, permanent ponds with abundant aquatic vegetation and high sun exposure. Cross-validation showed a good transferability of models across space. The high invasibility of a wide range of habitats by Pelophylax water frogs is alarming and unveils their invasiveness, contributing therefore to explain their success of invasion over broad geographic scales.
... At the scale of individual water bodies used for breeding, canopy cover is clearly a strong predictor of amphibian community composition and abundance. Numerous papers report increased amphibian diversity, richness, and abundance in wetlands with open canopies in comparison to closed canopies, although we note that many studies are from similar landscapes and not entirely independent (Werner & Glennemeier, 1999;Halverson et al., 2003;Skelly et al., 2005;van Buskirk, 2005;Stevens, Paszkowski & Scrimgeour, 2006;Werner et al., 2007a,b;Liner et al., 2008;Provete et al., 2014;Skelly, Bolden & Freidenburg, 2014;Simpson et al., 2021). For example, Liner et al. (2008) found nearly double the larval amphibian richness among open-canopy, herbaceous marshes compared to more closed-canopy Cypress-Gum swamps within a pine savanna landscape in Georgia, USA. ...
... While some amphibians can utilize plant detritus directly, most species depend indirectly on microbial, periphyton, algal, and invertebrate production (Stoler & Relyea, 2011, 2020. A growing body of relatively recent research demonstrates that changes in detritus quantity or qualitysometimes interacting with light levelscan have large effects on larval amphibian performance (Skelly et al., 1999(Skelly et al., , 2002(Skelly et al., , 2005Werner & Glennemeier, 1999;Semlitsch, 2000Semlitsch, , 2003Halverson et al., 2003;Rubbo & Kiesecker, 2004;Schiesari, 2006;Rubbo, Belden & Kiesecker, 2008;Williams, Rittenhouse & Semlitsch, 2008;Maerz, Cohen & Blossey, 2010;Earl et al., 2011Earl et al., , 2014Stoler & Relyea, 2011, 2013b, 2020Earl & Semlitsch, 2013;Stephens, Berven & Tiegs, 2013;Cohen, Rainford & Blossey, 2014;Rowland et al., 2016;Dodd & Buchholz, 2018;Burrow & Maerz, 2021) and several studies indicate that nutrient availability has a larger effect than light level (Skelly et al., 2002;Schiesari, 2006;Rowland et al., 2015Rowland et al., , 2016Stoler & Relyea, 2020). Many studies report that higher nitrogen or phosphorus content in plant detritus leads to larger larval size at metamorphosis, faster development, and increased survival (Maerz et al., 2010;Cohen et al., 2012Cohen et al., , 2014Stephens et al., 2013;Stoler & Relyea, 2013b;Earl et al., 2014;Martin, Rainford & Blossey, 2015;Burrow & Maerz, 2021). ...
Descriptions of amphibian habitat, both aquatic and terrestrial, often include plants as characteristics but seldom is it understood whether and how those plants affect amphibian ecology. Understanding how plants affect amphibian populations is needed to develop strategies to combat declines of some amphibian populations. Using a systematic approach, we reviewed and synthesized available literature on the effects of plants on pond-breeding amphibians during the aquatic and terrestrial stages of their life cycle. Our review highlights that plant communities can strongly influence the distribution, abundance, and performance of amphibians in multiple direct and indirect ways. We found three broad themes of plants’ influence on amphibians: plants can affect amphibians through effects on abiotic conditions including the thermal, hydric, and chemical aspects of an amphibian’s environment; plants can have large effects on aquatic life stages through effects on resource quality and abundance; and plants can modify the nature and strength of interspecific interactions between amphibians and other species – notably predators. We synthesized insights gained from the literature to discuss how plant community management fits within efforts to manage amphibian populations and to guide future research efforts. While some topical areas are well researched, we found a general lack of mechanistic and trait-based work which is needed to advance our understanding of the drivers through which plants influence amphibian ecology. Our literature review reveals the substantial role that plants can have on amphibian ecology and the need for integrating plant and amphibian ecology to improve research and management outcomes for amphibians.
... Organisms that rely on algal and periphyton resources for growth and development can be particularly sensitive to changes in detrital resources and wetland canopy cover. For example, a rapidly growing body of research demonstrates that changes in detritus quantity or quality and light levels can strongly affect larval amphibian performance (Skelly et al. 1999, Werner and Glennemeier 1999, Semlitsch 2000, Halverson et al. 2003, Rubbo and Kiesecker 2004, Schiesari 2006, Rubbo et al. 2008, Williams et al. 2008, Maerz et al. 2010, Earl et al. 2011, Stephens et al. 2013, Stoler and Relyea 2013, Cohen et al. 2014, Rowland et al. 2016, Dodd and Buchholz 2018. Generally, because emergent herbaceous detritus is higher in nitrogen and phosphorus and has lower concentrations of secondary metabolites, it decomposes more rapidly and supports higher primary autotrophic production compared to more recalcitrant deciduous leaf litter. ...
... Shifting to lower quality tree leaf litter concurrent with reduced light from canopy closure tends to reduce productivity within wetlands resulting in reduced larval amphibian survival, increased time to metamorphosis, and reduced size at metamorphosis, all of which are important determinants of adult amphibian fitness and population growth (Berven 1988, 1990, 1995, Taylor and Scott 1997, Skelly et al. 2002, Altwegg and Reyer 2003. Indeed, wetland canopy cover is among the strongest known natural gradients predicting local amphibian distributions and richness (Skelly et al. 1999, Werner and Glennemeier 1999, Halverson et al. 2003, Liner 2006, Werner et al. 2007a. While a majority of larval amphibian species studied appear negatively impacted by increased wetland canopy cover, effects can be amphibian and plant species specific (Maerz et al. 2005, Schiesari 2006, Skelly et al. 2014, Earl and Semlitsch 2015. ...
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Isolated wetlands embedded within longleaf pine savannas support a high proportion of regional biodiversity including many amphibian species. Today, remnant isolated wetlands are often overgrown and hydrologically altered due to fire exclusion or incompatible, cool season fire regimes. In the absence of warm season fires when wetlands are dry, shrubs and trees succeed herbaceous plants, which alters wetland productivity via effects on light and detritus quality. We used a factorial aquatic mesocosm study to test the effects of altered detritus and shade on the growth, development, and survival of tadpoles of two priority amphibian species: gopher frogs (Rana capito) and ornate chorus frogs (Pseudacris ornata). Gopher frog survival was higher among maidencane, sedge, and pine treatments compared to oak and sweetgum treatments. While gopher frog larval periods were lowest in the sedge treatment, there was a nominal general effect of litter type on gopher frog larval periods, growth rates, and mass at metamorphosis. Shading had a nominal and inconsistent effect on gopher frog growth rates, but did extend larval periods in all litter treatments, decreased survival in all litter treatments except oak, and decreased mass at metamorphosis in all litter treatments except pine and sweetgum. Ornate chorus frog survival was minimally affected by shading and litter treatments, but growth rates and mass at metamorphosis were highest in maidencane and sedge treatments, and larval periods were extended with shading in all litter treatments. Shading also decreased growth rates in maidencane and sedge litters and decreased mass at metamorphosis in pine and sweetgum litters. Our results demonstrate that succession of isolated wetlands can reduce tadpole performance for two priority species both through changes in leaf litter and shading, though the effect on survival, larval growth, larval period, and size at metamorphosis can differ between species. These results support management recommendations to restore and maintain open canopy, grassy conditions in isolated wetlands for conservation of priority amphibian species.
... For instance, the tadpoles of L. mystacinus, L. gracilis and S. nasicus have rapid development (between one and two months) (Fabrezi 2011; personal observations for L. gracilis). Consequently, the scarce light that reaches the forest floor may prevent tadpoles from having optimal conditions needed for growth and development (Skelly et al. 2002;Halverson et al. 2003). ...
... B. cordobae or S. nasicus). Second, light reduction limits primary productivity and thus reduces basal resources and food availability for tadpoles, jeopardizing their growth, performance and survival (Halverson et al. 2003;Mallory and Richardson 2004;Schiesari 2006). Finally, L. lucidum may indirectly modify anuran communities through leaf extracts or leachates. ...
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Amphibians are declining globally from diverse and complex causes, with one of the most prevalent being alien species invasion. Alien woody invaders, such as the evergreen Ligustrum lucidum, are deeply transforming native ecosystems worldwide. Our aim was to evaluate the effect of Ligustrum lucidum invasion on native anurans in a seasonal semiarid subtropical forest. Sampling was conducted in streams dominated by L. lucidum forest and streams dominated by native forest. Amphibian species richness and diversity, and community and species abundance were recorded. We characterized invaded and non-invaded streams by measuring biotic and abiotic variables. Invaded streams support only three of the seven species present in non-invaded streams. Species richness in non-invaded streams was twice as high as in invaded streams and community abundance was almost five times greater in the former than in the latter. Invasion by L. lucidum triggers profound changes in vegetation physiognomy and composition of this xerophytic seasonal forest, resulting in dark, unsuitable habitats for several native anurans. In agreement with the few studies focused on the effect of alien woody invaders on amphibians, this study supports that woody alien invasion results in a reduction of native anuran richness and the dominance of a generalist anuran species. This is the first report on the disruptive role of L. lucidum invasion in local anurans diversity and community dynamics.
... A reduction in forest canopy exposes the lowest habitat layers to intense radiation and wind (Semlitsch et al., 2009), generating warmer surfaces (Zheng et al., 2000). Given the importance of vegetation structure to many amphibian species (Halverson et al., 2003;Grundel et al., 2015), intense fires are expected to negatively affect many amphibians due to changes in micro-environmental conditions (Russell et al., 1999;Bury et al., 2002). However, we did not find negative responses in our study area, and interestingly the response of amphibians to fire differed according to the bioregion affinity of each species: whereas the presence of Atlantic species in burnt water points did not vary, Mediterranean species increased their presence in severely burnt water points. ...
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Climate and socioeconomic factors are modifying fire regimes. In this scenario, some taxa, such as amphibians, may be increasingly vulnerable. We examined the response of amphibian species to fire severity after a 1600-ha fire in a fire-active region located in the northwestern Iberian Peninsula. This area is a biogeographical crossroad where Atlantic and Mediterranean amphibian species can coexist in the same ponds. We sampled 33 water points in native (mainly oak) and exotic (eucalyptus) forests. Water points were sampled in two different periods: just after the fire to report direct mortality, and two years after the fire to evidence amphibian resilience. We examined the influence of forest type and fire severity on the number of amphibian species detected per point (species richness). Species richness per point varied depending on both forest type (i.e. higher richness in the native forest) and fire severity. Both Atlantic and Mediterranean groups showed higher species richness at native than at exotic points. The occurrence of Atlantic species did not change with fire whereas the number of Mediterranean species increased at sites affected by high-severity fires. This study identified the negative effect of eucalyptus plantations on amphibians and showed that the response of this taxon to fire is partially shaped by species-specific bioregion affinity
... Tadpole visible density in the study pools is shown to be influenced by the interaction of canopy cover and water temperature along with other factors. This supports the importance of canopy cover as a resource gradient for tadpoles through its effect on water temperature, dissolved oxygen, and abundance and composition of periphyton by previous studies (Halverson et al., 2003;Schiesari, 2006;Skelly et al., 2002). The diel patterns of dissolved oxygen and water temperature are influenced by canopy cover gradient (Broadmeadow et al., 2011;Werner and Glennemeier, 1999) which can influence the tadpole activities. ...
Delaying metamorphosis in low-temperature conditions by anuran larvae known as ‘overwintering’ have been poorly studied, especially in terms of habitat ecology and behavioural aspects. The present study investigates some of the ecological aspects of overwintering tadpoles of the genus Nanorana in the Western Himalaya in an anthropogenically modified stream where check dams have altered the natural habitat, which can potentially influence the amphibian ecology. We present insights on the influence of check dams on the tadpole activity pattern and morphometric traits useful in the conservation planning of the narrowly distributed and understudied species, which are most sensitive to habitat modification. We monitored natural and modified pool habitats in the stream during winter and post-winter seasons based on the visual density of tadpoles to assess the diurnal and seasonal emergence pattern with associated habitat variables. Generalized Linear Mixed Modeling (GLMM) was used to understand the influence of various habitat variables on the visible density of tadpoles. Fine-scale temporal scan sampling of tadpoles was carried out to complement the understanding of the visible density variation and analyzed using circular plots and activity overlap estimation. Variation in morphometric traits was assessed using field morphometry and photogrammetry. Mean tadpole visible density at nighttime was higher in modified pools than natural pools during winter, but there was no statistically significant difference during daytime; the nocturnal pattern changed in the post-winter, where visible density was higher in natural pools. Tadpole visible density was influenced by the interaction of mean canopy cover with water temperature, instream cover items richness, mean canopy cover percentage, water temperature, leaf litter depth, water velocity and interaction of time of the day with pool modification. Tadpole activity patterns varied significantly between pool types during post-winter, where modified pool population increased daytime activity and thus the activity overlap reduced from winter (90.8%) to post-winter (64.5%). During both seasons, the mean body size of the natural pool population was significantly lower than the modified pool population; mean relative tail length was significantly lower in natural pools during post-winter; mean tail depth was significantly lower in natural pools during winter. The study presents evidence of the influence of anthropogenic habitat alterations on behaviour and morphometric traits of overwintering tadpoles, which needs to be further investigated. We also discuss the variation in nocturnal emergence, habitat selection and morphometric trait patterns in the modified habitat, potential reasons and similar behaviour in other aquatic organisms, which need to be considered while developing conservation strategies for the overwintering tadpoles in the region. Data Availability Statement All relevant datasets used in this study are available through Zenodo (Jithin et al., 2022; DOI: 10.5281/zenodo.6327687).
... Pond canopy closure was measured as in Arietta et al. (2020) by using five hemispherical photographs taken along the shore at each cardinal point and at the center of each pond during leaf-off and leaf-on seasons. We estimated average leaf-on and leaf-off global site factor (GSF; the ratio of above-canopy radiation to undercanopy radiation; Anderson, 1964) and used a weighted GSF value integrated over the duration of wood frog embryonic and larval life stages (Halverson et al., 2003). GSF is scaled between 0 and 1, and we report it here as a percentage. ...
Understanding drivers of metapopulation dynamics remains a critical challenge for ecology and conservation. In particular, the degree of synchrony in metapopulation dynamics determines how resilient a metapopulation is to a widespread disturbance. In this study, we used 21 years of egg mass count data across 64 nonpermanent freshwater ponds in Connecticut, USA to evaluate patterns of abundance and growth and to assess regional as well as local factors in shaping the population dynamics of wood frogs (Rana sylvatica = Lithobates sylvaticus). In particular, we asked whether a species known to undergo metapopulation dynamics exhibited spatial synchrony in abundances. With the exception of a single year when breeding took place during severe drought conditions, our analyses revealed no evidence of synchrony despite close proximity (mean minimum distance <300 m) of breeding ponds across the 3213 ha study area. Instead, local, pond‐scale conditions best predicted patterns of abundance and population growth rate. We found negative density dependence on population growth rate within ponds as well as evidence that larger neighboring pond populations had a negative effect on focal ponds. Beyond density, pond depth was a critical predictor; deeper ponds supported larger populations. Drought conditions and warm winters negatively affected populations. Overall, breeding ponds vary in critical ways that either support larger, more persistent populations or smaller populations that are not represented by breeding pairs in some years. The infrequency of spatial synchrony in this system is surprising and suggests greater resilience to stressors than would have been expected if dynamics were strongly synchronized. More generally, understanding the characteristics of systems that determine synchronous population dynamics will be critical to predicting which species are more or less resilient to widespread disturbances like land conversion or climate change.
... Light values can be integrated over time to yield seasonal estimates of light environments from a single sampling event (Frazer et al. 1997) (or at least two samples in deciduous canopies (e.g. Halverson et al. 2003)). Thus, HP offers an efficient, non-destructive method of estimating forest microhabitat features. ...
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Accurate estimates of forest canopy structure are central for a wide range of ecological studies. Hemispherical photography (HP) is a popular tool to estimate canopy attributes. However, traditional HP methods require expensive equipment, are sensitive to exposure settings, and produce limited resolution which dramatically affects the accuracy of gap fraction estimates. As an alternative, hemispherical images can be extracted from spherical panoramas produced by many smartphone camera applications. I compared hemispherical photos captured with a digital single lens reflex camera and 180◦ lens to those extracted from smartphone spherical panoramas (SSP). The SSP HP method leverages built-in features of current generation smartphones to produce sharper images of higher resolution, resulting in more definition of fine canopy structure. Canopy openness and global site factor from SSP HP are highly correlated with traditional methods (R2 >0.9), while leaf area index estimates are lower, especially in more closed canopies where traditional methods fail to capture fine gaps.
... For example, Hyperolius zonatus, Kassina lamottei, and Phrynobatrachus guineensis are all considered to be pristine forest specialists (Schiøtz, 1999;Rödel and Ernst, 2001; and were absent in the secondary forest in P1 Rödel, 2005, 2008), but had established populations during P2. All three species reproduce in stagnant water and we previously argued that this group of frogs was particularly affected by logging, because an opening of the canopy results in increased temperature and evaporation (Halverson et al., 2003;Pringle et al., 2003;Rice et al., 2018), thus increasing the desiccation risk of tadpoles in puddles and ponds (Ernst and Rödel, 2005;. A species particularly prone to these processes is Phrynobatrachus guineensis. ...
Assessing resilience of ecological communities, the ability of species assemblages to return to a previous state following disturbance, is an essential task when designing biodiversity friendly concepts for silvicultural management regimes. However, ecological communities may demonstrate resilience only after long time periods, and long-term data, collected on permanent study sites within standardized sampling frameworks, are scarce. We assessed the resilience of amphibian assemblages to disturbance via selective logging within the largest remaining rainforest in Ivory Coast, the Taï National Park. We analyzed extensive amphibian and habitat data obtained during two major assessment periods > 15 years apart (period P1 = in 2000, period P2 = in 2016-2017) and spanning 45 years post-logging recovery (period P1 = 30 years post-logging). We revisited identical sites and used identical, standardized methodology in both study periods. Forest structure in formerly selectively logged sites slowly recovered towards old growth forest structure over the course of 45 years, with most visible changes occurring between P1 and P2. While species richness and diversity in both old growth and secondary forests remained largely unchanged across the study periods, frog assemblage composition in formerly disturbed sites followed the forest recovery process. Yet, their composition was still distinctly different from old growth forest assemblages. The different levels of resilience over time indicate that the slowest organisms in our system (forest trees) set the pace for the recovery of associated taxa (frogs). Effective post-harvesting restoration schemes need to incorporate these delayed response times. Thus, significantly longer felling cycles than those commonly applied in regular harvesting schemes are needed if the goal is to conserve and restore original forest diversity in logged forest ecosystems.
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The role of predators and competitors in the choice of oviposition site by the treefrog Hyla chrysoscelis was examined in a randomized complete block experiment using 90 replicated experimental ponds. Control ponds containing neither predators nor competitors were contrasted with treatment ponds into which one of four species of predators (Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, Tramea carolina larvae) or one of two species of competitors (Rana catesbeiana, Hyla chrysoscelis) was added. Treatments had significant effects on the mean number of eggs deposited in ponds. Fewer eggs were laid in ponds with Ambystoma, Enneacanthus, or Hyla, as a result of fewer females laying eggs and fewer eggs laid per visit, compared with control ponds. Notophthalmus, Rana, and Tramea had no effect on the number of eggs laid. Ovipositing Hyla discriminated among potential oviposition sites based on the species present. Choice of oviposition site can determine the success of a female's reproductive investment, and it can be a mechanism affecting the structure of ecological communities as well. Our results emphasize the importance of oviposition site choice in the evolution of reproductive patterns and implicate species avoidance by ovipositing females as a mechanism important in generating variability in ecological communities.
Concern over the fate of amphibians has led to increasing attention on the mechanisms that underly their distribution. Among pond-breeding anurans, surveys have revealed that most species are restricted to a minority of ponds, but distributions can shift over time. The mechanisms for these patterns fall into two broad classes: variation in environmental attributes among ponds, and the impact of spatial context surrounding ponds. There is abundant evidence that embryonic and larval anurans are sensitive to variation in a large number of abiotic and biotic variables. In some cases these effects on performance have been linked to variation among natural ponds, and compelling evidence suggests that anuran distributions may be restricted by the action of local conditions. While these explanations are often discussed in terms of single variates (e.g., hydroperiod), field experiments suggest that elimination will often result from the joint action of multiple factors (e.g., low pH and high metal concentrations). While variation among ponds is usually considered in terms of the direct impacts on the survival, growth, and development of offspring, there is mounting evidence that adult behavior may regulate distributions via the selection of oviposition sites. Spatial context for ponds can impact anuran distributions in distinct ways. Metapopulation models suggest that pond isolation may influence patterns of occupancy, a hypothesis supported by survey data. In addition, most pond-breeding anurans utilize terrestrial uplands as adults. The amount and quality of terrestrial habitats also can impact distributional patterns. Thus, via either mechanism, spatial context can impact distributional pattern in the absence of any impact due to between-pond variation. Current evidence suggests that the restricted yet dynamic nature of anuran distributions results from factors operating within as well as surrounding breeding ponds. The dynamic natures of both ponds and surrounding uplands appear to have sizable impacts on distributional patterns. Recognition of this dynamism has critical importance for the development of effective conservation measures for anurans.
When analysing the woodland light climate, the effect of the canopy on diffuse light from the sky and on direct sunlight must be considered separately. Instantaneous measurements can be made to estimate the percentage of diffuse light cut off, the `day-light factor', but problems of spectral composition and instrument response, of unequal distribution of light over the sky, and of short-term fluctuations of light in the open are all liable to bias such estimates. Integrated measurements of light totals at any considerable number of sites are costly, require considerable maintenance, and do not permit prediction of light condition at other times of year. A reasonably accurate estimate of the mean percentage of diffuse and direct light cut off by the canopy can be obtained from hemispherical photographs, and from these percentages, the actual total of light received over any desired period may be calculated. The photographs, taken with a special camera, cover a whole hemisphere. On the circular image grids can be placed to estimate the light conditions, and details of grid construction are given. These photographic estimates compare well with estimates made from the partial regression of daily or hourly totals of light at three sites, in a deciduous wood in east England, on diffuse and direct light over equivalent periods in the open nearby. The evidence suggests that the percentage reduction of diffuse and direct light can be treated as constant over a period of a month, but these are only averages, and over shorter periods discrepancies might appear. Figures for the mean percentage reduction of diffuse light over an hour for the month may be slightly biased by unequal light distribution over the sky. As `daylight factor' has often been misapplied by biologists, and has also an architectural definition, the term `site factor', with appropriate qualifications as to type of light and time, has been adopted instead for the percentage reduction of light. When presenting results, however, these should as far as possible be given as absolute amounts of light, not as percentage reductions. Light conditions in the open vary greatly with climate and latitude, and equal figures for the percentage reduction of light from two different areas may well represent quite different absolute quantities.
This study examines the influence of forest canopy cover on the breeding pond distributions of wood frogs (Rana sylvatica), leopard frogs (R. pipiens), and American toads (Bufo americanus). Survey results indicated that wood frogs bred in both openand closed-canopy ponds, whereas leopard frogs and toads did not. We conducted two experiments in 1.5-m2 cages set in natural ponds to evaluate species' performance in open- and closed-canopy ponds. The first experiment was conducted in two open-canopy and two closed-canopy ponds and demonstrated that tadpole growth and survivorship for all three species were higher in open-canopy ponds. Wood frogs, however, also grew and survived well in closed-canopy ponds, whereas performance of leopard frogs and toads was very poor in closed-canopy ponds. Competition between species was asymmetrical; wood frogs affected leopard frogs, but the reverse effect was negligible. The second experiment was conducted in the two closed-canopy ponds with wood frogs and leopard frogs and indicated that food supplementation dramatically increased both survivorship and growth rate of leopard frogs. There was no effect of food addition on wood frog survivorship and a minimal effect on growth rate. Resource differences between open- and closedcanopy ponds therefore appear to have a strong influence on distributions of these species. Dissolved oxygen concentrations also were lower in closed- than in opencanopy ponds, which may interact with resource levels to influence species performance. Finally, we note the potentially widespread and important influence of forest and pond succession on the distribution of amphibian species. © 1999 by the American Society of Ichthyologists and Herpetologists.
l. Of 61 studies of aquatic ectotherms, increased rearing temperature (apparently not stressful for growth and development) caused a reduction in organism size at a given developmental stage in 55 cases (90.2%) and an increase in only six (9.8%). 2. The six exceptions to the size-reduction rule included one diatom (Phaeoductylum tricornutum), one copepod (Sulmincola salmoneus) and four species of mayfly (Ephemeroptera). The extent to which these exceptions could be explained by each of four mechanisms was investigated by comparing their life cycles and niches with those of other closely-related species. 3. No satisfactory explanation could be found for the response of P. tricornutum, but mechanisms consistent with the response of S. sulmoneus were: reduced risk of oxygen shortage, low risk of ectotherm predation and seasonal constraints on the life cycle. The latter may also help explain the four mayfly exceptions.