Abstract
The performance of invertebrate herbivores in grasslands can be
influenced by climate warming, but there is a lack of experimental
evidence, particularly in high elevation areas. We conducted two
complementary experiments to investigate the effect of experimental
warming on the performance of the grassland caterpillar Gynaephora
alpherakii , a notorious pest species in the alpine Tibetan meadow. The
first field experiment examined the effect of warming (non-warmed vs.
warmed) on the feeding behavior, growth and development rate of the
caterpillars. The second chamber experiment explored the relationship
between temperature and caterpillar appetite, excrement mass,
respiration rate or change of caterpillar weight. Results show that
warming significantly decreased fresh body mass of caterpillars by
27.5%, cocoon volume by 61.1%, and egg production per female moth by
26.9 % at the end of the field experiment. Warming did not affect
cocooning time but significantly increased feeding time of caterpillars
during the field experimental period. The independent chamber experiment
revealed a significant and positive correlation between caterpillar
appetite, excrement mass, respiration rate, and temperature. However,
except the first examination, there was a significant negative
correlation between changes in caterpillar weight and temperature.
Stepwise regression analysis indicated that excrement mass had the
greatest influence on caterpillar weight. The weight loss of
caterpillars to warming might thus be attributed to elevated metabolic
rates at higher temperatures, and the behavioral adaptations failed to
compensate for the physiological-induced weight loss. These findings
suggest that climate warming can modify the performance and thus the
fitness of invertebrate herbivores in high elevation areas.
Key words: body size, feeding activity, grassland caterpillar,
invertebrate, Qinghai-Tibet Plateau
Introduction
Climate change poses a significant threat to ecosystems worldwide, with
rising temperatures influencing various biological processes and
interactions (IPCC, 2021; Harvey et al., 2023). In particular,
invertebrate herbivores that play critical roles in ecological dynamics,
nutrient cycling, and energy flow within grassland ecosystems, are one
of the most vulnerable groups (Hulme, 1994; Katz, 2016; McCary &
Schmitz, 2021; Kempel et al., 2023). These organisms are vital for
maintaining plant community structure, supporting predator populations,
and enhancing soil health through their feeding activities and nutrient
excretion (Kempel et al., 2015; Crawley, 2019; Neff et al., 2023). As
global temperatures continue to rise, understanding the effects of
climate warming on herbivores becomes crucial for predicting changes in
community structure and ecosystem functioning.
Previous researches indicate that global warming has profound effects on
body size across taxa (Dale & Frank, 2014; Diamond et al., 2014; Chown
& Duffy, 2015; Merckx et al., 2018). Body size is expected to decrease
in response to climate change due to the inverse relationship between
temperature and body size, commonly known as the temperature-size rule
for ectotherms (Angilletta et al., 2004; Kingsolver & Huey, 2008). This
rule posits that ectotherms exhibit faster growth rates but attain
smaller adult sizes under elevated temperatures (Angilletta, 2009),
aligning with the intraspecific version of Bergmann’s rule (Blackburn et
al., 1999). While a decrease in size due to climate change is frequently
observed in mammals and birds and is considered a widespread response
(Gardner et al., 2011; Sheridan & Bickford, 2011), there are notable
exceptions (Meiri et al., 2009; Yom-Tov & Geffen, 2011). Recent studies
indicate a growing number of exceptions to the trend of decreasing body
size with rising temperatures. For instance, findings show an opposite
pattern in species such as the common lizard (Lacerta vivipara )
in European mountains (Chamaille-Jammes et al., 2006), various taxa on
the Tibetan Plateau (Zhao et al., 2014; Xi et al., 2016), as well as
other species like the marten (Martes americana ) in Alaska
(Yom-Tov et al., 2008) and the Eurasian otter (Lutra lutra ) in
Norway (Yom-Tov et al., 2006). These exceptional cases challenge the
applicability of Bergmann’s and James’s rules (James, 1970) for
endotherms, as well as the temperature-size rule for ectotherms (Ray,
1960; Atkinson, 1994; Atkinson & Sibly, 1997). Consequently, there is
no consistent pattern observed between warming and body size, making it
crucial to elucidate the underlying mechanisms to fully comprehend the
impact of climate warming on body size.
Warming can modify the body size of herbivores through various
mechanisms, thereby influencing their response to rising temperatures
(Sheridan & Bickford, 2011). One primary mechanism involves changes in
metabolic rates; as ambient temperatures rise, ectothermic herbivores
often experience elevated metabolic activity, which initially leads to
faster growth rates. However, this rapid growth may also result in
smaller adult sizes due to accelerated development reducing the
available time for growth (Desai & Singh, 2009). Additionally, warming
can affect resource availability and induce nutritional stress. Higher
temperatures may alter plant phenology, potentially creating mismatches
between the timing of herbivore emergence and food availability
(Williamson et al., 2002). For instance, if plants mature earlier due to
warming, herbivores may encounter reduced access to nutritious foliage,
ultimately hindering their growth and resulting in smaller body sizes
(Sahin, 2001). Behavioral adaptations also significantly influence
changes in body size. As temperatures rise, herbivores may adjust their
habitat use and feeding strategies to seek cooler microhabitats or
alternative food sources, although these adjustments may not fully
counteract the physiological effects of increased temperatures on growth
rates (Sheridan & Bickford, 2011). Furthermore, climate-induced
stressors, such as intensified competition and altered predator-prey
dynamics, can further impact body size. In warmer environments,
herbivores may face increased competition for limited resources, leading
to suboptimal growth conditions. This interplay of factors underscores
the complexity of how warming impacts the size of herbivores. In this
end, the mechanisms through which warming affects herbivore body size
are multifaceted, involving metabolic, ecological, and behavioral
responses. Understanding these interactions is crucial for predicting
the ecological implications of climate change on herbivore populations
and their roles within ecosystems.
Alpine regions, such as the Tibetan Plateau, are currently experiencing
rapid climatic shifts characterized by increasing temperatures and
altered precipitation patterns. These changes can significantly impact
local biodiversity and ecological interactions (Wu et al., 2011).
However, there is a dearth of empirical data regarding the responses of
invertebrate herbivores in high-elevation areas, which raises concerns
considering the unique adaptations and ecological roles fulfilled by
these species in fragile alpine ecosystems.
The grassland caterpillar Gynaephora alpherakii exemplifies a
pertinent case in this context. This species is a notorious pest in the
alpine Tibetan meadows, capable of causing substantial damage to
vegetation and disrupting local ecosystems. The life cycle and
population dynamics of G. alpherakii are closely linked to
environmental conditions, making it an ideal model for investigating the
impacts of climate change on herbivorous insects (Chen et al., 2015).
Previous research has shown that increases in temperature can influence
herbivore physiology, behavior, and population dynamics. For instance,
higher temperatures have been associated with increased metabolic rates,
altered feeding behaviors, and shifts in developmental timing across
various insect taxa (Bale et al., 2002; Sheridan & Bickford, 2011).
However, most of this research has predominantly focused on lowland
ecosystems, thereby highlighting a critical knowledge gap concerning
high-elevation species.
To address this gap, we conducted a comprehensive study to examine the
effects of experimental warming on the performance of G.
alpherakii through two complementary experiments. The first field
experiment aimed to assess how warming influenced the feeding behavior,
growth, and developmental rates of these caterpillars in their natural
habitat. The second controlled chamber experiment focused on the
relationship between temperature and various physiological metrics,
including caterpillar appetite, excrement mass, respiration rate, and
changes in body weight. Two specific hypotheses were tested: (1) warming
would reduce caterpillar body size, and (2) behavioral adaptations
resulting from warming would not fully offset the physiological-induced
weight loss.
Material and methods
2.1 Study site and species
This study was conducted in an alpine meadow located in Hongyuan County
(32°48′–32°52′N and 102°01′–102°33′E), Sichuan Province, China, within
the eastern Tibetan Plateau. Detailed information regarding the climatic
conditions and plant community composition can be found in prior studies
(Wu et al., 2021).
In this meadow ecosystem, the grassland caterpillar Gynaephora
alpherakii emerges as one of the most significant and prevalent
herbivorous insects. During typical growing seasons, larval densities of
this species range from several to tens per square meter, with
population outbreaks potentially reaching up to 200 individuals per
square meter (Xi et al., 2013). These caterpillars exhibit voracious
feeding behavior, primarily targeting sedges while occasionally
consuming forbs. Furthermore, their activity patterns are highly
sensitive to temperature fluctuations; they are observed crawling to the
apices of plant shoots to feed during optimal thermal conditions (e.g.,
mid-morning and mid-afternoon), while seeking refuge beneath large
leaves or on the soil surface during extreme temperatures (e.g., noon or
nighttime) (Xi et al., 2013).
2.2 Field experiment
We conducted a one-factorial experiment comprising two treatments: 1)
Non-warmed and 2) Warmed, with six replicates for each treatment. The
field experiment commenced on June 1 and concluded on September 4, 2016.
In late May 2016, twelve cylindrical cages (0.5 m in diameter and
height) were systematically deployed in a pasture that was grazed by
livestock exclusively during the winter months, maintaining a minimum
spacing of 3 m between cages. The vegetation within the study area
exhibited homogeneity. Each cage was constructed from a robust steel
frame encircled and covered with fine steel mesh (0.1 mm thick, 2 × 2 mm
mesh size). Following the initiation of the experiment, the tops of the
cages were similarly enclosed with steel mesh, and the bases were
embedded 20 cm into the soil to prevent herbivore escape. Additionally,
six cages were fitted with transparent plastic film (with sunlight
transparency exceeding 95%) to create a warmed microenvironment, while
the remaining six served as controls (non-warmed).
Experimental caterpillars were collected from adjacent areas, ensuring
that only healthy, medium-sized individuals were selected for the study.
Ten individuals of Gynaephora alpherakii were introduced into
each replicate, reflecting densities observed during both typical and
outbreak years. Prior to the initiation of the experiment, larger
herbivores (e.g., grasshoppers) and predators (e.g., Lycosa sp.)
were removed from the cages to eliminate potential confounding effects.
We measured the fresh body mass of each living caterpillar biweekly,
carefully collecting individuals from each cage, weighing them using a
portable analytical balance (Sartorius, Germany; precision 0.001 g), and
then returning them to their respective cages. Additionally, we
monitored cocoon production and alive caterpillar number every three
days from the emergence of the first cocoon on August 7 until all
caterpillars had cocooned by September 7, measuring the length and width
of 18 cocoons to calculate their volume using the elliptical volume
formula.
We also evaluated the foraging behavior of Gynaephora alpherakii following the methodology described by Xi et al. (2013). On sunny days,
we recorded the number of feeding individuals—termed “feeding
frequency”—on an hourly basis from 08:00 to 18:00 (Beijing time),
conducting two observation sessions during the initial 10 days when all
caterpillars were alive. This approach ensured that measurements of
feeding frequency were independent of density fluctuations (Xi et al.,
2013).
Temperature was monitored using thermometers (model DS1921G, Maxim
Integrated Products, Sunnyvale, California, USA) in three cages for each
treatment. Over the experimental duration, the mean daily temperature in
the warmed treatments was observed to be 1.8 °C higher than in the
non-warmed controls (Cao et al., 2022).
2.3 Chamber experiment
To determine the influence of temperature on the performance of G.
alpherakii , i.e., caterpillar appetite, excrement mass, respiration
rate and body weight, we conducted a complementary chamber experiment
during the period of the field experiment. The chamber was set as a
day/night regime of 14/10 h, respectively, and the humidity was set as
45%. The temperature gradient was 2℃ from 12℃ to 28℃, resulting in a
temperature gradient of nine levels. Each level repeated four times.
Each replicate was a transparent glass box (20, 21, and 17 cm in length,
width, and height, respectively). There were 72 boxes in total, each of
which contained 0.5 g plant fresh leaves (Scirpus pumilus ). Five
healthy, medium-sized individuals of caterpillars were additionally
introduced into each of 36 boxes, and the left 36 ones without
caterpillars. The leaves were collected from the field and cleaned up
indoor and cut down into 5 cm long fragments. The end of leaves was
enfolded by cotton and then inserted into 2 ml centrifuge tube after
immersed in water. The chamber experiment lasted for 24 hours. We
recorded the weight of leaves, the body weight of each caterpillar, and
the weight of excrement before and after the experiment. The caterpillar
appetite was calculated following the protocols of Waldbauer (1968):
where W = weight of leaves before the experiment with
caterpillars; L = weight of leaves after the experiment with
caterpillars; a = (weight of leaves before the experiment without
caterpillars - weight of leaves after the experiment without
caterpillars)/weight of leaves before experiment without caterpillars;b = (weight of leaves before the experiment without caterpillars
- weight of leaves after the experiment without caterpillars)/weight of
leaves after experiment without caterpillars
In addition, the respiration rate of caterpillars was also measured
using a non-steady-state and automated soil CO2 flux
system (LI-8100, LI-COR Biosciences, Lincoln, NE, USA) with a survey
chamber of 10 cm in diameter (835.2 cm3 for chamber
volume). The temperature gradient was 4 ℃ from 6 ℃ to 28 ℃. Before
measuring, 10 caterpillars were placed in experiment chamber with enough
food for 24 hours at each treatment (Song et al., 2008). Each treatment
contained three replicates.
2.4 Data analysis
Kolmogorov–Smirnov and Levene’s tests were used to check for normality
of the distribution and variance homogeneity of the sample residuals,
respectively. The cocooning rate was arcsine-transformed to achieve
normality. One-way analysis of variance (ANOVA) was employed to
determine the effects of warming on fresh body mass (per observation
day), cocooning rate (per observation day), cocoon volume and egg
production per female moth. Once a significant effect was detected, the
difference between treatments was determined using post hoc Tukey
tests. A generalized linear model (GLM with Poisson errors) was used to
test the effect of warming on the number of feeding caterpillars (per
observation hour). In addition, linear or exponential fittings were used
to determine the relationship between temperature and caterpillar
appetite, excrement mass, respiration rate or change of caterpillar
weight, respectively. A stepwise approach was employed to investigate
the significant caterpillar physiological factors influencing
caterpillar body size. All the data analyses were conducted in SPSS 22.
Results
Warming significantly decreased fresh body mass of caterpillars by
12.4% in 2014/6/14 (Appendix S1: Table S1: F = 7.00, p =
0.025), by 27.5 % in 2014/6/23 (Appendix S1: Table S1: F =
15.69, p = 0.003), by 30.0 % in 2014/7/10 (Appendix S1: Table
S1: F = 15.12, p = 0.003), by 27.5 % in 2014/7/23
(Appendix S1: Table S1: F = 18.23, p = 0.002) during the
observation time (Fig. 1A). Warming did not affect cocooning rate (Fig.
1B) but significantly decreased cocoon volume by 61.1 % (Fig. 1C,
Appendix S1: Table S1: F = 8.59, p = 0.015) and egg
production per female moth by 26.9 % (Fig. 1D, Appendix S1: Table S1:F = 18.22, p = 0.002).
In addition, warming significantly changed the foraging behavior of
caterpillars (Appendix S1: Table S2). Specifically, the number of
feeding individuals was significantly higher at 9:00 (Appendix S1: Table
S2: Z = 2.35, p = 0.019), 13:00 (Appendix S1: Table S2: Z = 2.33,p = 0.020) and 14:00 (Appendix S1: Table S2: Z = 3.32, p < 0.001) during the first examining time (Fig. 2A), and at
10:00 (Appendix S1: Table S2: Z = 2.01, p = 0.044), 11:00
(Appendix S1: Table S2: Z = 2.99, p = 0.003), 12:00 (Appendix S1:
Table S2: Z = 2.96, p = 0.003), 13:00 (Appendix S1: Table S2: Z =
3.71, P < 0.001), 14:00 (Appendix S1: Table S2: Z =
3.25, P = 0.001), 15:00 (Appendix S1: Table S2: Z = 2.57,P = 0.010), 16:00 (Appendix S1: Table S2: Z = 2.40, P =
0.016) and 17:00 (Appendix S1: Table S2: Z = 3.27, p = 0.001)
during the second examining time (Fig. 2B) in the warmed plots than in
the non-warmed cages.
Caterpillar appetite, excrement mass and respiration rate increased with
temperature (Fig. 3A-I). A negatively linearly relationship was observed
between the change of caterpillar weight and temperature (Fig. 3K,L),
except the first examining time (Fig. 3J). In addition, results of
stepwise regression analysis showed that excrement mass influenced
caterpillar weight most (Fig. 4).
Discussion
Our findings demonstrate that climate warming significantly impacted the
performance of the grassland caterpillar Gynaephora alpherakii in
the alpine Tibetan meadow. Elevated temperatures can lead to substantial
declines in caterpillar body mass, cocoon volume, and reproductive
output. Specifically, the observed reductions of 27.5% in fresh body
mass, 61.1% in cocoon volume, and 26.9% in egg production per female
moth underscore the potential for warming to adversely affect the
population dynamics of this notorious pest species. These present
results align with existing literature indicating that climate change
can disrupt growth and reproductive strategies across various insect
taxa (Desai & Singh, 2009; Sheridan & Bickford, 2011).
Interestingly, while warming increased feeding time, it did not alter
the duration of cocooning. This finding suggests that caterpillars may
compensate for the physiological stress induced by higher temperatures
through increased feeding efforts (O’Connor, 2009). The increase in
feeding duration could indicate a behavioral adaptation aimed at
counteracting the metabolic stress resulting from warmer conditions
(Reuman et al., 2014). However, despite these increased feeding times,
the resultant weight loss and decreased reproductive output highlight
the inadequacy of this adaptation to fully mitigate the energy deficits
associated with elevated temperatures. This conclusion is consistent
with previous studies that have shown similar patterns of increased
feeding efforts without corresponding increases in body mass or
reproductive success under stress conditions (Sahin, 2001).
The significant negative correlation between weight change and
temperature further supports our hypothesis that warmer conditions have
detrimental effects on caterpillar performance. The identification of
excrement mass as a major influence on caterpillar weight raises
critical questions regarding nutrient assimilation and energy allocation
in a warming climate (Ohlberger, 2013). Increased metabolic rates may
lead to higher energy demands, which, when coupled with potential
declines in food quality or availability, could result in reduced growth
and fitness (Kingsolver et al., 2004). Such findings are particularly
relevant as they resonate with established theories regarding the
temperature-size rule, where ectothermic organisms tend to exhibit
reduced body sizes at elevated temperatures (Atkinson, 1994).
From an ecological perspective, the implications of our findings are
twofold. First, the decrease in body size and reproductive capacity ofG. alpherakii could lead to a decline in its population density,
potentially altering herbivory dynamics in alpine grasslands. Such
shifts could have cascading effects on plant communities, particularly
if this species serves as a significant herbivore within the ecosystem.
Changes in herbivory patterns may affect plant community structure and
dynamics, leading to broader ecological consequences (Post et al., 2009;
Cao et al., 2022). Second, the potential for warmer temperatures to
shift outbreak patterns of this pest could have significant consequences
for grassland management and agricultural practices in the region.
Understanding these dynamics is critical for developing effective pest
management strategies, especially in the face of ongoing climate change.
Additionally, the findings of our study may have implications for the
conservation of biodiversity in these high-elevation ecosystems, where
species interactions are already finely balanced.
While our study offers valuable insights, it also underscores the need
for further research to explore the long-term effects of warming onG. alpherakii and other herbivorous species in high elevation
areas. Future studies should investigate the interactive effects of
multiple stressors, such as changes in precipitation patterns and
nutrient availability, to better understand the resilience of these
herbivores in a warming world. This approach will provide a more
comprehensive view of how climate change affects herbivores and their
ecosystems, allowing for better-informed management decisions.
In conclusion, our findings provide critical evidence of the detrimental
effects of climate warming on the performance of grassland caterpillars.
This underscores the urgent need for proactive management approaches to
mitigate the impacts of climate change on these vital species and their
ecosystems. As climate change continues to pose challenges to
biodiversity and ecosystem health, understanding the responses of key
species like G. alpherakii will be essential for ensuring the
sustainability of grassland environments.