Macquarie University PURE Research Management System
This is the peer reviewed version of the following article:
Yadav, S., Stow, A. and Dudaniec, R.Y. (2020), Elevational partitioning in species distribution, abundance and body size of Australian alpine grasshoppers (Kosciuscola). Austral Ecology, vol. 45, no. 5, pp. 609-620.
which has been published in final form at: doi.org/10.1111/aec.12876
This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.
1 Elevational partitioning in species distribution, abundance and body size of Australian
2 alpine grasshoppers (Kosciuscola)
3
4 Sonu Yadav1*, Adam Stow1, Rachael Y. Dudaniec1
5
6 1 Department of Biological Sciences, Macquarie University, North Ryde, 2109, NSW,
7 Australia
8
9 *Corresponding author:
10
11 Sonu Yadav, Department of Biological Sciences, Macquarie University, North Ryde, 2109,
12 NSW, Australia, Email- [email protected]
13
14 Acknowledgments
15 We thank Kate D. L. Umbers for help with field work protocols and useful discussions. We
16 thank Justin McNab for help with fieldwork, Rachel Slatyer for providing useful information
17 on the species occurrence and Giselle Muschett for helping with species identification. We
18 thank Youning Su from the Australian National Insect Collection, CSIRO museum, Canberra
19 for providing access to type specimens. This project was funded by Macquarie University
20 with start-up funding to R.Y.D., a Macquarie graduate student research grant to S.Y, a
21 Holsworth Wildlife Research Endowment grant from the Ecological Society of Australia (to
22 SY), the Theodore J. Cohn Research Fund from the Orthopterist’s Society (to SY) and the
23 Joyce W. Vickery Scientific Research Fund from The Linnean Society of NSW (to SY). All
24 procedures were performed in accordance with the animal ethical guidelines of Macquarie 25 University, Australia, and sampling permissions were obtained from local government
26 authorities and national parks (License number: SL101832). 27 Abstract
28 Changes in the physical environment with elevation can influence species distributions and
29 their morphological traits. In mountainous regions, steep temperature gradients can result in
30 patterns of ecological partitioning among species that potentially increases their vulnerability
31 to climate change. We collected data on species distributions, relative abundance and body
32 size for three grasshopper species of the genus Kosciuscola (K. usitatus, K. tristis, and K.
33 cognatus) at three locations within the mountainous Kosciuszko National Park in Australia
34 (Thredbo, Guthega and Jagungal). All three species showed differences in their distributions
35 according to elevation, with K. usitatus ranging from 1400 m asl to 2000 m asl, K. tristis
36 from 1600 m asl to 2000 m asl, and K. cognatus from 1550 m asl to 1900 m asl. Decreasing
37 relative abundance with increasing elevation was found for K. usitatus, but the opposite
38 pattern was found for K. tristis. The relative abundance of K. cognatus did not change with
39 elevation but was negatively correlated with foliage cover. Body size decreased with
40 elevation in both male and female K. usitatus, which was similarly observed in female K.
41 tristis and male K. cognatus. Our results demonstrate spatial partitioning of species
42 distributions and clines in body size in relation to elevational gradients. Species-specific
43 sensitivities to climatic gradients may help to predict the persistence of this grasshopper
44 assemblage under climate change.
45
46 Keywords
47 Australian Alps, Kosciuscola, Orthoptera, relative abundance, body size 48 Introduction
49 Alpine species are often adapted to narrow thermal ranges, making them sensitive to
50 increasing temperatures (Laurance et al, 2011; Williams et al, 2008) and more likely to be
51 adversely affected by climate change (Dirnböck et al, 2011; La Sorte & Jetz, 2010). Alpine
52 ecosystems show sharp clines in climatic characteristics leading to changes in the
53 composition and diversity of biota at fine spatial scales (Beccacece et al, 2016; Hilt &
54 Fiedler, 2005; Sundqvist et al, 2013; Zenker et al, 2015). Changes in species composition and
55 diversity along elevation gradients have been reported for several taxa (moths: Brehm &
56 Fiedler, 2003; dung beetles: Herzog et al, 2013; plants: Mota et al, 2018; birds: Rahbek,
57 1997; Zenker et al, 2015), and climate, topography and productivity are among the major
58 drivers of such changes (Currie & Kerr, 2008; Lomolino, 2001; Ramirez-Bautista &
59 Williams, 2019). Species’ physiological tolerances and microhabitat usage are further
60 contributors (Birkett et al, 2018; Röder et al, 2017; Wachter et al, 1998; Werenkraut &
61 Ruggiero, 2014), whereby specialist species with narrow distributions and narrow thermal
62 niches are expected to show increased sensitivity to shifts in climatic conditions (Hoffmann,
63 2010). Therefore, knowledge of the fine-scale distributions of species within alpine
64 environments is necessary for assessing their vulnerability to climate change.
65
66 Species distributed along environmental gradients are often observed with morphological
67 changes such as intraspecific colour polymorphism and variation in body size, particularly in
68 insects (Sheridan & Bickford, 2011; Whitman, 2008; Yadav et al, 2018). Body size is an
69 important trait related to fitness, survival, growth, and physiology (Chown & Gaston, 2010;
70 Kleiber, 1947) and may therefore influence the relative success of certain species within
71 species assemblages in variable environments. Body size in ectotherms, including insects, is
72 generally positively correlated with temperature with smaller individuals in cooler climates 73 (Berner & Blanckenhorn, 2006; Buckley et al, 2014; Mousseau, 1997; Telfer & Hassall,
74 1999), however no single consistent pattern has been found across the insect taxa (Brehm et
75 al, 2018; Maveety & Browne, 2014). This inconsistency indicates that the underlying
76 mechanisms controlling body size may vary across species and locations.
77 Insects frequently show sexual size dimorphism, with females typically being the
78 larger sex (Chown & Gaston, 2010) including in Orthoptera (Hochkirch & Gröning, 2008).
79 Males and females may show distinct variations in body size along environmental gradients,
80 potentially due to sex-specific selection pressures (Fairbairn, 1997; Stamps, 1993; Teder &
81 Tammaru, 2005) or altered developmental periods along temperature gradients (Fairbairn et
82 al, 2007; García-Navas et al, 2017; Laiolo et al, 2013). Due to a strong association between
83 environmental variables (for e.g. temperature) and variation in dimorphism in insects
84 (Davidowitz et al, 2004; Stillwell et al, 2007b), the pattern of dimorphism may vary or shift
85 with changing environmental conditions. Understanding sex-specific variation in body size
86 along elevation gradients may therefore provide insight into the environmental mechanisms
87 generating clinal distributions within species assemblages.
88
89 Alpine ecosystems are particularly susceptible to climate change (Gobiet et al, 2014; IPCC,
90 2014) and the Australian Alps are no exception. Since 1979 the Australian Alps have
91 experienced a decline in precipitation by 6%, an increase in temperature at an average of
92 0.4°C since 1954 along with a substantial decline in snow depth (Hennessy et al, 2003;
93 Nicholls, 2005; Sánchez-Bayo & Green, 2018; Wahren et al, 2013). The Australian alpine
94 bioregion is further threatened by fire, sub-alpine vegetation encroachment and impacts of
95 invasive species (Hoffmann et al, 2019). Such threatening processes have been linked to
96 alterations in the survival rates, distributions and population dynamics of endemic alpine
97 species (Green, 2010; Nicholls, 2005). For invertebrates, overwintering survival rates under 98 reduced snow cover may be decreased because snow acts as a buffer from fluctuating
99 temperatures and insulates eggs during cold winter months (Bale & Hayward, 2010). Despite
100 the potential consequences of climate change to alpine species, studies of these assemblages
101 are few in the Australian Alps (Hoffmann et al, 2019), especially for invertebrates, in spite of
102 their value in assessing ecological responses to shifting thermal regimes (Hassall, 2015;
103 Khamis et al, 2014; Yadav et al, 2019).
104
105 Here we examine the effect of temperature-elevation gradients on Kosciuscola grasshoppers
106 in terms of their occurrence, relative abundance and body size. We focus on three of five
107 species of the genus: K. usitatus, K. tristis, and K. cognatus (Orthoptera: Acrididae), which
108 all co-occur, are flightless, and endemic to the Australian Alps. Our three focal species are
109 common throughout the Kosciuszko alpine region ranging from montane woodlands to alpine
110 areas (Slatyer et al, 2016). Field observations suggests that the species may differ in
111 elevational range, with K. cognatus occupying the broadest range (Tatarnic et al, 2013) and
112 K. tristis having the narrowest range of the five species of Kosciuscola. (Slatyer et al, 2014).
113 Further observations similarly indicate that K. cognatus occupies low to middle elevation
114 areas (900-1700m), K. tristis middle to high elevations (1500-2000m) and K. usitatus (1400-
115 2200m) from middle to the highest of elevations (Campbell & Dearn, 1980). Moreover,
116 previous work has found that these grasshopper species show variation in cold tolerance
117 limits (Slatyer et al, 2016), with K. usitatus having the highest cold tolerance (CTmin= −3
118 °C), followed by K. tristis (CTmin= −2 °C) and K. cognatus (CTmin= −1 °C). Despite these
119 patterns there has been no systematic survey to date on the fine-scale elevational distributions
120 and abundances of Kosciuscola species, which are required to better characterise the current
121 structure of this species assemblage.
122 123 Here, we examine how the distribution, relative abundance and body size of three
124 Kosciuscola grasshopper species vary along elevation gradients in the Kosciuszko alpine
125 region in the Australian Alps. We collected data during a single season from replicate
126 transects along elevational gradients across three mountain regions (Thredbo, Guthega and
127 Jagungal). We test if (1) Kosciuscola species differ in occurrence and relative abundance
128 along temperature, elevation and vegetation gradients, and (2), whether intra-specific
129 variation in body size among males and female Kosciuscola is related to temperature-
130 elevation gradients.
131
132 Materials and methods
133 Study species
134 All three focal grasshopper species have non-overlapping generations (Campbell & Dearn,
135 1980; Umbers et al, 2013). Adults lay eggs during autumn and summer and hatching takes
136 place in spring (Campbell & Dearn, 1980; Umbers et al, 2013). Of the other two species, K.
137 tasmanicus only occurs in Tasmania, and K. cuneatus is observed to be absent from the
138 higher elevational zones of Kosciuszko National Park (Tatarnic et al, 2013) and thus were
139 excluded from our study. Sexual dimorphism in body size is present in all three species with
140 females having a larger body size than males (Slatyer et al, 2016; Umbers et al, 2013).
141
142 Study sites and sample collection
143 Grasshoppers were collected over a two-week period in late February to early March 2017 in
144 the Kosciuszko alpine region within the south-east of New South Wales of Australia (Fig 1).
145 Transects were constructed along an elevational range of 1,400 to 2,000m, with time-based
146 searches conducted at 50-metre increments in altitude. Samples were collected from a total of
147 48 sites across three locations: 1) Thredbo (-36.5°S, 148.3 °E), 2) Guthega (-36.4°S, 148.38 148 °E) and 3) Jagungal (-36.14°S, 148.38 °E; Table S1). We searched for the three focal species
149 within a ~30m2 plot for 20 mins by two people (40 minutes in total; except in 11 sites where
150 the number of people varied from 3-5; see Tables S2, S3, S4 for species specific details).
151 Transect length ranged from 500-600 meters and depended on the steepness of the elevation
152 gradient for each transect.
153
154 In Thredbo, we collected grasshoppers from three transects with a total of 23
155 sampling points (T1= 13 points, ~400m length, T2= 4 points, ~500m length, and T3= 6
156 points, ~250m length) with an elevational gradient ranging from ~1,400-2,000 m a.s.l (Fig.
157 1). A total of three transects were laid in Guthega with 18 sampling points (T1= 7 points, ~
158 170m, T2= 5 points, ~180m, and T3= 6 points, ~250m) with an elevational range of ~1,600-
159 1,850 m a.s.l (Fig. 1) and in Jagungal one transect was covered (T1 = 7 points, ~350m length)
160 and an elevation range of ~1,550-1,900 m a.s.l. (Fig. 1). Only one transect was sampled in
161 Jagungal due to poor accessibility. Spatial coordinates (latitude, longitude) were recorded at
162 each transect point using a handheld GPS device (Garmin Etrex10).
163
164 Grasshoppers were captured using small plastic jars and the time of the day, species name
165 and sex were recorded from live specimens upon processing. To further confirm species
166 identification, features such as eye colour, cheek pattern, pronotum shape, body shape and
167 size were compared against type specimens stored in the Australian National Insect
168 Collection (CSIRO, Canberra). Hind leg femur length was measured on the left side for each
169 live specimen using Vernier calipers (precision to 0.02 mm), which is a commonly-used
170 proxy for body size in grasshoppers (Masaki, 1967; Mousseau & Roff, 1989). All samples
171 were stored in 95% ethanol for DNA preservation.
172 173 Environmental data
174 Environmental variables were chosen based on field observations, expert opinion and
175 knowledge of the species ecology and thermal tolerance (Slatyer et al, 2016; Slatyer et al,
176 2014; Umbers et al, 2013). Temperature, precipitation (including rainfall or snow cover) and
177 topography are known to affect reproduction, growth, activity, and distribution of insects,
178 including grasshoppers (Buckley et al, 2013; Buckley & Nufio, 2014; Hodkinson, 2005;
179 Joern & Gaines, 1990; Roland & Matter, 2016; Yadav et al, 2018). Foliage cover has also
180 been linked with movement, egg laying (Clark, 1967), the distribution of morphs (Yadav et
181 al, 2018) as well as adaptive genetic variation in grasshoppers (Yadav et al, 2019).
182 Maximum summer temperature (‘Temperature’ from here on) and annual rainfall
183 (‘Precipitation’ from here on) were obtained from ANUCLIM 6.1 (Lyons et al, 2017; Xu &
184 Hutchinson, 2013) and resampled to a 30m resolution. Elevation data were derived from a
185 Digital Elevation Model (DEM) obtained from Geoscience Australia at a 30m resolution
186 (Table S1 for per site information). Foliage Projective Cover (FPC) refers to the percentage
187 of land obscured by woody vegetation and trees, thus FPC provides a fine-scale measurement
188 of vegetation cover. FPC was obtained from the Terrestrial Ecosystems Research Network
189 AusCover database (TERNAusCover, 2011) at a resolution of five meters. To extract FPC
190 values, a buffer of 30m was used around each sampling site from which cell values were
191 averaged using the raster (Hijmans & van Etten, 2014) package in R (Team, 2017).
192 Correlations between environmental variables were examined using Pearson correlation
193 method and highly correlated variables (r >0.80) were removed from further analysis.
194
195 Temperature, precipitation and FPC association with elevation
196 We examined whether temperature, precipitation and FPC changed significantly with
197 elevation within our three study regions by fitting a linear mixed effect model (LMM) with 198 elevation as a fixed effect and the region (Thredbo, Guthega and Jagungal) as a random
199 intercept. The LMMs were fitted using the lmer function of the lme4 package (Bates et al,
200 2015) and goodness of fit was examined by calculating conditional R2 using the
201 r.squaredGLMM function in MUMIn (Barton, 2018). The statistical significance of the fixed
202 effect in LMM was examined using ANOVA with a Chi-square test in the R package car
203 (Fox & Weisberg, 2011). Diagnostic plots for each LMM were examined to ensure model
204 assumptions were met. We conducted all analyses in R.
205
206 Relative abundance along elevation and FPC
207 The relative abundance of each species at each site was recorded as capture rate per minute,
208 calculated as the number of grasshoppers caught divided by the number of searchers
209 multiplied by the total number of minutes spent collecting. This approach of obtaining
210 relative abundance estimates is commonly applied, for example in insects (Lancaster et al,
211 2015; Yadav et al, 2018) and salamanders (Dudaniec & Richardson, 2012). Due to the
212 correlation between elevation and precipitation, and elevation and temperature, we used
213 elevation as a proxy for temperature-precipitation in all further models (see Results section).
214 The influence of elevation and FPC on relative abundance in the three sampled mountain
215 regions pooled together was evaluated by fitting a LMM. Models were fitted individually for
216 each species with elevation and FPC as a fixed effect and region and time of the day as
217 random effects. Time of the day was used as a random effect to account for possible bias in
218 the number of samples collected due to variations in temperature and weather differences
219 during the day. Model significance were assessed using Chi-square test.
220
221 Body size correlation with elevation 222 To test if body size of each species changed significantly with elevation and to account for
223 sex-specific differences, we first fitted the model with body size (measured from femur
224 length) as a response variable, elevation as a predictor, sex as an interaction term with
225 elevation and region and site as a random effect. We then examined the change in body size
226 along elevation gradients for male and females separately for each of the three species,
227 pooling data from all transects sampled according to each mountain region. A LMM was
228 fitted using femur length (body size proxy) as a response variable and elevation as a fixed
229 effect, with region and site as random effects. In addition to region, site was also used as a
230 random effect to account for non-independence of individuals from the same site. The
231 correlation between body size and FPC was not examined due to less variation in FPC at
232 some of the sites (Table S1).
233
234 Results
235 Temperature, precipitation and FPC association with elevation
236 Precipitation was positively correlated with elevation (χ2 = 1154.5, DF=1, P = <0.0001, R2 =
237 0.96). Temperature (χ2 = 1749.5, DF=1, P = <0.0001, R2 = 0.97) and FPC (χ2 = 4.51, DF=1, P
238 = 0.03, R2 = 0.20) both showed a significant negative association with elevation (Fig. S1).
239
240 Relative abundance with elevation and FPC
241 The relationship between relative abundance and elevation was not consistent across species.
242 For K. usitatus, relative abundance decreased with elevation (χ2 = 6.72, DF=1, P= 0.009, R2 =
243 0.26, Fig. 2A) with lower relative abundance at higher elevations. The relative abundance of
244 K. tristis increased with elevation (χ2 =17.61, DF=1, P = <0.0001, R2 = 0.44; Fig. 2B). No
245 significant correlation was evident between the relative abundance of K. cognatus and
246 elevation (χ2= 0.006, DF=1, P= 0.93; Fig. 2C). Similarly, the relationship between relative 247 abundance and FPC varied across species. K. cognatus showed a significant negative
248 correlation with FPC (χ2= 4.7, DF=1, P=0.03, R2 = 0.36; Fig. 2D), with higher abundances at
249 sites with low vegetation cover, however relative abundance was not significantly correlated
250 with FPC for K. usitatus (P= 0.10) or K. tristis (P= 0.80).
251
252 Body size change with elevation and sex
253 The relation between body size and elevation varied between species and sexes. The
254 interaction term was non-significant for K. usitatus (P = 0.14) showing that the change in
255 body size with elevation was not influenced by sex differences, and sexes similarly showed a
256 decline in body size with elevation (male N = 174, P= 0.0001, R2 = 0.34 and female N =237,
257 P=0.026, R2 = 0.04, Table 1, Fig. 3A,3B). However, the interaction effect of sex and
258 elevation was significant for K. tristis (P=0.009, Table 1) showing that the observed change
259 in body size was affected by sex, with only females significantly declining in body size with
260 elevation (females: N=91, P= 0.03, R2 = 0.21; males: N=188, P= 0.99, R2 = < 0.01; Table 1,
261 Fig. 3C, Fig. S2A). For K. cognatus the interaction effect of sex and elevation was also
262 significant (P=0.045) but in contrast, male body size significantly declined with increasing
263 elevation (females: N=128, P= 0.97, R2 <0.01; males: N=143, P= 0.019, R2 =0.70, Table 1,
264 Fig. 3D, Fig. S2B).
265
266 Discussion
267 Here we characterise changes in the occurrence and relative abundance of Kosciuscola
268 grasshoppers along elevational gradients in the Australian Alps. Elevation was strongly
269 associated with temperature and precipitation in the sampled regions, and was the dominant
270 driver of change in the relative abundance of K. usitatus and K. tristis, which occur at higher
271 elevations. In contrast, the relative abundance of the lower elevation species, K. cognatus, 272 was not significantly influenced by elevation, but decreased in relative abundance with
273 increasing foliage cover. Furthermore, we found sex-specific associations between body size
274 and elevation, with both male and female K. usitatus, female K. tristis and male K. cognatus
275 showing declines in body size. Our study highlights that elevational gradients strongly
276 associated with climatic variables and vegetation gradients can influence the relative
277 abundance and body size of grasshoppers at fine spatial scales, which may be indicative of
278 species and sex-specific sensitivities to climate change and habitat degradation.
279 280 Relative abundance and elevation
281 The relative abundance of K. tristis was greater at higher elevations, but the opposite effect
282 was found for K. usitatus, and K. cognatus showed no relationship. In ectotherms, including
283 insects, thermal preferences and habitat variation along elevation gradients may affect
284 community structure and composition (Birkett et al, 2018; Röder et al, 2017). In Kosciuscola,
285 the differential patterns of relative abundance we observe in the three focal species are likely
286 due to differences in their thermal tolerances along elevational gradients, as shown in K.
287 usitatus, which maintains higher cold tolerance compared to K. tristis (Slatyer et al, 2016). In
288 addition, interspecific differences in heat tolerance have been observed, whereby populations
289 above the tree line are more tolerant to heat compared to lower-elevation populations (Slatyer
290 et al, 2016). Furthermore, a previous study on K. cognatus, K. usitatus and Praxibulus sp. by
291 Dearn (1977), showed a decline in egg production per female at higher elevations, which may
292 act to partially explain the decline in relative abundance with elevation in K. usitatus.
293
294 For insects to cope with variable thermal regimes in alpine ecosystems it is beneficial to
295 regulate body temperature via colour polymorphism in order to exploit particular elevation
296 zones (Trullas et al, 2007). The increase in relative abundance of K. tristis at higher 297 elevations may be assisted by its documented colour changing behaviour that occurs
298 primarily in response to ambient temperature, where males change colour from turquoise blue
299 to black at lower temperatures (Umbers, 2011; Umbers et al, 2013). The maintenance of
300 different colour morphs in species along elevational gradients has been shown in several
301 insect species (e.g. Dearn, 1990; Hodkinson, 2005; Valverde & Schielzeth, 2015), and may
302 be assist thermoregulation (Harris et al. 2013), particularly at higher elevations (Umbers et al.
303 2013 .
304
305 Vegetation cover along the sampled transects was not a significant predictor of abundance in
306 K. usitatus and K. tristis but was significant for K. cognatus, whereby a large number of
307 individuals were observed at sites with lower vegetation cover (Fig. 2D). The higher relative
308 abundance of K. cognatus at lower vegetation site is unlikely to be a consequence of
309 increased detectability in areas of low vegetation cover as other species collected from the
310 same sites did not show significant change in abundance with FPC. In grasshoppers,
311 vegetation cover is highly important for egg laying, early instar survival, food resources
312 (Clark, 1967) and basking which in turn is very important for thermoregulation (Chappell &
313 Whitman, 1990; Kearney et al, 2009). Grasshoppers maintain preferred body temperature
314 through basking (Chappell & Whitman, 1990; Samietz et al, 2005), therefore vegetation
315 cover can affect thermoregulatory behaviour (principal means of controlling body
316 temperature with respect to ambient temperature) by altering the availability of basking sites.
317 The degree to which ectotherms can withstand changes in their ambient thermal environment
318 is crucial in regulating their abundance (Willott & Hassall, 1998). Vegetation cover has been
319 found to be an important predictor of the relative abundance and population dynamics of
320 other grasshopper species (Anderson, 1964; Kemp et al, 1990; Yadav et al, 2018). Our
321 previous study on a widespread grasshopper, Phaulacridium vittatum detected associations of 322 candidate loci putatively under selection with foliage cover, highlighting its potential role in
323 shaping adaptive genetic variations (Yadav et al, 2019). The absence of a significant
324 relationship in K. usitatus and K. tristis with FPC is likely due to lower variability in the
325 foliage cover in the species preferred elevation zones. A further examination of the
326 correlation in other areas might shed light on the interspecific differences in the relative
327 abundance change with FPC.
328
329 Body size variation along the elevation gradient
330 Temperature and precipitation gradients associated with elevation are important for
331 thermoregulation and resource availability, which in turn may exert selective constraints on
332 body size in alpine grasshopper communities (Laiolo et al, 2013; Whitman, 2008). At higher
333 elevations with lower temperatures, heat gain in insects may dominate over heat conservation
334 and insects with smaller body size may rapidly gain and lose heat (Kubota et al, 2007). The
335 capacity for rapid heat gain and loss in small ectothermic species leads to rapid changes in
336 body temperature under variable conditions. This is likely to confer wider thermal tolerances
337 (Peters & Peters, 1986), allowing small ectothermic insects to occupy higher elevations. Our
338 observation of declining body size with elevation may indicate selection for more efficient
339 thermoregulation at lower temperatures, however further investigation in to how body size
340 variation affects basal metabolism is required.
341
342 As univoltine species, the three studied Kosciuscola grasshopper species may experience
343 growth limitation at higher elevations due to a shorter season (i.e. summer period).
344 Constraints on resources and season length at higher elevations may lead to variation in the
345 timing of egg hatching, with implications for developmental periods in grasshoppers (Berner
346 & Blanckenhorn, 2006; Berner et al, 2004) and in other insects (Hodkinson, 2005). Short 347 season length may reduce a species growth period, and since this is positively correlated with
348 body size (Roff, 1980), short seasons may lead to smaller body size (Blanckenhorn &
349 Demont, 2004). Adult body size and fecundity may be particularly limited in univoltine
350 species that are sensitive to the length of a single season, as opposed to multivoltine species
351 that have a longer time period to grow and reproduce (Chown & Gaston, 1999; Honěk, 1993;
352 Horne et al, 2018; Kozłowski et al, 2004). In addition, grasshoppers at higher elevations may
353 experience later hatching compared to those at lower elevations (Mousseau, 1997), leading to
354 further shortening of the season length for higher elevation populations. Such patterns have
355 not been studied in Kosciuscola but may contribute to the overall differences in body size we
356 found according to elevation.
357
358 While most insects show sexual size dimorphism, geographic trends in body size change
359 between males and females may also differ (Stillwell et al, 2010; Stillwell et al, 2007a). We
360 found sex-specific patterns of body size change with elevation for K. tristis and K. cognatus
361 suggesting that sex-specific selection and fitness pressures may act on the sexes. Our results
362 are consistent with those of a previous study that examined femur length of only female K.
363 cognatus and K. usitatus in the same study area (Dearn, 1977), which failed to observe a
364 change in body size with elevation for K. cognatus, but found a negative correlation for K.
365 usitatus. Sex-specific clines in body size across geographical scales have been documented,
366 yet their underlying causes are not clear (reviewed in Blanckenhorn et al, 2006b).
367 One explanation is that environmental gradients can influence males and females
368 differently, creating sex specific clines (for e.g. Laiolo et al, 2013; Teder & Tammaru, 2005).
369 Elevational gradients in our study area are strongly correlated with precipitation and
370 temperature, and our results suggest that these variables affect body size in one sex more than
371 the other, as indicated for K. tristis and K. cognatus. The absence of a significant correlation 372 between female K. cognatus body size with elevation may indicate that females benefit from
373 a more consistent body size than males due to female-specific reproductive trade-offs with
374 environmental pressures. Such patterns have been found in several other insects for example
375 in grasshopper Chorthippus parallelus (Laiolo et al, 2013) and in seed beetle Callosobruchus
376 maculatus (Stillwell & Fox, 2007), whereby environment pressures such as temperature and
377 development time constrain variation in female body size than males.
378
379 The extent of sexual selection on males and natural selection on females for increased body
380 size, and therefore increased fecundity, may further lead to sex-specific body size clines
381 (Blanckenhorn et al, 2006a; Teder, 2014). The predominance of female-biased sexual size
382 dimorphism in Orthoptera (Hochkirch & Gröning, 2008) and in our study system suggests
383 that fecundity selection is an important selection pressure in this Order, which often results in
384 a change in body size in females compared to males (Laiolo et al, 2013) as observed in K.
385 tristis. Taken together, the evolution of male and female body size may follow a divergent
386 evolutionary trajectory as a consequence of processes such as sexual selection on males to
387 attain maturity at smaller sizes, fecundity selection on females, or sex-specific plasticity due
388 to ecological pressures (Fairbairn, 2013; Fairbairn et al, 2007; García-Navas et al, 2017),
389 leading to a decoupling of body size evolution between the sexes.
390
391 Conclusion
392 Our study suggests that fine-scale elevation gradients and habitat variables can significantly
393 influence relative abundances and body size clines in alpine insect species. Our results imply
394 that a further change in climatic and vegetation conditions may lead to changes in species
395 distributions and fitness related traits such as body size. While these patterns were observed
396 only in one season, statistical support was robust, though a greater sampling effort across 397 years with a longer sampling period throughout the season would further strengthen our
398 conclusions across spatial and temporal scales. An understanding of species distributions and
399 abundances in alpine regions is necessary to monitor responses to future change, and our
400 study exemplifies the use of invertebrate species for measuring these processes. In the
401 Australian Alps this is particularly important as the region is characterised by relatively flat
402 and rounded mountains with shallow elevational gradients, (1,600-2,228m on the mainland;
403 Tatarnic et al, 2013), so the opportunity for alpine species to migrate towards cooler, higher
404 elevations under climate-change induced temperature increases is restricted (Tatarnic et al,
405 2013). Furthermore, the results of our study provide a basis for investigating molecular
406 signatures of local adaptation across species in this system, which can inform the broader
407 conservation management of alpine species under environmental change. 408 References
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688 689 Table and Figure
690 Table 1: Correlations between body size and elevation in three Kosciuscola species, shown
691 for males and females individually and for the interaction between sex and elevation
692 (indicated by * mark). ‘Region’ indicates the number of regions analysed in the model.
693
Sex Observations Region Estimate ± SE Chisq DF P K. usitatus Male 174 3 -0.002 ± 0.00 14.295 1 0.0001
Female 237 3 -0.001 ± 0.00 4.93 1 0.02 Elevation*Sex 411 3 - 2.17 1 0.14 K. tristis Male 188 2 - 0 1 0.99
Female 91 2 -0.003 ± 0.001 4.27 1 0.03 Elevation*Sex 279 2 - 6.7162 1 0.009 K. cognatus Male 143 2 -0.012± 0.00 5.5 1 0.01
Female 128 2 - 0.00006 1 0.97 Elevation*Sex 271 2 - 4.016 1 0.045
Collection sites −36.00
Collection sites Collection sites −36.00 − 36.0 −36.05 Collection sites −36.05
− 36.0 −36.1 Jagungal−36.10
Longitude −36.10 Longitude
Collection sites Jagungal −36.35 T1 −36.15 −36.15 −36.2 −36.2 −36.36 0 2 4km −36.20 −36.20
148.30T1 148.35 148.40 Latitude 148.30 148.35 148.40 Latitude −36.3 −36.37 Guthega Jagungal Thredbo Guthega Jagungal Thredbo Longitude atitude L T2 Longitude GuthGuthegaega −36.38 −atitude 36.4 L Longitude −36.4 T3
Collection− 36.39sites 0 1 2km −36.5 Thredbo −36.49 −36.40 Thredbo T2
148.35 148.36 148.37 148.38 148.39 148.40 −36.6 Latitude −36.6 0 10 20km T3 −36.51 Guthega T1Jagungal Thredbo Longitude 148.1 148.2 148.3 148.4 148.5 Latitude Longitude−36.53 Guthega Jagungal Thredbo 148.1 148.2 148.3 148.4 148.5 Latitude 0 2 4km
694 Longitude Guthega Jagungal Thredbo 148.20 148.25 148.30 695 Figure 1: Collection sites for all three species across the three sampledLatitude mountain regions Guthega Jagungal Thredbo
696 (Thredbo, Guthega and Jagungal) shown on a relief map of Kosciuszko National Park.
697
698
699
700
701 (A) 1.2 (C) 1.00
0.80.9 0.75
area
GuthegaGuthega area 0.6 0.4 JagungalJagungal GuthegaGuthega ThredboThredbo 0.50 JagungalJagungal ThredboThredbo
Relative abundance abundance Relative Relative 0.3 Relative abundance Relative 0.25 0.0
0.0 1400 1600 1800 2000 0.00 Elevation (m asl) 1400 1600 1800 2000 Elevation (m asl)
(B) (D) 1.00
0.8 0.75
area area GuthegaGuthega GuthegaGuthega 0.50 JagungalJagungal JagungalJagungal 0.4 Thredbo Thredbo ThredboThredbo Relative abundance Relative
Relative abundance Relative 0.25
0.0
0.00 0 20 40 60 1400 1600 1800 2000 FPC (%) 702 Elevation (m asl)
703 Figure 2: The relative abundance of the three Kosciuscola grasshopper species varied in
704 response to elevation and Foliage Projective Cover (FPC %). (A) K. usitatus; (B) K. tristis;
705 (C) K. cognatus; (D) K. cognatus vs FPC. Regions were pooled for analysis. (A) (B)
13 K. usitatus male K. usitatus female
16 12
15 11
14 10 Femur length (mm) Femur Femur length (mm) Femur
13 9
1400 1600 1800 1400 1600 1800 Elevation (m asl) Elevation (m asl)
12.0 (D) (C) K. cognatus male K. tristis female 16 11.5
11.0 14
10.5 Femur length (mm) Femur Femur length (mm) Femur 12
10.0
1600 1700 1800 19000 1600 1700 1800 1900 2000 Elevation (m asl) 706 Elevation (m asl)
707 Figure 3: Significant changes (P<0.05) in sex-specific body size with elevation in three
708 species of Kosciuscola: K. usitatus A) male and B) female; C) K. tristis female; D) K.
709 cognatus male.
710