Is Spatial Variation in Population Size Structures of a Stream-Dwelling Caddisfly Due to the Altered Effects of a Predator by a Third-Party Species?

Is spatial variation in population size structures of a stream-dwelling caddisfly due to the altered effects of a predator by a third-party species?

Kelly M. Murray, David Stoker, Catherine M. Pringle & Troy N. Simon

Hydrobiologia The International Journal of Aquatic Sciences

ISSN 0018-8158

Hydrobiologia DOI 10.1007/s10750-018-3674-0

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Hydrobiologia https://doi.org/10.1007/s10750-018-3674-0

PRIMARY RESEARCH PAPER

Is spatial variation in population size structures of a stream- dwelling caddisﬂy due to the altered effects of a predator by a third-party species?

Kelly M. Murray . David Stoker . Catherine M. Pringle . Troy N. Simon

Received: 11 August 2017 / Revised: 22 March 2018 / Accepted: 4 April 2018 Ó Springer International Publishing AG, part of Springer Nature 2018

Abstract Predators alter abundances and life history larvae differed between reaches in the majority of characteristics of prey, and effects of predator–prey replicate streams, with smaller median body lengths in interactions can be altered by third-party species. Killifish-Only reaches. Killifish–Guppy reaches had Here, we examine size structures of the caddisfly, higher proportions of the largest instar, but we did not Phylloicus hansoni, in Trinidadian streams with two find differences in body length within an instar. No distinct fish assemblages: upstream reaches where the evidence of size-selective predation was found predatory killifish, Anablepsoides hartii, is the only through analysis of killifish stomach contents, and fish species (Killifish-Only reaches), and downstream environmental variables were largely similar between reaches where killifish and the omnivorous guppy, upstream and downstream reaches of the five study Poecilia reticulata, coexist (Killifish–Guppy reaches). streams, aside from higher killifish abundances in We asked: Do P. hansoni larvae exhibit differences in upstream reaches. Our results, coupled with previous size structure between reaches with differing fish evidence of guppies altering killifish populations, assemblages? We found that size distributions of suggest that the mediating effects of a third-party species (guppies) on predator–prey (killifish–caddisfly) interactions can affect the population size struc- Handling editor: Marcelo S. Moretti ture of prey populations.

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10750-018-3674-0) con- Keywords Size distributions Á Phylloicus Á tains supplementary material, which is available to authorized Neotropics Á Species interactions Á Aquatic insects users.

K. M. Murray (&) Á D. Stoker Á C. M. Pringle Á T. N. Simon Odum School of Ecology, University of Georgia, Athens, Introduction GA 30602, USA e-mail: [email protected] Predation can affect prey traits such as body size and K. M. Murray life history attributes in addition to reducing prey Department of Entomology, University of Georgia, abundance (Abrams & Rowe, 1996; Lima, 1998; Athens, GA 30602, USA Peckarsky et al., 2008a). This is of particular interest in aquatic insect populations, as their complex life T. N. Simon Warnell School of Forestry and Natural Resources, cycles balance growth and predator avoidance, often University of Georgia, Athens, GA 30602, USA 123 Author's personal copy

Hydrobiologia under seasonal constraints (Ludwig & Rowe, 1990; functioning of Trinidadian headwater streams as the Rowe & Ludwig, 1991). Through indirect effects, only leaf-shredding invertebrate (Botosaneanu & predators such as fish or other invertebrates can induce Sakal, 1992), and both leaf decomposition and P. changes in size at emergence (Feltmate & Williams, hansoni biomass are affected by the fish community 1991; Peckarsky et al., 2001) and fecundity (Greig & (Simon, 2015). Fish assemblages consistently differ McIntosh, 2008). Predators also exert direct effects on across small spatial scales: Below barrier waterfalls, prey traits through size-selective predation (Sańchez the killifish Anablepsoides hartii (Boulenger, 1890; et al., 2006; Rodrı´guez-Pe´rez et al., 2007). These syn. Rivulus hartii) and the guppy Poecilia reticulata effects could induce positive or negative size changes Peters, 1859 coexist, and above waterfalls, the killifish depending on the nature of the prey’s life cycle and is the only fish species (Fig. 1a). Guppies consume attributes of the predator. Understanding these preda- algae, detritus, and small benthic invertebrates (typ- tor–prey relationships and the influence of potential ically Diptera larvae; Bassar et al., 2010; Zandona` mediating factors upon them are important because et al., 2011), while killifish feed mainly on terrestrial aquatic insects fulfill a variety of trophic roles and aquatic invertebrates (Gilliam et al., 1993). essential to ecosystem functioning (Wallace & Web- Killifish also consume guppies at all life stages (Liley ster, 1996) and are significant energy subsidies to the & Seghers, 1975), but guppies affect killifish popula- terrestrial environment upon emergence (Nakano & tions by preying on killifish larvae (Fraser & Lam- Murakami, 2001; Hoekman et al., 2012). phere, 2013). Because P. hansoni larval biomass is Species interactions, ranging from predator–prey negatively affected in reaches with only killifish interactions to mutualistic relationships, are variable compared to those also containing guppies (Simon, depending upon the context in which the interactions 2015), it is likely that killifish are the most prominent occur (Thompson, 1988; Agrawal et al., 2007; Cham- predators of P. hansoni larvae, and guppies lessen the berlain et al., 2014). Abiotic conditions that shift along impacts of killifish on P. hansoni. Here, we aim to environmental gradients can regulate species interac- determine whether the presence of guppies acting as a tions (Menge, 1976; Zamora, 1999; Breeuwer et al., third-party species to the killifish–caddisfly relation- 2008) and indirectly determine community composi- ship has additional effects on the larval size structure. tion (Leary et al., 2012; Griffiths et al., 2015). Our primary objective was to assess if larval Additionally, biotic factors such as the influence of a populations of P. hansoni exhibit different size third-party species on a predator–prey relationship can structures between stream reaches with only killifish play an important role in altering the outcome of [Killifish-Only (KO) reaches] and those where killifish interactions (Wilbur, 1972; Werner & Peacor, 2003; and guppies are sympatric [Killifish–Guppy (KG) Dambacher & Ramos-Jiliberto, 2007). Here, we define reaches]. In terms of overall larval size structure, we a third-party species as one that has a modifying predicted that, where killifish existed alone, their influence on an interaction between two other species, increased predation pressure would result in P. producing indirect effects (Wootton, 1994). The hansoni populations consisting of smaller sized indi- mediating effect of third-party species on top-down viduals. To examine the mechanism underlying any predator effects has been documented in terrestrial differences in overall population size structure, we (Finke & Denno, 2004) and aquatic (Stelzer & conducted analyses to estimate potential differences in Lamberti, 1999) systems by measuring the resulting the history of both ‘‘growth’’ and ‘‘development’’ over change in abundance of prey species; however, the the past dry season between larval populations of each impact of a third-party species on a prey population reach type. Growth refers to general increases in body size structure has received less attention. size, while development specifically refers to move- To examine such potential indirect effects of a ment through distinct life stages (Butler, 1984), which third-party species, we used a system of headwater are represented in this case by larval instars. There- streams in Trinidad’s Northern Range Mountains, and fore, if there is a varying median size of larvae within a focused on size of larvae of the caddisfly Phylloicus specific instar between reach types at the time of hansoni Denning, 1983 (Trichoptera: Calamocerati- sampling, this could indicate differences in growth, dae) as a response to altered predator effects. This which may be further manifested by differently sized caddisfly species plays an important role in the adults upon emergence (as shown in other systems, 123 Author's personal copy

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Fig. 1 a Schematic of the fish assemblages within the focal reach was interspersed over * 100 m. b Map of Trinidad streams in Trinidad’s Northern Range: Upstream from barrier showing the Caroni river basin and focal stream locations (CED waterfalls are reaches with killifish (Anablepsoides hartii) as the El Cedro, END Endler, GDC Guard Dog Creek, TRT Trip Trace, only fish species (Killifish-Only or ‘‘KO’’ reaches), and RDN Ramdeen). Area of call-out showing focal streams downstream are reaches with both killifish (at lower densities) represents * 17% of total Caroni drainage area, which is and guppies (Poecilia reticulata; Killifish–Guppy or ‘‘KG’’ * 600 km2 reaches). Sampling of Phylloicus hansoni larvae within each

Peckarsky et al., 2001). Conversely, if there are no We predicted that, within this cross-section of larvae differences in median size within speciﬁc instars populations, P. hansoni would exhibit smaller body between reach types but there are differential propor- sizes within each instar in KO reaches compared to tions of instars, this could indicate potential differ- instars of KG reaches. Our other objective was to ences in development patterns between environments. investigate possible mechanisms for differing larval

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Hydrobiologia size structures, including consistent difference in habitat availability and food resources, as these larvae abiotic variables and aspects of the fish community. consume leaf detritus of terrestrial origin and prefer Samples from previous studies were used to examine shallow pools along stream margins (Wantzen & killifish diet in the context of guppy presence or Wagner, 2006; Turner et al., 2008): (1) wetted width absence. We hypothesized that positive size-selectiv- [or, width of the stream (cm)], (2) canopy cover (%), ity from killifish (rather than environmental differ- (3) depth (cm), and (4) leaf litter standing stocks ences) would serve as a mechanism of any exhibited [coarse benthic organic matter (CBOM), g m-2]. differences in larval size, and this size-selectivity from Transects perpendicular to the stream were selected killifish would be more pronounced where killifish at three points along each 100-m reach. We used a exist without guppies. spherical densiometer to estimate canopy cover over the stream by taking measurements at the middle and edges of the stream for each transect. Average depth Methods along transects was calculated from five measurements at uniform distance along the wetted width Site description (Hauer & Lamberti, 2007). We collected CBOM by hand-picking all benthic leaf litter from the stream that We conducted this study in streams on the southern fell within a 30 cm width along the transect line slope of Trinidad’s Northern Range Mountains, within (Hauer & Lamberti, 2007). The CBOM samples were the Caroni River drainage (Fig. 1b). The Caroni rinsed over a 250-lm sieve to remove sediment and drainage is located on the northwestern side of the dislodge macroinvertebrates, air-dried for 36 h in a island and encompasses an area of * 600 km2.We cabinet with a heat lamp to reduce humidity, and used five focal headwater streams: El Cedro (CED); weighed to the nearest 1 mg. Endler (END); Guard Dog Creek (GDC); Ramdeen We estimated killifish abundance to ensure that our (RDN); and Trip Trace (TRT), all of which exhibited focal streams were consistent with previous findings the pattern of decreasing number of fish species with that killifish are generally more abundant in KO increasing elevation shown in other Trinidadian reaches (Gilliam et al., 1993; Walsh & Reznick, 2009), streams (Gilliam et al., 1993). This tropical area which would presumably increase predation pressure experiences a warm climate divided into wet and dry on P. hansoni larvae. We deployed a single, baited season each year. minnow trap in each of three pools along the sampling We conducted stream surveys and P. hansoni larvae reach. All killifish captured within 10 min of placing collections during 11–22 May 2013, corresponding to the trap were counted, measured, and released, the end of the dry season in Trinidad. During the rainy yielding a catch per unit effort (CPUE, or number of season, periodic flooding often precludes stream individuals caught per trap deployment). Each killifish sampling. We sampled in reaches above barrier was categorized into one of the following five size waterfalls where killifish were found alone (KO) and classes based on body length: (1) juvenile (\ 15 mm); in reaches below barrier waterfalls, where killifish and (2) small (15–30 mm); (3) medium (30–45 mm); (4) guppies coexist (KG). KO and KG reaches were large (45–60 mm); and (5) extra-large ([ 60 mm), in 100 m each and typically separated by \ 50 m. In a order to determine whether there were differences in study using streams with this paired-reach structure, killifish size distributions between KO and KG Walsh & Reznick (2009) determined that environ- reaches. mental characteristics including temperature, dis- solved oxygen, salinity, and pH did not differ Phylloicus hansoni size structure between KO and KG reaches. To evaluate differences in P. hansoni size structures, Stream survey we collected larvae in the KO and KG reaches of all five focal streams. In each stream reach, we collected We measured four environmental characteristics larvae from three shallow pools containing naturally between KO and KG reaches of each focal stream accumulated leaf litter by searching leaf surfaces for that could affect P. hansoni populations in terms of seven minutes per pool, or until at least 50 individuals 123 Author's personal copy

Hydrobiologia total were found. Specimens were preserved in 5% killifish predation, we utilized two sets of samples that formalin to maintain original body length. Addition- were collected in April 2011 and stored in the ally, in an attempt to measure potential differences in laboratory, 2 years prior to our size structure collec- size of emerging adults to complement the study of tions: killifish gut contents and leaf pack samples that larval size structure, we placed pyramid-style emer- contained P. hansoni individuals collected from KO gence traps in the KO and KG reaches of RDN for and KG reaches. Both sets of samples were collected 1 week. Emergence traps were placed over pool from three of the streams included in the present study habitats and were checked every 24 h for adults. (CED, END, and GDC) and one additional stream, Body length and head capsule width of larvae were Cuara Steep Creek, which also shares the paired measured in the laboratory to the nearest 0.1 mm structure of KO and KG reaches described previously. under a dissecting microscope. Body length, which has been found to be the most accurate predictor of Killifish gut contents individual biomass in another Phylloicus species (Becker et al., 2009), was measured from the top of We quantified the size of larval P. hansoni found in the head capsule to the posterior of the abdomen. Head killifish gut contents to determine whether larval capsules were measured across the widest point. We consumption was size-dependent and whether there designated instar groups based on head capsule width were differences between reaches. Female fish had measurements, because we assumed that Dyar’s Law been dissected and the digestive tract was preserved in applied to P. hansoni. Dyar’s Law maintains that since 10% formalin. Gut contents were extracted from the insect development proceeds by discrete steps and that stomach and searched to identify P. hansoni larvae. the size of highly sclerotized body parts (such as the Typically, only the head capsule remained intact, head capsule) only increase at ecdysis, changes in which we measured to compare the average size of sizes of these body parts represent development consumed larvae. We analyzed gut contents of 52 and through the life cycle (Dyar, 1890). Alternatively, 40 killifish from KO and KG reaches, respectively. since changes in body length are not restricted to Guppy gut contents were collected and analyzed in a ecdysis, differences among larvae can represent similar manner during a separate study and did not growth due to feeding and metabolic activity (Swee- provide evidence of P. hansoni predation. ney, 1984). Therefore, instar group was treated as a categorical variable (designating development) and Leaf pack samples body length as a continuous measure of growth (Butler, 1984). We designated instar groups according We also compared the size of P. hansoni larvae to head capsule width as small (S, 0.2–0.4 mm), present in the stream with those found in killifish gut medium (M, 0.5–0.8 mm), and large (L, 0.9–1.1 mm) contents to (1) ensure P. hansoni individuals were by constructing a plot of head width versus body available as prey and (2) identify the size distribution length (following Nolen & Pearson, 1992) that showed of larvae in the environment in order to detect clear delineations of instars (Online Resource 1). Only potential size-selectivity by killifish. Standardized, the three largest instars (of five total) were represented 5 g packs were made from freshly fallen Cecropia in our collections, either due to life cycle seasonality peltata Linnaeus 1759 (Urticaceae) leaves attached at (i.e., these instars are not present or abundant at the the petiole with a binder clip to facilitate access by time of the collection) or the fact that early instars of macroinvertebrates. Cecropia peltata is a neotropical this caddisfly family have been noted to feed within tree commonly found near our study streams, and P. leaf mesophyll tissue (Nolen & Pearson, 1992) and hansoni larvae were often observed feeding upon thus would not have been collected by our method. fallen leaves. Packs were deployed in seven pools within each stream reach in the same time span in 2011 Auxiliary killifish predation and leaf pack samples as the killifish gut content collections, then collected after 12 days. Invertebrates were rinsed from leaves To further investigate potential explanations of the and preserved in 70% ethanol, and we measured the results of our analyses of larval P. hansoni populations head capsule width of all collected P. hansoni larvae. in KO and KG reaches and examine size-selectivity in Given the differences in sampling techniques, we were 123 Author's personal copy

Hydrobiologia not able to conduct the same types of size frequency Killifish predation analyses used for the 2013 data. We compared relative predation pressure and size- Statistical analyses selective predation between KO and KG reaches using paired t-tests with bootstrapping. Relative predation Phylloicus hansoni size distribution pressure was measured by the percentage of killifish guts with P. hansoni, and size-selective predation was To characterize overall distributions of larval body estimated by the mean head capsule width of con- length, we calculated the median, interquartile range sumed P. hansoni for each reach. For each test, we (0.25, 0.75), skew, and kurtosis exhibited by each again calculated the difference between KO and KG population collected from the KO and KG reaches of reaches, and the bootstrapped distribution was gener- each of the five focal streams in 2013. We further ated from resampling the data with replacement 5000 analyzed differences in median body length and head times. We then calculated 95% CIs from the boot- capsule width of P. hansoni between the KO and KG strapped distribution. Differences were considered to reach type of each stream using paired t-tests with be significant if the bootstrapped 95% CI did not bootstrapping. Bootstrapping was used due to our overlap zero. small sample size and to better meet test assumptions, All above analyses were conducted using the R such as normality. We also compared median body statistical program (version 3.4.0, R Core Team, 2017) length and mean relative abundances of each instar using the agricolae (de Mendiburu, 2016) package. group between reaches, as each could contribute to Significance was considered at P \ 0.05 unless any observed differences in size structure of the otherwise stated. The datasets generated during and/ population. For each test, the differences between KO or analyzed during the current study and correspond- and KG reaches were calculated, and the bootstrapped ing R code are currently available in the figshare distribution was generated from resampling these data repository https://doi.org/10.6084/m9.figshare. with replacement 5000 times. We then calculated 95% 5856651. confidence intervals (CIs) from the bootstrapped distribution. Differences were considered to be significant if the bootstrapped 95% CI did not overlap Results zero. Phylloicus hansoni size structure Stream survey and killifish size distribution Populations of P. hansoni larvae collected in the focal Stream environmental characteristics were quantified streams during the 2013 survey were typically shifted for each reach of all five focal streams and compared to larger body and head capsule sizes in reaches where by stream and reach using a two-way analysis of both killifish and guppies are present, compared to variance (ANOVA). Stream characteristics were ana- those with only killifish (Fig. 2). CED, RDN, and TRT lyzed for normality using Shapiro–Wilk tests, and had larger median body length in KG reaches, while transformations were used when necessary; untrans- GDC showed the opposite trend. In KO reaches, body formed values of all environmental characteristics are length measurements of GDC, RDN, and TRT had reported in results. Additionally, mean CPUE for each significant, positive skew (Table 1), indicating that the killifish size class was compared by reach using a one- distribution was dominated by small individuals. END way ANOVA. Model assumptions (e.g., homoscedas- was the only population to have significant, negative ticity, normal error distribution) were inspected kurtosis (Table 1), indicating a flat distribution. In KG graphically, and significance for the stream survey reaches, only GDC and TRT had a significant, positive was considered at P \ 0.01 to account for multiple skew, and GDC was the only stream with significant, comparisons. positive kurtosis, indicating a peaked distribution; CED, RDN, and TRT populations had a significant, negative kurtosis (Table 1). No adults were recovered from the in-stream emergence traps over the entire 123 Author's personal copy

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Fig. 2 Box plots showing Phylloicus hansoni a body length and b head capsule width by reach, Killifish- Only (KO; light boxes) and Killifish–Guppy (KG; dark boxes), for each of the five focal streams. Whisker markers indicate the range of body sizes and head capsule width for all individuals of KO reaches (N = 337) and KG reaches (N = 730). Outside edges of the box represent the interquartile range (0.25, 0.75) and the bold line represents the median. Circles beyond the whiskers denote outliers

Table 1 Skew and kurtosis values for Phylloicus hansoni size distributions of both reaches [upstream Killifish-Only (KO) and downstream Killifish–Guppy (KG)] for each of the five focal streams Stream Reach Skew Kurtosis Value tPValue tP

CED KO 0.223 0.670 0.253 - 0.974 - 1.461 0.075 KG 0.018 0.084 0.467 - 1.214 - 2.793 0.003 END KO 0.244 0.935 0.176 - 1.154 - 2.209 0.015 KG 0.187 0.772 0.221 - 0.732 - 1.508 0.067 GDC KO 0.593 1.811 0.038 0.262 0.400 0.345 KG 1.264 5.927 \ 0.001 1.593 3.737 \ 0.001 RDN KO 0.537 1.711 0.046 - 0.646 - 1.030 0.154 KG - 0.205 - 1.138 0.128 - 1.269 - 3.524 \ 0.001 TRT KO 1.043 3.760 \ 0.001 0.271 0.488 0.313 KG 0.581 3.219 0.001 - 0.858 - 2.376 0.009 Bold entries indicate a signiﬁcant P value Streams are abbreviated as CED El Cedro, END Endler, GDC Guard Dog Creek, RDN Ramdeen, TRT Trip Trace sampling period; therefore, all analyses were con- majority of larval groups in other streams: the 75th ducted on larvae alone. percentile of head capsule widths measured from both The range of P. hansoni body size and head capsule reaches of GDC was 0.6 mm, and the same parameter width from both reaches of GDC was reduced was higher by 0.1 mm in TRT KO and by at least compared to those of most other streams (Fig. 2). 0.4 mm in all other stream reaches (Fig. 2), possibly The distinct contrast in the range of head capsule because of distinct abiotic characteristics reported in widths suggested that the cohorts of larvae in GDC the following section. This disjuncture classiﬁes GDC were not aligned with the patterns exhibited by the

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Fig. 3 Plots representing the median body length a (± SE) and mean percent composition of three of instar groups of Phylloicus hansoni for a Killifish-Only (KO) reaches and b Killifish–Guppy (KG) reaches. Instar groups were classified according to head capsule widths as Small (S), Medium (M), and Large (L). The y-axis represents median body length for each b instar group and the x-axis represents percent composition of each instar group for KO (P. hansoni N = 281) and KG (P. hansoni N = 598) reaches of the four focal streams included in analyses

as an outlier stream, and we therefore decided to Canopy cover and depth varied by stream exclude GDC from later size structure analyses. (F4,24 = 4.641, P = 0.006 and F4,24 = 4.203, There were overall differences in both median body P = 0.010, respectively) but not by reach length and head capsule width. Phylloicus hansoni (F1,24 = 0.004, P = 0.950 and F1,24 = 0.107, were shifted to larger sizes in KG reaches, with larger P = 0.746, respectively). GDC was * 2.29 deeper median body length (95% CI [0.350, 2.325]) and head than the mean depth for both reaches from the other capsule width (95% CI [0.075, 0.350]). None of the four streams (Table 2). Wetted width differed by instar groups had different median body length stream (F4,24 = 11.714, P \ 0.001) but not reach between reaches (S = 95% CI [- 0.813, 0.138], (F1,24 = 3.648, P = 0.068). There were no differences M = 95% CI [- 0.350, 0.500], L = 95% CI in CBOM by stream (F4,24 = 0.993, P = 0.430) or [- 0.688, 0.538], Fig. 3); however, there were signif- reach (F1,24 = 0.196, P = 0.662). icant differences in the relative abundance of instars. Killiﬁsh were more abundant and larger in KO There were fewer S instars (95% CI [- 17.358, reaches. Killiﬁsh CPUE differed by reach

- 7.932]) and more L instars (95% CI [5.294, 25.873]) (F1,24 = 9.115, P \ 0.006) and was, on aver- in KG reaches (Fig. 3); there was not a difference in age, * 1.89 higher in KO reaches than in KG the relative abundances of M instars between reaches reaches (Table 2). Killifish in KO reaches were shifted (95% CI [- 10.900, 6.684]). In KG reaches, L instars to larger size classes in greater abundances comprised (mean ± SE) 47.3% ± 6.3 of the popula- (F9,40 = 9.100, P \ 0.001; Fig. 4). tion, with S and M instars comprising 18.1% ± 9.0 and 34.6% ± 3.9, respectively (Fig. 3). KO reaches Auxiliary killifish predation and leaf pack samples were shifted to a more even distribution of instar groups: S = 30.8% ± 7.3, M = 36.4% ± 3.9, The body length of killifish ranged from 32.2 to L = 32.8% ± 5.8 (Fig. 3). 84.7 mm from KO reaches, and 25.4–59.0 mm from KG reaches. There was no significant difference in Stream characteristics relative predation pressure between reaches from estimating percent of killifish with P. hansoni in their Environmental characteristics as analyzed by an gut contents (KO: 9.64% ± 3.37, KG: ANOVA differed primarily by stream (Table 2). 26.78% ± 6.94, 95% CI [- 0.333, 4.000]). Similarly,

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Table 2 Environmental characteristics of both reaches [upstream Killifish-Only (KO) and downstream Killifish–Guppy (KG)] for each of the five focal streams Stream Reach Width (cm) Depth (cm) Cover (%) CBOM (g m-2) Killifish (CPUE)

CED KO 195.0 ± 21.1 7.8 ± 2.8 90.1 ± 1.8 0.14 ± 0.07 13.7 ± 5.0 KG 357.0 ± 44.2 3.5 ± 0.1 84.2 ± 3.1 0.18 ± 0.02 7.0 ± 2.0 END KO 221.0 ± 62.2 6.2 ± 1.7 94.2 ± 2.3 0.25 ± 0.08 19.7 ± 0.7 KG 378.7 ± 66.8 8.5 ± 0.8 93.3 ± 0.6 0.18 ± 0.08 13.0 ± 1.5 GDC KO 244.3 ± 34.7 12.4 ± 1.5 89.7 ± 2.5 0.07 ± 0.00 10.7 ± 1.2 KG 244.3 ± 52.3 12.1 ± 3.9 94.9 ± 0.5 0.20 ± 0.15 8.3 ± 1.2 RDN KO 86.0 ± 7.0 5.5 ± 1.9 94.6 ± 0.3 0.41 ± 0.14 13.7 ± 2.0 KG 100.3 ± 26.1 4.0 ± 1.4 95.7 ± 1.8 0.15 ± 0.11 3.0 ± 3.0 TRT KO 149.0 ± 9.3 3.4 ± 0.6 92.4 ± 1.3 0.13 ± 0.07 19.0 ± 8.0 KG 141.3 ± 12.9 5.7 ± 2.0 92.4 ± 1.5 1.10 ± 0.84 12.0 ± 5.0 Streams are abbreviated as CED El Cedro, END Endler, GDC Guard Dog Creek, RDN Ramdeen, TRT Trip Trace. Measurements of wetted width (Width), depth, canopy cover (Cover), and leaf litter standing stocks [coarse benthic organic matter (CBOM)] were averaged from three transects. Killiﬁsh abundances [catch per unit effort (CPUE)] are the mean of individuals caught from three baited minnow traps within each reach. Values represent mean ± SE

the range of 0.2–1.2 mm, similar to that of the larvae from the size distribution analysis (0.2–1.1 mm). Head capsule widths were 0.56 mm ± 0.051 for KO individuals and 0.50 mm ± 0.12 for KG individuals.

Discussion

Our prediction that the caddisfly P. hansoni would exhibit significantly different larval size structures between stream reaches with differing fish assembly, with smaller median sizes in KO reaches compared to KG reaches, was supported by the results of this study. Fig. 4 Estimate of killifish (Anablepsoides hartii) abundances, However, our hypotheses regarding the mechanisms measured as catch per unit effort (CPUE), for each size class causing differences and size structure were not between Killifish-Only (KO) and Killifish–Guppy (KG) reaches from the 2013 stream survey. Size classes are abbreviated as: supported. We predicted that populations of larvae J Juvenile, S Small, M Medium, L Large, XL Extra-Large. Solid from KG reaches would contain larger individuals lines and filled symbols represent KO reaches, and dashed lines within an instar compared to those in the same instar with open symbols represent KG reaches. Points represent the from KO reaches, showing a change in growth as a mean ± SE for each killifish size class from all five focal streams response to differing predation pressure as mediated by guppy presence. We instead found that the increase there was no evidence for size-selective predation in median body size in KG reaches was due to greater based on the head capsule width of P. hansoni inside proportions of larger, later-instar larvae in KG reaches killifish gut contents (KO: 0.090 mm ± 0.005, KG: compared to KO reaches, which is more reflective of 0.154 mm ± 0.046, 95% CI [- 0.014, 0.160]). development patterns than of growth. Our attempt to All P. hansoni larvae from the 2011 leaf pack estimate size at adult emergence was unsuccessful due samples (62 total individuals from KO reaches and to the lack of adult caddisflies emerging during our 171 from KG reaches) had head capsule widths within sampling period, but we believe the fact that there was no difference in size of larvae in their last instar 123 Author's personal copy

Hydrobiologia between reaches which suggests that there would also reaches (Fig. 4). Increased killifish body size could be no difference in adult sizes. Further exploration of provide a competitive predatory advantage, and inter- P. hansoni development in the context of fish ference competition among and within fish species can community is needed to address this question more induce niche partitioning and impact foraging behav- completely. ior (Fausch et al., 1997; Baxter et al., 2004). In We did not find evidence to support an alternative summary, in KO reaches, we believe the increased hypothesis that environmental changes differed con- killifish population and overall size create enhanced sistently enough between upstream KO and down- predation pressure for P. hansoni larval populations. stream KG reaches: Environmental characteristics Analysis of killifish gut contents and subsequent varied more between streams than between reach comparison with associated leaf pack samples showed type, suggesting that patterns of P. hansoni size that killifish consumed mostly small, early-instar structure are linked more directly to fish assemblage larvae, even though medium- and large-sized P. than environmental variation. For example, there was hansoni were available as prey. There was also no little difference in P. hansoni size distributions evidence to suggest stronger per capita predation between reaches of END specifically, and an exam- pressure from killifish in KO reaches (i.e., consuming ination of the killifish CPUE data showed that killifish more larvae at one time). The size range of killifish abundance in the KG reach of END was as high as from which gut contents were analyzed did not include abundances in KO reaches of other streams (Table 2). extra-large size classes of killifish, and ontogenetic However, in some instances, the impact of preda- shifts in diet could occur with growth of organisms tors on prey populations can be overwhelmed by (Werner & Gilliam, 1984), but with evidence from this abiotic factors (Menge, 1976; Peckarsky et al., 2008b). study, we cannot say that the difference in P. hansoni Encalada & Peckarsky (2006, 2011) have shown that size structures is a direct result of killifish selectively stream geomorphology can influence environmental consuming larger larvae, which we had predicted suitability for oviposition by female aquatic insects, initially. Indeed, killifish may instead avoid large P. regardless of predator abundance. Thus, if the habitat hansoni larvae due to the protection of caddisflies’ is not suitable for insects in the first place, any unpalatable leaf cases, which larvae build with community-level effects are unlikely to be detected. increasing size and leaf toughness as they grow and For example, P. hansoni larval body sizes in GDC develop. were smaller for both KO and KG populations, The observed differences in the relative abundances suggesting broader differences in the life cycle such of instar groups between KO and KG reaches are as patterns of recruitment and growth that we cannot significant in the context of the life cycle of this fully address here. GDC was also markedly deeper caddisfly. The phenology of P. hansoni development than other focal streams and had steep rock banks, in Trinidad has not yet been thoroughly evaluated, but which might not have provided optimal habitat since studies of larval development of three Phylloicus Phylloicus larvae are typically found in pools along species in Costa Rica provided evidence for multivol- the stream margin (Wantzen & Wagner, 2006; Turner tinism (Jackson & Sweeney, 1995). Emergence of P. et al., 2008). hansoni adults may occur throughout the year, but it Several lines of evidence suggest that predation could also exhibit a peak in a particular season, similar pressure from killifish populations on P. hansoni to other tropical Calamoceratidae species (McElravy larvae is lessened by the presence of guppies. First, we et al., 1982; Nolen & Pearson, 1992). A survey of found lower abundances of killifish in KG reaches Phylloicus sp. larvae in Brazilian streams by Hua- compared to KO reaches (Table 2), which is consis- mantinco & Nessimian (2000) indicated that adult tent with other studies in the region (Gilliam et al., emergence would occur at the beginning of the rainy 1993; Walsh & Reznick, 2009). Guppies likely reduce season, based on an increase in the largest individuals killifish densities by preying upon larval killifish and in the spring. If Trinidadian P. hansoni populations altering killifish life history traits (Walsh et al., 2011; also follow this seasonal trend (which would also Fraser & Lamphere, 2013). Furthermore, we found explain the lack of adults caught in our emergence that killifish populations exhibited higher abundances traps), the collections for our study taken near the end of larger size classes in KO reaches compared to KG of the dry season should have contained the largest, 123 Author's personal copy

Hydrobiologia later-instar larvae in greatest amounts, but only larvae larger community, our study of P. hansoni has shown from KG reaches exhibited this pattern (Fig. 3). how the consequences of predation are influenced by We did not find differences in larval size within third-party species even over small spatial scales. instar groups, which leads us to estimate that there is not a difference in size of emerging adult caddisflies Acknowledgements We thank James Murray for his between reaches (though we cannot confirm this with assistance with figure creation; John Kronenberger, Anika Bratt, Travis McDevitt-Galles, Emily Nash, Michael our current study). This would be an interesting venue Rautenberg, William Roberts, Tierney Schipper, and Josh of further investigation because life history theory Soden for assisting with data collection; the Ramdeen and (Ludwig & Rowe, 1990; Rowe & Ludwig, 1991) and Ramlal families for logistical assistance; Darold Batzer, Maura evidence from empirical studies (e.g., Peckarsky et al., Dudley, William Hudson, and Joseph Travis for their comments on early versions of this manuscript. This manuscript was also 2001) suggest that predators cause reduced size and improved by the comments of three anonymous referees. This younger age at maturity in prey populations. The research was facilitated by the National Science Foundation- effects of predation risk can cause some organisms funded Frontiers in Biological Research grant (EF0623632) with complex life cycles to increase time spent in the awarded to David Reznick, a National Science Foundation Graduate Research Fellowship awarded to TNS, and the Thelma larval stage (Benard, 2004; Relyea, 2007). Throughout Richardson and Frank Golley Undergraduate Support Award larval development, holometabolous insects (such as from the Odum School of Ecology given to DS. This research P. hansoni) are restrained to checkpoints: attaining a was funded by the following programs at the University of threshold size for metamorphosis, a minimum viable Georgia: an Honors International Scholars Program award, a College of Agricultural and Environmental Sciences weight to survive pupation, and a critical weight that Undergraduate Research Initiative grant, and The Center for allows for the cessation of growth (Nijhout, 2003; Undergraduate Research Opportunities Summer Fellowship, Mirth & Riddiford, 2007; Chown & Gaston, 2010). awarded to KMM. Fish used in this study were collected in 2011 Prey species can be prevented from attaining the under the Institutional Animal Care and Use Committee protocol number A20110007 (David Reznick, University of resources needed to complete this development by California, Riverside). reducing foraging activity (Boyero et al., 2008)or experiencing decreased nutrient assimilation due to stress (McPeek, 2004; Stoks et al., 2005) in the References presence of predators; these potential effects on caddisfly populations from killifish might be lessened Abrams, P. A. & L. Rowe, 1996. The effects of predation on the in the presence of guppies. age and size of maturity of prey. Evolution 50: 1052–1061. Agrawal, A. A., D. D. Ackerly, F. Adler, A. E. Arnold, C. Caceres, D. F. Doak, E. Post, P. J. Hudson, J. Maron, K. A. Mooney, M. Power, D. Schemske, J. Stachowicz, S. Conclusion Strauss, M. G. Turner & E. Werner, 2007. Filling key gaps in population and community ecology. 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