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Leaf Diet Affects Growth of a Shredder, Limnephilus indivisus, from a Seasonal New England Pond
Scott D. Smyers, Brett A. Trowbridge, and Brian O. Butler

Northeastern Naturalist, Volume 18, Issue 1 (2011): 27–36

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2011 NORTHEASTERN NATURALIST 18(1):27–36 Leaf Diet Affects Growth of a Shredder, Limnephilus indivisus, from a Seasonal New England Pond Scott D. Smyers1,*, Brett A. Trowbridge1, and Brian O. Butler1 Abstract - Deciduous leaf litter is often a primary source of energy at the base of the food web in seasonal woodland ponds (e.g., vernal or autumnal pools), which are common in the northeastern United States. It is therefore important to understand how leaf diet affects growth of detritivores, such as the caddisfly Limnephilus indivisus Walker, that feed almost exclusively on submerged plant matter in seasonal pond systems. Growth may relate to how rapidly a caddisfly is able to complete its larval development and pupate, and hence its ability to survive in small ponds with short hydroperiods. Growth is also related to the maximum size the larva achieves. In many organisms, attaining a larger size conveys several distinct advantages: larger individuals typically have higher survival rates and increased fecundity, and hence greater fitness compared to smaller conspecifics or competing sympatric species. To assess how leaf diet influences the growth of larvae of L. indivisus, we conducted a controlled experiment to test how different leaf diets influenced larval growth. The leaf diet experiment was set up as follows: Treatment 1: Acer rubrum (Red Maple), Treatment 2: Quercus spp. (oak), and Treatment 3: Red Maple and oak. L. indivisus that were fed oak leaves or a mix of oak and Red Maple leaves grew significantly larger than larvae fed Red Maple leaves only. In Treatment 3, the caddisfly was observed more frequently on the oak leaf compared to the Red Maple leaf. Although L. indivisus selected oak leaves more often during the experiment, oak leaves were less abundant than maple leaves at our study site. We hypothesize that the oak leaves provide more nutrients to L. indivisus due to a combination of physical, biological, and chemical properties which results in more larval growth. Introduction Dead leaves from trees and shrubs can provide considerable energetic input to seasonal ponds (Bonner et al. 1997, Rubbo and Kiesecker 2004, Rubbo et al. 2006). At the base of the aquatic food web, deciduous leaves decaying on the bottom of a pond provide substrate and a direct nutrient source for zooplankton, fungi, and bacteria. The physical characteristics of decomposing leaves influence the bacterial and fungal communities that colonize the leaves (Gulis and Suberkropp 2003a, Mille-Lindblom and Tranvik 2003), and these fungi and bacteria provide most of the nutrients to detritivores, specifically shredders, such as caddisflies (Gulis and Suberkropp 2003b, Wiggins 2002). Shredders play an important role in nutrient cycling of seasonal pond ecosystems, both directly by transferring energy from decomposing leaf litter to animal biomass and indirectly by breaking down leaves to small particles that are more readily available to micro-organisms and filter feeders. In Massachusetts, larval Limnephilus indivisus Walker caddisflies are particularly common and often abundant within seasonal ponds (Colburn 2004). They are most conspicuous in early spring because they construct portable cases using parts 1Oxbow Associates, Inc., PO Box 971, Acton, MA 01720. *Corresponding author - smyers@oxbowassociates.com. 28 Northeastern Naturalist Vol. 18, No. 1 of leaves, stems, needles, and twigs. Limnephilus indivisus have a complex life history. Eggs deposited by adults in the autumn hatch after the pond fills in the late fall or early spring, then larvae feed principally on leaf litter and submerged vegetation and are most active from ice melt (typically early April in Massachusetts) until May–June (Colburn 2004, Wiggins 1973). Larvae pass through several instars before they seal themselves in their cases and then undergo pupation and metamorphosis. Adults emerge and breed during mid-late summer. Females deposit eggs near or in a pond late in the fall (Wiggins 1973, 1996). Changes in larval diet have been demonstrated to influence the life history and fecundity of caddisflies that inhabit seasonal ponds in the mountains of the western US (Jannot 2009, Jannot et al. 2007, Wissinger et al. 2004). The nutritive quality and availability of food influence growth rate and fitness of caddisflies (Anderson and Cummins 1979; Bärlocher et al. 1978; Hutchens et al. 1997; Otto 1974, 1983; Wiggins 2002). It is important to understand how different leaf litter diets affect growth of larval caddisflies. Hutchens et al. (1997) demonstrated that both the species of leaf and the time that the leaf is “conditioned” by microbes after being submerged influences larval growth rate of some caddisflies inhabiting headwater streams. If insect pests or disease defoliates the trees, aquatic shredders that rely on submerged leaves can be directly influenced (Hutchens and Benfield 2000). Further, caddisfly larvae can detect nutritive differences of leaves and preferentially select leaves that are heavily colonized by microbes and therefore have higher nutritional values (Inkley et al. 2008). The amount of leaf litter consumed and the subsequent decomposition of leaf litter is dependent upon food quality (Anderson and Cummins 1979). Considering results from the aforementioned experiments and Rubbo and Kiesecker’s (2004) hypothesis that a substantial shift in the tree species composition around a seasonal pond, particularly at the landscape level, can impact the entire food web of seasonal pond systems, we tested how leaf diet influenced caddisfly growth. We hypothesized that the assemblage of tree species within the forest canopy and their relative proportions of litter deposited within a pond are important to the productivity of the biota within a pond. We predicted that there is a direct relationship between the type of litter input by the surrounding forest and the food quality of leaf litter in a pond. To test our hypothesis, we investigated the growth of L. indivisus in response to three different leaf diets using the two most prevalent leaf species within our study pond. Methods To assess the influence of forest canopy composition on L. indivisus, we quantified distribution of leaves, by species, submerged in a seasonal pond. We also conducted a laboratory experiment to test how restricting the larval diet to certain leaf types influenced larval caddisfly growth. Our leaf-diet experiment used the species of leaves that were most commonly found within the pond, including Acer rubrum L. (Red Maple) and either Quercus rubra L. (Red Oak) or Q. velutina L. (Black Oak), which are known to hybridize in the northeastern United States (Voss 1996). For the purposes of this study, we refer to oaks from this group as Quercus spp. (oak). We hypothesized that a diet that includes slow-decaying 2011 S.D. Smyers, B.A. Trowbridge, and B.O. Butler 29 oak leaves would result in a faster growth rate of caddisflies compared to a diet restricted to Red Maple. Field data collection The study pond has no inlet or outlet and is located in a forested hillside east of East Wachusett Brook in Princeton, Worcester County, MA. There is a periphery of deciduous and coniferous trees, but the majority of the pond is without a high canopy. The southern end of the pond is >1 m deep in the center and has an open canopy, while the northern end extends into a forested swamp that is less than 1 m deep and has a full canopy. The southern end of the pond contains a mix of emergent grasses, sedges, shrubs, and saplings, while the northern end contains a swamp with a mix of Red Maple and Tsuga canadensis L. (Eastern Hemlock). The pond typically fills in late summer to early fall and does not dry out completely except during neardrought conditions (based on 9 years of observations by S.D. Smyers). On 21 April 2003, when the water level was near the average seasonal high, we collected live L. indivisus (n = 120) opportunistically throughout the pond. Limnephilus indivisus was identified based on case structure and larval characteristics of specimens preserved prior to and at the end of the experiment, using illustrations and descriptions by Wiggins (1996). On the same day, we also collected pond-conditioned leaf litter (e.g., saturated and colonized with bacteria and fungi) and determined the relative abundance of deciduous leaf species by identifying 100 whole deciduous leaves and recording counts for each species at five randomly selected locations within the pond. Caddisflies and leaf litter were transported in plastic buckets containing pond water to our laboratory. Leaf-diet experiment At the laboratory, some of the transported pond water, all of the caddisflies, and some whole leaves were kept in 3 aerated buckets (19 L) containing a mixture of leaf species, maintained at 20 °C with a simulated photoperiod typical for this latitude during this time of year (14:10 L:D), until we began the experiment on 23 April 2003. The experiment was conducted under the same photoperiod and temperature conditions until 14 May 2003 (21 days). The remaining leaves were stored in buckets containing pond water. Prior to introducing the caddisflies to the experimental container, we used calipers to measure the case to the nearest millimeter. The case construction of this species allows for accurate measurements of case length because the structural material is predominantly fastened perpendicular to the long axis of the body. The central cylinder surrounding the larva normally has flat openings at the anterior and posterior ends. The primary response variable was the growth of the caddisfly, as determined by measuring the length of each individual’s case at the beginning and at the completion of the experiment. Furthermore, at the end of the experiment, we measured the body length of each larva, outside of its case. Using a general linear model analysis of variance with Tukey multiple comparisons (MINITAB 12.1, 1998), we compared the difference between case length and total body length by treatment. Because body length and case length are related, we set α = 0.025 for growth comparisons. Caddisflies were randomly assigned to one of three treatments, each of which consisted of an opaque plastic cup (90 mm diameter x 112 mm tall) filled with 30 Northeastern Naturalist Vol. 18, No. 1 500 ml of previously aerated pond water. Each cup contained a single L. indivisus and 1 leaf of Red Maple (Treatment 1, n = 30), 1 leaf of oak (Treatment 2, n = 30), or 1 leaf of Red Maple and 1 leaf of oak (Treatment 3, n = 30). Leaves were selected randomly from our leaf storage bucket and all were inspected to ensure they were positively Red Maple or oak, approximately the same size by species, and in similar condition as approximated by appearance (e.g., stained black). The use of two leaves in the mixed treatment versus one leaf in the single leaf species treatment is not likely to have affected the growth because we replaced the leaves ad libitum before leaves were entirely consumed/skeletonized (approximately 75% eaten). All experimental cups were inspected once per day between 0900 and 1800 h, during simulated daylight conditions when activity is expected to be the highest (Bailey 1982). Water was replaced in each cup every 4 days with pond water that had been aerated for at least 12 hours. We took qualitative, observational count data on each replicate once per day for 21 days: specifically, survival, active feeding on a leaf versus non-active, and, if feeding, on which leaf. We also noted whenever a leaf needed to be replaced, This experiment depended on the presumption that the case length was directly related to body length. Therefore, at the conclusion of the experiment we measured the case length of the remaining L. indivisus and carefully extracted each individual from their case to compare the lengths of each individual’s body to its case. Because caddisflies typically curl when extracted from their cases, even when kept alive, we placed each individual on a dry Petri dish and used forceps to gently nudge the larva until it crawled into a straight line so that an accurate measurement of their body length could be obtained to the nearest mm. Using a t-test, we found no significant difference in length of the case compared to length of the body for L. indivisus (n = 62): x̅ body (± 1 SE) = 14.50 mm (± 0.30), x̅ case = 14.37 mm (± 0.28), t = 0.65, P = 0.521. Therefore, we determined that body size of L. indivisus was accurately estimated by measuring cases, thus providing a non-destructive measure of size. Results Field data collection The deciduous leaf count data indicated that Red Maple was the most common species represented in the pond (x̅ = 55.8%), followed by oak (x̅ = 17.8%), Betula allegheniensis Britton (Yellow Birch; x̅ = 16.4%), Populus tremuloides Michaux (Quaking Aspen; mean = 3.4%), Ulmus americana L. (American Elm; x̅ = 3.4%), Fagus grandifolia Ehrhart (American Beech; x̅ =1.6%), Hamamelis virginianus L. (Witch Hazel; x̅ = 1.2%), and A. saccharum Marshall (Sugar Maple; x̅ = 0.6%). Leaf-diet experiment Survival of caddisflies appears to have been influenced by leaf diet, as survivorship was least for Red Maple (60%), intermediate for oak (67%), and highest for oak and Red Maple (80%). The frequency of size classes, by length, of caddisfly cases at the beginning of the experiment was not maintained at the end of the experiment, indicating individual change in case length varies between individual 2011 S.D. Smyers, B.A. Trowbridge, and B.O. Butler 31 caddisflies, thereby inferring variability of leaf nutrition (Fig. 1). Caddisflies fed a diet of oak leaves (n = 20, x̅ case length ± 1 SE = 3.75 mm ± 0.51) or a mix of oak and Red Maple (n = 24, x̅ case length ± 1 SE = 3.33 mm ± 0.38) had significantly longer cases (Figs. 2–3) than those fed a diet restricted to Red Maple (n = 18, x̅ case length ± 1 SE = 1.39 mm ± 0.40; df = 2, F = 8.0, P = 0.001). The total body length of surviving caddisflies fed oak leaves (n = 20, x̅ body length ± 1 SE = 14.8 mm ± 0.42) or a mix of oak and Red Maple (n = 24, x̅ body length ± 1 SE = 15.5 mm ± 0.51) was significantly Figure 1. Frequency of caddisfly larvae in each of three leafdiet treatments as a function of total case length (mm): A. Red Maple, B. oak, and C. oak and Red Maple. Black bars represented case length at time 0, and white bars are final case length at the conclusion of the experiment. 32 Northeastern Naturalist Vol. 18, No. 1 longer (Fig. 4) than those fed a diet restricted to Red Maple (n = 18, x̅ body length ± 1 SE = 12.9 mm ± 0.44; df = 2, F = 8.0, P = 0.001). The average percent observed feeding was almost exactly the same between treatments: oak = 72.1%, oak and Red Maple = 71.4%, and Red Maple = 72.5%. In Treatment 3, where the caddisflies were provided both Red Maple and oak leaves, they were observed more frequently on the oak leaves (64%) compared to Red Maple leaves (36%). Within the oak leaf treatment, a total of 7 leaves were replaced, 2 after 16 d, 4 after 18 d, and 1 after 21 d. In the oak and Red Maple leaf treatment, we replaced 14 leaves in total, 8 after 18–20 d, including 6 oak and 2 maple leaves. Subsequently, within 3 replicates where leaves were already replaced, we replaced both oak and maple leaves (6 leaves total) on days 20–21, 5–6 d after the first leaf replacement (double replacement). In the treatment containing Red Maple leaves only, we replaced 7 leaves total, including 4 after 16 d, 1 after 18 d, 1 after 19 d, and 1 double replacement on day 20, 4 d after the first replacement We documented 5 occasions (oak = 3, oak and Red Maple = 1, Red Maple = 1) where caddisflies were observed outside of their case, but were later found to be back in their case and feeding normally. In other instances, when caddisflies were observed live outside their case, they were later found dead outside their case (oak = 10, oak and Red Maple = 6, Red Maple = 12). These were the only instances of mortality documented in our study. There were no observations of pupation during the experiment. Discussion Limnephilus indivisus were significantly larger after being provided Oak leaves or a mix of Oak and Red Maple leaves. The effects of these different leaf diets will likely carry through to subsequent life phases given that growth of larvae directly influences time to pupation, size at metamorphosis, and adult Figure 2. Mean (± 1 SE) case length (mm) at the beginning and end of the leaf-diet experiment. Black bars are case length at time 0, and white bars are final case length at the conclusion of the experiment. Asterisks indicate significant differences between beginning and end case length for oak and mixed oak and Red Maple leaves (*P = 0.001). NS indicates a non-significant difference within the Red Maple treatment. 2011 S.D. Smyers, B.A. Trowbridge, and B.O. Butler 33 reproductive success (Jannot et al. 2007, Stearns 1992). We believe the qualitative data collected during the experiment provides additional insight and basis for further experiments on feeding behavior. Our data indicate a trend for preferred selection of oak leaves by feeding caddisflies. When exposed to both oak and Red Maple leaves, L. indivisus more often selected oak leaves, even though oak leaves were less abundant than Red Maple leaves at our study site. This result is in agreement with Inkley et al.’s (2008) observations that larvae displayed a preference for leaves with higher nutritional values. Furthermore, the combination Figure 3. Mean (± 1 SE) case growth (mm) at the beginning and end of the leaf-diet experiment. Different letters above the bars indicate significant differences between experimental treatments (P = 0.001). Figure 4. Mean (± 1 SE) total body length (mm) of live larvae extracted from cases at the end of the leaf-diet experiment. Different letters above the bars indicate significant differences between body length (P = 0.001). 34 Northeastern Naturalist Vol. 18, No. 1 of oak and Red Maple resulted in the highest survival and total leaves consumed through the duration of the experiment, suggesting that a mixed diet is likely the most beneficial for survival and increases the rate of feeding, but not necessarily growth. Caddisfly growth is influenced by nutritional composition of each leaf type; both Red Maple and oak leaves may provide important, but different nutrients, with larvae benefiting from the ability to select from both types to obtain the most nutritious diet when both leaf types are available. In addition to the different nutritional quality of oak or Red Maple leaves, the structural integrity of the case constructed from different leaf species may affect the growth of the case and larval caddisfly. However, because we measured the actual body of the larvae at the conclusion of the experiment, we do not believe this was an ecologically important variable in our experiment, but we cannot rule out the possibility that the integrity of various leaf materials alone may indirectly influence growth of larvae under certain environmental conditions. Our study demonstrates that L. indivisus are capable of selectively feeding on the leaves that ultimately result in greater growth, but will continue to feed on less productive leaves when they are readily available. Any significant changes in the relative species richness of leaf litter could directly impact the feeding behavior, growth, and survivorship of larval L. indivisus. Furthermore, the environment experienced by a larval caddisfly, including nutrition, has been demonstrated to influence adult fecundity (Jannot 2009). Although our study did not include any analyses to quantify fungi or bacteria available or consumed by L. indivisus, we hypothesize that the oak leaves provide higher quality or more abundant nutrients for shredders due to a combination of physical and chemical properties, resulting in more growth. Other species of deciduous leaves should be considered for future experiments to compare how caddisflies are influenced by different species assemblages in representative populations within North America. We did not know the exact conditioning time of the leaves used in the experiment, but because we selected leaves at random from the five locations within the study pond and then mixed them, we know the leaves were equivalent to those available in situ; in this regard, our experiment represents realistic, natural conditions. We considered cutting the leaves to control for leaf area in each treatment. However, the use of entire leaves provides what may be an important physical property in the shape of the leaf. The use of two leaves in the mixed treatment versus one leaf in the single leaf species treatment is not likely to have affected the growth because we replaced the leaves ad libitum, and our results indicate that the two-leaf treatments did not result in significantly greater growth compared to the single-leaf oak treatment. Future studies on caddisflies of seasonal ponds with different canopy composition (e.g., Ulmus, Fagus, and Betula), including factors affecting decomposition rates of leaves, will be particularly useful to determine mechanisms that influence growth, food selection, and life history of caddisflies. Our results should encourage other researchers to conduct additional laboratory experiments and study caddisfly populations on a larger scale, within an entire pond, to determine how their growth and life history are affected by shifts in environmental conditions (e.g., temperature and hydroperiod) and leaf-litter composition. Other research has 2011 S.D. Smyers, B.A. Trowbridge, and B.O. Butler 35 addressed some of these questions, providing an excellent baseline of information from which to build upon. For example, leaves that are inundated in a permanent or semi-permanent pond are more nutritious for L. indivisus compared to leaves submerged for a shorter duration (Inkley et al. 2008). Moreover, nutrition, pond depth, and water temperature during larval development have been demonstrated to interact to influence female fecundity and longevity of L. indivisus (Jannot 2009). Limnephilid caddisflies are likely to be responsive to major changes in the composition of the leaf-litter input because they depend on leaf litter for nutrients. Any change in available nutrient quality, specifically leaf species, may affect the growth of caddisflies, and thereby increase or decrease the rate of leaf degredation into fine particles, which are a source of primary production within a pond. Thus, caddisflies are likely to have a major influence on how energy is cycled within seasonal ponds. Any changes to leaf availability that affect how fast caddisflies grow will also affect how rapidly leaves are broken down into smaller particles. The amount of leaf litter consumed, the growth rate of the larvae, and subsequent decomposition of leaf litter is partially dependent on temperature (Cummins 1973). Our results suggest that a seasonal pond containing a large proportion of oak leaves will produce larger caddisflies and will break down more leaves compared to an equivalent pond with a small proportion of oak leaves. However, other leaf species need to be examined to determine how they affect larval growth and time to pupation. As the dominant trees within a forest community surrounding an aquatic habitat change over time due to catastrophic weather events, forest clearing, species-specific disease, insect pests, or changes in climate, the surrounding vegetative input will affect seasonal pond ecosystems. Caddisflies are abundant and important components of the food web, and our future research will aim to determine how forests and their communities of dead leaves influence aquatic lentic ecosystems through complex pathways with potentially profound consequences. 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