The Effects Of Simulated Herbivory On An Invasive Species

Introduction

An ecosystem is characterized by organisms interacting with living and nonliving organisms. These organisms do not only live in close proximity to each other, but also rely on each other for their survival and build relationships to ensure species longevity. More specifically, there are categories of different types of interactions that species can have with one another. Commensalism is defined as one organism benefiting from the relationship it shares with another organism, while the other does not benefit but remains unharmed. An example of this is the cattle egret and cattle: the egret follows the cattle about one meter away when it mows through fields, which causes insects to be displaced, allowing the egret to eat (Heatwole 1965). Mutualism occurs when both organisms benefit from the relationship; the most well-known example is the clownfish and the sea anemone. The anemone gives the clownfish shelter, while the clownfish exerts aeration behavior near the anemone, which is important for the anemone since it is stoic (Herbert et al. 2017). The type of interaction that is most noticeable in everyday environments is antagonism. Antagonism is essentially predation, as one organism benefits at the other’s expense. There are countless examples of antagonism presented in nature, especially when observing organisms that eat plants. An example of antagonism is the Mediterranean climbing cutworm (Spodoptera littoralis), which consumes the leaves of the lima bean (Phaseolus lunatus) to survive (Maffei et al. 2004) .

Herbivory is a type of species interaction which describes the desiccation of plants at the hand of herbivores (Barber et al. 2012). Herbivory differs from predation in that the herbivore does not remove an entire individual from the population, but rather consumes small portions of its biomass allowing the organism to survive following desiccation (McNaughton 1982). Therefore, populations of plants are not as severely affected by its predators as animals. Plants MuzzSticky NoteCitation?MuzzSticky NoteReserve brackets for citations. Anything that belongs to the sentence should be working into that sentence. are threatened by herbivory from a variety of organisms including insects and mammals (Inouye, 1982). Herbivores often negatively affect plants by consuming portions of vegetative tissue, roots, seeds, flowers and fruit (Barber et al. 2012). Some organisms specifically feed on one part of the plant, such as flower-feeding herbivores, which feed on the pistils and ovules of the flower (Krupnick et al. 1999). Herbivory effects the overall fitness of plants. Severe levels of herbivory may decrease the plants interactions with antagonists and mutualists (Barber et al. 2012). Herbivores may consume gametes along with other integral reproductive structures (Krupnick et al. 1999, Cunningham, 1995).

One result of this is reduced size of floral display, as well as general floral appearance, which in turn interferes with the rates of pollen donation, pollen reception, and response of pollinators (Krupnick et al. 1999). Moderate levels of herbivory do not typically translate in the final plant yield and may increase plant fitness; it is only affected by relatively severe removal of plant tissue (McNaughton 1982). The consumption of small portions of the plant’s biomass can have a large direct impact on its fitness and reproductive success (Krupnick et al. 1999). Plants have the ability to compensate for loss of tissue due to herbivory. They may overcompensate when the severity of herbivory is low, and in the process increase their fitness (McNaughton 1982). Therefore, if a plant undergoes severe levels of herbivory, it will be more difficult for it to recover. The patterns of reproductive phenology usually represent some sort of adaptation in response to an array of selective pressures, such as competition, herbivory, pollination, seed dispersal and additional various climatic pressures (Brody 1997). In a hermaphroditic plant, the wild radish (Raphanus raphanistrum), it was found that reproductive traits were directly related to leaf herbivory. The males’ pollen number and size was found to be reduced, overall petal size MuzzSticky Note

How does this sentence work in the grander narrative and flow of this paragraph? What does it mean for herbivory, which is antagonistic, to decrease plant interactions with antagonists reduced, and fewer fruits were found with the heaviest treatments of herbivory (Lehtila and Strauss 1999). Plants generally produce some variation of compensation when presented with herbivory that can affect yield or fitness, and even produce beneficial overcompensation, yielding more growth in response to said herbivory (Trumble et al. 1993). Factors that affect a plant’s ability to compensate are water and nutrient availability, which are favorable in wetlands (Whitham 1991). Agriculturalists have long studied plants’ phenological resistance to herbivory to maximize their overall yield while dealing with the complications of phenology (Agrawal 2000, Strauss and Agrawal 1999). The response of the plant can depend on many things such as the way herbivory is exerted, or which part of the plant is being eaten.

In a study done on cotton plants, researchers found that the plant responded to reproductive organ damage with increased branching, and although delayed, produced a greater yield without herbivory (Sandras 1996). Apical dominance is seen when the main central stem of a plant is dominant over the side stems; in other words, when the main stem controls the lateral bud outgrowth (Cline 1997). It is best seen when plants get decapitated(when the main shoot is cut off), and apical dominance is released. The lateral buds then grow and create a bushier plant (Dun et al. 2006). It may have evolved so that when the shoot tip is removed, the plant can still complete its life cycle. It may be beneficial since the plant then accrues more stems, flowers and seeds (Agrawal 2000). When plants face the challenge of herbivory, a by-product benefit could be the release from apical dominance since this allows more stems and seeds to grow (Agrawal 2000). The phenomenon of apical dominance could be due to compensate for the challenges of light competition, effective pollination and effective dispersal (Aarssen 1995).

Purple loosestrife (Lythrum salicaria) is a Eurasian wetland perennial that is characterized as a woody half shrub. It is well known for its bundles of vibrant purple flowers MuzzSticky NoteAgain, work in the contents of the brackets within the sentence if it is not a citation that bloom every summer. It produces small fruits that have numerous seeds, and relies on biotic factors for pollination in the same way most flowering plants do (Mal et al. 1992). Many beekeepers use purple loosestrife for its ability to hold large amounts of nectar at a time (Hayes 1979). Although L. salicaria has some positive aspects, scientists in North America have viewed it as an incredibly harmful invasive species that has the potential to dominate aquatic habitats and flush out all of its competition (Mal et al. 1997). Its introduction to an ecosystem creates a change of events that can lead to the extinction of endangered species (Mal et al. 1997). This review will explore how L. salicaria came and established itself as an invasive species in North America, as well as how it became a staple organism in biological control research. It is believed that L. salicaria was introduced into North America around the nineteenth century by sailors who were travelling from Europe to North America (Stucky 1980). It went unnoticed for many years until around the 1980’s, when scientists began to see significant declines in wetland populations, and attributed the trend to the growth of L. salicaria. Many different types of agents were used to control the spread of L. salicaria, such as flooding, mowing, burning and herbicide, but all attempts found little or no success (Gabor et al. 1996). This was later discovered to be due to the fact that L. salicaria grows stronger and more resilient in foreign habitats, away from their natural competitors.

A study by Blossey and Nötzold done in 1995 compared observations of L. salicaria in its natural habitat of Switzerland to those from one of its invasive homes, New York. It was found that the phenotypic plasticity of the plants’ growth indicated pressure from herbivores. The plants in New York were found to be taller, as they didn't experience stress from their natural herbivore predators, found in Switzerland. L. salicaria would outcompete the native flora and fauna and displace the common North American wetland plants such as cattail (Anderson 1995). This indicates how they were MuzzSticky Notenot fact, speculationMuzzSticky Noteto those in its introduced range, specifically New York potentially successful as an invasive species when they first were introduced to North America; they could adapt to their new environment without pressure from their typical competitors, which allowed them to outcompete the native North American organisms. In another study by Quinn et al. 2013, evidence was found that shed more light on how L. salicaria was such a successful invasive species. The authors showed that since the natural predators of the plant were not present, allocation of certain resources towards its defense mechanisms was not necessary. This allowed the plant to focus only on growth and reproduction, and could then evolve faster than the plants that were already established. In its native environment, L. salicaria is under stress from herbivory from Galerucella, a beetle who feeds solely on the vegetative tissue of purple-loosestrife (Blossey 1991).

In order to reduce populations of L. salicaria in aquatic environments to allow the resurgence of native species, Galerucella was introduced as a bio-control mechanism (Blossey 1993). The purpose of Galerucella is to continuously damage the apical meristem, thus delaying vertical growth of the plant and increasing lateral branching as compensation. The beetle must then damage the apical meristem on the resulting branches to halt lateral growth as well (Blossey 1993). Other results of damaging the apical meristem include delayed floral initiation and fruit maturation. In one study conducted by Aarssen in 1997, the effects of shoot apex removal were recorded in pre-flower, post flower, and all-season species of L. salicaria. It was found that clipped plants experienced less vertical growth and increased branching, and unclipped plants had higher fitness throughout the treatment and in the following seasons (Aarsen et al. 1997). Purple loosestrife inhabits aquatic environments such as wetlands which enhances its ability to compensate for herbivory from Galerucella (Whitham 1991). In another similar study done by Katovich et al. in 1999, the effects of Galerucella on the survival of L. salicaria was studied. The authors stipulated in their discussion that apical dominance may stimulate production of new shoots, which may lead to the compensation for leaf tissue lost from herbivory.

In our study, the effects of herbivory on the phenology of purple loosestrife will be observed to provide further evidence on the theories from previous literature. The study will be conducted in the Phytotron in the Biosciences Complex at Queen’s University. Herbivory will be simulated by manually clipping the leaves of the plants. Our hypothesis is that simulating herbivory on purple loosestrife will have an effect on the growth and phenology of the plants. Methods Purple loosestrife was collected prior to the start of this experiment (n=16). The plants were transplanted into 4” containers of soil in the Queen’s Phytotron and labeled. Two control plants were selected. For one week, plants were allowed to adapt to conditions in the Phytotron. During their second week, plants were fertilized with 1g/L 20-20-20 fertilizer. Herbivory treatment began, and to simulate herbivory holes were punched in leaves in the pattern of different levels of leaf damage ranging from 0% (control plants) to 55%. Plants were monitored by students during the week day, and watered 3x per week for 6 weeks. Phytotron staff monitored and watered plants on the weekends. Conditions of plants were recorded, including controls, to see if there were measurable differences between between them based on level and type of herbivory. The variables measured were height, number of branches, leaves, stems, flowers, and flower initiations.