Opposite Effects Of Daytime And Nighttime Warming On Top-Down Control Of Plant Diversity

Introduction

The climate warming has been suggested to cause a number of ecologically important consequences, which could have dramatic effects on ecosystem level carbon, nitrogen and water cycling, these consequences can affect plant-insect interactions and the diversity of plant community (Bardgett, Manning, Morrien, & De Vries, 2013). Grasshoppers are important components of most grassland ecosystems.

These abundant herbivores can influence many ecosystem processes such as nutrient cycling and primary productivity. But the effects of grasshoppers on ecosystem processes often depend on the outcome of their interactions with other species, including predators. For example, spiders are common predators of grasshoppers that alter grasshopper behavior and can limit grasshopper population size. But the outcome of species interactions can be sensitive to changes in many biotic and abiotic environmental factors, such as temperature. Increases in mean temperature may be displayed in several alternative ways; these alternatives can differentially affect organismal performance, and community level interactions due to idiosyncrasies arising from temperature‐dependence of biological processes like photosynthesis, thermoregulation, and feeding (Barton & Schmitz, 2017).

Moreover, species may adjust their abilities to cope with different thermal conditions, which in turn may influence the nature of species interactions and their ecological consequences (Barton & Schmitz, 2017). Meanwhile, some species’ behaviors, such as spiders who can obtain benefits from plants such as shelter and access to insect prey, are strongly influenced by temperature change. If it is in daytime that temperature increases, spiders retreat to the shade and do not hunt, freeing the grasshoppers to eat more plants (Hawlena, Strickland, Bradford, & Schmitz, 2012). In addition, grasshoppers ‘stressed’ by spiders affect the productivity of soil. This means a grasshopper who is in fear of an attacker, such as a spider, will enter a situation of stress and will consume a greater quantity of carbohydrate-rich plants which are similar to humans under stress who might eat more sweets. And the competitive dominance the plants that grasshoppers feed on are suppressed, and leading to increases in plant diversity (Barton & Schmitz, 2017). Therefore, in this article, the authors make experiments that tested they hypothesis that daytime and nighttime warming have different effects on interactions between spiders, grasshoppers, and plant communities. It is interesting because the experiments show that the ability of the predator in a food web to adapt to temperatures can preserve the ways the species in the web influence one another across a range of climate conditions. And we can understand the predator-prey interactions between spiders and grasshoppers at different temperatures and how their corresponding behaviors affect the plant diversity.

Review of the Research Paper

This research paper states that daytime and nighttime warming have influences on community cover and species richness. Moreover, the authors do some experiments to compare the ecological effects of daytime and nighttime warming on a grassland community in which spider predators indirectly influence plant diversity by altering the behavior of grasshopper herbivores. The authors give a hypothesis, do experiments and analysis using graphs in order to prove that daytime and nighttime warming have the ecological effects on interactions between spiders, grasshoppers, and plant communities. The authors use experimental method following the steps, which are making observations, forming a hypothesis, making a prediction, conducting experiments, making analysis, getting results and making a conclusion. They make observations about the spider's predatory behavior and grasshopper herbivores behaviors at different temperatures in a day, and its effects on dominant plant species.

They then form a hypothesis that daytime and nighttime warming have different effects on interactions between spiders, grasshoppers, and plant communities, after that, based on their research resources, spiders and grasshoppers can be active during the daytime and nighttime, they make a prediction that spiders and grasshoppers’ interactions may be affected differently by daytime and nighttime warming (Barton & Schmitz, 2017). In the next, they conduct experiments; there are two experiments, which are behavioral experiment and field experiment. Using adult Pisaurina mira spiders and nymphal Melanoplus femurrubrum grasshoppers caught from nearby fields. The life span of these spiders is over 1 year, thus, adults are common in the early spring when experiments began, and the plant species are common in both experiments (Barton & Schmitz, 2017). These are the dependent variables. For the behavioural experiment, they measure spider and grasshopper height in the plant canopy by recording their vertical position using 15 cm tall horizontal strata, and the number of grasshoppers feeding on goldenrod every 30 minutes for 24 hours. Then they calculate mean height of spiders and grasshoppers during the daytime and nighttime periods and mean feeding time per grasshopper per observation as the number of grasshoppers observed eating goldenrod divided by the total number of grasshoppers observed multiplied by 30 minutes (Barton & Schmitz, 2017).

For the field experiment, using a standard protocol for multi‐year mesocosm experiments, they estimate percent cover of each plant species within each mesocosm. The percent cover is converted to biomass using a conversion factor developed for each species by destructively sampling three reference plots adjacent to the mesocosms. Therefore, they calculate a conversion factor as grams per percent cover, and used the mean from the three reference plots to estimate biomass within mesocosms (Barton & Schmitz, 2017). Finally, they make graphs and analyse, and the results come out. For the behavioral experiment, comparing the grasshopper and spider height, it is apparent that locations of spider and grasshopper overlapped in the nighttime warming treatment. Consequently, grasshopper feeding decreased when in the nighttime warming treatment, and during a period of 24hours, grasshoppers exposed to daytime warming spent nearly twice as much time eating than grasshoppers exposed to nighttime warming.

Behavioral responses of spiders and grasshoppers to daytime (06: 00–18: 00 h) and nighttime (18: 00–06: 00 h) warming. (a) Daytime warming reduced overlap of spiders and grasshoppers relative to control and nighttime warming treatments. In contrast, (b) nighttime warming increased spatial overlap of spiders and grasshoppers. (c) Daytime warming increased grasshopper feeding, whereas (d) nighttime warming decreased grasshopper feeding. (e) In a 24-hour period, grasshoppers exposed to daytime warming spent two more time eating goldenrod compared nighttime warming. Dashed boxes indicate the treatment being warmed during the period represented by each figure panel. Letters (x, y, z) indicate results of Tukey HSD (Barton & Schmitz, 2017). For the field experiment, there were no differences in the initial biomass of plant community evenness among these treatments.

However, after two years of warming treatment, average goldenrod biomass was five times higher in nighttime warming treatments than in daytime warming treatments. Moreover, comparing to the no warming treatments, daytime warming treatments had higher plant community evenness while evenness in nighttime warming treatments was lower. Thus, the goldenrod biomass has an effect on plant community evenness, and the effect of the goldenrod biomass did not differ among treatments. Plant community data at the initiation and end of the two-year field experiment. At initiation, neither (a) goldenrod abundance nor (b) plant community evenness differed among treatments, but (c) evenness was negatively related to goldenrod abundance. After 2 years, (f) goldenrod continued to have a negative effect on the plant community evenness. However, (d) goldenrod biomass was reduced by daytime warming and increased by nighttime warming. Consequently, after 2 years, (e) plant community evenness was higher in daytime warming treatments than nighttime warming treatments. Letters (x, y, z) indicate results of Tukey HSD (Barton & Schmitz, 2017).

Consequently, according to this paper, there is a conclusion that the time of warming changed can do affect the interactions between spiders, grasshoppers, and plant communities. The daytime temperature creates a thermal environment above the spider's thermal optimum, thus, spiders will seek refuge, and decreasing predation risk and allowing grasshoppers to increase feeding time, which reduces the abundance of competitively dominant and increases evenness of plant community. Contrarily, nighttime warming increase nighttime temperatures towards the spider's thermal optimum, thus, increase spider activity at night. When increasing predation risk, nighttime warming should decrease grasshopper feeding, at the end, releasing goldenrod from herbivory and reducing plant community evenness. In this paper, all experiments were done well at the Yale‐Myers Forest in northeastern Connecticut, USA. They accurately controlled the experiment as possible, because they used the same species of spiders and grasshoppers, and the life span of all spiders was over 1 year. Also they chose a same site that would not have any big change on mean temperature. This increased the accuracy of the experiments so that the results would not affected by other irrelative factors.

Although the author considered that the experiments were almost consistent with global trends and climate models, and the mean temperature between 1910 and 2014 at this place had slightly risen without significant change, however, other abiotic factors such as the humidity, light levels, and air pressure, which have been studied less extensively, may also have an impact on spider’s foraging decisions. This might also influence a spider's prey capture success and risk of predation (Herberstein & Fleisch, 2003). Therefore, the author should set up another larger field cages and stocked them with grasshoppers only or grasshoppers and spiders. To keep humidity, light levels and air pressure the same, they should surround some of the cages with humidifiers and lighting control devices constructed of steel frames covered with shade cloth so that species’ behaviors would not be affected by other abiotic factors.

Research Proposal

Based on this article, can spiders’ body size influence their prey attraction rates? And how it affects the spiders’ predation based on the same temperatures? Based on the background that the spiders’ size has different impacts on web design and nocturnal spiders can use body markings as visual lures to attract prey. And the higher attraction rate for large spiders could be a consequence of a bigger visual lure capable of attracting more prey than smaller spiders (Yuen & Bonebrake, 2017). Thus, we hypothesis that spiders’ body size has different effects on prey attraction rates. Small spiders may attract fewer prey but they also tend to have a higher interception efficiency suggesting that their small size might be advantageous in disguising their presence once grasshoppers are near the web.

For large spiders, they attract more prey, but they have a lower interception efficiency. We can conduct a series of experiments to compare body size effects of large and small on prey attraction rates of spider predators. Nephila pilipes are orb-weaver spiders commonly found in Hong Kong during the wet season, and females are much larger than males in N. pilipes (Yuen & Bonebrake, 2017). Thus, we can choose this kind of specie of spiders that have a same surface color but different sizes. And the experiments are conducted at the same habitat, which is at country park forests in Hong Kong, China. And setting up suitable field cages and stocked them with grasshoppers and spiders. To keep humidity, temperatures and light levels the same, it is necessary to surround some of the cages with humidifiers, temperature chambers and lighting control devices constructed of steel frames covered with shade cloth. Then we wait for grasshoppers captured by spiders’ web and count the number of grasshoppers on the different webs of female and male spiders.

If the results from the experiment is significant and consistent with my hypothesis, I can make a conclusion that spiders’ body size has different effects on prey attraction rates. Larger spiders have more grasshoppers attracted to their webs than smaller spiders, which means larger spiders have higher prey attraction rate than smaller spiders. Spider size exhibited an interactive effect for all attraction variables demonstrating the complexity of the relationship between spider morphology and prey attraction outcomes.