Before I dive into this week’s paper, I’d like to give a quick overview of ecology. Ecology is a very broad field of study, which focuses on the relationships between an organism, the organisms that it interacts with, and the environment where the organism lives. Ecologists use these interactions to study broad concepts such as how an ecosystem influences evolution, how populations are regulated, how energy and nutrients move between organisms, and how changes to the environment affect an ecosystem. Today, I’m going to be focusing on that last point, and looking at a change that’s currently affecting almost every ecosystem in the world: climate change.
In a nutshell, climate change is the general trend towards higher global temperatures and more severe weather, and is the result of humans increasing carbon dioxide levels in the atmosphere through the use of fossil fuels. CO2 acts a bit like the roof of a greenhouse,allowing sunlight into the atmosphere but preventing the heat from exiting, leading to increased temperatures. Unlike other human activities that can have major impacts on an ecosystem, such as deforestation, climate change occurs slowly, with its effects taking decades to become noticeable. This makes studying the effects of temperature changes on an ecosystem difficult, as many organisms will go through several generations over the course of a few decades, and may be able to adapt to changes in temperature over that time frame.
Because of this, scientists must either spend decades observing an organism (which is generally expensive and does not have usable results for many years), use microbes that reproduce rapidly (which are not ideal for making general statements about larger, more complex ecosystems) or look at how increases in temperature affect an individual (which fails to take into account the ability for organisms to adapt to changes over several generations). This week’s paper, Local Adaptation to Temperature Conserves Top-Down Control in a Grassland Food Web by Brandon Barton, attempts to account for all of these issues.
Barton focused on a simple system that consisted of a nursery web spider Pisaurina mira, a red-legged grasshopper Melanoplus femurrubrum, and several grasses and herbs. He chose this system for a number of reasons: 1) nursery web spiders have been shown to alter the behavior of grasshoppers, which typically prefer a grass-based diet but will switch to eating herbs when spiders are present,as well as spend less time eating in order to reduce the risk of predation, 2) nursery web spiders have been shown to respond to increases in temperature by moving closer to the ground to escape the heat, while their grasshopper prey, which do not respond to temperature changes, remain further from the ground 3) when spiders move towards the ground to escape from increased temperatures, grasshoppers will continue to eat herbs rather than grasses but will spend more time feeding due to the reduced risk of predation, and 4) this system is present across a wide area of the United States with large variations in temperature throughout this range.
This range of temperatures this system occurs in was very important for this experiment, as it allows for Barton to explore whether spiders in warmer areas have adapted to the increased temperature. To do this, he first looked at how spiders from three different sites (Northern Vermont, Northern Connecticut, and Central New Jersey) affected grasshopper behavior when temperature was altered. To do this, he used heat lamps to alter the temperature of a terrarium containing grasses, herbs, a grasshopper, and a spider. While spiders from all three sites moved closer to the ground when the temperature was increased, the magnitude to which they moved was much greater for Vermont spiders than for New Jersey spiders. The time grasshoppers spent eating increased the further down the spiders moved, which when combined with the effects of temperature on spiders, resulted in significant increases in feeding time for grasshoppers in terrariums with Vermont spiders as temperatures increased, but no change in feeding time for grasshoppers paired with New Jersey spiders.
The next step was to look at how these spiders affected grasshopper behavior in the wild. To do this, Barton set up insect mesh enclosures over random locations in each field site, to ensure that spiders and grasshoppers wouldn’t escape from the test sites and would have access to the local plant communities. At each site, he created 24 total enclosures: 6 with just grasshoppers for a control, and 3 experimental groups of 6 with grasshoppers and spiders from Vermont, Connecticut, or New Jersey. Rather than observe the behavior of the grasshoppers and spiders as he did with the terrarium experiment, Barton looked at the biomass of the herbs and grasses in each enclosure as a way of determining how the spiders were affecting grasshopper behavior. Less herb biomass would mean that grasshoppers were spending more time eating, which would in turn mean that the spiders were spending more time closer to the ground.
The results of this experiment are fairly straightforward. Enclosures with New Jersey spiders had the greatest amount of herb biomass at both the New Jersey and Connecticut sites. Enclosures with Vermont spiders showed the lowest amount of herb biomass at both the Connecticut and New Jersey sites. In the Vermont sites, the amount of herb biomass within the enclosure was the same regardless of the origin of the spiders. This shows that spiders from all three sites were most comfortable at lower temperatures, but had varying degrees of heat tolerance as temperatures increased.
The main takeaway from all of this is that spiders from different climates have adapted to those climates. Vermont spiders had very low heat tolerance, and therefore resorted to hiding closer to the ground to avoid temperature increases, while spiders from New Jersey, which have adapted to warmer climates, do not have to resort to heat avoidance behavior. By looking at variations in heat tolerance and behavior relating to said tolerance between individuals of the same species living in different environments, we may be able to more effectively study the potential effects of climate change while taking into account the ability of organisms to adapt to changing environments and without needing to collect data over several decades. While this method has several flaws, namely that it assumes the species in question would be able to adapt fast enough to keep up with changing temperatures (something that is unlikely in larger animals where a single generation can last several years), it has the potential to be a powerful method of studying the effects of climate change on arthropods and other small, short-lived organisms.