Top-down response to spatial variation in productivity and bottom-up response to temporal variation in productivity in a long-term study of desert ants(2022) Gibb et al.,Biology Letters, https://doi.org/10.1098/rsbl.2022.0314
Ecosystem productivity can tell us a lot about how an ecosystem functions. The more productive an ecosystem is, the more life it can support. But productivity doesn’t just affect the diversity or number of species within an ecosystem, it affects how those species interact, from the large carnivores you find at the upper levels, to the plants and bacteria down the ‘bottom’.
Within ecosystems, the strength of a top-down process (something influencing those upper levels) vs. a bottom-up process (something influencing the lower levels) depends on how much primary productivity there is. Primary production occurs when a species makes its own energy instead of eating something else, and when there is a lot of it going around, it often allows the carnivores at the upper trophic levels to suppress the population numbers of herbivores. That means that while a bottom-up process may end up affecting the herbivores, a top-down process (like the hunting of carnivores) might impact the entire ecosystem.
On the other side of the spectrum, when there is little primary productivity, there aren’t usually as many carnivores suppressing the herbivore populations. A bottom-up process will increase herbivore numbers, making these bottom-up processes more important in these low-productivity systems. This is known as the Exploitation Ecosystem Hypothesis (EEH).
Invasive snails, parasite spillback, and potential parasite spillover drive parasitic diseases of Hippopotamus amphibius in artificial lakes of Zimbabwe(2021) Schols et al.,BMC Biology, https://doi.org/10.1186/s12915-021-01093-2
Artificial lakes can be a huge plus for the regions where they are constructed. People come to hang out at them, they can serve as habitat for local or migrating species, and they can also improve water accessibility. In fact, the majority of the research that I did for my PhD took place in artificial, human-made lakes (see here and here). Yet, these artificial lakes can also wreak havoc by destroying local ecosystems and introducing invasive species. Furthermore, because humans build communities around these lakes there is a risk of increased transmission of parasites to livestock and humans alike.
One group of common invasive species in these artificial lakes are snails, which serve as intermediate hosts for many parasites (see Did You Know?). Introduced water plants (like hyacinth) often harbor invasive species like the snails, and dams built to make artificial lakes often block snail predators from accessing the lakes, which means that the snails increase in number due to the release from predation pressure. Today’s authors wanted to understand how invasive snails modified parasite transmission within an artificial lake.
Turning an ecosystem that has been ruined by humans back into a thriving natural world is a long, difficult task, but it is possible. One method for making it easier is re-introducing species that we’ve wiped out. Often the reintroduction of the functions that these species perform helps restore many other species, and helps the ecosystem returns to a more ‘natural’ state.
But what happens when a really key species has gone extinct? One way of solving this conundrum is introducing a similar species that performs the same function. This sounds like a good workaround, but introducing a non-native species might have unexpected ecological repercussions.
This week’s researchers were based on Round Island, in Mauritius, where two species of giant tortoise (the saddle-backed and the domed Mauritius giant tortoise) had gone extinct. A third species, the Aldabra giant tortoise, was introduced in 2007. The main point of concern on the island is that the tortoise diet may overlap with that of a vulnerable species, the Telfair’s skink. This week’s team wanted to find out whether the tortoise was helping or hindering the island.
It’s no secret that the world is undergoing a biodiversity crisis. This comes not only from climate change and human land use, but also invasive species – non-native species that cause harm to native ecosystems. Specifically, there are seven times more invasive species now than there were 75 years ago. Because of how many there are, and just how fragile ecosystems have become, it’s important to know what effects that invasive species have.
Ecological restoration (see Did You Know?) is one effective solution that can be used to mitigate the biodiversity crisis. Reestablishing native species can often help with this restoration, as does removing invasive species, but it usually requires human intervention. By removing these invasive species, the idea is that the native species will be released from competition and benefit from better access to necessary resources.
Yet to monitor invasive species removal, you need long-term data on population persistence, which is very difficult (logistically and financially) to collect. Understanding how the removal of invasive species benefits restoration requires not only measuring how such removal benefits ecosystem function, but also how it can benefit population persistence in the long term. Today’s authors wanted to understand how the removal of an invasive species benefited local community resilience.
When an animal is facing a lack of prey, or the weather is making it too difficult for them to keep on keeping on, they might choose to enter a state known as torpor. This occurs when the animal lowers its metabolic rate drastically, sometimes to less that 1% of its normal rate. It’s not a perfect solution though, as the costs of torpor include sleep deprivation and memory loss. Nevertheless, it’s a go-to for many small mammals, since they’re warmed up much more quickly than larger ones, and can snap out of torpor when they need to.
It might sound like this is cold-weather behaviour, but it can also occur in summer. Especially if you’re a nocturnal mammal living in part of the world where nights can be very short, or even non-existent, like Scandinavia. Long days means reduced hunting times, so using torpor might be necessary to get through summers as well as winters! This week’s researchers wanted to better understand how small bats survive in northern Norway by looking at how and when they awake from torpor.
With increasingly clear effects of global climate change, everyone’s thinking about how we will handle extreme temperatures and weather events as they become more common. Less obvious is the fact that the changing climate is also rearranging global food webs, with many species readjusting in the fact of a new range of temperatures. This might not sound fantastic (and let’s face it, it’s not), but this changing climate may be able to teach us something about how species adapt to higher or lower temperatures.
Temperature plays a key role in determining whether an invasive species can take up residence in a new region. We know that low temperatures can be particularly limiting to newly-invasive species, especially insects and spiders. Yet few studies look at how lower temperature in a new environment can affect the survival, development, and behavior of new invaders.
We tested whether invasive widow spiders from a warm climate (Australia) adapted over generations to the lower temperatures of their invaded habitat in Japan. The move to Japan should require adapting to lower temperatures, but it might not, for a few reasons. Spiders from both locations may be equally good at coping with cooler or warmer temperatures, or, since urban areas are typically warmer than natural habitats, organisms that move between urban habitats might avoid facing the low temperature constraints.
Did You Know: Cities as Heat Islands
It’s hot in cities! One reason for this is the urban heat island effect, where urban areas are several degrees hotter than surrounding natural areas because of all of the heat-absorbing surfaces like roads and buildings. More than half of the human population lives in cities, and as they heat up, it is especially important to understand how some species adapt and even do better in urban environments. Urbanization and climate change can also increase the spread of invasive species. For example, some urban-adapted invasive species thrive in urban habitats that would otherwise be too cold for them to survive and reproduce in. Understanding how urbanization, climate change, and invasions interact can help us predict changes in biodiversity and species distributions in the future.
What We Did
The Australian redback spider, Latrodectus hasselti, is an invasive species of widow spider, native to Australia. Redback spiders are well-known in Australia for their bite and neurotoxic venom. Redbacks have been transported (likely accidentally along with used cars or produce) to Japan, New Zealand, the Philippines, Papua New Guinea, and India. We compared traits across native and invasive-habitat temperatures in a native population of spiders. The native spiders were collected from Sydney, Australia and the invasive population from Osaka, Japan, where redbacks became established in 1995.
We reared the spiders in the lab for three generations. We first checked for population differences in how spiders responded to extreme temperatures, measuring the lowest and highest temperatures at which spiders were able to maintain normal activity.
Next, we investigated how spiders respond to more moderate temperature differences, such as those in autumn, right before overwintering. When female spiders from each population produced egg sacs, we put the egg sacs for two weeks in either Japan-typical (15 degrees Celsius) or Australia-typical (25 degrees) autumn temperatures, then put all egg sacs at 25 degrees until spiderlings emerged. We predicted that the invasive spiders from Japan would be better adapted to low temperatures than the native Australian population, as they’re used to colder temperatures. We also measured hatching success, development time, and body size.
Once the spiderlings were juveniles, we measured behavioural traits that may be important for survival in nature: boldness – how quickly a spider resumed movement after a simulated predator threat (a puff of air), and exploration – building a web in a new environment.
What We Found
At extreme high temperatures, spiders from each population were similarly tolerant, with females able to move at temperatures of up to 55 degrees Celsius! Surprisingly, males from the invasive population from Japan were less tolerant of extreme low temperatures, suggesting that they may not overwinter successfully in colder regions. Egg sacs from the Japanese population hatched equally well at low and high temperatures, but egg sacs from the Australian population failed to hatch more often at low temperatures. Native spiders also took longer to emerge from the egg sacs than invasive spiders at low temperatures, which could expose egg sacs to more predation risk.
The Japanese population was bolder and more exploratory at low temperatures, but less bold and less exploratory at high temperatures, whereas the native population was similarly bold and exploratory at both temperatures.
Spiders from Japan, which live in cooler habitats, developed at both low and high temperatures, compared to a native population, which hatched less and developed more slowly when exposed to low temperatures. This study only tested one invasive and one native population, and it would be worthwhile to compare multiple invasive populations from both cooler and warmer habitats, as well as multiple native populations across Australia.
Although the invasive habitat in Japan is more extreme in temperature, spiders also live in more urbanized habitats compared to the native population. Urban habitats are hotter, and we would like to measure what conditions the animals are directly experiencing in the urban and natural habitats, to find out if spiders are able to colonize cooler climates because they thrive in urban heat islands habitats.
Some organisms may be better equipped to deal with changes we are facing with urbanization, habitat fragmentation, and climate change. In the case of Australian redback spiders, within twenty years, we found that an invasive population changed significantly in traits related to thermal performance, which may give them an advantage as temperatures change worldwide.
Behavioral traits are studied less frequently; finding increased variability in an invasive population may provide a clue to how the species can thrive in different environments. Understanding how organisms can establish and spread in environments different from their native ranges can help us predict which species will survive in our increasingly urbanized, changing world.
Dr. MonicaMowery is a Zuckerman STEM postdoctoral fellow in the labs of Yael Lubin and Michal Segoli at Ben-Gurion University of the Negev. She received a B.S. in biology and community health at Tufts University, working on butterfly visual signals and behaviour in Sara Lewis’ lab. Her PhD was conducted in the labs of Maydianne Andrade and Andrew Mason at the University of Toronto Scarborough, where she studied invasion success in widow spiders.You can read more about Monica’s work at her website.
Trophic cascades (see Did You Know?) are an important part of many ecological systems. However, most of the world’s large predator species were lost around 10,000 years ago (potentially due to human impacts), thus limiting the role that predators could play in driving trophic cascades. Though large predators were lost, many large herbivores are still around, which means it is difficult for a smaller predator to take down/consume these herbivores, much less have an effect large enough to drive a trophic cascade.
In the United States, large felines such as cougars (Puma concolor) are known to predate large equid species (such as feral horses or donkeys), but much of the ecological literature assumes/claims that cougars do no exert a strong enough pressure to consider them “significant” predators of these equid species. Specifically, some reports state that these species don’t have any natural predators, and other reports echo the claim. Today’s authors report on a novel trophic cascade between the cougar, feral donkeys (Equus africanus asinus), and wetland vegetation.
Wind turbines are a constant source of controversy. The planet needs more renewable energy, yet wind turbines represent a threat to many bird populations. There have been a plethora of studies just in the last two years focussing on the impact of the turbines on birds. Yet furrier fliers often get overlooked, and it should come as no surprise that wind turbines can be a large source of mortality for bats as well.
Previously wind turbines were usually built out at sea, or in open, cleared land. Yet there has been a move over the last decade to building more wind turbines in forested areas. Moving further into forests could represent a larger threat for bats, so do they show any behaviour that could help them avoid wind turbines? That’s what today’s researchers wanted to find out.
In the natural world, most organisms are limited by the environment as to where they can live. While this can be as drastic as a whale being limited to the ocean and humans being limited to the land, there are also more subtle limitations. That is, black and grizzly bears live in temperate environments, but polar bears are inhabit the arctic where it is MUCH colder. Due to the limitations imposed by the environment, black and grizzly bears cannot live further north.
Historically, most studies have focused on abiotic variables (i.e., non-living), like temperature and precipitation, as there is a clear role for the climate in determining where and when a species can live. However, biotic variables (i.e., living) like predation or competition can also play a role in defining the limits of a species range, though this has proven more difficult to test than abiotic factors, as many tests of biotic variables produce species-specific results. Charles Darwin proposed a framework in 1859 that the importance of biotic interactions would vary predictably with latitude and elevation. That is, at cooler, high-altitude locations abiotic interactions would be more important, while biotic interactions would be more important at warmer, low-altitude locations. Although a number of studies have attempted to test the three predictions (see Did You Know? ) derived from this framework, the results are contradictory and come from data testing different predictions using different data. Today’s authors sought to test all three predictions at once in order to resolve these contradictory results.
A role for the local environment in driving species-specific parasitism in a multi-host parasite system(2022) Hasik & Siepielski,Freshwater Biology, https://doi.org/10.1111/fwb.13961
Image credit: Adam Hasik, image cropped
Parasites are an ever-present part of every ecological community on Earth, yet there are some species that harbor more parasites than others. In systems where parasitism is density-dependent, meaning parasitism increases with host density, the most common/numerous species will harbor the greatest amount of parasites. Yet there are also cases of species-specificity, whereby parasites specifically target a single host species. In other host-parasite systems, local-adaption plays a role in parasitism dynamics, whereby parasites are better at attacking their local hosts than they are attacking foreign hosts and/or hosts are better at defending themselves from local parasites than foreign parasites.
With all of these different factors affecting how host-parasite systems operate, it is important to identify when and if each one is operating within specific ecological communities. This is especially necessary when ecological communities are comprised of multiple host species and multiple parasite species, all of which can/do interact with one another.
To investigate the above factors, we first conducted a survey of parasitism in damselflies (Enallagma spp.) and their water mite parasites (Arrenurus spp.). From there, we then carried out to field experiments to understand why parasitism operates the way it does within this system.