A trophic cascade occurs when a predator’s effects of its prey goes on to affect ‘lower’ levels of that ecosystem. A great example is the effect that sea otters have on kelp: the sea otters prey extensively on sea urchins, which in turn increases the populations of kelp, which the sea urchins prey on. While this is a result of direct predation by otters, often this can occur through a prey species changing its behaviour to avoid the predators.
Yet most ecosystems are more complex than a simple three-level trophic system. Cascades are therefore more likely to occur when the ecosystem is less complex, or when there are well-defined relationships between species, as a result of a predator having preferred prey species or only a few groups of species making up an ecosystem.
This week’s authors investigated how the platypus (our recently-found-to-be-fluorescent friend) influences the abundance and species richness of invertebrates across both rivers and lakes, and whether it’s capable of affecting an ecosystems algae and sediments as well.
Endless forms most stupid, icky, and small: The preponderance of noncharismatic invertebrates as integral to a biologically sound view of life (2020) Jesse Czekanski‐Moir & Rebecca J. Rundell, Ecology & Evolution, https://doi.org/10.1002/ece3.6892
When we think about evolution, too often our perception is that it drives species towards larger, more complex, more beautiful forms. It’s driven by popular media in part, but also by the way we teach it and the organisms we choose to focus on. This goes right back to early conceptions of evolution, with Darwin’s seminal text The Origin of Species referencing “endless forms most beautiful and most wonderful”, instead of “most basic and abhorrent”.
But the authors of today’s paper want to challenge that preconception of evolution as favouring larger or more complex or beautiful organisms, and they have some truly magnificent examples to do so with.
Instead of my normal procrastination for the past week (too much Twitter), I’ve spent every moment not buried in my thesis finalisation preparing one of the most unpleasant blogging pieces since I started this website. It’s the sort of thing that makes me ashamed whenever I publish something by one of the fantastic team of EcoMass authors. Because their amazing work now has to share ranks with this.
Projecting the continental accumulation of alien species through to 2050 (2020) Seebens at al., Global Change Biology, DOI: 10.1111/gcb.15333
A by-product of globalisation is that over the coming decades, no matter how many episodes of Border Patrol get recorded, new species are going to find their way into new habitats and potentially become invasive alien species, exerting negative effects on the locals. We’ve seen this in the past, and I’ve beaten many a dead horse writing about these species on this site.
What this paper set out to find is whether or not we can predict at what scale this trend will increase.
The transition of a coral reef to an algal reef as a result of bleaching and overfishing is one of the most readily identifiable examples of a local ecosystem collapse (Image Credit: Stop Adani, CC BY 2.0, Image Cropped)
It’s a bleak headline, and one which was plastered all over my Twitter and Facebook feeds at the start of this week. I’m used to grim news about the environment. It’s part of my job. So there’s nothing particularly surprising about this title.
What it does represent though, is another use of a somewhat sensational term that is ill-defined by scientists and poorly understood by the public. We’ve written about such terms in the past, biodiversity and functional extinction being two examples. Here, I’m referring to ‘ecosystem collapse‘. I get the draw to such a term: people need to be alarmed about climate change and the ongoing loss of biodiversity our planet faces. Ecosystem collapse sounds really alarming.
So I thought I’d swim around in the literature a bit and see if I could figure out what we mean when we talk about ecosystem collapse.
First record of niche overlap of native European plaice (Pleuronectes platessa) and non-indigenous European flounder (Platichthys flesus) on nursery grounds in Iceland (2020) Henke et al., Aquatic Invasions, In Press
Determining whether or not an introduced species is invasive is important, as it determines whether or not management steps need to be taken to slow or eliminate any negative impacts it might have on the local ecosystem. In Iceland, 15 introduced species have been recorded over the past decades but only six of them are currently classified as invasive or potentially invasive. One of these potentially invasive species is the European flounder (Platichthys flesus), a flatfish commonly found in coastal waters of Europe. The flounder is a catadromous fish, meaning it spawns in marine habitats but has the ability to survive in freshwater streams as well.
In 1999, the flounder was firstly identified in Icelandic waters in the southwest of the country. Since then it has rapidly spread clockwise around the country. Currently, it can be found in every part of Iceland, mostly in estuaries but also up in rivers and lakes. Juvenile flounder can be found on nursery grounds in shallow, brackish waters where they overlap with juvenile European plaice (Pleuronectes platessa). Plaice is a commercially important flatfish species native to Iceland. Despite the knowledge of the flounder’s arrival in Iceland in 1999, not much research has been conducted on the impact of this potentially invasive species.
One of the defining moments of my childhood was a holiday around Australia in the back of a Holden Commodore. My parents drove my sister and me around the whole country, and right in the middle of the holiday we took a trip out to the Great Barrier Reef. Swimming among such a mind-blowing variety of fish species was an unforgettable experience, and one I was able to pass onto my own kid last year. We’d get back into the boat after a swim and stare at an ID card my wife had bought us, trying to figure out which of the cornucopia of dazzlingly-coloured species we had seen.
I just want to start this article off by saying that I had TWO amazing pieces scheduled for today, and I’ve put them both off for next week (my apologies to Yi-Kai Tea and Charlie Woodrow). I’ve done so because the start of this week saw a paper come to Ecology Twitter’s attention that is just plain wild (excuse the pun).
Conservation trade-offs: Island introduction of a threatened predator suppresses invasive mesopredators but eliminates a seabird colony(2020) Scoleri et al., Biological Conservation, https://doi.org/10.1016/j.biocon.2020.108635
Invasive species are a nightmare for local wildlife wherever they are, but on islands they’re even worse. Introduced predators can wipe out entire populations of species, as Tibbles the cat and his fellow feral buddies demonstrated in the extreme when they drove the Lyall’s wren extinct. On coastal islands this is a recurring theme. An invasive ‘mesopredator’ – like the American Mink in Europe or the cat in Australia – is introduced and quickly goes to work, often on small mammals, birds, reptiles and amphibians alike.
Sometimes, but not always, introducing a top predator to an area can suppress the activities of the mesopredator. They can outcompete the mesopredator for resources, or begin to prey on them. The problem is, that if that top predator goes after the same food as the mesopredator, the local prey species suffer either way.
Trying to figure out where a species can comfortably live is one thing, but figuring out which habitats they can actually access is another. I like to think most marsupials would do quite well in South America or Africa, but the fact is that they’re not dispersing across the Atlantic or Pacific anytime soon. However a Species Distribution Model (a statistical model that can be used to predict the likelihood of a species being found somewhere) often requires a more nuanced approach than “big ocean separating these two habitats”.
To integrate a species’ ability to actually access an area into a Species Distributions Model (SDM), we often use the concept of connectivity. Often, this means simply measuring the distance between two populations. But sometimes a species ability to disperse might not reflect something as simple as how far it needs to go. A perfectly good habitat might be only 100 metres away, but cut off by a raging great cliff. Or a road.
In this study, we wanted to see whether we could relate connectivity parameters used in an SDM to the actual ability of the species to disperse.
What We Did
We used two separate study systems here. One consisted of roughly 300 lakes within Northern Norway housed within a single catchment, or watershed, whereby a single path between each lake could be traced. Here we had presence-absence records for two species, the northern pike (Esox lucius) and the European perch (Perca fluviatilis). Both are native to the region, but they are starting to expand into more lakes and have a more severe effect as the climate warms. We used an SDM to investigate which factors determined species presence, including connectivity variables like the length of the rivers between each lake and a downstream population, and the average slope of those rivers.
The second ecosystem was a series of lakes in Sweden which pike and perch had previously occupied, but had been removed from in the 60s and 70s through the use of rotenone, a chemical dumped in small lakes which wipes out fish populations. These were useful, as we knew that the lakes were otherwise suitable for the species given their presence beforehand. As such, here we used a much simpler model to focus on dispersal ability, simply comparing whether or not the species were able to access and then recolonise the lakes from which they had been removed. We compared successful recolonisation from the nearest downstream lake to the same connectivity parameters as in the larger model.
Did You Know: Island Biogeography & Lakes
They obviously don’t look it, but when it comes to biogeography, lakes are essentially a special type of island. Most of the rules of island biogeography apply to them (for fish anyway); larger lakes are more likely to have more species, lakes close to the ocean or other large lakes (the ‘mainland’) are more likely to have those species as well. The big difference between regular islands and lakes is that we can mark pathways between them much more easily. You’d think that would make it easy for us to stop fish spreading into new lakes as the climate warms, but the problem is as always people – people often spread fish from lake to lake, and the rules of island biogeography don’t apply in quite the same way to someone with a car.
What We Found
The slope of the river was a much more important factor in determining a species presence than the actual distance between populations. This makes sense, as a steep slope could make it difficult for a fish to swim up, or could indicate the presence of a waterfall. Furthermore, adding connectivity parameters to our SDM in our first study system did improve our models, but did it represent dispersal accurately?
For pike, the effect of slope was pretty consistent across the two study systems, indicating that the effects of connectivity in a large SDM can mirror a species dispersal ability. However for perch there was some inconsistency across the two study systems, indicating that perhaps there was some other aspect of the rivers between populations that had a larger effect on dispersal.
This study suffers from the same “lab vs. field” pitfalls as any other experiment that compares a complex study system to a smaller, ‘simpler’ one. Here, time is a factor. Our first study system looks at populations that have had centuries, in some cases millenia, to establish, whereas the second one looks at short-term re-establishments. It’s possible that given enough time, pike or perch could have eventually recolonised some of those lakes.
Having an idea of the effect of how different slope measurements can affect the dispersal of species is a great help, as it lets us know which lakes are protected by natural dispersal barriers, and which are likely to be invaded by species moving from downstream. However the fact that for perch, slope parameters varied in their effects across the study systems is a stern reminder that we need to always be mindful of how connectivity parameters actually relate to dispersal ability.
Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is now completely done with this paper and never wants to look at it again. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.