Polyps of Schuchertinia allmanii. (Image Credit: Luis Martell, CC BY 4.0, Image cropped)
Earth is a fountain of incredible abundances and varieties of life-forms, with many of them still undiscovered. Biodiversity is a key pillar for our life as we know it, and we are not only a small fraction of it, but also use and harness this richness for the benefit of our own species’ advancement. Many human advances are based on other organisms’ attributes and talents, which is why we use certain species as “model organisms” when pioneering scientific breakthroughs. One example of such a specific form of life has helped us make some serious inroads into forms of regeneration and even immortality over the last few billion years ago, and leading us to great discoveries in science.
Bachelor students studying ecology collect data on a field course with the Norwegian University of Science and Technology. (Image credit: Caitlin Mandeville, CC BY 2.0)
The first time I remember really thinking that I could be an ecologist was during a three-day trip to a field station in northern Wisconsin as part of my college limnology course. Sure, I already loved my ecology classes and learning about nature. But actually being a scientist? Real scientists, I thought, were people like my professors and graduate teaching assistants, who peppered their lectures with captivating tales of their own research.
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.
This weekend I’ve got some friends coming around to play Evolution: Climate with my wife and me. I’ve never been much of a board game type, but my friends learned recently that if you slap an ecology (or comic book) theme on anything I’ll be there with bells on. Evolution looks like it’ll be fun, but first I wanted to get weirdly passionate and talk about a childhood game that really drove me towards ecology as a kid.
While outdoor and feral cats are pretty universally accepted by scientists these days to be environmental hazards of the most destructive kind, the fact remains that they’re… well, cats. They’ve been companion animals for millenia, and often the general public react strongly against proposed measures for feral cats (or even to being told to keep their own cats indoors).
So why is it that despite a wealth of science making the case for feral cat management, many people simply can’t get on board with keeping them in check? And why do ecologists even need the public onside in the first place?
To dig a bit deeper, I spoke to Brooke Deak, a socioecologist based at the University of Adelaide. Brooke has spent the last three years studying the feral cat management debate, trying to better understand the relationship between feral cats and the general public.
Sam Perrin (SP): I guess before we start talking about feral cats, we should start talking about what constitutes a feral cat. What makes a cat a feral, a stray, or just an outdoor cat?
Brooke Deak, University of Adelaide (BD): In Australia it’s easier to tell the difference between the categories of cat. Feral cats here have minimal contact with humans, they don’t use human resources or take food from us. They may have been born as domestic cats, but they’ve escaped and their kittens have been born out in the wild. Stray cats in Australia are semi-domestic cats. They may wander around, but they’re reliant on humans to some degree. And then you have the domestic indoor cats.
But in the US, Europe and Canada, it’s very tricky to define them. Especially in the US, where every single county and state you go into has a different definition of what a feral cat is. It makes it hard for management purposes to get in there and to know what to do with them. Are they people’s pets if they spend some time in feral cat colonies? Do you Trap-Neuter-Release, or do you kill them, if there’s a chance a human somewhere feeds it sometimes?
SP: Your research focuses on public perception of feral cat management. Why is it so important to get the public onside when it comes to managing invasive species like cats?
Without public support, invasive species management programs often fail. Social license from the community is important to gain in order to use almost any management methods within a locality. Without this, management campaigns can become delayed for years or sometimes indefinitely.
That’s why we need public support, and why it’s important to engage with the community. We need to understand their ideas the public and their attitudes towards invasive species and the effects of those species. It is also important just to bring awareness to the community as to what’s going on, and to bridge the gap between the government and the public as well in terms of building trust. There’s a lot of dimensions to public support for invasive species.
SP:The anti-science brigade are fairly vocal these days, often with regards to the climate change debate. Yet even people who are all about listening to science in the climate debate will ignore the overwhelming evidence that feral cats – and really all outdoor cats – are bad for the environment. Why is there that rejection of the science in this case?
I think there’s a lot of misinformation out there. There are so many people who get information from places that aren’t completely scientific or don’t have all of the facts. In my experience in the US, there are a lot of people who really advocate for outdoor and feral cats, and a lot of the misinformation about these cats comes from them, saying that we need to save these animals. It gives people the impression that feral cats aren’t that bad, when in fact there are so many negative impacts that they have, such as on humans, on other species of cat, on native felids, and what they can do to pet cats themselves.
Then of course there are the people who might accept that outdoor and feral cats do have a negative impact but refuse to believe that their own cat is part of that problem, despite all the scientific evidence that says that it IS a problem, and perhaps there’s an emotional aspect to that too.
SP:The main thing we associate with outdoor and feral cats is their killing of wildlife, but of course there are a lot of other dangers they pose too.
So there’s not much hybridisation here in Australia, but in Europe they have the Scottish wildcat for one, who are already threatened. And there’s definitely a danger to that population due to them hybridising with feral cats. Then further south in Europe you have the Iberian lynx. Feral cats can spread feline leukaemia to the lynx, which hurts those populations even further.
I don’t think enough people know about that, it’s definitely a big problem wherever there are native felids. Populations are going downhill because of this domestic cat species. They’ll be in danger of extinction if we don’t do something about the feral cats.
SP:As opposed to killing feral cats, many organisations and researchers have proposed the Trap-Neuter-Release approach, whereby a feral cat is caught, desexed, and returned to the wild. The theory is that it prevents the cats from reproducing, keeping down the population long term. How effective is TNR?
Actually I don’t think it’s effective anywhere to be honest, especially not in Australia. Even if you neuter them so they can’t birth more kittens, they’re still going to be out there killing, and it only takes one cat to decimate an entire population of a small mammal or bird species. It might be effective in terms of getting rid of the cat’s reproductive abilities, but not in terms of their impact. After being neutered, they’re still out there killing and spreading disease.
It might work in the long, long term to manage the populations, but it’s not very realistic. I don’t think there are the resources or funding to make it work effectively, in the US or Australia. It’s definitely not an option for Australia, because we have so many endangered species that are really under immediate threat from cats.
SP:There are a lot of different techniques for dealing with feral cats. You were able to fill out an entire seven minute video explaining them all (linked below). How good is public knowledge of these different techniques?
Not very. It depends on familiarity with the methods and the feral cats debate in the first place. You have farmers who are much more familiar with control techniques. Certainly sheep farmers and people with livestock, because they’re directly affected by the feral cat presence.
People in residential areas aren’t as familiar. That’s understandable, most of the time it doesn’t impact them. But for some management plans, we’ve had issues using poisons or traps designed for cats. Because people aren’t familiar with the science behind the methods, they hear the word ‘poison’ and think “oh you can’t have that around kids and pets and other wildlife”.
SP:Invasive species are the second biggest threat to biodiversity worldwide, and Australia has had a plethora of bad experiences on that front. Yet cats have often somehow managed to frame themselves as outside of the invasive species debate. Why is that??
I think just because we think of them as companion animals. People have cats which creates more of an emotional bond. They see a feral cat and think it’s just like their house cat. They don’t really realise what they’d get themselves into if they went to pet it.
Also, feral cats in Australia are out in the bushland mostly. You do get some in cities, but not many. It means the nature of feral cats is just not something that they’re very familiar with. The only association they have with cats are cute domestic ones. You wouldn’t own a fox. You could try, but it’s not a great idea.
SP:During the bushfires earlier this year we heard a lot about the plight of the koala. But of course there were a lot of smaller mammals and birds that really suffered as well, and many of those effects were exacerbated by the cats picking off whatever escaped the fires. Are there any species we should be especially concerned about, with bushfire seasons around the counter in Australia?
One in particular is the kangaroo island dunnart. It was in peril before the bushfires and now it’s even worse because the feral cats have been on the edge of the fire line on the island. They’ve been hunting everything and the poor little dunnarts, they don’t really have anywhere to go. I think they’re recovering now slowly, very slowly.
That was the case with a lot of species when the bushfires tore through Australia. The cats had a smorgasbord, and just sat on the edge of the fireline and killed everything that came by. It was awful.
To find out more about Brooke’s work, follow her on Twitter @Deakology.
Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is really sick of explaining to his friends back home that Mittens is still a murder machine even though he’s got a bell on his collar. 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.
When temperatures increase, trees grow more. When a moose struts in and eats the twigs, trees grow less. So, if we just have enough moose around, climate warming won’t be able to increase the growth rate of trees. This is what we call the “cooling” effect. Rather simple – and cool – story, right?
However, every ecologist knows that the biological theatre is more complex than this. What if snow protects saplings against browsing? What if changes in temperature affect moose in such a way that they will not feed on trees in the same way as they used to? What if trees’ response to moose is actually different depending on whether it is warm or cold? In complex ecological systems, tree growth is determined in an intricate network of interactions, where the story line is so mind-bogglingly complicated that it seems almost impossible to say what is actually going on.
Luckily, it’s not quite impossible. In this paper, we set out to model those intricate networks, taking into account everything from the climate, the tree species, the effect of time, to the presence of herbivores and their browsing intensity, in an attempt to disentangle that complex biological theatre.
What We Did
To get a baseline impression of what happens to trees when moose aren’t around, we set up fences to keep the voracious ungulates away. Originally, this fencing was started by NTNU University Museum in Trøndelag, later expanding to 62 sites scattered across Norway and eastern Canada. As moose can only browse on relatively small trees, fences were placed in clear-cut areas where we could monitor the growth of the trees from the sapling phase.
After keeping track of the height of hundreds of trees inside and outside of the fences for more than a decade, we had assembled over 16 000 growth measurements. These were accompanied by annual estimations of the proportion of browsed twigs. Based on existing knowledge about which plant species the moose preferred, we also estimated the amount of food available for moose at each site. The tree growth and browsing data were complemented with data on three climatic variables, namely, growth period temperature, precipitation and winter snow-water equivalent, as well as data on regional moose density.
We analyzed the data with structural equation models (SEMs) that combine multiple predictors and response variables into one big model network. A SEM allows you to treat an environmental variable both as an explanatory variable and a response variable simultaneously. For instance, the amount of competing trees could be explained by moose presence, but it itself could explain tree growth.
Did You Know
Herbivore cooling effects are better documented in arctic and alpine systems, where smaller woody plants namely shrubs, play the role of the trees. Empirical studies have shown that for example reindeer can slow down climate-change driven shrubification that would otherwise result in loss of open tundra. However, also in the arctic, herbivore effects take multiple forms: sheep effect seem to be modified by climate, potentially via plant-plant competition.
What We Found
Three of the studied tree species played along with the simple cooling story: Canadian rowan and birch and Norwegian birch. They benefited from higher temperatures and suffered from moose. However, most of the tree species wrote their own, more nuanced narratives. Canadian fir responded more weakly to temperature when moose were missing. Norwegian rowan flipped its temperature response curve around if moose were present. Norwegian pine responded negatively to temperature, but did not seem to be bothered by moose. This is understandable, as heat turns into an enemy when it gets too hot. The soft, palatable species took more damage from moose than spiky spruce and pine.
From a tree’s point of view, the role of a moose can change from a foe to a friend if the moose browses on a neighbouring competitor tree. Canadian fir and rowan and Norwegian birch and pine benefited from the fact that moose lowered the growth of competing trees. Snow complicated the story even further. Norwegian rowan benefited from increasing winter precipitation, but only outside of the fences, suggesting that individual trees were indeed protected from browsing by a snow layer. This is potentially a result of snow lowering the proportion of browsed twigs. Interestingly, also temperature affected browsing intensity, but the effect size and direction varied between different tree species.
The bottom line given by these results is clear: the moose cooling effect exists, but how important it is really depends on ecological context.
We always need to careful when assessing results obtained by using datasets of different accuracies. Where locally estimated browsing data was highly precise, regional moose density and climate data were less so. Thus, the effect strengths may partly reflect differences in data quality rather than true differences between explanatory variables. Overcoming these weaknesses might reveal side-plots yet to be unravelled.
So, if the pathways of climate effects are this complex, what is actually going to happen in the boreal forests when temperatures rise? Some tree species may benefit from increase of temperature just to end up on the moose dinner menu. Less tasty ones may thrive, or suffer from excess heat and increased competition. If global warming brings us snowless springs, cooling potential of browsing may increase. In contrast, if we get more snow with increasing precipitation, moose may turn into a trivial side-character.
In the complex interplay of biotic and abiotic actors, only one thing is certain: that we do not know what will happen outside the observed variable boundaries. Interactions and non-linearities make any future predictions highly uncertain. If we are to place hope on herbivory as a cooler of climate change impacts, constraints imposed by species differences, snow, competition, as well as climate effects on browsing must be acknowledged – not so neat of a story, and perhaps less cool, but nearer the ecological reality.
Katariina E. M. Vuorinen is a PhD candidate at the Norwegian University of Science and Technology. She studies the effects of climate and large herbivores on plants by using data from across boreal and arctic biomes. You can read more about her work at this link.
Title Image Credit: James D. M. Speed, NTNU University Museum, CC BY 2.0, Image Cropped