Lauren Kuehne has an interesting guest post at the blog Conservation Bytes talking about one of the most persistent false dichotomies in the environmental world: native versus exotic species.
A drawback to the attention garnered by high-profile invasive species is the tendency to infer that every non-native species is bad news, the inverse assumption being that all native species must be ‘good’. While this storyline works well for Hollywood films and faerie tales, in ecology the truth is rarely that simple.
There’s a desert angle here that I’ll talk about after the jump, along with some videographic reptile squee. You have been warned.
One of the issues facing the North American desert these days is the slow decline of a number of populations of desert tortoises — excitingly split into two species last year, at least for herpetological values of “exciting.” Though they’re a threatened species — or at least the species west of the Colorado River is, as wildlife agencies haven’t yet dealt with the issue of listing the new species east of the river — you can still find tortoises wandering around the desert pretty easily. I’ve seen about a dozen in the last four years without even trying to, and a big solar project with which I’m familiar has found a whole lot more.
But torts are threatened, by everything from bulldozers to a contagious respiratory disease to illegal collecting, and one of the biggest threats to their existence is a native species of bird, the American raven. Ravens have always eaten juvenile tortoises, which are pretty vulnerable, but in centuries past there were never enough of them to threaten the species as a whole. But the ravens’ numbers have been swelled artificially as we move into the desert, bringing garbage dumps and lawn sprinklers and power poles to perch on. And so the pressure on the tortoises has increased by probably an order of magnitude, perhaps more.
And you could pretty much take the previous paragraph and find-replace “ravens” with “coyotes” and it would still make sense, possibly excepting the “power poles to perch on” part. Not all exotic species are harmful and invasive — you’ll be hard-pressed to find enviros campaigning against daffodils, for instance — and there are some native species that can be invasive and destructive onder the right conditions. Or the wrong ones.
Over the last couple decades I’ve found that a lot of my fellow environmentalists — like a lot of my fellow humans of all kinds — really like neat dichotomous categories. But those categories often say more about the people doing the categorizing than they do about the world.
And then sometimes they don’t: I’m informed that some of the junipers Kuehne refers to, which are thought to be encroaching on sagebrush habitat in more northern parts of the desert, turn out — once you cut them down and count the rings — to have been “encroaching” for three or four centuries. But there’s money to be made cutting them down, either by increasing grazing land or burning the trees for biomass power, or both.
Bonus baby desert tortoise squee courtesy the U.S. Geological Survey. If you’re allergic to harp music you might want to lower the sound.
Keith Peterson says
Good old Ravens. They take after humans I hear.
Chris Clarke says
They definitely do, if said humans happen to have french fries.
And where would we be without exotic (but non-invasive) species, such as rosemary?
Azkyroth, Former Growing Toaster Oven says
…isn’t sagebrush originally from Russia?
Chris Clarke says
It is from Asia originally, Azkyroth, but it’s been present in the Great Basin area since the Pliocene. Perhaps you’re thinking of tumbleweed, which is a 19th Century import?
Rev. BigDumbChimp says
Pigweed will kill us all!
Then there’s cheatgrass. Nasty stuff. It occurs in areas in (at least) the montane-desert interface along the California-Nevada border, where sagebrush used to be. I don’t know if it’s a native, but it makes crappy fodder, robs the sagebrush plant community of the ability to grow back, is a wildfire fuel, and just generally makes the lives of people living in, say, Mono County, CA difficult. I have NO idea how widespread this stuff is. We have tons of it on our “getaway” property in northern Mono County, in the Eastern Sierra, where some previous owner decided to take out all the beautiful sagebrush and such and plant water-devouring trees that are now dying. There’s a two or three week interval in the spring where cheatgrass is green, growing, and hasn’t yet set seed, an all-too-tiny interval in which to contain it. After that, seeds are scattered and the grass dries to become wildfire fuel.
Am I the only one who suddenly pictured a coyote nervously perched atop a power pole after reading this?
I am left with two images in my mind. One is of a line of power poles, each topped by a coyote waiting to make its delightful little pounce onto a squirrel; the other is of a hidden race of desert hobbits foiling, with the ravens’ help, the ever encroaching hoards of dragons.
bring it with the tortoise posts, man.
Conservation issues are (or ought to be) biological issues; biology is complicated; therefore conservation issues are complicated.
I’ll agree that invasive vs. native is not a meaningful dichotomy.
Still, native/invasives like ravens in the Mojave are, afaik, rare. White-tailed deer in the NE of the US, Canada geese, maybe coyotes in some places, raccoons in others, oppossums…that’s pretty much it (North America).
Brings to mind images of bobcats up poles, though.
Rather expected the tortoises’ shells to be leathery. Those appear to be at least somewhat calcified, like a bird’s egg.
chigau (違わない) says
Baby tortoises hatch.
“We have millions and millions of years of evolution. We know what to do!”
Even the categories of “native” and “exotic” can be a bit misleading. Many of the birds and plant species we see in North America today were brought over by European settlers.
exotic vs. native species problems are different in the East Texas swaps. Kudzu has a bad reputation because it can quickly overtake a forest. But at the same time, the leaves are edible, and if we just harvested it, we’d have a great source of nutrition.
Imported red fire ants are an “exotic” species that deserves absolutely NO sympathy. They completely wiped out our horned-toad population, and their stings are simply awful.
Chris Clarke says
Brown-headed cowbirds, too.
And even within species: I’ve heard a lot of grumbling about coastal California poppies being planted in the desert. After the 1990 Oakland fire, some of us tried to persuade the big wildflower seed companies donating native seeds for revegetation to make sure the seed stock came from within the same county. It didn’t really sway them.
Chris Clarke says
Kudzu’s roots are also a source of a high-quality cooking starch. Takes a backhoe to dig ’em out, though.
John Morales says
Humans are a non-native invasive species, outside of Africa, and do a fair bit of ecological damage.
Rey Fox says
There’s also the Caspian tern that eat juvenile salmon at the mouth of the Columbia River.
Azkyroth, Former Growing Toaster Oven says
…err, I mean, yes, I am. *cough*
Here in western Oregon, opossums are so ubiquitous (mostly on I-5) that it’s hard to remember that they were introduced during the 30s or 40s.
The bio teacher at the local community college insisted that I did not have ravens on my front lawn (because they’re not found in the valley floor, only further up in the Cascades) until I brought in a few photos of birds that were obviously not crows.
Gnumann, soon to be known as Gnumann+ (with no irony at all) says
No, but I certainly did after reading this.
What strikes me about the main feature is the lack of real focus on the main invasive species. The ravens are just a derivative problem of course. The problem is that people are invasive in the desert environment.
The ravens out here on the coast are filling in the gaps that were abandoned by raptors and sine they love forest interfaces – and humans love to make them – they’re doing quite well out here. But they’re also very good at seed-picking, so they’re our go-to bird for moving seeds.
It’s a bit of a quandary; and it’s hard to tell if the raven explosion is really hurting other species, since they’re filling gaps left behind. Even if their numbers help in some places, are they hurting in others?
And it’s tough to get seeds local to your area; they’re almost always the same seed across the country, let alone variants around our single state. We’ve been doing alot of private seed-collection and trading. Even so, we know we’re mixing seeds from one side of the mountain with ones from the other, which did not previously mix.
@ Crissa: In general, replacing a specialist predator (like most raptors) with a generalist is going to shift the pressures on prey species considerably. Most raptors that have been pushed out feed mainly on rodents and rabbits, or prey birds like doves, or fish. Ravens are really adapted for catching fast prey near they’re size, but they’re very smart, so they shift the pressure away from things like mice to things like bird and reptile eggs and baby animals of all types. It also doesn’t help that they tend to have much higher population density that raptors and can put fairly severe pressure on some species during their breeding season and then live off humans for the rest of the year.
I don’t get it. What does native mean? Native of the desert or native of the US?
I may be missing some finer points but I don’t see the real problem with the dichotomy, partly because I don’t believe it exists to any meaningful extent in modern ecology.
Anyway, parts of the post irked me a little. This line in particular had me shaking my head:
Its just far too simplistic. I have never heard an experienced ecologist categorise native vs. non-native species in this way. A more common and valid account would be ‘native species = good in their natural habitats if provenance is appropriate, and as long as ecosystem health has not been significantly compromised’ and ‘non-native species = possibly bad if its biology or ecology is significantly different from native species already present’. Not as snappy, but neither is the reality of most things in ecology.
An ecologist would generally strongly caution against introducing (or facilitating the introduction of) any species, race or even within-species genetic variant into any location to which it is not native, regardless of whether the habitat might seem appropriate, unless an appropriate impact assessment has determined no significant negative impact is likely. Lauren Kuehne’s article does clearly acknowledge that “native invaders” usually rear their ugly heads in response to some kind of human-mediated environmental change, but that point and its implications for conservation biology seem to be a little lost in the rest of the article and in much of the surrounding discussion.
In the specific cases which the article mentions the focus should not be on “invasion”, it should be on ecosystem change, its causes and its implications. From a conservation biology perspective, the focus is on reversing or mitigating the causes of the invasion (if anthropogenic and where possible), or letting nature take its course (if the invasion is spontaneous and not caused by disturbance), or further research with a view to greater understanding and management (if the impact is uncertain or if it appears benign or positive, though in what spatial, temporal, social, economic or scientific contexts one determines “positive impacts” is another can of worms altogether…).
I also take issue with the suggestion that the stated impact of ravens on tortoises is a demonstration that native species can be invasive. In this case, it is important to remember that the major invasive species into that desert habitat is Homo sapiens, and the impact that they have had on the ecosystem has precipitated a change in ecosystem processes and functions. It is also not just the ravens that have had an impact – early invaders arriving with humans to this modified desert habitat will have included a host of other species associated with human habitation, including rats, mice, and domestic cats and dogs. Future invaders may even include pet tortoises which can have an impact if they are released into the wild (as may be happening with African spurred tortoises released into parts of Arizona). I would argue that the ravens are not invasive, they are adapting to a newly available habitat – the altered desert landscape – recently created by people.
There are also a few problems with the background articles cited in the ConservationBytes.com post: for example, in the ABC News article, the claim that
is nonsense. Is the California condor native to Ireland? No. The entire species is not native to Ireland. It has never arrived there. Is the Himalayan Snowcock native to the mountains of Nevada? No. It was introduced there, and the entire species is not native to Nevada. However, the longer a species exists in a specific locality, the more likely it is that natural selection and genetic variation will begin to produce a population with unique genotypes or phenotypes – creating the odd situation where a population of a non-native species can have a unique local provenance – the reverse of what theABC article states. An interesting example is the European wild rabbit, introduced in Britain where some populations have unique behavioural characteristics, while others display local resistance to disease. While this clearly causes confusion vis-à-vis the extent to which a species or population should be considered native, fundamentally the line between native and non-native is not as indistinct as the articles would suggest. The definition of “native” is a basic concept in biogeography, and I don’t see how or where it has changed in the past century. A species is considered native to a region or locality if it can be stated to have arrived at that locality under its own steam or via some natural process. Hence the little egret and great white egret, species common in southern Europe which have recently extended their range into the UK and appear to be thriving, are new natives in Britain. The European starling, brought to the US by settlers, is not native there.
John Morales says
roland, in context here, ‘native’ clearly refers to a ‘non-invasive’ species (since it’s been phrased as a dichotomy), or in other words a species that has not co-evolved as part of the local ecosystem.
@ John Morales #25
Not really correct – the article at ConservationBytes.com discusses native species that are invasive. A native species can become invasive even in its own locality if a shift in ecosystem function or stability enables it to gain a foothold. I am currently working on a site in Ireland where a small area of raised bog is being invaded by birch woodland, which has spread from 100 yards away, as a result of drainage of the bog for peat extraction (turf cutting). wihtoiut the drainage, the birch would not have been able to establish itself.
This reminds me of a conversation I had with a commercial fisherman on the shores of Gooseneck Cove in Newport, RI. I was watching and photographing Double-crested Cormorants on the waters of the cove; it was flow tide and they were busy feeding on the food coming into the cove from the ocean. The fisherman was complaining that the the Cormorants were overfishing, especially as Gooseneck Cove serves as a sort of natural nursery for all kinds of fish. He was actually blaming the feeding Cormorants for the drastic down-tick in the Cod, Flounder, and Striped Bass populations that caused the government to issue regulations to lower limits for the overfished populations. When I pointed out that Cormorants had probably been feeding on this cove for millenia without making a dent in the fish populations, he just scowled at me and walked away. You really have to wonder what some people use for brains!
I’d argue that it would be nearly impossible, after thousands of years of human disturbance, to determine if a native species could actually be considered invasive. First, you’d have to prove it wasn’t historically THERE to begin with and wasn’t actually extirpated previously and is now just returning. You’d have to prove that it was human activity that allowed the invasion rather than just some coincidental natural change. Even with non-native inasive species, it’s sometimes a natural invasion, not human stupidity; birds blown by storms, rafts of lizards washed from one island to the next, an eagle unluckily dropping a pregnant fish in a lake that fish has never been in.
A native species can become invasive even in its own locality if a shift in ecosystem function or stability enables it to gain a foothold.
this is a silly usage of the term. under this definition, ants that set up a new colony are “invading” previously unoccupied territory.
So, either we need a better term to describe migration of species into previously unoccupied habitats, or a new term to describe the unintended but nevertheless assisted transplantation of species outside of their native habitats.
“invasive” just doesn’t cut it in the instance you are describing.
I have never heard an experienced ecologist categorise native vs. non-native species in this way.
I’ve heard plenty of conservation groups do this though.
common for city and county planners to respond with “restoration” projects, too.
look the real issue is, and always been… what, EXACTLY is it that we want to accomplish?
maximizing productivity of selected species?
it will vary in time, and with the place, and with the people involved in making the decisions.
I was there when conservation biology as a field really started taking off. It used to be that the main goal was preserving the maximum biodiversity possible within any given ecosystem, but in practice that was just too simplistic. Now, often the goal is managed resources; making sure critical reserves are set aside for specific species to act as population distribution nodes.
In the end, it’s always the people that utilize the ecosystem that decide what the “best” decision is for it.
For better or worse. As soon as humans can interact with an environment, decisions are made that change it.
Conservation issues are (or ought to be) biological issues; biology is complicated; therefore conservation issues are complicated.
well, they OUGHT to be complicated, but often the end results of conservation “projects” are all too horribly simplistic.
Arghh, the cheatgrass! I am near Reno, so I’m unfortunately all too familiar with the damn stuff. Wikipedia tells me that is is indeed invasive, and that “it is much reviled by ranchers and land managers.” Yeah, no shit. I am pleased to find that Wiki makes a note of what’s become a common way to try to deal with cheatgrass: set some sheep on it in the early spring and try to get as much of it eaten as possible before it sets seed.
@ Ichtyic #29:
Your point indicates that you don’t understand the term “invasive species” as used in conservation biology. The Society for Conservation Biology provides the following defnintion:
“An invasive species is one that arrives (often with human assistance) in a habitat it had not previously occupied, then establishes a population and spreads autonomously.”
So in your example, yes the ants could be considered invasive if they establish a new colony (and spread) in a habitat where they do not usually occur. However, invasive species usually refers to those with negative impact. The definition of an invasive species used by the UN is:
“species whose introduction and /or spread outside their natural past or present distribution (including habitat preference) threatens biological diversity.”
So in your example the ants would generally not be categorised as an invasive species from a policy perspective unless the impact on biodiversity was negative.
@ Ichtyic #30:
Granted that may be the case. This is where the scientific community needs to get tough on conservation groups that leave the science out of conservation. As regards city and town planning, in most countries invasives are dealt with on a priority basis informed by scientific evidence (for an example of one national initiative to prioritise species for action, see www.http://invasivespeciesireland.com)
However, invasive species usually refers to those with negative impact.
usually, but not always.
no, there is no consistency to the usage of the term.
yes, I am more than well aware of conservation biology, having gotten my grad degree in behavioral ecology and worked with nonprofit conservation groups for over 20 years since.
thanks for the attempted lesson though.
@ Ichtyic #31:
Bingo, spot on. Hence the huge headache at the interface of conservation science and conservation policy over most of the past 20 years. Much of this has been agreed upon to a large extent at the UN level – look at the Aichi Targets at the UNCBD website (http://www.cbd.int/sp/targets/) and you’ll see the results of a global effort to try to answer that question (though the US is, sadly, not formally involved with or implementing the CBD).
@ Ichtyic, #35
Well if I’m not correct with the definition please show me where I’m wrong with your ants, I’d be obliged.
Much of this has been agreed upon to a large extent at the UN level
…but completely irrelevant at the local level, which is often the most important.
OTOH, if you want a great example of an international case of what the reality is, how about the current attempts at producing a reserve system for the Ross Sea?
like I said, what will be decided to be the goals for “conservation planning” will constantly vary with the people involved, the stakes involved, and the time you begin your efforts.
it isn’t realistic to have an “umbrella” goal in mind, even something as supposedly simple as “maximizing biodiversity”.
Not at all, the Aichi Targets and the rest of the CBD process call for bottom-up implementation based on local realities. The Ross Sea issue has so far proved difficult because of a lack of political will between countries that are bound to the CBD targets (whihc include co-operation). Instead, they are foot-dragging because of other strategic interests in the Ross Sea (industrial fishing, military activities and mineral explorartion). The problem is not that conservation priorities are not well established, its that the governments that need to commit to forming a legally bding Marine Protected Area are not stepping up to the plate. But that could change within the next 12 months – the Ross Sea is one of the issues to be discussed at the next UN Biodiversity Summit, next month, and it will be raised at the UN General Assembly in 2013 and the Int’l Whaling Commission at its next meeting.
Dave, ex-Kwisatz Haderach says
Sort of, but the coyote I was picturing was wearing a hang-glider attached to an Acme rocket, and waiting for the Road Runner to go racing past.
yeah. That was the point.
The referent of ‘we’ was not ‘experienced ecologists’.
Different? The ones to worry about are precisely the ones whose ecology is similar to native species already present.
There might be feral dogs in the far West Mojave and around like Barstow, but none of those Eurasian animals can hack it in the physical environment there. Much more worrisome are plants.
Back in the pre-Interstate days of Rte. 66, gas stations used to sell tortoises for a couple of dollars. A very large number of them were then released days to decades later, wherever. Nobody knows how that’s affected local gene pools.
There was a brief sensation among testudinidophiles in 1989 when a fifth North American tortoise species (before the recent split of G.agassizi [second link following]) was announced from Baja. It turns out that they were (moved there from elsewhere in Mexico.
It’s not clear that those alternatives are mutually exclusive. Ravens might be a floor wax and a dessert topping.
Like white-tailed deer, they have become, what, native pseudo-invasives thanks to habitat alteration by people.
Well. political boundaries clearly ought to be irrelevant (though clearly they are not, practically speaking). So, yeah, the desert, but I’d even say a particular region of the particular desert.
It’s because of the complexities of the actual biology going on that discussions like this get bogged down in misunderstanding about words and concepts. Native, non-native, invasive, exotic, introduced, endemic, population, species.
Turkeys, house finches, and red-eared sliders are certainly native to eastern North America, but they have been introduced all across the continent. Armadillos and oppossums, and maybe coyotes, and cowbirds have expanded their natural ranges greatly recently without being moved by humans.
These cases are different from starlings and sparrows and pigeons and pythons and purple loosestrife and cheatgrass and tumbleweeds etc. etc.
And I suspect that here and there there are populations of introduced non-native species that are actually beneficial to local biodiversity. Honeybees?
I used to think that maybe Erodium cicutarium, a Eurasian weed, was the best thing that ever happened to tortoises in the West Mojave. It grows in times and plaes when native annuals couldn’t, and the torts certainly didn’t mind eating it.
shit that was submit instead of preview
The announcement of the Baja tortoise not-species was 20 years before the recent split of G. agassizi.
The tortoises found in Baja were (probably) moved there from elsewhere in Mexico.
Documentation for both of these statements is the Murphy et al. ‘Dazed and Confused’ paper linked (full text).
biodiversity, invasive species, native, none-native habitat change, all of that is complicated.
Humans have been involved with changing where we live for a very long time. We have been a factor in the disappearance of whole animal species and the distribution of plants and animals for millennia. I doubt that we will realistically be able to prevent the redistribution of the plants and animals all around the world to ecological habitats in which they are well adapted to survive we can slow it down some what maybe but unless we are willing to impose real draconian measures that are 100% effective we will eventually fail.
Of all the things that are happening in the world that is the sadist
Hi there Chas #41
You misread me. My point is that Kuehne’s point was far too simplistic.
Dr. Kuehne’s article was about the perceptions of ecologists, and that’s what I was talking about. Kuehne is herself an experienced ecologist.
Incorrect. If their ecology is similar to a species already present they are more likely to have the same vulnerabilities and therefore can be predated or succumb to disease etc., and are therefore less likely to become invasive. If they are sufficiently different, then they will either (a) not be as readily impacted by the same species of herbivore / carnivore / pest / pathogen, at least not to the same degree (b) have some other significant competitive advantage, perhaps being bigger or stringer, or able to utilise resources more efficiently, e.g. link preferentially with mycorrhiza, fix nitrogen more swiftly, or otherwise inhabit a physical / ecological niche not previously filled etc, (c) be better able to adapt to local environmental variation, such as the impacts of climate change, which can further facilitate invasion, OR (d) start killing / weakning one or more species of the native flora and fauna that theretofore was not under that specific attack. All major threats by invasive species around the world, from rats introduced to small islands, to New Zealand flatworms in Britain, to West Nile Virus in the US – have been caused by species that were significantly different in biology or ecology from the existing native complement in the specific ecosystem where they then became invasive.
Wrong on the first point – European rats, rabbits and mice thrive in dryland habitats in the US, Australia, and Africa. Look at the bunnies hopping all over the Australian outback – but in any case European mammals are not the only animals associated with human habitation! My point was that the ravens are late comers to an altered ecosystem; habitation and its associated menagerie – deliberately transported or not – created a new arena which ravens exploit.
As for your point about the plants, you may be right but I know of no tortoise-eating plants causing trouble in the desert ;)
I’ll defer to your expertise on the testudinidae point. My point is that one of the many ways that people change the ecosystem is by transporting or otherwise facilitating the introduction of other species, which may incluide pet torts as has happened in some areas.
I understand your point, but my argument would be that the desert ecosystem is now fundamentally changed as a result of human impact, and ravens are one of those tag-along, late-to-the-party species associated with human habitation. To me, its not “the ravens are invasive in the desert”, its a case of “the desert ecosystem is thrashed and is no longer functioning as it was, and look the people brought their bloody ravens”.
Chris Clarke says
This is the best kind of comment thread: I’ve learned a few things.
I don’t wish to disagree with your main point, but the North American deserts are actually among the LEAST thrashed ecosystems on the continent in terms of conversion to human use, invasive exotics as a percentage of biomass, intact biodiversity, roads, and a number of other factors. At present, human disruption in much of the desert is nodal, clustering around a few large cities and along road corridors, and ravens are one of the means by which the effects of that disruption radiates out into less-altered desert.
Of course there are human activities that cause changes at the landscape level: climate change and chemical and particulate air pollution are good examples in SoCal. Those will get worse in weeks and months to come. There’s also grazing, which has affected almost all of the desert. Still, to the extent that any landscape that’s been inhabited by humans since before the end of the pluvial can be considered “untouched,” the North American deserts mainly are relative to much of the rest of the continent.
Presumably we’re defining “plants” to exclude solar power plants.
Hi Chris, welcome to the realm of cyber bullies and feminazis! (tongue firmly in cheek).. and thanks for getting a good discussion going.
You’re right on the threat status of US deserts, and “thrashed” is overstating it significantly, my bad. A better way to state it: in the areas where the torts are under threat from ravens the birds are not, from my perspective, invasive. Humans are the primary invader, and the ravens et al are part of the altered ecosystem (even if periferal) they create.
Hmm. I understand Chris to say that the regions where the torts are under threat from the ravens are regions that humans have not invaded. (Presumably, the regions humans have invaded, the torts are pretty much extinct.) That is to say, the ravens have invaded regions around the regions humans have invaded.
Granted, this is splitting hairs.
Hi khms (# 47)
Think of it this way: human habitation encroaches on the desert, thereby shrinking the overall area of the desert; and although this may not significantly threaten the persistence of the entire desert habitat, the impacts on the desert ecosystem (for example through induced edge effects) can be significant at its perimeter. Those impacts may include changes in soil structure and soil biodiversity, hydrology, vegetation, evapotranspiration and other local atmospheric effects, and introduction of new species. You have desert, areas where desert ecosystems are directly impacted by human habitation, and a zone in between where the habitat may still be visibly recognised as a desert, but where desert ecosystem functioning breaks down. Ravens will not normally penetrate from their natural habitat far into the desert. They use human habitation, a man-made habitat that they are adapted to – as a stepping stone or a base from which to forage.
My job is to study and control invasive species, and I assure you that we don’t worry about daffodils. We worry about the Emerald Ash borer, the Asian Longhorn beetle, cheatgrass, and sudden oak death. If a non-native species threatens to cause the extinction of an ecologically or economically important species, we ought to worry. However, it’s a complicated issue. We often can’t stop a species from spreading: a fast voracious insect like the emerald ash borer is hard to stop. And sometimes what you think is a horrific danger may not be. I worry that sudden oak death will spread to the east coast and kill off an important forest species, but I do that knowing that we lost the American chestnut to two invasive species and yet no one now remembers. We’ve forgotten what the east used to look like when chestnut was the dominant forest species and a tremendously important economic species. And more recently, we lost the American Elm to an invasive species. I miss the tall and beautiful elms, but younger people don’t know to miss it. It is very difficult to assess risk and economic damage.
mudpuddle, we’re talking past each other for some reason.
Yes, I did. I thought you were talkig about Clarke’s OP.
Yes, sorry. As above. My bad.
I’m certain we’re also talking about different things with regard to ecological and/or biological similarity.
For plants, by ecological similarity I mean ‘ability to grow and reproduce in the same place’. Except in the most extreme of habitats, all invasive plants are outcompeting native populations for space (and maybe pollinators, and maybe mycorrhizae). Competition is most severe between more ecologically similar species, by my definition (which integrates compatibility with local mycorrhizae, pollinators, herbivores, etc.). Susceptibility to disease and herbivory is going to depend on lots of other variables that I guess you are including under ‘biological differences’ because I don’t consider them ecological. They’re certainly unpredictable a priori. Resource-use efficiency, for example, is a biological difference but not an ecological one.
Different considerations apply to herbivores, still others to omnivores and predators–of various trophic levels–and whatever. Introducing a brown treesnake onto an island full of endemic birds and lizards is a completely different disaster causally from the spread of Eurasian annual grasses, or ravens for that matter, in the Mojave Desert. I’d agree that the treesnake case is one of ecological difference from any previous predators on the island, but not the grasses and not the ravens.
Point is, I was talking about specifically ecological similarity, not whatever you’re lumping under ‘biological’. All species have biological differences; it’s why they’re different species.
I was talking specifically about the Mojave Desert and my remarks should not be extrapolated to the dryland habitats of the world.
But they’re not. There have been ravens (and tortoises) in the Mojave Desert for far longer than it’s been a desert. That’s the whole point of the title of the OP. What’s happened is that human supplementation (from towns, roads, dumps, and sewage-treatment plants) has allowed their formerly low and ecologically constrained numbers to mushroom. They’re strong flyers and as they spread out from the supplementation centers to breed, their increased numbers put increased pressure on tortoises (and doubtless many other species).
I don’t think that’s true. But density has increased.
I can’t really agree with that, even if you mean behavioral, rather than evolutionary, adaptation. They’re using the man-made ‘habitat’ specifically as a place in which to forage, but mostly when not breeding.
shit, I keep getting distracted and forgetting:
the vid: squee, and I have never publicly squeed before.
Yes, tortoises lay brittle-shelled rather than leathery eggs, but the calcified layer is thinner than in bird eggs. The x marks indicate the tops of the eggs, which are never rotated after starting incubation. The whiter circle you can see around the x is where the chorionic membrane attaches the embryo to the inside of the shell early on.
I should add that ravens also use man-made roads, poles, and towers where they can anywhere in the desert and that probably helps them too.
Maybe! By biology I mean the gamut of physical, genetic and phenotypic characteristics, including structure, life cycle, immunology, biochemistry, growth phases, resource uptake and use etc. By ecology I mean relationships with other species and non-living elements of the ecosystem, including to the ability to maximise resources available under given conditions (climate, geology etc).
I understand the example you give of plants being ecologically similar, but I still disagree. Two biologically similar plant species would have similar resource requirements, similar defence mechanisms in response to specific forms of attack, have resistance and susceptibility to the same pests and pathogens, have similar physiology and biochemistry, and / or reproduce in similar ways and at similar rates etc (ecosystem factors being equal). Ecological similarity between two plants would mean inter alia that they share predators, pests and / or pathogens in a given ecosystem, utilise the same living resources (e.g. mycorrhizae or other symbiotic partners) at similar rates in similar ways, fill a similar niche, are similarly distributed in the ecosystem, are dispersed by the same mechanism (e.g. birds) at similar rates etc.
Different biology or ecology can therefore confer an advantage on one species or the other (native or non-native) in a given habitat or ecosystem.
On the ravens, I’ll certainly take your word for it – I have studied ravens in Scotland and Pennsylvania but never heard or read of any desert populations not associated in some way with humans, so thanks for setting me right. So I guess they are not invasive because they have been there all along, but have become more abundant and are at higher densities because of human activites, and hence are also eating more torts.
Chris Clarke says
I’m tempted to call the dynamics irruptive rather than invasive, but I didn’t because raven numbers haven’t crashed yet.
What a great discussion! I’m late to the convo, and I’m an infrequent commenter here, but I’m a Ph.D. ecologist and my research focuses on a species I consider a “native invasive”, as Chris has described the raven here. A few thoughts:
1. “Invasive species” has historically been – and is still currently, in many/most places – considered synonymous with “non-native” species. But recently (last 10-15 years) some ecologists have begun to realize that, in some cases, species that are native to that region/community can greatly increase in abundance and begin to exhibit traits of invasive species. See, for example, Cretan and Kelty 1999’s work on hay-scented fern: http://www.springerlink.com/content/t7833v2252816767/. For those without journal access, there’s a discussion of hay-scented fern as a native invasive in this publicly-available article: http://naldc.nal.usda.gov/download/47586/PDF. Nowadays, many ecologists view native/non-native and invasive/non-invasive as two separate traits.
2. Species that are invasive (i.e., aggressively dominant, crowding and out-competing other species) are generally considered “harmful” as they reduce biodiversity. Though still controversial, many ecologists have recently argued that non-native species are not inherently harmful. That in many cases they may have neutral or even beneficial effects on biological communities, and therefore should be judged on their impact rather than their region of origin. See, e.g., Davis et al. 2011’s Nature article: http://www.nature.com/nature/journal/v474/n7350/full/474153a.html, and the growing literature on novel ecosystems.
3. There are many examples of “native invasive” species in North America alone, above and beyond those listed here. White-tailed deer and brown-headed cowbirds are probably the classic examples. But off the top of my head, I’d add Barred Owls (replacing Northern Spotted Owls in the PNW) and elk in Yosemite pre-wolf reintroduction. We’re seeing a trend towards biotic homogenization around the globe, where generalist species that are tolerant of human disturbance – whether native or non-native to that community – are greatly increasing and out-competing disturbance-sensitive species.
4. If I may jump in to Chas and mudpuddles’ conversation:
Two biologically similar plant species would have similar resource requirements, similar defence mechanisms in response to specific forms of attack, have resistance and susceptibility to the same pests and pathogens, have similar physiology and biochemistry, and / or reproduce in similar ways and at similar rates etc (ecosystem factors being equal).
Not necessarily. By “biologically similar” I presume you mean phylogenetically related? Actually, recent research into the evolution of plant defenses shows that closely related plant species have very different chemical and/or physical defenses and very different specialized herbivores. Closely related plants (i.e., same genus, different species) that occur in close proximity to one another, and may be considered “ecologically similar” in many ways (similar resource needs, growth form, etc.), actually have more different defenses than expected by random. See, e.g., Kursar et al. 2012: http://www.pnas.org/content/106/43/18073.full. You’re on the right track in that introduced species that are able to escape their specialized predators and parasites are better able to out-compete native species and become invasive, but that’s not necessarily linked with either biological or ecological similarity. It has more to do with the presence or absence of their predators and parasites.
Ok, I think I’ve said more than enough for now – this is obviously a topic I’m very interested in!
Chris, someday I’ll show you the 2 Joshua trees (or what’s left of them) where I saw raven nests in the West Mojave. You should go there to check it out anyway; it’s real different, seemingly marginal and/or deteriorating JT habitat; the JTs are very few and far between, but many are big and old (and many are dead and down) and easily the tallest items in the environment. So ravens nest in them.
I don’t really see the nebulous nature of the terms “native” versus “invasive” as a big problem in ecology. In the area that I’m from, to an ecologist, it is pretty clear what these terms mean. In SE Michigan, sugar maple, american beech, and hop-hornbeam are native plants in a mesic forested area. Non-native and invasive plants in this area are white mulberry, common buckthorn, and norway maple. These invasive plants provide very little food to native herbivores and outcompete plant species. Of course there are examples of non-native plants that are non-invasive (at least not yet) to this area. That would include bald-cypress, amur maple, and dawn redwoods. Additionally, there are native plants that are invasive in this area. For example, in Michigan southern dry-mesic forested areas red maples and sugar maples are invasive. I don’t think that these statements are controversial to someone familiar with the system. I do not understand the controversy that arises from using these terms.
Also, when a species is labeled as invasive in one ecosystem and in one locality it is not at all implied that it is invasive everywhere across the globe. You just have to have some knowledge of the system in question.
Just because the general public or environmentalists misuse or misunderstand terms that are helpful in a scientific context doesn’t mean that we should just throw them out. Does it?
Btw, it is great to final comment on Pharyngula. I’ve been lurking for years. It’s also great to see some ecology related posts with comments from ecologists! The micro people seem to dominate the biology blogoshere.
Chris Clarke says
Not at all! But for those of us with a foot in each of those three worlds,* it’s always helpful to discuss just what the terms mean. And for those of us who attempt to convey the science to those enviros and other lay people, detailing the ways in which a term’s definition in science differs from the common usage is almost always illuminating.
* Yeah, I know. Pipe down.
Chris Clarke says
And Chas, I’d love to see those trees with you even if we didn’t need an excuse to get you the fuck out here again. We have a couch now, by the way.
Chris Clarke says
viajera, thanks for those links. Guess I’ve got some reading for the afternoon.
Thanks for the welcome and for clarifying! I also belong to those three worlds. Communicating can be a bit frustrating at times. I’ve been involved in a non-profit that plants native plants and removes non-natives. We’ve recieved some criticism because of this issue.
I’d like to reiterate that I enjoyed your ecology related post and I eagerly await more!
SC (Salty Current), OM says
Imagine how much I‘m learning!
Late to the discussion, but I wish to add that a non-native species that is similar to a native species can create problems, especially if there is the risk of hybridisation. This can happen between domestic animals and their wild counterparts (see Przewalski’s horse and domestic horses, or Scottish wildcats with domestic cats), or with species introduced into habitats where a close relative is already present (e.g. brook trout hybridising with bull trout in the Pacific northwest, introduced sheepshead minnow breeding with endemic Pecos pupfish in New Mexico).
Also, it’s possible that diseases can spread from a non-native to a related species that’s native. Of course, this means that the pathogen and vector have to be introduced as well. See the diseases that have decimated American chestnuts & elms, as acrasis mentioned; both originally afflicted Asian species, only to wipe out North American species once introduced. Similarly, the varroa mites that cause so much trouble with European honeybees originated from Asian honeybees.