NOTES ON INVASIVE SPECIES – by Karen Haubensak
Terminology: Exotic: synonymous with introduced, non-indigenous, alien – basically the
opposite of “native”, which means that it arrived in its current location independent of humans.
Weeds and ruderals: species that possess traits that allow them to do well in temporary habitats
(says nothing about native/non-native status).
Invasive: generally refers to species that are rapidly expanding outside their native range
Naturalized: an introduced species that persists without human intervention (self-sustaining
populations without human intervention). In the common parlance, this is a species that is
recognized as having spread as far as it will go, it’s no longer expanding its range, and is now a
part of the flora.
Can a native plant be invasive? Of course. Pinyon-juniper expansion into the sagebrush-steppe
zones of the Great Basin is a perfect example. These are two species that are native, but due to
fire exclusion have expanded their range into the sagebrush community, where they tend to have
effects like: decreasing the understory, which leads to increased soil erosion and decreased soil
fertility, with decreased range value overall.
“If humans introduced the species and humans are natural, then aren’t introduced species
natural?” My favorite question.
What does natural mean? Nothing really. So, maybe we need to leave questions like this to the
philosophers and instead try to state the question more in terms of what we know about invaders
from a biological perspective (and I’m not saying all these points are non-contentious):
 Most invaders have not reached their new range via dispersal mechanisms that have
evolved in their lineage. For example, we have a lot of invaders here in north America
that have evolved mechanisms to be wind-dispersed, but they didn’t get here by winddispersal. They got here by hitching a ride in hay shipments on ships and trains.
 We know that over geologic time there have been biogeographic barriers to dispersal, and
that those have been broken down by humans.
 We also know that the rate at which we’re moving species around the globe is many
many times faster than that at which species would move without human intervention.
The scope of the problem: It might be that introduced species are the second-biggest threat to
biodiversity in the US, after habitat destruction (Wilcove et al., 1998).
The scope of the problem, or how many species have been introduced? Before we talk numbers,
I want to introduce the concept of the lag period, between introduction and when the plant is
recognized as being an invader. A lot of time can pass here, 40, 50 years, maybe even 100 years,
where all sorts of things can happen. In most cases, species poop out and go away in this time,
or become very small components of the local flora. But in a small fraction of the cases, they
become problems.
How many plant species have been introduced? Now let’s talk some numbers. 5000 exotics
have established in the US. But that doesn’t answer how many have been introduced? Florida
provides some pretty good numbers because they have such a huge horticultural industry. They
estimate that 25,000 have been introduced, and of that number, 900 have become established in
the wild (Simberloff et al., 1997). That’s less than 4 percent.
Tens rule: Some biologists have attempted to come up with a little mathematical rule to explain
what proportion of species will become invaders, given a certain number that is introduced
(Williamson and Fitter, 1996). There’s little empirical evidence for this so far.
The flip side: Mark Davis (2003) suggests that biodiversity has actually increased with
introductions in North America. He also states that there is no documented example of a native
plant driven to extinction by competition with an introduced plant.
Economic impacts: not very good numbers for forest systems (this is a huge industry, probably
an order of magnitude larger than the range industry). For the range industry, there is a cost
associated with lost forage crop, and a separate cost associated with control efforts. Ranchers
don’t tend to control weeds that their livestock can actually eat. Pimental et al. (2000) have
some estimates of these types of costs.
All the hypotheses in invasive species ecology / biology can really be fit into these two questions
– from the species perspective, what makes it a good invader? Then from the community
perspective, what makes a community particularly vulnerable?
What makes a species invasive: First let’s talk about what makes asking this question so
frustrating. Plants have such a wide array of strategies for success – in particular, reproduction –
some reproduce vegetatively, and some are really great at sexual reproduction. This is just an
example of why it’s so difficult for biologists and for managers for that matter to try to come up
with generalizations.
The first attempts to come up with predictions came from studies like this, a sort of retrospective
analysis. Rejmanek and Richardson (1996) took 24 pine species, with known invasive status
(invasive or non-invasive), then put in to their model all sorts of information about their life
histories (ten different traits). And they found out that they could predict with very high
accuracy (90%?) which ones would be invasive, just based on three different traits – seed size,
length of juvenile period, and interval between seeding. Reichard and Hamilton’s (1997)
analysis is similar but a bit more ambitious (380 species of woody plants in north America).
They found that “invasiveness in native range” was the factor that best explained which species
would become invasive in north America.
Daehler (1998): Another retrospective analysis. He had big global datasets of weeds, and threw
all these weeds into a big pot (statististically speaking), then sampled the species for overrepresentation in a particular family. So in, other words, which plant families tend to have the
most weeds in them? Then based on that information, he came up with a set of predictions for
non-agricultural invaders: nitrogen-fixers, climbers, grasses, clonal trees, and aquatic species.
Species with these characteristics were most represented among the invaders.
Natural Enemies Hypothesis: operates on the assumption that plants in their native ranges are
held in check by natural enemies, that is, herbivores, parasites, and pathogens, and when they
come to their new range they are released from that pressure and their poplations go nuts. This is
the hypothesis on which biocontrol is based. There are many many biologists who are working
on this issue. In particular I would suggest you check out John Maron’s and Ingrid Parker’s
work (they work separately, I don’t think they’ve published anything together) (for example,
Maron and Vila [2001] is a good review).
The other hypothesis is sort of neat, and this is based on the assumption that plants in their native
range have to allocate resources to defense. In their new range, they’re released from their
natural enemies and suddenly they can start allocating resources to growth. So introduced plants
are more successful because they are bigger than in their native range. Berndt Blossey and Ray
Callaway publish on this, and Buckley et al. (2003) is an example of an empirical test of the idea.
Why are some communities invasible? Most of these questions are about the biotic resistance
hypothesis, so if a community has higher biotic resistance, it won’t get invaded. If it has low
resistance, it will be invaded. Most people assume that disturbed or species-poor communities
have low resistance – they’re the ones that tend to be invaded.
There’s a couple of problems with this. One is that yes, there are disturbance-loving invaders,
and disturbed communities do tend to be invaded. But there are a lot of species that don’t
require disturbance for establishment, in other words, there are plenty of intact, pristine habitats
that are being invaded by exotics. Also, to complicate matters, there are some examples where
lots of disturbance is actually bad for the exotics --- like in the short-grass prairie in the US,
burning yearly keeps the exotics out.
As far as the second point goes, the diversity-invasibility question is one that’s being currently
debated. Yeah, our intuition would say, low diversity, few species, highly invasible. But there’s
been a lot of empirical work that’s shown that:
Hot-spots of diversity tend to be highly invaded. Tom Stohlgren’s group at Colorado State U.
have published a lot on this phenomenon in the Rocky Mountain Front Range.
Some particular issues for forest and range: (next lecture I’ll be talking about range systems in
the Great Basin so today I’ll emphasize forest issues).
“Exotic” in a forestry context can refer to pests (disease, insects), trees (as in exotic tree
plantations) and understory species. We’ll emphasize the latter two today.
Exotic tree plantations – dominate forest productivity in many areas around the world. For
example, in Britain sitka spruce is an important component of the timber industry. Likewise, in
Australia Carribean pine and slash pine comprise upwards of 90% of their forest productivity
(wood and wood products). Loblolly pine (which is native in our country), is grown in a number
of areas around the world, and is considered a really excellent plantation species because it
grows fast and “true” (straight up and down).
The problems with tree plantations arise when they are adjacent to native forests, and the exotic
trees start to invade the native areas. Other criticisms include the following: exotic trees tend to
have higher water and nutrient demands than their native counterparts, so tree plantations might
“exhaust” soil resources more quickly than natives; the understory is less abundant and its
diversity is lower. With less understory, there is increased erosion and decreased soil fertility.
However, in some degraded areas, exotic tree plantations have been shown to help the
regeneration of understory species, thus increasing the fertility and diversity of the plantations
(Harrington and Ewel, 1998). The caveat to this study is that the abundance of native understory
species was constrained by exotic understory species. Oh well.
When we consider exotic species in the understory of a native forest, what are some questions
we might ask? One is, how do invasive species change the understory component? Another is,
how do such changes affect overall system productivity (like NPP)?
If we look at some dramatic examples, we might speculate about the answers to those questions.
In Australian dry rainforests, a plant called “rubber vine” is a problem invader. Like the picture
shows, this vine climbs up on mature trees and appears to kill them. It’s pretty clear that the
understory is completely dominated by this invasive (so, that’s the answer to the first question)
and in terms of the second question, how does such a change affect NPP, it’s also pretty
reasonable to hypothesize that the productivity of this forest is being affected by this vine if there
is indeed substantial mortality of mature trees. The second species shown on this slide, Camphor
laurel, is a big problem in the subtropical rainforests of Australia, but I mainly put this picture in
here because it’s funny. I don’t have direct information about effects it’s having in that system.
For a local example, Japanese barberry is an understory invasive species in eastern deciduous
forests of the US. In this case, it has been shown to decrease diversity in the understory, making
it more homogeneous. As far as its effects on whole ecosystem productivity goes, it might be
harder to make the case here. It’s pretty clearly changing the understory. But it might not be
affecting the growth or productivity of the mature trees. However, it has been shown to alter soil
pH. This can have cascading effects on processes like decomposition, which can then affect the
amount of soil nutrients made available for the trees to take up. This is just speculation,
however. No one has yet shown such a link with this particular invader (but check out
Ehrenfeld, 2003; she’s done a lot of work on this species).
It’s also important to ask whether management practices in forests tend to make forests more
invaded. There is plenty of evidence that shows that road-building, logging, grazing, fire or fire
exclusion, can promote invasion. However, I did find one interesting study that demonstrated
clearly that in a managed forest compared to a protected one (that is, no management occurs
there, only recreation) there was no difference in abundance of invasive species. You’ll have to
check out Fornwaldt et al. 2003 to see the details.
In all of this discussion I have not mentioned “mechanisms.” These are the processes by which
invasive plants actually invade or have an impact on the invaded ecosystem. They can range
from competition, to allelopathy, to alteration of soil chemical properties (like increased salt
concentrations), to alteration of nutrient cycles. Levine et al. (2003) have noted that fewer than
5% of all studies that have looked at invader impacts have tried to identify the mechanism.
When we do identify mechanisms, we can start asking questions like: Why do only a small
fraction of plants have significant impacts? And Why do invaders have large impacts in some
systems and not in others? These are important questions for conservation and management.
References Cited:
Buckley et al. 2003. Are invasives bigger? A global study of seed size variation in two invasive
shrubs. Ecology 84:1434-1440.
Daehler, C. 1998. The taxonomic distribution of invasive angiosperm plants: ecological insights
and comparison to agricultural weeds. Biological Conservation 84:167-180.
Davis. 2003. Biotic globalization: does competition from introduced species threaten
biodiversity? Bioscience 53:481-489.
Ehrenfeld, J. 2003. Effects of exotic plant invasions on soil nutrient cycling processes.
Ecosystems 6: 503-523.
Fornwaldt et al. 2003. Non-native plant invasions in managed and protected ponderosa
pine/Douglas-fir forests of the Colorado Front Range. Forest Ecology and Management
177:515-527.
Harrington and Ewel. 1998. Invasibility of tree plantations by native and non-indigenous plant
species in Hawaii. Forest Ecology and Management 99:153-162.
Levine et al. 2003. Mechanisms underlying the impacts of exotic plant invasions. Proceedings
of the Royal Society (London) 270:775-781.
Maron and Vila. 2001. When do herbivores affect plant invasion? Evidence for the natural
enemies and biotic resistance hypotheses. Oikos 95:361-373.
Pimentel et al. 2000. Environmental and economic costs of nonindigenous species in the United
States. BioScience 50:53-65.
Reichard and Hamilton. 1997. Predicting invasions of woody plants introduced into North
America. Conservation Biology 11:193-203.
Rejmanek and Richardson. 1996. What attributes make some plant species more invasive?
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Simberloff et al. 1997. Strangers in Paradise. Washington (DC): Island Press.
Wilcove et al. 1998. Quantifying threats to imperiled species in the United States. Bioscience
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Williamson and Fitter. 1996. The varying success of invaders. Ecology 77:1661-1666.