plants are not supposed to aggressively TAKE OVER fields ... ken

I disagree with Ken; Native tree types "aggressively" colonize new space. We then say this is a good thing. The "invasions" by non-native plants are of a similar nature, but these are plants which them go on to crowd out more desirable native plants that provide food, nectar, pollen, etc. for animals that have evolved together with these plant species. When a plant species from another part of the world is brought in, and if that species has the ability to out-compete native plants because it has no enemies in the area......the end result is fewer species, not more.

Up where my land is-and this would hold true anywhere but the species would change-species like white pine, northern white cedar, birch, aspen, etc. rapidly colonize old abandoned fields. These fields were not "natural"...they were man-made. The abandonment of these old fields-when seed stock is nearby-are then rapidly filled in with colonizing species. The problem then is that the non-native invaders do the same thing when space is cleared for them, but because they are not "from here", all the insects and birds, etc. have trouble using them as food sources. Or to take it in another direction, up here, common buckthorn is invading woodlands. In this case, the invasive plants actually are providing a food to the birds, who then fly for miles, pooping out the seeds which spreads the plant far and wide. It is thought that when a bird eats the fruits of common buckthorn, it ends up being a net energy loss for the bird as these fruits cause the bird to have what amounts to diarrhea. In any case, it is the eating of non-native fruits by birds, which fly great distances, pooping out the seeds of native and non-native fruits alike.

But "aggressiveness" is not a bad thing, and without aggressiveness, we would not have had a green world following the last ice age. Plants "aggressively"colonizing new ground is a good thing. It's just necessary that we don't introduce plants that will be bullies to the existing flora.

If looked at in isolation, no, probably not a big deal. But nothing in nature occurs in isolation........unless maybe you live in Madagascar!! The fact that calleries have hybridized with other pears to produce viable fruit and thus seeding into open, unplanted areas is only the tip of iceberg as to their distribution in other, less obvious situations. The fact that they are listed as invasive in your state confirms that has already occurred.

And as Tom has mentioned, it is the nature of pioneer species like many native trees (often rather weedy ones) and readily invasive non-natives to colonize open cleared areas. Some survive long term even with other, later competition and some will die off with competition. However, with the invasive species it is the fact that they CAN compete - or even out-compete - with later appearing species.

Here they infest the understory along with ligustrums. Agree with basically everything said above LOL. It wouldn't be such an issue if it was a desirable plant, but that just isn't the case (at least in the minds of most people).

Are there any documented cases where wildlife suffered or went extinct because an invasive plant took over and caused catastrophe?

"The problem then is that non-natives invaders do the same thing when space is cleared for them, but because they are not 'from here' ,all the insects and birds have trouble using them as a food source."

But if the fields were previously empty for many years the birds and insects were not getting food from there anyway.......

Tom is right. The risk is they will crowd out and out-compete the native plants that normally would colonize an open field. Lots of Native insects and birds have special relationships with those native species. Several types of butterfly are threatened by invasive plants that look like the plants they evolved to lay their eggs on, but which their larvae can't eat. They lay eggs on the foreign look-alikes and all the babies starve.
I'm not sure if we are certain of the cause, but the native New England species of clover is now extremely rare. Black locust on Cape Cod is such an effective nitrogen fixer lots of other invasive plants that can't normally survive on the poor soil start to spring up around them.

You are right though...sometimes when I see a Tree of Heaven or Norway maple growing in a parking lot, I wonder if it isn't better then the sterile asphalt that would otherwise be there.

The risk is that birds will carry the seeds from these trees to an actual native woodland.

People actually plant tree of heaven on purpose? Ignorance must be bliss.

In the county that borders me to the east the biggest industry is nursery, and in the past one of the top production items was ornamental pear, probably not so much now, but still substantial. I've watched the area for a little over 30 years, and I've seen the effects the pears have had on the immediate landscape. The pears were and still are used extensively as ornamentals in my immediate area and in fact anywhere there are houses. The local garden centers as well as the box stores sell them.

The areas surrounding the nurseries are mostly rural, with open land, fencerows, and woodlots, some extensive. The pears have seeded into the open areas and fencerows, but not the woods. In my area, there are active pastures, haylots and fencerows, but not much in the way of woods. Here the pears have seeded into the fencerows only, the cows keep the pastures eaten, and the haylots are cut twice a year, so no pears.

The pears are acting just like the native pioneer species are, they are opportunists colonizing open land, sometimes aggressively. Like the natives, they are heliophytes, demanding sun and dieing out when overtopped by the climax species. They are rarely if ever found in a closed canopy forest, their ultimate height is at least half of that of the climax species. As pioneers, they cast less shade and are lower growing than red maples, redcedar, sweetgum, and tulip poplar, the premier invaders here.

Pears provide much forage for native and introduced pollinators and food for native and introduced birds. The two species I see most using pears are honeybees and starlings, both non-natives.

In my area I don't consider the pears a serious threat to the ecosystem, they pass from the scene the same as native pioneer species do, and they probably offer more to both vertabrates and invertabrates than the natives do.

More freakish Pear hybrids means more competition for our native dogwoods.

In a "dog eats dog" world that is sort of fine I suppose. Imagine going to Kings Canyon to see the Redwoods and seeing the descendants of Bradfords instead. Imagine going to wherever to see your cousins and seeing what I replaced them with instead.

I will agree it is an uphill battle and one I largely fight "symbolicly" you could say. After all I probably am not going to make the difference. Then again while you are laying there dying in the gutter, or while you are just not looking, I am not the guy who will take the wallett you no longer need. Sometimes you just gotta fight the good fight.

Why cant they come up with a sterile pear like the yoshino cherry or the purple leaf plum?

Maybe not great examples - both of those can and often do produce fruit :-)) And the seeds are viable but not overly so.

Thanks for taking the interest to wonder about this . . .

"But if the fields were previously empty for many years the birds and insects were not getting food from there anyway."

I can't say I've ever seen an empty field. Here the fields have grasses and forbs which provide insects, small mammals and seeds for critter food as well as nest sites and homes for a range of insects, birds, and mammals. Even the recently harvested fields have grain that was missed by the harvest which is enjoyed by many of the larger birds: turkeys, geese, ducks, ravens, and crows. Twice in early spring I've seen black bears grazing on new grass sprouts in the field next door. Once the secondary species start to come into abandoned farm fields (around here the most common first natives are cherries, white pine, aspen, and sumac) they continue to add habitat for the various critters. So to my mind, it's more a question of which plants will provide more use to the birds and mammals that depend on them, and which will provide more use to us? I'd much rather have the native cherries and pines than multiflora rose, buckthorn, bush honeysuckles, etc. since they provide more use to both the critters and, as they mature to full-sized trees, to us.

Multiflora rose can make a nice addition to the garden if deadheaded ;)

Also I may try to make a callery pear bonsai. Only good use for them besides a rootstock.

Multiflora rose can make a nice addition to the garden if deadheaded

Surely, this time, Logan, you're joking. Have you EVER seen or gotten near a massive multiflora rose with its vicious thorns and attempted deadheading such a monster!

Multiflora rose was brought into WV in the 30s as a disastrous attempt to halt erosion from over plowing. It's everywhere now, and a real bitch to deal with.

In all honestly, my biggest objection to invasive plants has to do specifically with shade-tolerant invasives, specifically because these then are able to invade woodland. I care about all the other plant communities getting boned up too, and deal with such daily, but my keenest issue is with those items like common buckthorn which can destroy woodland. tlbean, once common buckthorn invades a mesic, southern Wisconsin woodland, unless treated, that woods is doomed. Nothing...........and i do mean nothing, can compete with the buckthorn. More shade-tolerant than even maples, it can outcompete all species. It poisons the soil for other plant types via process called allelopathy. IT produces fruits which causes birds to have diarrhea and is said to result in a net loss of energy to these birds which consume the fruit! Once common buckthorn has successfully taken over such a woodland, there will ultimately be one species left-common buckthorn.

Excerpt from a recent MDA article about Berberis thunbergii

Researchers in
Connecticut found that there is a higher density of blacklegged (deer) ticks
carrying Lyme disease in barberry infestations than in other habitats. Japanese
barberry infestations create an ideal, humid environment for ticks. As a
result, blacklegged ticks can reach higher densities in these areas.

In Minnesota,
we commonly think of deer as being the main food source for ticks. In fact,
they are but one vector for ticks, and by extension, Lyme disease. White-footed
mice and other rodents are important vectors for immature ticks, and are the
primary sources for a tick larva’s first blood meal. Mice thrive in stands of
Japanese barberry; the dense, impenetrable stands protect them from predators
and provide optimal nesting sites.

The high
populations of ticks, combined with the high populations of rodent vectors
result in higher levels of Lyme disease in Japanese barberry infestations. This
is especially concerning for humans, as the majority of Lyme cases occur as a
result of nymphal tick bites.

Gardengal

What makes trees that are supposed to be sterile produce fruit sometimes if at all?

Very few naturally occurring plants are sterile - that is usually the purview of hybrids. Or plants that have been manipulated to be so. It is the basic biological drive/intention of plants to reproduce. But there are some that are less inclined to flower or produce seed/fruit or that are self-incompatible and will require cross pollination and that is not always well know initially. Geography, climate and location can also be a factor.

While callery pears may be self-sterile (aka self-incompatible), they are easily pollinated and fertilized by related species.

tlbean2004(Z7):

The question is, what makes plants that evolved to produce seeds suddenly become sterile? All sorts of things. Some "sterile" trees are really just self incompatible. Many species have mechanisms so they don't interbreed with themselves (to promote genetic diversity). As long as there is only one grafted cultivar of that species in the US, it can't reproduce. Genetically, all specimens of a grafted cultivar are the same individual. Once someone introduces a different cultivar, then they can reproduce just fine with each other or their descendants. I heard that was what happened with the callery pear.

Others are mutant freaks that aren't good at reproducing. As several couples I know can attest, infertility isn't always total. The young may not inherit whatever trait interfered with reproduction.

Laceyvail, have you ever seen an espaliered rose before? I planted a multiflora rose that I found on the side of a road (very nice form for it's purpose) and I am training it into an espalier. I am using it to stop people from going in between a sweetgum tree and my garden shed (so they don't trample my miscanthus grass, gardenias, and loropetaums). But an espaliered rose will not reach a large size because it must be constantly pruned. I understand your objection to this plant (because it can be a nuisance), however, it makes a good espalier and a rootstock.

Some debate the use of non natives but when you get one that is invasive to boot its end of story.

Most have poor form and weak wood as well.

For those of you seeing wild pears, what size are the fruits? The ones I have seen so far have quarter sized fruit, too big for most birds to eat. Deer and opossums will still be able to spread the seeds however.

They are about the size of a marble.

A crow or squirrel would be able to carry off a quarter sized fruit. Most cultivated ornamental pears I see have marble sized fruit, though.

I know the cultivated ones have marble sized fruit, I was speaking of the wild ones coming up in fields or on roadsides.

I have found a couple of wild trees growing in the woods behind my property, but honestly I think they were from my old Asian pear (aka sand pear), the deer love to eat those.