We now make a jump shift intoCommunity Ecology

Community Ecology is the study of ecological interactions among different species.

Those interactions vary in their effects on the species…One kind of interaction is mutually beneficial – mutualism

Some kinds are harmful to one species, but beneficial to the other – predation, parasitism, herbivory

One kind is mutually detrimental – competition

Others have no effect on at least one participant – commensalism, detritivory, and theoretically neutralism. We’ll consider these interactions later, but first some more general principles…

Why is community ecology important?

Community ecology has important applications because of the global loss of species in ecological communities.

To understand why species are lost, we must understand how interactions among species affect the members of a community.

For example, invasion of communities by exotic species can be especially damaging. Consider the effects of the zebra mussel, the lamprey, and the alewife in the Great Lakes, or the effect of purple loosestrife along the shorelines of the lakes. Those are ‘local’ examples. Are they unique?

Other global pests you’ve probably heard about:

Africanized honey bee –The African honey bee is far more aggressive than

native honeybee pollinators. It escaped from research captivity in Brazil, and has moved northward, as well as cross-breeding with native bees. The cross breeds are still more aggressive than natives, displace native hives, and depress honey production.

The aggressiveness can cause them to attack humans en masse, with fatal consequences.

Here are recent maps of their distribution in the U.S….

Kudzu –kudzu is an Asian vine that can grow as much as a

foot/day. In the southern U.S. it has already destroyed 4 million acres of trees and shrubs. Rare species in infested areas have been driven at least locally extinct.

Asian Tiger Mosquito –Was accidentally introduced into the U.S. in 1985. It replaces native aquatic invertebrate larvae, and thus the adults that would mature from them. In addition to its ecological consequences, unlike the native

mosquitoes it has replaced, it is a potential disease carrier.

Kudzu vines completely covering trees (and everything else)

The Asian tiger mosquito, almost uniquely, is active (and biting) throughout the day.

So, we study species interactions to learn how and why community changes can arise from them.

We’ll begin with mutualism.

Mutualism benefits both species participating in the interaction. It may occur in two ways:

• obligate mutualism – both species depend on the interaction for survival

• facultative mutualism – the interaction is beneficial, but not necessary.

A case study – the swollen thorn Acacia and ants that live only on Acacias…

swollen thorn – they have a soft pith inside that ants excavate for nests.

The Acacia also has Beltian bodies at the tips of each leaflet, very high in protein to help feed the ants, and…

Nectaries at the base of each leaf that provide nectar for the ants.

The plants thus clearly benefit the ants:• They provide a home (the hollow inside of the thorns), and• They provide food (protein from the Beltian bodies and nectar from the nectaries)

How do the ants benefit the Acacia?

• Ants attack any herbivores (usually other insects) that attempt to eat the acacia. They not only attack leaf-eating insects, but also eggs and larvae of herbivorous insects. They even bite and sting browsing mammals that attempt to eat the acacia.

• They cut back the growing tips of other plants that grow near, or, in the case of vines, on the acacia.

This defense of the plant has been shown experimentally to be important to the growth and survival of the acacia. Small acacias were cut down, then allowed to re-grow, some with ants having re-colonized, and others where that was prevented. Here’s what the shoots looked like…

with ants without ants

Dan Janzen quantified the importance of the ants to Acacia survival:

Group Acacia survival (10 months)

Ants removed from 43% Acacia

Ants present 72% .

Ant-plant mutualisms have evolved many times on different continents.

39 families of plants have sugar-secreting nectaries

The acacia-ant interaction is probably an obligate mutualism. There are more obviously obligate mutualisms, made obvious by the extent of evolutionary design involved. That is frequently the case with pollinator-plant interactions. One of the classic cases is flowers designed to have the shape of female wasps; they attract males that attempt to copulate with the female, but only either gather or deliver pollen.

Other mutualisms are facultative – at least one of the two species may have other possibilities to achieve an end. Think of pollinators that may have many flowers from which they can gather pollen and nectar. Think of all the different species that might choose to eat some attractive fruit. For example, bright red fruits attract many different birds…

There is another type of interaction – commensalism – that benefits one species without either benefiting or harming the other.

How can that happen?

When the actions of one species are of no importance to the other, but those actions are beneficial to the first species, a commensalism is occurring. There are numerous examples:

The remora is a small fish that swims with sharks. Sharks are messy eaters, and the scraps they miss are the diet of the remora.

This is a whale shark being ‘accompanied’ by two remoras.

In other cases the remora attaches itself to a ‘host’ by a sucker disc. By attaching itself and potentially affecting the efficiency of movement of the host, this may move the interaction beyond the bounds of commensalism.

Another example is the interaction between cowbirds or cattle egrets and the large grazers of grasslands. The animals walk though the grassland and, in moving through the plant community, flush insects from within it. Feeding rates for the insect-eating birds clearly increase, but there is no apparent effect on the grazers.

This interaction, too, may grade from commensalism to mutualism. The birds may ride on the backs of the grazers, picking off ectoparasites. When they do, there is benefit to the grazer, as well as the bird.

There is also an interaction called amensalism – an interaction in which one species is harmed without effect on the other. Usually it is apparent that the negative effect was unintentional, but nevertheless important. Examples:

• Mammals bare the spaces around waterholes by using the waterhole intensively enough to trample the vegetation around it. Feel for the trampled plants. The mammal didn’t mean to kill plants, it just happened.

• Small, ground plants in a forest are damaged by falling parts of large, older trees. Herbs, shrubs and small trees in tropical forests are frequently damaged more by falling objects than by herbivores.

Now, on to the interactions that dominate community ecology:

Two types, one has positive effect on one participant and negative on the other – predation, parasitism and herbivory.

The other has negative impact on both participants – competition.

In parasitism, the parasite consumes a resource – the host – but the host is not killed immediately (and in theory not by the parasite directly) as is the case in a predator – prey interaction.

In the case of disease – host interactions (a parasitism) the duration of the interaction can lead the host to develop immunity to the parasite.

Very often, human bacterial and viral diseases have very high initial infection rates, followed by increased immunity.

Examples…

A (relatively) recent outbreak of bubonic plague in India…

The source of infection was a bacterium living in fleas on rats. When the fleas moved onto humans and bit them, the infection was passed.

Year Number of Cases Mortality rate (%)

1953 21,000 70

1954 6,100 84

1955 700 23

1956 300 20

1957 40 0

1958 30 0

1959 40 0

The transmission of the plaque bacterium from fleas to humans does not often occur. The fleas must move from rats, their preferred hosts, to humans. That only occurs when:

1) the fleas heavily infest rats and2) rats are in close contact with humans, for example in

crowded, garbage-strewn conditions of medieval Europe or conditions in densely populated cities in India.

We know how the great plague of the 14th century originated… It began in 1346 during the siege of Caffa along the Crimean Straits. It spread to Italy and the south of France in 1347, then to all of Europe in 1348. It had disappeared entirely by 1357.

Why is the plague so feared?1. Its initial high mortality and2. Its reappearance at intervals.

There were 3 more episodes in Europe during the 14th century. They occurred at intervals of 10-13 years. Enough immunity persisted to make each of those episodes have a lower incidence and mortality.

However, there were 3 episodes in England during the 17th century. Those occurred at intervals of 22-40 years. Enough time had passed between episodes that both a loss of immunity in survivors had occurred and an immune population had largely been replaced with a susceptible one. Each episode killed 13-15% of the population of London.

The recent time course of the West Nile virus is probably a similar example. Here are dead bird and human case data for Canada, Ontario, and Windsor/Essex County:

Bird dataYear Canada Ontario Windsor/Essex2001 127 127 202002 555 281 92003 1633 242 112005 447 300 12

Human dataYear Canada Ontario Windsor/Essex2002 340 319 352003 1388 5 (10)2004 ??? 14 12005 159 89 (28)

A (vaguely) personal example:

The human schistosome (Schistosoma mansoni) causes debilitating diseases called schistosomiasis or bilharziasis by infesting the liver.

Schistosomes (blood flukes) excite an immune response when they enter a human through pores, but the first wave don’t succumb, both because they face a small initial immune response, and because they rapidly coat themselves with host proteins. Later attacks invoke a very large and rapid immune response, and they succumb.

There is also cross-resistance. Infection by one species evokes immune responses against later attacks by other species.

Now the personal part…

There are schistosomes that infect ducks in mid-continental lakes. All schistosomes have complex life histories that involve a life stage that infests an intermediate host, usually a mollusc. The first time you are exposed to the duck schistosomes, you don’t even know it.

Later exposures, for me when I helped a field class sample for fish in an Iowa stream on a warm day when the snails were shedding the intermediate stage, evoke an extremely strong reaction. The duck schistosomes never get past the lower dermal layers, but the later exposure causes what is called “swimmer’s itch”, a ‘rash’ far itchier than poison ivy.

Herbivore – Plant Interactions

Some herbivores act as predators, uprooting the plants they eat, for example sheep. Most herbivores, though, do not kill the plants they eat. They can, however, strongly influence, or even control plant populations.

Many examples of control come from conservation biology and biological control…

Klammath weed is a European plant that accidentally became established in California in the early 1900s. By 1944 it spread over 2 million acres of range land in 30 counties. Biological control specialists brought in a beetle from Australia, a Chrysolina, and within 10 years the weed was controlled.

Klammath weed was an important ‘pest’ species because it contains high concentrations of a toxic alkaloid, hypericin, that is used in low doses as a medicine. The concentrations in the plant are dangerous for cattle.

The plant and the control agent:

Year Status

1944 2 million acres covered1945 Chrysolina introduced1954 99% reduction in Klammath weed

Australia has had its own control problems. Opuntia cactuses were introduced to create cheap, natural fencerows to manage cattle in pastures. The cactuses did not grow only along the designed fencerows. At one time, millions of acres of grassland were unusable due to the cactuses. Australian control specialists went to South America, where the cactuses had come from, and found a control agent, the moth Cactoblastis cactorum. Within 10 years both the cactus and the moth remained present, but at low abundance.

Here’s what cactus pad looks like after moth attack. Eggs laid on a cactus pad hatch into larvae that burrow into the pad and eat the inside, hollowing it out and killing that stem.

moth larva

Other examples of herbivory having significant influence on the plant community:

Algonquin Park spruce forest before and after spruce budworm attack -

Relative biomass in and outside vole exclosures. The y-axis here is summed height of plants in 100 cm2 areas as an indication of plant biomass. This was the difference after 2 years.

Finally, see Figure 17.18 in the textbook. That exclosure was for cattle in Hawaii.

Even when there isn’t physical protection, plants have evolved a host of defenses against being eaten:

1. Structural defenses a – hooks, prickles, spines, …

2. Structural defenses b – the design of the C4 leaf discourages herbivores

Most of the useful food is concentrated in the cells surrounding the vein. These cells are hardened with high levels of silica and lignin. They are hard to chew and digest.

3. Chemical defenses – plants produce a huge variety of chemicals that are harmful or toxic to herbivores. They can be loosely organized into 3 categories:

a. digestibility reducers – tannins and phenols bind proteins as they are freed from organelles by the

herbivore chewing. They reduce the nutritional value of the plant.

b. toxins – alkaloids, glycosides, pyrethrins, non-protein amino acids – some are directly toxic, others affect functions and result in toxic impact. A few examples:

A non-protein amino acid, that when incorporated causes failure of critical enzymes during insect larval development

replacement ofa carbon in the side chain with O

Daucus carota (Queen Anne’s lace, a common weed around here) contains furanocoumarins that are toxic to a wide variety of insects. The coumarins are secondary chemicals produced by an extra step in a normal pathway that produces lignin (see Figure 20.15 in the text). The furanocoumarins affect DNA replication and herbivore growth and reproduction.

Indian grass (Sorghastrum nutans) produces and stores cyanogenic glycosides during drought. The salivary enzymes of the herbivores break complex sugars into simple ones. When an herbivore eats Indian grass during dry conditions, its digestive enzymes free cyanide, which poisons the consumer.

We have frequent experience with toxin defenses:a) alkaloids - flavors of vanilla, chocolate (caffeine and ???) are toxins effective against insect herbivoresb) the flavor of cinnamon comes from trans-cinnamic acid, again effective against insectsc) tobacco is protected by nicotine in the leaves. We can become addicted, but aphids are paralyzed by it.Other examples: morphine (and its derivatives), strychnine, digitalis, capsisic acid (the flavor in peppers,...

4. Pharmaco-active compounds – many of these are mimics of animal hormones. You can imagine how they might disrupt normal developmental pathways in insects. Among the most devious:

- plant-made functioning analogs of animal hormones- examples: insect molting and juvenile hormones. If it’s a plant that produces juvenile hormone, an insect eating it never molts to adulthood and can never reproduce. If it’s a molting hormone, and insect eating this plant is ‘fooled’ into molting too early, and it matures as a very small adult capable of producing far fewer young than if it hadn’t taken in the hormone.