Defense Mechanisms

1. Defense Mechanisms • Most organisms have adaptations that help protect them against their predators • Cacti have thorns • Porcupines have quills • Monarch butterflies have toxins and protective colouration • The interactions between producers and consumers typically result in co-evolution • Ex: The milkweed plant produces bitter-tasting chemicals that discourage many herbivores, but some have adapted to tolerate the toxin

2. Defense Mechanisms • Many organisms use protective colouration as a natural defense mechanism • This can include camouflage, mimicry, and warning colouration • For example, the dead leaf butterfly is nearly invisible to predators • When it remains motionless, its brown colouration and the veined pattern on its wings camouflage it from predators

3. Defense Mechanisms • Some species use warning colours, such as red, yellow, and black • The highly venomous eastern coral snake has red, yellow, and black stripes • Yellow jacket wasps are black and yellow • Other species that are not toxic or poisonous use these colours to their advantage • The non-venomous Scarlet king snake has red, yellow, and black stripes similar to the eastern coral snake • The syrphid fly has a similar yellow and black colouration to a yellow jacket • This type of mimicry, where a species looks like another species that has an effective defence strategy, is called Batesian mimicry I’m super venomous! I’m just faking it!

4. Defense Mechanisms • Even two species that are both poisonous, harmful, or unpalatable may benefit from mimicking each other • For example, the Zimmerman’s poison frog or the poison dart frog closely resembles the mimic poison arrow frog • An animal that becomes sick preying on the Zimmerman’s poison frog will avoid all frogs with that colouration, including the mimic frog • Scientists hypothesize that the converse is also true • Predators finding the mimic frog distasteful, will also avoid Zimmerman’s poison frogs • This co-evolved defense mechanism is called Müllerian mimicry We’re both poisonous! But we’re also adorable!

5. Symbiotic Relationships • Close interactions between two species living in direct contact often result in an ecological relationship called symbiosis • Symbiosis means living together • Symbiotic relationships have one organism, the symbiont, which lives or feeds in or on another organism, the host • There are 3 forms of symbiosis: • Parasitism • Mutualism • Commensalism

6. Parasitism • In parasitism, a symbiont (the parasite) benefits from the relationship but the host is harmed by it • Mistletoe is a parasite that obtains food by growing roots directly into the host tree and gaining nutrients from its sap • The interactions weakens the tree and predisposes it to disease

7. Parasitism • Parasites include: • Viruses • Unicellular organisms • Insects • Various types of worms • Ectoparasites live outside their hosts • Ex: Mistletoe, ticks • Endoparasites live inside their hosts • Ex: Viruses, tapeworms • Usually depend on their interactions with their hosts to survive and can’t exist outside their host

8. Parasitism • The parasite-host cycles are similar to predator-prey cycles and show a direct relationship to population density • An increase in the host population results in an increase in the parasite population • The increase in parasites eventually reduces the host population growth, either through decreasing the hosts’ abilities to reproduce or by reducing survivorship • The cycle continues as the survivors in a now-reduced population of hosts don’t have to compete with as many individuals for resources • Figure 11.37 shows the population cycles of one host-parasite relationship: the adzuki bean weevil and its wasp parasitoid • The adult female wasp lays her eggs into or on the host bean weevil • The larva hatches and eats the tissue of its host • This type of wasp parasite that kills their host is called a parasitoid

9. Parasitism

10. Mutualism • When both partners in a symbiotic relationship benefit from the relationship, or depend on it to survive, their relationship is called mutualism • Such relationships are common in nature • A lichen, for example, is actually a combination of an alga and a fungus • Their mutualistic relationship allows them to grow on exposed, bare rock, where neither would survive on its own • While the algal partner in the relationship carries out photosynthesis to feed both organisms, the fungus protects the alga from drying out or blowing away • The fungus also produces an acid that dissolves rock, releasing minerals the alga requires

11. Mutualism • Mutualism is also common in aquatic ecosystems • The hermit crab and sea anemone have a mutualistic relationship • The sea anemone’s stinging tentacles protect the crab from predators • The crab provides the sea anemone with a ready source of food, the detritus from its meals • The crab also provides a “mobile home”

12. Mutualism • Animal behaviour is an important part of most mutualistic relationships • For example, in Latin America, bull-horn acacia trees show mutualism with stinging ants • The leaves of the Acacia produce a sugary liquid that the stinging ants consume • The stinging ants also find protection inside the tree’s hollow thorns • The ants are beneficial to the tree because they attack any other herbivores that land on it • The ants also cut down the branches of other plants that come in contact with the Acacia, ensuring the Acacia has adequate light for photosynthesis

13. Mutualism • How do mutualistic relationships affect the growth of the populations involved? • Because both partners have co-evolved, growth in one population typically spurs growth in the other population • Similarly, if one population decreases in size, the other population tends to do the same

14. Commensalism • A symbiotic relationship in which one partner benefits and the other partner is unaffected is commensalism • For example, the lemon shark doesn’t appear to benefit or suffer from its relationship with the remora • The remora uses a modified, sucker-like dorsal fin to hold fast to the shark’s body • It receives protection and bits of food from the shark and gains a source of transportation • The remora is clearly benefiting, while the shark seems unaffected

15. Commensalism • Another example of commensalism is the relationship between the cattle egret and cattle • The birds follow the cattle around, feeding on insects roused by the cattle’s movement • The cattle seem unaffected by the ever-present birds, neither profiting nor being harmed by the relationship • Cattle egrets don’t limit their relationships to cattle • Also engage in commensalism with other large animals such as rhinoceroses and even kangaroos

16. Commensalism • In cases of commensalism, it’s often difficult to determine how each species is affected • Some ecologists argue that there are few true cases of commensalism • They believe both partners in symbiotic relationships are usually affected in some way, making the relationship a mutualistic one • Although how both organisms are affected is not always clear • If true commensalism does exist… • Growth of the host population would affect growth of the symbiont population in a positive way • Growth of the symbiont population would have no effect on the host population whatsoever

17. Summary Table 11.2: Interspecific Interactions