Species interactions and disease dynamics

Question 1

What is a species interaction?

Answer 1

biological interactions that affect a pair of organisms living together in a community

Question 2

Give an example of an intraspecific interaction and an interspecific interaction

Answer 2

• intraspecific examples: mating, competition.
• interspecific examples: competition predation, mutualism

Question 3

How do disease dynamics and species interactions interact with each other?

Answer 3

• disease epidemics and trophic cascades
• competition, predation and disease transmission
• apparent competition and parasites
• parasitism, predation, competition and invasions

Question 4

What is a trophic cascade?

Answer 4

trophic cascade - when predators reduce the abundance of their prey to the extent that the prey’s food source indirectly increases in abundance

Question 5

Describe the effect of sea urchins and lobsters and the effect it has on disease dynamics?

Answer 5

• we care because sea urchins eat algae which help ground the plants in the sea which then destroys the habitat; important for conservation
• increased bacterial epidemics in sea urchins when you take away the lobsters
  
  • minimum size of closed population within which can sustain transmission
  • when the size of the closed population falls below the critical community size level the low density of infected hosts causes extinction of the pathogen

Question 6

What can be the effect on the critical population size when released from predation and what would be the effect of this on the disease dynamics

Answer 6

• most host populations are at equillibrium at densities below this epidemic threshold
• however, if released from predation allows populations to grow then the increased density (and contacts) create the conditions for an outbreak

Question 7

What needs to happen for the epidemic to occur?

Answer 7

frequency of contact between infected and susceptible individuals must be higher than the cure/death rate for an epidemic to occur

Question 8

What does it mean that disease acts as a redundant population regulator

Answer 8

• when other population regulators like completion or predation are too weak to cap populations at low levels
• once the prevelance of the disease increases to a certain point it starts to decrease the population growth of the urchins

Question 9

Foxes, coyotes, Lyme disease, explain the connections

Answer 9

• What is going on here? 1) A major change in predator–prey interactions in North America over the last half-century has resulted from the range expansion and population growth of a new top predator—the coyote, Canis latrans, which has spread across the continent following the extirpation of gray wolves, Canis lupus (19). The expansion of coyotes likely suppressed the abundance of several small-mammal predators, with the reduction of foxes by interference competition with coyotes being the best documented (20–22).
• red foxes are really good at hunting the small rodents which act as reserviors of lime disease
• The replacement of foxes by coyotes would likely reduce predation rates on small-mammal prey (i.e., the reverse of mesopredator release) because red fox (Vulpes vulpes) densities are typically an order of magnitude higher than coyote densities (23–25), and small mammals make up a larger fraction of their diets, particularly in the eastern United States, where coyotes have hybridized with wolves (26) and rely far more on deer (27, 28). Further, red fox cache prey for later consumption and are thus capable of killing large quantities of prey when prey are abundant (e.g., after an acorn mast). Thus, somewhat paradoxically, the expansion of coyotes likely decreased predation rates on small mammals by suppressing more-efficient predators (foxes). We had also just started to figure out that while deer make excellent hosts for ticks and can influence transmission by increasing tick densities, it is these exact small mammal populations that likely serve as amplifying hosts for lyme.

Question 10

What was the effect of coyote invasion on Lyme disease?

Answer 10

cayoties eat foxes, foxes eat rodents; if you have a lot of cayoties then you lose foxes and you have more rodents → more lime disease

Question 11

Apparent competition in ecological setting?

Answer 11

• We discussed the concept of apparent competition in the context of the immune system at the beginning of the course. In this case one parasite trigged an immune response that depressed another. This concept actually comes from ecology and typically it applied to a two competing prey species with a common predator. So species A increases the density of the predator which also preys on species B.
• Parasites can fill in for the predator

Question 12

Describe apparent competition using the example of lizards and malaria

Answer 12

• The two species are similar in size are known to be competitive with one another. A. gingivinus is a stronger competitor and owns most of the island, where as P. wattsi is only found in the central hills.
• The larger species, Anolis gingivinus, is distributed throughout the island in almost every habitat, whereas the smaller species, A. wattsi, is found only in a patchy dis- tribution in the wooded hills in the island’s center. The hills tend to be more vegetated and more mesic, but the restricted range of A. wattsi cannot be a result simply of its inability to exist in drier habitats. On other nearby islands (for example St. Eustatius where it coexists with the much larger A. bimaculatus) I have found A. wattsi throughout the island in a broad range of habitats, including dry scrubby zones. Also, in experimental manipulations, A. wattsi can survive and reproduce in the lower, drier habitats of St. Maarten provided A. gingivinus has been removed (Roughgarden et al. 1984).
• Body size is a proxy for age in these animals. As A. gingivinus gets bigger/older there is an increase in prevalence of malaria….now remember we know this makes sense! A long time has passed for the lizards to get infected and if they are not curing themselves or dying this means that the proportion infected accumulates. The shape decrease in prevalence could be do to lizards of a certain size/age spontaneously clearing the infection…fishy…or death it could be that infection with malaria is killing off these guys.
• Parasite-mediated competition might in- crease the diversity of species that coexist in a biological community when normally competitively superior species are hindered by infection. This would occur when the competitive ability or carrying capacity of the dominant species is reduced, perhaps even slightly, to allow the coexistence of other species.

Question 13

Describe parasite invasion, predation, competition and invasion on the example of amphipod species

Answer 13

3 amphipod species have been invaded Ireland since the 1900s- Gammarus pulex (European) and North American Grammarus tigrinus and Crangonyx pseudogracilis. These invaders have excluded the native Gammarus duebeni celticus from some sites, but not others.
- Mutual intraguild predation (holy mouthful it means that they all eat each other..predation between potentially competing species) is important for determining these exclusions
- In the field they looked at sites with only native, only invaders (excluded native already), and co-occurring natives and invaders (colors of circles). They also looked at the presence of a microsporidian parasite (sites with crosses)
- Parasitism was significantly more frequent in invaded, mixed-species, sites (parasite present at 50% of sites), compared with sites containing only the native G. d. celticus (parasite present at 17%) In field collection and lab tests it was found that the invading species do NOT get infected with the parasite only the native does.
- They set up field experiment where they housed the amphipods in different combinations in little PVC tubes houses in the lake so natural conditions were there. In the world most crowded figure they show: 1. that the presence of parasites alone does not impact survival of then native (1st three bars). 2. And that being held with a competitor alone does not impact survival of the native whether the competitor is infected or not (4th bar) 3. In the treatment where the native is unparasitized it effectively consumes the invader (last two bars sparse dots is when invader gets eaten in the absence of the parasite). In this SAME treatment for the native species there is what they call “cryptic virulence”. They back this up with lab studies. Suggest that high prevalnces of native parasitism may permit the spread of G. tigrinus into new habitats and coecistince of the natve and invader.

Question 14

What can be the effect of parasite mediated apparent competition?

Answer 14

• Parasites can reverse the outcome of competitive interactions (Hanley et al. 1995; Yan et al. 1998) and parasite mediated apparent competition has been implicated in species invasions (Settle & Wilson 1990; Rushton et al. 2000).
• Here, we have shown that parasitism may mediate the outcome of invasions through its effect on predation hierarchies, which are key to the outcome of interactions between native and invading amphipods (Dick 1996). We have shown that parasitism alters the strength of these dominance relationships, weakening the predatory impact of the native species on the smaller invading species, thereby facilitating coexistence. Conversely, we have also shown that, by weakening the ability of the native species to withstand invasions, parasitism also has the potential to facilitate exclusion of the native.