Plant and Algae Interactions

Question 1

Macroalgal communities are affected by stress (desiccation, competition etc.) and disturbance (sudden rise in temperature, storm).
Define stress and disturbance

Answer 1

Stress can be defined as a pressure that is there fairly constantly throughout the environment.
Disturbance can be defined as a discrete event that ultimately leads to more mortality than reproduction at that time.

Question 2

Describe a cyclic interaction which can occur between A cyclic interaction is formed by the competition for space between, algae, barnacles and mussels.

Answer 2

• Reduction in fucoid cover leads to settlement of barnacles
○ Calcareous plates undercut algal holdfasts as adults spread during growth. By reducing the abundance of macro-algae they will also reduce the ability of the algae to buffer each other in waves.
○ Barnacles compete with algae for primary space on rock surface
• Mussels settle on secondary substrate (require a rough substrate for attachment) e.g. barnacles or algae
○ Overgrow and smother organisms on which settled
○ Out-compete barnacles and algae for space
• Fucoid’s can be good at sticking to the complex mussel beds, which make good points of attachment. The macroalgae can outgrow the mussels and return to a macro-algae dominated shore.
○ Disturbance events can also remove mussel communities allowing algae to re-colonise e.g. wave action, grazing
Adaptive mechanisms e.g. Postelsia palmaeformis. Really good at settling on mussel dominated shore, with specially adapted gametes and holdfasts.

Question 3

Talk about nutrient recycling in mangrove systems.

Answer 3

• Nutrient exporters
• Leaves unpalatable for most invertebrates due to a high carbon : nitrogen ratio.
• 28% leaf litter taken underground and consumed by sesarmid crabs (Robertson, 1986) - extremely important for mineral and carbon recycling. Bacteria and fungi begin to break down the leaves, making them digestible. Ecosystem engineer.
○ Two main families; Grapsidae and Ocypodidae
○ Utilise leaf litter as primary food source
○ Reduces organic matter export (Lee, 1998
○ Bioturbation (Smith et al, 1991)
• Majority of litter exported by the tide on to seagrass beds and adjacent coral reefs

Question 4

Niche separation

Answer 4

• Birds are most diverse and abundant group
• Display niche separation (birds taking insects from bark or flowers or foliage) through feeding guilds (a group of animals that eat in a specific way, cross taxonomic groups) (Noske, 1995)
○ Foliage-foraging insectivores
○ Bark-foraging insectivores
○ Facultative nectarivores (take insects, along with nectar)
○ Aerial hawkers
• Gastropods - detritivores or grazers of epiphytic algae
Display niche separation to feed on leaf litter or bark (read Cook, 1986)

Question 5

The giant mangrove whelk, is a species of brackish-water snail - talk about its diet

Answer 5

Terebralia palustris

• Displays an ontogenetic change in diet. Young gastropods don’t have a strong enough radula to rasp these leaves, which gives a shift from marine to terrestrial diet.  (Soemodihardjo & Kastoro, 1977; Nishihira, 1983) 
• They cluster on eaves to avoid being moved by the sesarmid crabs (utilise air- and water-borne olfactory cues to locate food (Fratini et al, 2001))
	○ Overlap in zonation, food items and active foraging time
	○ Different feeding strategies (Fratini et al, 2000) but compete for food (Slim et al, 1997)

• Organic content and abundance of leaves correlate with total density of gastropod, with abundance of leaves correlating more strongly with gastropod size distribution (Penha-Lopes et al, 2009)

Question 6

Algal defenses

Answer 6

• Structural and morphological
○ Growth forms unable to be manipulated by grazers
○ Tougher plant tissue less palatable (Steneck & Dethier, 1994)
○ Calcification (Padilla, 1989)
○ Heteromorphic life cycles - alternating morphological generations

• Chemical (Hay & Fenical, 1988; Paul et al, 2001)
	○ Constitutive or induced
		§ Phlorotannins (fucoids), halophenolics (Polysiphonia), diterpenes (Osmundea)
• Factors affecting habitat choice:
	○ Availability
	○ Complexity
	○ Suitability as a food source

Question 7

Chemical defenses

Answer 7

• Fucoids produce phlorotannins, which can act as chemical defences (read ‘defence theories in algae’, on BB)
• Phlorotannins work by acting on the microbial community inside the animals gut.
• Expenditure on defence reduces expenditure on growth and/or reproduction (and vice versa)
• Defences can be constitutive
○ Present all the time (still variable with season),
• Or inducible
○ Produced when required. These are good with slow moving grazers (Littorina).
○ Ascophyllum nodosum only induces defences when under attack by certain herbivores (read Pavia & Toth, 2000). Will for gastropods, but not crusaceans for examples. Cues are thought to be in the mucus of the snail.

Question 8

Constitutive Chemical Defences

Answer 8

• Ascophyllum nodosum concentrates chemical defences in the tissues which matter most (read Pavia et al 2002).
So many reproductive organs they are less precious than organs like the holdfast, some in the stipe and a little in the apex. (read Pavia et al 2002)

Question 9

Sex-specific trade-offs in Chemical Defences ascophyllum nodosum

Answer 9

• Ascophyllum nodosum Males put less energy into sex than the females at certain sites
  
  • This means more energy for defence after reproduction at these sites, and this leaves females vulnerable to attack (read Kurr & Davies, 2018).
  • The difference between sites may be due to the survivability of the germlings, with high wave action and low survivability more energy may need to be funnelled into sex, and the pressure goes on the females.

Question 10

WIDER READING: chemical defenses

Answer 10

Hay and Fenical, 1988. Marine Plant-Herbivore Interactions: The Ecology of Chemical Defense.Annual Review of Ecology and Systematics, 19, pp.111–145.
· Concentrates on the ecology of seaweed chemical defence
· Rhodophyta: e.g. genus Laurencia produce terpenoids and acetogenins
o Elatol is a well-known metabolite of Laurencia; it is cytotoxic, ichthyotoxic, insecticidal, and deters feeding by reef fishes (Anon, 1988)
· Chlorophyta: e.g. Family Caulerpaceae; genera Caulerpa
o Produce sesquiterpenoid and diterpenoid compounds that show wide range of bioactivity, deterring heterring herbivores.
· Phaeophyta:
o only algae to produce phlorotannins, which have been found to show antibiotic, antifungal, antialgal, and antilarval activities.
In tropical/warmer waters, Order Dictyotales produce chemical compounds that deter herbivores but lack other inhibitory effects
Small relatively sedentary herbivores like some amphipods, polychaetes, and ascoglossans (mesograzers) often selectively consume seaweeds that are low preference foods for fishes and larger invertebrates. Compounds from these seaweeds deter feeding by larger herbivores but stimulate, or do not affect, feeding by several mesograzers. Although a few mesograzers sequester chemical defenses from their algal hosts, most are not highly specialized and appear to be advantaged indirectly by their close association with seaweeds that are not visited by their predators.

Question 11

WIDER READING: algal / predator interaction

Answer 11

McGuinness, 1997. Seed predation in a tropical mangrove forest: a test of the dominance-predation model in northern Australia.Journal of Tropical Ecology, 13(2), pp.293–302.
· Tests the generality of the dominance-predation model by looking at predation on the propagules of 3 mangrove species (Avicennia marina, Bruguiera exaristata, Ceriops tagal and Rhizpphora stylosa)
· This model states that a species’ dominance in the canopy and the predation rate on the propagules of this species are inversely protected
· Predators were likely to significantly affect community structure, since they were found to attack more than 50% of all species’ propagules (except R. stylosa)
· Results did not match dominance-predation model’s predictions…
· For 3 out of the 4 species, it could be concluded that there was no significant relationship between the two variables
…this contrasts with the results found by Smith 1987 and Smith et al. 1989, whose results were supportive of the subject model

Question 12

WIDER READING: nutrient recycling

Answer 12

Lee, 1998. Ecological role of grapsid crabs in mangrove ecosystems: a review.Marine and Freshwater Research, 49(4), pp.335–343
• Organic matter is recycles by the feeding of sesernid crabs on the fallen leaves from mangroves; this forms the basis of a coprophagous food chain (coprophagia = consumption of faeces)
• Crabs have preference for feeding on certain mangrove species’ propagules and so this influences community structure
Bioturbation by the crabs also affects surface topography, particle size distribution and degree of aeration (and therefore the sediment’s phytotoxin concentration