Habitat selection 2 Flashcards
Leaf preference in Poplar aphids
(Pemphigus betae) (Whitham 1980):
(aphids are sap sucking insects)
Research on the dispersal and choice of breeding site in females following their emergence from overwinter eggs laid in the bark of cottonwood poplar trees in North America.
After hatching, females, known as stem mothers, migrate up the trunk of their host tree and along the branches and twigs until they encounter a suitable leaf.
The ‘Stem mother’ stimulates the formation of a protective gall on the midrib of the leaf, within which they produce daughters asexually via parthenogenesis this creates a concealed shelter for the young which is rich in nutrients to aid their development
On maturing, some of these daughters become winged migrants and disperse to new host plants.
However, the extent to which stem mothers succeed in producing daughters depends on the kind of leaf they choose
Why do Poplar Aphids show leaf preference?
larger leaves = less likely to abort & therefore produce more daughters – no. Of daughters increases proportionally to increased leaf size / rate of abortion reduces in proportion to increased leaf size
But - large leaves in short supply. Largest leaves = c. 2% of leaf crop of the host trees.
Small leaves c. 33% of the crop but were generally avoided.
So there is competition for the best sites.
Since a female Poplar Aphid encountering a large leaf is very likely to find it already occupied, what should she do?
Either settle on the occupied leaf & try to establish a 2nd (or 3rd) gall, or search for another leaf.
already occupied = risk failure due to nutrients being shared with other broods of young but larger leaves can support more galls
average reproductive success for 1, 2 & 3 stem mothers/leaf: see the dotted line – choosing a smaller leaf where you are the only stem mother or second on a smaller leaf may be equivalent or better than sharing a higher quality habitat (larger leaf) with more competition.
Examples suggest that habitat choice and extant competitors impact reproductive success
Ideal free distribution impacts reproductive success - but how?
Habitat quality varies resulting in ‘good’ & ‘bad’ habitats
Quality changes due to exploitation by organisms (inter/intraspecial)
e.g: value of large leaf to aphid is reduced once another female settles on it.
High quality habitats are almost certain to be exploited resulting in:
- depletion of resources (or access to them)
- increased levels of interference
What started as best place may soon become no better than a site of previously lower quality.
So it may benefit an individual to go to 2nd best site rather than struggle for access to what had originally been the best.
Competition then increases at new site until it too declines in value & individuals would do better to go to 3rd best site etc.
High quality sites can support more competitors than low quality sites but they eventually reach a point where distribution of competitors reflects distribution of habitat quality.
This is ideal free distribution, an evolutionarily stable strategy (ESS) theory of dispersion
(Fretwell & Lucas 1969)
studies show that competitors distribute themselves either:
-in proportion to the ratio of resources
-or so as to achieve an equal reward rate
(e.g. Parker 1974; Milinski 1979; Godin & Keenleyside 1984)
Assumptions of ideal free distribution
1) all individuals are equally good competitors,
2) individuals are ‘free’ to move wherever they like,
3) individuals are `ideal’ in having perfect knowledge of the relative value of each habitat.
4) individuals choose a habitat so as to maximise their net reproductive benefit
Point 4 can be easily measured whereas it is unlikely that point 3 is true in all cases. IFD is still useful.
Deciding where to feed
value (profitability) of a site depends on 2 things:
-food intake achievable without competition
-extent to which intake is reduced by presence of competitors
differences in food availability will be cancelled out by different levels of competition
There’s a lot at rich sites, less at poor sites -so feeding rate will end up the same at all sites.
if feeding rate becomes better at one site for any reason, more individuals will move there & equilibrium feeding rate will be restored
example paper: Stickleback feeding experiment (Milinski 1979)
Laboratory based experiment: 6 fish were provided with 2 feeding sites, one providing Daphnia at twice the rate of the other.
Sticklebacks are small and easy to raise in the lab, they feed together and are not territorial
Ideally a lidded tank with oneway glass to view the fish would prevent fish from observing feeders and being influenced by external environment – we don’t know if this was the case
The lower figure shows the mean number of fish at what was initially the poorer site and then at the same site when it switched to the higher reward rate (arrow).
The mean was 3 at each site when feeding rates were the same at both ends. When feeding rates were adjusted to twice the amount of food at one end the mean changes to 4 fish at the end with more feed and two at the other end. When the feeding rate is switched (arrow) the fish means are switched.
In both cases the number quickly changed to match that predicted by ideal free theory (dotted lines). Milinski (1988).
Example paper: Yellow dungflies (Scatophaga stercoraria) availability of mates (Parker 1974)
Mates are another example of a finite resource
Females arrive at dung pat during the first 20 minutes or so after it has been deposited to lay eggs fertilised by previous mating.
Males compete to mate in order to fertilise the next batch and search for females in different zones on and around the pat
Number Of females captured by males
Competition on the pat (Zone A) is fierce so some males search in the surrounding grass (zones B-E) where late arriving females that have not yet been inseminated can be found
No. Of females arriving in each zone differs – prediction of IFD – distribution of males such as to achieve equal capture rates
The solid curve shows the number of females captured by males in each zone (Parker 1974), and the dashed curve the number expected if the distribution of males conforms to ideal free theory (i.e. males achieve an equal capture rate).
Issues with the assumptions of ideal free distribution
In many studies: competing individuals do not always experience equal reproductive gain once they reach an equilibrium distribution (e.g. Thornhill 1980; Harper 1982; Godin & Keenleyside 1984)
Why is this – are the assumptions realistic?
Assumption 1 : equally good competitors? No - dominant individuals often monopolise resources
Assumption 2: ‘free’ to move? No - dominant individuals often limit movement of subordinates.
IFD: effects of unequal competition
A wide range of predicted distributions of competitors
e.g: large individuals consume food at 2x rate of small individuals
best viewed as ‘ideal free distributions of competitive abilities’, rather than IDF of individuals
(Sutherland & Parker 1985)
Unequal competition for overlapping habitat resources is a key reason for species segregation
Example paper:
Interspecific competition for habitat and resources between Brown Trout and Arctic Charr in Norway
(Langeland et al.1991)
Deep glacial lakes such as those in Norway support these two species – they are both native species
Both species prefer the shallow littoral zone.
In the spring, summer, and early autumn the trout excludes the charr from the littoral zone pushing them further out into the deep water zones. Hence the trout benefit from the richer feeding zone
In late autumn and winter, the charr excludes the trout from the littoral zone because charr are active at colder water temps.
This is ofcourse impacted by global warming
Very few studies have attempted to test IFD (Ideal free distribution) at ecosystem scale
It is difficult to do (expensive, long time)
but how do we know its true applicability if we don’t?
Only known paper example: Haugen et al. (2006) – tested IFD at ecosystem scale. Utilising a data set of 50-60 years of data
Studied Pike Esox lucius in Windermere, lake district – 2 large basins (‘habitats’) joined by narrow connection.
Utilised a long-term mark-recapture survival studies + population size manipulated + natural variation in density.
Pike will change their location according to the richness of food available. Removing some Pike results in reduced competition, larger individuals which produce more young.
In both basins, when pike density was low, reproductive output (fitness) was greatest. As total pop. density changed, no. of pike in each basin changed (movement), in manner predicted
Positive ‘Isodar’
This is a rare study as most research projects only receive 3 years of funding – a major issue
Extreme asymmetry between species in accessing habitat can result in exclusion and population decline as observed in non-native invasive species
Example paper:
Native crayfish and benthic fishes are being out-competed by non-native crayfish. (Bubb et al. 2006)
All utilise similar microhabitats - benthic refuges (e.g. under rocks) by day and are nocturnal.
Bubb et al. measured dominance hierarchy (winners / losers of shelters) in laboratory
Signal crayfish (non-native) > White-clawed crayfish (native) > Bullhead benthic fish (native)
Bullhead rarely enter shelters occupied by crayfish, but shelter-sharing more common between White-clawed and bullhead
Where shelters are limited in availability, subordinates will be exposed to predation (e.g. from heron, otter)
Laboratory findings were supported by field data collected via radio-tracking - close habitat overlaps were observed
Parasites: hosts as habitats
*organisms also act as habitats to others
e.g. schistosome flukes in aquatic snails and mammals (different stages)
– also differences in host-specific habitat between parasite species.
Human parasite habitats:
Head louse - Head
Loa loa - Eye
Dirofilaria - Heart
Ascaris - Gut
Malaria - Blood & liver
Wuchereria - Lymphatics
Hosts are islands of suitable habitat for parasites - so how do they find them?
Parasites may alter host behaviour to enhance likelihood of obtaining next ‘habitat’ for next lifecycle stage
Parasites may also modify host behaviour to enhance dispersal
For example:
Leucochloridium paradoxum: develop as parasitic sporocysts in the tentacles of snails. This attract birds which are their final hosts.
Ligula (cestode): causes host fish to swim more actively near surface and avoid cover (eaten by fish-eating birds) – final host.
