MSc Research
The effects of plant density upon several determinants of reproductive success in Cistus landanifer (Cistaceae).
General Science Questions
1) Does plant density affect pollination success?
2) Does plant density affect reproductive effort?
3) Does plant density affect rate of fruit predation?
4) Does plant density affect the within plant distribution of the above variables
Background
A number of studies have shown that population density may have a stabilizing influence on population size and growth (e.g.: Nicholson 1933, 1957, Smith 1935, Lack 1954). This inherent stability could be disrupted, however, if population size falls too low (Allee et al. 1949, Allee 1951). Plants are ideal test subjects for this area of research because, being sessile, they are both particularly vulnerable to density effects (since they are not directly able to find mates themselves) and they are relatively easy to locate and monitor.
Population density, by constraining frequency and duration of pollinator visits, is likely to have a crucial role to play in individual plant pollination success (e.g.: Zimmerman 1981). However, pollination success is not the only density-dependant component of overall reproductive success. Also important is pre-pollination reproductive effort, and post-pollination seed predation. There has been relatively little research into how population density affects these two processes, and what the repercussions are for plant reproductive success.
Naturally rare, sparsely populated plant species may have evolved mechanisms to overcome difficulties associated with isolation; such as long-distance seed dispersal mechanisms, a persistent seed bank, specialized plant-pollinator mutualisms, and self-compatibility (Barrett & Kohn 1991, Ayres & Ryan 1997, but see Wolf & Harrison 2001). The most immediate risk of pollination failure and species extinction is faced by plant species which lack those characteristics described above (Wilcock & Neiland 2002, and references therein). For these species the main problem arises when human activity artificially imposes low population density, through habitat fragmentation and/or degradation. Research in this area, therefore, has important implications not just for theories relating to population dynamics but also for the design of effective population management and conservation strategies.
Methods & Study System
The plant species studied was Cistus landanifer L, which forms a major component of western Mediterranean scrubland (matorral). The species is easily distinguishable because of its growth form, large flowers and resinous exudates produced by the leaves and stems in summer which release a characteristic odor. The breeding system is self-incompatible (individuals cannot fertilise their own ovules), and the generalist floral architecture and pollen/nectar rewards attract a wide diversity of insect pollinators (Talavera et al. 1993). The fruiting capsules are susceptible to fruit parasitism and seed predation by a number of insect species (Bastida & Talavera 2002).
The study was carried out in an area of scrubland near the town of Luz, in the Alentejo region of Portugal. Three sites were chosen to represent the full range of environmental variation within which C. landanifer exists. Twenty plants at each site were randomly chosen. For each plant I measured distance to the nearest C. landanifer plant, height, and the number of flowers and fruits produced. Sampling was timed to take place after the main period of flowering, but before seed dispersal (June-July 2003). Three fruits were selected from each plant (one each from the top, middle and bottom portion of the plant). For each capsule, I noted its height on the plant and any evidence of parasitism. I also estimated the number of seeds and undeveloped ovules per fruit by manually counting seeds and ovules in two segments per fruit with a low power microscope, then taking the average and multiplying it by the number of segments in the fruit.
Key Results
1) Pollination success declined as population density decreased.
2) Reproductive effort and fruit parasitism increased as population density decreased.
3) Pollination success and reproductive effort per fruit increased with fruit height, but only in dispersed plants.
4) Overall reproductive success was the product of a significant interaction between pollination success and reproductive effort. This means that dispersed plants were able to partially compensate for reduced pollination success by increasing reproductive effort.
