Plant Fruit/Seed Dispersal. First, some defini6ons

Transcription

1 Plant Fruit/Seed Dispersal First, some defini6ons Propagules, refer to the dispersing structures, which may be a seed, fruit, or something else. Seed dispersal: usually refers to propagule dispersal, and may mean both yearly seed movements, as well as migra6on or range expansion. Default is seed leaving a parent; context for migra6on or range expansion. 1

2 Primary Dispersal Secondary Dispersal Seed Dispersal refers to movement of a single individual s seed or a single species. Seed Rain Seed Rain refers to all seed arriving in a loca6on by any means. 2

3 Seed Shadow Seed Shadows refer to a density distribu6on of seeds aeer dispersal from a single plant. Two dimensions are illustrated, but remember this is a three dimensional process. Reminder of Plant Dynamics emphasizing role of seed rain. 3

4 How do plants disperse? Wind dispersal Ballis6c dispersal Water dispersal Gravity dispersal Animal dispersal endozoochory epizoochory Seed Dispersal Syndromes General rules of thumb. 4

5 Seed shadows for several herbs. wind ant ballis6c Seed shadows for miscellaneous trees. wind w/ high terminal velocity rodents wind then rodents ballis6c 5

6 Wind Dispersal Examples Aerodynamics of Wind Dispersal Size and Weight modify the Shape and Distance of seed dispersal. Weight decreases distance. Wing area increases distance Shape or weight loca6on cri6cal. Mass Surface area Shape Real fruit 6

7 Animal Dispersed Fruit Animal Dispersed Fruit (secondary) Tamais Peromyscus boylii Peromyscus californicus Dipodomys Scurius 7

8 MORE Animal Dispersed Fruit 8

9 Variety of possums and rodents Mammalian Frugivores Many monkeys and primates, Possoms, rodents, many others Lots of other mammals like the Spectacled Bat 9

10 Parrots Pigeons Frugivorous Birds Cassowary Hornbills Issues for animal dispersal AXrac6ng frugivores Surviving the animal handling, especially if the fruit is consumed. Diversity among frugivores leads to some6mes conflic6ng selec6on (guts of birds and mammals quite different). Cost of fruits Poten6al size of seed depends on vector 10

11 The specifics of the vectors are interes6ng, but what drives the ecological and evolu6onary selec6on? Why Disperse? Intraspecific Issues like density- dependent compe66on, sibling compe66on, and inbreeding. Escape from interspecific constraints, like seed predators or pathogens Colonize new or bexer habitats Metapopula6on dynamics depends on dispersal. 11

12 Ul6mately, all habitats are transient. Varia6on in scale and intensity of transience. Large Temporal Scale Post- glacial shies in climate resulted in rapid migra6on among forest trees. Rates of migra6on suggest phenomenal dispersal rates, even by species you might not expect. 12

13 Post- glacial migra6on rates. Quercus exhibited one of the faster rates of migra6on, equivalent to several kilometers per genera6on. Numbers refer to when, in thousands of years ago, pollen appears in soil cores at that loca6on. 13

14 Three hypotheses are common in plant studies of dispersal Escape Hypothesis: Dispropor6onate mortality risks near parent. Coloniza6on hypothesis: seeds disperse to sites different from parents; perhaps shade- intolerant species dispersing to patches. Directed Dispersal hypothesis: vectors take seeds to special sites that must be predictable and rela6vely stable. Supported by mistletoes and chili peppers. 14

15 Escape and Coloniza6on hypotheses depend on the shape and density of seeds in the seed shadow. Long- distance dispersal may be the most important aspect, the tail of the seed shadow. (think post- glacial migra6ons). A fat tail is important for long- distance dispersal. Shape of seed shadows Fat tail The shape of the dispersal curve at the tail is believed to be the most important character. This has led to detailed work trying to determine the best fit for seed shadows because models differ in their tails. 15

16 Empirical evidence oeen doesn t help dis6nguish among a variety of models, too many data points are required. Seed traps are a labor intensive way of collec6ng sufficient dispersal data; either seed rain or seed shadows, but it can be done right. Connie Clark worked on this ques6on using isolated trees in a highly diverse African forest, comparing wind-, monkey-, and large bird- dispersed trees. For traps beneath trees: Note that during the frui6ng season, only bird- dispersed seeds differen6ally ended up beneath bird- dispersed trees. Clark et al Oecologia 139:

17 Here, birds differen6ally disperse bird- fruit to other frui6ng trees bearing bird- fruit. Monkeys don t do that. During the non- frui6ng season, it s all chance dispersal. Clark et al Oecologia 139: Summary of Dispersal PaXerns. Both bird and monkey dispersal paxerns yielded fa1er tails (especially the bird dispersed species) which means farther overall distances. Clark et al Ecology 86:

18 No mortality associated with parent. Mortality associated with parent. The Janzen- Connell Model predicts recruitment at a distance from the parent plant. Based on mortality due to predators or pathogens associated with the parent. Recruitment Some evidence supports the Janzen- Connell Model but other studies do not. 18

19 May be more complicated than simple Janzen- Connell models For many tropical trees, seedlings have a lower rate of survival beneath parent, but fairly uniform distribu6on for the next 1000 m (this species has a seed shadow to 39 m). Long distance vertebrate dispersal is differen6ally successful. Hardesty et al Ecol. LeXers 9: Summary Dispersal theory has developed several hypotheses based on poten6al selec6ve forces, inbreeding, compe66on, seed predators, coloniza6on, patch dynamics. Many studies focus on natural history or on mathema6cal modeling of seed shadows. While s6ll know very lixle about shapes of seed shadows, how plants axract frugivores, linkages between dispersal and subsequent life history stages, clearly long- distance dispersal is a crucial aspect of long- term plant fitness. 19