Iowa Lakeside Laboratory and Department of Zoology, University of Iowa, ... tween ruderals and mature prairie plant species in its life history characteristics. (c/.
Oecologia (Berl.) 22, 3 9 9 4 0 9 (1976) 9 by Springer-Verlag 1976
The Natural History of a Fugitive Prairie Plant (Mirabilis hirsuta (Pursh) MaeM.) W i l l i a m J. P l a t t Iowa Lakeside Laboratory and Department of Zoology, University of Iowa, Iowa City, Iowa 52242 Received August 15, 1975
Summary. Some perennial fugitive plants that colonize badger disturbances in xeric prairies have a limited dispersal capacity, and consequently propagules are dispersed over a small area. I hypothesized that high density-dependent mortality might occur early in the life history of such species, and thus increased survival might occur in subsequent age classes because intraspecifie competition would be reduced. These hypotheses were tested using natural and experimental cohorts of Mirabilis hirsuta (Pursh) MacM. From these data and field observations, inferences were obtained concerning selective forces operating upon life history characteristics of this species. The distance between individuals of M. hirsuta increases in successive age classes; the greatest decrease in density occurs between the propagule and seedling age classes. Mortality of propagules due to predation by ants and mice was density-dependent. Predation rates were highest at high propagule densities and predation upon propagules located on badger disturbances was higher than the mortality of propagules at similar densities in undisturbed prairie. The results of mortality in the propagule age class are seedlings present only at low densities and located away from parent plants. Seedlings survive to maturity only if they are located on badger disturbances; this species apparently can not successfully compete with plants present in undistrubed prairie. On badger disturbances seedlings present at low densities have much higher survival (roughly 50 % ) to maturity than do seedlings present at high densities (essentially zero). Thus, if high densities of propagules occur on a disturbance, predation upon propagu]es results, indirectly, in increased survival of seedlings to maturity. Such predation potentially could have important effects upon interspecifie competition of M. hirsuta with other fugitives also colonizing badger disturbances. Reproductive success of M. hirsuta on the Cayler Prairie Preserve is contingentupon successful colonization of disturbance sites. It would appear that selection has operated upon the life history characteristics to favor both successful immigration onto new sites and establishment of seedlings on those sites. Relatively few, but large propagules are produced annually over a long adult life span. While large propagules enhance seedling establishment on xeric sites, production of few propagules annuallyfor a number of years increases the likelihood of immigration onto sites that are variable in the time of appearance within the dispersal range of the plant.
Introduction Fugitives are defined as species i n h a b i t i n g t r a n s i e n t e n v i r o n m e n t s (Hutchinson, 1951). I n prairies a t y p e of localized t r a n s i e n t e n v i r o n m e n t is produced w h e n a badger ( Taxidea taxus) digs a g r o u n d squirrel (Spermophilus sp.) o u t of its burrow. A m o u n d of dirt 0.2-0.3 m 2 is formed t h a t is persistent for at least 10-20 years, b u t e v e n t u a l l y reverts to u n d i s t u r b e d prairie. Several p e r e n n i a l p l a n t species colonize badger disturbances, b u t are rare i n u n d i s t u r b e d prairie (Platt, 1975). Some of these fugitive species produce propagules designed for long-distance dispersal, while others produce propagules t h a t m a y t r a v e l a t most only a few meters. If the
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dispersal capacity is limited, propagules will be concentrated in a relatively small area around the parent plant. I hypothesized that populations of those fugitives with limited dispersal capacity might be subjected to considerable density-dependent mortality early in the life history. If such mortality occurred in the propagule age class, this might result in increased survival rates for later age classes because intraspecific competition would be reduced. From study of survivorship and life history characteristics I also suggest how natural selection might operate upon the reproductive biology of such fugitive species. The above hypotheses were tested by measuring the distribution of individuals comprising different age classes in a population of a fugitive species and relating age specific changes in the distribution to age-specific mortality. Mirabilis hirsuta (Pursh) MaeM. (Nyctaginaceae) was the species studied. This species occurs on badger disturbances that are close together (1-3 m apart) on the Cayler Prairie Preserve in northwestern Iowa (Platt, 1975). Mirabilis hirsuta is intermediate between ruderals and mature prairie plant species in its life history characteristics (c/. Cruden, 1973; Platt, 1975). During the second year of growth plants produce fruits by self pollination (cleistogamy). In subsequent years they produce fruits via self-pollination and by outcrossing (xenogamy). Usually 3 flowers with a single ovule each are present in each involucre, and potentially 3 fruits can be produced per involucre. Seed set of cleistogamous flowers is about 75 % (2.26 4- 0.10 (S.E.) fruits/involucre). Seed set of the facultativelyxenogamous flowers is increased by noctuid moth pollination. Caged plants with xenogamous flowers from which pollinators were excluded had a significantly lower (P
