Powerpoint template for scientific posters (Swarthmore College)

1
Responding to temporal resource pulses in a
spatially subsidized community surprising
strategies of desert annuals D. Alexander Wait1
and Wendy B. Anderson2 1Dept. of Biology,
Missouri State University and 2Dept. of Biology,
Drury University, Springfield, Missouri

• Conclusions
• Species richness on subsidized islands is lower
  than on unsubsidized islands only in dry years
  (Fig. 2), but the species found on subsidized
  islands are not commonly found on either the
  mainland or unsubsidized islands in any year.
  Therefore, subsidies impart a very different
  selection pressure and limit the species pool
  from which to draw on.
• In dry environments natural selection should
  maximize water-use efficiency (WUE) (Ehleringer
  1993) or operate to maximize carbon assimilation
  (Gibson 1998). In this system the regionally
  common (i.e., unsubsidized species) annual
  species have both relatively high photosynthetic
  rates and low WUE (Figs. 4-6).
• Interestingly, even though subsidies increase
  soil moisture (Fig. 3), the dominant species in
  subsidized areas are even more WUE than the
  dominant species in unsubsidized areas (Figs. 4
  and 6), with no difference in assimilation rates
  (Fig. 4). Therefore, selection apparently
  operates to maximize WUE instead of
  photosynthesis (see Casper et al. 2005). This
  phenotypic selection is also indicated in
  Atriplex (Figs 7-8)
• Differences in species composition are
  maintained by periodic wet years because water
  pulses strengthen selection in subsidized areas
  for species that are considered more weedy
  (e.g., Amaranthus and Chenopodium) but in this
  case these species have more conservative water
  use strategies and greater plasticity to respond
  in wet years.
• Greater productivity in subsidized areas than in
  unsubsidized areas (Figs 1-2) is apparently a
  function of subsidized species being able to keep
  their stomates open longer into the day (see Fig.
  5), and by having longer growing seasons (i.e.,
  they have slower growth rates but grow for
  longer periods of time).
• Without pulsed years, subsidized islands would
  probably not look very different from
  unsubsidized islands with respect to plant
  species richness and productivity.

Results A) Water pulses increase soil organic
matter more in subsidized than unsubsidized areas
which results in greater soil moisture in
subsidized areas even during interpulse years
(Fig. 3). Differences in soil moisture help
explain differences in cover (Fig. 1) and
productivity (Fig. 2) between unsubsidized and
subsidized areas.
Introduction The physiological responses of
plants to pulse events are well studied (e.g.,
Oecologia vol. 141 2), while responses to pulse
events in spatially subsidized environments have
received little attention. Here we examine the
physiological characteristics of herbaceous
plants growing in a hyper arid region that
experiences periodic (every 2-7 years) pulses of
rain and contains areas (small islands) that are
highly subsidized with nutrient resources via
seabird guano (see Fig. 1). We ask how water
pulses affect selection for physiological traits
in subsidized vs. unsubsidized areas. The results
suggest how spatial subsidies in arid
environments alter and magnify the impact of
temporal variation in rainfall on primary
productivity and species richness (see Fig. 2)
via selection for conservative water use traits.
D) Carbon isotopic discrimination, an integrated
measure of water-use efficiency (WUE), indicates
that subsidized species are more WUE than
unsubsidized species. These data are consistent
with instantaneous measures of WUE (Fig. 4). As
expected WUE is greater in a dry than a wet year.
Fig. 3. Mean (SE) gravimetric soil moisture at a
depth of 8-12 cm on unsubsidized (n6) and
subsidized (n6) islands in two dry and two wet
years.
Fig. 6. Mean (SE) carbon isotope discrimination
calculated from carbon isotope ratios of leaf
material of dominate C3 species (Cr, Pl, Ch)
collected at the end of the growing season in a
dry and wet year. Lower discrimination values
indicate higher water-use efficiency.
B) Species growing in subsidized areas do not
have higher instantaneous net photosynthetic
rates than species growing in unsubsidized areas
(Fig. 4), and therefore must grow over a longer
period of time to attain greater biomass (see
Fig. 2). In addition, species in subsidized areas
are significantly more water-use efficient than
species growing in unsubsidized areas (Fig. 4),
even though soil moisture availability is greater
in the subsidized areas (Fig. 3).
E) Atriplex (a species common to both
unsubsidized and subsidized areas) net
assimilation rates were not significantly
different between area types, while instantaneous
WUE was greater in Atriplex individuals growing
in subsidized areas (Fig. 7). These results mimic
those found for the species common to each area
type (Fig. 4), and suggest that phenotypic
selection for physiological traits is occurring
in Atriplex due to both subsidies and water
pulses.
Fig. 1. Subsidized islands have up to 80
herbaceous cover in a wet year (top), while
unsubsidized islands have less then 30 cover in
a wet year (bottom).

• Study site
• Highly water pulsed Mean ( SD) annual
  precipitation 1991-2004 was 53 72 mm range was
  0-300 mm.
• 1999- 2002 were dry years, 2003 and 2005 were
  wet years. Pulse (wet) years have on average
  over 150 mm rain, while interpulse (dry) years
  average less than 20 mm. All data were collected
  in March-May 2001, 2002, 2003 and 2005.
• Six subsidized and six unsubsidized islands in
  midriff region of Gulf of California subsidies
  are primarily in the form of seabird guano
  deposition.
• Subsidized islands have up to 18x higher soil N
  and P, higher soil moisture in both interpulsed
  and pulsed years (see Fig. 3), and lower soil pH
  (Wait et al. 2005).
• Productivity is higher, but herbaceous species
  richness is lower on subsidized than unsubsidized
  islands (Fig. 2).
• There is virtually no overlap in herbaceous
  species between unsubsidized and subsidized
  islands. Cryptantha (Cr) and Plantago (Pl) are
  dominant on unsubsidized islands, while
  Chenopodium (Ch) and Amaranthus (Am) are dominant
  on subsidized islands Atriplex (At) is the only
  species that is common across all areas Perytile
  (Pr) is dominant on subsidized islands only in
  pulse years.
• Island size, physical disturbance, and geology
  do not explain island wide differences in
  productivity or species richness (Anderson and
  Wait 2001, Anderson and Polis 2004).

Literature cited Anderson, W. B. and G.A. Polis.
2004. Allochthonous nutrient and food inputs
consequences for temporal stability. In Polis,
G. A., M. E. Power, and G. R. Huxel (eds), Food
webs at the landscape scale. University of
Chicago Press. Anderson, W.B., and D.A. Wait.
2001. Subsidized island biogeography hypothesis
another new twist on an old theory. Ecology
Letters. 4289-291. Casper, B.B., Forseth, I.N.
and D.A. Wait. 2005. Variation in carbon isotope
discrimination in relation to plant performance
in a natural population of Cryptantha flava.
Oecologia. (Online First). Ehleringer, J.R. 1993.
Carbon and water relations in desert plants an
isotopic perspective. In Ehleringer et al. (eds)
Stable isotopes and plant carbon-water relations.
Academic, San Diego, pp 155-172. Gibson, A.C.
1998. Photosynthetic organs of desert plants.
Bioscience 48911-920. Wait, D.A., Aubrey, D.P.,
and W.B. Anderson. 2005. Seabird guano influences
on desert islands soil chemistry and herbaceous
species richness and productivity. Journal of
Arid Environments. 60681-695.
Fig. 4. Mean (SE) mid-day instantaneous net
assimilation and water-use efficiency for
dominant species growing in unsubsidized (Cr and
Pl n26) and subsidized (Ch and Am n26) areas
across four different islands in a wet year.
Fig. 7. Mean (SE) mid-day instantaneous net
assimilation and water-use efficiency for
Atriplex growing in unsubsidized and subsidized
(n6) areas across two different islands in a wet
year.
C) Species that dominate subsidized areas have
lower water contents and are more water stressed
in dry years than species that dominate in
unsubsidized areas (Fig. 5), indicating that
these species keep their stomates open longer
into the growing season and into the day.
However, in a wet year plant water potential is
much higher in species growing in subsidized than
unsubsidized areas (Fig. 5), indicating greater
plasticity in responding to a water pulse.
F) Atriplex individuals in subsidized areas in
dry years are more water stressed than Atriplex
individuals in unsubsidized areas (Fig. 8).
Therefore, physiological traits associated with
Atriplex production patterns (which follow
overall production patterns Fig. 2) apparently
mimic those found in species only growing in
unsubsidized or subsidized areas (Figs. 4-6).
D) Leaf Carbon Isotope
Acknowledgments We thank the Mexican Department
of the Environment for a permit to work on the
islands. Funding for this project was provided by
the Andrew W. Mellon Foundation. We thank Doug
Aubrey and Kate Heckman for help in the field
For further information Please contact
daw385f_at_smsu.edu or alexanderwait_at_missouristate.e
du. More information on this (and a link to the
poster) can be obtained at http//courses.smsu.ed
u/daw385f/homepage.htmDr
Fig. 5. Mean (SE) plant water content and
mid-day water potential for dominant species
growing on unsubsidized (Cr and Pl n12) and
subsidized (Ch and Am n12) areas towards the
end of the growing season across six islands in
two dry and two wet years.
Fig. 8. Mean (SE) mid-day water potential for
Atriplex growing in unsubsidized and subsidized
(n8) areas towards the end of the growing season
across six islands in two dry and two wet years.
Fig. 2. Mean (SE) productivity and apparent
species richness on unsubsidized (n6) and
subsidized (n6) islands in two dry and two wet
years.