Should larger patches of plants have more arthropods

and be more attractive to foraging lizards?

 

Kara Davis, Lindsy Greene, and Omid Veiseh

 

BIOL 417a,b   Summer Session 2002

Biology Department, Western Washington University

 

 

(poster modified by Dr. Anderson to fit website format)

 

Introduction

           

            Acquiring food is one of the four basic tasks required for animal.  The desert-dwelling lizard Cnemidophorus tigris is a wide, intensive forager that forages widely (hence, “wide”) and uses vision and chemoreception to seek prey hidden from view (hence, “intensive”).   The foraging pathway of C. tigris is hypothesized to be influenced by distribution and abundance of arthropods.  The spatiotemporal patterns of arthropods are hypothesized to be related to the distribution and abundance of perennial plants.  Desert perennials are somewhat clustered and unevenly distributed across various substrata; that is, perennials may form mixed species patches that are reminiscent of archipelagoes, with each island as individual or contiguous plants, and each patch or cluster of plants as an archipelago.  Each archipelago, then, is somewhat isolated from surrounding vegetation.  It may be that these archipelagoes may hold richer pockets of arthropods than would most individual plants in the habitat.  Furthermore, it is expected that the foraging locations of C. tigris should correlate with spatial patterns of arthropod abundance.  Examining arthropod abundance in a set of plant patches, and varying the characteristics among plant patches may elucidate patterns and causes of arthropod distribution.  Moreover, explicit hypotheses about the spatial patterns of foraging Cnemidophorus tigris may be feasible.   Because distribution, abundance, and diversity of many desert arthropods can be measured proficiently by pit trapping, we chose to use pit traps to examine the patterns of arthropod abundance in archipelagoes of perennial plants.  The research site chosen was near the Biol 417 field course study site, in the Alvord Basin, a northern locale in the Great Basin Desert.   

 

  • We chose the following explicit hypotheses to focus our research:
  • Archipelagoes with greater percent cover and greater volume of plants per archipelago area should have more arthropods.
  • Arthropod abundance will increase with area covered by the archipelago;
  • Archipelagoes dominated by the densely branched perennial Sarcobatus vermiculatus should have greater abundance and diversity of arthropods than archipelagoes dominated by the sparsely branched Artemisia tridentata. 

 

Methods

 

Clusters of plants and plants near these clusters were modified to isolate the cluster from the surrounding plants.  Each cluster formed a sort of archipelago of plant-islands.  It was the characteristics of the cluster plants of the archipelago that we were testing for their effects on arthropod abundance and diversity. 

 

The conditions for each archipelago:

ü      it is located on the sandy flats,

ü      has no more than 1/3 of its perimeter adjacent to hardpan,

ü      has a distance of at least 1m from archipelago perimeter to neighboring plants.

ü      has at least 1 SAVE and 2 ARTR plants (islands) in the archipelago,

ü      has 20-80% vegetation cover within the archipelago perimeter,

ü      has each plant in the archipelago less than 75cm from its nearest neighboring plant within the archipelago (but at least 1 m from the nearest plant outside of the archipelago).

 

Additional criteria for each of the 24 Artemisia tridentata (ARTR) dominated archipelagoes:

ü      2 ARTR with pitfall traps

ü      each pit-trapped ARTR with 2 pitfall traps

ü      each pit-trapped ARTR covered an area of 4500-7000 cm2

ü      > 66% cover by ARTR plants

 

Additional criteria for each of the 12 Sarcobatus vermiculatus (SAVE) dominated archipelagoes:

ü      1 SAVE with pitfall traps

ü      the pit-trapped SAVE with 4 pitfall traps

ü      the SAVE with pitfall traps had an area of 9500-13500 cm2

ü      50-66% cover by SAVE plants

 

Pit trapping procedure: 

Open-top soup cans were buried vertically, with the tops level to the ground surface.  Seven ounce plastic drinking cups were placed into the soup cans.  The cups were shorter than the soup cans, but the cups had almost exactly same top outside diameter as the inside diameter of the soup can; the top, perimeter lip of the drinking cup rested on the top, perimeter of the soup can.  The plastic cups were half-filled with propylene glycol, which was the insect preservative.  Six days later the contents of the cups were poured into 40 dram snap cap vials, and the vials were labeled by archipelago type and precise pitfall location.  The arthropods were sorted and identified (at least to level of order, often to family) in lab at WWU. 

 

Results and Discussion

 

Archipelagoes were isolated clusters of plant-islands (Figs 1 and 2), and archipelagoes were large compared to the nearest shrub (Fig 2 and 3), so the effect of individual nearby plants on the plants of the archipelago was assumed to be low.  Thus effects on numbers of arthropods in pit traps were expected to be related to the plant and archipelago sampled as well as an overall habitat effect (from all surrounding plants). 

 

The number of plants, plant cover, and plant volume varied directly with archipelago size (Figs 4, 5, and 6), but the actual proportion of standing crop decreased with archipelago size (Fig 7).  Hence, larger archipelagoes had lower standing crop of perennial plants per unit area, and should have fewer arthropods per unit area of archipelago.

 

The arthropod abundance appeared to vary directly with amount of plant cover (Figs 8 and 9), but not with archipelago size (Fig 10).  Given Figures 4-7, the observed patterns of arthropod abundance were expected.

 

There appeared to be no relationship between ARTR plant size and the number of arthropods captured under it (Fig 11).  These data were in contrast to data from 2001.  But whereas individual plants were somewhat isolated in 2001, two ARTRs per ARTR-dominated archipelago were pit trapped in the current study.  Thus, the close proximity of pit-trapped plants in the current study may have contributed to the lack of a relationship between plant size and arthropod number.

 

Given an apparent direct relationship between proportion of plant cover in an archipelago and arthropod abundance (Fig 7), it would be expected that a plant patch of contiguous, tightly packed plants would have the greatest arthropod abundance.  Thus, a dense cluster of plants, as seen in an archipelago of smaller area (Fig 7) should have a similar effect on arthropod abundance as a single large plant (see 2001 pit trap study).   Stated in another way, because large archipelagoes did not comprise densely packed clusters of plants, arthropod density was low in large archipelagoes. 

 

Arthropod diversity (number of orders and number of arthropods within and among orders), as measured by the Shannon-Weaver Index was marginally higher for SAVE archipelagoes than for ARTR archipelagoes. This result is expected, given the greater standing crop of edible plant biomass per area of cover in SAVEs.

 

 

Conclusion

 

We tentatively infer from our results that if a foraging Western Whiptail Lizard were to forage most efficiently, it should tend to use larger plants and densely packed clusters (patches) of plants, particularly if those plants are SAVEs and patches are SAVE-dominated. 

 

 

Figure 1.   Distance (cm) from perimeter of ARTR-dominated archipelago to     

                    the nearest shrub outside of the archipelago.

 

 

 

 

Figure 2.  Distance (cm, dark blue) from perimeter of SAVE-dominated archipelago

        to the nearest shrub outside of the archipelago, and size of that shrub as

        measured by area of its cover (cm2, light blue).

 

 

 

 

 

 

 

 

 

 

 

Figure 3.   Direct relationship of total plant cover to total plant volume

                     in 24 ARTR-dominated archipelagoes.

 

 

 

 

 

 

 

Figure 4.   Amount of plant cover in archipelago as related to area of archipelago. 

       Sample includes 24 ARTR-dominated archipelagoes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5.  Relationship of plant number to plant cover in archipelagoes.  

      Sample size is 24 ARTR-dominated archipelagoes.

 

 

 

 

Figure 6.  Direct relationship of total plant volume of plants to area of archipelago.  

      Sample size is 24 ARTR-dominated archipelagoes.

 

 

 

Figure 7.  Inverse relationship of archipelago area to proportion of archipelago

      in plant cover.  Sample includes 24 ARTR-dominated archipelagoes.

 

 

Figure 8.  Relationship of arthropod abundance to amount of cover in an archipelago. 

      Sample includes 24 ARTR-dominated archipelagoes.

 

 

 

 

 

Figure 9.  Relationship of amount of plant cover in archipelago with number of

      arthropods captured in pit-traps in the archipelago.   Sample includes

     24 ARTR-dominated archipelagoes and 12 SAVE-dominated archipelagoes.

 

 

 

 

Figure 10.   Effect of archipelago area on arthropod abundance. 

                       Sample size is 24 ARTR-dominated archipelagoes.

 

 

 

 

 

 

 

Figure 11.  Number of arthropods caught in pit traps under individual ARTRs. 

        Sample includes 2 ARTRs pit-trapped per archipelago for each of

        24 ARTR-dominated archipelagoes.

 

 

 

Figure 12.  Arthropod diversity in pit traps in ARTR-dominated archipelagoes v. SAVE-dominated archipelagoes; the mean Shannon Weaver Index among 24 ARTR archipelagoes compared with the mean S-W Index among 12 SAVE archipelagoes.