Seed-predators reduce broadleaf weed growth and competitive ability

Agriculture Ecos~'stems & Envwonment ELSEVIER

Agriculture, Ecosystems and Environment, 48 (1994) 27-34

Seed-predators reduce broadleaf weed growth and competitive ability Gerald E. Brtlst 1 Department ofEntomology, Purdue University, WestLafayette, IN 47607, USA (Accepted 6 August 1993 )

Abstract Based on field observations that fewer broadleafweeds were found in low-input no-tillage systems (LINTS), a series of greenhouse experiments was conducted to investigate the effect of weed seed-predators on the competitive ability of broadleaf weeds grown with grass weeds. Two major broadleaf weeds, which were found in LINTS, red root pigweed Amaranthus retroflexus and lambsquarters Chenopodium album, and two major grass weeds, large crabgrass Digatara sanguinales and fall panicum Panicum dichotoflorum were tested singularly and in pairs (always one broadleafwith one grass) in the study. These seed pairings were exposed to ( 1 ) seed-predators (carabids and field crickets found in LINTS) which were at different densities, (2) wheat straw residue, (3) seed-predators and residue, and (4) a control (no seed-predators or residue). Seed-predators reduced the plant relative yield (PRY) ofbroadleaf weeds to the same extent as the wheat residue. Seed-predators preferred to feed on the broadleaf seeds which reduced the broadleaf weeds competitive ability when grown with grasses. Decreasing seed-predators by 50% resulted in almost no change in broadleafplant relative yields compared with the control (no seedpredators). The decrease in seedling PRY caused by seed-predators was still evident as the plants matured. There was a significant interaction between seed-predators and residue in reducing broadleaf PRY probably because of differing effects of each on plant growth.

1. Introduction

Many studies have demonstrated the effect of post-dispersal seed predators on plant community structure in natural habitats (Brown et al., 1975; Reichman, 1979; Melhop and Scott, 1983). Normally ants or rodents are the major seed-predators in these systems (Inouye et al., 1980; Anderson, 1982). Risch and Carrol (1986) made one of the few studies to investigate the significance of post-dispersal seed predators in an agroecosystem. In modern commercial agriculture, most fac~Present address: SWPAP R.R. 6, Box 139A, Vincennes, IN 47591, USA.

tors that interfere with yields (e.g. weeds, insects, disease, etc. ) are either eliminated or rigidly controlled. These practices result in a reduction in the diversity and number of arthropods that can act as beneficial natural controls. Low-input no-tillage systems (LINTS) more closely resemble a natural community with its species richness and diversity (House and Stinner, 1987). Ground beetles (Coleoptra: Carabidue) and field crickets (Orthoptera: Gryllidae) are greatly increased in LINTS (House and Stinner, 1987). While ground beetles are usually considered insect predators that prey on many different pest species (Best and Beegle, 1977; Brust et al., 1985) they, along with crickets, are also major weed-seed predators (Brust and

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G.E. Brust / Agriculture, Ecosystemsand Environment48 (1994) 27-34 House, 1988). However, neither has been considered as a natural control agent important in influencing weed species density. Research by Shilling et al. (1985) in North Carolina no-tillage systems demonstrated a significant reduction in broadleaf weeds and a less significant reduction in grass weed species compared with conventional-tillage systems. Wheat residue reduced the plant population and biomass of two common broadleaf weed species, red root pigweed, Amaranthus retroflexus L., and common lambsquarters, Chenopodium album L. and to a lesser extent large crabgrass, Digatara sanguinales L. and Fall panicum, Panicum dichotorlorum L. Most studies attribute reduced weed numbers to allelopathy or the physical attributes of the residue which interfere with plant growth. When a low-input no-tillage study was initiated in North Carolina in 1985, the broadleaf weeds, red root pigweed and lambsquarters, comprised 48% of all weeds in the field; 3 years later, they comprised only 28% of the weed species. Observations made in these LINTS and studies conducted by Picket and Squieres (1989) indicate that soil arthropods (i.e. seed-predators) found in great abundance under the residue cover may be influencing the weed-species population. Exclusion techniques were used in greenhouse studies to compare more closely the interactions between broadleaf and grass weed species in the presence and absence of seed-predators and wheat residue. The experiments were conducted to discern if seed-predators can influence the competitive interaction of the broadleaf species with the grass weed species in the presence or absence of a wheat residue.

2. Materials and methods

2. I. Low input no-tillage fields The seed density of lambsquarters, red root pigweed, large crabgrass, and fall panicum in LINTS was determined as follows: Samples were taken in late autumn after weed dispersal. Twenty 0.01 m 2 quadrats were randomly taken in each

50 m × 100 m LINTS (four reps) over a 2 year period (1985-1986). The top 20 m m of soil was excavated within each quadrat, spread out over 2 m × 3 m white cardboard and seeds of each weed species counted. These four species made up 83% of all weed-seed present in the samples. Density of weed-seed predators was determined by randomly taking 20 0.1 m 2 quadrat samples in each LINTS. Arthropods identified as seed-predators (Brust and House, 1988) in order of abundance were Carabids: Amara cuproleotaL. Amara impuncticollis, (as a genus Amara represented 33% of all seed-predators), Anisodactylus mercula, Germer, Anisodactylus rusticus (17%), Selenophorus spp. (5%), Harpalus pennsylvanicus say (5%), Agonum spp. (4%), Stenolophus spp. (4%) and a cricket: Gryllus pennsylvanicus L. (15%).

2.2. Greenhouse experiments Greenhouse experiments were conducted in 1988 and 1989 using the endemic densities of the different weed-seed predators and weed-seed species equivalent to those recorded in the field. A 35 m 2 area of greenhouse was filled with a sterile mixture of 70% sand and 30% silty soil to a depth of 10 cm. This area was fertilized similarly to field plots (N 65 k g h a -~, P 12 k g h a -1, K 12 kg ha-1. Sixteen enclosures ( 1.6 m × 1.4 m; 2.25 m 2) were created by using 30 cm wide metal sheathing with 10 cm of the sheathing below the soil surface and 20 cm above. In these experiments, seed proportions based on the relative mass of seeds rather than numbers were used because the interest is in seedling establishment and competition (Harper, 1977; Risch and Carrol, 1986). The mass of seeds used for each weed species was 86 nag per 0.1 m 2 (seeds were obtained from weeds of a crop production system). The seeds of each species were evenly distributed over a 0.1 m 2 area in each enclosure (four mono-seeded areas per enclosure) and a similar density of a mixture of one broadleaf and one grass species was seeded in the other four areas within an enclosure. This resulted in eight seeded areas per enclosure. Half (eight) of the enclosures received 15 carabids (same den-

G.E. Brust /Agriculture, Ecosystemsand Environment 48 (1994) 27-34

sity as was found in the field) and five crickets. Carabids and crickets were field-collected from LINTS. One-half (four) of all predator enclosures and one-half (four) of all non-predator enclosures received a cover of dried wheat straw ( 10 cm in depth) equivalent to that found in the LINTS in early spring (6000 kg h a - l ) . Seeds were sprinkled into the enclosures before straw placement and were left on the soil surface to simulate field conditions. Regular watering buried the weed-seeds in the soil for germination. An 8 cm strip of straw (6 cm deep) was placed as a border around the edge of each arena so seed predators could hide in these areas during the day. In addition to weed-seeds, 25 g of a dog food (Top Choice ®) was also placed in each arena to provide a meat source for carabids. The dog food was replaced as needed. The top 6 cm of soil was removed and replaced after each experiment. Additional experiments (in 1989) were conducted as previously described with the only exception being the type or density of seed-predator used. Differing proportions of seed-predators were used, with the number endemic to the LINTS being set at lx (20 seed-predators per 2.25 m2). This resulted in three levels of seedpredators being examined in the areas: 1/2x, lx and 2 x . Another set of experiments examined the effects of crickets ( 15 ) and carabids ( 15 ) alone at a 1x rate. All of the modified predator studies were set up as previously described. These two sets of experiments were conducted twice. Total plant biomass and the number of weed seedlings were determined 30 days after placing the seeds in the enclosures. A 0.0625 ( 1/16) m 2 quadrat was placed in the center of each weed area and the plants within the quadrat were removed, counted, sorted to species (if mixedseeding) oven dried to a constant mass and weighed. Experiments were conducted at temperatures and a photoperiod similar to those occurring in late autumn or early spring in North Carolina. To determine whether the competitive interactions of seedlings were similar to those of more mature plants, the harvesting of the fourth experimental replication was changed from the previous three. Instead of sampling a 0.0625 m 2

29

area of seedlings, only 1/2 this area (0.03125 m 2) was sampled for seedlings. One month later (a total of 2 months growth), the other half was harvested. 2.3. Data analysis

Total weight and number of weeds were analyzed using a randomized complete block (RCB) ANOVA. The comparison of seedling and older plants was analyzed using a repeated measures ANOVA and regression analysis. An analysis of competition was used to determine the effect of seed-predators and residue on broadleafand grass competition. The performances of broadleaf weeds and grasses in monoculture and in mixture were compared by calculating the respective plant relative yield (PRY) (Trenbath, 1974). The PRY of a broadleaf in competition with a grass is the total dry weight per plant of the broadleaf grown in mixture with the grass divided by its (broadleaf) per plant dry weight when grown in monoculture. When a treatment is applied and the PRY of the broadleaf weed changes in comparison with its PRY in the control (no seed-predators, no residue) then the new variable has changed the competitiveness of broadleaf with respect to the grass (Harper, 1977). Orthogonal contrasts were used to separate differences in PRY values. In order to focus on the broadleaf/grass interaction, only the plant relative yields of the broadleaf weeds compared with the grass weeds will be examined in this study.

3. Results 3.1. Weed growth

There were no significant differences among the four individual trials. Therefore, the results were pooled and treated as blocks giving a total of 16 replications for each treatment. Weed emergence usually occurred within 5-7 days of watering. Both grass species usually emerged at the same time and prior to the broadleaf species. Pigweed emerged before lambsquarters. Cara-

G.E. Brust / Agriculture, Ecosystems and Environment 48 (1994) 27-34

b i d s a n d crickets w e r e o b s e r v e d f e e d i n g o n w e e d seeds a n d o c c a s i o n a l l y o n t h e seedlings at night b u t v e r y s e l d o m d u r i n g the day, a l t h o u g h crickets were o c c a s i o n a l l y f o u n d f e e d i n g o n c l o u d y days. O n c e d i s t u r b e d , c a r a b i d s w o u l d leave t h e f e e d i n g a r e a f o r a few m i n u t e s t h e n r e t u r n to the s a m e area. C r i c k e t s t e n d e d n o t to r e t u r n t o the same area once disturbed. T h e r e w a s a 56% a n d 66% r e d u c t i o n in t h e seedling w e i g h t o f l a m b s q u a r t e r s a n d p i g w e e d (respectively) when carabids were present comp a r e d w i t h the c o n t r o l (Fig. 1 ). H o w e v e r , t h e r e w a s o n l y a 2 0 % a n d 25% d e c r e a s e in the seedling w e i g h t o f large c r a b g r a s s a n d fall p a n i c u m (res p e c t i v e l y ) w h e n s e e d - p r e d a t o r s were p r e s e n t compared with the control. Residue treatments h a d a s i m i l a r effect o n b r o a d l e a f a n d grass w e e d s c o m p a r e d with the s e e d - p r e d a t o r t r e a t m e n t (Fig. 1 ). T h e r e was n o significant i n t e r a c t i o n b e t w e e n s e e d - p r e d a t o r s a n d residue.

1.2• LAM/LC

1

[]

LAM/FP

tu -J ~ o.8 ~tr la. u. 0.6 iii .a ¢"t 0,4 ,~ 0t r m 0,2

[]

PIG/LC

[]

PIG/FP

0 CONTROL

PREDATORS

PREDS.+RESIDUE

Fig. 2 The plant relative yield (PRY) of two broadleafweeds (LAM = lambsquarters; PIG = red root pigweed ) grown with two grass weed species (LC=large crabgrass; FP= fall panicum ) and exposed to seed-predators, a wheat straw residue, a combination of both and a control (no seed-predators or residue present). Bars represent + 1 S.E.

12

,M W

~ 03

RESIDUE

TREATMENT

16

03 ¢3

8

Table I Plant relative yields (PRY) for broadleafweeds grown with grass weeds with and without wheat residue and seedpredators

0

o

...

CONTROL

PREDATORS

RESIDUE

PREDS.+RES.

TREATMENT

Fig. 1 Weight of 1 month-old seedlings of four weed species after exposure to seed-predators, wheat straw residue and a combination of predators. Preds.= seed predators, res.=wheat straw and control=no seed-predators and no wheat straw. Bars represent +_1 S.E.

--

Treatment

PRY values I

Control Residue-only Seed-predators-only Seed-predators + residue

1.037a 0.648 b 0.709 b 0.420 ¢

Broadleaf/grass lambsquarter/large crabgrass lambsquarter/fall panicum pigweed/large crabgrass pigweed/fall panicum

0.551 ¢ 0.671 b 0.822 a 0.670 b

1Means with different letters are significantly different from one another at the P< 0.05 level.

G.E. Brust / Agriculture, Ecosystems and Environment 48 (1994) 27-34 1.2

3.2. Competitive interactions Plant relative yields of the broadleaf weeds were greatest in the control and significantly less in the other three treatments (Table 1 ). The PRY of the broadleaf weeds in the residue and predator treatments were not significantly different from one another. There was a significant reduction in the PRY of the broadleaf weeds in the seed-predatorX residue interaction (Table 1 ). The PRY of lambsquarters/large crabgrass was the lowest while the PRY of pigweed/large crabgrass was the highest for any of the four combinations (Table 1 ). The pigweed/fall panicum and lambsquarters/fall panicum, PRY did not differ. Seed-predators, residue or their combination reduced the PRY of the lambsquarters/ grass combination compared with the pigweed/ grass combinations (Fig. 2).

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3. 3. Mature weed growth

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Fig. 4 Plant relative yields of two broadleaf weeds when exposed to seed-predators of three different densities ( 1/2x = ten seed-predators per 2.25 m 2, lx = 20 seed-predators per 2.25 m 2 and 2x= 40 seed-predators per 2.25 m2). Bars represent + I S.E.

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RESIDUE

TREATMENTS

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31

1.2

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PRY VALUES SMALL PLANTS Fig. 3 The plant relative yield (PRY) values of 1 month-old broadleaf seedlings compared with 2 month-old broadleaf seedlings.

There was a significant relationship (y=0.118+0.523 x, r2=0.626, P<0.001) between seedling PRY values and older plant PRY values (Fig. 3). Plant relative yields remained relatively the same as the seedlings grew into larger more mature plants in the control, predators or residue + predators treatments. However, PRY increased slightly as plants matured in the residue treatment. A comparison of PRY for each of the broadleaf/grass combinations between seedling and older plants showed a general decrease or no change in PRY.

3.4. Change in predator ratios Increasing seed predators to twice the endemic density decreased all the PRY of the broadleaf

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G.E. Brust / Agriculture, Ecosystems and Environment 48 (1994) 27-34

weeds by 30% in the modified-predator treatmerit and by 52% in the predators and residue treatment (Fig. 4). Decreasing predators by half increased broadleaf PRY in this treatment by approximately 19% compared with the lx predator treatment (Fig. 4). The PRY in the 1/2 x modified-predator x residue treatment were almost the same as the PRY in the residue only treatment (Fig. 4a). 3.5. Change in weed-seed predator composition

Broadleaf weeds in carabid-only treatments had lower overall PRY than in cricket-only treatments (Table 2). Having a lx population ofcarabid-only seed-predators decreased the PRY of the broadleaf weeds compared with having a mixed Ix population of carabids and crickets (Table 2 ). When residue was added to the three different predator groups there was a significant ( P < 0 . 0 5 ) reduction in PRY for the broadleaf weeds in the carabid-only+residue (52%) and mixed predator + residue (41%) compared with the predator-only (no residue) treatment. Table 2 Plant relative yields (PRY) of broadleafweeds (Chenopodium sp. and Amaranthus sp.) when grownwith grass weeds with Carabids-onlyCricketsor a mixedpopulationof the two with and withoutresidue Treatment

Control (no residueor predators) Residue Seed-predator Seed-predator +residue

PRY valuest Crickets Carabids only only

1x mix

1.035 A a 0.975A a 0.659 Abc 0,632Ab 0.741 A b 0.602B b

1.077A a 0.644A b 0.713ABb

0.579 A c 0.302B c

0.382B c

Broadleaf/grass mixture Lambs/largecrabgrass 0.630Aa Lambs/fallpanicum 0.679A a Pigweed/largecrabgrass 0.704A a Pigweed/fallpanicum 0.864A b

0.505Aa 0.697A b 0.863B c 0.761ABbc

0.531Aa 0.640A ab 0.785ABc 0.706 B bc

tMeans with differentletters are significantlydifferentfrom one anotherat the P< 0.05 level.Meanswith upper caseletters are comparisonswithin a row; means with lower case letters are comparisonswithina column.

Cricket-only treatments had more effect, i.e. increasing the PRY of the broadleaf weed, on the lambsquarters/large crabgrass mixture than the carabid-only or a 1x-mixed population of predators (Table 2). The PRY oflambsquarters when grown with large crabgrass decreased in the carabid-only treatment compared with the cricketonly or Ix-mixed population treatments. However, carabids increased the PRY of pigweed when it was grown with large crabgrass compared with the cricket-only treatment (Table 2).

4. Discussion Seed-predators and residue greatly reduced the weight of broadleaf seedlings compared with the control. Grass seedling weight was reduced when seed-predators were present but residue reduced seedling weight even more. The seed-predators appeared to prefer broadleaf seeds over grass seeds, as had been previously demonstrated (Brust and House, 1988). This feeding preference resulted in a competitive disadvantage for broadleaf weeds. Seed-predators preferentially fed on and eliminated many more of the broadleaf weed seeds compared with the grass weed seeds. Mittlebach and Gross (1984) found that seed-predators fed selectively on weed seeds in old field environments. Other seed-predators (mostly ants) were found to lower the overall abundance of many weed species in disturbed habitats (Risch and Carroll, 1986), which resulted in a reversal of plant competition compared with when ants were not present. Similar results can be observed in this study when the broadleaf weeds were grown with grasses. In the control, PRY were close to one but when seedpredators were added, the broadleaf PRY dropped significantly in all the broadleaf/grass pairs. When PRY of the broadleafweeds are significantly reduced by seed-predators they are less able to compete with grasses (Trenbath, 1974). This selective feeding by seed predators is thought to occur because they are maximizing the return/handling time investment in feeding and opening a particular seed (Reichman, 1977; Whitford, 1978). However, the weights of the

G.E. Brust / Agriculture, Ecosystems and Environment 48 (1994) 27-34

four different seeds do not support this hypothesis if mass is the only consideration (i.e. pigweed=0.5 mg/seed; lambsquarters=0.6 rag/ seed; fall panicum=0.8 mg/seed and large crabgrass = 0.7 rag/seed). Other researchers have found that while seed mass is important, a number of other factors influence seed selection, including seed abundance, seed morphology and nutritional quality of the seed species (Tevis, 1958; Inouye et al., 1980; Anderson, 1982). Both broadieaf and grass seeds were of equal total mass and each was distributed randomly with the other. However, lambsquarters and pigweed have a 41% and 36% (respectively) higher oil content compared with either of the two grasses (Levin, 1974). These oils are in the form of triglycerides of fatty acids which are important components of insect diets (Chapman, 1982 ). Therefore, the seed-predators were probably maximizing their handling time by feeding selectively on the seed species which could return the most nutrients for the effort. As demonstrated in this study and others (Risch and Carroll, 1986), there is a relationship between the early growth reduction of plants and the reduced growth potential and competitive ability of older, more mature plants. Thus, if seed-predators can reduce early competitiveness of certain weed species, then this disadvantage will continue as the plant matures. Therefore, seed-predators can influence the number of mature weeds and thus, indirectly, affect the seed production of the weed species they feed upon as a seed. Doubling the density of seed-predators reduced the competitive ability of the broadleaf weeds by approximately 30%. At the 1/2 x rate of predators, PRY were not different between residue and residue + 1/2 x seed-predators. This demonstrates there were not enough seed-predators (ten per 2.2 m 2) to exert an influence on the PRY of the broadleaf weeds. This may explain why carabids and crickets have been ignored in agroecosystems as possibly influencing weed patterns or distributions. Their numbers are usually greatly reduced in cultivated agricultural fields compared with natural or no-tillage systems (House and All, 1981; House and Stinner, 1983 ). However, with the increase of no-tillage

33

systems and low-input sustainable agricultural systems, soil seed-predators can now be found in much greater densities. The contribution of weed-seed predators to reduce selectively certain weed species and consequently their survival can be significant. The overall average reduction of weed growth, mostly broadleaf, in field trials using wheat mulch was 60% (Shilling et al., 1985). The average reduction of weed growth (i.e. broadleaf) in the seedpredators + residue treatment of this study was 63%. This is not meant to imply a strong relationship between this study (greenhouse) and the field study, but it does support the hypothesis that seed-predators probably do contribute to the overall weed reduction in no-tillage agroecosysterns as demonstrated in the LINTS started in 1985. As might be expected, and as found in other studies of seed-predators (Janzen, 1971; Brown et al., 1975; Davidson, 1977), there were different seed-species preferences by the two seed-predator groups. When large crabgrass seeds were present, carabids selected the broadleaf seeds to a much greater extent than when fall panicum seeds were present, which greatly reduced the competitiveness of the broadleafweeds. Crickets preferred lambsquarters seeds compared with pigweed seeds. Thus, carabids may be the most important factor affecting some of the different seed mixtures while crickets affected other mixtures more strongly. This study also shows that some seed combinations (e.g. lambsquarters and large crabgrass) were greatly affected by both seed-predators, but other seed combinations were affected little by either seed-predator. Therefore, depending on the makeup of the soil arthropod population and the different seed mixtures, there could be several different outcomes as to which weed species best establishes itself in any particular microhabitat. Other studies have also shown that seed-predator preference has a strong influence in determining the distribution of certain plants in natural communities (Harper, 1987; Smith, 1987).

5. Conclusions Plant residues on the soil surface affect weed

G.E. Brust / Agriculture, Ecosystems and Environment 48 (1994) 27-34

growth in three ways: ( 1 ) by releasing chemicals that slow weed growth in some weed species which reduces their competitiveness; (2) by physically interfering with the growth o f the seeds; ( 3 ) by a u g m e n t i n g the n u m b e r o f soil art h r o p o d s which feed selectively o n some weed seeds which also reduces their competitiveness. This interaction significantly reduces the b r o a d leaf P R Y which is p r o b a b l y the results o f differential effects o f the three factors on weed growth. The seed-predators c o n s u m e the seeds which rem o v e s t h e m f r o m any further competition. The residue affects g e r m i n a t i o n a n d retards seedling growth. Therefore, the two factors act u p o n different phases o f plant growth and together greatly reduce the competitiveness o f the b r o a d l e a f weeds c o m p a r e d with the grass weeds. I f the goal o f weed biological control is to change the plant species c o m p o s i t i o n in a particular area (Harris, 1988) then carabids and crickets could be c o n s i d e r e d biocontrol agents o f certain weeds. This study has d e m o n s t r a t e d that seed-predators (i.e. carabids and crickets) f o u n d in m o s t no-tillage row crop agroecosystems can influence weed species c o m p o s i t i o n . The combination o f residue a n d seed-predators greatly reduces b r o a d l e a f weed growth while also reducing, to a lesser extent, the grass weeds. As growers look for new ways to control weeds without relying totally on chemicals, the utilization o f biological controls b e c o m e s m o r e p r o m i n e n t .

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tworm interactions in corn agroecosystems.J. Econ. Entomol., 78: 1389-1392. Chapman, R. F., 1982. The Insects: Structure and Function. Harvard Univ. Press, Cambridge, MA, pp. 84-99. Davidson, D. W., 1977. Foragingecologyand community organization in desert seed-eating ants. Ecology, 58: 725737. Harper, J. L., 1977. Populations of plants. Academic Press, London. Harris, P., 1988. Environmental impact of weed-control insects. BioScience, 38: 542-548. House, G. J. and All, J. N., 1981. Carabid beetles in soybean agroecosystems:community composition and ecosystem interactions. Environ. Manage., 7: 23-28. House, G. J. and Stinner, B. R., 1987. Arthropods in conservation tillage systems. Miscell. Pub No. 65. Entomol. Soc. Am. Inouye, R. S., Byers, G. S. and Brown, J. H., 1980. Effects of predation and competition on survivorship, fecundity and community structure of desert annuals. Ecology,61: 13441351. Janzen, D. J., 1971. Seed predation by animals. Ann Rev. Ecol. Syst., 2: 465-492. Levin, D. A., 1974. The oil content of seeds: an ecological perspective. Am. Nat., 108: 193-206. Mehlhop, P. and Scott, N. J. Jr. 1983. Temporal patterns of seed use and availability in a guild of desert ants. Ecol. Ent., 8: 69-85. Mittlebach, G. G. and Gross, K. L., 1984. Experimental studies of seed predation in old-fields. Oecologia, 65: 7-13. Picket, S. T. A. and Squeirs, E. R., 1989. Experimental approaches to understanding community organization and dynamics: Confirmation and surprise in Piedmont old fields. EcologicalSoc. Am. Abstr., 72:136. Reichman, O. J., 1977. Optimization of diets through food preferences by heteromyid rodents. Ecology,58: 454-457. Reichman, O. J., 1979. Foraging specializations of individual seed-harvest ants. Behav. Ecol. Sociobiol., 9:149-152. Risch, S. J. and Carrol, C. R., 1986. Effects of seed predation by a tropical ant on competition among weeds. Ecology, 67: 1319-1327. Shilling, D. G., Liebl, R. A. and Worsham, A. D., 1985. Rye and wheat mulch: The suppression of certain broadleaf weeds and the isolation and identification ofphytotoxins. In: The Chemistry of Allelopathy. Am. Chem. Sot., pp. 251-265. Smith, T. J., 1987. Seed predation in relationship to tree dominance and distribution in mangrove forests. Ecology, 68: 266-273. Tevis, L., Jr., 1958. Interactions between the harvester ant Veromesor pergandei (Mayr) and some desert ephemerals. Ecology,39: 695-704. Trenbath, B. R., 1974. Biomassproductivityof mixtures. Adv. Agron., 25: 177-181. Whitford, S.C., 1978. Foraging in seed harvester ants Pogonomyrmex spp. Ecology,59:185-189.