Dynamical role of predators in population cycles of a forest insect: An experimental test

Science

Volumn 285, Issue 5430, 1999, Pages 1068-1071

  Turchin, P ^a   Taylor, A D ^a   Reeve, J D ^a  

^a NONE

Author keywords

[No Author keywords available]

Indexed keywords

BEETLE; DENSITY DEPENDENCE; POPULATION CYCLE; PREDATOR-PREY INTERACTION;

ARTICLE; BEETLE; ECOLOGY; FOREST; INSECT; NONHUMAN; POPULATION DYNAMICS; PREDATION; PREDATOR; PRIORITY JOURNAL;

COLEOPTERA; DENDROCTONUS FRONTALIS; HEXAPODA; INSECTA; PINUS ECHINATA; SCOLYTINAE;

EID: 0033551902     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.285.5430.1068     Document Type: Article

References (31)

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16. First-order predation acting in an overcompensating manner may cause cycles, but such first-order cycles are characterized by short periods (for example, two-point cycles; even four-or eight-point first-order cycles are dominated by a period of 2) as opposed to multiannual (or multigenerational) second-order cycles with which we are concerned in this report. Note that SPB oscillations are characterized by a period of 6 to 8 years (7) or 30 to 48 generations, assuming five to six SPB generations per year (29).
17. The cage design and experimental protocol are similar to previous work (30) and only briefly summarized here. At the beginning of each experiment, we located stands of loblolly pine in the Kisatchie National Forest, Louisiana, USA, and selected trees of the same diameter (25 to 30 cm) separated by 100 m. On some ("cage") trees we installed 2-m-tall cylindrical exclosures made from polyethylene screening, and other trees were used as controls. The cage was divided into a central 1-m experimental area with 0.5-m buffer zones above and below, which acted as barriers to the movement of insects into the experimental area. (Some predators managed to enter the exclusion zone, however, and thus our results are probably conservative with respect to measuring predation impact.) The trees were baited with SPB aggregation pheromone to induce attack, and then adult beetles (2000 total) were added to the experimental area of the cage in installments, generating an attack pattern and density that mimicked exposed areas. The densities of successful attacks and eggs inside and outside the cage were estimated by taking bark samples (30) after egg laying was complete. When brood development was complete, sections of the trunk were cut from inside and outside the cage and placed in individual rearing cans, and the number of emerging SPB recorded. Apart from having no cage installed, the control trees were treated exactly as the cage trees. In addition to using replicate trees at each location, we replicated the study spatially (two locations separated by at least 3 km) and seasonally (early summer versus fall), for a total of four studies each year. The complete data set over the 5-year course of the study consisted of 56 cage and 64 control trees. We quantified predation impact in two ways: by calculating the survival (emerging adults divided by eggs) and ratio of increase (emerging adults divided by successful attacks) both inside and outside cages. The densities of successful attacks and eggs were statistically indistinguishable inside compared with outside the cages, except for 1994, when egg density was significantly lower outside cages. This does not affect our main result however, because lower egg density elevated SPB survival outside cages, resulting in a conservative estimate of predation impact.
18. The statistical significance of the effect of predator exclusion was assessed by t tests, treating each year as a separate comparison. The survival on control trees versus outside cages on experimental trees was not significantly different (apart from fall of 1992), so we treated these replicates as a single category, "exposed." Our main result was not affected by this data pooling, because in 1993, the first year after the peak, survival inside cages was significantly higher than survival both outside cages on experimental trees and on control trees (P < 0.001 for both comparisons). The comparisons in the ratio of increase were performed in the same manner, except the data were first log-transformed to stabilize the variance.
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31. This research was supported by USDA-Forest Service RWU-4501 and by NSF grant DEB 9509237. We thank D. Rhodes and the staff of Kisatchie National Forest for assistance in the field. J. Elkinton, J. Hayes, J. Cronin, A. Berryman, and C. Godfray provided helpful comments and suggestions on an early version of the manuscript.