NEAREST NEIGHBOR AS A MEASURE OF SPATIAL RELATIONSHIPS 445 but since logging operations and the burning and reburning that followed this logging, aspen and white birch have taken over the role as dominants.Thirty-two islands and 5 mainland areas were chosen for the study. Three species of hibernators, Eutamias minimus, Tamias striatus and Zapus hudsonias were taken on the mainland with only the Zapus being found on the islands. All six of the non-hibernating species taken were found on the islands. These include Microtus pennsylvanicus, Synaptomys cooperi, Clethrionomys gapperi, Peromyscus maniculatus, Blarina brevicauda and Sorex cinereus.The degree of occupancy changed from 4 out of 21 islands in 1950 to 21 of 24 islands in 1952. Along with this increase in the proportion of islands occupied there was an increase in the number of species found on most of the islands although the average number of species iound on each occupied island did not change appreciably. This increase in islands occupied was correlated with an increase in the density of the mainland populations.The water seemed to be an efficient barrier to travel during the summer but the ice of winter became a highway for dispersal of the various nonhibernating species in the winter.Considerable difference was found in the vulnerability of the several species to trappin~. The Peromyscus were readily caught. The Clethrionomys were not as easy to capture as the former but were still readily taken. The Microtus, how~ ever, posed a different problem. They were difficult to capture and on two islands of about 2.2 and 4 acres respectively we were not able to elim~ inate them after 16 days of trapping on a 15-foot grid.The populations of small mammals fluctuate quite widely and the several populations appear to be somewhat independent of each other. The islands appeared to lag behind the mainland in the development of their populations.

The plants and Homoptera on three old fields in southeast Michigan were sampled. Within fields, correlations between plant and insect diversity were generally weak. But using all samples from three fields, evenness (J) and diversity (H) of the insects were highly correlated with plant evenness and plant diversity, respectively. For example, 72% of the variance in insect H could be accounted for by variation in plant H. Number of species (S) showed a positive but weaker correlation. When correlations were based on the pooled samples from each field, all three statistics for insects were highly correlated with those for the plants. Insect H was also highly correlated with plant structure (foilage height diversity FHD) over all three fields. These two measures of plant diversity (H and FHD) were highly correlated and were equally good correlates of insect H. Together they accounted fir 79% of the variance in insect H. This extends to insects a correlation between plant and animal diversity, already well established for birds and possibly true for lizards and rodents. It leaves open the unresolved question as to whether plant structure or plant species diversity is more important.

We examined the spatial variability of N mineralization, nitrification, and denitrification at a resolution of 1 m over a 0.5—ha portion of an old field in southeast Michigan. Net mineralization and nitrification rates were estimated from changes in ammonium and nitrate during 45—d laboratory incubations of soil from >300 individual sample locations. Denitrification was estimated from nitrous oxide accumulation rates during 24—h incubations of intact cores (n = 252) under acetylene atmospheres at a pressure of 10 kPa. We used geostatistical procedures to characterize the spatial distributions of these and other soil variates. Semivariograms for all three N transformations showed a high degree of spatial dependence among points sampled within 1—40 m of one another. Nugget variances were 27—37% of structural variances, indicating that most of the variation within the sample populations for these rates could be attributed to spatial autocorrelation at a scale >1 m. Isopleths calculated using punctual kriging algorithms show a nonuniform distribution of these transformations across the field. High rates of all processes occurred in swales on the northern edge of the sample area, but also occurred elsewhere in the field on drier, more level sites. These results indicate that spatial characteristics of the measured nitrogen transformations in this old field are complex, and that only some of this complexity is associated with surface topography. Whether spatial complexity affects or mainly reflects plant community structure is not known, but this small—scale heterogeneity may influence existing plant and microbial population dynamics and should be considered by those attempting to understand community dynamics or to quantify ecosystem—level nutrient fluxes.

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