Species Co-Occurrence Patterns in Temperate Forest Herb Communities: Are They Stable Through Time?

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With increasing frequency, conservation biologists are documenting declines in native species over time. We found evidence for 18% native species loss over 50 years in forest herb communities in northern Wisconsin, relying on baseline data collected by John Curtis and the Plant Ecology Laboratory at the University of Wisconsin during the 1940s and 50s. The relationship between these losses and patterns of community structure is unknown. We explore changes in community structure by examining co-occurrence patterns. We ask 3 questions: (1) is there evidence for non-random co-occurrence patterns of forest herbs? (2) do co-occurrence patterns shift through time? and (3) are shifts in co-occurrence patterns correlated with species loss? We assembled presence-absence matrices for each of 59 sites for both time periods (n = 118 matrices). Species were listed in rows, and quadrats in columns. The number of species in a matrix varied across sites, but included exactly 20 quadrats in all cases. For each matrix, we recorded the observed C-score and the number of checkerboard species pairs (NCSP), based on co-occurrence patterns in individual quadrats. To calculate a null distribution for C-scores and NCSPs, we created 5000 random matrices for each matrix, and calculated a standard effect size for observed C-scores and NCSPs. Species tend to co-occur more than would be expected due to chance alone, supporting our first hypothesis. Standard effect sizes for C-scores are significantly less than zero in both time periods (95% CI = −1.89 to −0.93 in 1950; −1.73 to −0.87 in 2000). The same is true for the NCSPs (95% CI = −1.68 to −0.66 in 1950; −1.51 to −0.69 in 2000). We find no evidence supporting our second and third hypotheses. Community structure as measured by patterns of co-occurrence remained remarkably stable through time, despite species losses and changes in species composition.


Presented at the 89th Ecological Society of America Annual Meeting, Portland, OR.

Oral Session 45: Forest Ecology II: Communities and Structure.

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