Long-term priority effects among insects and fungi colonizing decaying wood

Abstract

1. Priority effects have been hypothesized to have long-lasting impact on community structure in natural ecosystems. Long-term studies of priority effects in natural ecosystems are however sparse, especially in terrestrial ecosystems. 2. Wood decay is a slow process involving a high diversity of insect and fungus species. Species interactions that drive change in communities of insects and fungi during wood decay are poorly understood because of a lack of sufficient long-term studies. 3. In this paper, we followed the colonization and succession of wood-living insects and fungi on cut trees during 15 years, from tree death and onwards, in a boreal forest landscape. We test the long-term priority effects hypothesis that the identity and abundance of species that colonize first affect the colonization success of later-arriving species. We also hypothesize that species interact in both facilitative and inhibitory ways, which ultimately affect habitat quality for a red-listed late-succession beetle species. 4. Possible causal associations between species were explored by path analysis. The results indicate that one bark beetle species, Hylurgops palliatus, and one wood-borer species, Monochamus sutor, which colonized the wood during the first year after cutting, influenced the occurrence of a rare, wood-living beetle, Peltis grossa, that started to emerge from the stumps about 10 years later. The positive effects of Hylurgops palliatus and negative effects of M. sutor were largely mediated through the wood-decaying fungus species Fomitopsis pinicola. 5. The study shows that variable priority effects may have long-lasting impact on community assembly in decaying wood. The study also exemplifies new possibilities for managing populations of threatened species by exploring links between early, well-understood species guilds and late, more poorly understood species guilds.

Fig. 1

Hypothetical relationships between species. Hylurgops palliatus, the earliest colonizer, reduces habitat availability for Ips typographus, Pityogenes chalcographus and Monochamus sutor. Ips typographus and P. chalcographus, which fly at the same time, may reduce habitat availability for the later-arriving M. sutor. Field observations indicated that stumps with signs of M. sutor lost their bark early, which was seemingly negative for Fomitopsis pinicola. Trichaptum abietinum causes white-rot and F. pinicola causes brown-rot, which is a prerequisite for Peltis grossa. Direct associations are indicated with arrows, and negative or positive associations are indicated with signs.

Fig. 2

Development of 363 high stumps over time. (a) Total number of Peltis grossa emergence holes and living and dead fruiting bodies of Fomitopsis pinicola. (b) Proportion of stumps colonized by P. grossa, F. pinicola and Trichaptum abietinum. (c) Proportion remaining bark ± Standard Error (SE).

Fig. 3

Suggested pathways during 15 years of wood decay, from Hylurgops palliatus colonization to Peltis grossa emergence. Coefficients indicate direct effects; for total and indirect effects, see Table 3. Stump diameter and height were included as independent variables for each species but are not illustrated here. Regressions performed with ln-transformed values for stump diameter and stump height and with ln(x + 1)-transformed values for number of P. grossa and Monochamus sutor emergence holes per stump, and number of Fomitopsis pinicola fruiting bodies per stump. Untransformed values for Trichaptum abietinum frequency and area of bark colonized by H. palliatus. U = unexplained variance (1 − R²). *P<0·05, ***P<0·001. Goodness-of-fit index (GFI) 0·98, χ²=23, P<0·0001.