Species–area relationships across four trophic levels – decreasing island size truncates food chains

Abstract

That larger areas will typically host more diverse ecological assemblages than small ones has been regarded as one of the few fundamental 'laws' in ecology. Yet, area may affect not only species diversity, but also the trophic structure of the local ecological assemblage. In this context, recent theory on trophic island biogeography offers two clear-cut predictions: that the slope of the species-area relationship should increase with trophic rank, and that food chain length (i.e. the number of trophic levels) should increase with area. These predictions have rarely been verified in terrestrial systems. To offer a stringent test of key theory, we focused on local food chains consisting of trophic specialists: plants, lepidopteran herbivores, and their primary and secondary parasitoids. For each of these four trophic levels, we surveyed species richness across a set of 20 off-shore continental islands spanning a hundred-fold range in size. We then tested three specific hypotheses: that species richness is affected by island size, that the slope of the species-area curve is related to trophic rank, and that such differences in slope translate into variation in food chain length with island size. Consistent with these predictions, estimates of the species-area slope steepened from plants through herbivores and primary parasitoids to secondary parasitoids. As a result of the elevated sensitivity of top consumers to island size, food chain length decreased from large to small islands. Since island size did not detectably affect the ratio between generalists and specialists among either herbivores (polyphages vs oligophages) or parasitoids (idiobionts vs koinobionts), the patterns observed seemed more reflective of changes in the overall number of nodes and levels in local food webs than of changes in their linking structure. Overall, our results support the trophic-level hypothesis of island biogeography. Per extension, they suggest that landscape modification may imperil food web integrity and vital biotic interactions.

The geographical variation of network structure is scale dependent: understanding the biotic specialization of host–parasitoid networks

“…Similarly, our results show the importance of considering different spatial scales to get a broader understanding of the specialization pattern and their determinants. Additionally, if network structure varies across spatial scales (Pillai et al 2011, Roslin et al 2014, Wood et al 2015, Galiana et al 2018), then network studies estimating the causes of variation in network structure along any environmental gradient (Dormann et al 2017, Tylianakis and Morris 2017, Pellissier et al 2018) might benefit from understanding the spatial scaling of network structure along the gradient. However, caution must be exerted when interpreting the comparison between different spatial scales.…”

“…This is crucial, as several spatial processes, such as dispersal or species sorting, generate changes in network structure as the spatial scale of observation changes (Gravel et al 2011, Pillai et al 2011, Roslin et al 2014, Montoya and Galiana 2017, Galiana et al 2018). For instance, given that landscape configuration has different effects on each species depending on their dispersal abilities, (Ewers and Didham 2006), differences in the slopes of the species-area relationships (SAR) across trophic levels can emerge, promoting changes in network structure across spatial scales (Holt et al 1999, Holt 2009, Roslin et al 2014, Galiana et al 2018). These differences in the slope of SARs across trophic levels can reflect differences in β-diversity (i.e., site-to-site variation in community composition) across trophic levels, when the total extent of area and the size of the regional pool of species are accounted for (Storch et al 2012, Lazarina et al 2013).…”

A test of trophic and functional island biogeography theory with the avifauna of a continental archipelago

“…As habitat destruction and fragmentation continue to degrade ecosystems globally (Brooks et al, ; Fahrig, ), diversity–area relationships will be useful in understanding and predicting the effects of global change on the multiple dimensions of biodiversity (Didham, Lawton, Hammond, & Eggleton, ; Keil, Storch, & Jetz, ; van Noordwijk et al, ). Specifically, as the size of natural habitats decreases through human action, the trophic TIB suggests that predatory species will be the first at risk of population reduction and local extinction (Didham et al, ; Dobson et al, ; Fountain‐Jones et al, ; Roslin et al, ). The trophic TIB thus likely determines whether community disassembly occurs randomly or systematically.…”

Spatial and temporal genetic structure at the fourth trophic level in a fragmented landscape

“…This is because with increasing trophic level, the resource pool becomes sparse and locally unstable [1][2][3]. Parasitoids live at the third and fourth trophic levels and are an important part of virtually all insect communities [4].…”