Apparency revisited

Abstract

How easy a plant is to find, or its apparency, is thought to shape plant defenses. Recent meta-analyses suggest that the types of plant defenses employed are not well-predicted by apparency, or apparency can be confounded with life history traits like woodiness and stature. Here, we suggest that the searching environments in which plants grow also influence plant apparency and should thus affect investment in plant defense. Specifically, bare, unvegetated environments may result in greater apparency of inhabitants of all statures to enemies, as a result of loss of associational resistance. We make several predictions about plant defenses in simple searching environments. (1) Plants living in simple searching environments should be more highly defended than plants living in more vegetated, complex searching environments. (2) Plant defenses involving signals-both, signals serving to hide plants and aposematic signals-should be favored in simple searching environments. (3) Levels of damage from enemies in simple searching environments should be related to defensive strategy (resistance, aposematism, mimicry, or crypsis); apparent plants should have low damage, because, as they are easily found, they should be well-defended though physical or chemical defense. In contrast, predictions about damage levels in cryptic plants are harder to make, as damage reflects both whether plants are encountered or not, as well as overall palatability. If crypsis is favored in more palatable species, as has been suggested previously, we predict that cryptic plants should have greater variance in damage and greater maximum damage, if, once found, plants are palatable. (4) Organisms from diverse evolutionary lineages inhabiting the same simple searching environments should adapt to selection from apparency by converging on similar background matching or aposematic defenses. We then test some of these predictions with descriptive data collections in two simple searching environments: largely unvegetated graywacke scree mountaintops of New Zealand and serpentine barrens of northern California (USA). We find that plants that are more apparent (i.e., do not match local rock color as measured across 300-700 nm wavelengths) are more defended, as inferred from mean damage received. In contrast, cryptic species in the same habitats get 79 more heavily damaged, once found, suggesting overall greater palatability. There was no evidence of greater variation in damage, as measured by coefficient of variation, but maximum damage was much greater on cryptic species in both habitats. Convergence on gray substrate is found in diverse species of plants in New Zealand, as well as by scree-living grasshoppers; in California, grasshoppers have also converged on substrate color, and seed color of a non-cryptic plant also matches local outcrops. Considering searching environment and enemy searchingabilities when evaluatingplant apparencytoenemiesmay shed more lightonthis challenge to plants.

Ontogenetic changes in insect herbivory in birch (Betula pubesecens): The importance of plant apparency

“…We found increases in the background losses of downy birch foliage to insects in natural environments as we moved from small saplings to large saplings and then to mature trees, while the among‐plant variation in herbivory decreased. Both these patterns fit the predictions of the plant apparency hypothesis (Feeny, ; Fowler, ; Strauss et al., ). First, the larger (more apparent) individuals of downy birch suffered higher insect herbivory than was seen in the smaller (less apparent) individuals, presumably due to the higher probabilities of herbivore encounters with larger plants.…”

“…The plant apparency hypothesis also predicts a higher among‐individual variation in insect herbivory in smaller (juvenile) than in larger (mature) plants: many juveniles would avoid damage because they are hidden, but those discovered by herbivores would receive relatively high amounts of damage (Fowler, ; Strauss et al., ). In contrast, the resource allocation theory would not predict that variation in foliar damage among individuals of the same size class would differ between juvenile and mature plants.…”

Mucilage‐bound sand reduces seed predation by ants but not by reducing apparency: a field test of 53 plant species

“…A positive value denotes that sand-covered seeds had a lower removal rate than controls, a negative indicates the opposite. Strauss et al (2015), in surveys of California and New Zealand plants, found that leaf damage was more variable in unapparent species; they suggested that the need to defend may be higher in less-well-matched plants. Numbers along the x-axis of panel B denote number of species tested per family.…”

“…While much is known about the composition, development, location, and underlying genetic mechanisms of mucilage production in a few select systems (e.g., Arabidopsis), fewer studies have examined the role of mucilage in ecological interactions in the field (Fuller and Hay 1983, Gutterman and Shem-Tov 1997, Engelbrecht and Garc ıa-Fayos 2012, Yang et al 2013, Engelbrecht et al 2014. By tightly binding a protective coating of substrate to the seed surface when dried, mucilage-bound substrate may create either a physical barrier to seeds or provide seed camouflage by reducing their visual or olfactory apparency (sensu Fuller and Hay 1983, Strauss and Cacho 2013, Strauss et al 2015. Many seeds are consumed by predators and any defensive functions of seed mucilage have only been tested in a small number of species.…”

Plant neighbour identity and invasive pathogen infection affect associational resistance to an invasive gall wasp

“…How easily a host plant is found by herbivores, i.e. its apparency (Strauss et al 2015), has been described as a key mechanism driving AR (Castagneyrol et al 2013). Both visual (Dulaurent et al 2012;Damien et al 2016) and chemical (Zhang and Schlyter 2004;Jactel et al 2011) cues used by insect herbivores to locate and colonize a host tree can be disrupted by the presence of heterospecific neighbours.…”